Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2024 • 336 Curved GaAs cantilever waveguides for the vertical coupling to photonic integrated circuits
    Qvotrup, Celeste and Liu, Zhe and Papon, Camille and Wieck, Andreas D. and Ludwig, Arne and Midolo, Leonardo
    Optics Express 32 3723 – 3734 (2024)
    We report the nanofabrication and characterization of optical spot-size converter couplers based on curved GaAs cantilever waveguides. Using the stress mismatch between the GaAs substrate and deposited Cr-Ni-Au strips, single-mode waveguides can be bent out-of-plane in a controllable manner. A stable and vertical orientation of the out-coupler is achieved by locking the spot-size converter at a fixed 90° angle via short-range forces. The optical transmission is characterized as a function of temperature and polarization, resulting in a broad-band chip-to-fiber coupling extending over 150 nm wavelength bandwidth at cryogenic temperatures, with the lower bound for the coupling efficiency into the TE mode being 16± 2% in the interval 900-1050 nm. The methods reported here are fully compatible with quantum photonic integrated circuit technology with quantum dot emitters, and open opportunities to design novel photonic devices with enhanced functionality. © 2024 Optica Publishing Group.
    view abstractdoi: 10.1364/OE.510799
  • 2023 • 335 Considerably Increased Dynamics of CO-Water Complexes over CO and Water Alone
    Bertram, Cord and Zaum, Christopher and Fang, Wei and Michaelides, Angelos and Morgenstern, Karina
    Nano Letters 23 4793 – 4799 (2023)
    Solvents are increasingly known to influence chemical reactivity. However, the microscopic origin of solvent effects is scarcely understood, particularly at the individual molecule level. To shed light on this, we explored a well-defined model system of water (D2O) and carbon monoxide on a single-crystal copper surface with time-lapsed low-temperature scanning tunneling microscopy (STM) and ab initio calculations. Through detailed measurements on a time scale of minutes to hours at the limit of single-molecule solvation, we find that at cryogenic temperatures CO-D2O complexes are more mobile than individual CO or water molecules. We also obtain detailed mechanistic insights into the motion of the complex. In diffusion-limited surface reactions, such a solvent-triggered increase in mobility would substantially increase the reaction yield. © 2023 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.3c00158
  • 2023 • 334 Electronic correlations and superconducting instability in La3Ni2 O7 under high pressure
    Lechermann, Frank and Gondolf, Jannik and Bötzel, Steffen and Eremin, Ilya M.
    Physical Review B 108 (2023)
    Motivated by the report of superconductivity in bilayer La3Ni2O7 at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100 K, a multiorbital non-Fermi-liquid state resulting from a loss of Ni-ligand coherence within a flat-band-dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between Ni-dz2 and Ni-dx2-y2 orbital character. In a model Hamiltonian picture, spin fluctuations originating mostly from the Ni-dz2 orbital give rise to strong tendencies towards a superconducting instability with a B1g or B2g order parameter. The dramatic enhancement of Tc in pressurized La3Ni2O7 is due to stronger Ni-dz2 correlations compared to those in the infinite-layer nickelates. © 2023 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.108.L201121
  • 2023 • 333 Enhanced activity and coke resistivity of NiCoFe nanoalloy catalyst in CO2 reforming of methane
    Das, Subhasis and Tillmann, Lukas and Xia, Wei and Muhler, Martin
    Journal of the Indian Chemical Society 100 (2023)
    NiCo nanoalloy catalysts were prepared from hydrotalcite precursors and used in CO2 reforming of methane (DRM) under atmospheric and 2 MPa pressure in a fixed-bed reactor at 700-850 °C. The Ni6Co1 catalyst with a molar ratio of Ni/Co to 6 showed the highest stability and activity in DRM under atmospheric pressure. This was due to the homogeneous dispersion of nanoalloy particles (∼14 nm) on the MgAl(O) support, which had a strong metal-support interaction. Nonetheless, a slow and continuous deactivation was spotted under 2 MPa pressure due to the coke deposition. Further modification of Ni6Co1 with optimum amount of Fe (in Ni6Co0.5Fe0.5) formed ternary NiCoFe nanoalloy with improved metal-support interaction and reduced alloy size (∼10 nm). The presence of Fe significantly improved the coke resistance capability and provided high stability under 2 MPa pressure. © 2023 Indian Chemical Society
    view abstractdoi: 10.1016/j.jics.2023.101049
  • 2023 • 332 Ensemble averages of ab initio optical, transport, and thermoelectric properties of hexagonal SixGe1-x alloys
    Borlido, Pedro and Bechstedt, Friedhelm and Botti, Silvana and Rödl, Claudia
    Physical Review Materials 7 (2023)
    Ge-rich hexagonal SiGe alloys have recently emerged as new direct-gap semiconductors with unprecedented potential for integration of photonics on silicon. We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal SixGe1-x alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasichemical approximation to obtain alloy averages of electronic properties. At low temperatures, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal Ge is direct with very weak optical transitions at the absorption edge. The alloy band gap remains direct for a Si content below 45% and the oscillator strength of the lowest optical transitions is efficiently enhanced by alloying. The optical spectra show clear trends and both absorption edges and prominent peaks can be tuned with composition. The dependence of transport coefficients on carrier concentration and temperature is similar in cubic and hexagonal alloys. However, the latter display an anisotropic response due to the reduced hexagonal symmetry. In particular, the transport mass exhibits a significant directional dependence. Seebeck coefficients and thermoelectric power factors of n-doped alloys show nonmonotonous variations with the Si content independently of temperature. © 2023 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.7.014602
  • 2023 • 331 Hydrogenation of different carbon substrates into light hydrocarbons by ball milling
    Li, Linfeng and Vozniuk, Olena and Cao, Zhengwen and Losch, Pit and Felderhoff, Michael and Schüth, Ferdi
    Nature Communications 14 (2023)
    The conversion of carbon-based solids, like non-recyclable plastics, biomass, and coal, into small molecules appears attractive from different points of view. However, the strong carbon–carbon bonds in these substances pose a severe obstacle, and thus—if such reactions are possible at all—high temperatures are required1–5. The Bergius process for coal conversion to hydrocarbons requires temperatures above 450 °C6, pyrolysis of different polymers to pyrolysis oil is also typically carried out at similar temperatures7,8. We have now discovered that efficient hydrogenation of different solid substrates with the carbon-based backbone to light hydrocarbons can be achieved at room temperature by ball milling. This mechanocatalytic method is surprisingly effective for a broad range of different carbon substrates, including even diamond. The reaction is found to proceed via a radical mechanism, as demonstrated by reactions in the presence of radical scavengers. This finding also adds to the currently limited knowledge in understanding mechanisms of reactions induced by ball milling. The results, guided by the insight into the mechanism, could induce more extended exploration to broaden the application scope and help to address the problem of plastic waste by a mechanocatalytic approach. © 2023, Springer Nature Limited.
    view abstractdoi: 10.1038/s41467-023-40915-5
  • 2023 • 330 Measurement of the efficiency of a bright quantum-dot-based single-photon source
    Antoniadis, Nadia O. and Tomm, Natasha and Javadi, Alisa and Schott, Rüdiger and Valentin, Sascha R. and Wieck, Andreas D. and Ludwig, Arne and Warburton, Richard J.
    Proceedings of SPIE - The International Society for Optical Engineering 12446 (2023)
    A single-photon source has been developed using a single quantum dot to mimic a two-level atom. Low noise is achieved by operation at low temperature, the use of very high quality material, and by embedding the quantum dots in a diode structure. A single quantum dot is tuned into resonance with an open microcavity, a highly miniaturised Fabry-Perot cavity. The Purcell factor is approximately ten resulting in a radiative lifetime of just 50 ps. An end-to-end efficiency of above 50% is achieved and the photons exhibit two-photon interference with a visibility of 98%. © 2023 SPIE.
    view abstractdoi: 10.1117/12.2657247
  • 2023 • 329 Microstructural and Tensile Properties Evolutions of Direct-Aged Waspaloy Produced by Wire Arc Additive Manufacturing
    Sazerat, Marjolaine and Nait-Ali, Azdine and Barot, Lucie and Cervellon, Alice and Lopez-Galilea, Inmaculada and Eyidi, Dominique and Joulain, Anne and Villechaise, Patrick and Cormier, Jonathan and Weber, Sebastian and Fortunier, Roland
    Minerals, Metals and Materials Series 717 – 737 (2023)
    The microstructure and tensile properties of direct-aged Waspaloy manufactured using wire arc-based Cold Metal Transfer (CMT) have been investigated. Samples were exposed to temperatures ranging from 700 to 900 °C, for up to 96 h. In the as-deposited condition, pronounced chemical segregation is inherited from the process, leading to heterogeneous γ′ precipitation between dendrite cores and interdendritic spacings. γ′ size and distribution were measured in both areas for each heat treatment, and a diffusion-controlled coarsening behavior following the Lifshitz–Slyozov–Wagner theory was observed for temperatures above 760 °C. Activation energies were calculated. Tensile tests at room temperature were carried out not only on the additively processed alloy before and after aging but also on wrought sub-solvus and super-solvus treated material for reference. Results showed that heat treatment significantly increased the yield strength and ultimate tensile strength of the CMT samples, of up to +340 MPa compared to the as-built conditions. Elongation, however, decreased from 40–45% to 16–28%. Direct-aged CMT Waspaloy exhibited similar behavior to that of wrought super-solvus Waspaloy, due to their large grains (~200–250 µm). Anisotropy in tensile properties was estimated by calculating the ratio of properties for horizontal and vertical specimens. Finally, the formation of intermetallic phases was assessed. Thermodynamic calculations predicted the formation of M23C6, η, and σ phases in interdendritic spacings at thermodynamic equilibrium in the range 700–900 °C. Using electron diffraction patterns and energy-dispersive X-ray spectrometry, intergranular (Cr, Mo)23C6 secondary carbides decorating grain boundaries and located near (Ti, Mo)C primary carbides in the interdendritic spacings were observed to nucleate and grow. © 2023, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-031-27447-3_43
  • 2023 • 328 Modulating Liquid-Liquid Transitions and Glass Formation in Zeolitic Imidazolate Frameworks by Decoration with Electron-Withdrawing Cyano Groups
    Song, Jianbo and Frentzel-Beyme, Louis and Pallach, Roman and Kolodzeiski, Pascal and Koutsianos, Athanasios and Xue, Wen-Long and Schmid, Rochus and Henke, Sebastian
    Journal of the American Chemical Society 145 9273 – 9284 (2023)
    The liquid phase of metal-organic frameworks (MOFs) is key for the preparation of melt-quenched bulk glasses as well as the shaping of these materials for various applications; however, only very few MOFs can be melted and transformed into stable glasses. Here, the solvothermal and mechanochemical preparation of a new series of functionalized derivatives of ZIF-4 (Zn(im)2, where im- = imidazolate and ZIF = zeolitic imidazolate framework) containing the cyano-functionalized imidazolate linkers CNim- (4-cynanoimidazolate) and dCNim- (4,5-dicyanoimidazolate) is reported. The strongly electron-withdrawing nature of the CN groups facilitates low-temperature melting of the materials (below 310 °C for some derivatives) and the formation of microporous ZIF glasses with remarkably low glass-transition temperatures (down to only about 250 °C) and strong resistance against recrystallization. Besides conventional ZIF-4, the CN-functionalized ZIFs are so far the only MOFs to show an exothermic framework collapse to a low-density liquid phase and a subsequent transition to a high-density liquid phase. By systematic adjustment of the fraction of cyano-functionalized linkers in the ZIFs, we derive fundamental insights into the thermodynamics of the unique polyamorphic nature of these glass formers as well as further design rules for the porosity of the ZIF glasses and the viscosity of their corresponding liquids. The results provide new insights into the unusual phenomenon of liquid-liquid transitions as well as a guide for the chemical diversification of meltable MOFs, likely with implications beyond the archetypal ZIF glass formers. © 2023 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/jacs.3c01933
  • 2023 • 327 Quantitative three-dimensional imaging of chemical short-range order via machine learning enhanced atom probe tomography
    Li, Yue and Wei, Ye and Wang, Zhangwei and Liu, Xiaochun and Colnaghi, Timoteo and Han, Liuliu and Rao, Ziyuan and Zhou, Xuyang and Huber, Liam and Dsouza, Raynol and Gong, Yilun and Neugebauer, Jörg and Marek, Andreas and Rampp,...
    Nature Communications 14 (2023)
    Chemical short-range order (CSRO) refers to atoms of specific elements self-organising within a disordered crystalline matrix to form particular atomic neighbourhoods. CSRO is typically characterized indirectly, using volume-averaged or through projection microscopy techniques that fail to capture the three-dimensional atomistic architectures. Here, we present a machine-learning enhanced approach to break the inherent resolution limits of atom probe tomography enabling three-dimensional imaging of multiple CSROs. We showcase our approach by addressing a long-standing question encountered in body-centred-cubic Fe-Al alloys that see anomalous property changes upon heat treatment. We use it to evidence non-statistical B2-CSRO instead of the generally-expected D03-CSRO. We introduce quantitative correlations among annealing temperature, CSRO, and nano-hardness and electrical resistivity. Our approach is further validated on modified D03-CSRO detected in Fe-Ga. The proposed strategy can be generally employed to investigate short/medium/long-range ordering phenomena in different materials and help design future high-performance materials. © 2023, The Author(s).
    view abstractdoi: 10.1038/s41467-023-43314-y
  • 2023 • 326 Strong and ductile high temperature soft magnets through Widmanstätten precipitates
    Han, Liuliu and Maccari, Fernando and Soldatov, Ivan and Peter, Nicolas J. and Souza Filho, Isnaldi R. and Schäfer, Rudolf and Gutfleisch, Oliver and Li, Zhiming and Raabe, Dierk
    Nature Communications 14 (2023)
    Fast growth of sustainable energy production requires massive electrification of transport, industry and households, with electrical motors as key components. These need soft magnets with high saturation magnetization, mechanical strength, and thermal stability to operate efficiently and safely. Reconciling these properties in one material is challenging because thermally-stable microstructures for strength increase conflict with magnetic performance. Here, we present a material concept that combines thermal stability, soft magnetic response, and high mechanical strength. The strong and ductile soft ferromagnet is realized as a multicomponent alloy in which precipitates with a large aspect ratio form a Widmanstätten pattern. The material shows excellent magnetic and mechanical properties at high temperatures while the reference alloy with identical composition devoid of precipitates significantly loses its magnetization and strength at identical temperatures. The work provides a new avenue to develop soft magnets for high-temperature applications, enabling efficient use of sustainable electrical energy under harsh operating conditions. © 2023, The Author(s).
    view abstractdoi: 10.1038/s41467-023-43953-1
  • 2023 • 325 X-radiography front tracking gradient furnace for directional solidification of bulk Al-alloys
    Jafarizadeh-Koohbanani, A. and Steinbach, S. and Drescher, J. and Frenzel, J. and Kargl, F.
    Review of Scientific Instruments 94 (2023)
    A unique gradient furnace for directional solidification experiments with bulk Al-alloy samples developed at German Aerospace Center is presented. It allows for in situ process control in solidifying samples by using x-radiography, and further insight into the solidification process is gained in combination with x-ray computational tomography on the solidified samples. Tracking of interfaces during directional solidification of bulk samples via in situ x-radiography (TIREX) enables the investigation of the melting process and observation of the movement of the entire mushy zone through the sample, tracing the solid-liquid interface during directional solidification and correlating the observations with the microstructure of the samples. Monitoring the temperature profile inside the sample by in situ observation of the length of the mushy zone is particularly important because the temperature gradient G and the rate of interfacial growth v determine the microstructure of solidification. The x-radiography setup offers temporal and spatial resolutions of 0.5 s and 70 μm, respectively, with a field of view of 10 × 50 mm2. Constant solidification velocities of up to 0.15 mm s−1 at a temperature gradient of up to 8 K mm−1 can be achieved in a temperature range of 537-1373 K. A flat solid-liquid interface inside a rod-like sample with 5 mm diameter is achieved by surrounding the sample by thermal isolating graphite foam. Performance tests with hypoeutectic Al-10 wt. % Cu alloy samples show the functionality of the furnace facility. © 2023 Author(s).
    view abstractdoi: 10.1063/5.0151523
  • 2022 • 324 A High Temperature SOI-CMOS Chipset Focusing Sensor Electronics for Operating Temperatures up to 300°C
    Kappert, H. and Braun, S. and Kordas, N. and Kosfeld, A. and Utz, A. and Weber, C. and Rämer, O. and Spanier, M. and Ihle, M. and Ziesche, S. and Kokozinski, R.
    Journal of Microelectronics and Electronic Packaging 19 1-7 (2022)
    Sensors are the key elements for capturing environmental properties and are increasingly important in the industry for the intelligent control of industrial processes. While in many everyday objects highly integrated sensor systems are already state of the art, the situation in an industrial environment is clearly different. Frequently, the use of sensor systems is impossible, because the extreme ambient conditions of industrial processes like high operating temperatures or strong mechanical load do not allow the reliable operation of sensitive electronic components. Fraunhofer is running the Lighthouse Project "eHarsh"to overcome this hurdle. In the course of the project, an integrated sensor readout electronic has been realized based on a set of three chips. A dedicated sensor frontend provides the analog sensor interface for resistive sensors typically arranged in a Wheatstone configuration. Furthermore, the chipset includes a 32-bit microcontroller for signal conditioning and sensor control. Finally, it comprises an interface chip including a bus transceiver and voltage regulators. The chipset has been realized in a high-temperature 0.35-micron SOI-CMOS technology focusing operating temperatures up to 300_C. The chipset is assembled on a multilayer ceramic low-temperature cofired ceramics (LTCC) board using flip chip technology. The ceramic board consists of four layers with a total thickness of approximately 0.9 mm. The internal wiring is based on silver paste while the external contacts were alternatively manufactured in silver (sintering/soldering) or in gold alloys (wire bonding). As an interconnection technology, silver sintering has been applied. It has already been shown that a significant increase in lifetime can be reached by using silver sintering for die attach applications. Using silver sintering for flip chip technology is a new and challenging approach. By adjusting the process parameter geared to the chipset design and the design of the ceramic board high-quality flip chip interconnects can be generated. © 2022 International Microelectronics Assembly and Packaging Society.
    view abstractdoi: 10.4071/imaps.1547377
  • 2022 • 323 A thermo-viscoplasticity model for metals over wide temperature ranges- application to case hardening steel
    Oppermann, P. and Denzer, R. and Menzel, A.
    Computational Mechanics 69 541-563 (2022)
    In this contribution, a model for the thermomechanically coupled behaviour of case hardening steel is introduced with application to 16MnCr5 (1.7131). The model is based on a decomposition of the free energy into a thermo-elastic and a plastic part. Associated viscoplasticity, in terms of a temperature-depenent Perzyna-type power law, in combination with an isotropic von Mises yield function takes respect for strain-rate dependency of the yield stress. The model covers additional temperature-related effects, like temperature-dependent elastic moduli, coefficient of thermal expansion, heat capacity, heat conductivity, yield stress and cold work hardening. The formulation fulfils the second law of thermodynamics in the form of the Clausius–Duhem inequality by exploiting the Coleman–Noll procedure. The introduced model parameters are fitted against experimental data. An implementation into a fully coupled finite element model is provided and representative numerical examples are presented showing aspects of the localisation and regularisation behaviour of the proposed model. © 2021, The Author(s).
    view abstractdoi: 10.1007/s00466-021-02103-4
  • 2022 • 322 Atom Pair Frequencies as a Quantitative Structure-Activity Relationship for Catalytic 2-Propanol Oxidation over Nanocrystalline Cobalt-Iron-Spinel
    Geiss, J. and Falk, T. and Ognjanovic, S. and Anke, S. and Peng, B. and Muhler, M. and Winterer, M.
    Journal of Physical Chemistry C 126 10346-10358 (2022)
    The purpose of this study is to find a direct and quantitative correlation of the structure of Co3-xFexO4nanoparticles with catalytic performance in 2-propanol oxidation. Eight nanocrystalline samples with varying iron contents are synthesized, and quantitative information regarding their structure is obtained from nitrogen physisorption, X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analyzed by reverse Monte Carlo simulations. The catalytic performance is tested in 2-propanol oxidation in the gas phase. Overall, catalytic conversion data as a function of temperature are deconvoluted to obtain conversion and half-conversion temperatures as quantitative parameters for the different catalytic reaction channels. The crystal structure is described by a spinel structure with interstitial cation defects. These defects result in a reduced electronic state of the nanoparticles. The defect density depends on the cationic composition. We also observe a complex cationic distribution on tetrahedral and octahedral sites, which is strongly influenced by the overall cationic composition. In the catalytic tests, the samples exhibit a low-temperature pathway, which is deactivated in subsequent runs but can be recovered by an oxidative treatment of the catalyst. We find that the frequency of cation pairs CoO-CoOand CoO-CoTof the individual samples correlates directly to their catalytic activity and selectivity. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acs.jpcc.2c00788
  • 2022 • 321 Catalytic effects for cellulose-based model fuels under low and high heating rate in air and oxy-fuel atmosphere
    Eckhard, T. and Pflieger, C. and Schmidt, S. and Böttger, J. and Senneca, O. and Schiemann, M. and Scherer, V. and Muhler, M. and Cerciello, F.
    Fuel 324 (2022)
    The detailed catalytic influence of minerals on solid biomass in oxy-fuel combustion is yet to be fully understood. The catalytic influence of metal sulfates on a mineral-free, cellulose-based model biomass was investigated during slow and high heating in air and oxy-fuel combustion. Measurements were performed in a thermogravimetric setup in air with slow heating rates and in a flat-flame burner in oxy-fuel combustion atmosphere with high heating rates. Temperature-programmed experiments identified the catalytic activity scale of Fe > K > Na > Mg ∼ Ca in synthetic air (20% O2/He) for the sulfates. The highly active metals Fe and K were chosen for more detailed investigations in oxy-fuel combustion experiments using an additional loading of Mg as less-volatile mineral tracer. Samples doped with Fe and Mg (FeMg-MH) exhibited lower thermal stability and higher particle combustion temperatures in the flat-flame burner compared with the undoped model fuel, while the combination of K and Mg (KMg-MH) decreased the particle combustion temperature drastically during oxy-fuel combustion. X-ray diffraction patterns acquired between 25 and 800 °C showed that in FeMg-MH the mineral phases FeSO4 and MgSO4 were still separated and independently active, while the addition of MgSO4 to K2SO4 formed the stable mineral phase Langbeinite inhibiting the K mobility. The influence of metal chlorides and nitrates was also investigated by slow heating rate TGA experiments showing an overlapping of metal salts decomposition and carbon devolatilization and oxidation. © 2022 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2022.124437
  • 2022 • 320 Conductivity enhancement of Al- and Ta-substituted Li7La3Zr2O7 solid electrolytes by nanoparticles
    Bauer, A. and Ali, M.Y. and Orthner, H. and Uhlenbruck, S. and Wiggers, H. and Fattakhova-Rohlfing, D. and Guillon, O.
    Journal of the European Ceramic Society 42 1033-1041 (2022)
    A nanopowder consisting of La2Zr2O7 particles with lithium containing species on their surface was prepared by spray flame synthesis and subsequently added to Li7La3Zr2O12 powder obtained by a conventional solid-state reaction. The spray flame synthesis method utilized in this work yields nanoparticles with a small size of approximately 5 nm, which is unprecedented within the scope of oxide-based ionic conductors for solid-state batteries. Remarkably, the addition of nanoparticles for sintering at a relatively low temperature of 1000 °C significantly improved the ionic conductivity by 50 %. In contrast, there was no influence of incorporating nanoparticles on the conductivity at sintering temperatures at or above 1100 °C, which is the typical temperature range applied for conventional sintering of Li7La3Zr2O12. Compared to prior published work with analogous materials, a more than twofold improvement in conductivity was demonstrated while the sintering temperature was decreased by 100 °C. © 2021 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2021.11.029
  • 2022 • 319 Early particle formation and evolution in iron-doped flames
    Lalanne, M.R. and Wollny, P. and Nanjaiah, M. and Menser, J. and Schulz, C. and Wiggers, H. and Cheskis, S. and Wlokas, I. and Rahinov, I.
    Combustion and Flame 244 (2022)
    In flame synthesis of nanoparticles, the temperature history experienced by the nascent particle aerosol defines the morphology, composition, and crystallinity of the resulting nanomaterial. Commonly, flame-synthesis processes are modeled with an isothermal approximation assuming that the particle temperature replicates that of the surrounding gas phase, avoiding inclusion of an additional internal coordinate in the population balance model, and thus reducing the computational cost. This simplification neglects the influence of matter- and energy-exchange as well as thermochemistry between the particle and reactive gas phase, impacting the particle temperature. In this work, we investigate the temperature history of the particles in incipient formation stages and their evolution in iron-doped flames, prototypical for many other transition-metal (oxide) synthesis systems. The temperature and relative volume-fraction distributions of early particles forming in H2/O2/Ar flames doped with iron pentacarbonyl were determined for the first time, based on spectrally and spatially resolved flame emission measurements and pyrometric analysis of the continuum emission emanating from the nascent particle aerosol. The nascent particle temperature was found to be several hundred degrees above the gas-phase temperature for all physically reasonable assumptions concerning particle composition and emission efficiency. Early particles volume fraction rises sharply shortly after the decomposition of iron pentacarbonyl and decreases steeply in the flame front, in excellent agreement with previous particle-mass spectrometry/quartz-crystal microbalance measurements. By modeling the evaporation process of isothermal iron particles, we show that vanishing of particles in the flame front cannot be explained by evaporation of particles that are in thermal equilibrium with the gas phase. A single-particle Monte-Carlo simulation based on a simple model comprising Fe-monomer condensation, concurrent with oxidation, reduction, etching, and evaporation occurring at the particle surface, captures both the flame structure with respect to early particle formation and their excess temperature compared to the gas phase. © 2022
    view abstractdoi: 10.1016/j.combustflame.2022.112251
  • 2022 • 318 Experimental Investigation of Temperature and Contact Pressure Influence on HFI Welded Joint Properties
    Egger, C. and Kroll, M. and Kern, K. and Steimer, Y. and Schreiner, M. and Tillmann, W.
    Materials 15 (2022)
    This paper presents an experimental electro-thermo-mechanical simulation of high-frequency induction (HFI) welding to investigate the effect of temperature and contact normal stress on the weld seam quality. Therefore welding experiments at different temperatures and contact pressures are performed using flat specimens of 34MnB5 steel sheet. In order to characterize the weld seam strength of the welded specimens, tensile and bending tests are performed. To obtain a relative weld seam strength, the bending specimens were additionally hardened prior to testing. With the hardened specimens, it can be shown that the weld seam strength increases with increasing temperature and contact normal stress until a kind of plateau is formed where the weld seam strength remains almost constant. In addition to mechanical testing, the influence of the investigated process parameters on the weld seam microstructure is studied metallographically using light optical microscopy, scanning electron microscopy, EBSD and hardness measurements. It is shown that the weld seam strength is related to the amount of oxides in the bonding line. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma15103615
  • 2022 • 317 Green steel at its crossroads: Hybrid hydrogen-based reduction of iron ores
    Souza Filho, I.R. and Springer, H. and Ma, Y. and Mahajan, A. and da Silva, C.C. and Kulse, M. and Raabe, D.
    Journal of Cleaner Production 340 (2022)
    Iron- and steelmaking cause ∼7% of the global CO2 emissions, due to the use of carbon for the reduction of iron ores. Replacing carbon by hydrogen as the reductant offers a pathway to massively reduce these emissions. However, the production of hydrogen using renewable energy will remain as one of the bottlenecks at least during the next two decades, because making the gigantic annual crude steel production of 1.8 billion tons sustainable requires a minimum stoichiometric amount of ∼97 million tons of green hydrogen per year. Another fundamental aspect to render the ironmaking sector more sustainable lies in an optimal utilization of green hydrogen and energy, thus reducing efforts for costly in-process hydrogen recycling. We therefore demonstrate here how the efficiency in hydrogen and energy consumption during iron ore reduction can be dramatically improved by the knowledge-based combination of two technologies: partially reducing the ore at low temperature via solid-state direct reduction (DR) to a kinetically defined degree, and subsequently melting and completely transforming it to iron under a reducing plasma (i.e. via hydrogen plasma reduction, HPR). Results suggest that an optimal transition point between these two technologies occurs where their efficiency in hydrogen utilization is equal. We found that the reduction of hematite through magnetite into wüstite via DR is clean and efficient, but it gets sluggish and inefficient when iron forms at the outermost layers of the iron ore pellets. Conversely, HPR starts violent and unstable with arc delocalization, but proceeds smoothly and efficiently when processing semi-reduced oxides, an effect which might be related to the material's high electrical conductivity. We performed hybrid reduction experiments by partially reducing hematite pellets via DR at 700 °C to 38% global reduction (using a standard thermogravimetry system) and subsequently transferring them to HPR, conducted with a lean gas mixture of Ar-10%H2 in an arc-melting furnace, to achieve full conversion into liquid iron. This hybrid approach allows to exploit the specific characteristics and kinetically favourable regimes of both technologies, while simultaneously showing the potential to keep the consumption of energy and hydrogen low and improve both, process stability and furnace longevity by limiting its overexposure to plasma radiation. © 2022 Elsevier Ltd
    view abstractdoi: 10.1016/j.jclepro.2022.130805
  • 2022 • 316 High stress twinning in a compositionally complex steel of very high stacking fault energy
    Wang, Z. and Lu, W. and An, F. and Song, M. and Ponge, D. and Raabe, D. and Li, Z.
    Nature Communications 13 (2022)
    Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m2, far above the SFE regime for twinning (<~50 mJ/m2) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41467-022-31315-2
  • 2022 • 315 Influence of process temperature and residence time on the manufacturing of amorphous solid dispersions in hot melt extrusion
    Gottschalk, T. and Grönniger, B. and Ludwig, E. and Wolbert, F. and Feuerbach, T. and Sadowski, G. and Thommes, M.
    Pharmaceutical Development and Technology 27 313-318 (2022)
    The manufacturing of amorphous solid dispersions via hot melt extrusion is a topic of high interest in pharmaceutical development. By this technique, the drug is dissolved in the molten polymer above solubility temperature within the process time. In this study, an experimental framework is proposed determining the minimum required process temperature and the residence time using particularly low quantities of material. Drug/polymer mixtures in different ratios were processed in a micro-scale extruder while the process temperature and residence time were varied systematically. The phase situation was assessed by the turbidity of the final extrudate. Four different drug/polymer mixtures were investigated in three drug/polymer ratios. The minimum required process temperature was close to solubility temperature for each specific formulation. Moreover, an influence of residence time on the phase situation was found. About three minutes were required in order to dissolve the drug in the polymer at these process conditions. © 2022 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/10837450.2022.2051549
  • 2022 • 314 Lattice dynamics, elastic, magnetic, thermodynamic and thermoelectric properties of the two-dimensional semiconductors MPSe3 (M = Cd, Fe and NI): a first-principles study
    Musari, A.A. and Kratzer, P.
    Materials Research Express 9 (2022)
    Adopting Density Functional Theory (DFT) with Hubbard U correction implemented in Quantum Espresso, we have performed a comprehensive first-principles study of MPSe3 (M = Cd. Fe and Ni) monolayers. The computed electronic properties revealed the semi-conductive nature of the monolayers with small indirect bandgaps. A free-standing single layer of MPSe3 can be exfoliated from the parent compound by virtue of its structural stability and high in-plane stiffness. Hence, the elastic and dynamical properties were computed to establish the mechanical and dynamical stability. The results showed that CdPSe3 and NiPSe3 are stable in the trigonal structure while a single negative frequency observed in the phonon dispersion of FePSe3 indicates the possibility to relax to another, less symmetric structure. In addition, these 2D systems showed relatively good response when subjected to strain hence, they can be said to be mechanically stable. The thermodynamic properties, such as internal energies, vibrational free energies, entropies and constant-volume heat capacities have been computed within the harmonic approximations using the phonon density of states. The computed thermoelectric properties show that CdPSe3 and FePSe3 have the peak figure of merit at low temperature of 50 K. This work predicts a thermoelectric performance with an electronic figure of merit of 0.28 for p-doped CdPSe3. Moreover, the DFT+U method predicts an electronic figure of merit of 0.39 and 0.2 for p-doped FePSe3 and NiPSe3, respectively. © 2022 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1591/ac96d3
  • 2022 • 313 On the diversity of fossil and alternative gasoline combustion chemistry: A comparative flow reactor study
    Zinsmeister, J. and Gaiser, N. and Melder, J. and Bierkandt, T. and Hemberger, P. and Kasper, T. and Aigner, M. and Köhler, M. and Oßwald, P.
    Combustion and Flame 243 (2022)
    Recent progress in molecular combustion chemistry allows for detailed investigation of the intermediate species pool even for complex chemical fuel compositions, as occur for technical fuels. This study provides detailed investigation of a comprehensive set of complex alternative gasoline fuels obtained from laminar flow reactors equipped with molecular-beam sampling techniques for observation of the combustion intermediate species pool in homogeneous gas phase reactions. The combination of ionization techniques including double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy enables deeper mechanistic insights into the underlying reaction network relevant to technical fuels. The selected fuels focus on contemporary automotive engine application as drop-in fuels compliant to European EN 228 specification for gasoline. Therefore, potential alternative gasoline blends containing oxygenated hydrocarbons as octane improvers obtainable from bio-technological production routes, e.g., ethanol, iso-butanol, methyl tert‑butyl ether (MTBE), and ethyl tert‑butyl ether (ETBE), as well as a Fischer-Tropsch surrogate were investigated. The fuel set is completed by two synthetic naphtha fractions obtained from Fischer-Tropsch and methanol-to-gasoline processes alongside with a fossil reference gasoline. In total, speciation data for 11 technical fuels from two atmospheric flow reactor setups are presented. Detailed main and intermediate species profiles are provided for slightly rich (ϕ = 1.2) and lean (ϕ = 0.8) conditions for intermediate to high temperatures. Complementary, the isomer distribution on different mass channels, like m/z = 78 u fulvene/benzene, of four gasolines was investigated. Experimental findings are analyzed in terms of the detailed fuel composition and literature findings for molecular combustion chemistry. Influences of oxygenated fuel components as well as composition of the hydrocarbon fractions are examined with a particular focus on the soot precursor chemistry. This dataset is available for validation of chemical kinetic mechanisms for realistic gasolines containing oxygenated hydrocarbons. © 2021
    view abstractdoi: 10.1016/j.combustflame.2021.111961
  • 2022 • 312 Rapid Water Diffusion at Cryogenic Temperatures through an Inchworm-like Mechanism
    Fang, W. and Meyer auf der Heide, K.M. and Zaum, C. and Michaelides, A. and Morgenstern, K.
    Nano Letters 22 340-346 (2022)
    Water diffusion across the surfaces of materials is of importance to disparate processes such as water purification, ice formation, and more. Despite reports of rapid water diffusion on surfaces the molecular level, details of such processes remain unclear. Here, with scanning tunneling microscopy, we observe structural rearrangements and diffusion of water trimers at unexpectedly low temperatures (<10 K) on a copper surface, temperatures at which water monomers or other clusters do not diffuse. Density functional theory calculations reveal a facile trimer diffusion process involving transformations between elongated and almost cyclic conformers in an inchworm-like manner. These subtle intermolecular reorientations maintain an optimal balance of hydrogen-bonding and water–surface interactions throughout the process. This work shows that the diffusion of hydrogen-bonded clusters can occur at exceedingly low temperatures without the need for hydrogen bond breakage or exchange; findings that will influence Ostwald ripening of ice nanoclusters and hydrogen bonded clusters in general. © 2021 American Chemical Society
    view abstractdoi: 10.1021/acs.nanolett.1c03894
  • 2022 • 311 Shock-tube study of the influence of oxygenated additives on benzene pyrolysis: Measurement of optical densities, soot inception times and comparison with simulations
    Nativel, D. and Herzler, J. and Krzywdziak, S. and Peukert, S. and Fikri, M. and Schulz, C.
    Combustion and Flame 243 (2022)
    The influence of the addition of oxygenated hydrocarbons (methanol, ethanol, and n-butanol) and ethers (diethyl ether, dimethoxymethane, furan, and tetrahydrofuran) on soot formation from benzene pyrolysis was studied. The pyrolysis process was investigated behind reflected shock waves at pressures around 1.4 bar and in the temperature range of 1670–2680 K. Extinction was measured at 633 nm to determine soot optical densities and soot-inception times. For extinction measurements, the studied gas mixtures contain 2.00 mol% C6H6 and 0.75 mol% additives diluted in argon. Since particle-inception times strongly depend on temperature and the reactive system cannot be considered isothermal because of the high reactant concentration in the shock tube, the temperature was measured as a function of time by two-color infrared absorption based on CO using two quantum-cascade lasers. For this purpose, 0.80 mol% CO and 5.00 mol% He were added to the studied gas mixtures as thermometry target species and enhancing species for vibrational relaxation, respectively. Both, temperature and measured optical densities were compared to simulations based on a detailed chemical kinetics mechanism from the CRECK Modeling Group. Additionally, simulations with a new mechanism composed of the CRECK mechanism (Pejpichestakul et al. 2009) and the recent PAH sub-mechanism of Sun et al. (2021) were performed. The agreement of experiments and simulations of the optical density were considerably improved using the aforementioned merged mechanism. © 2022 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2022.111985
  • 2022 • 310 Surface and Bulk Chemistry of Mechanochemically Synthesized Tohdite Nanoparticles
    De Bellis, J. and Ochoa-Hernández, C. and Farès, C. and Petersen, H. and Ternieden, J. and Weidenthaler, C. and Amrute, A.P. and Schüth, F.
    Journal of the American Chemical Society 144 9421-9433 (2022)
    Aluminum oxides, oxyhydroxides, and hydroxides are important in different fields of application due to their many attractive properties. However, among these materials, tohdite (5Al2O3·H2O) is probably the least known because of the harsh conditions required for its synthesis. Herein, we report a straightforward methodology to synthesize tohdite nanopowders (particle diameter ∼13 nm, specific surface area ∼102 m2g-1) via the mechanochemically induced dehydration of boehmite (γ-AlOOH). High tohdite content (about 80%) is achieved upon mild ball milling (400 rpm for 48 h in a planetary ball mill) without process control agents. The addition of AlF3can promote the crystallization of tohdite by preventing the formation of the most stable α-Al2O3, resulting in the formation of almost phase-pure tohdite. The availability of easily accessible tohdite samples allowed comprehensive characterization by powder X-ray diffraction, total scattering analysis, solid-state NMR (1H and 27Al), N2-sorption, electron microscopy, and simultaneous thermal analysis (TG-DSC). Thermal stability evaluation of the samples combined with structural characterization evidenced a low-temperature transformation sequence: 5Al2O3·H2O → κ-Al2O3→ α-Al2O3. Surface characterization via DRIFTS, ATR-FTIR, D/H exchange experiments, pyridine-FTIR, and NH3-TPD provided further insights into the material properties. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/jacs.2c02181
  • 2022 • 309 Temperature Rise Inside Shear Bands in a Simple Model Glass
    Lagogianni, A.E. and Varnik, F.
    International Journal of Molecular Sciences 23 (2022)
    One of the key factors, which hampers the application of metallic glasses as structural components, is the localization of deformation in narrow bands of a few tens up to one hundred nanometers thickness, the so-called shear bands. Processes, which occur inside shear bands are of central importance for the question whether a catastrophic failure of the material is unavoidable or can be circumvented or, at least, delayed. Via molecular dynamics simulations, this study addresses one of these processes, namely the local temperature rise due to viscous heat generation. The major contribution to energy dissipation is traced back to the plastic work performed by shear stress during steady deformation. Zones of largest strain contribute the most to this process and coincide with high-temperature domains (hottest spots) inside the sample. Magnitude of temperature rise can reach a few percent of the sample’s glass transition temperature. Consequences of these observations are discussed in the context of the current research in the field. © 2022 by the authors.
    view abstractdoi: 10.3390/ijms232012159
  • 2022 • 308 Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution
    Schröter, M. and Herper, H.C. and Grünebohm, A.
    Journal of Physics D: Applied Physics 55 (2022)
    Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ac2a66
  • 2021 • 307 A Low-Temperature Structural Transition in Canfieldite, Ag8SnS6, Single Crystals
    Slade, T.J. and Gvozdetskyi, V. and Wilde, J.M. and Kreyssig, A. and Gati, E. and Wang, L.-L. and Mudryk, Y. and Ribeiro, R.A. and Pecharsky, V.K. and Zaikina, J.V. and Bud’ko, S.L. and Canfield, P.C.
    Inorganic Chemistry 60 19345-19355 (2021)
    Canfieldite, Ag8SnS6, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag8SnS6 of mass up to 1 g from a ternary Ag–Sn–S melt. On cooling from high temperature, Ag8SnS6 undergoes a known cubic (F4̅3m) to orthorhombic (Pna21) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5–300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn21 (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag8SnSe6 and Ag8GeSe6. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag8SnS6 with its argyrodite analogues, Ag8TQ6 (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna21 arrangement. We support this picture by showing that the transition to the Pmn21 phase is avoided in Ge alloyed Ag8Sn1–xGexS6 samples as well as in pure Ag8GeS6 © 2021 American Chemical Society
    view abstractdoi: 10.1021/acs.inorgchem.1c03158
  • 2021 • 306 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 305 Boosting the kinetic efficiency of formate dehydrogenase by combining the effects of temperature, high pressure and co-solvent mixtures
    Jaworek, M.W. and Gajardo-Parra, N.F. and Sadowski, G. and Winter, R. and Held, C.
    Colloids and Surfaces B: Biointerfaces 208 (2021)
    The application of co-solvents and high pressure has been shown to be an efficient means to modify the kinetics of enzyme-catalyzed reactions without compromising enzyme stability, which is often limited by temperature modulation. In this work, the high-pressure stopped-flow methodology was applied in conjunction with fast UV/Vis detection to investigate kinetic parameters of formate dehydrogenase reaction (FDH), which is used in biotechnology for cofactor recycling systems. Complementary FTIR spectroscopic and differential scanning fluorimetric studies were performed to reveal pressure and temperature effects on the structure and stability of the FDH. In neat buffer solution, the kinetic efficiency increases by one order of magnitude by increasing the temperature from 25° to 45 °C and the pressure from ambient up to the kbar range. The addition of particular co-solvents further doubled the kinetic efficiency of the reaction, in particular the compatible osmolyte trimethylamine-N-oxide and its mixtures with the macromolecular crowding agent dextran. The thermodynamic model PC-SAFT was successfully applied within a simplified activity-based Michaelis-Menten framework to predict the effects of co-solvents on the kinetic efficiency by accounting for interactions involving substrate, co-solvent, water, and FDH. Especially mixtures of the co-solvents at high concentrations were beneficial for the kinetic efficiency and for the unfolding temperature. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2021.112127
  • 2021 • 304 Design of a Co–Al–W–Ta Alloy Series with Varying γ′ Volume Fraction and Their Thermophysical Properties
    Volz, N. and Xue, F. and Bezold, A. and Zenk, C.H. and Fries, S.G. and Schreuer, J. and Neumeier, S. and Göken, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 52 3931-3944 (2021)
    The γ′ volume fraction is a key parameter in precipitation-strengthened Co- and Ni-base superalloys and mainly determines the alloys’ properties. However, systematic studies with varying γ′ volume fractions are rare and the influence on thermal expansion has not been studied in detail. Therefore, a series of six Ta-containing Co-based alloys was designed with compositions on a γ–γ′ tie-line, where the γ′ volume fraction changes systematically. During solidification, Laves (C14-type) and µ (D85-type) phases formed in alloys with high levels of W and Ta. Single-phase γ or two-phase γ/γ′ microstructures were obtained in four experimental alloys after heat treatment as designed, whereas secondary precipitates, such as χ (D019-type), Laves, and μ, existed in alloys containing high levels of γ′-forming elements. However, long-term heat treatments for 1000 hours revealed the formation of the χ phase also in the former χ-free alloys. The investigation of the thermal expansion behavior revealed a significant anomaly related to the dissolution of γ′, which can be used to determine the γ′ solvus temperature with high accuracy. Compared to thermodynamic calculations, differential scanning calorimetry (DSC) and thermal expansion analysis revealed a larger increase of the γ′ solvus temperatures and a lesser decline of the solidus temperatures when the alloy composition approached the composition of the pure γ′ phase. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11661-021-06353-y
  • 2021 • 303 Development of a concept for the use of low-temperature emulsion in drilling of Inconel 718
    Rinschede, T. and Biermann, D. and Iovkov, I. and Bücker, M.
    Procedia CIRP 104 774-779 (2021)
    Due to their low thermal conductivity, nickel-based alloys can cause an increased thermal energy flow to the cutting tool during machining. This may result in intensive tool wear and a poor bore quality. In order to increase the process cooling during drilling, a new concept has been developed to supply a cooled emulsion at temperatures of T ≈ -20 °C to the process under high pressure. Therefore, a low-temperature emulsion itself and a circuit for the supply of the emulsion were developed. This should enable a higher productivity as well as component quality. © 2021 Elsevier B.V.. All rights reserved.
    view abstractdoi: 10.1016/j.procir.2021.11.130
  • 2021 • 302 Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance
    Luo, T. and Serrano-Sánchez, F. and Bishara, H. and Zhang, S. and Villoro, B. and Kuo, J.J. and Felser, C. and Scheu, C. and Snyder, G.J. and Best, J.P. and Dehm, G. and Yu, Y. and Raabe, D. and Fu, C. and Gault, B.
    Acta Materialia 217 (2021)
    Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries. © 2021 The Authors
    view abstractdoi: 10.1016/j.actamat.2021.117147
  • 2021 • 301 In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass
    Orava, J. and Balachandran, S. and Han, X. and Shuleshova, O. and Nurouzi, E. and Soldatov, I. and Oswald, S. and Gutowski, O. and Ivashko, O. and Dippel, A.-C. and Zimmermann, M. and Ivanov, Y.P. and Greer, A.L. and Raabe, D. and...
    Nature Communications 12 (2021)
    A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-021-23028-9
  • 2021 • 300 Investigations of electron-electron and interlayer electron-phonon coupling in van der Waals hBN/WSe2/hBN heterostructures by photoluminescence excitation experiments
    Jadczak, J. and Kutrowska-Girzycka, J. and Schindler, J.J. and Debus, J. and Watanabe, K. and Taniguchi, T. and Ho, C.-H. and Bryja, L.
    Materials 14 1-12 (2021)
    Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′ 1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14020399
  • 2021 • 299 Kondo holes in strongly correlated impurity arrays: RKKY-driven Kondo screening and hole-hole interactions
    Eickhoff, F. and Anders, F.B.
    Physical Review B 104 (2021)
    The emerging and screening of local magnetic moments in solids have been investigated for more than 60 years. Local vacancies as in graphene or in heavy fermions can induce decoupled bound states that lead to the formation of local moments. In this paper, we address the puzzling question how these local moments can be screened and what determines the additionally emerging low-temperature scale. We review the initial problem for half-filled conduction bands from two complementary perspectives: By a single-particle supercell analysis in the uncorrelated limit and by the Lieb-Mathis theorem for systems with a large Coulomb interaction U. Applying Wilson's numerical renormalization group approach to a recently developed mapping of the problem onto an effective low-energy description of a Kondo hole with up to Nf=7 correlated impurities as background, we proof that the stable local moments are subject to screening by three different mechanisms. Firstly the local moments are delocalized by a finite U beyond the single-particle bound state. We find a Kosterlitz-Thouless type transition governed by an exponentially suppressed low-energy scale of a counterintuitive Kondo form with Jeff∝Un for small U, where n>1 depends on the precise model. Secondly, we show that away from half-filling the local moment phase becomes unstable and is replaced by two types of singlet phases that are adiabatically connected. At a critical value for the band center, the physics is governed by an exponentially suppressed Kondo scale approaching the strong coupling phase that is replaced by a singlet formation via antiferromagnetic RKKY interaction for large deviation from the critical values. Thirdly, we show that the local magnetic moment can be screened by a Kondo hole orbital at finite energy, even though the orbital occupation is negligible: An additional low-energy scale emerges below which the localized moment is quenched. Similarities to the experimental findings in Ce1-xLaxPd3 are pointed out. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.045115
  • 2021 • 298 Low temperature sintering of fully inorganic all-solid-state batteries – Impact of interfaces on full cell performance
    Ihrig, M. and Finsterbusch, M. and Tsai, C.-L. and Laptev, A.M. and Tu, C.-H. and Bram, M. and Sohn, Y.J. and Ye, R. and Sevinc, S. and Lin, S.-K. and Fattakhova-Rohlfing, D. and Guillon, O.
    Journal of Power Sources 482 (2021)
    One of the necessary prerequisites to advance the electrochemical performance of Li7La3Zr2O12 (LLZ) based all-solid-state lithium batteries is the manufacturing of dense composite cathodes from cathode active material (CAM) and the LLZ ceramic solid electrolyte. However, free co-sintering of LLZ and CAM mixtures requires temperatures above 1000 °C which often leads to decomposition and secondary phase formation, especially for high energy CAMs. In our study we present a completely dry processing route which is fast, free of any sintering additives and coatings and suitable to fabricate dense mixed cathodes, pure LLZ separators and multilayers of the two. Through application of high mechanical pressure during Field-Assisted Sintering we were able to reduce the sintering temperature down to 675–750 °C with dwell times as low as 10 min, while still obtaining 95% theoretical density for LCO/LLZ mixtures. The low sintering temperature is suitable for high energy CAMs, but leads to a significant effect of surface impurities, especially from powder handling in air, and affects the crystallinity of the CAM/LLZ interface. In the present paper we investigate the impact of resulting interfaces on the ionic conductivity, the interfacial impedance and the cycling stability of produced cells and propose the optimization strategy. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2020.228905
  • 2021 • 297 Low-temperature and low-pressure effective fluorescence lifetimes and spectra of gaseous anisole and toluene
    Beuting, M. and Dreier, T. and Schulz, C. and Endres, T.
    Applied Physics B: Lasers and Optics 127 (2021)
    Fluorescence spectra and lifetimes of anisole and toluene vapor in nitrogen have been measured at conditions below ambient (257–293 K and 100–2000 mbar) upon excitation with 266-nm laser light to expand the applicable range of anisole and toluene laser-induced fluorescence (LIF) for conditions below room temperature that occur in expanding flows and cases with strong evaporative cooling. Anisole fluorescence spectra broaden with decreasing pressure while fluorescence lifetimes decrease simultaneously. This is consistent with a more pronounced effect of internal vibrational redistribution on the overall fluorescence signal and can be explained by significantly reduced collision rates. In the case of toluene, the transition from photo-induced heating to photo-induced cooling was observed for the first time for 266 nm. The data confirm predictions of earlier work and is particularly important for the advancement of the available photo-physical (step-ladder) models: since those transitions mark points where the molecules are already thermalized after excitation (i.e., no vibrational relaxation occurs during deactivation), they are important support points for fitting empirical parameters and allow analytical determination of the ground state energy transferred to the excited state. The data enable temperature and/or pressure sensing, e.g., in accelerating cold flows using laser-induced fluorescence of both tracers. © 2021, The Author(s).
    view abstractdoi: 10.1007/s00340-021-07605-w
  • 2021 • 296 Phase decomposition in nanocrystalline Cr0.8Cu0.2 thin films
    Chakraborty, J. and Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 888 (2021)
    Metastable Cr0.8Cu0.2 alloy thin films with nominal thickness of 360 nm have been deposited on Si(100) substrate by co-evaporation of Cu and Cr using molecular beam epitaxy (MBE). Phase evolution, microstructure, stress development, and crystallographic texture in Cr0.8Cu0.2 thin films have been investigated by X-ray diffraction (XRD), atom probe tomography (APT) and transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) during annealing of the films in the temperature range 200–450 °C. X-ray diffraction of the as-deposited thin film shows single phase bcc crystal structure of the film whereas APT observation of fine precipitates in the film matrix due to inherent compositional fluctuation indicates onset of phase separation via spinodal decomposition regime. XRD (in-situ) and APT investigation of 300 °C annealed film reveals that the early stage of phase separation involves localized formation of metastable intermediate bcc precipitate phase having 60 at% Cr and 40 at% Cu approximately (~Cr0.6Cu0.4). For longer duration of annealing at temperature ≥350 °C, such metastable bcc precipitates act as heterogeneous nucleation sites for the onset of precipitation of Cu rich fcc Cu(Cr) phase which indicates a change of phase separation mechanism from ‘spinodal decomposition’ to ‘nucleation and growth’. Annealing of the film at temperature ≥400 °C for longer duration leads to the formation of a two phase structure with Cu rich fcc precipitate phase in a Cr rich bcc matrix. Observed phase decomposition is accompanied by significant changes in the microstructure, residual stress and crystallographic texture in the Cr rich bcc film matrix which leads to the minimization of both surface and strain energies and thereby a reduction of total Gibbs free energy of the thin film. Thermodynamic model calculation has been presented in order to understand the nucleation pathway of Cu rich stable fcc Cu(Cr) precipitates via non-classical nucleation of metastable intermediate bcc Cr0.6Cu0.4 phase. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2021.161391
  • 2021 • 295 Studying the mechanism of phase separation in aqueous solutions of globular proteins via molecular dynamics computer simulations
    Brudar, S. and Gujt, J. and Spohr, E. and Hribar-Lee, B.
    Physical chemistry chemical physics : PCCP 23 415-424 (2021)
    Proteins are the most abundant biomacromolecules in living cells, where they perform vital roles in virtually every biological process. To maintain their function, proteins need to remain in a stable (native) state. Inter- and intramolecular interactions in aqueous protein solutions govern the fate of proteins, as they can provoke their unfolding or association into aggregates. The initial steps of protein aggregation are difficult to capture experimentally, therefore we used molecular dynamics simulations in this study. We investigated the initial phase of aggregation of two different lysozymes, hen egg-white (HEWL) and T4 WT* lysozyme and also human lens γ-D crystallin by using atomistic simulations. We monitored the phase stability of their aqueous solutions by calculating time-dependent density fluctuations. We found that all proteins remained in their compact form despite aggregation. With an extensive analysis of intermolecular residue-residue interactions we discovered that arginine is of paramount importance in the initial stage of aggregation of HEWL and γ-D crystallin, meanwhile lysine was found to be the most involved amino acid in forming initial contacts between T4 WT* molecules.
    view abstractdoi: 10.1039/d0cp05160h
  • 2021 • 294 Thermoelastic properties and γ’-solvus temperatures of single-crystal Ni-base superalloys
    Horst, O.M. and Schmitz, D. and Schreuer, J. and Git, P. and Wang, H. and Körner, C. and Eggeler, G.
    Journal of Materials Science 56 7637-7658 (2021)
    Abstract: The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. Graphical abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
    view abstractdoi: 10.1007/s10853-020-05628-w
  • 2020 • 293 A combined experimental and modelling approach for the Weimberg pathway optimisation
    Shen, L. and Kohlhaas, M. and Enoki, J. and Meier, R. and Schönenberger, B. and Wohlgemuth, R. and Kourist, R. and Niemeyer, F. and van Niekerk, D. and Bräsen, C. and Niemeyer, J. and Snoep, J. and Siebers, B.
    Nature Communications 11 (2020)
    The oxidative Weimberg pathway for the five-step pentose degradation to α-ketoglutarate is a key route for sustainable bioconversion of lignocellulosic biomass to added-value products and biofuels. The oxidative pathway from Caulobacter crescentus has been employed in in-vivo metabolic engineering with intact cells and in in-vitro enzyme cascades. The performance of such engineering approaches is often hampered by systems complexity, caused by non-linear kinetics and allosteric regulatory mechanisms. Here we report an iterative approach to construct and validate a quantitative model for the Weimberg pathway. Two sensitive points in pathway performance have been identified as follows: (1) product inhibition of the dehydrogenases (particularly in the absence of an efficient NAD+ recycling mechanism) and (2) balancing the activities of the dehydratases. The resulting model is utilized to design enzyme cascades for optimized conversion and to analyse pathway performance in C. cresensus cell-free extracts. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-14830-y
  • 2020 • 292 Application Progress of Annealing Treatment Process in the Study of Nano-multilayer Films [退火处理工艺在纳米多层膜材料研究中的应用进展]
    Li, H. and Xing, Z. and Hodúlová, E. and Hu, A. and Tillmann, W.
    Cailiao Daobao/Materials Reports 34 03099-03105 (2020)
    Compared with traditional bulk materials, nano-multilayer films exhibit unique optical, magnetic, electrical, mechanical and thermal properties due to their small-size effects, surface effects, quantum size effects, and quantum tunneling effects. Therefore, nano-multilayer films have been widely used in the areas of optical devices, semiconductors, electromagnetic protection, processing and manufacturing, surface protection and electronic packaging as optical absorbing materials, electromagnetic absorbing materials, magnetic recording materials, photovoltaic materials and low-temperature joining materials. There exists intrinsic size dependence in the physical and mechanical properties with the microstructure of nano-multilayer films. Due to the limitation of the preparation process, defects such as vacancies and dislocations can cause difficulty in fully meeting the requirements of heat resistance, wear resistance and corrosion resistance in the complex service environment, which limits the further development of nano-multilayer films. In the field of concentrating circuits and chip fabrication, nano-multilayer films devices are often working in a severe environment deviating from the normal temperature. However, metastable nano-multilayer films with high surface free energy tend to reach a state of low-energy and form a stable structure by interdiffusion of immiscible dual phases, interlayer detachment and interface evolution under heat. It might result in the extinction of melting point depression property, superhardness property and so on due to the destructions of the nano-multilayer structure. Therefore, studying on the microstructure evolution, thermal stability and failure mechanism of nano-multilayer films is particularly important for increasing the service life and reliability of nano-multilayer systems. As a common heat treatment method, the annealing process is widely used to eliminate defects in metals, so as to achieve to modify the properties. For nano-multilayer films operating at high temperatures, the annealing process is also an effective means of extending its service life. At present, the main directions of annealing process in nano-multilayer films research and application are: (i) improving nano-multilayer film performance by adopting different annealing temperature, holding time and cooling rate; (ii) investigating the effect of annealing temperature on the thermal stability of nano-multilayer films by increasing the annealing upper limit temperature and obtain a critical temperature that maintains stability of the interface of nano-multilayer. It is found that the appropriate annealing process can refine the nano-multilayer films grain structure, increase the density, decrease the defect density, induce the formation of special structures, reinforcing the interaction of atoms and dislocations. Therefore, the light transmittance of the film is increased with improvement of optical properties, as well as the magnetic, electrical and mechanical properties are significantly improved; (iii) in addition, the nano-multilayer film is annealed in a certain temperature range to observe the bilayer interface evolution, atomic diffusion and new phase formation using TEM, XRD and other means. Thus the structural stability, chemical stability and mechanical stability of nano-multilayer film can be studied. In this paper, the current progress and challenges of annealing process in nano-multilayer films modification and thermal stability research are reviewed. The influence of annealing parameters on the enhancement of nano-multilayer properties including optical properties, magnetic properties, electrical properties, mechanical properties is elaborated. Furthermore, it mainly focuses on the influencing mechanism of elevated temperature annealing on the thermal stability and microstructure evolution of immiscible nano-multilayer system. At last, the further development of annealing process for designing and preparing of high-strength and thermally stable nano-multilayer films are prospected, which has important theoretical significance and application value in materials welding/joining, integrated circuits, cutting tools, absorbing coatings, etc. © 2020, Materials Review Magazine. All right reserved.
    view abstractdoi: 10.11896/cldb.19010159
  • 2020 • 291 Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing
    Ivanova, A. and Frka-Petesic, B. and Paul, A. and Wagner, T. and Jumabekov, A.N. and Vilk, Y. and Weber, J. and Schmedt Auf Der Günne, J. and Vignolini, S. and Tiemann, M. and Fattakhova-Rohlfing, D. and Bein, T.
    ACS Applied Materials and Interfaces 12 12639-12647 (2020)
    Porous tin dioxide is an important low-cost semiconductor applied in electronics, gas sensors, and biosensors. Here, we present a versatile template-assisted synthesis of nanostructured tin dioxide thin films using cellulose nanocrystals (CNCs). We demonstrate that the structural features of CNC-templated tin dioxide films strongly depend on the precursor composition. The precursor properties were studied by using low-temperature nuclear magnetic resonance spectroscopy of tin tetrachloride in solution. We demonstrate that it is possible to optimize the precursor conditions to obtain homogeneous precursor mixtures and therefore highly porous thin films with pore dimensions in the range of 10-20 nm (ABET = 46-64 m2 g-1, measured on powder). Finally, by exploiting the high surface area of the material, we developed a resistive gas sensor based on CNC-templated tin dioxide. The sensor shows high sensitivity to carbon monoxide (CO) in ppm concentrations and low cross-sensitivity to humidity. Most importantly, the sensing kinetics are remarkably fast; both the response to the analyte gas and the signal decay after gas exposure occur within a few seconds, faster than in standard SnO2-based CO sensors. This is attributed to the high gas accessibility of the very thin porous film. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.9b11891
  • 2020 • 290 CO-concentration and temperature measurements in reacting CH4/O2 mixtures doped with diethyl ether behind reflected shock waves
    He, D. and Shi, L. and Nativel, D. and Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 194-205 (2020)
    The oxidation of CH4/diethyl ether mixtures was studied with laser absorption-based time-resolved temperature and CO concentration measurements behind reflected shock waves. Fuel-rich (equivalence ratio ϕ = 2.0) mixtures were studied because of their relevance for mechanism development for partial oxidation reactions in the context of polygeneration processes and measurements at ϕ = 0.5 and 1.0 were used to verify the mechanism performance in an extended range of equivalence ratios. Temperature and CO concentration were measured via absorption using two fundamental vibrations of CO (ν" = 0, P20 and ν" = 1, R21) with two mid-IR quantum-cascade lasers near 4.8546 and 4.5631 µm. Interference from broadband absorption of CO2 in the region near 4.56 µm was quantified based on measured temperature-dependent CO2 absorption cross-sections and mechanism-based prediction of CO2 concentrations. The measured temporal CO-concentration and temperature profiles were compared with simulations based on two mechanisms (Fikri et al., 2017; Yasunaga et al., 2010). For mixtures with ϕ = 0.5, the two mechanisms show similar results, and well reproduce the experimental data. At ϕ = 1.0 and 2.0, the Fikri et al. mechanism shows very good agreement with the experiments whereas the Yasunaga et al. mechanism predicts a too fast CO-concentration and temperature rise. © 2020 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2020.02.024
  • 2020 • 289 Densification of a high chromium cold work tool steel powder in different atmospheres by SLPS: Microstructure, heat treatment and micromechanical properties
    Farayibi, P.K. and Blüm, M. and Weber, S.
    Materials Science and Engineering A 777 (2020)
    The degradation of moulds, dies and tools employed in plastic, food and chemical processing industries has necessitated the development of suitable wear and corrosion-resistant materials. As improving the wear and corrosion resistance of iron base alloys tend to have opposing demands regarding chemical composition and heat treatment, optimisation of both parameters has to be kept in mind. One alloying element that is known to improve both corrosion and wear resistance of steels is nitrogen. Hence, an investigation into the densification of high chromium X190CrVMo20-4-1 cold work tool steel in a vacuum and under a nitrogen atmosphere at different pressures via supersolidus liquid-phase sintering (SLPS) process is reported in this paper. The investigation aimed to elucidate the influence of different atmospheres and nitrogen partial pressures employed during densification on the microstructure, optimal heat treatment parameters and micromechanical properties of the steel. Experimental findings were supplemented by computational thermodynamics calculations. The results revealed that increasing nitrogen pressure promoted the diffusion of vanadium from Cr-rich carbides (M7C3) to form V-rich carbonitrides, M(C,N). Optimum quench-hardening temperature was strongly influenced by the matrix chemistry. Upon tempering, the nitrogen-sintered samples had higher secondary hardening potential than the vacuum-sintered at a higher temperature, but a low-temperature tempering is beneficial to the corrosion resistance of the steel. The mechanical properties of the carbides in the densified steels in different atmospheres were influenced by their chemical composition. Experimental observations are in good agreement with computational thermodynamic evaluations. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.139053
  • 2020 • 288 Dominant In-Plane Symmetric Elastoresistance in CsFe2As2
    Wiecki, P. and Haghighirad, A.-A. and Weber, F. and Merz, M. and Heid, R. and Böhmer, A.E.
    Physical Review Letters 125 (2020)
    We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a divergent nematic susceptibility. Rather, our results can be interpreted naturally within the framework of a coherence-incoherence crossover, where the low-temperature coherent state is sensitively tuned by the in-plane atomic distances. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.125.187001
  • 2020 • 287 Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in nanoclusters
    Muckel, F. and Lorenz, S. and Yang, J. and Nugraha, T.A. and Scalise, E. and Hyeon, T. and Wippermann, S. and Bacher, G.
    Nature Communications 11 (2020)
    The fundamental bandgap Eg of a semiconductor—often determined by means of optical spectroscopy—represents its characteristic fingerprint and changes distinctively with temperature. Here, we demonstrate that in magic sized II-VI clusters containing only 26 atoms, a pronounced weakening of the bonds occurs upon optical excitation, which results in a strong exciton-driven shift of the phonon spectrum. As a consequence, a drastic increase of dEg/dT (up to a factor of 2) with respect to bulk material or nanocrystals of typical size is found. We are able to describe our experimental data with excellent quantitative agreement from first principles deriving the bandgap shift with temperature as the vibrational entropy contribution to the free energy difference between the ground and optically excited states. Our work demonstrates how in small nanoparticles, photons as the probe medium affect the bandgap—a fundamental semiconductor property. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-17563-0
  • 2020 • 286 High-strength Damascus steel by additive manufacturing
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Nature 582 515-519 (2020)
    Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2409-3
  • 2020 • 285 Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6 H1.6
    Terwey, A. and Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Eggert, B. and Ollefs, K. and Brabänder, V. and Radulov, I. and Skokov, K. and Faske, T. and Hu, M.Y. and Zhao, J. and Alp, E.E. and Giacobbe, C. and G...
    Physical Review B 101 (2020)
    We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 in terms of the vibrational (phonon) density of states (VDOS), which we determined with Fe57 nuclear resonant inelastic X-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy ΔSlat across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change ΔSlat of (3.2±1.9)JkgK across the FM-to-PM transition contributes with ∼35% significantly and cooperatively to the total isothermal entropy change ΔSiso. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.064415
  • 2020 • 284 Meissner currents induced by topological magnetic textures in hybrid superconductor/ferromagnet structures
    Dahir, S.M. and Volkov, A.F. and Eremin, I.M.
    Physical Review B 102 (2020)
    Topological spin configurations in proximity to a superconductor have recently attracted great interest due to the potential application of the former in spintronics and also as another platform for realizing nontrivial topological superconductors. Their application in these areas requires precise knowledge of the existing exchange fields and/or the stray fields, which are therefore essential for the study of these systems. Here, we determine the effective stray field Hstr and the Meissner currents jS in a superconductor/ferromagnet/superconductor (S/F/S) junction produced by various nonhomogenous magnetic textures M(r) in the F. The inhomogeneity arises either due to a periodic structure with flat domain walls (DW) or is caused by an isolated chiral magnetic skyrmion (Sk). We consider both Bloch- and Néel-type Sk and also analyze in detail the periodic structures of different types of DW's, that is, Bloch-type DW (BDW) and Néel-type DW (NDW) of finite width with in- and out-of-plane magnetization vector M(x). The spatial dependence of the fields Hstr(r) and Meissner currents jS(r) are shown to be qualitatively different for the case of Bloch- and Néel-type magnetic textures. While the spatial distributions in the upper and lower S are identical for Bloch-type Sk and DW's they are asymmetric for the case of Néel-type magnetic textures. The depairing factor, which determines the critical temperature Tc and which is related to the vector potential of the stray field, can have its maximum at the center of a magnetic domain but also, as we show, above the DW. For Sk's, the maximum is located at a finite distance within the Sk radius rSk. Based on this, we study the nucleation of superconductivity in the presence of DW's. Because of the asymmetry for Néel-type structures, the critical temperature Tc in the upper and lower S is expected to be different. The obtained results can also be applied to S/F bilayers. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.014503
  • 2020 • 283 Micro-, macromechanical and aeroelastic investigation of glass - fiber based, lightweight turbomachinery components
    Iseni, S. and Prasad, M.R.G. and Hartmaier, A. and Holeczek, K. and Modeler, N. and di Mare, F.
    Proceedings of the ASME Turbo Expo 10A-2020 (2020)
    A major technical challenge for modern aero engines is the development of designs which reduce noise and emission whilst increasing aerodynamic efficiency and ensuring aeroelastic stability of low-temperature engine components such as fans and low-pressure compressors. Composites are used in aviation due to their excellent stiffness and strength properties, which also enable additional flexibility in the design process. The weight reduction of the turbomachine components, due to composite materials and lighter engines, is especially relevant for the design and developments of hybrid-electric or distributed propulsion systems [1]. To accomplish this, a representative volume element (RVE) of a glass-fiber reinforced polymer is created, describing the geometrical arrangement of the textile reinforcement structure within the polymer matrix. For both phases, realistic linear elastic properties are assumed. This RVE will be investigated with the finite element method under various loading conditions to assess its anisotropic elastic properties and also its damping behaviour for elastic waves. To study the influence of delamination on the mechanical properties, small defects will be introduced into the model at the interface between reinforcement and matrix. Based on this micromechanical approach, a constitutive model for the composite will be formulated that describes the anisotropic properties as well as the damping behaviour. This constitutive model is then used to describe the material response in a macro-mechanical model, which serves as the basis for an aeroelastic analysis of a 1/3-scaled high-speed fan using a conventional (Ti-6Al -4V) and fiber composite material. Copyright © 2020 ASME
    view abstractdoi: 10.1115/GT2020-14951
  • 2020 • 282 Observation of the Kondo screening cloud
    V. Borzenets, I. and Shim, J. and Chen, J.C.H. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S. and Sim, H.-S. and Yamamoto, M.
    Nature 579 210-213 (2020)
    When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect1,2. Unlike electric-charge screening, the spin-screening cloud3–6 occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the Kondo cloud can theoretically spread out over several micrometres. The cloud has not so far been detected, and so its physical existence—a fundamental aspect of the Kondo effect—remains controversial7,8. Here we present experimental evidence of a Kondo cloud extending over a length of micrometres, comparable to the theoretical length ξK. In our device, a Kondo impurity is formed in a quantum dot2,9–11, coupling on one side to a quasi-one-dimensional channel12 that houses a Fabry–Pérot interferometer of various gate-defined lengths L exceeding one micrometre. When we sweep a voltage on the interferometer end gate—separated by L from the quantum dot—to induce Fabry–Pérot oscillations in conductance we observe oscillations in the measured Kondo temperature TK, which is a signature of the Kondo cloud at distance L. When L is less than ξK the TK oscillation amplitude becomes larger as L becomes smaller, obeying a scaling function of a single parameter L/ξK, whereas when L is greater than ξK the oscillation is much weaker. Our results reveal that ξK is the only length parameter associated with the Kondo effect, and that the cloud lies mostly within a length of ξK. Our experimental method offers a way of detecting the spatial distribution of exotic non-Fermi liquids formed by multiple magnetic impurities or multiple screening channels13–16 and of studying spin-correlated systems. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2058-6
  • 2020 • 281 Optimisation based material parameter identification using full field displacement and temperature measurements
    Rose, L. and Menzel, A.
    Mechanics of Materials 145 (2020)
    A material parameter identification is presented for a fully thermo-mechanically coupled material model based on full field displacement and temperature measurements. The basic theory of the inverse problem is recapitulated, focusing on the choice of the objective function, proposing a new formulation, and explaining in detail the necessary numerical treatment of experimental data during the pre-processing of an identification. This includes the handling of the intrinsically different data sets of displacement (Lagrangian type) and temperature (Eulerian type). Experimental data is obtained by means of a Digital-Image-Correlation (DIC) as well as by a thermography system and three algorithmic boxes are provided for the necessary pre-processing. The experimental setup is discussed, measured data presented and analysed. From this setup, a successive approach to the identification process is motivated. Based on the experimental observations, a thermo-mechanically coupled material model is chosen, the required constitutive relations summarised and the material parameters interpreted. For the fixed choice of model and experiments, the inverse problem is solved. A very good fit was obtained for both the displacement and the temperature field. Results are interpreted and remaining errors discussed. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.mechmat.2019.103292
  • 2020 • 280 Standard Gibbs energy of metabolic reactions: V. Enolase reaction
    Greinert, T. and Vogel, K. and Seifert, A.I. and Siewert, R. and Andreeva, I.V. and Verevkin, S.P. and Maskow, T. and Sadowski, G. and Held, C.
    Biochimica et Biophysica Acta - Proteins and Proteomics 1868 (2020)
    The glycolytic pathway is one of the most important pathways for living organisms, due to its role in energy production and as supplier of precursors for biosynthesis in living cells. This work focuses on determination of the standard Gibbs energy of reaction ΔRg′0 of the enolase reaction, the ninth reaction in the glycolysis pathway. Exact ΔRg′0 values are required to predict the thermodynamic feasibility of single metabolic reactions or even of metabolic reaction sequences under cytosolic conditions. So-called “apparent” standard data from literature are only valid at specific conditions. Nevertheless, such data are often used in pathway analyses, which might lead to misinterpretation of the results. In this work, equilibrium measurements were combined with activity coefficients in order to obtain new standard values ΔRg′0 for the enolase reaction that are independent of the cytosolic conditions. Reaction equilibria were measured at different initial substrate concentrations and temperatures of 298.15 K, 305.15 K and 310.15 K at pH 7. The activity coefficients were predicted using the equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT). The ePC-SAFT parameters were taken from literature or fitted to new experimentally determined osmotic coefficients and densities. At 298.15 K and pH 7, a ΔRg′0(298.15 K, pH 7) value of −2.8 ± 0.2 kJ mol− 1 was obtained. This value differs by up to 5 kJ mol− 1 from literature data. Reasons are the poorly defined “standard” conditions and partly undefined reaction conditions of literature works. Finally, using temperature-dependent equilibrium constants and the van ‘t Hoff equation, the standard enthalpy of reaction of ΔRh′0(298.15 K, pH 7) = 27 ± 10 kJ mol− 1 was determined, and a similar value was found by quantum-chemistry calculations. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.bbapap.2020.140365
  • 2020 • 279 The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten
    Bonnekoh, C. and Reiser, J. and Hartmaier, A. and Bonk, S. and Hoffmann, A. and Rieth, M.
    Journal of Materials Science 55 12314-12337 (2020)
    Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10853-020-04801-5
  • 2020 • 278 The effect of metal-oxide incorporation on the morphology of carbon nanostructures
    Tigges, S. and Wöhrl, N. and Hagemann, U. and Ney, M. and Lorke, A.
    Journal of Physics D: Applied Physics 53 (2020)
    Metal-organic, single-source, low-temperature, morphology-controlled growth of carbon nanostructures is achieved, using an inductively coupled plasma-enhanced chemical vapor deposition system. Three distinctive morphologies, namely nanoflakes, nanowalls (CNWs) and nanorods (and intermediates between these morphologies), can be reproducibly deposited, depending on the process parameters. The synthesized structures can be described as hybrid materials consisting of metal oxide incorporated in a carbon matrix material. Since the incorporation of metal oxide into the carbon structure significantly influences their growth, the synthesis cannot be described solely with the existing models for the growth of CNWs. Optical emission spectroscopy is used to measure the relative number density of suspected growth and etching species in the plasma, while physical and chemical surface analysis techniques (scanning electron microscopy, Raman spectroscopy, scanning Auger microscopy and x-ray photoelectron spectroscopy) were employed to characterize the properties of the different nanostructures. Therefore, by using methods for both plasma and surface characterization, the growth process can be understood. The precursor dissociation in the plasma can be directly linked to the deposited morphology, as the incorporation of Al2O3 into the nanostructures is found to be a major cause for the transition between morphologies, by changing the dominant type of defect within the carbon structure. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ab6946
  • 2020 • 277 The Investigation of the Influence of Low-Temperature Plasma and Steam Sterilization on the Properties of Track Membranes Made of Polyethylene Terephthalate
    Filippova, E.O. and Karpov, D.A. and Pichugin, V.F. and Ulbricht, M.
    Inorganic Materials: Applied Research 11 1116-1123 (2020)
    Abstract: The influence of modification by low-temperature atmospheric-pressure plasma and steam sterilization on the properties of track membranes based on polyethylene terephthalate is studied. It is found that the action of hot steam under pressure changes the topography of the surface of the membranes with the formation of artifacts in the form of large oval-shaped protrusions with a height of 300–400 nm and a density of up to 0.007 protrusions/μm2 on the surface, increases the surface roughness by 40% and the wetting angle by 9°–18° for the initial membranes and by 36.8°–39.6° for the membranes modified in plasma, and decreases their surface energy to the initial value of 33 mJ/m2. Despite the morphological and structural changes in the surface, sterilization by hot steam under pressure does not lead to any noticeable change in the surface charge and ζ potential of the track membranes. Hot steam under pressure does not promote further crystallization of the membrane, keeping the polymer with a crystalline phase of 40–42%. Thus, to preserve the properties acquired by the membrane after the plasma treatment, it is necessary to search for a different sterilization method (gamma radiation, ethylene oxide sterilization). © 2020, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S2075113320050111
  • 2020 • 276 Thermodynamics and kinetics of glycolytic reactions. Part i: Kinetic modeling based on irreversible thermodynamics and validation by calorimetry
    Vogel, K. and Greinert, T. and Reichard, M. and Held, C. and Harms, H. and Maskow, T.
    International Journal of Molecular Sciences 21 1-20 (2020)
    In systems biology, material balances, kinetic models, and thermodynamic boundary conditions are increasingly used for metabolic network analysis. It is remarkable that the reversibility of enzyme‐catalyzed reactions and the influence of cytosolic conditions are often neglected in kinetic models. In fact, enzyme‐catalyzed reactions in numerous metabolic pathways such as in glycolysis are often reversible, i.e., they only proceed until an equilibrium state is reached and not until the substrate is completely consumed. Here, we propose the use of irreversible thermodynamics to describe the kinetic approximation to the equilibrium state in a consistent way with very few adjustable parameters. Using a flux‐force approach allowed describing the influence of cytosolic conditions on the kinetics by only one single parameter. The approach was applied to reaction steps 2 and 9 of glycolysis (i.e., the phosphoglucose isomerase reaction from glucose 6‐ phosphate to fructose 6‐phosphate and the enolase‐catalyzed reaction from 2‐phosphoglycerate to phosphoenolpyruvate and water). The temperature dependence of the kinetic parameter fulfills the Arrhenius relation and the derived activation energies are plausible. All the data obtained in this work were measured efficiently and accurately by means of isothermal titration calorimetry (ITC). The combination of calorimetric monitoring with simple flux‐force relations has the potential for adequate consideration of cytosolic conditions in a simple manner. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms21218341
  • 2020 • 275 Thermodynamics and kinetics of glycolytic reactions. Part II: Influence of cytosolic conditions on thermodynamic state variables and kinetic parameters
    Vogel, K. and Greinert, T. and Reichard, M. and Held, C. and Harms, H. and Maskow, T.
    International Journal of Molecular Sciences 21 1-20 (2020)
    For systems biology, it is important to describe the kinetic and thermodynamic properties of enzyme-catalyzed reactions and reaction cascades quantitatively under conditions prevailing in the cytoplasm. While in part I kinetic models based on irreversible thermodynamics were tested, here in part II, the influence of the presumably most important cytosolic factors was investigated using two glycolytic reactions (i.e., the phosphoglucose isomerase reaction (PGI) with a uni-uni-mechanism and the enolase reaction with an uni-bi-mechanism) as examples. Crowding by macromolecules was simulated using polyethylene glycol (PEG) and bovine serum albumin (BSA). The reactions were monitored calorimetrically and the equilibrium concentrations were evaluated using the equation of state ePC-SAFT. The pH and the crowding agents had the greatest influence on the reaction enthalpy change. Two kinetic models based on irreversible thermodynamics (i.e., single parameter flux-force and two-parameter Noor model) were applied to investigate the influence of cytosolic conditions. The flux-force model describes the influence of cytosolic conditions on reaction kinetics best. Concentrations of magnesium ions and crowding agents had the greatest influence, while temperature and pH-value had a medium influence on the kinetic parameters. With this contribution, we show that the interplay of thermodynamic modeling and calorimetric process monitoring allows a fast and reliable quantification of the influence of cytosolic conditions on kinetic and thermodynamic parameters. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms21217921
  • 2019 • 274 Ablation target cooling by maximizing the nanoparticle productivity in laser synthesis of colloids
    Waag, F. and Gökce, B. and Barcikowski, S.
    Applied Surface Science 466 647-656 (2019)
    Even if ultrashort laser pulses are used during the laser synthesis of colloids, a significant amount of laser energy is converted into thermal energy, which results in heating the ablation target and the colloid. To date, little attention has been paid to these heating effects in the literature. This study was focused on measurements of the process temperature during the high-power, ultrashort-pulsed laser ablation of a nickel target in a continuous water flow setup. Time-resolved monitoring of the temperature of the ablation target and of the colloid indicated that there was an initial rapid uptake of thermal energy followed by a thermally-stable state in which there was very little additional heating. Shifting the focal plane from behind the target onto its surface and further into the fluid provided insight concerning the different mechanisms of heat generation, dissipation, and transfer in the laser synthesis of colloids. It even was possible to distinguish the fluence effects and the colloid re-irradiation effects. New possibilities of process control were identified by correlating the productivity of laser ablation at different focal plane shifts with the measured thermal data. Counterintuitively, the temperature of the target was minimized via ablation cooling when the productivity of the process was maximized. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.030
  • 2019 • 273 Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)
    Bertram, C. and Fang, W. and Pedevilla, P. and Michaelides, A. and Morgenstern, K.
    Nano Letters 19 3049-3056 (2019)
    A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b00392
  • 2019 • 272 Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag-Core-Porous-Cu-Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules
    O'Mara, P.B. and Wilde, P. and Benedetti, T.M. and Andronescu, C. and Cheong, S. and Gooding, J.J. and Tilley, R.D. and Schuhmann, W.
    Journal of the American Chemical Society 141 14093-14097 (2019)
    Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as-0.6 V vs RHE. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b07310
  • 2019 • 271 Coherent spin dynamics of electrons and holes in CsPbBr3 perovskite crystals
    Belykh, V.V. and Yakovlev, D.R. and Glazov, M.M. and Grigoryev, P.S. and Hussain, M. and Rautert, J. and Dirin, D.N. and Kovalenko, M.V. and Bayer, M.
    Nature Communications 10 (2019)
    The lead halide perovskites demonstrate huge potential for optoelectronic applications, high energy radiation detectors, light emitting devices and solar energy harvesting. Those materials exhibit strong spin-orbit coupling enabling efficient optical orientation of carrier spins in perovskite-based devices with performance controlled by a magnetic field. Here we show that elaborated time-resolved spectroscopy involving strong magnetic fields can be successfully used for perovskites. We perform a comprehensive study of high-quality lead halide perovskite CsPbBr3 crystals by measuring the exciton and charge carrier g-factors, spin relaxation times and hyperfine interaction of carrier and nuclear spins by means of coherent spin dynamics. Owing to their ‘inverted’ band structure, perovskites represent appealing model systems for semiconductor spintronics exploiting the valence band hole spins, while in conventional semiconductors the conduction band electrons are considered for spin functionality. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-08625-z
  • 2019 • 270 Development and construction of AISI H11/ZrO2 joints for injection molding tools
    Tillmann, W. and Anar, N.B. and Manka, M. and Wojarski, L. and Lehmert, B.
    Welding in the World 63 1861-1871 (2019)
    Increasing demands in industrial applications and simultaneous efforts to provide long-lasting and cost-efficient tools in the injection molding industry lead to the use of metal–ceramic joints with the aim to combine the specific properties of both materials. Due to its high CTE, zirconium oxide (ZrO2) is used for the ceramic part and is joined with the tool steel AISI H11 (1.2343). In this work, suitable joining techniques with a low heat input and therefore a low thermal load are applied and characterized for the production of metal–ceramic composites. The selection of joining techniques is based on the boundary conditions during the injection molding process, in which the composites have to resist the temperature, pressure, as well as shear and tensile loads. Therefore, besides brazing, other joining processes such as gluing, screwing, shrinking, and clamping were analyzed as possible low temperature joining techniques for ceramic-metal-compounds. The best results for the tensile strengths with 90 MPa were achieved by a brazing process, carried out in vacuum with approximately 10−5 mbar, at a temperature of 920 °C for 5 min, using the commercially available brazing filler alloy CB4. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-019-00800-6
  • 2019 • 269 Development of a MOF-FF-compatible interaction model for liquid methanol and Cl− in methanol
    Siwaipram, S. and Bopp, P.A. and Soetens, J.-C. and Schmid, R. and Bureekaew, S.
    Journal of Molecular Liquids 285 526-534 (2019)
    If complex systems are to be studied in molecular simulation, one usually attempts to combine existing interaction models in order to describe the new system. This is, however, not always feasible. We thus propose here a new pairwise-additive interaction model for liquid methanol and solvated Cl− to be used to study the immersion of Metal-Organic Frameworks (MOFs) in methanol. Practically, it entails that all interactions must be written to be compatible with the family of MOF-FF models, which have been specifically developed and then widely employed in molecular simulations of such MOFs, in particular flexible ones. The new model for liquid methanol has been mostly tailored to provide densities and dielectric constants as close to experiment as possible in a large temperature domain. This is important since the flexible MOFs modify their shapes according to their loading with guest molecules of various types, and also according to the thermodynamic conditions. The model yields excellent agreement for the density-temperature, dielectric constant-temperature, and self-diffusion-temperature relationships, properties. Other properties such as e.g. the compressibilities or thermal expansion coefficients are of the correct order of magnitude. Since some MOF frameworks are electrically charged, counterions will be present in these cases. The interactions of Cl− with the liquid are thus also considered here. The solvation of this ion is also found to be satisfactory when compared to other MD studies. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.molliq.2019.04.068
  • 2019 • 268 Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator
    Huttmann, F. and Rothenbach, N. and Kraus, S. and Ollefs, K. and Arruda, L.M. and Bernien, M. and Thonig, D. and Delin, A. and Fransson, J. and Kummer, K. and Brookes, N.B. and Eriksson, O. and Kuch, W. and Michely, T. and Wende, H.
    Journal of Physical Chemistry Letters 10 911-917 (2019)
    We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M 5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)μ B and (+7.0 ± 0.6)μ B , respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M 5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.8b03711
  • 2019 • 267 Features of spin dynamics of magnetic ions and charge carriers in self-organized quantum dots CdSe/ZnMnSe
    Kozyrev, N.V. and Kirstein, E. and Namozov, B.R. and Kusrayev, Y.G. and Zhukov, E.A. and Sedova, I.V. and Yakovlev, D.R. and Bayer, M.
    Journal of Physics: Conference Series 1400 (2019)
    Self-organized disk-shaped quantum dots of CdSe embedded in diluted magnetic ZnMnSe barrier were studied by means of pump-probe time-resolved Kerr rotation (TRKR) technique at low temperature T = 7 K. In absence of the external magnetic field TRKR signal exhibits long-living spin dynamics with the decay time exceeding the period between laser pulses. Such spin dynamics is not typical for diluted magnetic semiconductors and nano-structures based on them and could be a trace of a bound magnetic polaron. Resonant spin amplification measured in transversal magnetic field up to 1 T shows the only one peak near B = 0. In B = 1 T the long-living non-precessing signal practically vanishes, while the precessing one appears with the Larmor frequency corresponding to the Mn2+ ions' net spin precession around the magnetic field. It was found that the signal consists of three components with slightly different precession frequencies that could be due to the fine structure of the manganese spin sublevels occurring because of a stress in quantum dots. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1400/7/077010
  • 2019 • 266 First-principles characterization of reversible martensitic transformations
    Ferrari, A. and Sangiovanni, D.G. and Rogal, J. and Drautz, R.
    Physical Review B 99 (2019)
    Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high- and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.094107
  • 2019 • 265 Highly Compact TiO 2 Films by Spray Pyrolysis and Application in Perovskite Solar Cells
    Möllmann, A. and Gedamu, D. and Vivo, P. and Frohnhoven, R. and Stadler, D. and Fischer, T. and Ka, I. and Steinhorst, M. and Nechache, R. and Rosei, F. and Cloutier, S.G. and Kirchartz, T. and Mathur, S.
    Advanced Engineering Materials 21 (2019)
    Transparent and pinhole free hole-blocking layers such as TiO 2 grown at low temperatures and by scalable processes are necessary to reduce production costs and thus enabling commercialization of perovskite solar cells. Here, the authors compare the transport properties of TiO 2 compact layers grown by spray pyrolysis from commonly used titanium diisopropoxide bisacetylacetonate ([Ti(OPr i ) 2 (acac) 2 ]) precursor to films grown by spray pyrolysis of TiCl 4 . Spray pyrolysis provides insights into the interdependence of precursor chemistry and electron transport properties of TiO 2 films and their influence on the performance of the perovskite solar cells. X-ray diffraction and X-ray photoelectron spectroscopy data confirm the chemical and structural composition of the obtained films. Thin film deposition at lower temperature (150 °C) are conducted using TiCl 4 to evaluate the influence of crystal growth and topography by scanning electron microscopy and atomic force microscopy as well as thickness (profilometry) and transmittance (UV/Vis spectroscopy) on the power conversion efficiency of perovskite solar cells. TiO 2 compact layers grown from TiCl 4 enhance the power conversion efficiency by acting as superior electron transfer medium and by reducing hysteresis behavior, when compared to films grown using titanium diisopropoxide bisacetylacetonate. UV/Vis spectroscopy and external quantum efficiency studies reveal the correlation of transmittance on the power conversion efficiency. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201801196
  • 2019 • 264 Influence of Lipase Immobilization Mode on Ethyl Acetate Hydrolysis in a Continuous Solid-Gas Biocatalytic Membrane Reactor
    Vitola, G. and Mazzei, R. and Poerio, T. and Barbieri, G. and Fontananova, E. and Büning, D. and Ulbricht, M. and Giorno, L.
    Bioconjugate Chemistry 30 2238-2246 (2019)
    Solid-gas biocatalysis was performed in a specially designed continuous biocatalytic membrane reactor (BMR). In this work, lipase from Candida rugosa (LCR) and ethyl acetate in vapor phase were selected as model enzyme and substrate, respectively, to produce acetic acid and ethanol. LCR was immobilized on functionalized PVDF membranes by using two different kinds of chemical bond: electrostatic and covalent. Electrostatic immobilization of LCR was carried out using a membrane functionalized with amino groups, while covalent immobilization was carried out using membrane, with or without surface-immobilized polyacrylamide (PAAm) microgels, functionalized with aldehyde groups. These biocatalytic membranes were tested in a solid-gas BMR and compared in terms of enzyme specific activity, catalytic activity, and volumetric reaction rate. Results indicated that lipase covalently immobilized is more effective only when the immobilization is mediated by microgels, showing catalytic activity doubled with respect to the other system with covalently bound enzyme (4.4 vs 2.2 μmol h-1). Enzyme immobilized by ionic bond, despite a lower catalytic activity (3.5 vs 4.4 μmol h-1), showed the same specific activity (1.5 mmol·h-1·g-1 ENZ) of the system using microgels, due to a higher enzyme degree of freedom coupled with an analogously improved enzyme hydration. Using the optimized operating conditions regarding immobilized enzyme amount, ethyl acetate, and molar water flow rate, all three BMRs showed continuous catalytic activity for about 5 months. On the contrary, the free enzyme (in water/ethyl acetate emulsion) at 50 °C was completely inactive and at 30 °C (temperature optimum) has a specific activity 2 orders of magnitude lower (8.4 × 10-2 mmol h-1 g-1) than the solid-gas biocatalytic membrane reactor. To the best of our knowledge, this is the first example of solid-gas biocatalysis, working in the gaseous phase in which a biocatalytic membrane reactor, with the enzyme/substrate system lipase/ethyl acetate, was used. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.bioconjchem.9b00463
  • 2019 • 263 Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films
    Glushko, O. and Dehm, G.
    Acta Materialia 169 99-108 (2019)
    Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.03.004
  • 2019 • 262 Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2-Based Thin-Film Transistor Devices
    Mai, L. and Zanders, D. and Subaşl, E. and Ciftyurek, E. and Hoppe, C. and Rogalla, D. and Gilbert, W. and Arcos, T.D.L. and Schierbaum, K. and Grundmeier, G. and Bock, C. and Devi, A.
    ACS Applied Materials and Interfaces (2019)
    A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsami.8b16443
  • 2019 • 261 Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion
    Losch, P. and Huang, W. and Vozniuk, O. and Goodman, E.D. and Schmidt, W. and Cargnello, M.
    ACS Catalysis 9 4742-4753 (2019)
    Complete catalytic oxidation of methane in the presence of steam at low temperatures (T &lt; 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C-H bond of methane at low temperature is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technological applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure-property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from commercially available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepared. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T50 of 355 °C and T90 of 375 °C (where T50 and T90 are temperature values at which the samples reach 50% and 90% methane conversion, respectively) in steam-containing reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b00596
  • 2019 • 260 Nanosecond plasmas in water: Ignition, cavitation and plasma parameters
    Grosse, K. and Held, J. and Kai, M. and Von Keudell, A.
    Plasma Sources Science and Technology 28 (2019)
    Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. The understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (HVs) between 14 and 26 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. Ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip. The temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 μm with an initial pressure of 5 ×108 Pa at a temperature of 1200 K for a HV of 20 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied HV. The dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy. After igniting the nanosecond plasma, the electrical power oscillates in the feed line between HV pulser and plasma chamber with a ring down time of the order of 60 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. Broadband emission due to recombination and Bremsstrahlung becomes visible within the first 30 ns. At later times, line emission dominates. Stark broadening of the spectral lines of Hα (656 nm) and OI (777 nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab26fc
  • 2019 • 259 Nonlinear electrical and rheological spectroscopies identify structural and supramolecular relaxations in a model peptide
    Honorio, G. and Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Soft Matter 15 4334-4345 (2019)
    Supercooled liquid secondary amides display an electrical absorption peak characterized by an almost Debye-like shape, indicative of a close-to-exponential polarization response. This response, believed to be supramolecular in nature, is so enormously intense that the amide's structural process, contributing only a few percent to the total relaxation strength, is hard to resolve reliably using standard dielectric spectroscopy. To overcome this issue, nonlinear dielectric spectroscopy involving field-induced structural recovery and temperature-induced physical aging, was applied near the calorimetric glass transition of a mixture of N-methylformamide and N-ethylacetamide. Without the need to rely on cumbersome deconvolution procedures, it is thus demonstrated that the supramolecular response is by a factor of 6 slower than the structural relaxation. Conversely, in linear rheological experiments only the structural relaxation could be resolved, but not the supramolecular one. However, medium-amplitude oscillatory shear experiments carried out at 160 K do reveal the supramolecular process. Hence, the combination of linear and nonlinear mechanical measurements corroborates the dielectrically uncovered spectral separation of the two processes. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9sm00434c
  • 2019 • 258 Rare-earth ion exchanged Cu-SSZ-13 zeolite from organotemplate-free synthesis with enhanced hydrothermal stability in NH 3 -SCR of NO : X
    Zhao, Z. and Yu, R. and Shi, C. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and McGuire, R. and Parvulescu, A.-N. and Maurer, S. and Müller, U. and Zhang, W.
    Catalysis Science and Technology 9 241-251 (2019)
    The relatively low hydrothermal stability of Al-rich Cu-SSZ-13 catalysts hinders their practical application in ammonia selective catalytic reduction (NH 3 -SCR) reaction. Rare-earth ions were introduced into the Al-rich SSZ-13 zeolite using an organotemplate-free synthesis prior to the exchange of Cu 2+ ions. Among the rare-earth ions tested (Ce, La, Sm, Y, Yb), Y shows significant enhancement of the hydrothermal stability and NH 3 -SCR activities after severe hydrothermal aging at 800 °C for 16 h when compared with Cu-SSZ-13 without Y. Cu-Y-SSZ-13 catalysts with various amounts of Y were prepared, and it is found that with increasing Y content, the low temperature NO conversions can be improved even after hydrothermal aging. SEM-EDX analysis together with two-dimensional multiple quantum magic-angle-spinning nuclear magnetic resonance ( 23 Na MQ MAS NMR) confirms that the Y ions are successfully incorporated into the ion-exchange sites of the SSZ-13 zeolite. Results from 27 Al MAS, 29 Si MAS NMR, temperature-programmed desorption of ammonia (NH 3 -TPD) and quantitative 1 H MAS NMR indicate that Y can stabilize the framework Al and also preserve the Brønsted acid sites in the Al-rich SSZ-13 zeolite. The hydrogen temperature programmed reduction (H 2 -TPR), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of nitric oxide (NO) or NH 3 adsorption demonstrate that introduction of Y ions causes Cu 2+ ions to preferentially occupy the 6-MR, which has high hydrothermal stability. However, too much of Y may lead to activity loss at both low and high temperatures. The optimized Al-rich Cu-Y-SSZ-13 with 2.8 wt% of copper (Cu) and 1.3 wt% of Y displays almost the same deNO x activities as the conventional organotemplated high silica Cu-SSZ-13 catalyst in a wide reaction temperature window of 150-650 °C after severe hydrothermal treatment. Rare-earth ions could be an effective additive for Cu-SSZ-13 catalysts to further improve their hydrothermal stability for practical applications. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cy02033g
  • 2019 • 257 Role of Electron-Phonon Coupling in the Thermal Evolution of Bulk Rashba-Like Spin-Split Lead Halide Perovskites Exhibiting Dual-Band Photoluminescence
    Steele, J.A. and Puech, P. and Monserrat, B. and Wu, B. and Yang, R.X. and Kirchartz, T. and Yuan, H. and Fleury, G. and Giovanni, D. and Fron, E. and Keshavarz, M. and Debroye, E. and Zhou, G. and Sum, T.C. and Walsh, A. and Hofk...
    ACS Energy Letters 4 2205-2212 (2019)
    The optoelectronic properties of lead halide perovskites strongly depend on their underlying crystal symmetries and dynamics, sometimes exhibiting a dual photoluminescence (PL) emission via Rashba-like effects. Here we exploit spin- and temperature-dependent PL to study single-crystal APbBr3 (A = Cs and methylammonium; CH3NH3) and evaluate the peak energy, intensity, and line width evolutions of their dual emission. Both perovskites exhibit temperature trends governed by two temperature regimes - above and below approximately 100 K - which impose different carrier scattering and radiative recombination dynamics. With increasing temperature, high-energy optical phonons activate near 100 K to drive energy splitting of the dual bands and induce line width broadening via electron-phonon coupling, with a stronger coupling constant inferred for carriers recombining by the spin-split indirect bands, compared to the direct ones. We find that the unusual thermal evolutions of all-inorganic and hybrid bulk lead bromide perovskites are comparable, suggesting A-site independence and the dominance of dynamic effects, and are best understood within a framework that accounts for Rashba-like effects. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsenergylett.9b01427
  • 2019 • 256 Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy
    Thurston, K.V.S. and Gludovatz, B. and Yu, Q. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Journal of Alloys and Compounds 794 525-533 (2019)
    Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019
    view abstractdoi: 10.1016/j.jallcom.2019.04.234
  • 2019 • 255 Tuning the charge flow between Marcus regimes in an organic thin-film device
    Atxabal, A. and Arnold, T. and Parui, S. and Hutsch, S. and Zuccatti, E. and Llopis, R. and Cinchetti, M. and Casanova, F. and Ortmann, F. and Hueso, L.E.
    Nature Communications 10 (2019)
    Marcus’s theory of electron transfer, initially formulated six decades ago for redox reactions in solution, is now of great importance for very diverse scientific communities. The molecular scale tunability of electronic properties renders organic semiconductor materials in principle an ideal platform to test this theory. However, the demonstration of charge transfer in different Marcus regions requires a precise control over the driving force acting on the charge carriers. Here, we make use of a three-terminal hot-electron molecular transistor, which lets us access unconventional transport regimes. Thanks to the control of the injection energy of hot carriers in the molecular thin film we induce an effective negative differential resistance state that is a direct consequence of the Marcus Inverted Region. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10114-2
  • 2019 • 254 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2018.2856461
  • 2019 • 253 Variable chemical decoration of extended defects in Cu-poor C u2ZnSnS e4 thin films
    Schwarz, T. and Redinger, A. and Siebentritt, S. and Peng, Z. and Gault, B. and Raabe, D. and Choi, P.-P.
    Physical Review Materials 3 (2019)
    We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich Cu2ZnSnSe4 thin films. For a precursor film, which was co-evaporated at 320C, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at 500C, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed Cu2ZnSnSe4 film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.035402
  • 2019 • 252 Voltage waveform tailoring in radio frequency plasmas for surface charge neutralization inside etch trenches
    Krüger, F. and Wilczek, S. and Mussenbrock, T. and Schulze, J.
    Plasma Sources Science and Technology 28 (2019)
    The etching of sub micrometer high-aspect-ratio (HAR) features into dielectric materials in low pressure radio frequency technological plasmas is limited by the accumulation of positive surface charges inside etch trenches. These are, at least partially, caused by highly energetic positive ions that are accelerated by the sheath electric field to high velocities perpendicular to the wafer. In contrast to these anisotropic ions, thermal electrons typically reach the electrode only during the sheath collapse and cannot penetrate deeply into HAR features to compensate the positive surface charges. This problem causes significant reductions of the etch rate and leads to deformations of the features due to ion deflection, i.e. the aspect ratio is limited. Here, we demonstrate that voltage waveform tailoring can be used to generate electric field reversals adjacent to the wafer during sheath collapse to accelerate electrons towards the electrode to allow them to penetrate deeply into HAR etch features to compensate positive surface charges and to overcome this process limitation. Based on 1D3V particle-in-cell/Monte Carlo collision simulations of a capacitively coupled plasma operated in argon at 1 Pa, we study the effects of changing the shape, peak-to-peak voltage, and harmonics' frequencies of the driving voltage waveform on this electric field reversal as well as on the electron velocity and angular distribution function at the wafer. We find that the angle of incidence of electrons relative to the surface normal at the wafer can be strongly reduced and the electron velocity perpendicular to the wafer can be significantly increased by choosing the driving voltage waveform in a way that ensures a fast and short sheath collapse. This is caused by the requirement of flux compensation of electrons and ions at the electrode on time average in the presence of a short and steep sheath collapse. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab2c72
  • 2018 • 251 Application of toluene LIF to transonic nozzle flows to identify zones of incomplete molecular mixing
    Beuting, M. and Richter, J. and Weigand, B. and Dreier, T. and Schulz, C.
    Optics Express 26 10266-10273 (2018)
    Toluene laser-induced fluorescence (LIF) has been applied to image the mixing deficit on the molecular level in the transonic wake of two different blunt-body injectors in a compressible accelerated nozzle flow. A single-color excitation and two-color detection scheme is employed to measure the signal red-shift caused by the quenching effect of molecular oxygen on the fluorescence of toluene, which reduces and red-shifts the LIF signal if both substances interact on a molecular level. To this end, toluene is injected alternatingly with O2-contaning and O2-free carrier gas into the air main flow. Differences of both signals mark regions where mixing on molecular level is incomplete. A zone of molecular mixing deficit extending several millimeters in stream-wise direction is identified. The effect of local variations in temperature on the sensitivity of this technique is discussed using photo-physical data measured in a stationary low-temperature cell. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstractdoi: 10.1364/OE.26.010266
  • 2018 • 250 Brittle fracture of high-strength bolts of large diameters at low temperatures
    Stranghöner, N. and Lorenz, C. and Feldmann, M. and Citarelli, S. and Bleck, W. and Münstermann, S. and Brinnel, V.
    Stahlbau 87 17-29 (2018)
    Brittle fracture of high-strength bolts of large diameters at low temperatures. High-strength structural bolting assemblies of the system HV are executed as preloaded bolting assemblies in highly loaded steel structures. More often these bolting assemblies are used up to M72 e. g. in wind energy towers. In addition to static and fatigue loads, these connections are exposed to low temperatures. In case of equal boundary conditions, the risk of brittle fracture increases with increasing thickness of the steel material. In principle, this relation also applies to increasing bolt diameters. Since no systematic investigations into the tendency to brittle fracture of high-strength bolts with large diameter exist and the choice of steel material to avoid brittle fracture of high-strength bolts is not covered in EN 1993-1-10, high-strength bolts of large diameters are made of higher alloyed steels. In the frame of the IGF-project ”Brittle Fracture of High-Strength Bolts of Large Diameters at Low Temperatures“ systematic investigations into the low temperature behaviour of high-strength bolts of the system HV were carried out for bolt diameters from M24 to M64 to assess the risk of brittle fracture for these components. This article presents the main results and conclusions achieved in the research project. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstractdoi: 10.1002/stab.201810559
  • 2018 • 249 Coil-coated steel sheets measured by DMA using an immersion testing cell
    Haakmann, F. and Mayer, C. and Raulf, M.
    Journal of Thermal Analysis and Calorimetry (2018)
    The effect of humidity on the glass transition temperature of coatings is well known and analyzed. In this study, an analysis method is introduced for analyzing coil-coated metal sheets submerged in water and water/methyl ethyl ketone (MEK) by using an immersion cell coupled to a DMA. For this purpose, a model polyurethane acrylate coating was applied to zinc/magnesium-coated steel plates and measured before and after immersion in water and water/MEK by DMA and immersion cell. The results show a shift of glass transition temperature from 138 to 13 °C of the coating by storing the coated steel plates under water and immersion testing. A time-dependent diffusion of water from the cell into the coating until saturation can be observed. An increasing concentration of MEK within the immersion cell results in a greater shift of the glass transition temperature. Overall, the immersion cell seems to be a useful tool for in situ characterization of the behavior of coil coating varnishes in liquid environment (Foster et al. in Prog Organ Coat 51:244–249, 2004, van der Wel et al. in Prog Organ Coat 37:1–14, 1999). © 2018, Akadémiai Kiadó, Budapest, Hungary.
    view abstractdoi: 10.1007/s10973-018-7944-x
  • 2018 • 248 Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography
    Kwiatkowski da Silva, A. and Inden, G. and Kumar, A. and Ponge, D. and Gault, B. and Raabe, D.
    Acta Materialia 147 165-175 (2018)
    We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.022
  • 2018 • 247 From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca-Au-Al: In Situ Variable-Temperature Transformation
    Pham, J. and Meng, F. and Lynn, M.J. and Ma, T. and Kreyssig, A. and Kramer, M.J. and Goldman, A.I. and Miller, G.J.
    Journal of the American Chemical Society 140 1337-1347 (2018)
    The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3 (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b10358
  • 2018 • 246 Heterogeneous to homogeneous melting transition visualized with ultrafast electron diffraction
    Mo, M.Z. and Chen, Z. and Li, R.K. and Dunning, M. and Witte, B.B.L. and Baldwin, J.K. and Fletcher, L.B. and Kim, J.B. and Ng, A. and Redmer, R. and Reid, A.H. and Shekhar, P. and Shen, X.Z. and Shen, M. and Sokolowski-Tinten, K....
    Science 360 1451-1455 (2018)
    The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega–electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds. © 2017 The Authors
    view abstractdoi: 10.1126/science.aar2058
  • 2018 • 245 Influence of biomass torrefaction parameters on fast pyrolysis products under flame-equivalent conditions
    Pielsticker, S. and Möller, G. and Gövert, B. and Kreitzberg, T. and Hatzfeld, O. and Yönder, Ö. and Angenent, V. and Hättig, C. and Schmid, R. and Kneer, R.
    Biomass and Bioenergy 119 392-410 (2018)
    Pretreating raw biomass via torrefaction changes fuel specific properties like grindability, volatile content, energy density and biochemical stability and thus enables an enhanced fuel replacement for pulverized fossil fuel fired furnaces. In this study, the influence of torrefaction temperature on devolatilization behavior is investigated in a small-scale fluidized bed reactor approximating flame-equivalent conditions. Therefore the pyrolysis products of two different biofuels with varying degree of torrefaction are determined via ex-situ FTIR gas analysis in an N2 atmosphere in the temperature range from 873 to 1473 K. Furthermore, the mass fraction of residual char particles is determined by adding O2 to the fluidizing gas and analyzing the burnout products. Char fraction and volatile composition are used to estimate the energy release distribution between homogeneous volatile combustion and heterogeneous char burnout. The experiments revealed enlarging char yields at the expense of volatile yields with increasing degree of torrfaction at all investigated pyrolysis temperatures. Furthermore, torrefaction favors higher fractions of CO2 and lower fractions of CO and C2Hx in the light gas. Further on, no significant impact of torrefaction conditions on the tar composition could be identified. The calculation of higher heating value (HHV) based on char yield and gas composition reveals an overall increase of HHV, while the relative contribution from the volatile fraction decreases with increasing degree of torrefaction. Following this, an increase of torrefaction degree will shift combustion from a high intense volatile combustion in the near burner region towards a less intense but prolonged char conversion in the far burner region. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.biombioe.2018.08.014
  • 2018 • 244 Low-temperature MOCVD deposition of Bi2Te3 thin films using Et2BiTeEt as single source precursor
    Bendt, G. and Gassa, S. and Rieger, F. and Jooss, C. and Schulz, S.
    Journal of Crystal Growth 490 77-83 (2018)
    Et2BiTeEt was used as single source precursor for the deposition of Bi2Te3 thin films on Si(1 0 0) substrates by metal organic chemical vapor deposition (MOCVD) at very low substrate temperatures. Stoichiometric and crystalline Bi2Te3 films were grown at 230 °C, which is approximately 100 °C lower compared to conventional MOCVD processes using one metal organic precursors for each element. The Bi2Te3 films were characterized using scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The elemental composition of the films, which was determined by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, was found to be strongly dependent of the substrate temperature. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2018.03.021
  • 2018 • 243 Multifunctional Stimuli-Responsive Cellulose Nanocrystals via Dual Surface Modification with Genetically Engineered Elastin-Like Polypeptides and Poly(acrylic acid)
    Malho, J.-M. and Brand, J. and Pecastaings, G. and Ruokolainen, J. and Gröschel, A. and Sèbe, G. and Garanger, E. and Lecommandoux, S.
    ACS Macro Letters 7 646-650 (2018)
    Cellulose nanocrystals (CNCs) are promising candidates for a myriad of applications; however, successful utilization of CNCs requires balanced and multifunctional properties, which require ever more applied concepts for supramolecular tailoring. We present here a facile and straightforward route to generate dual functional CNCs using poly(acrylic acid) (PAA) and biosynthetic elastin-like polypeptides (ELPs). We utilize thiol-maleimide chemistry and SI-ATRP to harvest the temperature responsiveness of ELPs and pH sensitivity of PAA to confer multifunctionality to CNCs. Cryo-TEM and light microscopy are used to exhibit reversible temperature response, while atomic force microscopy (AFM) provides detailed information on the particle morphology. The approach is tunable and allows variation of the modifying molecules, inspiring supramolecular engineering beyond the currently presented motifs. The surge of genetically engineered peptides adds further possibilities for future exploitation of the potential of cellulose nanomaterials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsmacrolett.8b00321
  • 2018 • 242 On the nucleation of planar faults during low temperature and high stress creep of single crystal Ni-base superalloys
    Wu, X. and Dlouhy, A. and Eggeler, Y.M. and Spiecker, E. and Kostka, A. and Somsen, C. and Eggeler, G.
    Acta Materialia 144 642-655 (2018)
    The present work studies the nucleation of planar faults in the early stages of low temperature (750 °C) and high stress (800 MPa) creep of a Ni-base single crystal superalloy (SX). Two families of 60° dislocations with different Burgers vectors were detected in the transmission electron microscope (TEM). These can react and form a planar fault in the γ′ phase. A 2D discrete dislocation model helps to rationalize a sequence of events which lead to the nucleation of a planar fault. First, one 60° channel dislocation approaches another 60° interface dislocation with a different Burgers vector. At a distance of 5 nm, it splits up into two Shockley partials. The interface dislocation is pushed into the γ′-phase where it creates a small antiphase boundary. It can only move on when the leading Shockley partial joins it and creates an overall 1/3<112> superdislocation. This process is fast and therefore is difficult to observe. The results obtained in the present work contribute to a better understanding of the processes which govern the early stages of low temperature and high stress primary creep of SX. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.063
  • 2018 • 241 On the potential of using dual-function hydrogels for brackish water desalination
    Ali, W. and Gebert, B. and Altinpinar, S. and Mayer-Gall, T. and Ulbricht, M. and Gutmann, J.S. and Graf, K.
    Polymers 10 (2018)
    Although current desalination technologies are mature enough and advanced, the shortage of freshwater is still considered as one of the most pressing global issues. Therefore, there is a strong incentive to explore and investigate new potential methods with low energy consumption. We have previously reported that reversible thermally induced sorption/desorption process using polymeric hydrogels hold promise for water desalination with further development. In order to develop a more effective hydrogels architecture, polyelectrolyte moieties were introduced in this work as pendent chains and a thermally responsive polymer as network backbone using reversible addition-fragmentation chain transfer (RAFT) polymerisation. The ability of the comb-type polymeric hydrogels to desalinate water was evaluated. These hydrogels were proved to absorb water with low salinity from brine solution of 2 g L-1 NaCl and release the absorbed water at relatively low temperature conditions of 50 °C. The fraction of the grafted polyacrylic acid and the comb-chain length were varied to understand their influence on the swelling/deswelling behaviour for these hydrogels. The ionic fraction in the hydrogels and the resulting hydrophilic/hydrophobic balance are crucial for the proposed desalination process. In contrast, the comb-chain length impacted the swelling behaviour of hydrogels but showed relatively little influence on the dewatering process. © 2018 by the authors.
    view abstractdoi: 10.3390/polym10060567
  • 2018 • 240 Optimization of pellets manufacturing process using rough set theory
    Pałkowski, Ł. and Karolak, M. and Kubiak, B. and Błaszczyński, J. and Słowiński, R. and Thommes, M. and Kleinebudde, P. and Krysiński, J.
    European Journal of Pharmaceutical Sciences 124 295-303 (2018)
    Pharmaceutical pellets are spherical agglomerates manufactured in extrusion/spheronization process. The composition of the pellets, the amount of active pharmaceutical ingredient (API) and the type of used excipients have an influence on the shape and quality of dosage form. A proper quality of the pellets can also be achieved by identifying the most important technological process parameters. In this paper, a knowledge discovery method, called dominance-based rough set approach (DRSA) has been applied to evaluate critical process parameters in pellets manufacturing. For this purpose, a set of condition attributes (amount of API; type and amount of excipient used; process parameters such as screw and rotation speed, time and temperature of spheronization) and a decision attribute (quality of the pellets defined by the aspect ratio) were used to set up an information system. The DRSA analysis allowed to induce decision rules containing information about process parameters which have a significant impact on the quality of manufactured pellets. Those rules can be used to optimize the process of pellets manufacturing. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejps.2018.08.027
  • 2018 • 239 Overview on micro- and nanomechanical testing: New insights in interface plasticity and fracture at small length scales
    Dehm, G. and Jaya, B.N. and Raghavan, R. and Kirchlechner, C.
    Acta Materialia 142 248-282 (2018)
    Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.019
  • 2018 • 238 Proton Mobility, Intrinsic Acid Strength, and Acid Site Location in Zeolites Revealed by Varying Temperature Infrared Spectroscopy and Density Functional Theory Studies
    Losch, P. and Joshi, H.R. and Vozniuk, O. and Grünert, A. and Ochoa-Hernández, C. and Jabraoui, H. and Badawi, M. and Schmidt, W.
    Journal of the American Chemical Society 140 17790-17799 (2018)
    The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. &lt; 40 kJ mol -1 ) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O-(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O-D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T &gt; 423 K), which has so far never been achieved by any other technique. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/jacs.8b11588
  • 2018 • 237 Solubility predictions of acetanilide derivatives in water: Combining thermochemistry and thermodynamic modeling
    Held, C. and Brinkmann, J. and Schröder, A.-D. and Yagofarov, M.I. and Verevkin, S.P.
    Fluid Phase Equilibria 455 43-53 (2018)
    Knowledge about solubility in water is required for crystallization processes, for the development of structure-property relationships, for the establishment of solubility scales, assessing environmental contamination, and for validating thermodynamic models. Approaches are desired that allow predicting solubility without the use of any experimental solubility data. Most methods that have been proposed to predict aqueous solubility of organic compounds face low prediction reliability and the lack of model interpretability. This work proposes the use of a thermodynamic approach for the prediction of solubility of acetanilide and its derivatives in water. This approach requires fusion enthalpy and fusion temperature as well as the activity coefficient of the respective acetanilide derivative. The latter was obtained by the equation of state PC-SAFT, which uses thermochemistry data as input for model parametrization. The thermochemical data on acetanilide and its derivatives (vapor and sublimation pressures, sublimation and fusion enthalpies) were collected from the literature and evaluated for internal consistency. In order to validate the final solubility prediction model, aqueous solubility of acetanilide and 17 derivatives were predicted and compared to experimental solubility data from literature at 298.15 K as well as to an ideal solubility model, which assumes ideal mixture behavior. The results showed that mixtures of acetanilides + water are highly non-ideal, and the average deviations between solubility data and ideal solubility model could be reduced by two orders of magnitude by using PC-SAFT for the solubility predictions. More promising, PC-SAFT was found to allow predicting the temperature dependence of the aqueous solubility accurately, while ideal solubility model failed to quantitatively describe temperature-dependent solubility. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2017.09.023
  • 2018 • 236 Structural and thermoelectrical characterization of epitaxial Sb2Te3 high quality thin films grown by thermal evaporation
    Bendt, G. and Kaiser, K. and Heckel, A. and Rieger, F. and Oing, D. and Lorke, A. and Rodriguez, N.P. and Schierning, G. and Jooss, C. and Schulz, S.
    Semiconductor Science and Technology 33 (2018)
    Thermal evaporation of Sb2Te3 powder was systematically studied under various pressure and temperature conditions. Low pressure experiments (5 •10-6 mbar) conducted inside a horizontal tube reactor at a temperature range of 500 °C-600 °C generated rough polycrystalline films on Si(100) substrates. Based on these experiments, the chemical composition of the resulting films were determined by the furnace temperature. Enhancing the reactor pressure to 20 mbar shifted the growth zone towards higher temperature ranges and yielded highly c-oriented Sb2Te3 films on Si(100) and Al2O3(0001) substrates. Additional experiments were conducted inside a special reactor containing two independent heaters to study the effects of the evaporator and substrate temperatures independently. In contrast to the samples generated in the previous reactor, a two-zone heating reactor allowed the growth of epitaxial Sb2Te3 films with a very smooth surface topology on Al2O3(0001) substrates, as shown by SEM, EDX, XPS, and HRTEM. The electrical in-plane conductivity of the Sb2Te3 films decreased with increasing temperature, ultimately reaching 3950 S •cm-1 at 300 K. The films showed a p-type carrier concentration of 4.3 •10-19 cm-3 at 300 K and a very high carrier mobility of 558 cm2 •V-1 •s-1. The Seebeck coefficient increased monotonically from 94 μV •K-1 at 270 K to 127 μV •K-1 at 420 K. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6641/aad7a3
  • 2018 • 235 Supercritical CO2 impregnation of PLA/PCL films with natural substances for bacterial growth control in food packaging
    Milovanovic, S. and Hollermann, G. and Errenst, C. and Pajnik, J. and Frerich, S. and Kroll, S. and Rezwan, K. and Ivanovic, J.
    Food Research International 107 486-495 (2018)
    Biodegradable polymers with antibacterial properties are highly desirable materials for active food packaging applications. Thymol, a dietary monoterpene phenol with a strong antibacterial activity is abundant in plants belonging to the genus Thymus. This study presents two approaches for supercritical CO2 impregnation of poly(lactic acid)(PLA)/poly(ε-caprolactone)(PCL) blended films to induce antibacterial properties of the material: (i) a batch impregnation process for loading pure thymol, and (ii) an integrated supercritical extraction-impregnation process for isolation of thyme extract and its incorporation into the films, operated in both batch or semi-continuous modes with supercritical solution circulation. The PCL content in films, impregnation time and CO2 flow regime were varied to maximize loading of the films with thymol or thyme extract with preserving films’ structure and thermal stability. Representative film samples impregnated with thymol and thyme extract were tested against Gram (−) (Escherichia coli) and Gram(+) (Bacillus subtilis) model strains, by measuring their metabolic activity and re-cultivation after exposure to the films. The film containing thymol (35.8 wt%) showed a strong antibacterial activity leading to a total reduction of bacterial cell viability. Proposed processes enable fast, controlled and organic solvent-free fabrication of the PLA/PCL films containing natural antibacterial substances at moderately low temperature, with a compact structure and a good thermal stability, for potential use as active food packaging materials. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.foodres.2018.02.065
  • 2018 • 234 Thermophysical and Mechanical Properties of Advanced Single Crystalline Co-base Superalloys
    Volz, N. and Zenk, C.H. and Cherukuri, R. and Kalfhaus, T. and Weiser, M. and Makineni, S.K. and Betzing, C. and Lenz, M. and Gault, B. and Fries, S.G. and Schreuer, J. and Vaßen, R. and Virtanen, S. and Raabe, D. and Spiecker, E...
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4099-4109 (2018)
    A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-018-4705-1
  • 2018 • 233 Time-resolved impact electrochemistry - A new method to determine diffusion coefficients of ions in solution
    Saw, E.N. and Blanc, N. and Kanokkanchana, K. and Tschulik, K.
    Electrochimica Acta 282 317-323 (2018)
    Diffusion is often the rate-limiting factor of reactions in condensed phase. Thus, knowing the diffusion coefficient is key in numerous aspects ranging from drug release to steering of reactions in both homogeneous liquid phase and electrochemical reactions. Cyclic voltammetry at macro electrodes and chronoamperometry at micro electrodes are well-established methods to determine the diffusion coefficients of redox-active species dissolved in a solution. However, if the formal potentials of the redox species are outside of the potential window of the solvent, then these methods cannot be readily applied. Here we demonstrate a new concept to determine the diffusion coefficient of ions to overcome this limitation. We use their reaction with a well-defined amount of a redox-active indicator substance, which is confined in a nanoparticle suspended in a solution containing the species of interest. Employing transformative nanoparticle impact analysis, the diffusion-limited reaction of an indicator nanoparticle with the species of interest is initiated and followed by chronoamperometry. Measuring the time it takes to fully convert the indicator particle enables the determination of the diffusion coefficient of interest. This concept is demonstrated for variety of (pseudo-)halides in aqueous solution using Ag nanoparticles as redox indicator. Using chloride as an example, is further shown that this new methodology can be applied to study effects of temperature and viscosity on the diffusion coefficients. Given the multitude of nanoparticles that may serve as electrochemical redox indicator, this approach can be used to determine the diffusion coefficients for a large variety of species in different liquid environments. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.06.013
  • 2017 • 232 A quantum chemical and kinetics modeling study on the autoignition mechanism of diethyl ether
    Sakai, Y. and Herzler, J. and Werler, M. and Schulz, C. and Fikri, M.
    Proceedings of the Combustion Institute 36 195-202 (2017)
    A detailed chemical kinetics model has been developed to elucidate the auto-ignition behavior of diethyl ether (DEE) under conditions relevant for internal combustion engines. The present model is composed of a C0-C4 base module from literature and a DEE module. For the low-temperature oxidation mechanism, the reactions of ROO and QOOH radicals were studied previously with a quantum-chemical and transition state theory approach by Sakai et al. (2015). In the present study, the potential energy surfaces for the unimolecular reactions of OOQOOH isomers and 1- and 2-ethoxyethyl radicals were determined with a CBSQB3 composite method. In the presence of an OOH group, the reaction barrier of the hydrogen shift from the β site (terminal carbon atom) decreases as it does in alkane oxidation but there is no effect on the hydrogen shift from the α site (next to the ether oxygen atom). Therefore, the reaction barriers of OOQOOH isomers have the same trend as the corresponding ROO radical and rate constants for the reactions of OOQOOH isomers were determined. The constructed model was validated against the recent data of ignition delay times provided in literature by Werler et al. (2015). The agreement is good over the temperature range 500-1300K and pressure range 1-40bar, although, open questions remain regarding the non-consensus at 900-1150K and 40bar. Reaction-path and sensitivity analyses attribute the importance of the reactivity at the α site to the decrease of the C H bond dissociation energy due to the ether oxygen atom. © 2016.
    view abstractdoi: 10.1016/j.proci.2016.06.037
  • 2017 • 231 Avoiding Self-Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low-Temperature CO Oxidation
    Wang, Y. and Widmann, D. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Angewandte Chemie - International Edition 56 9597-9602 (2017)
    Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low-temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low-temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self-poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate-limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201702178
  • 2017 • 230 Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters
    Yang, J. and Muckel, F. and Baek, W. and Fainblat, R. and Chang, H. and Bacher, G. and Hyeon, T.
    Journal of the American Chemical Society 139 6761-6770 (2017)
    doi: 10.1021/jacs.7b02953
  • 2017 • 229 Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 305-315 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.11.013
  • 2017 • 228 Conversion of an instantaneous activating K+ channel into a slow activating inward rectifier
    Baumeister, D. and Hertel, B. and Schroeder, I. and Gazzarrini, S. and Kast, S.M. and Van Etten, J.L. and Moroni, A. and Thiel, G.
    FEBS Letters 591 295-303 (2017)
    The miniature channel, Kcv, is a structural equivalent of the pore of all K+ channels. Here, we follow up on a previous observation that a largely voltage-insensitive channel can be converted into a slow activating inward rectifier after extending the outer transmembrane domain by one Ala. This gain of rectification can be rationalized by dynamic salt bridges at the cytosolic entrance to the channel; opening is favored by voltage-sensitive formation of salt bridges and counteracted by their disruption. Such latent voltage sensitivity in the pore could be relevant for the understanding of voltage gating in complex Kv channels. © 2016 Federation of European Biochemical Societies
    view abstractdoi: 10.1002/1873-3468.12536
  • 2017 • 227 Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers
    Anusca, I. and Balčiunas, S. and Gemeiner, P. and Svirskas, S. and Sanlialp, M. and Lackner, G. and Fettkenhauer, C. and Belovickis, J. and Samulionis, V. and Ivanov, M. and Dkhil, B. and Banys, J. and Shvartsman, V.V. and Lupascu, D.C.
    Advanced Energy Materials (2017)
    Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic-inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (&gt;27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700600
  • 2017 • 226 Doubling of the magnetic energy product in ferromagnetic nanowires at ambient temperature by capping their tips with an antiferromagnet
    Wang, F.Z. and Salikhov, R. and Spasova, M. and Liébana-Viñas, S. and Bran, C. and Chen, Y.-S. and Vazquez, M. and Farle, M. and Wiedwald, U.
    Nanotechnology 28 (2017)
    We present an approach to prepare free-standing tips of micrometer-long nanowires electrodeposited in anodic aluminum oxide nanopores. Such open tips can be further processed, e.g. for vertical interconnects of functional layers or for tailoring the magnetization reversal of ferromagnetic nanowires. The magnetic switching of nanowires is usually initiated by vortex or domain formation at the nanowire tips. We show that coating the tips of Fe30Co70 nanowires (diameter 40 nm, length 16 μm) with thin antiferromagnetic Fe50Mn50 capping layers (thickness ≈10 nm) leads to magnetic hardening with a more than doubled energy product at ambient temperature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa77b7
  • 2017 • 225 Effect of temperature and texture on the reorientation of martensite variants in NiTi shape memory alloys
    Laplanche, G. and Birk, T. and Schneider, S. and Frenzel, J. and Eggeler, G.
    Acta Materialia 127 143-152 (2017)
    Martensitic Ni50Ti50 wires and sheets with different textures were tensile tested in the temperature range between −100 °C and 60 °C. The effect of texture and temperature on reorientation of martensite variants was investigated. After deformation, all material states were heated into the austenite regime to study their shape memory behavior. During room temperature tensile testing, in-situ digital image correlation revealed that the reorientation of martensite variants is associated with the nucleation and propagation of a macroscopic Lüders band. A comparison between the mechanical data obtained for wire and sheet specimens revealed a strong effect of texture. The plateau stresses of sheets were found to be 25–33% larger and their recoverable strains were 30% lower than for wires. However, the product of plateau stress and recoverable strain, which represents the external work per unit volume required for martensite variants reorientation does not depend on texture. The tensile tests performed at different temperatures revealed that in the temperature range considered the recoverable strain does not depend significantly on temperature. In contrast, the plateau stress as well as the external work required to reorient martensite decrease with increasing testing temperature. We use a thermodynamic approach involving the elastic strain energy associated with the growth of reoriented martensite variants to rationalize these temperature dependencies. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.01.023
  • 2017 • 224 Effect of temperature on the fatigue-crack growth behavior of the high-entropy alloy CrMnFeCoNi
    Thurston, K.V.S. and Gludovatz, B. and Hohenwarter, A. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Intermetallics 88 65-72 (2017)
    Near-equiatomic multi-component high-entropy alloys (HEAs) have engendered much attention of late due to the remarkable mechanical properties of some of these new metallic materials. In particular, one of the first reported HEAs, the equiatomic, single-phase, face-centered cubic (fcc) alloy CrMnFeCoNi, often termed the Cantor alloy, has been shown to display an exceptional combination of strength, ductility and fracture toughness, i.e., damage tolerance, at room temperature, properties that are further enhanced at cryogenic temperatures. Despite this alloy being the most studied HEA to date, its resistance to crack growth under cyclic fatigue loading has not yet been characterized. Here, we examine its fatigue-crack propagation behavior, primarily at lower, near-threshold, growth rates, both at room temperature (293 K) and at 198 K. At 293 K, the alloy shows a fatigue threshold, ΔKTH, of ∼4.8 MPa√m, which increases by more than 30% to ΔKTH ∼6.3 MPa√m with decrease in temperature to 198 K; additionally, the Paris exponent m was found to increase from roughly 3.5 to 4.5 with decreasing temperature. Examination of the fracture surfaces and crack paths indicate a transition from predominantly transgranular crack propagation at room temperature to intergranular-dominated failure at the lower temperature. Such a change in crack path is generally associated with an increasing degree of physical contact between the two fracture surfaces, i.e., roughness-induced fatigue crack closure, which is likely to be the main reason for the difference in the measured thresholds. Additionally, we believe that the higher thresholds found at 198 K are associated with the alloy's higher strength at lower temperatures, which both reduces the crack-tip opening displacements at a given stress-intensity range and prevents plastic deformations of the grains in the wake of the crack. At room temperature, such plastically deformed grains can be associated with a loss of contact shielding of the crack-tip through closure, resulting in a lower threshold compared to 198 K. © 2017
    view abstractdoi: 10.1016/j.intermet.2017.05.009
  • 2017 • 223 Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 214-228 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.09.017
  • 2017 • 222 Flame-temperature, light-attenuation, and CO measurements by spontaneous Raman scattering in non-sooting diesel-like jets
    Raffius, T. and Schulz, C. and Ottenwälder, T. and Grünefeld, G. and Koß, H.-J. and Brands, T. and Pischinger, S.
    Combustion and Flame 176 104-116 (2017)
    Quantitative spatially resolved measurements of temperature and species are lacking particularly in the core of combusting diesel jets. Major problems are light attenuation and interfering light emissions. However, these factors are reduced in non-sooting diesel-like jets, as demonstrated in the present work, because light is not attenuated by soot and interfering LIF (laser-induced fluorescence) from PAHs (polycyclic aromatic hydrocarbons) is substantially lower. The current results show that thermometry by SRS (spontaneous Raman scattering) excited by a UV (ultraviolet) laser is therefore feasible even in the core of a non-sooting diesel-like jet in a combustion vessel. Two diagnostic approaches are assessed. The first one is based on the spectral band shape of the Stokes (red-shifted) ro-vibrational SRS from N2, whereas the ratio of integrated ro-vibrational Stokes to anti-Stokes (blue-shifted) N2-SRS bands is exploited in the second one. It turns out that the first method is advantageous in terms of light attenuation by molecular species, the influence of interfering emissions, and resulting single-shot capability. However, these investigations also show that the anti-Stokes N2-SRS signal can be used for quantification of light attenuation. This is particularly attractive because this SRS band at ∼235 nm nearly coincides with a LIF emission from NO at ∼237 nm, leading to improved attenuation correction of NO-LIF. Furthermore, the recorded spectra indicate that additional quantitative species measurements by SRS are feasible in the non-sooting jet. For instance, the mole fraction of CO is quantified in this work for the first time in the jet core. © 2016 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2016.09.027
  • 2017 • 221 Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing
    Jaya, B.N. and Goto, S. and Richter, G. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 707 164-171 (2017)
    In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4 MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.09.010
  • 2017 • 220 High activity and negative apparent activation energy in low-temperature CO oxidation - Present on Au/Mg(OH)2, absent on Au/TiO2
    Wang, Y. and Widmann, D. and Wittmann, M. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Catalysis Science and Technology 7 4145-4161 (2017)
    Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)2 catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from -90 °C to 200 °C. Catalysts with a very narrow Au particle size distribution were prepared by colloidal deposition. Kinetic measurements, performed under differential, dry reaction conditions at different constant temperatures, enabled the separation of thermal and deactivation effects. They revealed that the distinct reaction behavior, with an exceptionally high activity at temperatures below 0 °C and decreasing CO oxidation rates in the range between -50 °C and 30 °C, equivalent to a negative apparent activation energy, does not result from either deactivation effects or H2O trace impurities, but is an intrinsic feature of the reaction. An unusual temperature dependence was also observed for the tendency for deactivation, with a pronounced maximum at -20 °C, which mainly results from an accumulation of surface carbonate species blocking active reaction sites or access of adsorbed reactants to them. Similar measurements on Au/TiO2 catalysts revealed that the high activity of Au/Mg(OH)2 in the low-temperature range compared to Au/TiO2 is first of all due to the weaker interactions of Mg(OH)2 with CO2 compared to TiO2. This leads to an increasing tendency of CO2 product molecules to adsorb on the latter catalyst at reaction temperatures below 0 °C and hence to rapid 'self-poisoning' with CO2 desorption as the rate-limiting step. For Au/Mg(OH)2, CO2 desorption is much faster, allowing much higher rates in the continuous CO oxidation. Based on temporal analysis of products (TAP) reactor measurements, the decay of the reaction rates in the range -50 °C to +50 °C is tentatively attributed to a decreasing steady-state coverage of weakly bound molecularly adsorbed O2 with increasing temperature, while stable adsorbed active surface oxygen is negligible over the entire range of reaction temperatures investigated. The implications of these and earlier findings for the mechanistic understanding of the low-temperature CO oxidation on Au/Mg(OH)2 and support effects therein are discussed. © The Royal Society of Chemistry 2017.
    view abstractdoi: 10.1039/c7cy00722a
  • 2017 • 219 High-Throughput Structural and Functional Characterization of the Thin Film Materials System Ni-Co-Al
    Decker, P. and Naujoks, D. and Langenkämper, D. and Somsen, C. and Ludwig, Al.
    ACS Combinatorial Science 19 618-624 (2017)
    High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600 °C. Optical, electrical, and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.6b00176
  • 2017 • 218 Insights into the deformation behavior of the CrMnFeCoNi high-entropy alloy revealed by elevated temperature nanoindentation
    Maier-Kiener, V. and Schuh, B. and George, E.P. and Clemens, H. and Hohenwarter, A.
    Journal of Materials Research 32 2658-2667 (2017)
    A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a âŒ100)-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young's modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.260
  • 2017 • 217 Low temperature growth of gallium oxide thin films via plasma enhanced atomic layer deposition
    O'Donoghue, R. and Rechmann, J. and Aghaee, M. and Rogalla, D. and Becker, H.-W. and Creatore, M. and Wieck, A.D. and Devi, A.
    Dalton Transactions 46 16551-16561 (2017)
    Herein we describe an efficient low temperature (60-160 °C) plasma enhanced atomic layer deposition (PEALD) process for gallium oxide (Ga2O3) thin films using hexakis(dimethylamido)digallium [Ga(NMe2)3]2 with oxygen (O2) plasma on Si(100). The use of O2 plasma was found to have a significant improvement on the growth rate and deposition temperature when compared to former Ga2O3 processes. The process yielded the second highest growth rates (1.5 Å per cycle) in terms of Ga2O3 ALD and the lowest temperature to date for the ALD growth of Ga2O3 and typical ALD characteristics were determined. From in situ quartz crystal microbalance (QCM) studies and ex situ ellipsometry measurements, it was deduced that the process is initially substrate-inhibited. Complementary analytical techniques were employed to investigate the crystallinity (grazing-incidence X-ray diffraction), composition (Rutherford backscattering analysis/nuclear reaction analysis/X-ray photoelectron spectroscopy), morphology (X-ray reflectivity/atomic force microscopy) which revealed the formation of amorphous, homogeneous and nearly stoichiometric Ga2O3 thin films of high purity (carbon and nitrogen &lt;2 at.%) under optimised process conditions. Tauc plots obtained via UV-Vis spectroscopy yielded a band gap of 4.9 eV and the transmittance values were more than 80%. Upon annealing at 1000 °C, the transformation to oxygen rich polycrystalline β-gallium oxide took place, which also resulted in the densification and roughening of the layer, accompanied by a slight reduction in the band gap. This work outlines a fast and efficient method for the low temperature ALD growth of Ga2O3 thin films and provides the means to deposit Ga2O3 upon thermally sensitive polymers like polyethylene terephthalate. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt03427j
  • 2017 • 216 Low-Temperature Atomic Layer Deposition of Cobalt Oxide as an Effective Catalyst for Photoelectrochemical Water-Splitting Devices
    Kim, J. and Iivonen, T. and Hämäläinen, J. and Kemell, M. and Meinander, K. and Mizohata, K. and Wang, L. and Räisänen, J. and Beranek, R. and Leskelä, M. and Devi, A.
    Chemistry of Materials 29 5796-5805 (2017)
    We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV-vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (&lt;2% C and &lt;5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition of effective catalyst particles and layers in photoelectrochemical water-splitting devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b05346
  • 2017 • 215 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 214 Magnetic polaron on dangling-bond spins in CdSe colloidal nanocrystals
    Biadala, L. and Shornikova, E.V. and Rodina, A.V. and Yakovlev, D.R. and Siebers, B. and Aubert, T. and Nasilowski, M. and Hens, Z. and Dubertret, B. and Efros, A.L. and Bayer, M.
    Nature Nanotechnology 12 569-574 (2017)
    Non-magnetic colloidal nanostructures can demonstrate magnetic properties typical for diluted magnetic semiconductors because the spins of dangling bonds at their surface can act as the localized spins of magnetic ions. Here we report the observation of dangling-bond magnetic polarons (DBMPs) in 2.8-nm diameter CdSe colloidal nanocrystals (NCs). The DBMP binding energy of 7 meV is measured from the spectral shift of the emission lines under selective laser excitation. The polaron formation at low temperatures occurs by optical orientation of the dangling-bond spins (DBSs) that result from dangling-bond-assisted radiative recombination of spin-forbidden dark excitons. Modelling of the temperature dependence of the DBMP-binding energy and emission intensity shows that the DBMP is composed of a dark exciton and about 60 DBSs. The exchange integral of one DBS with the electron confined in the NC is ∼0.12 meV. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstractdoi: 10.1038/nnano.2017.22
  • 2017 • 213 Measuring and Predicting Thermodynamic Limitation of an Alcohol Dehydrogenase Reaction
    Voges, M. and Fischer, F. and Neuhaus, M. and Sadowski, G. and Held, C.
    Industrial and Engineering Chemistry Research 56 5535-5546 (2017)
    The knowledge of thermodynamic limitations on enzymatic reactions and of influencing factors thereon is essential for process optimization to increase space-time yields and to reduce the amount of solvent or energy consumption. In this work, the alcohol dehydrogenase (ADH) catalyzed reaction from acetophenone and 2-propanol to 1-phenylethanol and acetone in aqueous solution was investigated in a temperature range of 293.15-303.15 K at pH 7. It serves as a model reaction to demonstrate the use of biothermodynamics in order to investigate and predict limitations of enzymatic reactions. Experimental molalities of the reacting agents at equilibrium were measured yielding the position of reaction equilibrium (Km) at different reaction conditions (temperature, initial reactant molalities). The maximum initial acetophenone molality under investigation was 0.02 mol·kg-1 due to solubility limitations with a 1- to 50-fold excess of 2-propanol. It was shown that Km strongly depends on the initial reactant molalities as well as on reaction temperature. Experimental Km values were in the range of 0.20 to 0.49. Thermodynamic key properties (thermodynamic equilibrium constant, standard Gibbs energy and standard enthalpy of reaction) were determined by measured Km values and activity coefficients of the reacting agents predicted with the thermodynamic model ePC-SAFT. In addition, ePC-SAFT was used to predict Km at different initial molalities. Experimental and predicted results were in quantitative agreement (root-mean-square error of experimental versus predicted Km was 0.053), showing that ePC-SAFT is a promising tool to identify process conditions that might increase/decrease Km values and, thus, shift the position of reactions for industrial applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b01228
  • 2017 • 212 Microresonator array: A particular optical sensor
    Weigel, T. and Schweiger, G. and Esen, C. and Ostendorf, A.
    Technisches Messen 84 373-380 (2017)
    Spherical microresonators are high sensitivity sensors for the measurement of important physical quantities e.g. temperature or pressure. Measuring methods based on single optical resonators need expensive and delicate laser systems or spectral devices. The aim of this paper is to present a novel multi-purpose sensing technology based on whispering gallery modes in spherical microparticle arrays. Examples for different applications are given to prove the flexibility and usability of the method.
    view abstractdoi: 10.1515/teme-2016-0063
  • 2017 • 211 Molecular dynamics simulations of entangled polymers: The effect of small molecules on the glass transition temperature
    Mahmoudinezhad, E. and Marquardt, A. and Eggeler, G. and Varnik, F.
    Procedia Computer Science 108 265-271 (2017)
    Effect of small molecules, as they penetrate into a polymer system, is investigated via molecular dynamics simulations. It is found that small spherical particles reduce the glass transition temperature and thus introduce a softening of the material. Results are compared to experimental findings for the effect of different types of small molecules such as water, acetone and ethanol on the glass transition temperature of a polyurethane-based shape memory polymer. Despite the simplicity of the simulated model, MD results are found to be in good qualitative agreement with experimental data. © 2017 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procs.2017.05.152
  • 2017 • 210 On the bifunctional nature of Cu/ZrO2 catalysts applied in the hydrogenation of ethyl acetate
    Schittkowski, J. and Tölle, K. and Anke, S. and Stürmer, S. and Muhler, M.
    Journal of Catalysis 352 120-129 (2017)
    The catalytic hydrogenation of ethyl acetate to ethanol was studied at ambient pressure in the temperature range from 463 K to 513 K using Cu/ZrO2 catalysts obtained by co-precipitation as a function of the Cu loading. The hydrogenation was established as a reproducible probe reaction by determining optimal reaction parameters without deactivation or thermodynamic limitations. Power-law kinetics were determined yielding an apparent activation energy of 74 kJ mol−1 and reaction orders of 0.1–0.3 for H2 and −0.4 to 0.1 for ethyl acetate in the temperature range from 473 K to 503 K. Metallic Cu was found to be essential for the hydrogenation, but the catalytic activity was not proportional to the Cu surface area derived from N2O decomposition and temperature-programmed H2 desorption experiments identifying Cu/ZrO2 as bifunctional catalyst. The acidic sites of the ZrO2 matrix were probed by temperature-programmed experiments with ethyl acetate and NH3. Cu0 is assumed to provide atomic hydrogen by dissociative adsorption and spillover, but the reaction rate is more affected by the tight contact between the embedded Cu nanoparticles and the X-ray amorphous ZrO2 matrix. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.05.009
  • 2017 • 209 Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1
    Zyman, Z. and Epple, M. and Goncharenko, A. and Rokhmistrov, D. and Prymak, O. and Loza, K.
    Journal of Materials Science: Materials in Medicine 28 (2017)
    Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20–980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO4) after heating to 200 °C. CaHPO4 gradually condensed to amorphous calcium pyrophosphate Ca2P2O7 (CPP) between 200 to 620 °C. Amorphous CPP crystallized at 620–740 °C to a metastable polymorph α′-CPP of the high-temperature phase α-CPP and β-CPP. The α′-CPP/ β-CPP phase ratio reached a maximum at 800 °C (60 wt% α′-CPP/40 wt% β-CPP), and α′-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900 °C, so that a three-phasic mixture was obtained in the powder heated to 980 °C. The occurrence of metastable α′-CPP is attributed to Ostwald’s step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed. © 2017, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10856-016-5820-4
  • 2017 • 208 Potential of an alumina-supported Ni3Fe catalyst in the methanation of CO2: Impact of alloy formation on activity and stability
    Mutz, B. and Belimov, M. and Wang, W. and Sprenger, P. and Serrer, M.A. and Wang, D. and Pfeifer, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 6802-6814 (2017)
    A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358°C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 &gt; 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01896
  • 2017 • 207 Propene/isobutane mixtures in heat pumps: An experimental investigation
    Venzik, V. and Roskosch, D. and Atakan, B.
    International Journal of Refrigeration 76 84-96 (2017)
    Zeotropic mixtures in heat pumps, based on thermodynamic analysis, should lead to higher coefficients of performance (COP) due to the temperature glide which decreases exergy losses in the heat exchangers. However, fluid mixtures influence every component of a plant and the total system performance. In addition to the various theoretical studies in this field, a laboratory scale vapour compression heat pump test rig was designed and set up. In the present experimental investigations, the operating performance for the pure fluids isobutane and propene, and their mixtures are systematically investigated. COPs and exergetic efficiencies as a function of evaporation temperature, compressor speed and composition of the mixture are presented and compared with a theoretical approach. Contrary to theoretical expectations, the experimental results show only a slight increase of the COP for the mixture, compared to the better pure fluid, because heat exchanger pressure drops reduce the temperature glide. © 2017 Elsevier Ltd and IIR
    view abstractdoi: 10.1016/j.ijrefrig.2017.01.027
  • 2017 • 206 Reaction Equilibrium of the ω-Transamination of (S)-Phenylethylamine: Experiments and ePC-SAFT Modeling
    Voges, M. and Abu, R. and Gundersen, M.T. and Held, C. and Woodley, J.M. and Sadowski, G.
    Organic Process Research and Development 21 976-986 (2017)
    This work focuses on the thermodynamic equilibrium of the ω-transaminase-catalyzed reaction of (S)-phenylethylamine with cyclohexanone to acetophenone and cyclohexylamine in aqueous solution. For this purpose, the equilibrium concentrations of the reaction were experimentally investigated under varying reaction conditions. It was observed that the temperature (30 and 37 °C), the pH (between pH 7 and pH 9), as well as the initial reactant concentrations (between 5 and 50 mmol·kg-1) influenced the equilibrium position of the reaction. The position of the reaction equilibrium was moderately shifted toward the product side by either decreasing temperature or decreasing pH. In contrast, the initial ratio of the reactants showed only a marginal influence on the equilibrium position. Further experiments showed that increasing the initial reactant concentrations significantly shifted the equilibrium position to the reactant side. In order to explain these effects, the activity coefficients of the reacting agents were calculated and the activity-based thermodynamic equilibrium constant Kth of the reaction was determined. For this purpose, the activity coefficients of the reacting agents were modeled at their respective experimental equilibrium concentrations using the equation of state electrolyte PC-SAFT (ePC-SAFT). The combination of the concentrations of the reacting agents at equilibrium and their respective activity coefficients provided the thermodynamically consistent equilibrium constant Kth. Unexpectedly, the experimental Km values deviated by a factor of up to four from the thermodynamic equilibrium constant Kth. The observed concentration dependency of the experimental Km values could be explained by the influence of concentration on activity coefficients. Further, these activity coefficients were found to be strongly temperature dependent, which is important for the determination of standard enthalpy of reactions, which in this work was found to be +7.7 ± 2.8 kJ·mol-1. Using the so-determined Kth and activity coefficients of the reacting agents (ePC-SAFT), the equilibrium concentrations of the reaction were predicted for varying initial reactant concentrations, which were found to be in good agreement with the experimental behavior. These results showed a non-negligible influence of the activity coefficients of the reacting agents on the equilibrium position and, thus, on the product yield. Experiments and ePC-SAFT predictions showed that the equilibrium position can only be described accurately by taking activity coefficients into account. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.oprd.7b00078
  • 2017 • 205 Reversible hydrogen storage in yttrium aluminum hydride
    Cao, Z. and Ouyang, L. and Wang, H. and Liu, J. and Felderhoff, M. and Zhu, M.
    Journal of Materials Chemistry A 5 6042-6046 (2017)
    Reversible hydrogen storage has been found in transition metal alanates, Y(AlH4)3, for the first time. An amount of 3.4 wt% H2 can be released at 140 °C from the first dehydrogenation step of Y(AlH4)3, and 75% of it is reversible at 145 °C and 100 bar H2, which holds promise for low-temperature applications. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ta10928d
  • 2017 • 204 Shape memory micro-and nanowire libraries for the high-throughput investigation of scaling effects
    Oellers, T. and König, D. and Kostka, A. and Xie, S. and Brugger, J. and Ludwig, Al.
    ACS Combinatorial Science 19 574-584 (2017)
    The scaling behavior of Ti-Ni-Cu shape memory thin-film micro- and nanowires of different geometry is investigated with respect to its influence on the martensitic transformation properties. Two processes for the highthroughput fabrication of Ti-Ni-Cu micro- to nanoscale thin film wire libraries and the subsequent investigation of the transformation properties are reported. The libraries are fabricated with compositional and geometrical (wire width) variations to investigate the influence of these parameters on the transformation properties. Interesting behaviors were observed: Phase transformation temperatures change in the range from 1 to 72 °C (austenite finish, (Af), 13 to 66 °C (martensite start, Ms) and the thermal hysteresis from -3.5 to 20 K. It is shown that a vanishing hysteresis can be achieved for special combinations of sample geometry and composition. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.7b00065
  • 2017 • 203 Size- and density-controlled deposition of Ag nanoparticle films by a novel low-temperature spray chemical vapour deposition method—research into mechanism, particle growth and optical simulation
    Liu, Y. and Plate, P. and Hinrichs, V. and Köhler, T. and Song, M. and Manley, P. and Schmid, M. and Bartsch, P. and Fiechter, S. and Lux-Steiner, M.C. and Fischer, C.-H.
    Journal of Nanoparticle Research 19 (2017)
    Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation. © 2017, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-017-3834-6
  • 2017 • 202 Standard Gibbs energy of metabolic reactions: II. Glucose-6-phosphatase reaction and ATP hydrolysis
    Meurer, F. and Do, H. T. and Sadowski, G. and Held, C.
    Biophysical Chemistry 223 30--38 (2017)
    ATP (adenosine triphosphate) is a key reaction for metabolism. Tools from systems biology require standard reaction data in order to predict metabolic pathways accurately. However, literature values for standard Gibbs energy of ATP hydrolysis are highly uncertain and differ strongly from each other. Further, such data usually neglect the activity coefficients of reacting agents, and published data like this is apparent (condition-dependent) data instead of activity-based standard data. In this work a consistent value for the standard Gibbs energy of ATP hydrolysis was determined. The activity coefficients of reacting agents were modeled with electrolyte Perturbed Chain Statistical Associating Fluid Theory (ePC-SAFT). The Gibbs energy of ATP hydrolysis was calculated by combining the standard Gibbs energies of hexokinase reaction and of glucose-6-phosphate hydrolysis. While the standard Gibbs energy of hexokinase reaction was taken from previous work, standard Gibbs energy of glucose-6-phosphate hydrolysis reaction was determined in this work. For this purpose, reaction equilibrium molalities of reacting agents were measured at pH 7 and pH 8 at 298.15 K at varying initial reacting agent molalities. The corresponding activity coefficients at experimental equilibrium molalities were predicted with ePC-SAFT yielding the Gibbs energy of glucose-6-phosphate hydrolysis of -13.72 +/- 0.75 kJ. mol(-1). Combined with the value for hexokinase, the standard Gibbs energy of ATP hydrolysis was finally found to be - 31.55 +/- 127 kJ. mol(-1). For both, ATP hydrolysis and glucose-6-phosphate hydrolysis, a good agreement with own and literature values were obtained when influences of pH, temperature, and activity coefficients were explicitly taken into account in order to calculate standard Gibbs energy at pH 7, 298.15 K and standard state. (C) 2017 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.bpc.2017.02.005
  • 2017 • 201 State transition and electrocaloric effect of BaZrxTi1-xO3: Simulation and experiment
    Ma, Y.-B. and Molin, C. and Shvartsman, V.V. and Gebhardt, S. and Lupascu, D.C. and Albe, K. and Xu, B.-X.
    Journal of Applied Physics 121 (2017)
    We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1- xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1- xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973574
  • 2017 • 200 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 199 Syntheses and structures of N,C,N-stabilized antimony chalcogenides
    Ganesamoorthy, C. and Wölper, C. and Dostál, L. and Schulz, S.
    Journal of Organometallic Chemistry 845 38-43 (2017)
    The oxidation reactions of ArSb [Ar = 2,6-(HC=N-t-Bu)2C6H3] with S8, grey Se and Te as well as E2Ph2 (E = S, Se, Te) are demonstrated. The reactions of ArSb with elemental sulfur and selenium occurred at elevated temperatures and yielded ArSbE (E = S, 1; Se 2), whereas the reactions with E2Ph2 proceeded at room temperature with subsequent formation of the corresponding insertion complexes ArSb(EPh)2 (E = S 3; Se 4). In addition, ArSb(TePh)2 (5) was formed at very low temperature and showed a temperature-dependent reversible equilibrium with ArSb and Te2Ph2 between -80 °C and 20 °C. The formation and structure of compounds 1-4, which were isolated in good yields, are assigned through multinuclear NMR (1H, 13C, 77Se), IR spectroscopy and microanalyses data. In addition, the molecular structures of 2-4 are further confirmed by single crystal X-ray diffraction studies. © 2017.
    view abstractdoi: 10.1016/j.jorganchem.2017.01.007
  • 2017 • 198 The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel
    Han, J. and da Silva, A.K. and Ponge, D. and Raabe, D. and Lee, S.-M. and Lee, Y.-K. and Lee, S.-I. and Hwang, B.
    Acta Materialia 122 199-206 (2017)
    The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γR) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γR were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.09.048
  • 2017 • 197 The roles of Co-precipitation pH, phase-purity and alloy formation for the ammonia decomposition activity of Ga-promoted Fe/MgO catalysts
    Rein, D. and Friedel Ortega, K. and Weidenthaler, C. and Bill, E. and Behrens, M.
    Applied Catalysis A: General 548 52-61 (2017)
    A series of mesoporous MgFe1.75Ga0.25O4 mixed spinel oxides obtained upon calcination of hydrotalcite-like precursors was investigated in the ammonia decomposition reaction at 1 atm after reduction in H2 atmosphere. The corresponding precursors were synthesized from metal salt solutions at five constant pH values in the range between 8.5 and 10.5 by co-precipitation in aqueous media to study the impact of pH variation on the catalyst's structure and activity. N2 physisorption, thermogravimetric analysis, powder X-ray diffraction, Mössbauer spectroscopy, and temperature programmed techniques (H2-TPR and NH3-TPD) were applied to gather information about the textural, (micro-)structural, and adsorption properties of the samples. While phase purity in the precursor and oxide stages is only observed for pH = 10, undesired by-phases (MgFe2O4 and/or Fe3O4) are additionally formed during co-precipitation at the remaining pH values. This is partly related to an incomplete precipitation of Mg2+ cations in less alkaline environments. In situ XRD measurements during reduction revealed that Fe-Ga alloys are formed between 500 and 600 °C. The absence of by-phases avoids the formation of α-Fe, thus improving the structural and compositional homogeneity of the nitridated samples. This beneficial effect is reflected by the low activation energy (70 kJ/mol) and the enhanced low temperature activity (&lt;450 °C) of the phase pure material in the catalytic decomposition of ammonia. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2017.09.004
  • 2017 • 196 Thermal management approaches of Cu(Inx,Ga1-x)Se2 micro-solar cells
    Sancho-Martínez, D. and Schmid, M.
    Journal of Physics D: Applied Physics 50 (2017)
    Concentrator photovoltaics (CPV) is a cost-effective method for generating electricity in regions that have a large fraction of direct solar radiation. With the help of lenses, sunlight is concentrated onto miniature, highly efficient multi-junction solar cells with a photovoltaic performance above 40%. To ensure illumination with direct radiation, CPV modules must be installed on trackers to follow the sun's path. However, the costs of huge concentration optics and the photovoltaic technology used, narrow the market possibilities for CPV technology. Efforts to reduce these costs are being undertaken by the promotion of Cu(Inx,Ga1-x)Se2 solar cells to take over the high cost multi-junction solar cells and implementing more compact devices by minimization of solar cell area. Micrometer-sized absorbers have the potential of low cost, high efficiencies and good thermal dissipation under concentrated illumination. Heat dissipation at low (&lt;10×) to medium (10 × to 100×) flux density distributions is the key point of high concentration studies for macro- and micro-sized solar cells (from 1 μm2 to 1 mm2). To study this thermal process and to optimize it, critical parameters must be taken in account: absorber area, substrate area and thickness, structure design, heat transfer mechanism, concentration factor and illumination profile. A close study on them will be carried out to determine the best structure to enhance and reach the highest possible thermal management pointing to an efficiency improvement. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8ac5
  • 2017 • 195 Thermodynamics of enzyme-catalyzed esterifications: I. Succinic acid esterification with ethanol
    Altuntepe, E. and Greinert, T. and Hartmann, F. and Reinhardt, A. and Sadowski, G. and Held, C.
    Applied Microbiology and Biotechnology 101 5973-5984 (2017)
    Succinic acid (SA) was esterified with ethanol using Candida antarctica lipase B immobilized on acrylic resin at 40 and 50 °C. Enzyme activity in the reaction medium was assured prior to reaction experiments. Reaction-equilibrium experiments were performed for varying initial molalities of SA and water in the reaction mixtures. This allowed calculating the molality-based apparent equilibrium constant Km as function of concentration and temperature. Km was shown to depend strongly on the molality of water and SA as well as on temperature. It could be concluded that increasing the molality of SA shifted the reaction equilibrium towards the products. Water had a strong effect on the activity of the enzyme and on Km. The concentration dependence of Km values was explained by the activity coefficients of the reacting agents. These were predicted with the thermodynamic models Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), NRTL, and Universal Quasichemical Functional Group Activity Coefficients (UNIFAC), yielding the ratio of activity coefficients of products and reactants Kγ. All model parameters were taken from literature. The models yielded Kγ values between 25 and 115. Thus, activity coefficients have a huge impact on the consistent determination of the thermodynamic equilibrium constants Kth. Combining Km and PC-SAFT-predicted Kγ allowed determining Kth and the standard Gibbs energy of reaction as function of temperature. This value was shown to be in very good agreement with results obtained from group contribution methods for Gibbs energy of formation. In contrast, inconsistencies were observed for Kth using Kγ values from the classical gE-models UNIFAC and NRTL. The importance of activity coefficients opens the door for an optimized reaction setup for enzymatic esterifications. © 2017, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00253-017-8287-4
  • 2017 • 194 Trace Adsorption of Ethane, Propane, and n-Butane on Microporous Activated Carbon and Zeolite 13X at Low Temperatures
    Birkmann, F. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 62 1973-1982 (2017)
    Removing of trace-leveled light hydrocarbons from exhaust air or gas streams becomes an increasingly important issue in the field of process and environmental technology, e.g., storage and transport of liquefied natural gas. Adsorption processes at temperatures below 0°C have great potential to meet process specifications or environmental regulatory limits. Designing of such adsorption processes requires a profound insight into the thermodynamics of adsorption at low temperatures, which is not available yet. Therefore, this work provides adsorption isotherms of ethane, propane, and n-butane on microporous activated carbon and zeolite 13X in a temperature range from -40 to +60°C and at partial pressures from 5 to 1000 Pa. The influence of temperature on the adsorbed amount on activated carbon and zeolite 13X is discussed for each adsorptive considering isosteric heats of adsorption and specific interactions between the adsorptive and the adsorbent surface. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b01068
  • 2016 • 193 3-Dimensional microstructural characterization of CdTe absorber layers from CdTe/CdS thin film solar cells
    Stechmann, G. and Zaefferer, S. and Konijnenberg, P. and Raabe, D. and Gretener, C. and Kranz, L. and Perrenoud, J. and Buecheler, S. and Tiwari, A.N.
    Solar Energy Materials and Solar Cells 151 66-80 (2016)
    The present work reports on a study on the microstructure and its evolution during processing of CdTe absorber layers from CdTe/CdS thin film solar cells grown by low-temperature processes in substrate configuration. Investigations were performed at different stages of the cell manufacturing, from deposition to the final functional solar cell, with the aim to understand the microstructure formation of the photoactive layer. To this end 3-dimensional microstructure characterization was performed using focused ion beam/electron backscatter diffraction tomography ("3D-EBSD") together with conventional 2D-EBSD. The analyses revealed strong microstructural and textural changes developing across the thickness of the absorber material, between the back contact and the p-n junction interfaces. Based on the 3-dimensional reconstruction of the CdTe thin film, a coherent growth model was proposed, emphasizing the microstructural continuity before and after a typical CdCl2-annealing activation treatment. One of the principal results is that the absorber layer is created by two concomitant processes, deposition and recrystallization, which led to different textures and microstructures. Further changes are the result of subsequent annealing treatments, favoring twinning and promoting well-defined texture components. The results open the possibility for a grain boundary engineering approach applied to the design of such cells. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2016.02.023
  • 2016 • 192 A higherature ferromagnetic topological insulating phase by proximity coupling
    Katmis, F. and Lauter, V. and Nogueira, F.S. and Assaf, B.A. and Jamer, M.E. and Wei, P. and Satpati, B. and Freeland, J.W. and Eremin, I. and Heiman, D. and Jarillo-Herrero, P. and Moodera, J.S.
    Nature 533 513-516 (2016)
    Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ∼2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies. © 2016 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/nature17635
  • 2016 • 191 Advanced Simulation-based Design of High Performance Machining Processes
    Biermann, D. and Bleckmann, T. and Schumann, S. and Iovkov, I.
    Procedia CIRP 46 165-168 (2016)
    The development of high performance machining processes is a key aspect to achieve higher productivity, efficiency and performance in modern production systems. In order to reduce the corresponding effort and costs, simulation systems are one possibility to support the design and the optimization of manufacturing processes. In this article, three different application examples with respect to milling, grinding and deep-hole drilling operations are presented. In this context, both finite-element and geometric-kinematic simulation approaches are applied to model the different challenging issues of the corresponding machining process. © 2016 The Authors.
    view abstractdoi: 10.1016/j.procir.2016.03.167
  • 2016 • 190 Altering the luminescence properties of self-assembled quantum dots in GaAs by focused ion beam implantation
    Rothfuchs, C. and Kukharchyk, N. and Greff, M.K. and Wieck, A.D. and Ludwig, Ar.
    Applied Physics B: Lasers and Optics 122 (2016)
    Using quantum dots (QDs) as single-photon sources draws the attention in many quantum communication technologies. One pathway towards manufacturing single-photon sources is focused ion beam (FIB) implantation in molecular beam epitaxy-grown QD samples to disable all QDs around an intentional one for single photoluminescence (PL) emission. In this paper, we investigate the lattice disorders in the vicinity of InAs/GaAs QDs introduced by FIB implantation of gallium and indium ions. For high fluences, we achieve total elimination of the QDs photoluminescence. The impact of the different ion species and fluences is studied by low-temperature PL measurements. Furthermore, we deduce a simple model based on the trap-assisted recombination for the description of the degradation of the PL emission. It allows the determination of the fluences at which the PL emission is suppressed. Moreover, we identify the implantation-induced non-radiative defects by temperature-dependent PL measurements. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-015-6305-8
  • 2016 • 189 Colloquium: Water's controversial glass transitions
    Amann-Winkel, K. and Böhmer, R. and Fujara, F. and Gainaru, C. and Geil, B. and Loerting, T.
    Reviews of Modern Physics 88 (2016)
    Water is the most common and, judged from its numerous anomalous properties, the weirdest of all known liquids and the complexity of its pressure-temperature map is unsurpassed. A major obstacle on the way to a full understanding of water's structure and dynamics is the hard-to-explore territory within this map, colloquially named the no man's land. Many experiments suggest that just before stepping across its low-temperature border, amorphous ices undergo glass-to-liquid transitions while other interpretations emphasize the importance of underlying disordered (nano)crystalline states. Prospects for reconciling the conflicting views regarding the nature of water's glass transitions are discussed. © 2016 American Physical Society.
    view abstractdoi: 10.1103/RevModPhys.88.011002
  • 2016 • 188 Combinatorial design of transitory constitution steels: Coupling high strength with inherent formability and weldability through sequenced austenite stability
    Springer, H. and Belde, M. and Raabe, D.
    Materials and Design 90 1100-1109 (2016)
    We introduce a novel alloying and processing scheme for high strength steels, which allows for precise and cost-effective cold forming due to high fractions of metastable austenite, and a subsequent low-distortion, coating-preserving strengthening through martensitic transformation induced by low temperature treatments. As the constitution is thus synchronised with the processing requirements, we refer to these materials as Transitory Constitution Steels. Suitable alloy compositions were identified by high throughput screenings through the exemplarily material systems Fe-5Ni-0.3C-(3-15)Mn and Fe-13.5Cr-6Mn-2Cu-0.2C-(0-2)Ni (wt.%) using combinatorial bulk metallurgical methods. The transformation behaviour, mechanical properties and underlying microstructural phenomena were studied in more detail after upscaling of selected compositions. The steel Fe-13.5Cr-6Mn-1.5Cu-0.2C (wt.%) exhibited an increase in yield strength from 300 to 1050. MPa after immersion into liquid nitrogen, as well as an ultimate tensile strength of more than 1700. MPa at a total elongation of more than 9%. Despite the ultra high strength, no embrittlement induced by Laser beam welding was observed, highlighting the inherent weldability of steels synthesised by the alloying and processing scheme presented here. Possibilities for flexible alloy design and processing variations are discussed. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.11.050
  • 2016 • 187 Compatible solutes: Thermodynamic properties relevant for effective protection against osmotic stress
    Held, C. and Sadowski, G.
    Fluid Phase Equilibria 407 224-235 (2016)
    Organisms developed very different strategies to protect themselves against osmotic stress. To sustain high salt concentrations of their surrounding some organisms accumulate so-called compatible solutes (CSs), which increase the internal osmotic pressure without disturbing the organism's metabolism. At constant temperature, osmotic pressure is mainly determined by the concentration of the compatible solute and the osmotic coefficient of the aqueous solution, and to a minor extent also by solution densities. Thus, osmotic coefficients and densities were measured for aqueous CS solutions in a broad range of concentration and at three temperatures (273. K, 310. K, 323. K) at atmospheric pressure. Further, the solubility of CSs in water was measured as function of temperature to determine the maximum CS concentration that can be applied in aqueous solutions. CSs under investigation were trimethylamine N-oxide (TMAO), trehalose, citrulline, N,. N-dimethylglycine, DMSO, glycerol, methylglycine, and ectoine. The data was used to calculate real osmotic pressures induced by these CSs. PC-SAFT was applied to model thermodynamic properties and phase equilibria of aqueous CS solutions in quantitative agreement to experimental data. Among the CSs investigated in this work, TMAO induced the highest osmotic pressure and thus can be considered the best protector against osmotic stress. The data was finally analyzed concerning the influence of CSs molecular size, charge, and hydrophobicity on osmotic pressure. This included also the comparison to incompatible solutes (urea, glycine). © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2015.07.004
  • 2016 • 186 Controlling the Photocorrosion of Zinc Sulfide Nanoparticles in Water by Doping with Chloride and Cobalt Ions
    Weide, P. and Schulz, K. and Kaluza, S. and Rohe, M. and Beranek, R. and Muhler, M.
    Langmuir 32 12641-12649 (2016)
    Photodegradation under UV light irradiation is a major drawback in photocatalytic applications of sulfide semiconductors. ZnS nanoparticles were doped with very low amounts of chloride or cobalt ions in the ppm range and codoped with chloride and cobalt ions during their synthesis by precipitation in aqueous solution followed by calcination. The high-temperature wurtzite phase annealed at 800 °C had a high susceptibility to UV irradiation in water, while the low-temperature zincblende phase annealed at 400 °C was found to be stable. Chlorine doping increased the rate of photocorrosion in water, whereas cobalt doping led to a stabilization of the ZnS nanoparticles. Based on photochemical and spectroscopic investigations applying UV/vis, X-ray photoelectron, and photoluminescence spectroscopy, the increased susceptibility of Cl-doped ZnS is ascribed to a higher number of surface point defects, whereas the stabilization by Co2+ is caused by additional recombination pathways for the charge carriers in the bulk, thus avoiding photocorrosion processes at the surface. Additional doping of Cl-doped ZnS with cobalt ions was found to counteract the detrimental effect of the chloride ions efficiently. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.6b03385
  • 2016 • 185 Devolatilization and volatiles reaction of individual coal particles in the context of FGM tabulated chemistry
    Knappstein, R. and Kuenne, G. and Ketelheun, A. and Köser, J. and Becker, L. and Heuer, S. and Schiemann, M. and Scherer, V. and Dreizler, A. and Sadiki, A. and Janicka, J.
    Combustion and Flame 169 72-84 (2016)
    The method of Flamelet Generated Manifolds (FGM) is coupled with a coal devolatilization model to perform transient simulations of a well-defined single coal particle combustion experiment for the first time. The gas phase chemistry is mapped onto a three-dimensional manifold controlled by the mixture fraction, a reaction progress variable and the enthalpy. A simulation of an electrically heated inert pyrolysis reactor is performed in order to evaluate transferability and applicability of experimentally obtained devolatilization kinetic parameters to Large Eddy Simulation (LES) codes for combustion configurations. Finally, the FGM modeling approach is applied to a premixed flat flame configuration, in which the coal particles cross a laminar flame front and are exposed to the hot gases. Numerical results regarding the volatiles reaction are compared to experimental findings. Particle and gas phase states are studied. Overall, good agreement between numerical results and experimental findings regarding the volatiles ignition range could be observed. © 2016 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2016.04.014
  • 2016 • 184 Double minimum creep of single crystal Ni-base superalloys
    Wu, X. and Wollgramm, P. and Somsen, C. and Dlouhy, A. and Kostka, A. and Eggeler, G.
    Acta Materialia 112 242-260 (2016)
    Low temperature (750°C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the (001) direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.012
  • 2016 • 183 Dual-Templated Cobalt Oxide for Photochemical Water Oxidation
    Deng, X. and Bongard, H.-J. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 9 409-415 (2016)
    Mesoporous Co3O4 was prepared using a dual templating approach whereby mesopores inside SiO2 nanospheres, as well as the void spaces between the nanospheres, were used as templates. The effect of calcination temperature on the crystallinity, morphology, and textural parameters of the Co3O4 replica was investigated. The catalytic activity of Co3O4 for photochemical water oxidation in a [Ru(bpy)3]2+[S2O8]2- system was evaluated. The Co3O4 replica calcined at the lowest temperature (150°C) exhibited the best performance as a result of the unique nanostructure and high surface area arising from the dual templating. The performance of Co3O4 with highest surface area was further examined in electrochemical water oxidation. Superior activity over high temperature counterpart and decent stability was observed. Furthermore, CoO with identical morphology was prepared from Co3O4 using an ethanol reduction method and a higher turnover-frequency number for photochemical water oxidation was obtained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201500872
  • 2016 • 182 Effects of CO2 on submicronic carbon particulate (soot) formed during coal pyrolysis in a drop tube reactor
    Senneca, O. and Apicella, B. and Heuer, S. and Schiemann, M. and Scherer, V. and Stanzione, F. and Ciajolo, A. and Russo, C.
    Combustion and Flame 172 302-308 (2016)
    In oxycombustion and gasification processes coal pyrolysis occurs in CO2-rich atmospheres. The present work investigates the effect of such conditions on the quantity and quality of the submicronic carbon particulate produced. Pyrolysis experiments were carried out in either N2 or CO2 atmospheres in a laminar drop tube reactor, with wall temperatures of 1573 K, heating rates of 104–105 K/s and residence times below 130 ms, so as to reproduce pyrolysis conditions comparable to those of pulverized coal-fired boilers. The carbon particulate sampled in the reactor was found to have bimodal distribution in the micronic and submicronic ranges. A method based on solvent extraction was applied to carbon particulate for separating the two modes and determining the relative mass contribution of micronic and submicronic fractions. In CO2 atmosphere the amount of submicronic fraction of carbon particulate, referred to as soot, was found to be up to four times as much as upon N2 experiments. Beside the larger formation of soot, relevant differences in terms of combustion reactivity, size distribution and chemical structure of the residual carbon particulate produced in CO2 environment in respect to N2 environment were observed by means of a large array of techniques including thermogravimetry, microscopy (SEM+EDX), FT-IR, UV–visible and Raman spectroscopy along with XRD and XPS techniques. © 2016 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2016.07.023
  • 2016 • 181 Electrocaloric Effect in Ba(Zr,Ti)O3–(Ba,Ca)TiO3 Ceramics Measured Directly
    Sanlialp, M. and Shvartsman, V.V. and Acosta, M. and Lupascu, D.C. and Alford, N.
    Journal of the American Ceramic Society 99 4022-4030 (2016)
    In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase. © 2016 The American Ceramic Society
    view abstractdoi: 10.1111/jace.14456
  • 2016 • 180 Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures
    Gludovatz, B. and Hohenwarter, A. and Thurston, K.V.S. and Bei, H. and Wu, Z. and George, E.P. and Ritchie, R.O.
    Nature Communications 7 (2016)
    High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning. © 2016, Nature Publishing Group. All rights reserved.
    view abstractdoi: 10.1038/ncomms10602
  • 2016 • 179 Fe-doped Beta zeolite from organotemplate-free synthesis for NH3-SCR of NOX
    Zhu, Y. and Chen, B. and Zhao, R. and Zhao, Q. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Shi, C. and Zhang, W.
    Catalysis Science and Technology 6 6581-6592 (2016)
    Two types of Beta zeolites, one from organotemplate-free synthesis with a Si/Al ratio of 9 and the other from a commercial one with a Si/Al ratio of 19, were employed here to dope Fe for NH3-SCR of NOx. Fe-Beta (Si/Al = 9) exhibits much higher activity than Fe-Beta (Si/Al = 19), especially at low-temperature regions (< 250 °C). In addition, it also exhibits better hydrothermal stability as compared with Fe-Beta (Si/Al = 19), which demonstrates that it is a promising SCR catalyst with good activity as well as hydrothermal stability. The correlation between the quantitative calculation of the content of isolated Fe3+ in Beta zeolites and the NO conversion rate at 150 °C shows a linear relationship, suggesting that the isolated Fe3+ species affect the SCR activity directly. The higher activity of the Fe-Beta-9 catalyst is supposed to be related not only to the isolated Fe3+ but also to the acidity. Furthermore, the template-free synthesized Beta zeolite shows less dealumination during hydrothermal aging and therefore better hydrothermal stability during the SCR reaction. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy00231e
  • 2016 • 178 Fine structure of the Mn acceptor in GaAs
    Krainov, I.V. and Debus, J. and Averkiev, N.S. and Dimitriev, G.S. and Sapega, V.F. and Lähderanta, E.
    Physical Review B 93 (2016)
    We reveal the electronic level structure of the Mn acceptor, which consists of a valence-band hole bound to an Mn2+ ion, in presence of applied uniaxial stress and an external magnetic field in bulk GaAs. Resonant spin-flip Raman scattering is used to measure the g factor of the AMn0 center in the ground and excited states with the total angular momenta F=1 and F=2 and characterize the optical selection rules of the spin-flip transitions between these Mn-acceptor states. We determine the random stress fields near the Mn acceptor, the constant of the antiferromagnetic exchange interaction between the valence-band holes and the electrons of the inner Mn2+ shell as well as the deformation potential for the exchange energy. The p-d exchange energy, in particular, decreases significantly with increasing compressive stress. By combining the experimental Raman study with the developed theoretical model on the scattering efficiency, in which also the random local and external uniaxial stresses and magnetic field are considered, the fine structure of the Mn acceptor is determined in full detail. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.93.235202
  • 2016 • 177 Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
    Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 138 9572-9580 (2016)
    Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b04251
  • 2016 • 176 Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers
    Weyrich, C. and Drögeler, M. and Kampmeier, J. and Eschbach, M. and Mussler, G. and Merzenich, T. and Stoica, T. and Batov, I.E. and Schubert, J. and Plucinski, L. and Beschoten, B. and Schneider, C.M. and Stampfer, C. and Grütz...
    Journal of Physics Condensed Matter 28 (2016)
    Ternary (Bi1-xSbx)2Te3 films with an Sb content between 0 and 100% were deposited on a Si(1 1 1) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by x-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1-xSbx)2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/49/495501
  • 2016 • 175 High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 3367-3373 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b00447
  • 2016 • 174 Improvement of catalytic activity over Cu–Fe modified Al-rich Beta catalyst for the selective catalytic reduction of NOx with NH3
    Xu, L. and Shi, C. and Chen, B. and Zhao, Q. and Zhu, Y. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 236 211-217 (2016)
    Copper and iron bimetal modified Al-rich Beta zeolites from template-free synthesis were prepared for selective catalytic reduction (SCR) of NOx with NH3 in exhaust gas streams. Comparing to the Cu-based and Fe-based mono-component Beta catalysts, Cu(3.0)-Fe(1.3)-Beta bi-component catalyst shows better low-temperature activity and wider reaction-temperature window. Over 80% of NO conversion can be achieved at the temperature region of 125–500 °C. Due to the synergistic effect of copper and iron evidenced by XRD, UV–Vis–NIR, EPR and XPS measurements, the dispersion state of active components as well as the ratio of Cu2+/Cu+ and Fe3+/Fe2+ were improved over Cu(3.0)-Fe(1.3)-Beta. Isolated Cu2+ and Fe3+ ions which located at the exchange sites could be the active species at the low-temperature region, while FeOx cluster species may be more important to the high-temperature activity. During the test of sulfur resistance, Fe-containing samples including Cu(3.0)-Fe(1.3)-Beta and Fe(2.7)-Beta-4 present better performance compared to Cu(4.1)-Beta-4. Deactivation of Cu-based catalyst is attributed to the easier deposition of sulfates over the surface according to the results of TGA coupled with TPD experiments. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2016.08.042
  • 2016 • 173 Influence of the degree of infiltration of modified activated carbons with CuO/ZnO on the separation of NO2 at ambient temperatures
    Sager, U. and Däuber, E. and Bathen, D. and Asbach, C. and Schmidt, F. and Tseng, J.-C. and Pommerin, A. and Weidenthaler, C. and Schmidt, W.
    Adsorption Science and Technology 34 307-319 (2016)
    The reduction of NO2 in air at ambient temperatures with activated carbons can be increased by the infiltration of metal oxide nanoparticles into the sorbents. The NO2 is first adsorbed to the activated carbon and subsequently catalytically reduced to physiologically neutral substances by the metal oxides. The catalytic reduction at ambient temperatures is rather slow. In a former study concerning the application in cabin air filters, it was shown that the modification of activated carbon with 5 wt% CuO/ZnO leads to reduced breakthrough of NO2 and that the adsorbent was able to regenerate between repeated NO2 adsorption cycles. Here we show that the efficiency of the sorbent can be more than doubled by increasing the metal oxide infiltration to 20 wt% whereas a further increase in loading yields no additional improvement, due to a partial transformation of the oxidic compounds. © 2016, © The Author(s) 2016.
    view abstractdoi: 10.1177/0263617416653120
  • 2016 • 172 Lattice dynamics and thermoelectric properties of nanocrystalline silicon-germanium alloys
    Claudio, T. and Stein, N. and Petermann, N. and Stroppa, D.G. and Koza, M.M. and Wiggers, H. and Klobes, B. and Schierning, G. and Hermann, R.P.
    Physica Status Solidi (A) Applications and Materials Science 213 515-523 (2016)
    The lattice dynamics and thermoelectric properties of sintered phosphorus-doped nanostructured silicon-germanium alloys obtained by gas-phase synthesis were studied. Measurements of the density of phonon states by inelastic neutron scattering were combined with measurements of the elastic constants and the low-temperature heat capacity. A strong influence of nanostructuring and alloying on the lattice dynamics was observed. The thermoelectric transport properties of samples with different doping as well as samples sintered at different temperature were characterized between room temperature and 1000°C. A peak figure of merit zT=0.88 at 900°C is observed and is comparatively insensitive to the aforementioned parameter variations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532500
  • 2016 • 171 Magnetization and transport properties of single crystalline RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb)
    Drachuck, G. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C.
    Journal of Magnetism and Magnetic Materials 417 420-433 (2016)
    Single crystals of RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb) were grown out of a high temperature solution rich in Pd and P and characterized by room-temperature powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization and temperature-dependent in-plane resistivity measurements. In this series, YPd2P2 and LaPd2P2 YbPd2P2 (with Yb2+) are non-local-moment bearing. Furthermore, YPd2P2 and LaPd2P2 are found to be superconducting with Tc≃0.75 and 0.96 K respectively. CePd2P2 and PrPd2P2 magnetically order at low temperature with a ferromagnetic component along the crystallographic c-axis. The rest of the series manifest low temperature antiferromagnetic ordering. EuPd2P2 has Eu2+ ions and both EuPd2P2 and GdPd2P2 have isotropic paramagnetic susceptibilities consistent with L=0 and [formula presented] and exhibit multiple magnetic transitions. For R=Eu–Dy, there are multiple, T&gt;1.8K transitions in zero applied magnetic field and for R=Nd, Eu, Gd, Tb, and Dy there are clear metamagnetic transitions at T=2.0 K for H&lt;55kOe. Strong anisotropies arising mostly from crystal electric field (CEF) effects were observed for most magnetic rare earths with L≠0. The experimentally estimated CEF parameters B20 were calculated from the anisotropic paramagnetic θab and θc values and compared to theoretical trends across the rare earth series. The ordering temperatures as well as the polycrystalline averaged paramagnetic Curie–Weiss temperature, θave, were extracted from magnetization and resistivity measurements, and compared to the de-Gennes factor. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2016.05.089
  • 2016 • 170 Metal hydrides for concentrating solar thermal power energy storage
    Sheppard, D.A. and Paskevicius, M. and Humphries, T.D. and Felderhoff, M. and Capurso, G. and Bellosta von Colbe, J. and Dornheim, M. and Klassen, T. and Ward, P.A. and Teprovich, J.A., Jr. and Corgnale, C. and Zidan, R. and Grant...
    Applied Physics A: Materials Science and Processing 122 (2016)
    The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost of concentrating solar thermal power. We focus on the underlying technology that allows metal hydrides to function as thermal energy storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room temperature and as high as 1100 °C. The potential of metal hydrides for thermal storage is explored, while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature TES are also addressed. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9825-0
  • 2016 • 169 Modeling Hardenable Stainless Steels Using Calculated Martensite Start Temperatures in Thermodynamic Equilibrium Calculations
    Seifert, M. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 47 5953-5959 (2016)
    In this work, martensite start temperatures of several martensitic stainless steels containing different amounts and types of carbides were calculated by means of thermodynamic equilibrium calculations. Two different equations were introduced into the Thermo-Calc® software. The calculations were performed for the respective compositions at austenitization temperature and compared to martensite start temperatures measured using a quenching dilatometer. The purpose was to estimate hardenability and hardness of newly developed steels. Even though the equations used were determined empirically for specific alloying systems, general trends for the investigated steels were found to be reproduced very well. Thus, the comparison of martensite start temperatures of different steels in comparable alloying systems is highly effective for modeling new steels and for predicting their hardenability. © 2016, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-016-3805-z
  • 2016 • 168 On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 1381-1387 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600071
  • 2016 • 167 Optical visualization of radiative recombination at partial dislocations in GaAs
    Karin, T. and Linpeng, X. and Rai, A.K. and Ludwig, Ar. and Wieck, A.D. and Fu, K.-M.C.
    Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 1989-1992 (2016)
    Individual dislocations in an ultra-pure GaAs epi-layer are investigated with spatially and spectrally resolved photoluminescence imaging at 5 K. We find that some dislocations act as strong non-radiative recombination centers, while others are efficient radiative recombination centers. We characterize luminescence bands in GaAs due to dislocations, stacking faults, and pairs of stacking faults. These results indicate that low-temperature, spatially-resolved photoluminescence imaging can be a powerful tool for identifying luminescence bands of extended defects. This mapping could then be used to identify extended defects in other GaAs samples solely based on low-temperature photoluminescence spectra. © 2016 IEEE.
    view abstractdoi: 10.1109/PVSC.2016.7749976
  • 2016 • 166 Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics
    Sawada, H. and Kawakami, K. and Körmann, F. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Acta Materialia 102 241-250 (2016)
    Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.09.010
  • 2016 • 165 Photoluminescence of gallium ion irradiated hexagonal and cubic GaN quantum dots
    Rothfuchs, C. and Kukharchyk, N. and Koppe, T. and Semond, F. and Blumenthal, S. and Becker, H.-W. and As, D.J. and Hofsäss, H.C. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 383 1-5 (2016)
    We report on ion implantation into GaN QDs and investigate their radiation hardness. The experimental study is carried out by photoluminescence (PL) measurements on molecular beam epitaxy-grown GaN quantum dots after ion implantation. Both quantum dots grown in the hexagonal (H) and the cubic (C) crystal structure were subjected to gallium ions with an energy of 400 kV (H) and 75 kV (C) with fluences ranging from 5×1010 cm−2 to 1×1014 cm−2 (H) and to 1×1015 cm−2 (C), respectively. Low-temperature PL measurements reveal a PL quenching for which a quantitative model as a function of the ion fluence is developed. A high degradation resistance is concluded. A non-radiative trap with one main activation energy is found for all QD structures by temperature-dependent PL measurements. Further analysis of fluence-dependent PL energy shifts shows ion-induced intermixing and strain effects. Particular for the hexagonal quantum dots, a strong influence of the quantum confined Stark effect is present. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2016.06.004
  • 2016 • 164 Physisorption versus chemisorption of oxygen molecules on Ag(100)
    Mehlhorn, M. and Morgenstern, K.
    Journal of Chemical Physics 144 (2016)
    We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945339
  • 2016 • 163 Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
    Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
    Combustion and Flame 169 307-320 (2016)
    Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times &gt;4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2016.03.030
  • 2016 • 162 Simultaneous Study of Brownian and Néel Relaxation Phenomena in Ferrofluids by Mössbauer Spectroscopy
    Landers, J. and Salamon, S. and Remmer, H. and Ludwig, F. and Wende, H.
    Nano Letters 16 1150-1155 (2016)
    We demonstrate the ability of Mössbauer spectroscopy to simultaneously investigate Brownian motion and Néel relaxation in ferrofluidic samples. For this purpose, Mössbauer spectra of coated iron oxide nanoparticles with core diameters of 6.0-26.4 nm dissolved in 70 vol % glycerol solution were recorded in the temperature range of 234-287 K and compared to low-temperature spectra without Brownian motion. By comparison to theory, we were able to determine the particle coating thickness and the dynamic viscosity of the fluid from the broadening of the absorption lines (Brownian motion), as well as the state of Néel relaxation. Results from Mössbauer spectroscopy were crosschecked by AC-susceptometry at several temperatures for Brownian motion and in the high-frequency regime (100 Hz-1 MHz) for Néel relaxation. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.5b04409
  • 2016 • 161 Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe
    Kothapalli, K. and Böhmer, A.E. and Jayasekara, W.T. and Ueland, B.G. and Das, P. and Sapkota, A. and Taufour, V. and Xiao, Y. and Alp, E. and Bud'ko, S.L. and Canfield, P.C. and Kreyssig, A. and Goldman, A.I.
    Nature Communications 7 (2016)
    A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mössbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≥31.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1-xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.
    view abstractdoi: 10.1038/ncomms12728
  • 2016 • 160 Strong correlations, strong coupling, and s -wave superconductivity in hole-doped BaFe2As2 single crystals
    Hardy, F. and Böhmer, A.E. and De'Medici, L. and Capone, M. and Giovannetti, G. and Eder, R. and Wang, L. and He, M. and Wolf, T. and Schweiss, P. and Heid, R. and Herbig, A. and Adelmann, P. and Fisher, R.A. and Meingast, C.
    Physical Review B 94 (2016)
    We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient γn is observed with both decreasing band filling and isovalent substitution (K, Rb, and Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our density functional theory + slave-spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit. ©2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.94.205113
  • 2016 • 159 Tempering behavior of a low nitrogen boron-added 9%Cr steel
    Fedorova, I. and Kostka, A. and Tkachev, E. and Belyakov, A. and Kaibyshev, R.
    Materials Science and Engineering A 662 443-455 (2016)
    The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.03.092
  • 2016 • 158 The detachment behavior of polycarbonate on thin films above the glass transition temperature
    Tillmann, W. and Hagen, L. and Hoffmann, F. and Dildrop, M. and Wibbeke, A. and Schöppner, V. and Resonnek, V. and Pohl, M. and Krumm, C. and Tiller, J.C. and Paulus, M. and Sternemann, C.
    Polymer Engineering and Science 56 786-797 (2016)
    When producing mono-axially stretched films made of amorphous polycarbonate, a self-reinforcement is generated due to the stretching process. This leads to an increase of the strength and stiffness. The mono-axial stretching process is conducted at temperatures above the glass transition temperature, whereas better mechanical properties are obtained at higher stretching temperatures. However, the film tends to adhere to the rolls, especially at temperatures from 10°C above the glass transition temperature. The rolls of the mono-axial stretching unit are made of an induction hardened and polished quenched and tempered steel 1.7225 – 42CrMo4. This work reports on the investigation of the detachment behavior of polycarbonate on different coatings as a function of the temperature and contact time. The main intention is to find a suitable coating on which the polycarbonate film adheres only slightly at temperatures clearly exceeding the glass transition temperature. POLYM. ENG. SCI., 56:786–797, 2016. © 2016 Society of Plastics Engineers. © 2016 Society of Plastics Engineers
    view abstractdoi: 10.1002/pen.24307
  • 2016 • 157 The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens
    Wollgramm, P. and Bürger, D. and Parsa, A.B. and Neuking, K. and Eggeler, G.
    Materials at High Temperatures 33 346-360 (2016)
    In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/09603409.2016.1186414
  • 2016 • 156 Thermal shift of the resonance between an electron gas and quantum dots: What is the origin?
    Brinks, F. and Wieck, A.D. and Ludwig, Ar.
    New Journal of Physics 18 (2016)
    The operation of quantum dots (QDs) at highest possible temperatures is desirable for many applications. Capacitance-voltage spectroscopy (C(V)-spectroscopy) measurements are an established instrument to analyse the electronic structure and energy levels of self-assembled QDs. We perform C(V) in the dark and C(V) under the influence of non-resonant illumination, probing exciton states up to X4+ on InAs QDs embedded in a GaAs matrix for temperatures ranging from 2.5 to 120 K. While a small shift in the charging spectra resonance is observed for the two spin degenerate electron s-state charging voltages with increasing temperature, a huge shift is visible for the electron-hole excitonic states resonance voltages. The s2-peak moves to slightly higher, the s1-peak to slightly lower charging voltages. In contrast, the excitonic states are surprisingly charged at much lower voltages upon increasing temperature. We derive a rate-model allowing to attribute and value different contributions to these shifts. Resonant tunnelling, state degeneracy and hole generation rate in combination with the Fermi distribution function turn out to be of great importance for the observed effects. The differences in the shifting behaviour is connected to different equilibria schemes for the peaks - s-peaks arise when tunnelling-in- and out-rates become equal, while excitonic peaks occur, when electron tunnelling-in- and hole-generation rates are balanced. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa4f63
  • 2016 • 155 Thermodynamics of Bioreactions
    Held, C. and Sadowski, G.
    Annual Review of Chemical and Biomolecular Engineering 7 395-414 (2016)
    Thermodynamic principles have been applied to enzyme-catalyzed reactions since the beginning of the 1930s in an attempt to understand metabolic pathways. Currently, thermodynamics is also applied to the design and analysis of biotechnological processes. The key thermodynamic quantity is the Gibbs energy of reaction, which must be negative for a reaction to occur spontaneously. However, the application of thermodynamic feasibility studies sometimes yields positive Gibbs energies of reaction even for reactions that are known to occur spontaneously, such as glycolysis. This article reviews the application of thermodynamics in enzyme-catalyzed reactions. It summarizes the basic thermodynamic relationships used for describing the Gibbs energy of reaction and also refers to the nonuniform application of these relationships in the literature. The review summarizes state-of-the-art approaches that describe the influence of temperature, pH, electrolytes, solvents, and concentrations of reacting agents on the Gibbs energy of reaction and, therefore, on the feasibility and yield of biological reactions. Copyright © 2016 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-chembioeng-080615-034704
  • 2016 • 154 Thermodynamics of the alanine aminotransferase reaction
    Voges, M. and Schmidt, F. and Wolff, D. and Sadowski, G. and Held, C.
    Fluid Phase Equilibria 422 87-98 (2016)
    The thermodynamic equilibrium of the aminotransferase reaction from l-alanine and 2-oxoglutarate to l-glutamate and pyruvate in aqueous solution was investigated in a temperature range between 25 and 37 °C and pH between at 5 and 9.Prior to considering the reaction equilibria, measurements were carried out to ensure the enzyme activity in the aqueous reaction media. After that, equilibrium concentrations of reacting agents were measured by HPLC-analysis. At constant temperature and pH, reaction equilibrium was shown to depend on the absolute molalities (0.005-0.130 mol kg-1) as well as on the ratio of initial molalities of the reactants. It could be concluded that reaction equilibrium was shifted towards the product site upon increasing reactant molalities, increasing temperature, and increasing pH. Further, yields of pyruvate were increased upon excess initial molality of l-alanine compared to 2-oxoglutarate.The thermodynamic equilibrium constant Ka* was determined by extrapolating the ratio of product equilibrium molalites and reactant equilibrium molalites to infinite dilution of all reacting agents. The activity-coefficient ratio of products and reactants in the reaction media was predicted with ePC-SAFT. Combining Ka* and the activity-coefficient ratio allowed quantitatively predicting the influence of temperature, pH, and reacting-agent molalities on the reaction equilibrium. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2016.01.023
  • 2016 • 153 Thermoresponsive amperometric glucose biosensor
    Pinyou, P. and Ruff, A. and Pöller, S. and Barwe, S. and Nebel, M. and Alburquerque, N.G. and Wischerhoff, E. and Laschewsky, A. and Schmaderer, S. and Szeponik, J. and Plumeré, N. and Schuhmann, W.
    Biointerphases 11 011001 (2016)
    The authors report on the fabrication of a thermoresponsive biosensor for the amperometric detection of glucose. Screen printed electrodes with heatable gold working electrodes were modified by a thermoresponsive statistical copolymer [polymer I: poly(ω-ethoxytriethylenglycol methacrylate-co-3-(N,N-dimethyl-N-2-methacryloyloxyethyl ammonio) propanesulfonate-co-ω-butoxydiethylenglycol methacrylate-co-2-(4-benzoyl-phenoxy)ethyl methacrylate)] with a lower critical solution temperature of around 28 °C in aqueous solution via electrochemically induced codeposition with a pH-responsive redox-polymer [polymer II: poly(glycidyl methacrylate-co-allyl methacrylate-co-poly(ethylene glycol)methacrylate-co-butyl acrylate-co-2-(dimethylamino)ethyl methacrylate)-[Os(bpy)2(4-(((2-(2-(2-aminoethoxy)ethoxy)ethyl)amino)methyl)-N,N-dimethylpicolinamide)](2+)] and pyrroloquinoline quinone-soluble glucose dehydrogenase acting as biological recognition element. Polymer II bears covalently bound Os-complexes that act as redox mediators for shuttling electrons between the enzyme and the electrode surface. Polymer I acts as a temperature triggered immobilization matrix. Probing the catalytic current as a function of the working electrode temperature shows that the activity of the biosensor is dramatically reduced above the phase transition temperature of polymer I. Thus, the local modulation of the temperature at the interphase between the electrode and the bioactive layer allows switching the biosensor from an on- to an off-state without heating of the surrounding analyte solution.
    view abstractdoi: 10.1116/1.4938382
  • 2016 • 152 Two-Step Annealing Leading to Refined Bi2Te3-In2Te3 Lamellar Structures for Tuning of Thermoelectric Properties
    Liu, D. and Li, X. and Schmechel, R. and Rettenmayr, M.
    Journal of Electronic Materials 45 1654-1660 (2016)
    A two-step annealing process was applied to control the morphology of Bi2Te3-In2Te3 composite materials via precipitation of In2Te3 from supersaturated (Bi,In)2Te3. Finer lamellae were obtained via two-step as compared with single-step isothermal annealing. The microstructure was optimized by exploiting thermodynamic and kinetic effects during nucleation and growth of In2Te3. The relationship between the morphologies and thermoelectric properties was analyzed. With preannealing at a lower temperature, refined morphologies lead to an enhanced power factor and zT in the temperature range from room temperature to ∼100°C. The enhancement is mainly caused by an increased Seebeck coefficient, most probably due to energy-dependent scattering processes. However, the thermal conductivity is dominated by bipolar thermal transport that compensates the low lattice thermal conductivity completely. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11664-015-4151-4
  • 2015 • 151 A novel approach for analyzing the dissolution mechanism of solid dispersions
    Ji, Y. and Paus, R. and Prudic, A. and Lübbert, C. and Sadowski, G.
    Pharmaceutical Research 32 2559-2578 (2015)
    Purpose To analyze the dissolution mechanism of solid dispersions of poorly water-soluble active pharmaceutical ingredients (APIs), to predict the dissolution profiles of the APIs and to find appropriate ways to improve their dissolution rate. Methods The dissolution profiles of indomethacin and naproxen from solid dispersions in PVP K25 were measured in vitro using a rotating-disk system (USP II). A chemical-potential-gradient model combined with the thermodynamic model PC-SAFT was developed to investigate the dissolution mechanism of indomethacin and naproxen from their solid dispersions at different conditions and to predict the dissolution profiles of these APIs. Results The results show that the dissolution of the investigated solid dispersions is controlled by dissolution of both, API and PVP K25 as they codissolve according to the initial API loading. Moreover, the dissolution of indomethacin and naproxen was improved by decreasing the API loading in polymer (leading to amorphous solid dispersions) and increasing stirring speed, temperature and pH of the dissolution medium. The dissolution of indomethacin and naproxen from their amorphous solid dispersions is mainly controlled by the surface reaction, which implies that indomethacin and naproxen dissolution can be effectively improved by formulation design and by improving their solvation performance. Conclusions The chemical-potential-gradient model combined with PC-SAFTcan be used to analyze the dissolution mechanism of solid dispersions and to describe and predict the dissolution profiles of API as function of stirring speed, temperature and pH value of the medium. This work helps to find appropriate ways to improve the dissolution rate of poorly-soluble APIs. © Springer Science+Business Media New York 2015.
    view abstractdoi: 10.1007/s11095-015-1644-z
  • 2015 • 150 Adsorption of a Switchable Industrial Dye on Au(111) and Ag(111)
    Boom, K. and Müller, M. and Stein, F. and Ernst, S. and Morgenstern, K.
    Journal of Physical Chemistry C 119 17718-17724 (2015)
    We investigate astraphloxine, an industrial dye, on two metal surfaces, Au(111) and Ag(111). Low-temperature scanning tunneling microscopy with submolecular resolution in comparison to semiempirical calculations reveal that only two of the nine possible conformers of this molecule are adsorbed. The two conformers adsorb via one of their indol groups, which serves as a platform that decouples the rest of the molecule from the surfaces. A change from one to the other conformer is demonstrated by injecting inelastic electrons from the tunneling tip selectively into individual molecules. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b04883
  • 2015 • 149 Benchmark Thermochemistry for Biologically Relevant Adenine and Cytosine. A Combined Experimental and Theoretical Study
    Emel'yanenko, V.N. and Zaitsau, D.H. and Shoifet, E. and Meurer, F. and Verevkin, S.P. and Schick, C. and Held, C.
    Journal of Physical Chemistry A 119 9680-9691 (2015)
    The thermochemical properties available in the literature for adenine and cytosine are in disarray. A new condensed phase standard (p° = 0.1 MPa) molar enthalpy of formation at T = 298.15 K was measured by using combustion calorimetry. New molar enthalpies of sublimation were derived from the temperature dependence of vapor pressure measured by transpiration and by the quarz-crystal microbalance technique. The heat capacities of crystalline adenine and cytosine were measured by temperature-modulated DSC. Thermodynamic data on adenine and cytosine available in the literature were collected, evaluated, and combined with our experimental results. Thus, the evaluated collection of data together with the new experimental results reported here has helped to resolve contradictions in the available enthalpies of formation. A set of reliable thermochemical data is recommended for adenine and cytosine for further thermochemical calculations. Quantum-chemical calculations of the gas phase molar enthalpies of formation of adenine and cytosine have been performed by using the G4 method and results were in excellent agreement with the recommended experimental data. The standard molar entropies of formation and the standard molar Gibbs functions of formation in crystal and gas state have been calculated. Experimental vapor-pressure data measured in this work were used to estimate pure-component PC-SAFT parameters. This allowed modeling solubility of adenine and cytosine in water over the temperature interval 278-310 K. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.5b04753
  • 2015 • 148 Butadiene from acetylene-ethylene cross-metathesis
    Trotuş, I.-T. and Zimmermann, T. and Duyckaerts, N. and Geboers, J. and Schüth, F.
    Chemical Communications 51 7124-7127 (2015)
    Acetylene to butadiene direct synthesis, via enyne cross-metathesis, is demonstrated with commercial ruthenium carbene catalysts. Using excess of ethylene, yields greater than 50% are obtained. High activity is observed in the first minute of the reaction (TOF &gt; 800 h-1 based on butadiene). Catalyst reusability and poisoning are discussed. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5cc00853k
  • 2015 • 147 Chemical cross-linking of polypropylenes towards new shape memory polymers
    Raidt, T. and Hoeher, R. and Katzenberg, F. and Tiller, J.C.
    Macromolecular Rapid Communications 36 744-749 (2015)
    In this work, syndiotactic polypropylene (sPP) as well as isotactic polypropylene (iPP) are cross-linked to gain a shape memory effect. Both prepared PP networks exhibit maximum strains of 700%, stored strains of up to 680%, and recoveries of nearly 100%. While x-iPP is stable for many cycles, x-sPP ruptures after the first shape-memory cycle. It is shown by wide-angle X-ray scattering (WAXS) experiments that cross-linked iPP exhibits homoepitaxy in the temporary, stretched shape but in contrast to previous reports it contains a higher amount of daughter than mother crystals. Shape memory polypropylene is prepared by cross-linking of syndiotactic as well as isotactic polypropylene (iPP). Cross-linked iPP is a shape-memory polymer with excellent stored strain, fixity-, and recovery-ratios. Wide angle X-ray scattering (WAXS) experiments reveal that the crystals in programmed x-iPP show a microstructure with mother and daughter crystals. In contrast to previous reports, the amount of daughter crystals exceeds that of the mothers. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201400727
  • 2015 • 146 Classification of the pre-settlement behaviour of barnacle cyprids
    Maleschlijski, S. and Bauer, S. and Aldred, N. and Clare, A.S. and Rosenhahn, A.
    Journal of the Royal Society Interface 12 (2015)
    Barnacle cyprids exhibit a complex swimming and exploratory behaviour on surfaces and settlement is a consequence of extensive surface probing and selection of suitable settlement sites. In this work, the behaviour of cyprids in their pre-settlement phase was studied by three-dimensional video stereoscopy. With this technique, three-dimensional trajectories were obtained that were quantitatively analysed. The velocity during vertical sinking of cyprids of Balanus amphitritewas used with a modified form of Stokes' law to calculate their mean body density. Furthermore, a classification of the swimming patterns allowed the extension of existing models describing cyprid locomotion and swimming behaviour. The patterns were characterized with respect to their occurrence, transition between patterns and their velocity distribution, and motions were identified that led to surface contacts. This analysis provides a classification framework, which can assist future attempts to identify behavioural responses of cyprids to specific settlement cues. © 2014 The Author(s) Published by the Royal Society.
    view abstractdoi: 10.1098/rsif.2014.1104
  • 2015 • 145 Consecutive mechanism in the diffusion of D2O on a NaCl(100) bilayer
    Heidorn, S.-C. and Bertram, C. and Cabrera-Sanfelix, P. and Morgenstern, K.
    ACS Nano 9 3572-3578 (2015)
    The motion of D<inf>2</inf>O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy. The diffusion distance histogram reveals a squared diffusion lattice that agrees with the primitive unit cell of the (100) surface. From the Arrhenius dependence, we derive the diffusion energy, the pre-exponential factor, and the attempt frequency. The mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b00691
  • 2015 • 144 Development of sustained and dual drug release co-extrusion formulations for individual dosing
    Laukamp, E.J. and Vynckier, A.-K. and Voorspoels, J. and Thommes, M. and Breitkreutz, J.
    European Journal of Pharmaceutics and Biopharmaceutics 89 357-364 (2015)
    In personalized medicine and patient-centered medical treatment individual dosing of medicines is crucial. The Solid Dosage Pen (SDP) allows for an individual dosing of solid drug carriers by cutting them into tablet-like slices. The aim of the present study was the development of sustained release and dual release formulations with carbamazepine (CBZ) via hot-melt co-extrusion for the use in the SDP. The selection of appropriate coat- and core-formulations was performed by adapting the mechanical properties (like tensile strength and E-modulus) for example. By using different excipients (polyethylene glycols, poloxamers, white wax, stearic acid, and carnauba wax) and drug loadings (30-50%) tailored dissolution kinetics was achieved showing cube root or zero order release mechanisms. Besides a biphasic drug release, the dose-dependent dissolution characteristics of sustained release formulations were minimized by a co-extruded wax-coated formulation. The dissolution profiles of the co-extrudates were confirmed during short term stability study (six months at 21.0 ± 0.2 °C, 45% r.h.). Due to a good layer adhesion of core and coat and adequate mechanical properties (maximum cutting force of 35.8 ± 2.0 N and 26.4 ± 2.8 N and E-modulus of 118.1 ± 8.4 and 33.9 ± 4.5 MPa for the dual drug release and the wax-coated co-extrudates, respectively) cutting off doses via the SDP was precise. While differences of the process parameters (like the barrel temperature) between the core- and the coat-layer resulted in unsatisfying content uniformities for the wax-coated co-extrudates, the content uniformity of the dual drug release co-extrudates was found to be in compliance with pharmacopoeial specification. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2014.12.027
  • 2015 • 143 Differences between thermal and laser-induced diffusion
    Zaum, C. and Meyer-Auf-Der-Heide, K.M. and Mehlhorn, M. and McDonough, S. and Schneider, W.F. and Morgenstern, K.
    Physical Review Letters 114 (2015)
    A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.146104
  • 2015 • 142 Dynamics in Supercooled Secondary Amide Mixtures: Dielectric and Hydrogen Bond Specific Spectroscopies
    Gainaru, C. and Bauer, S. and Vynokur, E. and Wittkamp, H. and Hiller, W. and Richert, R. and Böhmer, R.
    Journal of Physical Chemistry B 119 15769-15779 (2015)
    Alkylacetamide-based model peptides display an intense Debye-type dielectric relaxation. In order to explore the extent to which this feature has to be regarded analogous to that in other supramolecular liquids, notably the monohydroxy alcohols, we applied broadband dielectric, time-dependent solvation, and near-infrared spectroscopies as well as shear rheology and various nuclear magnetic resonance techniques to mixtures of N-methylacetamide (NMA) or N-ethylacetamide (NEA) with N-methylformamide. Compared in the modulus format, dielectric relaxation, solvation dynamics, and mechanical response indicate a common global and local dynamics. The present spin-relaxation measurements reflect motional processes which are significantly faster than the dominant Debye dielectric response, and a similar conclusion is drawn from measurements of the shear viscosity. The NH overtone stretching vibrations reveal a temperature-dependent hydrogen-bond equilibrium that changes its characteristics near temperatures of 325 K. Finally, dielectric low-temperature data recorded for (NEA)0.4(NMF)0.6 mixed with 2-picoline indicate the existence of a critical concentration akin to the situation in various monohydroxy alcohol mixtures. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b10034
  • 2015 • 141 Element-resolved thermodynamics of magnetocaloric lafe13-xsix
    Gruner, M.E. and Keune, W. and Roldan Cuenya, B. and Weis, C. and Landers, J. and Makarov, S.I. and Klar, D. and Hu, M.Y. and Alp, E.E. and Zhao, J. and Krautz, M. and Gutfleisch, O. and Wende, H.
    Physical Review Letters 114 (2015)
    By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13-xSix. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.057202
  • 2015 • 140 Evolution of the Laves Phase in Ferritic Heat-Resistant Steels During Long-term Annealing and its Influence on the High-Temperature Strength
    Nabiran, N. and Klein, S. and Weber, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 102-114 (2015)
    Heat-resistant ferritic steels containing Laves phase precipitates were designed supported by thermodynamic modeling. High-temperature compression tests at 1173.15 K (900 °C) and a detailed characterization of the microstructural evolution during annealing at 1173.15 K (900 °C) were carried out to investigate the effect of Laves phase formation on the high-temperature strength. Due to the addition of W/Mo and/or Nb, the high-temperature strength of the newly designed alloys is significantly higher than that of the reference steels. However, the high-temperature strength of all investigated steels decreases slightly as the annealing time is increased up to 1440 hours. To determine the influence of Laves phase formation and coarsening on the high-temperature strength during long-term annealing, the precipitates were extracted from the ferritic matrix in different annealing states. The phases in the powder residue were determined by XRD, and the chemical composition of the Laves phase in dependence of the annealing time was analyzed by EDS measurements. During annealing, steel Fe18CrMoW forms Nb(C,N), Ti(C,N), Laves phase (Fe2Nb) and Fe3Nb3C, whereas alloy Fe19CrWAl forms Nb(C,N), Ti(C,N), and Laves phase (Fe2Nb). The Laves phase within the alloys Fe18CrMoW and Fe19CrWAl differs in its morphology as well as its chemical composition. The Laves phase in steel Fe18CrMoW attains its chemical equilibrium after 192 hours, whereas alloy Fe19CrWAl required 24 hours. Overall, the formation of the Laves phase prevents significant grain growth during high-temperature annealing, thus preserving the high-temperature strength over a long time period. © 2014, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-014-2505-9
  • 2015 • 139 Extreme flexibility in a zeolitic imidazolate framework: Porous to dense phase transition in desolvated ZIF-4
    Wharmby, M.T. and Henke, S. and Bennett, T.D. and Bajpe, S.R. and Schwedler, I. and Thompson, S.P. and Gozzo, F. and Simoncic, P. and Mellot-Draznieks, C. and Tao, H. and Yue, Y. and Cheetham, A.K.
    Angewandte Chemie - International Edition 54 6447-6451 (2015)
    Abstract Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23% contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K. ZIF-4(Zn) undergoes a porous to non-porous transition on cooling from the high-temperature (HT) to low-temperature (LT) phase. The nature of this transition is elucidated by a combined approach of structure solution from powder diffraction, DSC measurement, and DFT calculations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201410167
  • 2015 • 138 Fluctuating multicomponent lattice Boltzmann model
    Belardinelli, D. and Sbragaglia, M. and Biferale, L. and Gross, M. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 91 (2015)
    Current implementations of fluctuating lattice Boltzmann equations (FLBEs) describe single component fluids. In this paper, a model based on the continuum kinetic Boltzmann equation for describing multicomponent fluids is extended to incorporate the effects of thermal fluctuations. The thus obtained fluctuating Boltzmann equation is first linearized to apply the theory of linear fluctuations, and expressions for the noise covariances are determined by invoking the fluctuation-dissipation theorem directly at the kinetic level. Crucial for our analysis is the projection of the Boltzmann equation onto the orthonormal Hermite basis. By integrating in space and time the fluctuating Boltzmann equation with a discrete number of velocities, the FLBE is obtained for both ideal and nonideal multicomponent fluids. Numerical simulations are specialized to the case where mean-field interactions are introduced on the lattice, indicating a proper thermalization of the system. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.91.023313
  • 2015 • 137 FMR Investigations of Two-dimensional Periodic Arrays of Disc-shaped Co Particles at Different Temperatures
    Martyanov, O.N. and Balaev, D.A. and Pylypenko, O.V. and Odnodvorets, L.V. and Chernov, S.V. and Nepijko, S.A. and Elmers, H.-J. and Schneider, C.M. and Schönhense, G.
    Journal of Superconductivity and Novel Magnetism 28 3587-3591 (2015)
    Using ferromagnetic resonance method, we performed measurements of two-dimensional periodic arrays of disc-shaped cobalt particles with different diameters a=450 and 900 nm and the distance between them l=2a and 3a with temperature variation in the range T = 140 −300 K. The first derivative of the microwave absorption spectrum was registered. With the increase of T additional peaks on both sides from the main peak that move aside from it, all peaks show an increase of the intensity and a decrease of the width. Dependences of the resonance field on T shows saturation-like behavior with increasing temperature. They move to higher temperatures and show sharper behavior with a increase and l decrease, respectively. An increase of a leads to the intensity decrease and width increase of all three adsorption peaks. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10948-015-3178-3
  • 2015 • 136 From high-entropy alloys to high-entropy steels
    Raabe, D. and Tasan, C.C. and Springer, H. and Bausch, M.
    Steel Research International 86 1127-1138 (2015)
    Inspired by high-entropy alloys, we study the design of steels that are based on high configurational entropy for stabilizing a single-phase solid solution matrix. The focus is placed on the system Fe-Mn-Al-Si-C but we also present trends in the alloy system Fe-Mn-Al-C. Unlike in conventional high-entropy alloys, where five or more equiatomically proportioned components are used, we exploit the flat configurational entropy plateau in transition metal mixtures, stabilizing solid solutions also for lean, non-equiatomic compositions. This renders the high-entropy alloying concept, where none of the elements prevails, into a class of Fe-based materials which we refer to as high-entropy steels. A point that has received little attention in high-entropy alloys is the use of interstitial elements. Here, we address the role of C in face-centered cubic solid solution phases. High-entropy steels reveal excellent mechanical properties, namely, very high ductility and toughness; excellent high rate and low-temperature ductility; high strength of up to 1 GPa; up to 17% reduced mass density; and very high strain hardening. The microstructure stability can be tuned by adjusting the stacking fault energy. This enables to exploit deformation effects such as the TRIP, TWIP, or precipitation determined mechanisms. We present a class of massive solid solution steels with high configurational entropy. Focus is placed on the system Fe-Mn-Al-Si-C, i.e., considering also C interstitials. By exploiting the flat configurational entropy plateau in metal mixtures, solid solutions of lean, non-equiatomic compositions can be stabilized. This renders the high-entropy alloying concept, where none of the elements prevails, into high-entropy steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201500133
  • 2015 • 135 Heat input modeling and calibration in dry NC-milling processes
    Schweinoch, M. and Joliet, R. and Kersting, P. and Zabel, A.
    Production Engineering 9 495-504 (2015)
    Due to friction and material deformation in the shear zone, workpieces in NC-milling processes are subjected to heat input and thermal loading. Ongoing geometric changes as well as time-varying contact and cutting conditions result in an inhomogeneous temperature field that is constantly in flux. Such thermally loaded workpieces often exhibit complex and transient thermomechanical deformations, which may result in erroneous material removal with respect to the desired shape. In order to meet critical manufacturing tolerances, it is therefore necessary to avoid and compensate these effects. Predicting the deformation exhibited by a thermally loaded workpiece is a problem of linear thermoelasticity, which can be solved by use of the finite element (FE) method. A prerequisite to this is the accurate calculation of the temperature field that results within the workpiece material during the course of the milling process. Although the FE method may be used for this as well, the practical application to realistic milling processes is limited due to the required computational resources. This paper presents a fast geometric process simulation for the prediction of cutting forces, heat input and thermal loading in dry NC milling. The temperature field of the workpiece is continuously updated, such that it is possible to determine the temperature of any material point at any point in time of the milling process. Individual models comprising the simulation system are described in detail, along with the experiments that are required to calibrate them. The accuracy of the geometric process simulation is validated by comparison with experimental data for a non-trivial milling process. © 2015, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-015-0621-z
  • 2015 • 134 Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
    Gu, D. and Jia, C.-J. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Journal of the American Chemical Society 137 11407-11418 (2015)
    Co<inf>3</inf>O<inf>4</inf> with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H<inf>2</inf> reduction of nanocast Co<inf>3</inf>O<inf>4</inf> at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3¯m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo<inf>2</inf>O<inf>4</inf>, CoCr<inf>2</inf>O<inf>4</inf>, and CoFe<inf>2</inf>O<inf>4</inf>) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe<inf>2</inf>O<inf>4</inf> with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b06336
  • 2015 • 133 Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates
    Macleod, S.J. and See, A.M. and Hamilton, A.R. and Farrer, I. and Ritchie, D.A. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D.
    Applied Physics Letters 106 (2015)
    Accumulation mode devices with epitaxially grown gates have excellent electrical stability due to the absence of dopant impurities and surface states. We overcome typical fabrication issues associated with epitaxially gated structures (e.g., gate leakage and high contact resistance) by using separate gates to control the electron densities in the Ohmic and Hall bar regions. This hybrid gate architecture opens up a way to make ultrastable nanoscale devices where the separation between the surface gates and the 2D electron gas is small. In this work, we demonstrate that the hybrid devices made from the same wafer have reproducible electrical characteristics, with identical mobility and density traces over a large range of 2D densities. In addition, thermal cycling does not influence the measured electrical characteristics. As a demonstration of concept, we have fabricated a hybrid single-electron transistor on a shallow (50 nm) AlGaAs/GaAs heterostructure that shows clear Coulomb blockade oscillations in the low temperature conductance. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4905210
  • 2015 • 132 Influence of Alloying Elements, Heat Treatment, and Temperature on the Thermal Conductivity of Heat Treatable Steels
    Wilzer, J. and Küpferle, J. and Weber, S. and Theisen, W.
    Steel Research International 86 1234-1241 (2015)
    The thermal conductivity of heat treatable martensitic steels plays an important role for many industrial applications. In case of hot stamping, the thermal conductivity of forging tools determines not only the product quality but also the productivity. Hence, the aim of ongoing developments is to increase the thermal conductivity of tool steels for several industrial applications. Therefore, it is highly beneficial to know how thermal conductivity is influenced by alloying elements, heat treatment, and temperature. This work deals with the thermal conductivity of non-alloyed heat treatable steel C45 and high-alloyed corrosion resistant steel X42Cr13 in the as-quenched and in the tempered condition. Additionally, the influence of dislocations on thermal conductivity is analyzed using commercially pure iron (Armco-Iron) in the annealed and in the cold-rolled condition. The results reveal that tempering affects both the electronic and the phononic contribution to thermal conductivity. Furthermore, due to the high chromium content, the thermal conductivity of steel X42Cr13 increases with temperature, which can be traced back to the electronic contribution. These results are useful for the development and improvement of tool steels, when the thermal conductivity of tools is not only a property but also a process parameter. This work deals with the thermal conductivity of non-alloyed steel C45 and corrosion resistant steel X42Cr13. Tempering affects both the electronic and the phononic contribution to thermal conductivity. Furthermore, thermal conductivity of steel X42Cr13 increases with temperature, which can be traced back to the electronic contribution. These results are useful for the development and improvement of tool steels. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201400294
  • 2015 • 131 Influence of water on supra-molecular assembly of 4, 4′-dihydroxy azobenzene on Ag(111)
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of Chemical Physics 142 (2015)
    We explore co-deposition of water and 4, 4′-dihydroxy azobenzene on Ag(111) by low-temperature scanning tunneling microscopy at different water-to-azobenzene ratios. At all ratios, the water interacts with the hydroxyl end groups of the molecule replacing the direct hydrogen bonding. The change in bonding reduces the azobenzene density as compared to the one in the closed-packed waterless azobenzene structure. At intermediate water-to-azobenzene ratios, pores are formed in the azobenzene layer at nanometer distance from the water. At high water-to-azobenzene ratios, a water superstructure with a 1.4 nm × 1.4 nm unit cell develops. Our results point to a method to vary the density of an organic layer by tuning the amount of an inorganic additive. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4907368
  • 2015 • 130 Joining tube to tube sheet for coil wound heat exchangers by hybrid friction diffusion bonding
    Roos, A. and Alba, D.R. and Hanke, S. and Wimmer, G. and Dos Santos, J.F.
    American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 6A-2015 (2015)
    Coil-wound heat exchangers (CWHE) for low temperature applications such as the liquefaction of natural gas (LNG) are often made of aluminium alloys. The fabrication of these aluminium coil-wound heat exchangers holds several challenges, one of which is joining the tubes to the tube sheet. For this specific task, conventional joining technologies such as laser beam welding (LBW) or tungsten inert gas (TIG) welding cannot be easily performed in fully-mechanised mode or are not cost-effective. A joint project between the Helmholtz-Zentrum Geesthacht (HZG) and LINDE Engineering aims at the development of a new solid state joining process, Hybrid Friction Diffusion Bonding (HFDB), to fabricate tube-to-tubesheet connections for aluminium coil-wound heat exchangers. In the present study, the HFDB process has been developed to industrial maturity and the quality of the joints has been demonstrated by gas leak tightness tests and tensile pull-out tests. The joints meet the requirements for industrial application. Furthermore, the thermal field development in the weld area and the applied process forces have been monitored and correlated to process parameters. The microstructure of the joint has been investigated, and dynamic recrystallization is assumed to be the primary grain refinement mechanism in the thermomechanically affected zone. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/PVP201545064
  • 2015 • 129 LES of the Sydney piloted spray flame series with the PFGM/ATF approach and different sub-filter models
    Rittler, A. and Proch, F. and Kempf, A.M.
    Combustion and Flame 162 1575-1598 (2015)
    Detailed numerical investigations of the Sydney spray flame series [1] are presented for ethanol flames referred to as "EtF3, EtF6 and EtF8", which feature identical ethanol mass flow rates but different carrier gas mass flow rates. Large eddy simulations (LES) are performed, where the gaseous and liquid phases are modeled by an Eulerian/Lagrangian approach. The turbulent sub-filter stresses (sgs) are modeled with Nicoud's sigma model [2] on grids with two different resolutions. Combustion is modeled with the premixed flamelet generated manifold approach (PFGM), which is combined with the artificially thickened flame (ATF) method. The sub-filter distributions of the control variables are modeled with (a) a β function (β-fdf) and (b) a top-hat function (TH). First, the influence of the variance in the mixture fraction and reaction progress variable is investigated separately, where the variances are either determined from an algebraic model or a transport equation model. Subsequently, the TH model is used to account for the joint impact of Z and Yp. The results are compared against the experimental measurements and reference simulations without sub-filter model. The particle statistics are in good agreement with the experimental data. The variances predicted by the two algebraic models are quite similar, whereas the transport equation model predicts variances which are one order of magnitude higher. The results obtained with the TH and the β-fdf model are comparable. It is found that the impact of the sgs models for the mixture fraction and the progress variable increases with an increasing carrier gas mass flow rate. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.11.025
  • 2015 • 128 Molecular motions in supercooled and glassy ibuprofen: Deuteron magnetic resonance and high-resolution rheology study
    Bauer, S. and Storek, M. and Gainaru, C. and Zimmermann, H.b and Böhmer, R.
    Journal of Physical Chemistry B 119 5087-5095 (2015)
    Using deuteron nuclear magnetic resonance, the molecular motions of specifically isotope-labeled ibuprofen were probed at the carboxylic group and at the methin group next to it. Spin relaxometry revealed slight differences between the molecular motions of the two isotopomers that are rationalized with reference to the hydrogen bonding of the COOH moiety. In the glassy state, a small-angle jump process among about four sites, related to the so-called γ-process, was identified using stimulated-echo spectroscopy. Indications for a Debye-like process, previously found to leave a weak signature in the dielectric loss, could not unambiguously be detected in magnetic resonance or shear mechanical experiments carried out for supercooled liquid ibuprofen. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b01072
  • 2015 • 127 NOx conversion properties of a novel material: Iron nanoparticles stabilized in carbon
    Busch, M. and Kompch, A. and Suleiman, S. and Notthoff, C. and Bergmann, U. and Theissmann, R. and Atakan, B. and Winterer, M.
    Applied Catalysis B: Environmental 166-167 211-216 (2015)
    Nitrogen oxides (NOx) belong to the most common pollutants from combustion processes and are a major threat to human health. Carbon-based catalysts exhibit strong advantages for NOx removal like low-toxic application and easy handling. However, gasification of the carbon matrix at elevated temperatures is still one of the greatest concerns. Hence, we have directed our focus on especially low temperature NOx-removal using a novel material, iron nanoparticles stabilized in a carbon matrix (nano-Fe/C). The investigations included NO2 uptake properties and catalytic conversion of NO2 in recycle flow at 425K and 328K, scanning transmission electron microscopy and 77K-N2-adsorption. Nano-Fe/C exhibits superior NOx-removal properties compared with untreated or iron-infiltrated activated carbon or magnetite reference catalysts. No severe catalyst deactivation or catalyst aging at 425K is observed. Even at 328K nano-Fe/C still exhibits NO2-conversion, although without converting the product NO. NO2 adsorption at 297K is suggested to occur in three stages with different kinetics: (1) NO2 adsorption and reduction to NO, (2) physisorption on the oxidized catalyst surface and (3) saturation of the catalyst and diffusion into the substrate matrix. At 425K, NO2 is quickly reduced to NO and the resulting NO is further converted to N2O. After complete consumption of NO, the residual NO2 is also converted to N2O. A possible reaction mechanism is suggested based on the conversion kinetics. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2014.11.013
  • 2015 • 126 Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys
    Ma, D. and Yao, M. and Pradeep, K.G. and Tasan, C.C. and Springer, H. and Raabe, D.
    Acta Materialia 98 288-296 (2015)
    Abstract The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic Fe<inf>x</inf>Mn<inf>62-x</inf>Ni<inf>30</inf>Co<inf>6</inf>Cr<inf>2</inf> based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.030
  • 2015 • 125 Photoluminescence of two-dimensional GaTe and GaSe films
    Del Pozo-Zamudio, O. and Schwarz, S. and Sich, M. and Akimov, I.A. and Bayer, M. and Schofield, R.C. and Chekhovich, E.A. and Robinson, B.J. and Kay, N.D. and Kolosov, O.V. and Dmitriev, A.I. and Lashkarev, G.V. and Borisenko, D.N...
    2D Materials 2 (2015)
    Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We reportonthe low-temperature micro-photoluminescence (PL) of GaTe and GaSefilms with thicknesses ranging from 200 nm toasingle unit cell.Inboth materials,PL shows adramatic decrease by 104-105 when film thicknessis reduced from 200to10 nm. Basedon evidence from continuouswave (cw) and time-resolved PL, wepropose amodel explaining the PLdecrease as a result of nonradiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1583/2/3/035010
  • 2015 • 124 Poly(N,N-dimethylaminoethyl methacrylate) Brushes: PH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte
    Thomas, M. and Gajda, M. and Amiri Naini, C. and Franzka, S. and Ulbricht, M. and Hartmann, N.
    Langmuir 31 13426-13432 (2015)
    The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b03448
  • 2015 • 123 Radio frequency microelectromechanical system-platform based on silicon-ceramic composite substrates
    Fischer, M. and Gropp, S. and Nowak, J. and Capraro, B. and Sommer, R. and Hoffmann, M. and Mülle, J.
    Journal of Microelectronics and Electronic Packaging 12 37-42 (2015)
    In the last few years, several low-temperature coefficient of expansion of low temperature cofired ceramic (LTCC) materials have been developed for direct wafer bonding to silicon. BGK, a sodium-containing LTCC, was originally developed for anodic bonding of the sintered LTCC, whereas BCT (bondable ceramic tape) was tailored for direct silicon bonding of green LTCC tapes to fabricate a quasi-monolithic, silicon ceramic compound substrate. This so-called silicon-on-ceramic (SiCer) technique is based on homogeneous nanostructuring of a silicon substrate, a lamination step of BCT and Si, and a subsequent pressure-assisted sintering. We present a new approach for an integrated radio frequency (RF)-platform setup combining passive, active, and mechanical elements on one SiCer substrate. In this context, RF parameters of the Si-adapted LTCC tapes and the use of commercial metal pastes on BCT with respect to bondability and solderability are investigated. We show first technological results of creating cavities at the SiCer interface for SiCer-specific contacting options (e.g., exposed contact pads at the interface), as well as windows in the ceramic layer of the SiCer substrate for additional Si processing (e.g., Si backside thin-film wiring, plasma etching). A further investigated platform technology is deep reactive-ion etching of the SiCer composite substrate. The etching behavior of Si and BCT is demonstrated and discussed. With the SiCer technique, it is possible to reduce the Si content at the setup of RF microelectromechanical system to a minimum (low signal damping). © 2015 International Microelectronics Assembly and Packaging Society.
    view abstractdoi: 10.4071/imaps.442
  • 2015 • 122 Redox-stable high-performance thin-film solid oxide fuel cell
    Keuter, T. and Roehrens, D. and Menzler, N.H. and Vaßen, R.
    ECS Transactions 68 2001-2009 (2015)
    In this work, a mechanically redox-stable SOFC with a 1 μm thin-film sol-gel electrolyte is presented. With this electrolyte a power output larger than 1.25 W/cm2 at 0.7 V and an operating temperature of 600°C could be demonstrated. Half cells were re-oxidized in excess air, in order to test the redox stability of these SOFCs. No cracks were found in the sol-gel electrolyte after re-oxidation for 4 hours at 600°C and 30 minutes at 800°C, respectively. Due to the fact, that the energy release rate is proportional to the thickness of the thin-film, a thinner film is more stable against cracking than a thicker film at constant tensile stresses. The SOFC with the thin-film sol-gel electrolyte can be considered as stable against re-oxidation, because the long re-oxidation time of 4 hours at an operating temperature of 600°C is unlikely to happen under real conditions. © The Electrochemical Society.
    view abstractdoi: 10.1149/06801.2001ecst
  • 2015 • 121 Reverse engineering of fluid selection for thermodynamic cycles with cubic equations of state, using a compression heat pump as example
    Roskosch, D. and Atakan, B.
    Energy 81 202--212 (2015)
    Fluid selection for thermodynamic cycles like refrigeration cycles, heat pumps or organic Rankine cycles remains an actual topic. Generally the search for a working fluid is based on experimental approaches or on a not very systematic trial and error approach, far from being elegant. An alternative method may be a theory based reverse engineering approach, proposed and investigated here: The design process should start with an optimal process and with (abstract) properties of the fluid needed to fit into this optimal process, best described by some general equation of state and the corresponding fluid-describing parameters. These should be analyzed and optimized with respect to the defined model process, which also has to be optimized simultaneously. From this information real fluids can be selected or even synthesized which have fluid defining properties in the optimum regime like critical temperature or ideal gas capacities of heat, allowing to find new worldng fluids, not considered so far. The number and kind of the fluid-defining parameters is mainly based on the choice of the used EOS (equation of state). The property model used in the present work is based on the cubic Peng-Robinson equation, chosen due to its moderate numerical expense, sufficient accuracy as well as a general availability of the fluid-defining parameters for many compounds. The considered model-process works between the temperature levels of 273.15 and 333.15 K and can be used as heat pump for supplying buildings with heat, typically. The objective functions are the COP (coefficient of performance) and the VHC (volumetric heating capacity) as a function of critical pressure, critical temperature, acentric factor and two coefficients for the temperature-dependent isobaric ideal gas heat capacity. Also, the steam quality at the compressor entrance has to be regarded as a problem variable. The results give clear hints regarding optimal fluid parameters of the analyzed process and deepen the thermodynamic understanding of the process. Finally, for the COP optimization a strategy for screening large databases is explained. Several fluids from different substance groups like hydrogen iodide (COP = 3.68), formaldehyde (3.61) or cyclopropane (3.42) were found to have higher COPs than the often used R134a (3.12). These fluids will also have to fulfill further criteria, prior to their usage, but the method appears to be a good base for fluid selection. (C) 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/
  • 2015 • 120 Spheronization of solid lipid extrudates: A novel approach on controlling critical process parameters
    Petrovick, G.F. and Pein, M. and Thommes, M. and Breitkreutz, J.
    European Journal of Pharmaceutics and Biopharmaceutics 92 15-21 (2015)
    Solid lipids are non-toxic excipients, which are known to potentially enhance delivery and bioavailability of poorly water-soluble drugs and moreover to mask unpleasant tasting drugs. Multiple unit matrix dosage forms based on solid lipids, such as lipid pellets, can be obtained by solvent-free cold extrusion and spheronization. This method presents advantages in the processing of sensitive substances, such as low process temperatures, the absence of solvents and a drying step. However, the material temperature during the spheronization showed to be critical so far. The process leads to increased material temperatures, causing particle agglomeration and discontinuity of the spheronization. In the present study, extrudates of 0.5 mm in diameter containing metformin hydrochloride, and either semisynthetic hard fat (Witocan® 42/44) or different ternary mixtures based on hard fat, glyceryl trimyristate, and glyceryl distearate, were spheronized. By applying common process parameters, particle agglomeration or material stickiness on equipment walls was observed in preliminary experiments after 2-6 min, depending on the lipid composition. Therefore, an innovative instrumental setup to control the spheronization process was developed utilizing an infrared light source, which was positioned over the particle bed. The new approach enabled a spheronization process that reached the desired spheronization temperature after 2-3 min and neither particle agglomeration nor material adherence occurred even after longer process times. The different formulations, even those based on high amount of solid lipids, were successfully spheronized over 15 min, resulting in small diameter lipid pellets with smooth surface and aspect ratios below 1.3. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2015.02.004
  • 2015 • 119 Synthesis, morphology and structure of the dense (Y1-xEux)2O3 spherical shape particles
    Bezkrovnyi, O.S. and Matveevskaya, N.A. and Yermolayeva, Y.V. and Tolmachev, A.V. and Prymak, O. and Epple, M. and Baumer, V.N.
    Crystal Research and Technology 50 621-625 (2015)
    The method to decrease of the porosity (densification) of crystalline spherical particles of the solid substitution solution, obtained by the method of precipitation from aqueous solution followed by low temperature crystallization of the amorphous intermediate product was proposed. The comparative analysis of morphology and structure of the particles before and after densification have been carried. It has been established that porosity of (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>O<inf>3</inf> particles has decreased 5 times compared to their initial state. It has been shown that densification process of the (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>O<inf>3</inf> spherical particles changes their morphology and structure: the size of the crystals doubles, the number and area of crystalline boundaries decrease, the intercrystalline spaces, which forming pores, are almost absent. The dense (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>O<inf>3</inf>(x=0-0,1) isolated spherical particles of 120-300 nm in diameter and dispersion less than 15 % by size, with the porosity 5 times lower, compared to the initial spherical particles of (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>O<inf>3</inf>, were obtained for the first time. It was established that in the process of the densification of porous spherical particles their morphology and structure state are improved: intercrystalline spaces are almost absent, the size of the crystals doubles. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/crat.201500026
  • 2015 • 118 The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe-Al alloys
    Brito, P. and Pinto, H. and Kostka, A.
    Corrosion Science (2015)
    The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe-Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2016.01.007
  • 2015 • 117 The glass transition in high-density amorphous ice
    Loerting, T. and Fuentes-Landete, V. and Handle, P.H. and Seidl, M. and Amann-Winkel, K. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 423-430 (2015)
    There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136 K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136 K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at > 0.2 GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tinfg/inf of 160 K at 0.40 GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122 K at 1.0 GPa. Based on the pressure dependence of HDA's Tinfg/inf measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116 K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20 K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p-T plane for LDA, HDA, and VHDA. © 2015 Published by Elsevier Inc.
    view abstractdoi: 10.1016/j.jnoncrysol.2014.09.003
  • 2015 • 116 The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 193-197 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2015.03.003
  • 2015 • 115 Wet-chemical synthesis of different bismuth telluride nanoparticles using metal organic precursors-single source vs. dual source approach
    Bendt, G. and Weber, A. and Heimann, S. and Assenmacher, W. and Prymak, O. and Schulz, S.
    Dalton Transactions 44 14272-14280 (2015)
    Thermolysis of the single source precursor (Et<inf>2</inf>Bi)<inf>2</inf>Te 1 in DIPB at 80 °C yielded phase-pure Bi<inf>4</inf>Te<inf>3</inf> nanoparticles, while mixtures of Bi<inf>4</inf>Te<inf>3</inf> and elemental Bi were formed at higher temperatures. In contrast, cubic Bi<inf>2</inf>Te particles were obtained by thermal decomposition of Et<inf>2</inf>BiTeEt 2 in DIPB. Moreover, a dual source approach (hot injection method) using the reaction of Te(SiEt<inf>3</inf>)<inf>2</inf> and Bi(NMe<inf>2</inf>)<inf>3</inf> was applied for the synthesis of different pure Bi-Te phases including Bi<inf>2</inf>Te, Bi<inf>4</inf>Te<inf>3</inf> and Bi<inf>2</inf>Te<inf>3</inf>, which were characterized by PXRD, REM, TEM and EDX. The influence of reaction temperature, precursor molar ratio and thermolysis conditions on the resulting material phase was verified. Moreover, reactions of alternate bismuth precursors such as Bi(NEt<inf>2</inf>)<inf>3</inf>, Bi(NMeEt)<inf>3</inf> and BiCl<inf>3</inf> with Te(SiEt<inf>3</inf>)<inf>2</inf> were investigated. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5dt02072g
  • 2014 • 114 A thermodynamic investigation of the glucose-6-phosphate isomerization
    Hoffmann, P. and Held, C. and Maskow, T. and Sadowski, G.
    Biophysical Chemistry 195 22-31 (2014)
    In this work, ΔRg+ values for the enzymatic G6P isomerization were determined as a function of the G6P equilibrium molality between 25 °C and 37 °C. The reaction mixtures were buffered at pH = 8.5. In contrast to standard literature work, ΔRg+ values were determined from activity-based equilibrium constants instead of molality-based apparent values. This yielded a ΔRg+ value of 2.55 ± 0.05 kJ mol- 1 at 37 °C, independent of the solution pH between 7.5 and 8.5. Furthermore, ΔRh + was measured at pH = 8.5 and 25 °C yielding 12.05 ± 0.2 kJ mol- 1. Accounting for activity coefficients turned out to influence ΔRg+ up to 30% upon increasing the G6P molality. This result was confirmed by predictions using the thermodynamic model ePC-SAFT. Finally, the influence of the buffer and of potassium glutamate as an additive on the reaction equilibrium was measured and predicted with ePC-SAFT in good agreement. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2014.08.002
  • 2014 • 113 Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
    Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 35 695-700 (2014)
    The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.
    view abstractdoi: 10.1007/s11669-014-0342-6
  • 2014 • 112 Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles
    Cui, C. and Gan, L. and Neumann, M. and Heggen, M. and Roldan Cuenya, B. and Strasser, P.
    Journal of the American Chemical Society 136 4813-4816 (2014)
    Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja4124658
  • 2014 • 111 Depinning of stiff directed lines in random media
    Boltz, H.-H. and Kierfeld, J.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    Driven elastic manifolds in random media exhibit a depinning transition to a state with nonvanishing velocity at a critical driving force. We study the depinning of stiff directed lines, which are governed by a bending rigidity rather than line tension. Their equation of motion is the (quenched) Herring-Mullins equation, which also describes surface growth governed by surface diffusion. Stiff directed lines are particularly interesting as there is a localization transition in the static problem at a finite temperature and the commonly exploited time ordering of states by means of Middleton's theorems [Phys. Rev. Lett. 68, 670 (1992)PRLTAO0031-900710.1103/PhysRevLett.68.670] is not applicable. We employ analytical arguments and numerical simulations to determine the critical exponents and compare our findings with previous works and functional renormalization group results, which we extend to the different line elasticity. We see evidence for two distinct correlation length exponents. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.90.012101
  • 2014 • 110 Enhancement of low-temperature activity over Cu-exchanged zeolite beta from organotemplate-free synthesis for the selective catalytic reduction of NOx with NH3 in exhaust gas streams
    Xu, L. and Shi, C. and Zhang, Z. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Feyen, M. and Maurer, S. and Yilmaz, B. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 200 304-310 (2014)
    A series of Cu-exchanged Al-rich Beta zeolites from organotemplate-free synthesis was prepared and investigated for selective catalytic reduction (SCR) of NO<inf>x</inf> with NH<inf>3</inf> in exhaust gas streams. In comparison to conventional Cu-Beta zeolite with Si/Al ratio of 19, Cu-Beta zeolite with Si/Al ratio of 4 is a superior low-temperature NH<inf>3</inf>-SCR catalyst. Very high NO conversion (>95%) can be achieved at temperatures as low as 150 to ∼400 °C. XRD, UV-Vis-NIR and NH<inf>3</inf>-TPD measurements show that more isolated Cu2+ ions are present at the exchange sites of Al-rich Beta zeolite. The combination of CO-FTIR and H<inf>2</inf>-TPR analysis demonstrates that Cu2+ ions could be reduced more readily on the Al-rich Beta than on the conventional Beta probably due to the proximity of the isolated Cu2+ ions. These can be correlated to the enhancement of NO conversion at lower temperatures over Cu-exchanged Al-rich Beta zeolite. © 2014 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2014.04.034
  • 2014 • 109 Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation
    Ochedowski, O. and Osmani, O. and Schade, M. and Bussmann, B.K. and Ban-Detat, B. and Lebius, H. and Schleberger, M.
    Nature Communications 5 (2014)
    The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E kin ‰ 500 ‰keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.
    view abstractdoi: 10.1038/ncomms4913
  • 2014 • 108 Increased productivity in hot aluminum extrusion by using extrusion dies with inner cooling channels manufactured by rapid tooling
    Hölker, R. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 611-612 981-988 (2014)
    The influence of local inner die cooling on the heat balance in hot aluminum extrusion was investigated. For the manufacturing of the die with cooling channels close to the forming zone, the layer-laminated manufacturing method was applied. The new tooling technology was applied in order to decrease the profile's exit temperature and to avoid thermally induced surface defects with the aim to raise the productivity in hot aluminum extrusion processes. Numerical and experimental investigations revealed that, while maintaining the exit temperature of the extrudate, a distinct increase of the production speed up to 300 % can be realized, while the extrusion force increases only slightly. An effect on the profiles microstructure was also detected. By applying die cooling, grain coarsening can be significantly limited or even be avoided. © 2014 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 107 Influence of molecular hydrogen on acetylene pyrolysis: Experiment and modeling
    Aghsaee, M. and Dürrstein, S.H. and Herzler, J. and Böhm, H. and Fikri, M. and Schulz, C.
    Combustion and Flame 161 2263-2269 (2014)
    The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration-time profiles for various species with a time resolution of 10μs. Concentration histories of reactants and polyacetylene intermediates (C2xH2, x=1-4) are measured during the pyrolysis of acetylene with and without H2 added to the gas mixture for a wide range of conditions. In the 1760-2565K temperature range, reasonable agreement between the experiment and the model predictions for C2H2, C4H2, C6H2, and C8H2 is achieved. H2 addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H2, which are required for the fast build-up of carbonaceous material. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.03.012
  • 2014 • 106 Influence of solution nitriding of supersolidus-sintered cold work tool steels on their hardenability
    Blüm, M. and Conrads, J. and Weber, S. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 69 273-281 (2014)
    Powder metallurgical steel grades offer higher quality and performance compared to cast and forged steel grades due to their finer microstructure without segregations or textures. Because high-alloyed, cold work tool steels cannot be compacted by solid-state sintering, hot isostatic pressing is state of the art. This process, however, is comparatively expensive and there is thus a high demand for alternative densification processes. Supersolidus liquid-phase sintering represents an alternative to hot isostatic pressing for densification of these steels to theoretical density. During sintering, the steel powder interacts with the sintering atmosphere, which can be a vacuum, hydrogen, hydrogen plus nitrogen, or nitrogen. In a nitrogen atmosphere, there may be nitrogen uptake by the sintered material, which changes the chemical composition of the steel and thus results in a decrease in the sintering temperature. The aim of this work is to investigate the influence of nitrogen uptake on the hardenability of a high-alloyed and supersolidus-sintered cold work tool steel. Computational thermodynamics using the Calphad method were applied to calculate the optimal parameters for direct quenching from the sintering temperature. In addition, the tempering response was investigated as a function of the heat-treatment parameters. It was found that nitriding exerts a significant influence on the hardenability, which is also dependent on the cooling rate from the sintering temperature to the quenching temperature. Hardenability was predicted qualitatively on the basis of equilibrium carbon concentrations of the austenitic matrix calculated with the Calphad method. © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/105.110234
  • 2014 • 105 Influence of substrate and its temperature on the optical constants of CuIn1-xGaxSe2 thin films
    Yin, G. and Manley, P. and Schmid, M.
    Journal of Physics D: Applied Physics 47 (2014)
    We investigate the influence of substrate and its temperature on the optical constants of CuIn1-xGaxSe2 (CIGSe) thin films using the transfer-matrix method. The optical constants of a CIGSe layer on top of a transparent conducting oxide (TCO) layer were calculated considering the realistic optical constants of the TCO layer after CIGSe deposition. It was found that TCO substrates could influence the optical constants of CIGSe layers and that the ITO (Sn doped In2O3) substrate had a greater impact than IMO (Mo doped In2O3) for the CIGSe (x = 0.4) film when compared to a reference on bare glass substrate. Additionally, the varied substrate temperatures did not impact the optical constants of CGSe (x = 1). For CIGSe (x = 0.4), the refractive index n stayed relatively independent although at low temperature the grain size was reduced and the Ga/(Ga+In) profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient k at low temperature showed higher absorption at longer wavelengths because of a lower minimum bandgap (E g,min) originating from reduced inter-diffusion of Ga-Se at a low substrate temperature. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/47/13/135101
  • 2014 • 104 Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries
    Wen, Y.H. and Peng, H.B. and Raabe, D. and Gutierrez-Urrutia, I. and Chen, J. and Du, Y.Y.
    Nature Communications 5 (2014)
    Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (<3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5964
  • 2014 • 103 Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 3245-3249 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201308206
  • 2014 • 102 Manipulation of the nuclear spin ensemble in a quantum dot with chirped magnetic resonance pulses
    Munsch, M. and Wüst, G. and Kuhlmann, A.V. and Xue, F. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Poggio, M. and Warburton, R.J.
    Nature Nanotechnology 9 671-675 (2014)
    The nuclear spins in nanostructured semiconductors play a central role in quantum applications. The nuclear spins represent a useful resource for generating local magnetic fields but nuclear spin noise represents a major source of dephasing for spin qubits. Controlling the nuclear spins enhances the resource while suppressing the noise. NMR techniques are challenging: the group III and V isotopes have large spins with widely different gyromagnetic ratios; in strained material there are large atom-dependent quadrupole shifts; and nanoscale NMR is hard to detect. We report NMR on 100,000 nuclear spins of a quantum dot using chirped radiofrequency pulses. Following polarization, we demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back and forth a hundred times. We demonstrate that chirped NMR is a powerful way of determining the chemical composition, the initial nuclear spin temperatures and quadrupole frequency distributions for all the main isotopes. The key observation is a plateau in the NMR signal as a function of sweep rate: we achieve inversion at the first quantum transition for all isotopes simultaneously. These experiments represent a generic technique for manipulating nanoscale inhomogeneous nuclear spin ensembles and open the way to probe the coherence of such mesoscopic systems. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nnano.2014.175
  • 2014 • 101 Nano-scale morphology of melanosomes revealed by small-angle X-ray scattering
    Gorniak, T. and Haraszti, T. and Garamus, V.M. and Buck, A.R. and Senkbeil, T. and Priebe, M. and Hedberg-Buenz, A. and Koehn, D. and Salditt, T. and Grunze, M. and Anderson, M.G. and Rosenhahn, A.
    PLoS ONE 9 (2014)
    Melanosomes are highly specialized organelles that produce and store the pigment melanin, thereby fulfilling essential functions within their host organism. Besides having obvious cosmetic consequences - determining the color of skin, hair and the iris - they contribute to photochemical protection from ultraviolet radiation, as well as to vision (by defining how much light enters the eye). Though melanosomes can be beneficial for health, abnormalities in their structure can lead to adverse effects. Knowledge of their ultrastructure will be crucial to gaining insight into the mechanisms that ultimately lead to melanosome-related diseases. However, due to their small size and electron-dense content, physiologically intact melanosomes are recalcitrant to study by common imaging techniques such as light and transmission electron microscopy. In contrast, X-ray-based methodologies offer both high spatial resolution and powerful penetrating capabilities, and thus are well suited to study the ultrastructure of electron-dense organelles in their natural, hydrated form. Here, we report on the application of small-angle X-ray scattering - a method effective in determining the three-dimensional structures of biomolecules - to whole, hydrated murine melanosomes. The use of complementary information from the scattering signal of a large ensemble of suspended organelles and from single, vitrified specimens revealed a melanosomal sub-structure whose surface and bulk properties differ in two commonly used inbred strains of laboratory mice. Whereas melanosomes in C57BL/6J mice have a well-defined surface and are densely packed with 40-nm units, their counterparts in DBA/2J mice feature a rough surface, are more granular and consist of 60-nm building blocks. The fact that these strains have different coat colors and distinct susceptibilities to pigment-related eye disease suggest that these differences in size and packing are of biological significance.
    view abstractdoi: 10.1371/journal.pone.0090884
  • 2014 • 100 One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Jensen, A.D. and Grunwaldt, J.-D.
    Chemical Communications 50 15404-15406 (2014)
    Flame spray pyrolysis (FSP) of Bi(iii)- and Mo(vi)-2-ethylhexanoate dissolved in xylene resulted in various nanocrystalline bismuth molybdate phases depending on the Bi/Mo ratio. Besides α-Bi2Mo3O12 and γ-Bi2MoO6, FSP gave direct access to the metastable β-Bi2Mo2O9 phase with high surface area (19 m2 g-1). This phase is normally only obtained at high calcination temperatures (&gt;560 °C) resulting in lower surface areas. The β-phase was stable up to 400 °C and showed superior catalytic performance compared to α- and γ-phases in selective oxidation of propylene to acrolein at temperatures relevant for industrial applications (360 °C). This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4cc07527g
  • 2014 • 99 Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2330845
  • 2014 • 98 Photo-enhanced antinodal conductivity in the pseudogap state of high-T c cuprates
    Cilento, F. and Dal Conte, S. and Coslovich, G. and Peli, S. and Nembrini, N. and Mor, S. and Banfi, F. and Ferrini, G. and Eisaki, H. and Chan, M.K. and Dorow, C.J. and Veit, M.J. and Greven, M. and Van Der Marel, D. and Comin, R...
    Nature Communications 5 (2014)
    A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe, we are able to track the dynamics of the dielectric function and unveil an anomalous decrease in the scattering rate of the charge carriers in a pseudogap-like region of the temperature (T) and hole-doping (p) phase diagram. In this region, delimited by a well-defined T* neq (p) line, the photoexcitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasiparticles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the single-band Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between nodal quasiparticles and antinodal excitations. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5353
  • 2014 • 97 Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization
    Schröter, A. and Franzka, S. and Hartmann, N.
    Langmuir 30 14841-14848 (2014)
    Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e2 spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates. (Graph Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la503814n
  • 2014 • 96 Quantum size effects in chemicurrent measurements during low-temperature oxidation of Mg(0001) epilayers
    Hagemann, U. and Nienhaus, H.
    New Journal of Physics 16 (2014)
    The reactivity of Mg epilayers on Si(111)-7 × 7 towards molecular oxygen is investigated as a function of the metal film thickness in the range between 7 and 45 monolayers. Quantum well and surface states are characterized with ultra-violet photoelectron spectroscopy demonstrating the epitaxial and single-crystalline structure of the Mg films. The oxidation rate is monitored during the reaction by measuring chemicurrents at 110K in the Mg/p-Si(111) Schottky diodes due to the non-adiabatic character of at least one step in the reaction chain. For film thicknesses around 9 and 13 monolayers the chemicurrent transients demonstrate that the reaction rate is strongly enhanced by a factor of more than two. With Mg 2p core level spectroscopy, a similar enhancement can be found for the total oxygen uptake for long exposures indicating that the chemicurrent increase measures solely a quantum size effect on the reactivity and no device-related effects. The enhanced reactivity can be explained by the increased first charge transfer into the affinity level of the approaching molecule when a quantum well state appears at the Fermi level and increases the density of electronic states. A linear relationship between the photoelectron intensity at the Fermi level and the maximum chemicurrent is clearly observed. On the other hand, the surface work function and the Schottky barrier height exhibit almost no correlation with the enhanced reactivity. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/11/113035
  • 2014 • 95 Reliability of CMOS on silicon-on-insulator for use at 250°C
    Grella, K. and Dreiner, S. and Vogt, H. and Paschen, U.
    IEEE Transactions on Device and Materials Reliability 14 21-29 (2014)
    This paper deals with the reliability of a 1.0-μ CMOS-silicon-on- insulator (SOI) process, which is intended for use at 250 °C. The goal is to give an overview of the most important reliability aspects that concern devices and circuits at temperatures of 250 °C and above. The investigated reliability aspects are the gate oxide integrity in terms of time-dependent dielectric breakdown measurements, electro- and stress migration, and the EEPROM reliability such as the data retention and the endurance, as well as transistor aspects (e.g., hot carrier, negative bias temperature instability) and the long-term stability of a ring oscillator and a band-gap reference. As most of the commonly applied methods for accelerated reliability testing and analysis are not designed to be used at such high temperatures, this paper evaluates in which way the known models can be applied and which physical mechanisms have to be considered. Since temperatures of 250°C and more are necessary for testing, the investigations also yield an estimate of the temperature limit of use for CMOS on SOI. The results indicate that the use of CMOS on SOI is, in principle, possible up to 400°C. © 2013 IEEE.
    view abstractdoi: 10.1109/TDMR.2013.2284665
  • 2014 • 94 Strain response of thermal barrier coatings captured under extreme engine environments through synchrotron X-ray diffraction
    Knipe, K. and Manero II, A. and Siddiqui, S.F. and Meid, C. and Wischek, J. and Okasinski, J. and Almer, J. and Karlsson, A.M. and Bartsch, M. and Raghavan, S.
    Nature Communications 5 (2014)
    The mechanical behaviour of thermal barrier coatings in operation holds the key to understanding durability of jet engine turbine blades. Here we report the results from experiments that monitor strains in the layers of a coating subjected to thermal gradients and mechanical loads representing extreme engine environments. Hollow cylindrical specimens, with electron beam physical vapour deposited coatings, were tested with internal cooling and external heating under various controlled conditions. High-energy synchrotron X-ray measurements captured the in situ strain response through the depth of each layer, revealing the link between these conditions and the evolution of local strains. Results of this study demonstrate that variations in these conditions create corresponding trends in depth-resolved strains with the largest effects displayed at or near the interface with the bond coat. With larger temperature drops across the coating, significant strain gradients are seen, which can contribute to failure modes occurring within the layer adjacent to the interface. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5559
  • 2014 • 93 Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures
    Wu, Z. and Bei, H. and Pharr, G.M. and George, E.P.
    Acta Materialia 81 428-441 (2014)
    Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of "solvent" and "solute" atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary and quaternary alloys based on the elements Fe, Ni, Co, Cr and Mn that were previously shown to be single-phase face-centered cubic (fcc) solid solutions. The alloys were arc-melted, drop-cast, homogenized, cold-rolled and recrystallized to produce equiaxed microstructures with comparable grain sizes. Tensile tests were performed at an engineering strain rate of 10-3 s-1 at temperatures in the range 77-673 K. Unalloyed fcc Ni was processed similarly and tested for comparison. The flow stresses depend to varying degrees on temperature, with some (e.g. NiCoCr, NiCoCrMn and FeNiCoCr) exhibiting yield and ultimate strengths that increase strongly with decreasing temperature, while others (e.g. NiCo and Ni) exhibit very weak temperature dependencies. To better understand this behavior, the temperature dependencies of the yield strength and strain hardening were analyzed separately. Lattice friction appears to be the predominant component of the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to a thermally induced increase of dislocation width. In the early stages of plastic flow (5-13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the temperature dependence of the shear modulus. In all the equiatomic alloys, ductility and strength increase with decreasing temperature down to 77 K. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.08.026
  • 2014 • 92 Temperature-Dependent Electron Shuffle in Molecular Group 13/15 Intermetallic Complexes
    Ganesamoorthy, C. and Bläser, D. and Wölper, C. and Schulz, S.
    Angewandte Chemie - International Edition 53 11587-11591 (2014)
    Monovalent RAl (R=HC[C(Me)N(2,6-iPr2C6H3)]2) reacts with E2Et4 (E=Sb, Bi) with insertion into the weak E-E bond and subsequent formation of RAl(EEt2)2 (E=Sb 1; Bi 2). The analogous reactions of RGa with E2Et4 yield a temperature-dependent equilibrium between RGa(EEt2)2 (E=Sb 3; Bi 4) and the starting reagents. RIn does not interact with Sb2Et4 under various reaction conditions, but formation of RIn(BiEt2)2 (5) was observed in the reaction with Bi2Et4 at low temperature. Doing the shuffle: Reactions of monovalent RAl with E2Et4 (E=Sb, Bi; Ar=2,6-iPr2C6H3) proceed with E-E bond cleavage and formation of RAl(EEt2)2, whereas RGa forms a reversible chemical equilibrium with E2Et4 and RGa(EEt2)2. RIn does not react with Sb2Et4, but also forms a reversible equilibrium with Bi2Et4 at low temperatures. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201406304
  • 2014 • 91 Thermodynamic phase behavior of API/polymer solid dispersions
    Prudic, A. and Ji, Y. and Sadowski, G.
    Molecular Pharmaceutics 11 2294-2304 (2014)
    To improve the bioavailability of poorly soluble active pharmaceutical ingredients (APIs), these materials are often integrated into a polymer matrix that acts as a carrier. The resulting mixture is called a solid dispersion. In this work, the phase behaviors of solid dispersions were investigated as a function of the API as well as of the type and molecular weight of the carrier polymer. Specifically, the solubility of artemisinin and indomethacin was measured in different poly(ethylene glycol)s (PEG 400, PEG 6000, and PEG 35000). The measured solubility data and the solubility of sulfonamides in poly(vinylpyrrolidone) (PVP) K10 and PEG 35000 were modeled using the perturbed-chain statistical associating fluid theory (PC-SAFT). The results show that PC-SAFT predictions are in a good accordance with the experimental data, and PC-SAFT can be used to predict the whole phase diagram of an API/polymer solid dispersion as a function of the kind of API and polymer and of the polymers molecular weight. This remarkably simplifies the screening process for suitable API/polymer combinations. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/mp400729x
  • 2014 • 90 Urease-induced calcification of segmented polymer hydrogels - A step towards artificial biomineralization
    Rauner, N. and Meuris, M. and Dech, S. and Godde, J. and Tiller, J.C.
    Acta Biomaterialia 10 3942-3951 (2014)
    Natural organic/inorganic composites, such as nacre, bones and teeth, are perfectly designed materials with exceptional mechanical properties. Numerous approaches have been taken to synthetically prepare such composites. The presented work describes a new way of mineralizing bulk materials on a large scale following the approach of bioinduced mineralization. To this end, a series of polymer conetworks with entrapped urease were prepared. After polymerization, the entrapped urease shows high enzymatic activity. The bioactive polymer conetworks were then treated with an aqueous mixture of urea and CaCl2. The urease-induced calcification indeed allows formation of carbonate crystals exclusively within the hydrogel even at room temperature. The influence of network composition, degree of cross-linking, immobilized urease concentration and temperature of calcification were investigated. By varying these parameters, spherical, monolithic clusters, as well as bar-like nanocrystals with different aspect ratios in spherical or dendritic arrays, are formed. The grown nanocrystals improve the stiffness of the starting material by up to 700-fold, provided that the microstructure shows a dense construction without pores and strong interaction between crystals and network. The process has the potential to generate a new class of hybrid materials that would be available on the macroscopic scale for use in lightweight design and medicine. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.05.021
  • 2014 • 89 Using the first steps of hydration for the determination of molecular conformation of a single molecule
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 136 13341-13347 (2014)
    Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4?-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja506762t
  • 2014 • 88 Validation and implementation of algebraic LES modelling of scalar dissipation rate for reaction rate closure in turbulent premixed combustion
    Ma, T. and Gao, Y. and Kempf, A.M. and Chakraborty, N.
    Combustion and Flame 161 3134-3153 (2014)
    The closure of the filtered reaction rate of the reaction progress variable using an algebraic model for Favre-filtered Scalar Dissipation Rate (SDR) N~c in turbulent premixed combustion has been assessed in the context of Large Eddy Simulations (LES). This assessment consists of a priori Direct Numerical Simulation (DNS) analysis based on freely propagating statistically planar turbulent premixed flames and a posteriori analysis, involving the LES simulations of a well-documented rectangular dump combustor configuration with sudden expansion (i.e. ORACLES burner) and a premixed flame stabilised on a triangular bluff body flame holder (i.e. Volvo Rig). It has been found that the newly developed SDR model satisfactorily captures N~c obtained from explicitly filtered DNS data. The predictions of this SDR based LES closure in the ORACLES burner and Volvo Rig configurations exhibit good agreement with experimental results without requiring any major modification to the model parameters. The predictions of the SDR model for the LES of the ORACLES burner and Volvo Rig have been compared to those of two algebraic Flame Surface Density (FSD) models, which yielded satisfactory agreement with experimental data in a previous analysis. The performance of the SDR based closure remains either comparable to or better than the FSD based closures for the two test configurations considered in this analysis. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.05.023
  • 2013 • 87 An elevated temperature study of a Ti adhesion layer on polyimide
    Taylor, A.A. and Cordill, M.J. and Bowles, L. and Schalko, J. and Dehm, G.
    Thin Solid Films 531 354-361 (2013)
    Titanium layers are used to promote adhesion between polymer substrates for flexible electronics and the Cu or Au conducting lines. Good adhesion of conducting lines in flexible circuits is critical in improving circuit performance and increasingcircuit lifetime. Nominally 50 nm thick Ti films on polyimide (PI) are investigated by fragmentation testing under uniaxial tensile load in the as-deposited state, at 350 C, and after annealing. The cracking and buckling of the films show clear differences between the as-deposited and the thermally treated samples, cracks are much straighter and buckles are smaller following heat treatment. These changes are correlated to a drop in adhesion of the samples following heat treatment. Adhesion values are determined from the buckle dimensions using a total energy approach as described in the work of Cordill et al. (Acta Mater. 2010). Cross-sectional transmission electron microscopy of the Ti/PI interface found evidence of a ~ 5 nm thick interlayer between the largely columnar Ti and the amorphous PI. This interlayer is amorphous in the as-deposited state but nano-crystalline in those coatings tested at elevated temperature or annealed. It is put forward that this alteration of the interfacial structure causes the reduced adhesion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.01.016
  • 2013 • 86 Atomic engineering of platinum alloy surfaces
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
    Ultramicroscopy 132 205-211 (2013)
    A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.012
  • 2013 • 85 Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Ultramicroscopy 132 233-238 (2013)
    A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.010
  • 2013 • 84 Communication: Substrate induced dehydrogenation: Transformation of octa-ethyl-porphyrin into tetra-benzo-porphyrin
    Van Vörden, D. and Lange, M. and Schmuck, M. and Schaffert, J. and Cottin, M.C. and Bobisch, C.A. and Möller, R.
    Journal of Chemical Physics 138 (2013)
    Individual molecules of octa-ethyl-porhphyrin-iron(III)-chloride adsorbed on a Cu(111) surface are studied by scanning tunneling microscopy. Upon moderate heating the molecules are found to transform into Fe-tetra-benzo-porphyrin at a surprisingly low temperature of 380 K. If the annealing is interrupted, the different steps of the transformation can be imaged. By evaluating the ratio of transformed molecules as function of annealing temperature, an approximate activation energy of 1.2 eV ± 0.1 eV could be determined. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4810879
  • 2013 • 83 Composition-dependent crystal structure and martensitic transformation in Heusler Ni-Mn-Sn alloys
    Zheng, H. and Wang, W. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Acta Materialia 61 4648-4656 (2013)
    In the present work, modulated four- and five-layered orthorhombic, seven-layered monoclinic (4O, 10M and 14M) and unmodulated double tetragonal (L10) martensites are characterized in Heusler Ni-Mn-Sn alloys using X-ray diffraction, high-resolution transmission electron microscopy, electron diffraction techniques and thermal analysis. All modulated layered martensites exhibit twins and stacking faults, while the L10 martensite shows fewer structural defects. The substitution of Sn with Mn in Ni 50Mn37+xSn13-x (x = 0, 2, 4) enhances the martensitic transition temperatures, while the transition temperatures decrease with increasing Mn content for constant Sn levels in Ni50-yMn37+ySn13 (y = 0, 2, 4). The compositional dependence of the martensitic transition temperatures is mainly attributed to the valence electron concentration (e/a) and the unit-cell volume of the high-temperature phase. With increasing transition temperatures (or e/a), the resultant martensitic crystal structure evolves in a sequence of 4O → 10M → 14M → L10 in bulk Ni-Mn-Sn alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.035
  • 2013 • 82 Conformational disorder in alkylsiloxane monolayers at elevated temperatures
    Weber, J. and Balgar, T. and Hasselbrink, E.
    Journal of Chemical Physics 139 (2013)
    Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C-H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850-3000 cm-1. We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4846298
  • 2013 • 81 Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols
    Bauer, S. and Burlafinger, K. and Gainaru, C. and Lunkenheimer, P. and Hiller, W. and Loidl, A. and Böhmer, R.
    Journal of Chemical Physics 138 (2013)
    A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance study on the hydrogen-bonded liquid 2-ethyl-1-hexanol C. Gainaru, Phys. Rev. Lett. 107, 118304 (2011)10.1103/PhysRevLett.107.118304 revealed anomalous behavior in various static quantities near 250 K. To check whether corresponding observations can be made for other monohydroxy alcohols as well, these experimental methods were applied to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular backbone. All studied liquids exhibit a change of behavior near 250 K, which is tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing the NIR band intensities, a linear cluster size is derived that agrees with estimates from dielectric spectroscopy. All studied alcohols, except 4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat 2-ethyl-1-butanol exhibits a well resolved structural relaxation in its dielectric loss spectrum, which so far has only been observed for diluted monohydroxy alcohols. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4793469
  • 2013 • 80 Design and experimental evaluation of a new nanoparticle thermophoretic personal sampler
    Azong-Wara, N. and Asbach, C. and Stahlmecke, B. and Fissan, H. and Kaminski, H. and Plitzko, S. and Bathen, D. and Kuhlbusch, T.A.J.
    Journal of Nanoparticle Research 15 (2013)
    A personal sampler that thermophoretically samples particles between a few nanometers and approximately 300 nm has been designed and first prototypes built. The thermal precipitator (TP) is designed to take samples in the breathing zone of a worker in order to determine the personal exposure to airborne nanomaterials. In the sampler, particles are deposited onto silicon substrates that can be used for consecutive electron microscopic (EM) analysis of the particle size distribution and chemical composition of the sampled particles. Due to very homogeneous size-independent particle deposition on a large portion of the substrate, representative samples can be taken for offline analysis. The experimental evaluation revealed a good general agreement with numerical simulations concerning homogeneity of the deposit and a very high correlation (R^2 = 0.98) of the deposition rate per unit area with number concentrations simultaneously measured with an SMPS for particle sizes between 14 and 305 nm. The samplers' small size of only 45 x 32 × 97 mm3 and low weight of only 140 g make it perfectly suitable as a personal sampler. The power consumption for temperature control and pump is around 1.5 W and can be easily provided by batteries. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1530-8
  • 2013 • 79 Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes
    Salaj-Kosla, U. and Pöller, S. and Schuhmann, W. and Shleev, S. and Magner, E.
    Bioelectrochemistry 91 15-20 (2013)
    The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28μA/cm2. The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl-, the current was reduced to 50% of the response in the absence of Cl-. The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2012.11.001
  • 2013 • 78 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2013 • 77 Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M. and Aktürk, S.
    Journal of Applied Physics 114 (2013)
    Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50- xGax in the composition range 12 ≤ x ≤ 25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7 M → L 1 0, 5 M → 7 M, and 5 M → 7 M → L 1 0 with decreasing temperature. The L1 0 non-modulated structure is most stable at low temperature. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4831667
  • 2013 • 76 Fabrication of Borassus fruit lignocellulose fiber/PP composites and comparison with jute, sisal and coir fibers
    Sudhakara, P. and Jagadeesh, D. and Wang, Y. and Venkata Prasad, C. and Devi, A.P.K. and Balakrishnan, G. and Kim, B.S. and Song, J.I.
    Carbohydrate Polymers 98 1002-1010 (2013)
    Novel composites based on borassus fruit fine fiber (BFF) and polypropylene (PP) were fabricated with variable fiber composition (5, 10, 15 and 20 wt%) by injection molding. Maleated PP (MAPP) was also used as compatibilizer at 5 wt% for effective fiber-matrix adhesion. FTIR analysis confirms the evidence of a chemical bonding between the fiber and polymeric matrix through esterification in presence of MAPP. The tensile and flexural properties were found to increase with 15 and 10 wt% fiber loadings respectively, and decreased thereafter. Coir, jute and sisal fiber composites were also fabricated with 15 wt% fiber loading under the same conditions as used for BFF/PP composites. It was found that the mechanical properties of BFF (15 wt%)/PP composites were equivalent to jute/PP, sisal/PP and superior to coir/PP composites. Jute/PP and sisal/PP composites showed higher water absorption than BFF/PP and coir/PP composites. These results have demonstrated that the BFF/PP composites can also be an alternative material for composites applications. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbpol.2013.06.080
  • 2013 • 75 Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers
    Müller, T. and Dohmen, R. and Becker, H.W. and ter Heege, J.H. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 166 1563-1576 (2013)
    Chemical interdiffusion of Fe-Mg along the c-axis [001] in natural diopside crystals (XDi = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10-11 to 10-17 bar. Diffusion couples were prepared by ablating an olivine (XFo = 0.3) target to deposit a thin film (20-100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe-Mg on composition of clinopyroxene (Fe/Mg ratio between Di93-Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe-Mg, can be described by a single Arrhenius relation with (Formula presented). DFe-Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa-Mg) while it is slower than DFe-Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks). © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00410-013-0941-y
  • 2013 • 74 Generation of AuGe nanocomposites by co-sparking technique and their photoluminescence properties
    Kala, S. and Theissmann, R. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The feasibility of spark discharge technique for preparing metal-semiconductor nanocomposites is demonstrated. In the AuGe system, Au shows only 10-3 atomic percent solid solubility in Ge, whereas 3.1 at.% Ge is soluble in Au. During the co-sparking, Au is used as anode material; the cathode is composed of Ge. The relative atomic percent of Au and Ge in the initially generated mixture can be changed by changing the charging current to the capacitor used to trigger the sparking. Depending upon the atomic ratio of Au and Ge in the initial mixture, AuGe agglomerates form AuGe composite nanoparticles on subsequent sintering, in which AuGe alloy nanoparticles are found dispersed in a Ge matrix. The size of the dispersed AuGe alloy nanoparticles depend on the relative atomic concentration of Au and Ge in the initial mixture as well as on the sintering temperature. AuGe alloy nanoparticles dispersed in the Ge matrix are observed to exhibit an intense photoluminescence between 550 and 600 nm. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1963-0
  • 2013 • 73 Hofmeister effect of sodium halides on the switching energetics of thermoresponsive polymer brushes
    Naini, C.A. and Thomas, M. and Franzka, S. and Frost, S. and Ulbricht, M. and Hartmann, N.
    Macromolecular Rapid Communications 34 417-422 (2013)
    A laser temperature-jump technique is used to probe the impact of sodium halides on the temperature-dependent switching kinetics and thermodynamics of poly(N-isopropylacrylamide) brushes. An analysis on the basis of a two-state model reveals van't Hoff enthalpy and entropy changes. Sodium halides increase the endothermicity and the entropic gain of the switching process below and above Tc following the Hofmeister series: NaCl &gt; NaBr &gt; NaI. In contrast, enthalpic and entropic changes at Tc remain virtually unaffected. This provides an unprecedented insight into the underlying switching energetics of this classic stimuli-responsive polymer. Because of its model character, these results represent an essential reference on the way to unpuzzle the molecular driving forces of the Hofmeister effect. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201200681
  • 2013 • 72 Improved group contribution parameter set for the application of solubility parameters to melt extrusion
    Just, S. and Sievert, F. and Thommes, M. and Breitkreutz, J.
    European Journal of Pharmaceutics and Biopharmaceutics 85 1191-1199 (2013)
    Hot-melt extrusion is gaining importance for the production of amorphous solid solutions; in parallel, predictive tools for estimating drug solubility in polymers are increasingly demanded. The Hansen solubility parameter (SP) approach is well acknowledged for its predictive power of the miscibility of liquids as well as the solubility of some amorphous solids in liquid solvents. By solely using the molecular structure, group contribution (GC) methods allow the calculation of Hansen SPs. The GC parameter sets available were derived from liquids and polymers which conflicts with the object of prediction, the solubility of solid drugs. The present study takes a step from the liquid based SPs toward their application to solid solutes. On the basis of published experimental Hansen SPs of solid drugs and excipients only, a new GC parameter set was developed. In comparison with established parameter sets by van Krevelen/Hoftyzer, Beerbower/Hansen, Breitkreutz and Stefanis/Panayiotou, the new GC parameter set provides the highest overall predictive power for solubility experiments (correlation coefficient r = -0.87 to -0.91) as well as for literature data on melt extrudates and casted films (r = -0.78 to -0.96). © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2013.04.006
  • 2013 • 71 In vitro and in vivo evaluation of biodegradable, open-porous scaffolds made of sintered magnesium W4 short fibres
    Bobe, K. and Willbold, E. and Morgenthal, I. and Andersen, O. and Studnitzky, T. and Nellesen, J. and Tillmann, W. and Vogt, C. and Vano, K. and Witte, F.
    Acta Biomaterialia 9 8611-8623 (2013)
    A cytocompatible and biocompatible, degradable, open-porous, mechanically adaptable metal scaffold made of magnesium alloy W4 melt-extracted short fibres was fabricated by liquid phase sintering. Cylindrical samples (3 × 5 mm) of sintered W4 short fibres were evaluated under in vitro (L929, HOB, eudiometer, weight loss) and in vivo conditions (rabbits: 6 and 12 weeks). The in vitro corrosion environment (e.g., temperature, flow, composition of corrosion solution, exposure time) significantly influenced the corrosion rates of W4 scaffolds compared with corrosion in vivo. Corrosion rates under cell culture conditions for 72 h varied from 1.05 to 3.43 mm y-1 depending on the media composition. Corrosion rates measured in eudiometric systems for 24 h were ∼24-27 times higher (3.88-4.43 mm y-1) than corrosion in vivo after 6 weeks (0.16 mm y-1). Moreover, it was found that the cell culture media composition significantly influences the ionic composition of the extract by selectively dissolving ions from W4 samples or their corrosion products. A pilot in vivo study for 6 and 12 weeks demonstrated active bone remodelling, no foreign body reaction and no clinical observation of gas formation during W4 scaffold implantation. Long-term in vivo studies need to be conducted to prove complete degradation of the W4 scaffold and total replacement by the host tissue. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2013.03.035
  • 2013 • 70 Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment
    Gavriljuk, V.G. and Theisen, W. and Sirosh, V.V. and Polshin, E.V. and Kortmann, A. and Mogilny, G.S. and Petrov, Yu.N. and Tarusin, Ye.V.
    Acta Materialia 61 1705-1715 (2013)
    The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of -100 down to -170 °C with its largest intensity near -150 °C. No transformation is observed at -196 °C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.11.045
  • 2013 • 69 Photo-physical properties of anisole: Temperature, pressure, and bath gas composition dependence of fluorescence spectra and lifetimes
    Faust, S. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 112 203-213 (2013)
    Anisole is a promising candidate for use as fluorescent tracer for gas-phase imaging diagnostics. Its high-fluorescence quantum yield (FQY) and its large Stokes shift lead to improved signal intensity (up to 100 times stronger) compared with the often used toluene. Fluorescence spectra and effective fluorescence lifetimes of gaseous anisole were investigated after picosecond laser excitation at 266 nm as a function of temperature (296-977 K) and bath gas composition (varying amounts of N2 and O2) at total pressures in the range of 1-10 bar to provide spectroscopic data and FQY for applications, e.g., in in-cylinder measurements in internal combustion engines. Fluorescence spectra of anisole extend from roughly 270-360 nm with a peak close to 290 nm at 296 K. The spectra show a red-shift with increasing temperature (0.03 nm/K) and O2 partial pressure (5 nm from N2 to air). In the investigated temperature range and in pure N2 at 1 bar total pressure the effective fluorescence lifetime drops with increasing temperature from 13.3 ± 0.5 to 0.05 ± 0.01 ns. Increasing the total pressure of N2 leads to a small decrease of the lifetime at temperatures above 400 K (e.g., at 525 K from 4.2 ± 0.2 ns at 1 bar to 2.7 ± 0.2 ns at 10 bar). At constant temperature and in the presence of O2 the lifetimes decrease significantly (e.g., at 296 K from 13.3 ± 0.5 ns in N2 to 0.40 ± 0.02 ns in air), with this trend diminishing with increasing temperature (e.g., at 675 K from 1.02 ± 0.08 ns in N 2 to 0.25 ± 0.05 ns in air). A phenomenological model that predicts fluorescence lifetimes, i.e., relative quantum yields as a function of temperature, pressure, and O2 concentration is presented. The photophysics of anisole is discussed in comparison with other aromatics. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-013-5420-7
  • 2013 • 68 Quantized electronic heat flow
    Sothmann, B. and Flindt, C.
    Science 342 569-570 (2013)
    A measurement of the quantum-limited heat flow in an electronic conductor opens a pathway to the nanoscale control of heat currents.
    view abstractdoi: 10.1126/science.1246105
  • 2013 • 67 Small-angle water reorientations in KOH doped hexagonal ice and clathrate hydrates
    Nelson, H. and Schildmann, S. and Nowaczyk, A. and Gainaru, C. and Geil, B. and Böhmer, R.
    Physical Chemistry Chemical Physics 15 6355-6367 (2013)
    Using deuteron nuclear magnetic resonance and dielectric spectroscopy KOH doped tetrahydrofuran clathrate hydrates and KOH doped hexagonal ice are studied at temperatures above 60 and 72 K, respectively. Below these temperatures proton order is established on the lattice formed by the water molecules. In the clathrate hydrate a new type of small-angle motion is discovered using deuteron spin-spin relaxation, line-shape analysis, and stimulated-echo experiments. Based on the latter results a model is developed for the local proton motion that could successfully be tested using random-walk simulations. It is argued that the newly identified small-angle motion, obviously absent in undoped samples, is an important feature of the mechanism which accompanies the establishment of proton order not only in doped clathrate hydrates but also in doped hexagonal ice. Specific motions of OH- defects are demonstrated to explain the experimentally observed behavior. The relative importance of localized versus delocalized OH- defect motions is discussed. © 2013 the Owner Societies.
    view abstractdoi: 10.1039/c3cp00139c
  • 2013 • 66 Study of soot formation and oxidation in the engine combustion network (ECN), spray A: Effects of ambient temperature and oxygen concentration
    Cenker, E. and Bruneaux, G. and Pickett, L. and Schulz, C.
    SAE International Journal of Engines 6 352-365 (2013)
    Within the Engine Combustion Network (ECN) spray combustion research frame, simultaneous line-of-sight laser extinction measurements and laser-induced incandescence (LII) imaging were performed to derive the soot volume fraction (fv). Experiments are conducted at engine-relevant high-temperature and high-pressure conditions in a constantvolume pre-combustion type vessel. The target condition, called "Spray A", uses well-defined ambient (900 K, 60 bar, 22.8 kg/m3, 15% oxygen) and injector conditions (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, ndodecane, 363 K). Extinction measurements are used to calibrate LII images for quantitative soot distribution measurements at cross sections intersecting the spray axis. LII images are taken after the start of injection where quasi-stationary combustion is already established. In addition, by changing the LII timing relative to the injection, the temporal variation of the soot cloud is observed from initial soot formation until soot oxidization. OH-chemiluminescence imaging was used to determine the lift-off length, relative to the soot-forming region and used to interpret the soot measurements. Results show that Spray A is a moderately sooting flame where signal trapping is not significant, aiding the potential for quantitative soot diagnostics. Maximum soot volume fractions around 2-3 ppm are obtained at the nominal ambient temperature defined for Spray A (i.e. 900 K) that rise to 12 ppm at elevated temperature (1030 K). At 1.5 ms nominal injection duration the Spray A soot cloud is mainly transient. Therefore, an extended injection duration of 4 ms at identical rail pressure was used to characterize the soot structure in quasi-steady mode. Variations of ambient temperature and oxygen concentration are carried out showing effects on soot formation and oxidation that are consistent with the literature. Copyright © 2013 SAE International.
    view abstractdoi: 10.4271/2013-01-0901
  • 2013 • 65 Surface grafting of Corchorus olitorius fibre: A green approach for the development of activated bioadsorbent
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Majumder, S.B. and Adhikari, B.
    Carbohydrate Polymers 92 2118-2127 (2013)
    The present work is an endeavor to prepare lignocellulosic biomass based adsorbent, suitable for removal of organic and inorganic pollutants from industrial effluents. Lignocellulosic Corchorus olitorius fibre (jute fibre) surface was grafted with naturally available polyphenol, tannin, preceded by the epoxy-activation of fibre surface with epichlorohydrin under mild condition in an aqueous suspension. The reaction parameters for the modification, viz., concentration of epichlorohydrin and tannin, time, and temperature were optimized. The successful occurrence of surface modification of jute fibre (JF) was characterized and estimated from weight gain percent, elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron and atomic force microscopy, and thermogravimetric analysis. An extensive analysis of deconvoluted FTIR spectra using the Voigt model was utilized to ensure the surface grafting. The microbiological susceptibility study revealed high persistency of JF towards biodegradation after efficient grafting with tannin. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.carbpol.2012.11.039
  • 2013 • 64 The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy
    Otto, F. and Dlouhý, A. and Somsen, C. and Bei, H. and Eggeler, G. and George, E.P.
    Acta Materialia 61 5743-5755 (2013)
    An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.018
  • 2013 • 63 Thermoresponsive ultrafiltration membranes for the switchable permeation and fractionation of nanoparticles
    Frost, S. and Ulbricht, M.
    Journal of Membrane Science 448 1-11 (2013)
    Poly(ethylene terephthalate) track-etched ultrafiltration membranes (110. nm pore diameter) have been functionalized with the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) via surface-initiated Atom Transfer Radical Polymerization (SI-ATRP). The PNIPAAm chain lengths, i.e. degree of graft functionalization, inside the membrane pores could be controlled very well with polymerization time. Importantly, gas flow/pore dewetting permporometry measurements demonstrated that the pore diameter in the dry state could be reduced and that the narrow pore size distribution of the membranes was not changed during the grafting process. Both hydrodynamic pore diameters of the membranes and grafted hydrodynamic layer thickness on the pore walls as well as their response to temperature could be estimated by measuring water permeability and applying Hagen-Poiseuille law. Defined temperature-induced swelling/deswelling ratios of ~2 had been observed. These data indicate that PNIPAAm chains in the brush state had been achieved. The ultrafiltration membrane pores could be switched between more open and more closed states. For example, the hydrodynamic pore diameter could be switched from 21. nm at 23. °C to 69. nm at 45. °C. For the same type of membrane the rejection of monomodal 21. nm silica nanoparticles could be switched from 99% at 23. °C to only 35% at 45. °C. The rejection for larger monomodal 35. nm silica nanoparticles was above 90% for every functionalized membrane irrespective of the temperature. For an exemplary functionalized membrane evidence for a switchable size-selective NP fractionation has been found. A mixture of the 21 and the 35. nm silica nanoparticles was ultrafiltered through the membrane and at 23. °C only the smaller ones could be found in the permeate whereas at 45. °C also the larger nanoparticles were able to pass the membrane. © 2013.
    view abstractdoi: 10.1016/j.memsci.2013.07.036
  • 2012 • 62 Adiabatic temperature increase associated with deformation twinning and dislocation plasticity
    Eisenlohr, A. and Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 3994-4004 (2012)
    We studied local deformation and temperature effects associated with mechanical twinning in Fe-3 wt.% Si at room temperature. During tensile testing, two large stress drops occurred. They were accompanied by local strain and temperature bursts, which we mapped via simultaneous displacement and temperature field characterization. To identify the microstructural origin of these phenomena, we performed high resolution electron backscatter scanning diffraction and electron channeling contrast imaging measurements. The microstructure at the positions where strong adiabatic heating occurred was characterized by the formation of primary twins and high dislocation activity within a range of about 10 μm around the twin-matrix interface. We suggest that the local temperature and strain jumps result from the formation and dissipative motion of lattice dislocations that accommodate twinning. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.008
  • 2012 • 61 Advancing density functional theory to finite temperatures: Methods and applications in steel design
    Hickel, T. and Grabowski, B. and Körmann, F. and Neugebauer, J.
    Journal of Physics Condensed Matter 24 (2012)
    The performance of materials such as steels, their high strength and formability, is based on an impressive variety of competing mechanisms on the microscopic/atomic scale (e.g. dislocation gliding, solid solution hardening, mechanical twinning or structural phase transformations). Whereas many of the currently available concepts to describe these mechanisms are based on empirical and experimental data, it becomes more and more apparent that further improvement of materials needs to be based on a more fundamental level. Recent progress for methods based on density functional theory (DFT) now makes the exploration of chemical trends, the determination of parameters for phenomenological models and the identification of new routes for the optimization of steel properties feasible. A major challenge in applying these methods to a true materials design is, however, the inclusion of temperature-driven effects on the desired properties. Therefore, a large range of computational tools has been developed in order to improve the capability and accuracy of first-principles methods in determining free energies. These combine electronic, vibrational and magnetic effects as well as structural defects in an integrated approach. Based on these simulation tools, one is now able to successfully predict mechanical and thermodynamic properties of metals with a hitherto not achievable accuracy. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/5/053202
  • 2012 • 60 An improved electrical and thermal model of a microbolometer for electronic circuit simulation
    Würfel, D. and Vogt, H.
    Advances in Radio Science 10 183-186 (2012)
    The need for uncooled infrared focal plane arrays (IRFPA) for imaging systems has increased since the beginning of the nineties. Examples for the application of IRFPAs are thermography, pedestrian detection for automotives, fire fighting, and infrared spectroscopy. It is very important to have a correct electro-optical model for the simulation of the microbolometer during the development of the readout integrated circuit (ROIC) used for IRFPAs. The microbolometer as the sensing element absorbs infrared radiation which leads to a change of its temperature due to a very good thermal insulation. In conjunction with a high temperature coefficient of resistance (TCR) of the sensing material (typical vanadium oxide or amorphous silicon) this temperature change results in a change of the electrical resistance. During readout, electrical power is dissipated in the microbolometer, which increases the temperature continuously. The standard model for the electro-optical simulation of a microbolometer includes the radiation emitted by an observed blackbody, radiation emitted by the substrate, radiation emitted by the microbolometer itself to the surrounding, a heat loss through the legs which connect the microbolometer electrically and mechanically to the substrate, and the electrical power dissipation during readout of the microbolometer (Wood, 1997). The improved model presented in this paper takes a closer look on additional radiation effects in a real IR camera system, for example the radiation emitted by the casing and the lens. The proposed model will consider that some parts of the radiation that is reflected from the casing and the substrate is also absorbed by the microbolometer. Finally, the proposed model will include that some fraction of the radiation is transmitted through the microbolometer at first and then absorbed after the reflection at the surface of the substrate. Compared to the standard model temperature and resistance of the microbolometer can be modelled more realistically when these higher order effects are taken into account. A Verilog-A model for electronic circuit simulations is developed based on the improved thermal model of the microbolometer. Finally, a simulation result of a simple circuit is presented. © 2012 Author(s).
    view abstractdoi: 10.5194/ars-10-183-2012
  • 2012 • 59 Autoignition of surrogate biodiesel fuel (B30) at high pressures: Experimental and modeling kinetic study
    Ramirez Lancheros, H.P. and Fikri, M. and Cancino, L.R. and Moréac, G. and Schulz, C. and Dagaut, P.
    Combustion and Flame 159 996-1008 (2012)
    Ignition delay times of surrogate biodiesel fuels were measured in a high-pressure shock tube over a wide range of experimental conditions (pressures of 20 and 40. bar, equivalence ratios in the range 0.5-1.5, and temperatures ranging from 700 to 1200. K). A detailed chemical kinetic mechanism developed for the oxidation of a biodiesel fuel and a B30 biodiesel surrogate (49% n-decane, 21% 1-methylnaphthalene, and 30% methyloctanoate in mol%) was used to simulate the present experiments. Cross reactions between radicals from the three fuel components and reactions of methylnaphthalene oxidation recently proposed in the literature were introduced into the model in order to improve ignition delay time predictions at low temperatures. The new scheme (7865 reversible reactions and 1975 species) yields improved model predictions of concentration profiles measured earlier in a jet-stirred reactor, and also represents fairly well the present experimental data over the entire range of conditions of this study. Sensitivity analyses and reaction path analyses were used to rationalize the results. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.10.006
  • 2012 • 58 Coulomb attraction during the carpet growth mode of NaCl
    Matthaei, F. and Heidorn, S. and Boom, K. and Bertram, C. and Safiei, A. and Henzl, J. and Morgenstern, K.
    Journal of Physics Condensed Matter 24 (2012)
    The submonolayer growth of NaCl bilayer high-rectangular shaped islands on Ag(111) is investigated at around room temperature by using low temperature scanning tunneling microscopy. The growth at the step edges is preferred. Two kinds of islands are observed. They either grow with their non-polar edge at the step edge of Ag(111) or the islands overgrow in a carpet-like mode with the polar direction parallel to the edge. In the latter case, the Ag step is rearranged and considerable, while the NaCl layer is bent. This study clarifies the nature of the interaction of an alkali halide nanostructure with a metal step edge. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/35/354006
  • 2012 • 57 Critical motor number for fractional steps of cytoskeletal filaments in gliding assays
    Li, X. and Lipowsky, R. and Kierfeld, J.
    PLoS ONE 7 (2012)
    In gliding assays, filaments are pulled by molecular motors that are immobilized on a solid surface. By varying the motor density on the surface, one can control the number N of motors that pull simultaneously on a single filament. Here, such gliding assays are studied theoretically using Brownian (or Langevin) dynamics simulations and taking the local force balance between motors and filaments as well as the force-dependent velocity of the motors into account. We focus on the filament stepping dynamics and investigate how single motor properties such as stalk elasticity and step size determine the presence or absence of fractional steps of the filaments. We show that each gliding assay can be characterized by a critical motor number, Nc. Because of thermal fluctuations, fractional filament steps are only detectable as long as N < Nc. The corresponding fractional filament step size is ℓ/N where ℓ is the step size of a single motor. We first apply our computational approach to microtubules pulled by kinesin-1 motors. For elastic motor stalks that behave as linear springs with a zero rest length, the critical motor number is found to be Nc=4, and the corresponding distributions of the filament step sizes are in good agreement with the available experimental data. In general, the critical motor number Nc depends on the elastic stalk properties and is reduced to Nc=3 for linear springs with a nonzero rest length. Furthermore, Nc is shown to depend quadratically on the motor step size ℓ. Therefore, gliding assays consisting of actin filaments and myosin-V are predicted to exhibit fractional filament steps up to motor number N=31. Finally, we show that fractional filament steps are also detectable for a fixed average motor number 〈N〉 as determined by the surface density (or coverage) of the motors on the substrate surface. © 2012 Li et al.
    view abstractdoi: 10.1371/journal.pone.0043219
  • 2012 • 56 Dispersions of silica nanoparticles in ionic liquids investigated with advanced rheology
    Wittmar, A. and Ruiz-Abad, D. and Ulbricht, M.
    Journal of Nanoparticle Research 14 (2012)
    The colloidal stabilities of dispersions of unmodified and surface-functionalized SiO 2 nanoparticles in hydrophobic and hydrophilic imidazolium-based ionic liquids were studied with advanced rheology at three temperatures (25, 100, and 200 °C). The rheological behavior of the dispersions was strongly affected by the ionic liquids hydrophilicity, by the nanoparticles surface, by the concentration of the nanoparticles in the dispersion as well as by the temperature. The unmodified hydrophilic nanoparticles showed a better compatibility with the hydrophilic ionic liquid. The SiO 2 surface functionalization with hydrophobic groups clearly improved the colloidal stability of the dispersions in the hydrophobic ionic liquid. The temperature increase was found to lead to a destabilization in all studied systems, especially at higher concentrations. The results of this study imply that ionic liquids with tailored properties could be used in absorbers directly after reactors for gas-phase synthesis of nanoparticles or/and as solvents for their further surface functionalization without agglomeration or aggregation. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-011-0651-1
  • 2012 • 55 Fluctuation-induced magnetization dynamics and criticality at the interface of a topological insulator with a magnetically ordered layer
    Nogueira, F.S. and Eremin, I.
    Physical Review Letters 109 (2012)
    We consider a theory for a two-dimensional interacting conduction electron system with strong spin-orbit coupling on the interface between a topological insulator and the magnetic (ferromagnetic or antiferromagnetic) layer. For the ferromagnetic case we derive the Landau-Lifshitz equation, which features a contribution proportional to a fluctuation-induced electric field obtained by computing the topological (Chern-Simons) contribution from the vacuum polarization. We also show that fermionic quantum fluctuations reduce the critical temperature Tc at the interface relative to the critical temperature Tc of the bulk, so that in the interval T c≤T< Tc it is possible to have a coexistence of gapless Dirac fermions at the interface with a ferromagnetically ordered layer. For the case of an antiferromagnetic layer on a topological insulator substrate, we show that a second-order quantum phase transition occurs at the interface, and compute the corresponding critical exponents. In particular, we show that the electrons at the interface acquire an anomalous dimension at criticality. The critical behavior of the Néel order parameter is anisotropic and features large anomalous dimensions for both the longitudinal and transversal fluctuations. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.109.237203
  • 2012 • 54 Formation of ZnMn 2O 4 ball-in-ball hollow microspheres as a high-performance anode for lithium-ion batteries
    Zhang, G. and Yu, L. and Wu, H.B. and Hoster, H.E. and Lou, X.W.
    Advanced Materials 24 4609-4613 (2012)
    Novel ZnMn 2O 4 ball-in-ball hollow microspheres are fabricated by a facile two-step method involving the solution synthesis of ZnMn-glycolate hollow microspheres and subsequent thermal annealing in air. When evaluated as an anode material for lithium-ion batteries, these ZnMn 2O 4 ball-in-ball hollow microspheres show significantly enhanced electrochemical performance with high capacity, excellent cycling stability and good rate capability. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adma.201201779
  • 2012 • 53 Frequency response of polymer films made from a precursor colloidal monolayer on a nanomechanical cantilever
    Liu, T. and Pihan, S. and Roth, M. and Retsch, M. and Jonas, U. and Gutmann, J.S. and Koynov, K. and Butt, H.-J. and Berger, R.
    Macromolecules 45 862-871 (2012)
    Nanomechanical cantilevers (NMC) were used for the characterization of the film formation process and the mechanical properties of colloidal monolayers made from polystyrene (PS). Closely packed hexagonal monolayers of colloids with diameters ranging from 400 to 800 nm were prepared at the air-water interface and then transferred in a controlled way on the surface of NMC. The film formation process upon annealing of the monolayer was investigated by measuring the resonance frequency of the NMC (≈12 kHz). Upon heating of non-cross-linked PS colloids, we could identify two transition temperatures. The first transition resulted from the merging of polymer colloids into a film. This transition temperature at 147 ± 3 °C as measured at ≈12 kHz remained constant for subsequent heating cycles. We attributed this transition temperature to the glass transition temperature T g of PS which was confirmed by dynamic mechanical thermal analysis (DMTA) and using the time temperature superposition principle. The second transition temperature (175 ± 3 °C) was associated with the end of the film formation process and was measured only for the first heating cycle. Furthermore, the transition of the colloidal monolayer into a homogeneous film preserved the mass loading on the NMC which allowed determination of the Young's modulus of PS (≈3 GPa) elegantly. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ma202396h
  • 2012 • 52 Interplay of wrinkles, strain, and lattice parameter in graphene on iridium
    Hattab, H. and N'Diaye, A.T. and Wall, D. and Klein, C. and Jnawali, G. and Coraux, J. and Busse, C. and Van Gastel, R. and Poelsema, B. and Michely, T. and Meyer zu Heringdorf, F.-J. and Horn-von Hoegen, M.
    Nano Letters 12 678-682 (2012)
    Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl203530t
  • 2012 • 51 Merocyanine/C60 planar heterojunction solar cells: Effect of dye orientation on exciton dissociation and solar cell performance
    Ojala, A. and Petersen, A. and Fuchs, A. and Lovrincic, R. and Pölking, C. and Trollmann, J. and Hwang, J. and Lennartz, C. and Reichelt, H. and Höffken, H.W. and Pucci, A. and Erk, P. and Kirchartz, T. and Würthner, F.
    Advanced Functional Materials 22 86-96 (2012)
    In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C60 organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post-annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric-field-dependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as-deposited cells. Fourier-transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules' orientations on the charge-transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201101697
  • 2012 • 50 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 49 NMR and conductivity studies of the mixed glass former effect in lithium borophosphate glasses
    Storek, M. and Böhmer, R. and Martin, S.W. and Larink, D. and Eckert, H.
    Journal of Chemical Physics 137 (2012)
    Alkali ion charge transport has been studied in a series of mixed glass former lithium borophosphate glasses of composition 0.33Li 2O 0.67xB 2O 3 (1 - x)P 2O 5. The entire concentration range, 0.0 x 1.0, from pure glassy Li 2P 4O 11 to pure glassy Li 2B 4O 7 has been examined while keeping the molar fraction of Li 2O constant. Electrical conductivity measurements and nuclear magnetic resonance techniques such as spin relaxometry, line shape analysis, and stimulated-echo spectroscopy were used to examine the temperature and frequency dependence of the Li ion motion over wide ranges of time scale and temperature. By accurately determining motional time scales and activation energies over the entire composition range the ion dynamics and the charge transport are found to be fastest if the borate and the phosphate fractions are similar. The nonlinear variation of the charge conduction, the most notable feature of the mixed glass former effect, is discussed in terms of the composition dependence of network former units which determine the local glass structure. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4754664
  • 2012 • 48 Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 11181-11188 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302723r
  • 2012 • 47 Stress-induced melting of crystals in natural rubber: A new way to tailor the transition temperature of shape memory polymers
    Heuwers, B. and Quitmann, D. and Katzenberg, F. and Tiller, J.C.
    Macromolecular Rapid Communications 33 1517-1522 (2012)
    Lightly cross-linked natural rubber (NR, cis-1,4-polyisoprene) was found to be an exceptional cold programmable shape memory polymer (SMP) with strain storage of up to 1000%. These networks are stabilized by strain-induced crystals. Here, we explore the influence of mechanical stress applied perpendicular to the elongation direction of the network on the stability of these crystals. We found that the material recovers its original shape at a critical transverse stress. It could be shown that this is due to a disruption of the strain-stabilizing crystals, which represents a completely new trigger for SMPs. The variation of transverse stress allows tuning of the trigger temperature Ttrig(σ) in a range of 45 to 0 °C, which is the first example of manipulating the transition of a crystal-stabilized SMP after programming. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201200313
  • 2012 • 46 Study of energy fluctuation effect on the statistical mechanics of equilibrium systems
    Lysogorskiy, Yu.V. and Wang, Q.A. and Tayurskii, D.A.
    Journal of Physics: Conference Series 394 (2012)
    This work is devoted to the modeling of energy fluctuation effect on the behavior of small classical thermodynamic systems. It is known that when an equilibrium system gets smaller and smaller, one of the major quantities that becomes more and more uncertain is its internal energy. These increasing fluctuations can considerably modify the original statistics. The present model considers the effect of such energy fluctuations and is based on an overlapping between the Boltzmann-Gibbs statistics and the statistics of the fluctuation. Within this "overlap statistics", we studied the effects of several types of energy fluctuations on the probability distribution, internal energy and heat capacity. It was shown that the fluctuations can considerably change the temperature dependence of internal energy and heat capacity in the low energy range and at low temperatures. Particularly, it was found that, due to the lower energy limit of the systems, the fluctuations reduce the probability for the low energy states close to the lowest energy and increase the total average energy. This energy increasing is larger for lower temperatures, making negative heat capacity possible for this case.
    view abstractdoi: 10.1088/1742-6596/394/1/012006
  • 2012 • 45 The effect of ionic strength, temperature, and pressure on the interaction potential of dense protein solutions: From nonlinear pressure response to protein crystallization
    Möller, J. and Schroer, M.A. and Erlkamp, M. and Grobelny, S. and Paulus, M. and Tiemeyer, S. and Wirkert, F.J. and Tolan, M. and Winter, R.
    Biophysical Journal 102 2641-2648 (2012)
    Understanding the intermolecular interaction potential, V(r), of proteins under the influence of temperature, pressure, and salt concentration is essential for understanding protein aggregation, crystallization, and protein phase behavior in general. Here, we report small-angle x-ray scattering studies on dense lysozyme solutions of high ionic strength as a function of temperature and pressure. We show that the interaction potential changes in a nonlinear fashion over a wide range of temperatures, salt, and protein concentrations. Neither temperature nor protein and salt concentration lead to marked changes in the pressure dependence of V(r), indicating that changes of the water structure dominate the pressure dependence of the intermolecular forces. Furthermore, by analysis of the temperature, pressure, and ionic strength dependence of the normalized second virial coefficient, b2, we show that the interaction can be fine-tuned by pressure, which can be used to optimize b 2 values for controlled protein crystallization. © 2012 Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2012.04.043
  • 2012 • 44 The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain
    Chan, W.-L. and Ligges, M. and Zhu, X.-Y.
    Nature Chemistry 4 840-845 (2012)
    One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S 1) into two triplet (T 1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S 1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2×T 1, but fission is not activated thermally. Rather, an S 1 ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S 1 ⇔ ME into 2×T 1, which leads to a high singlet-fission yield. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nchem.1436
  • 2011 • 43 Ab initio simulation of effects of structural singularities in aerogel absorption potential
    Debras, C. and Tayurskii, D. and Minisini, B. and Lysogorskiy, Y.
    Journal of Physics: Conference Series 324 (2011)
    In the present work simulation of Van der Waals potential between helium atom and part of silica aerogel strand by means of ab initio methods was performed. Cell with alpha quartz structure was used as building block of aerogel strand, because it is the most stable structure at low temperature, and only the surface layer of aerogel has been considered. For modeling absorption potential field in plane, summation of potential from individual building blocks has been provided. Two dimensional Van der Waals energy field was calculated for different geometries of aerogel strands. A rather deep potential well has been found in the corner formed due to aerogel strand crossing.
    view abstractdoi: 10.1088/1742-6596/324/1/012029
  • 2011 • 42 Accumulation of silver nanoparticles by cultured primary brain astrocytes
    Luther, E.M. and Koehler, Y. and Diendorf, J. and Epple, M. and Dringen, R.
    Nanotechnology 22 (2011)
    Silver nanoparticles (AgNP) are components of various food industry products and are frequently used for medical equipment and materials. Although such particles enter the vertebrate brain, little is known on their biocompatibility for brain cells. To study the consequences of an AgNP exposure of brain cells we have treated astrocyte-rich primary cultures with polyvinylpyrrolidone (PVP)-coated AgNP. The incubation of cultured astrocytes with micromolar concentrations of AgNP for up to 24h resulted in a time-and concentration-dependent accumulation of silver, but did not compromise the cell viability nor lower the cellular glutathione content. In contrast, the incubation of astrocytes for 4h with identical amounts of silver as AgNO 3 already severely compromised the cell viability and completely deprived the cells of glutathione. The accumulation of AgNP by astrocytes was proportional to the concentration of AgNP applied and significantly lowered by about 30% in the presence of the endocytosis inhibitors chloroquine or amiloride. Incubation at 4 °C reduced the accumulation of AgNP by 80% compared to the values obtained for cells that had been exposed to AgNP at 37 °C. These data demonstrate that viable cultured brain astrocytes efficiently accumulate PVP-coated AgNP in a temperature-dependent process that most likely involves endocytotic pathways. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/37/375101
  • 2011 • 41 Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 820-827 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.01.012
  • 2011 • 40 Auto-ignition of toluene-doped n-heptane and iso-octane/air mixtures: High-pressure shock-tube experiments and kinetics modeling
    Hartmann, M. and Gushterova, I. and Fikri, M. and Schulz, C. and Schießl, R. and Maas, U.
    Combustion and Flame 158 172-178 (2011)
    Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times τign were measured behind reflected shock waves in mixtures with air at φ=1.0 and 0.5 at p=40bar, over a temperature range of T=700-1200K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on τign on the base fuels as function of temperature. For typical toluene tracer concentrations ≤10%, the ignition delay time τign changes by less than 10% in the relevant pressure and temperature range. © 2010 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2010.08.005
  • 2011 • 39 Characterisation of exosomes derived from human cells by nanoparticle tracking analysis and scanning electron microscopy
    Sokolova, V. and Ludwig, A.-K. and Hornung, S. and Rotan, O. and Horn, P.A. and Epple, M. and Giebel, B.
    Colloids and Surfaces B: Biointerfaces 87 146-150 (2011)
    Exosomes from three different cell types (HEK 293T, ECFC, MSC) were characterised by scanning electron microscopy (SEM), dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA). The diameter was around 110. nm for the three cell types. The stability of exosomes was examined during storage at -20 °C, 4 °C, and 37 °C. The size of the exosomes decreased at 4 °C and 37 °C, indicating a structural change or degradation. Multiple freezing to -20 °C and thawing did not affect the exosome size. Multiple ultracentrifugation also did not change the exosome size. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2011.05.013
  • 2011 • 38 Deformation resistance in the transition from coarse-grained to ultrafine-grained Cu by severe plastic deformation up to 24 passes of ECAP
    Blum, W. and Li, Y.J. and Zhang, Y. and Wang, J.T.
    Materials Science and Engineering A 528 8621-8627 (2011)
    Pure Cu was subjected to severe plastic predeformation by p=1, 2, 4, 8, 16 and 24 passes of equal channel angular pressing (ECAP) on route BC at ambient temperature and subsequently tested in uniaxial compression parallel to the extrusion direction at constant rate or constant stress and temperatures from ambient temperature up to 418K. The maximum compressive strength of the ECAPed Cu varies in a systematic fashion with p, until a steady state is finally reached between p=8 and 16 where the rate sensitivity of flow stress is maximal. The results are quantitatively interpreted in terms of the boundary structure, considering the superposition of hardening due to refinement of low-angle boundaries and softening due to enhanced thermal recovery at high-angle boundaries. Beyond the maximum the compressive strength declines with strain for relatively low rate and/or elevated temperature of compression. This is explained by dynamic grain coarsening towards the new steady state developing in compression. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.08.010
  • 2011 • 37 Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane: II. A comparison of rotational and translational motions
    Lederle, C. and Hiller, W. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 134 (2011)
    Mixtures of the monohydroxy alcohol n-butanol with n-bromobutane are investigated via dielectric and nuclear magnetic resonance (NMR) techniques. Static- and pulsed-field gradient proton NMR yielded self-diffusion coefficients as a function of concentration and temperature. To monitor reorientational motions, broadband dielectric and 13C-spin relaxation time measurements were carried out. The latter demonstrate that the structural relaxation stems from the motion of the alkyl chains. By combining data from translational diffusion coefficients with published shear viscosities, hydrodynamic radii were determined that compare favorably with the van der Waals radii of single molecules. The results for the neat alcohol and for the binary mixtures are discussed with respect to a recent transient chain model. The approach of Debye and structural relaxation times at high temperatures, identified as a general feature of monohydroxy alcohols, is also discussed within that framework. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3549123
  • 2011 • 36 Effects of specific versus nonspecific ionic interactions on the structure and lateral organization of lipopolysaccharides
    Jeworrek, C. and Evers, F. and Howe, J. and Brandenburg, K. and Tolan, M. and Winter, R.
    Biophysical Journal 100 2169-2177 (2011)
    We report x-ray reflectivity and grazing incidence x-ray diffraction measurements of lipopolysaccharide (LPS) monolayers at the water-air interface. Our investigations reveal that the structure and lateral ordering of the LPS molecules is very different from phospholipid systems and can be modulated by the ionic strength of the aqueous subphase in an ion-dependent manner. Our findings also indicate differential effects of monovalent and divalent ions on the two-dimensional ordering of lipid domains. Na+ ions interact unspecifically with LPS molecules based on their ability to efficiently screen the negative charges of the LPS molecules, whereas Ca2+ ions interact specifically by cross-linking adjacent molecules in the monolayer. At low lateral pressures, Na+ ions present in the subphase lead to a LPS monolayer structure ordered over large areas with high compressibility, nearly hexagonal packing of the hydrocarbon chains, and high density in the LPS headgroup region. At higher film pressures, the LPS monolayer becomes more rigid and results in a less perfect, oblique packing of the LPS hydrocarbon chains as well as a smaller lateral size of highly ordered domains on the monolayer. Furthermore, associated with the increased surface pressure, a conformational change of the sugar headgroups occurs, leading to a thickening of the entire LPS monolayer structure. The effect of Ca2+ ions in the subphase is to increase the rigidity of the LPS monolayer, leading to an oblique packing of the hydrocarbon chains already at low film pressures, an upright orientation of the sugar moieties, and much smaller sizes of ordered domains in the plane of the monolayer. In the presence of both Na+-and Ca2+ ions in the subphase, the screening effect of Na+ is predominant at low film pressures, whereas, at higher film pressures, the structure and lateral organization of LPS molecules is governed by the influence of Ca2+ ions. The unspecific charge-screening effect of the Na+ ions on the conformation of the sugar moiety becomes less dominant at biologically relevant lateral pressures. © 2011 by the Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2011.03.019
  • 2011 • 35 Error analysis of large-eddy simulation of the turbulent non-premixed sydney bluff-body flame
    Kempf, A.M. and Geurts, B.J. and Oefelein, J.C.
    Combustion and Flame 158 2408-2419 (2011)
    A computational error analysis is applied to the large-eddy simulation of the turbulent non-premixed Sydney bluff-body flame, where the error is defined with respect to experimental data. The error-landscape approach is extended to heterogeneous compressible turbulence, which is coupled to combustion as described by a flamelet model. The Smagorinsky model formulation is used to model the unknown turbulent stresses. We introduce several measures to quantify the total simulation error and observe a striking 'valley-structure' in the error that arises as function of the spatial resolution and the Smagorinsky length parameter. The optimal refinement strategy that can be extracted from this error-landscape is reminiscent of that for non-reacting turbulent flow. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.04.012
  • 2011 • 34 Experimental and theoretical studies of the colloidal stability of nanoparticles?a general interpretation based on stability maps
    Segets, D. and Marczak, R. and Schäfer, S. and Paula, C. and Gnichwitz, J.-F. and Hirsch, A. and Peukert, W.
    ACS Nano 5 4658-4669 (2011)
    The current work addresses the understanding of the stabilization of nanoparticles in suspension. Specifically, we study ZnO in ethanol for which the influence of particle size and reactant ratio as well as surface coverage on colloidal stability in dependence of the purification progress was investigated. The results revealed that the well-known 〈-potential determines not only the colloidal stability but also the surface coverage of acetate groups bound to the particle surface. The acetate groups act as molecular spacers between the nanoparticles and prevent agglomeration. Next to DLVO calculations based on the theory of Derjaguin, Landau, Verwey and Overbeek using a core-shell model we find that the stability is better understood in terms of dimensionless numbers which represent attractive forces as well as electrostatic repulsion, steric effects, transport properties, and particle concentration. Evaluating the colloidal stability in dependence of time by means of UV-vis absorption measurements a stability map for ZnO is derived. From this map it becomes clear that the dimensionless steric contribution to colloidal stability scales with a stability parameter including dimensionless repulsion and attraction as well as particle concentration and diffusivity of the particles according to a power law with an exponent of ?0.5. Finally, we show that our approach is valid for other stabilizing molecules like cationic dendrons and is generally applicable for a wide range of other material systems within the limitations of vanishing van der Waals forces in refractive index matched situations, vanishing 〈-potential and systems without a stabilizing shell around the particle surface. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nn200465b
  • 2011 • 33 Guanidiniocarbonyl-pyrrole-aryl conjugates as nucleic acid sensors: Switch of binding mode and spectroscopic responses by introducing additional binding sites into the linker
    Gröger, K. and Baretić, D. and Piantanida, I. and Marjanović, M. and Kralj, M. and Grabar, M. and Tomić, S. and Schmuck, C.
    Organic and Biomolecular Chemistry 9 198-209 (2011)
    Two novel guanidiniocarbonyl pyrrole-pyrene conjugates 3 and 4 as spectroscopic probes for ds-polynucleotides were synthesized and their interaction with different ds-DNAs/RNAs studied. Compared to a previously reported first set of conjugates (1 and 2) the significant extension and increased rigidity of the central part of the structure resulted in a switch of DNA binding mode from intercalative (previously studied derivatives 1 and 2 with a nonbinding and flexible linker) to minor groove binding of the two novel guanidiniocarbonyl-pyrrole-pyrene conjugates 3 and 4. These two compounds interact strongly with ds-DNAs, but only weakly with ds-RNA. The newly incorporated heterocyclic moieties within the central part of the structure of 3 and 4 were able to control by steric and hydrogen-bonding effects the alignment of the molecules within various, structurally different forms of DNA minor grooves, whereby even small differences in the position of the attached pyrene within the groove were reflected in different fluorimetric responses. In addition, 3 and 4 revealed intriguing in vitro selectivity among various human tumour cell lines. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0ob00103a
  • 2011 • 32 Hierarchical interactions and their influence upon the adsorption of organic molecules on a graphene film
    Roos, M. and Künzel, D. and Uhl, B. and Huang, H.-H. and Brandao Alves, O. and Hoster, H.E. and Gross, A. and Behm, R.J.
    Journal of the American Chemical Society 133 9208-9211 (2011)
    The competition between intermolecular interactions and lateral variations in the molecule-substrate interactions has been studied by scanning tunneling microscopy (STM), comparing the phase formation of (sub)monolayers of the organic molecule 2,4′-BTP on buckled graphene/Ru(0001) and Ag(111) oriented thin films on Ru(0001). On the Ag films, the molecules form a densely packed 2D structure, while on graphene/Ru(0001), only the areas between the maxima are populated. The findings are rationalized by a high corrugation in the adsorption potential for 2,4′-BTP molecules on graphene/Ru(0001). These findings are supported by temperature programmed desorption (TPD) experiments and theoretical results. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2025855
  • 2011 • 31 Hierarchical micro- and mesoporous carbide-derived carbon as a high-performance electrode material in supercapacitors
    Rose, M. and Korenblit, Y. and Kockrick, E. and Borchardt, L. and Oschatz, M. and Kaskel, S. and Yushin, G.
    Small 7 1108-1117 (2011)
    Ordered mesoporous carbide-derived carbon (OM-CDC) materials produced by nanocasting of ordered mesoporous silica templates are characterized by a bimodal pore size distribution with a high ratio of micropores. The micropores result in outstanding adsorption capacities and the well-defined mesopores facilitate enhanced kinetics in adsorption processes. Here, for the first time, a systematic study is presented, in which the effects of synthesis temperature on the electrochemical performance of these materials in supercapacitors based on a 1 M aqueous solution of sulfuric acid and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid are reported. Cyclic voltammetry shows the specific capacitance of the OM-CDC materials exceeds 200 F g-1 in the aqueous electrolyte and 185 F g-1 in the ionic liquid, when measured in a symmetric configuration in voltage ranges of up to 0.6 and 2 V, respectively. The ordered mesoporous channels in the produced OM-CDC materials serve as ion-highways and allow for very fast ionic transport into the bulk of the OM-CDC particles. At room temperature the enhanced ion transport leads to 75% and 90% of the capacitance retention at current densities in excess of ∼10 A g-1 in ionic liquid and aqueous electrolytes, respectively. The supercapacitors based on 250-300 μm OM-CDC electrodes demonstrate an operating frequency of up to 7 Hz in aqueous electrolyte. The combination of high specific capacitance and outstanding rate capabilities of the OM-CDC materials is unmatched by state-of-the art activated carbons and strictly microporous CDC materials. © 2011 Wiley-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201001898
  • 2011 • 30 Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst
    Tedsree, K. and Li, T. and Jones, S. and Chan, C.W.A. and Yu, K.M.K. and Bagot, P.A.J. and Marquis, E.A. and Smith, G.D.W. and Tsang, S.C.E.
    Nature Nanotechnology 6 302-307 (2011)
    Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H 2 from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions. © 2011 Macmillan Publishers. All rights reserved.
    view abstractdoi: 10.1038/nnano.2011.42
  • 2011 • 29 Improvement of the dissolution rate of poorly soluble drugs by solid crystal suspensions
    Thommes, M. and Ely, D.R. and Carvajal, M.T. and Pinal, R.
    Molecular Pharmaceutics 8 727-735 (2011)
    We present a novel extrusion based approach where the dissolution rate of poorly soluble drugs (griseofulvin, phenytoin and spironolactone) is significantly accelerated. The drug and highly soluble mannitol are coprocessed in a hot melt extrusion operation. The obtained product is an intimate mixture of the crystalline drug and crystalline excipient, with up to 50% (w/w) drug load. The in vitro drug release from the obtained solid crystalline suspensions is over 2 orders of magnitude faster than that of the pure drug. Since the resulting product is crystalline, the accelerated dissolution rate does not bear the physical stability concerns inherent to amorphous formulations. This approach is useful in situations where the drug is not a good glass former or in cases where it is difficult to stabilize the amorphous drug. Being thermodynamically stable, the dissolution profile and the solid state properties of the product are maintained after storage at 40 °C, 75% RH for at least 90 days. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/mp1003493
  • 2011 • 28 Investigation of the criteria for fluid selection in rankine cycles for waste heat recovery
    Siddiqi, M.A. and Atakan, B.
    International Journal of Thermodynamics 14 117-123 (2011)
    The organic Rankine cycle is a promising way for the conversion of low temperature heat to electricity. Different fluids can be used in Rankine cycles for the utilization of waste heat. The suitability of a certain fluid will depend on its thermodynamic properties as well as on the conditions at which the heat is available, thus it is often unclear if an organic fluid has any advantage compared to inorganic fluids like water. Various substances starting from the refrigerants to high boiling organic liquids have been investigated as possible working fluids for the different temperature ranges at which the waste heat is available. The present communication reports the results for three different classes of substances: 1) a hydrocarbon (n-heptane); 2) two refrigerants (R245fa) and pentafluoro butane mixture (Solkatherm, SES36); and, 3) water in the intermediate temperature range (473 to 773 K) where the exhaust gases of combustion engines may be used as energy source for cogeneration. In this range it turns out that for many conditions, water and heptane are well suited working fluids for cogeneration systems. In the present investigation, the attention was not laid on the cycle efficiency alone, but also on the total exergy usage from an enthalpy stream (e.g. exhausts gas). This is taken into account while discussing the quality of the process. The results for different thermodynamic parameters and the surface area of the heat exchanger have been discussed. T-H. diagrams were also used for judging the suitability of a fluid. It turns out that water is well suited for many cases in the intermediate temperature regime.
    view abstractdoi: 10.5541/ijot.323
  • 2011 • 27 Liquid-crystalline elastomer microvalve for microfluidics
    Sánchez-Ferrer, A. and Fischl, T. and Stubenrauch, M. and Albrecht, A. and Wurmus, H. and Hoffmann, M. and Finkelmann, H.
    Advanced Materials 23 4526-4530 (2011)
    Microactuators are an essential component in microsystems or microdevices, and in applications that include artificial muscles, pumps, valves, or switchers. Liquid-crystalline elastomers are a new class of actuator material in the field of microsystem technologies, which can be used in standard processes. This newly developed actuator provides new possibilities in microfluidics because of its dimensional changes activated by the increase in temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adma.201102277
  • 2011 • 26 Multiple benzene-formation paths in a fuel-rich cyclohexane flame
    Li, W. and Law, M.E. and Westmoreland, P.R. and Kasper, T. and Hansen, N. and Kohse-Höinghaus, K.
    Combustion and Flame 158 2077-2089 (2011)
    Detailed data and modeling of cyclohexane flames establish that a mixture of pathways contributes to benzene formation and that this mixture changes with stoichiometry. Mole-fraction profiles are mapped for more than 40 species in a fuel-rich, premixed flat flame (φ=2.0, cyclohexane/O2/30% Ar, 30Torr, 50.0cm/s) using molecular-beam mass spectrometry with VUV-photoionization at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The use of a newly constructed set of reactions leads to an excellent simulation of this flame and an earlier stoichiometric flame (M.E. Law et al., Proc. Combust. Inst. 31 (2007) 565-573), permitting analysis of the contributing mechanistic pathways. Under stoichiometric conditions, benzene formation is found to be dominated by stepwise dehydrogenation of the six-membered ring with cyclohexadienyl⇄benzene+H being the final step. This finding is in accordance with recent literature. Dehydrogenation of the six-membered ring is also found to be a dominant benzene-formation route under fuel-rich conditions, at which H2 elimination from 1,3-cyclohexadiene contributes even more than cyclohexadienyl decomposition. Furthermore, at the fuel-rich condition, additional reactions make contributions, including the direct route via 2C3H3⇄benzene and more importantly the H-assisted isomerization of fulvene formed from i-/n-C4H5+C2H2, C3H3+allyl, and C3H3+C3H3. Smaller contributions towards benzene formation arise from C4H3+C2H3, 1,3-C4H6+C2H3, and potentially via n-C4H5+C2H2. This diversity of pathways is shown to result nominally from the temperature and the concentrations of benzene precursors present in the benzene-formation zone, which are ultimately due to the feed stoichiometry. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.03.014
  • 2011 • 25 Physical metallurgy and properties of β-solidifying TiAl based alloys
    Clemens, H. and Schmoelzer, T. and Schloffer, M. and Schwaighofer, E. and Mayer, S. and Dehm, G.
    Materials Research Society Symposium Proceedings 1295 95-100 (2011)
    In this paper, the physical metallurgy and properties of a novel family of high-strength γ-TiAl-based alloys is reviewed succinctly. These so-called TNM™ alloys contain Nb and Mo additions in the range of 3-7 atomic percent as well as small additions of B and C. For the definition of the alloy composition thermodynamic calculations using the CALPHAD method were conducted. The predicted phase transformation and ordering temperatures were verified by differential scanning calorimetry and in situ high-energy X-ray diffraction. TNM alloys solidify via the β-phase and exhibit an adjustable β-phase volume fraction at temperatures, where hot-working processes are performed. Due to the high volume fraction of β-phase these alloys can be processed isothermally as well as under near conventional conditions. In order to study the occurring deformation and recrystallization processes during hot-working, in situ diffraction experiments were conducted during compression tests at elevated temperatures. With subsequent heat-treatments a significant reduction of the β-phase is achieved. These outstanding features of TNM alloys distinguish them from other TiAl alloys which must exclusively be processed under isothermal conditions and/or which always exhibit a high fraction of β-phase at service temperature. After hot-working and multi-step heat-treatments, these alloys show yield strength levels > 800 MPa at room temperature and also good creep resistance at elevated temperatures. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.29
  • 2011 • 24 Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr-An approach based on natural analogue study
    Sengupta, P. and Fanara, S. and Chakraborty, S.
    Journal of Hazardous Materials 190 229-239 (2011)
    Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for 90Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite-wollastonite-gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO=35wt%, Al2O3=20wt%, SiO2=45wt%). We find that up to 30wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10-7gcm-2day-1 for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca2+ and Sr2+ were of the same order and bit higher compared to those calculated on the basis of Si4+ and Al3+. During accelerated leaching tests, zeolite and zeolite+epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ~10-15 to 10-13cm2/s at temperature intervals as high as 725-850°C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of 90Sr for various applications of radioactive Sr-pencils. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jhazmat.2011.03.031
  • 2011 • 23 Preparation of thermo-responsive polypropylene membranes via surface entrapment of poly(N-isopropylacrylamide)-containing macromolecules
    Guo, H. and Ulbricht, M.
    Journal of Membrane Science 372 331-339 (2011)
    Thermo-responsive polypropylene (PP) microfiltration membranes have been fabricated via surface entrapment of poly(N-isopropylacrylamide) (PNIPAAm)-containing homopolymer and block copolymers. A previously developed approach based on using solutions of the polymeric modifier in a solvent which swells the base membrane polymer had been used, and conditions have been varied. One block copolymer of PNIPAAm with polybutylacrylate (PBA) had been selected as best suited modifier. Models related to the underlying deswelling/entrapment process which leads to fixation of the modifier have been considered. For PP membrane, characterization of pore size distribution in dry state revealed a significant decrease of pore size as a consequence of the entrapment modification. Surface properties have been analysed by ATR-FTIR spectroscopy and water contact angle measurements, which confirmed the presence of modifier and a strong improvement of surface and pore wettability. The thermo-sensitive properties of either outer surface and inner pore wall of modified PP membranes have been verified by temperature-dependence of captive bubble contact angle and water permeability, respectively, both due to hydratation/dehydratation and volume phase transition of PNIPAAm around the lower critical solution temperature (LCST) of the block copolymer PBA- b-PNIPAAm which was around 31-32. °C. The effects of protein desorption from the modified membrane where bovine serum albumin (BSA) had been previously adsorbed were studied by measuring water flux upon manipulating water temperature during water filtration cum washing. In addition, non-porous PP plates had been modified using the same procedure and all the surface characterization results showed similar modification efficiency and surface properties as for porous PP membranes, confirming the dominating role of entrapment of the amphiphilic functional macromolecules into the PP surface layer instead of simple deposition onto the surface. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2011.02.018
  • 2011 • 22 Structure and phase behavior of archaeal lipid monolayers
    Jeworrek, C. and Evers, F. and Erlkamp, M. and Grobelny, S. and Tolan, M. and Chong, P.L.-G. and Winter, R.
    Langmuir 27 13113-13121 (2011)
    We report X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) measurements of archaeal bipolar tetraether lipid monolayers at the air-water interface. Specifically, Langmuir films made of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at three different temperatures, i.e., 68, 76, and 81 °C, were examined. The dependence of the structure and packing properties of PLFE monolayers on surface pressure were analyzed in a temperature range between 10 and 50 °C at different pH values. Additionally, the interaction of PLFE monolayers (using lipids derived from cells grown at 76 °C) with the ion channel peptide gramicidin was investigated as a function of surface pressure. A total monolayer thickness of approximately 30 Å was found for all monolayers, hinting at a U-shaped conformation of the molecules with both head groups in contact with the interface. The monolayer thickness increased with rising film pressure and decreased with increasing temperature. At 10 and 20 °C, large, highly crystalline domains were observed by GIXD, whereas at higher temperatures no distinct crystallinity could be observed. For lipids derived from cells grown at higher temperatures, a slightly more rigid structure in the lipid dibiphytanyl chains was observed. A change in the pH of the subphase had an influence only on the structure of the lipid head groups. The addition of gramicidin to an PLFE monolayer led to a more disordered state as observed by XRR. In GIXD measurements, no major changes in lateral organization could be observed, except for a decrease of the size of crystalline domains, indicating that gramicidin resides mainly in the disordered areas of the monolayer and causes local membrane perturbation, only. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la202027s
  • 2011 • 21 Studies on the cycling, processing and programming of an industrially applicable shape memory polymer Tecoflex® (or TFX EG 72D)
    Schmidt, C. and Chowdhury, A.M.S. and Neuking, K. and Eggeler, G.
    High Performance Polymers 23 300-307 (2011)
    The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles. © The Author(s) 2011.
    view abstractdoi: 10.1177/0954008311405245
  • 2011 • 20 Superheated rubber for cold storage
    Katzenberg, F. and Heuwers, B. and Tiller, J.C.
    Advanced Materials 23 1909-1911 (2011)
    Highly stretched rubber cools down upon relaxation. A natural rubber material that stores high elongations up to 1000% strain upon strain-induced crystallization at room temperature is reported. The strain recovered and, with this, the stored "cold" is released only by a thermal or athermal trigger. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adma.201100408
  • 2011 • 19 Supramolecular structures: Robust materials from weak forces
    Schmuck, C.
    Nature Nanotechnology 6 136-137 (2011)
    doi: 10.1038/nnano.2011.28
  • 2011 • 18 The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 3701-3710 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01875a
  • 2011 • 17 Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles
    Chamaani, A. and Marzbanrad, E. and Rahimipour, M. R. and Yaghmaee, M. S. and Aghaei, A. and Kamachali, R. D. and Behnamian, Y.
    Journal of Nanoparticle Research 13 6059--6067 (2011)
    In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1-100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1-5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.
    view abstractdoi: 10.1007/s11051-011-0258-6
  • 2011 • 16 Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    Nature Communications 2 (2011)
    Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1205
  • 2011 • 15 Unburned gas temperature measurements in a surrogate Diesel jet via two-color toluene-LIF imaging
    Tea, G. and Bruneaux, G. and Kashdan, J.T. and Schulz, C.
    Proceedings of the Combustion Institute 33 783-790 (2011)
    Non-intrusive temperature measurements of the unburned fuel/air mixture in vaporized Diesel jets have been performed using two-color toluene laser-induced fluorescence (LIF). This diagnostics technique exploits the temperature- dependent spectral shift of the LIF signal which occurs after ultraviolet (UV) excitation of toluene that is added as tracer to a non-fluorescing base fuel. The method requires the determination of the ratio of LIF intensities collected by two detectors separate spectral bands. In the current study, measurements were performed in a high-pressure, high-temperature cell capable of reproducing the thermodynamic conditions in the combustion chamber of a Diesel engine during the injection event. Various aspects of the experimental set-up and the data evaluation were optimized. The temperature sensitivity of the measurement strategy is optimum at temperatures below 700 K. Temperature data acquired from two-color LIF thermometry were compared to single-color toluene-LIF measurements using an adiabatic mixing model. The latter is determined from toluene LIF-based fuel concentration measurements, the evaporation enthalpy, and thermocouple measurements of the bath-gas/ambient cell temperature prior to fuel injection. Based on simultaneous measurements with two cameras using identical optical filters a methodology to optimize the image superposition and to minimize the statistical error was developed. These measurements also allowed to determine the 1 - σ precision of the two-color LIF measurement to be in the 20-40 K range. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2010.05.074
  • 2010 • 14 An efficient nickel catalyst for the reduction of carbon dioxide with a borane
    Chakraborty, S. and Zhang, J. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 132 8872-8873 (2010)
    Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja103982t
  • 2010 • 13 Artificial single variant martensite in freestanding Fe70Pd 30 films obtained by coherent epitaxial growth
    Bechtold, C. and Buschbeck, J. and Lotnyk, A. and Erkartal, B. and Hamann, S. and Zamponi, C. and Schultz, L. and Ludwig, Al. and Kienle, L. and Fähler, S. and Quandt, E.
    Advanced Materials 22 2668-2671 (2010)
    (Figure Presented) The mechanically soft behavior of the magnetic shape-memory material Fe70Pd30 allows huge tetragonal distortions to be stabilized in sputtered thin films by coherent epitaxial growth on various metallic buffers. Furthermore, it is demonstrated that epitaxial films more than 1 μm thick can be grown, which makes possible freestanding films in an artificial single variant state suitable for microactuators and sensors. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adma.201000599
  • 2010 • 12 Biocompatibility of nanoactuators: Stem cell growth on laser-generated nickel-titanium shape memory alloy nanoparticles
    Barcikowski, S. and Hahn, A. and Guggenheim, M. and Reimers, K. and Ostendorf, A.
    Journal of Nanoparticle Research 12 1733-1742 (2010)
    Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.
    view abstractdoi: 10.1007/s11051-009-9834-4
  • 2010 • 11 Compatible solutes: Thermodynamic properties and biological impact of ectoines and prolines
    Held, C. and Neuhaus, T. and Sadowski, G.
    Biophysical Chemistry 152 28-39 (2010)
    Compatible solutes like ectoine and its derivatives are deployed by halophile organisms as osmolytes to sustain the high salt concentration in the environment. This work investigates the relation of the thermodynamic properties of compatible solutes and their impact as osmolytes. The ectoines considered in this work are ectoine, hydroxyectoine, and homoectoine. Besides solution densities (15-45. °C) and solubilities in water (3-80. °C), component activity coefficients in the aqueous solutions were determined in the temperature range between 0 and 50. °C. The latter is important for adjusting a certain water activity and therewith a respective osmotic pressure within a cell. The characteristic effect of ectoines is compared to that of prolines, as well as to that of incompatible solutes as salts and urea. The experimental results show that the influence on the activity (coefficient) of water is quite different for compatible and incompatible solutes: whereas compatible solutes cause decreasing water activity coefficients, incompatible solutes lead to an increase in water activity coefficients. Based on this quantity, the paper discusses the impact of various osmolytes on biological systems and contributes to the explanation why some osmolytes are more often and at other temperatures used than others. Moreover, it was found that the anti-stress effect of an osmolyte is weakened in the presence of a salt.Finally, it is shown that the thermodynamic properties of compatible solutes can be modeled and even predicted using the thermodynamic model PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory). © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2010.07.003
  • 2010 • 10 Development and evaluation of a fluorescence microplate assay for quantification of heparins and other sulfated carbohydrates
    Lühn, S. and Schrader, T. and Sun, W. and Alban, S.
    Journal of Pharmaceutical and Biomedical Analysis 52 1-8 (2010)
    Due to their complex composition, quantification of heparins is difficult. On the one hand there are many biological tests, which only indirectly detect effects of the antithrombin-binding material. On the other hand direct quantitative methods are available but they are often insensitive, challenging, time-consuming or expensive. The aim of this study was to develop a sensitive, rapid, simple as well as inexpensive direct quantification assay suitable for routine analysis. Based on Polymer-H, a novel heparin complexing, fluorescent labeled synthetic polymer (λ(ex) 320 nm, λ(em) 510 nm), a microplate assay was developed and optimized. The specificity of the assay was evaluated by structure-assay response relationships studies using structurally defined glucan sulfates, heparins, and other natural and synthetic sulfated carbohydrates. The fluorescence intensity of Polymer-H (7.5 μg/ml) showed to be concentration-dependently amplified by heparins as well as by other sulfated carbohydrates. The best sensitivity, accuracy and linearity were observed in a range from 0.63 to 5.0 μg/ml heparins. No differences in the fluorescence between various heparins were observed, so that only one calibration curve is needed. In addition, all types of carbohydrates with a degree of sulfation (DS) &gt; ∼1.2 and a Mr &gt; 3000 can be quantified as well. By own calibration curves also other sulfated carbohydrates like fondaparinux or other glycosaminoglycans (DS &gt; 0.4) can be determined. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpba.2009.12.014
  • 2010 • 9 Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol
    Palkovits, R. and von Malotki, C. and Baumgarten, M. and Müllen, K. and Baltes, C. and Antonietti, M. and Kuhn, P. and Weber, J. and Thomas, A. and Schüth, F.
    ChemSusChem 3 277-282 (2010)
    The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900123
  • 2010 • 8 Elimination of Biological Contaminations from Surfaces by Plasma Discharges: Chemical Sputtering
    Rauscher, H. and Kylian, O. and Benedikt, J. and von Keudell, A. and Rossi, F.
    Chemphyschem 11 1382--1389 (2010)
    Plasma treatment of surfaces as a sterilisation or decontamination method is a promising approach to overcome limitations of conventional techniques. The precise characterisation of the employed plasma discharges, the application of sensitive surface diagnostic methods and targeted experiments to separate the effects of different agents, have led to rapid progress in the understanding of different relevant elementary processes. This contribution provides an overview of the most relevant and recent results, which reveal the importance of chemical sputtering as one of the most important processes for the elimination of biological residuals. Selected studies on the interaction of plasmas with bacteria, proteins and polypeptides are highlighted, and investigations employing beams of atoms and ions confirming the prominent role of chemical sputtering are presented. With this knowledge, it is possible to optimize the plasma treatment for decontamination/sterilisation purposes in terms of discharge composition, density of active species and UV radiation intensity.
    view abstractdoi: 10.1002/cphc.200900757
  • 2010 • 7 High-pressure SAXS study of folded and unfolded ensembles of proteins
    Schroer, M.A. and Paulus, M. and Jeworrek, C. and Krywka, C. and Schmacke, S. and Zhai, Y. and Wieland, D.C.F. and Sahle, C.J. and Chimenti, M. and Royer, C.A. and Garcia-Moreno, B. and Tolan, M. and Winter, R.
    Biophysical Journal 99 3430-3437 (2010)
    A structural interpretation of the thermodynamic stability of proteins requires an understanding of the structural properties of the unfolded state. High-pressure small-angle x-ray scattering was used to measure the effects of temperature, pressure, denaturants, and stabilizing osmolytes on the radii of gyration of folded and unfolded state ensembles of staphylococcal nuclease. A set of variants with the internal Val-66 replaced with Ala, Tyr, or Arg was used to examine how changes in the volume and polarity of an internal microcavity affect the dimensions of the native state and the pressure sensitivity of the ensemble. The unfolded state ensembles achieved for these proteins with high pressure were more compact than those achieved at high temperature, and were all very sensitive to the presence of urea and glycerol. Substitutions at the hydrophobic core detectably altered the conformation of the protein, even in the folded state. The introduction of a charged residue, such as Arg, inside the hydrophobic interior of a protein could dramatically alter the structural properties, even those of the unfolded state. The data suggest that a charge at an internal position can interfere with the formation of transient hydrophobic clusters in the unfolded state, and ensure that the pressure-unfolded form of a protein occupies the maximum volume possible. Only at high temperatures does the radius of gyration of the unfolded state ensemble approach the value for a statistical random coil. © 2010 by the Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2010.09.046
  • 2010 • 6 Influence of Ni on martensitic phase transformations in NiTi shape memory alloys
    Frenzel, J. and George, E.P. and Dlouhy, A. and Somsen, C. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 58 3444-3458 (2010)
    High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 [19] regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 [68] and the theory of Ball and James [25]. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.019
  • 2010 • 5 Micro- to nanostructured devices for the characterization of scaling effects in shape-memory thin films
    König, D. and Ehmann, M. and Thienhaus, S. and Ludwig, Al.
    Journal of Microelectromechanical Systems 19 1264-1269 (2010)
    Two microfabricated devices designed as test platforms for the investigation of scaling effects in micro- to nanosized substrate-attached shape-memory alloy (SMA) thin films as well as freestanding (suspended) thin-film microbridges are presented. These micromachined test platforms allow for simultaneous nanomechanical, electrical, and thermal tests on thin-film microbridges and can be seen as a basis for nanoscale SMA thin-film applications. The functionality of these devices is demonstrated for Ti 52 Ni32 Cu16 thin films as active material. The martensitic phase-transition temperatures for the thin films as substrate-attached or suspended microstructures as well as the dependence on the lateral dimensions were examined. It was found that decreasing the bridge width from 4 to 1 μm leads to a substantial and asymmetrical decrease of the phase-transition temperatures: 20% [austenite finish temperature (Af) and martensite start temperature (Ms)] and 80% [austenite start temperature (As
    view abstractdoi: 10.1109/JMEMS.2010.2067441
  • 2010 • 4 Probing the reactivity of ZnO and Au/ZnO nanoparticles by methanol adsorption: A TPD and DRIFTS study
    Kähler, K. and Holz, M.C. and Rohe, M. and Strunk, J. and Muhler, M.
    ChemPhysChem 11 2521-2529 (2010)
    The adsorption of methanol on pure ZnO and A--u-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H2peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJmol-1 and 127 kJmol-1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(101̄0) surface and on the highly reactive polar ZnO(0001̄) surface. The simultaneous formation of H2, CO, and CO2 at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO2 ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000282
  • 2010 • 3 Shape-dependent catalytic properties of Pt nanoparticles
    Mostafa, S. and Behafarid, F. and Croy, J.R. and Ono, L.K. and Li, L. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 132 15714-15719 (2010)
    Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106679z
  • 2010 • 2 Unique features of the folding landscape of a repeat protein revealed by pressure perturbation
    Rouget, J.-B. and Schroer, M.A. and Jeworrek, C. and Pühse, M. and Saldana, J.-L. and Bessin, Y. and Tolan, M. and Barrick, D. and Winter, R. and Royer, C.A.
    Biophysical Journal 98 2712-2721 (2010)
    The volumetric properties of proteins yield information about the changes in packing and hydration between various states along the folding reaction coordinate and are also intimately linked to the energetics and dynamics of these conformations. These volumetric characteristics can be accessed via pressure perturbation methods. In this work, we report high-pressure unfolding studies of the ankyrin domain of the Notch receptor (Nank1-7) using fluorescence, small-angle x-ray scattering, and Fourier transform infrared spectroscopy. Both equilibrium and pressure-jump kinetic fluorescence experiments were consistent with a simple two-state folding/unfolding transition under pressure, with a rather small volume change for unfolding compared to proteins of similar molecular weight. High-pressure fluorescence, Fourier transform infrared spectroscopy, and small-angle x-ray scattering measurements revealed that increasing urea over a very small range leads to a more expanded pressure unfolded state with a significant decrease in helical content. These observations underscore the conformational diversity of the unfolded-state basin. The temperature dependence of pressure-jump fluorescence relaxation measurements demonstrated that at low temperatures, the folding transition state ensemble (TSE) lies close in volume to the folded state, consistent with significant dehydration at the barrier. In contrast, the thermal expansivity of the TSE was found to be equivalent to that of the unfolded state, indicating that the interactions that constrain the folded-state thermal expansivity have not been established at the folding barrier. This behavior reveals a high degree of plasticity of the TSE of Nank1-7. © 2010 by the Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2010.02.044
  • 2010 • 1 Which controls the depolymerization of cellulose in ionic liquids: The solid acid catalyst or cellulose?
    Rinaldi, R. and Meine, N. and vom Stein, J. and Palkovits, R. and Schüth, F.
    ChemSusChem 3 266-276 (2010)
    Cellulose is a renewable and widely available feedstock. It is a biopolymer that is typically found in wood, straw, grass, municipal solid waste, and crop residues. Its use as raw material for biofuel production opens up the possibility of sustainable biorefinery schemes that do not compete with food supply. Tapping into this feedstock for the production of biofuels and chemicals requires-as the first-step-its depolymerization or its hydrolysis into intermediates that are more susceptible to chemical and/or biological transformations. We have shown earlier that solid acids selectively catalyze the depolymerization of cellulose solubilized in 1-butyl-3-methylimidazolium chloride (BMIMCl) at 100°C. Here, we address the factors responsible for the control of this reaction. Both cellulose and solid acid catalysts have distinct and important roles in the process. Describing the depolymerization of cellulose by the equivalent number of scissions occurring in the cellulosic chains allows a direct correlation between the product yields and the extent of the polymer breakdown. The effect of the acid strength on the depolymerization of cellulose is discussed in detail. Practical aspects of the reaction, concerning the homogeneous nature of the catalysis in spite of the use of a solid acid catalyst, are thoroughly addressed. The effect of impurities present in the imidazolium-based ionic liquids on the reaction performance, the suitability of different ionic liquids as solvents, and the recyclability of Amberlyst 15DRY and BMIMCl are also presented. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900281