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.

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  • 2020 • 163 Diffusion, defects and understanding the growth of a multicomponent interdiffusion zone between Pt-modified B2 NiAl bond coat and single crystal superalloy
    Esakkiraja, N. and Gupta, A. and Jayaram, V. and Hickel, T. and Divinski, S.V. and Paul, A.
    Acta Materialia 195 35-49 (2020)
    Composition-dependent diffusion coefficients are determined in B2-Ni(CoPt)Al system following the pseudo-binary and pseudo-ternary diffusion couple methods, which would not be possible otherwise in a quaternary inhomogeneous material fulfilling the conditions to solve the equations developed based on the Onsager formalism. The end-member compositions to produce ideal/near-ideal diffusion profiles are chosen based on thermodynamic details. The pseudo-binary interdiffusion coefficients of Ni and Al decrease in the presence of Co but increase in the presence of Pt. The pseudo-ternary interdiffusion coefficients indicate that the main interdiffusion coefficients increase significantly in the presence of Pt. Marginal changes of the cross interdiffusion coefficients substantiate a minor change of the diffusional interactions between the components. The thermodynamic driving forces show opposite trends with respect to composition as compared to the changes of the interdiffusion coefficients advocating a dominating role of the Pt(Co)-induced modifications of point defect concentrations. DFT-based calculations revealed that Pt alloying increases the Ni vacancy concentration and decreases the activation energy for the triple defect diffusion mechanism. These findings explain the increase in the thickness of the interdiffusion zone between the B2-Ni(Pt)Al bond coat and the single crystal superalloy René N5 because of Pt addition. Furthermore, the EPMA and TEM analyses reveal the growth of refractory elements-enriched precipitates. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.04.016
  • 2020 • 162 Are Onsager's reciprocal relations necessary to apply Thermodynamic Extremal Principles?
    Hackl, K. and Fischer, F.D. and Zickler, G.A. and Svoboda, J.
    Journal of the Mechanics and Physics of Solids 135 (2020)
    Onsager's Reciprocal Relations between thermodynamic forces and fluxes, for which Onsager was awarded the Nobel Prize, automatically follow from Thermodynamic Extremal Principles. Thus, the Principles are up to now non-applicable for the treatment of experimentally determined or theoretically modeled non-reciprocal systems as e.g. those in the magnetic field. However, we can demonstrate that adding of a certain barrier constraint as bilinear form of thermodynamic forces and fluxes accounted by the Thermodynamic Extremal Principles provides to non-reciprocal relations between the thermodynamic forces and fluxes. Such a novel idea may contribute to a better understanding of physics behind non-reciprocal systems. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2019.103780
  • 2020 • 161 Crystal structure and composition dependence of mechanical properties of single-crystalline NbCo2 Laves phase
    Luo, W. and Kirchlechner, C. and Zavašnik, J. and Lu, W. and Dehm, G. and Stein, F.
    Acta Materialia 184 151-163 (2020)
    Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.036
  • 2020 • 160 Composition dependence of hardness and elastic modulus of the cubic and hexagonal NbCo2 Laves phase polytypes studied by nanoindentation
    Luo, W. and Kirchlechner, C. and Li, J. and Dehm, G. and Stein, F.
    Journal of Materials Research 35 185-195 (2020)
    Regarding the effect of composition on the mechanical properties of intermetallic phases such as Laves phases, there is conflicting information in the literature. Some authors observed defect hardening when deviating from stoichiometric Laves phase composition, whereas others find defect softening. Here, we present a systematic investigation of the defect state, hardness, and elastic modulus of cubic and hexagonal NbCo2 Laves phases as a function of crystal structure and composition. For this purpose, diffusion couples were prepared which exhibit diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases, with concentration gradients covering their entire homogeneity ranges from 24 to 37 at.% Nb. Direct observations of dislocations and stacking faults in the diffusion layers as a function of composition were performed by electron channeling contrast imaging, and the hardness and elastic modulus were probed in the diffusion layers along the concentration gradients by nanoindentation. © 2020 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2019.384
  • 2020 • 159 Observations of grain-boundary phase transformations in an elemental metal
    Meiners, T. and Frolov, T. and Rudd, R.E. and Dehm, G. and Liebscher, C.H.
    Nature 579 375-378 (2020)
    The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4–6. The terminology ‘complexion’ was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8–19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20–24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [11 1 ¯] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27–29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2082-6
  • 2020 • 158 Random walk methods for Monte Carlo simulations of Brownian diffusion on a sphere
    Novikov, A. and Kuzmin, D. and Ahmadi, O.
    Applied Mathematics and Computation 364 (2020)
    This paper is focused on efficient Monte Carlo simulations of Brownian diffusion effects in particle-based numerical methods for solving transport equations on a sphere (or a circle). Using the heat equation as a model problem, random walks are designed to emulate the action of the Laplace–Beltrami operator without evolving or reconstructing the probability density function. The intensity of perturbations is fitted to the value of the rotary diffusion coefficient in the deterministic model. Simplified forms of Brownian motion generators are derived for rotated reference frames, and several practical approaches to generating random walks on a sphere are discussed. The alternatives considered in this work include projections of Cartesian random walks, as well as polar random walks on the tangential plane. In addition, we explore the possibility of using look-up tables for the exact cumulative probability of perturbations. Numerical studies are performed to assess the practical utility of the methods under investigation. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.amc.2019.124670
  • 2020 • 157 Moisture transport in PA6 and its influence on the mechanical properties
    Sharma, P. and Sambale, A. and Stommel, M. and Maisl, M. and Herrmann, H.-G. and Diebels, S.
    Continuum Mechanics and Thermodynamics 32 307-325 (2020)
    Various studies have reported the changes in the mechanical properties and the modification in morphology of polyamide due to the absorption of water. However, the relation between the local water content and the alteration in the properties has not been consolidated in a coupled model yet. In the current work, a simulation model is proposed that can capture the diffusion of water as well as simulate the effect of the local moisture content on the stiffness of polyamide (PA6). To this end, a finite element model was developed by coupling of a nonlinear diffusion model and a viscoelastic material model. The Galerkin finite element method was used to formulate the weak form of the equations for the two physical processes. The coupled nonlinear equations were solved with the help of the Newton method. The diffusion process was studied experimentally with the help of gravimetric measurements. Relaxation tests were conducted on the polyamide specimens that were saturated under different moisture levels. Based on these experimental results, the dependency of the material parameters on the local moisture content was identified and an efficient and stable numerical simulation model has been developed. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00161-019-00815-w
  • 2020 • 156 A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer — Part I: A priori and budget analyses
    Wen, X. and Rieth, M. and Scholtissek, A. and Stein, O.T. and Wang, H. and Luo, K. and Kempf, A.M. and Kronenburg, A. and Fan, J. and Hasse, C.
    Combustion and Flame 216 439-452 (2020)
    In this work, a comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer is conducted. At first, a priori analyses are conducted to evaluate the suitability of the premixed and non-premixed flamelet models for the studied pulverized coal flame with multiple combustion modes. Then, to clarify why a certain flamelet model does work or not work in certain regions, a more in-depth investigation of the premixed and non-premixed flamelet models is conducted through a budget analysis. The results show that the first and second derivatives in physical space can be well reproduced by the tabulated manifolds in trajectory variable space for both the premixed and non-premixed flamelet models, between which the non-premixed flamelet model performs slightly better. For the time derivative, large discrepancies can be observed, although the predicted variation trend overall follows the reference results. Through the analysis of the individual budget terms in the trajectory variable space, the individual trajectory variable's contributions to the convection and diffusion of thermo-chemical variables are quantified. Through the analysis of the individual budget terms for the sensible enthalpy and the CO mass fraction governing equations, the influences of the space transformation on the individual transport process (e.g., convection, diffusion, etc.) are clarified. Overall, the findings obtained from the budget analyses are consistent with those obtained from the a priori analyses. © 2019 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2019.05.046
  • 2019 • 155 Application of response surface methodology for optimization of hybrid friction diffusion bonding of tube-to-tube-sheet connections in coil-wound heat exchangers
    Alba, D.R. and Roos, A. and Wimmer, G. and Gonzalez, A.R. and Hanke, S. and Santos, J.F.D.
    Journal of Materials Research and Technology 8 1701-1711 (2019)
    This study evaluates the application of a new solid state joining process referred to as hybrid friction diffusion bonding. Based on heat processing and pressure, accelerated diffusion joins the materials. In the present study, two aluminum alloys were welded and characterized using leak tightness tests, tensile pull out tests, and metallographic analysis. Response surface methodology was used to optimize the tensile strength of single-hole tube-sheet samples. A Box-Behnken design was selected to evaluate the relations between the important process parameters and the ultimate tensile strength response to obtain optimal welding parameters. The data were analyzed with analysis of variance and were fitted to a second-order polynomial equation. The three-dimensional response surfaces derived from the mathematical models were applied to determine several optimum input parameters conditions. Under these conditions, the experimental ultimate tensile strength value was 202 MPa, which represents 95% of the base material strength. The experimental results obtained under optimum operating conditions were in agreement with the predicted values. Axial force was found to be the most significant factor affecting the joint strength followed by rotational speed. This can be attributed to their influence on the amount of mechanical energy introduced during the process, which is the parameter that primarily determines the joint strength. © 2019 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltd. Published by Elsevier Editora Ltd.
    view abstractdoi: 10.1016/j.jmrt.2018.11.012
  • 2019 • 154 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 • 153 Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials
    Chang, Y. and Lu, W. and Guénolé, J. and Stephenson, L.T. and Szczpaniak, A. and Kontis, P. and Ackerman, A.K. and Dear, F.F. and Mouton, I. and Zhong, X. and Zhang, S. and Dye, D. and Liebscher, C.H. and Ponge, D. and Korte-Ker...
    Nature Communications 10 (2019)
    Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-08752-7
  • 2019 • 152 Micromechanical modelling of coupled crystal plasticity and hydrogen diffusion
    Hassan, H.U. and Govind, K. and Hartmaier, A.
    Philosophical Magazine 99 92-115 (2019)
    Hydrogen transport behaviour in metals is greatly influenced by the mechanical stress and the underlying microstructural features. In this work, a micromechanical model based on coupled crystal plasticity and hydrogen diffusion is developed and applied to model hydrogen diffusion and storage in a polycrystalline microstructure. Particular emphasis is laid on mechanical influences on hydrogen transport, invoked by internal stresses and by trapping of dislocations generated by plastic strains. First, a study of a precharged material is carried out where hydrogen is allowed to redistribute under the influence of mechanical loading. These simulations demonstrate to which extent hydrogen migrates from regions with compressive strains to those with tensile strains. In the next step, the influence of plastic prestraining on hydrogen diffusion is analysed. This prestraining produces internal residual stresses in the microstructure, that mimic residual stresses introduced into components during cold working. Lastly, a series of permeation simulations is performed to characterise the influence of hydrogen trapping on effective diffusivity. It is shown that the effective diffusivity decreases with stronger traps and the effect is more prominent at a larger predeformation, because the trapped hydrogen concentration increases considerably. The reduction of effective diffusivity with plastic deformation agrees very well with experimental findings and offers a way to validate and parameterise our model. With this work, it is demonstrated how micromechanical modelling can support the understanding of hydrogen transport on the microstructural level. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/14786435.2018.1530466
  • 2019 • 151 Mass transport properties of quasiharmonic vs. anharmonic transition-metal nitrides
    Sangiovanni, D.G.
    Thin Solid Films 688 (2019)
    I present a development of the color-diffusion algorithm, used in non-equilibrium (accelerated) ab initio molecular dynamics simulations of point-defect migration in crystals [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)], to determine the temperature dependence of anion vacancy jump frequencies in rocksalt-structure (B1) TiN and VN characterized by quasiharmonic (TiN) vs. strongly anharmonic (VN) lattice dynamics. Over a temperature range [≈0.6·Tm < T < ≈0.9·Tm] relatively close to the materials melting points Tm, the simulations reveal that anion vacancy migration in TiN and VN exhibits an Arrhenius-like behavior, described by activation energies Ea TiN = 4.2 ± 0.3 eV and Ea VN = 3.1 ± 0.3 eV, and attempt frequencies νTiN = 8·1015±0.7 s−1 and νVN = 2·1017±0.8 s−1. A comparison of activation energies Ea extracted by Arrhenius linear regression at elevated temperatures with ab initio Ea0K values calculated at 0 Kelvin reveals that, while the nitrogen migration energy Ea TiN varies modestly with temperature {∆Ea TiN = [Ea(Tm) – Ea(0 K)]/Ea(0 K) ≈ 0.1}, the changes in Ea VN vs. T are considerable (∆Ea VN ≈ 1). The temperature-induced variations in vacancy migration energies and diffusivities are discussed in relation to the TiN and VN vibrational properties determined via ab initio molecular dynamics at different temperatures. It is argued that static 0-K calculations, which account for thermal expansion effects within the framework of quasiharmonic transition-state theory, accurately reproduce the finite-temperature mass transport properties of TiN. Conversely, the use of molecular dynamics simulations, which explicit treat lattice vibrations at any temperature of interest, is necessary to achieve reliable atomic diffusivities in B1 VN, a crystal phase dynamically stabilized by anharmonic vibrations [Mei et al., Phys. Rev. B 91, 054101 (2015)]. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.05.016
  • 2019 • 150 Hermeticity of SI1-XGEX Diaphragms for the Fabrication of a Capacitive Post-Cmos Pressure Sensor
    Walk, C. and Netaev, A. and Wiemann, M. and Gortz, M. and Vogt, H. and Mokwa, W. and Seidl, K.
    2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems and Eurosensors XXXIII, TRANSDUCERS 2019 and EUROSENSORS XXXIII 1862-1865 (2019)
    In this work, the hermeticity of diaphragm structures is investigated and optimized. The diaphragms are developed for the monolithic post-CMOS integration of capacitive pressure sensors. Si1-XGeX is used as diaphragm material and was deposited at temperatures below 400 °C.The hermeticity of the diaphragms was evaluated at a He pressure of 1800 hPa and in a temperature range from 50 °C to about 100 °C. The diffusion coefficients were determined by measuring the changes of diaphragm deflections due to He-diffusion inside the cavity.In the CVD process of Si1-XGeX cover layer on a polycrystalline p+Si1-XGeX diaphragm for closing the etch access holes, a variation of the SiH4 and GeH4 gas flows at a substrate temperature of about 380 °C was investigated regarding the selectivity of the layer growth on different surfaces (p+Si1-XGeX, Si, and SiO2). The selectivity of the layer growth against Si and SiO2 increases with the GeH4 ratio in the process gas flow. With a pure GeH4 gas flow, an optimisation of the parameters selectivity, He-diffusion and intrinsic stress of the Si1-XGeX cover layer was found. © 2019 IEEE.
    view abstractdoi: 10.1109/TRANSDUCERS.2019.8808725
  • 2018 • 149 Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers
    Colombara, D. and Werner, F. and Schwarz, T. and Cañero Infante, I. and Fleming, Y. and Valle, N. and Spindler, C. and Vacchieri, E. and Rey, G. and Guennou, M. and Bouttemy, M. and Manjón, A.G. and Peral Alonso, I. and Melchior...
    Nature Communications 9 (2018)
    Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03115-0
  • 2018 • 148 Numerical Benchmark of Phase-Field Simulations with Elastic Strains: Precipitation in the Presence of Chemo-Mechanical Coupling
    Darvishi Kamachali, R. and Schwarze, C. and Lin, M. and Diehl, M. and Shanthraj, P. and Prahl, U. and Steinbach, I. and Raabe, D.
    Computational Materials Science 155 541-553 (2018)
    Phase-field studies of solid-state precipitation under strong chemo-mechanical coupling are performed and benchmarked against the existing analytical solutions. The open source software packages OpenPhase and DAMASK are used for the numerical studies. Solutions for chemical diffusion and static mechanical equilibrium are investigated individually followed by a chemo-mechanical coupling effect arising due to composition dependence of the elastic constants. The accuracy of the numerical solutions versus the analytical solutions is quantitatively discussed. For the chemical diffusion benchmark, an excellent match, with a deviation <0.1%, was obtained. For the static mechanical equilibrium benchmark Eshelby problem was considered where a deviation of 5% was observed in the normal component of the stress, while the results from the diffuse interface (OpenPhase) and sharp interface (DAMASK) models were slightly different. In the presence of the chemo-mechanical coupling, the concentration field around a static precipitate was benchmarked for different coupling coefficients. In this case, it is found that the deviation increases proportional to the coupling coefficient that represents the strength of coupling concentration and elastic constants. Finally, the interface kinetics in the presence of the considered chemo-mechanical coupling were studied using OpenPhase and a hybrid OpenPhase–DAMASK implementation, replacing the mechanical solver of OpenPhase with DAMASK's. The observed deviations in the benchmark studies are discussed to provide guidance for the use of these results in studying further phase transformation models and implementations involving diffusion, elasticity and chemo-mechanical coupling effect. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2018.09.011
  • 2018 • 147 Grain boundary-constrained reverse austenite transformation in nanostructured Fe alloy: Model and application
    Huang, L. and Lin, W. and Wang, K. and Song, S. and Guo, C. and Chen, Y. and Li, Y. and Liu, F.
    Acta Materialia 154 56-70 (2018)
    Reverse austenite transformation (RAT) is critical for designing advanced high-strength steels (AHSS), which, however, has not been sufficiently studied in nanostructured (NS) steels or Fe alloys, and hence not fully understood yet. Herein, the RAT (e.g. ferrite to austenite) kinetics in the NS Fe alloy upon continuous heating was experimentally and theoretically investigated, where, the ultrafine austenite characterized by a sluggish growth velocity and a high thermal stability, and additionally, an appreciable solute partitioning detected using atom probe microscopy, indicate the diffusion-controlled mechanism of RAT. The double-edged role of grain boundaries (GBs) in the NS alloy is elucidated, i.e. enhancing the diffusivity due to the type-A kinetics, and simultaneously, facilitating the formation of constrained diffusion field mainly due to the segmented effect of GB nucleation. On this basis, a modified diffusion model incorporating the effect of GBs is implemented to understand the GB-constrained austenite growth and the associated partitioning behavior, and further complemented with Cahn model, an austenite growth model is applied to predict the overall kinetics of RAT in the NS Fe alloy. It then follows that a strategy by combination of diffusion-controlled growth model and microstructure model could serve as a framework to predict the kinetics of RAT in the NS alloys. Regarding the RAT kinetics in the NS alloys, the present work uncovers the ‘GB-constrained’ mechanism, which is expected to offer the potential application for nanostructure manipulation in the development of AHSS. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.021
  • 2018 • 146 Nano Impact Electrochemistry: Effects of Electronic Filtering on Peak Height, Duration and Area
    Kanokkanchana, K. and Saw, E.N. and Tschulik, K.
    ChemElectroChem 5 3000-3005 (2018)
    Nano impact electrochemistry is used to measure a transient signal while a nanoparticle (NP) hits an electrode due to its motion in a solution. A variety of information can be obtained from this current pulse, yet its accurate measurement is challenging due to its short duration (μs to s) and small amplitude (≤10 nA). A typically used low bandwidth low-pass filter can improve the signal-to-noise ratio, but it may cost severely in the accuracy of the data. Here, we demonstrate the effects of electronic filters by using generated current impulses with duration from 125 μs to 8 ms. Initially, a system dedicated to measure short and low current impulses was employed. There, an 8th order Bessel filter was used and the effect of varying the cut-off frequency between 50 Hz and 20 kHz on the impulse response is studied. Even though the charge is generally conserved by the filter, amplitude and duration of the pulse vary greatly in dependence of the cut-off frequency. In comparison, the response of widely used potentiostats was tested and significant deviations of the measured signal from the input were detected. Supported by destructive nano impact experiments with Ag NPs in KCl(aq), we show how the filtering affects the experimentally determined size of Ag NPs and Cl− diffusion coefficient, using impact charges and duration, respectively. As a result, we suggest a general guideline to researchers for accurate electrochemical nano impact measurements, in particular with respect to current peak duration analysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201800738
  • 2018 • 145 The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys
    Kontis, P. and Li, Z. and Collins, D.M. and Cormier, J. and Raabe, D. and Gault, B.
    Scripta Materialia 145 76-80 (2018)
    The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.10.005
  • 2018 • 144 Bioelectrocatalytic and electrochemical cascade for phosphate sensing with up to 6 electrons per analyte molecule
    Kopiec, G. and Starzec, K. and Kochana, J. and Kinnunen-Skidmore, T.P. and Schuhmann, W. and Campbell, W.H. and Ruff, A. and Plumeré, N.
    Biosensors and Bioelectronics 117 501-507 (2018)
    Despite the availability of numerous electroanalytical methods for phosphate quantification, practical implementation in point-of-use sensing remains virtually nonexistent because of interferences from sample matrices or from atmospheric O2. In this work, phosphate determination is achieved by the purine nucleoside phosphorylase (PNP) catalyzed reaction of inosine and phosphate to produce hypoxanthine which is subsequently oxidized by xanthine oxidase (XOx), first to xanthine and then to uric acid. Both PNP and XOx are integrated in a redox active Os-complex modified polymer, which not only acts as supporting matrix for the bienzymatic system but also shuttles electrons from the hypoxanthine oxidation reaction to the electrode. The bienzymatic cascade in this second generation phosphate biosensor selectively delivers four electrons for each phosphate molecule present. We introduced an additional electrochemical process involving uric acid oxidation at the underlying electrode. This further enhances the anodic current (signal amplification) by two additional electrons per analyte molecule which mitigates the influence of electrochemical interferences from the sample matrix. Moreover, while the XOx catalyzed reaction is sensitive to O2, the uric acid production and therefore the delivery of electrons through the subsequent electrochemical process are independent of the presence of O2. Consequently, the electrochemical process counterbalances the O2 interferences, especially at low phosphate concentrations. Importantly, the electrochemical uric acid oxidation specifically reports on phosphate concentration since it originates from the product of the bienzymatic reactions. These advantageous properties make this bioelectrochemical-electrochemical cascade particularly promising for point-of-use phosphate measurements. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2018.06.047
  • 2018 • 143 Phase-field modeling of pores and precipitates in polycrystalline systems
    Kundin, J. and Sohaib, H. and Schiedung, R. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    In this work, we develop an efficient phase-field approach to simulate the grain growth in polycrystalline ceramic materials in the presence of pores with various mobilities and diffusion coefficients. The multi-phase-field model is coupled to the Cahn-Hilliard equation for pore dynamics by interaction functions which describe the interaction of pores with grain boundaries. Two types of the model are suggested with one and two order parameters responsible for the pores. We also show that the model can be applied to the simulation of the interaction of the grain boundaries with coherent and non-coherent particles. The parameters of the model allow us to reproduce the equilibrium dihedral angle in the triple-junction of a pore or a particle and a grain boundary. A drag velocity of the grain boundary in the presence of pores or precipitates was also measured for various diffusion coefficients and grain boundary mobilities. The effects of the pore dynamics on the grain size evolution in ceramic materials was investigated and compared with reported theoretical predictions and experimental data. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aacb94
  • 2018 • 142 Influence of composition and crystal structure on the fracture toughness of NbCo2 Laves phase studied by micro-cantilever bending tests
    Luo, W. and Kirchlechner, C. and Fang, X. and Brinckmann, S. and Dehm, G. and Stein, F.
    Materials and Design 145 116-121 (2018)
    Cubic and hexagonal NbCo2 Laves phases are known to have composition dependent hardness and yield strength. However, it is unknown whether this dependence is also reflected in their fracture toughness values. In order to elucidate the fracture behavior, single-crystalline micro-cantilevers of the cubic and hexagonal NbCo2 Laves phases having different compositions were fabricated in the diffusion layers grown by the diffusion couple technique. Micro-cantilever bending tests were performed to study the composition- and crystal-structure-dependence of the fracture toughness. To exclude the influence of micro-cantilever geometry, pentagonal and rectangular beams were tested and found to result in the same fracture toughness value. The present results reveal that neither a change of the crystal structure nor a change in chemical composition has a significant influence on the fracture toughness of NbCo2 Laves phase. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.02.045
  • 2018 • 141 Simultaneous measurement of thermal conductivity and diffusivity of an undoped Al0.33Ga0.67As thin film epitaxially grown on a heavily Zn doped GaAs using spectrally-resolved modulated photothermal infrared radiometry
    Pawlak, M. and Pal, S. and Scholz, S. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 662 69-74 (2018)
    In this paper, we propose a method for measuring thermal and infrared properties of infrared transparent and semi-transparent thin film. We have investigated an undoped Al0.33Ga0.67As thin film epitaxially grown on a heavily Zn doped GaAs substrate using spectrally-resolved modulated photothermal infrared radiometry (SR-PTR). We perform supplementary measurements in order to determine values of layer thickness and infrared absorption coefficient and estimate successively values of the thermal conductivity and diffusivity of the Al0.33Ga0.67As thin layer, using the SR-PTR method. The obtained values of the thermal conductivity and diffusivity of the Al0.33Ga0.67As thin layer demonstrate that PTR method can be used for the thermal characterization of infrared transparent layers deposited on a highly infrared absorbing substrate. Supplementary Fourier Transform Infrared (FTIR) Spectroscopy measurements yield information only about the thickness of the Al0.33Ga0.67As layer. The results demonstrate that the SR-PTR method is a very good method for characterizing the thermal, geometrical and infrared properties of infrared-transparent thin film samples. However, some of the layer properties should be known a priori. It is worth emphasizing that the spectrally resolved measurements increase the reliability in estimating parameters of the thin layer by introducing additional channels of information. Finally, we conclude that the SR-PTR method combines features of infrared spectroscopic and calorimetric methods. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.tca.2018.02.009
  • 2018 • 140 Effective Diffusivity of Porous Materials with Microcracks: Self-Similar Mean-Field Homogenization and Pixel Finite Element Simulations
    Timothy, J.J. and Meschke, G.
    Transport in Porous Media 125 413-434 (2018)
    We investigate the influence of distributed microcracks on the overall diffusion properties of a porous material using the self-similar cascade continuum micromechanics model within the framework of mean-field homogenization and computational homogenization of diffusion simulations using a high-resolution pixel finite element method. In addition to isotropic, also anisotropic crack distributions are considered. The comparison of the results from the cascade continuum micromechanics model and the numerical simulations provides a deeper insight into the qualitative transport characteristics such as the influence of the crack density on the complexity and connectivity of crack networks. The analysis shows that the effective diffusivity for a disordered microcrack distribution is independent of the absolute length scale of the cracks. It is observed that the overall effective diffusivity of a microcracked material with the microcracks oriented in the direction of transport is not necessarily higher than that of a material with a random orientation of microcracks, independent of the microcrack density. © 2018, Springer Nature B.V.
    view abstractdoi: 10.1007/s11242-018-1126-y
  • 2018 • 139 Understanding the Enhancement of Surface Diffusivity by Dimerization
    Zaum, C. and Morgenstern, K.
    Physical Review Letters 121 (2018)
    Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2) meV, considerably smaller than the one for monomer diffusion, at (72±1) meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3) meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.185901
  • 2018 • 138 Production, deformation and mechanical investigation of magnetic alginate capsules
    Zwar, E. and Kemna, A. and Richter, L. and Degen, P. and Rehage, H.
    Journal of Physics Condensed Matter 30 (2018)
    In this article we investigated the deformation of alginate capsules in magnetic fields. The sensitivity to magnetic forces was realised by encapsulating an oil in water emulsion, where the oil droplets contained dispersed magnetic nanoparticles. We solved calcium ions in the aqueous emulsion phase, which act as crosslinking compounds for forming thin layers of alginate membranes. This encapsulating technique allows the production of flexible capsules with an emulsion as the capsule core. It is important to mention that the magnetic nanoparticles were stable and dispersed throughout the complete process, which is an important difference to most magnetic alginate-based materials. In a series of experiments, we used spinning drop techniques, capsule squeezing experiments and interfacial shear rheology in order to determine the surface Young moduli, the surface Poisson ratios and the surface shear moduli of the magnetically sensitive alginate capsules. In additional experiments, we analysed the capsule deformation in magnetic fields. In spinning drop and capsule squeezing experiments, water droplets were pressed out of the capsules at elevated values of the mechanical load. This phenomenon might be used for the mechanically triggered release of water-soluble ingredients. After drying the emulsion-filled capsules, we produced capsules, which only contained a homogeneous oil phase with stable suspended magnetic nanoparticles (organic ferrofluid). In the dried state, the thin alginate membranes of these particles were rather rigid. These dehydrated capsules could be stored at ambient conditions for several months without changing their properties. After exposure to water, the alginate membranes rehydrated and became flexible and deformable again. During this swelling process, water diffused back in the capsule. This long-term stability and rehydration offers a great spectrum of different applications as sensors, soft actuators, artificial muscles or drug delivery systems. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aaa6f5
  • 2017 • 137 Surface micro-patterning as a promising platform towards novel polyamide thin-film composite membranes of superior performance
    ElSherbiny, I.M.A. and Khalil, A.S.G. and Ulbricht, M.
    Journal of Membrane Science 529 11-22 (2017)
    Novel and efficient micro-patterned polyamide (PA) thin-film composite (TFC) membranes are successfully fabricated. Polyethersulfone support membranes are micro-patterned using two microfabrication methods, combined processes of vapor- and non-solvent-induced phase separation micro-molding, as well as micro-imprinting lithography. PA layer is successfully adapted on the developed micro-patterned supports and the impact on the membrane performance as a result of the difference in micro-patterning resolution is explored. The patterned PA TFC membranes exhibit superior water permeability, ~2–2.4 times compared to the flat PA TFC membranes, without sacrificing the membrane selectivity. This is mainly due to the distinguishable enhancement in membrane active surface area (~40–70%) and the increasing of the surface roughness upon micro-patterning. Furthermore, the concentration polarization analysis using different membrane orientations, with patterned grooves “parallel” and “perpendicular” to the direction of feed flow, and various feed concentrations is carried out. The results explicit the merits of implementing the micro-patterned TFC membranes in producing specific surface-induced mixing effects, which are found to reduce the concentration polarization, even at a high feed concentration. Moreover, the fidelity of the micro-patterning methods used in this work is comprehensively studied and different mechanisms for membrane surface patterning are proposed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.01.046
  • 2017 • 136 Diffusion across the glass transition in silicate melts: Systematic correlations, new experimental data for Sr and Ba in calcium-aluminosilicate glasses and general mechanisms of ionic transport
    Fanara, S. and Sengupta, P. and Becker, H.-W. and Rogalla, D. and Chakraborty, S.
    Journal of Non-Crystalline Solids 455 6-16 (2017)
    Viscosity and diffusivity of silicate melts and glasses are related to each other through relaxation timescales. The systematic is explored based on published data. Diffusion coefficients for Sr and Ba were measured in calcium aluminosilicate glasses at conditions near the glass/supercooled liquid boundary in temperature – time space making use of thin film technology and Rutherford Backscattering Spectroscopy (RBS) to measure concentration profiles on nanoscales. These data extend the range of published diffusion coefficients and combined with the systematic noted above allow the nature of change of diffusion coefficients across the glass transition region to be studied. Activation energies for diffusion in the glassy state (~ 360 kJ/mol) are higher than in the molten liquid (~ 213 kJ/mol). A defect based model of glass transition derived by Ojovan and coworkers, where attainment of a percolation threshold of configuron-type defects accounts for the glass – liquid transition, can explain the observed diffusion behaviour. Data treatment using this model yields a defect formation enthalpy of ~ 146 kJ/mol and a migration enthalpy of ~ 213 kJ/mol. The results of this study provide generalized expressions for the prediction of diffusion coefficients of cations in silicate melts for any composition at any temperature. © 2016
    view abstractdoi: 10.1016/j.jnoncrysol.2016.10.013
  • 2017 • 135 3D discrete dislocation dynamics study of creep behavior in Ni-base single crystal superalloys by a combined dislocation climb and vacancy diffusion model
    Gao, S. and Fivel, M. and Ma, A. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 102 209-223 (2017)
    A three-dimensional (3D) discrete dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of dislocation glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ′ precipitates. The vacancy diffusion model takes internal stresses by dislocations and mismatch strains into account and it is coupled to the dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of dislocation glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superdislocations, the dislocation glide in the γ matrix and the dislocation climb along the γ/γ′ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets dislocations further glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed dislocations exit and tend to climb. The increasing internal stresses due to the increasing dislocation density are easily overcome by dislocations under a high external stress that leads to a long-term dislocation glide accompanied by multiplication. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2017.02.010
  • 2017 • 134 A variational approach to the modelling of grooving in a three-dimensional setting
    Hackl, K. and Fischer, F.D. and Svoboda, J.
    Acta Materialia 129 331-342 (2017)
    We present a theory of thermal grooving, i.e. surface motion due to surface diffusion, based solely on geometrical and energetic arguments and a variational approach involving a thermodynamic extremal principle. The theory is derived for a fully three-dimensional setting. All interface and contact conditions at junction lines and points of the material aggregate are derived rigorously and without ambiguity. A finite element implementation of the model is employed. Numerical examples are presented and compared with experimental results from the literature. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.01.064
  • 2017 • 133 Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 48-58 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.12.048
  • 2017 • 132 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 131 Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Breitbach, B. and Dehm, G. and Scheu, C.
    Acta Materialia 133 356-366 (2017)
    Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.05.026
  • 2017 • 130 Transport mechanisms through PE-CVD coatings: Influence of temperature, coating properties and defects on permeation of water vapour
    Kirchheim, D. and Jaritz, M. and Mitschker, F. and Gebhard, M. and Brochhagen, M. and Hopmann, C. and Böke, M. and Devi, A. and Awakowicz, P. and Dahlmann, R.
    Journal of Physics D: Applied Physics 50 (2017)
    doi: 10.1088/1361-6463/aa511c
  • 2017 • 129 Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel
    Klein, S. and Mujica Roncery, L. and Walter, M. and Weber, S. and Theisen, W.
    Journal of Materials Science 52 375-390 (2017)
    The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic γ → α ′ or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 ∘C and correlated with the precipitation kinetics of cementite. To include processes in the short-term as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels. © 2016, The Author(s).
    view abstractdoi: 10.1007/s10853-016-0338-1
  • 2017 • 128 Investigation of local heat transfer in random particle packings by a fully resolved LBM-approach
    Kravets, B. and Kruggel-Emden, H.
    Powder Technology 318 293-305 (2017)
    Static particle/fluid systems occur in a wide range of technical processes in chemical industry and in energy technology. Direct numerical simulations (DNS) can be applied to model the fluid flow in particle packings and thus to examine heat and momentum transfer in those systems. Besides traditional CFD simulations, the Lattice-Boltzmann method (LBM) has been established as an alternative and efficient approach. Thermal LBM (TLBM) relies on a set of two distribution functions which represent the fluid flow and its internal energy and cover convection-diffusion problems. In the present study a D3Q19 model with a multiple-relaxation-time (MRT) collision model for density distributions and a Bhatnagar–Gross–Krook (BGK) collision model for energy distributions is used. The fluid-solid boundaries are represented by interpolated bounce-back methods. Numerical investigations are performed for single sphere and random particle packings. Particle averaged and local heat transfer coefficients are considered. Obtained results are compared against available state of the art correlations and recent numerical results from the literature. The results demonstrate the accuracy of the derived LBM approach. Particle averaged and local heat transfer coefficients in sphere packings are presented in a range of Rep = 20 − 100 and ε = 0.6 − 1. A correlation of local heat transfer coefficients in random sphere packings is derived. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2017.05.039
  • 2017 • 127 Gradient-based nodal limiters for artificial diffusion operators in finite element schemes for transport equations
    Kuzmin, D. and Shadid, J.N.
    International Journal for Numerical Methods in Fluids (2017)
    This paper presents new linearity-preserving nodal limiters for enforcing discrete maximum principles in continuous (linear or bilinear) finite element approximations to transport problems with steep fronts. In the process of algebraic flux correction, the oscillatory antidiffusive part of a high-order base discretization is decomposed into a set of internodal fluxes and constrained to be local extremum dim inishing. The proposed nodal limiter functions are designed to be continuous and satisfy the principle of linearity preservation that implies the preservation of second-order accuracy in smooth regions. The use of limited nodal gradients makes it possible to circumvent angle conditions and guarantee that the discrete maximum principle holds on arbitrary meshes. A numerical study is performed for linear convection and anisotropic diffusion problems on uniform and distorted meshes in two space dimensions. © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/fld.4365
  • 2017 • 126 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 • 125 Diffusion-related SOFC stack degradation
    Menzler, N.H. and Beez, A. and Grünwald, N. and Sebold, D. and Fang, Q. and Vaßen, R.
    ECS Transactions 78 2223-2230 (2017)
    As part of two different stack tests with four-plane short stacks and their intensive post-test characterization, two varying diffusionrelated degradation mechanisms were investigated. The first was a short-term test (∼1250h) with two different chromium evaporation protection layers on the air-side metallic interconnect and frame and the second was a long-term endurance test (∼ 35,000h). For the first stack, two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS), while the other two planes were coated with a manganese-cobalt-iron spinel layer by atmospheric plasma spraying (APS). The voltage loss in the planes with a WPS-coated interconnect was markedly higher than in those coated by means of APS. Finally, it was shown that the microstructure of the layers plays a key role in minimizing Cr evaporation. In this stack, gas-phase diffusion prevails over degradation. In the long-term stack, severe degradation due to solid-state manganese diffusion was observed. This paper draws an interaction hypothesis. © The Electrochemical Society.
    view abstractdoi: 10.1149/07801.2223ecst
  • 2017 • 124 On measurement of the thermal diffusivity of moderate and heavily doped semiconductor samples using modulated photothermal infrared radiometry
    Pawlak, M. and Panas, A. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 650 33-38 (2017)
    In this work, the accuracy of the thermal diffusivity estimation in moderately and heavily doped semiconductor samples using the modulated photothermal infrared radiometry is investigated. The studies were carried out on heavily doped Si and GaAs wafers, and on moderately doped Si and recently studied GaAs and CdSe samples. It is shown, that depending on the infrared properties of the semiconductor sample, the modulated photothermal infrared radiometry signal can yield information about thermal diffusivity, (effective) infrared absorption coefficient and electronic transport parameters (recombination lifetime, carrier diffusivity and surface recombination velocities). For the heavily doped samples, the modulated photothermal infrared radiometry signal consists only of the thermal response yielding information about the (effective) infrared absorption coefficient and thermal diffusivity. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case is about Ur = 0.05. For moderately doped samples the modulated photothermal infrared radiometry signal consists of the thermal and of the photocarrier response. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case varies between about Ur = 0.10 and about Ur = 0.30, depending on the existence of the maximum in the signal phase, but information about the electronic transport properties is derived. It is shown that not only infrared properties have the influence on the accuracy in estimating the thermal diffusivity of moderate doped semiconductor samples, but also the thermal, geometrical (thickness) and carrier recombination properties can play an important role. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.tca.2017.02.003
  • 2017 • 123 On the infrared absorption coefficient measurement of thick heavily Zn doped GaAs using spectrally resolved modulated photothermal infrared radiometry
    Pawlak, M. and Pal, S. and Ludwig, Ar. and Wieck, A.D.
    Journal of Applied Physics 122 (2017)
    In this paper, we report on measurements of the infrared absorption coefficient in the mid-infrared range of a heavily Zn-doped GaAs wafer using spectrally resolved modulated photothermal infrared radiometry (PTR). The method allows us to measure the infrared absorption coefficient of (i) much thicker samples as compared to the one used in Fourier Transform Infrared (FTIR) spectroscopy in transmission configuration and (ii) with non-mirror-like surfaces as would be required for measurements in the reflection configuration. From the best fits of the theoretical model to the PTR results, the values of the infrared absorption coefficient and thermal diffusivity of GaAs wafer are obtained. These values of infrared absorption coefficients are compared both with the literature values on very thin, similarly doped GaAs:Be sample and with infrared absorption coefficients calculated from FTIR specular reflectance measurements on the same sample. FTIR reflectance measurements demand additional assumptions for the evaluation of absorption coefficient and mirror-like surfaces. The results obtained from both experimental methods yield the same order of the infrared absorption coefficients. It is observed that the infrared absorption coefficient decreases with increasing wavelength because of inter-valence band transitions. However, only the infrared spectrum estimated using PTR exhibits free carrier absorption effect at a shorter wavelength as observed in previous works on very thin Be-doped GaAs samples. It is worth mentioning that the presented method is not limited to semiconductors, but can be used for other highly infrared absorbing samples. In addition, the spectrally resolved PTR measurements simultaneously provide the same values of thermal diffusivity of the GaAs wafer within estimation uncertainties thus demonstrating the reliability of the PTR method in the measurement of thermal diffusivity of such samples. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4989448
  • 2017 • 122 Growth and shape of indium islands on molybdenum at micro-roughened spots created by femtosecond laser pulses
    Ringleb, F. and Eylers, K. and Teubner, T. and Schramm, H.-P. and Symietz, C. and Bonse, J. and Andree, S. and Heidmann, B. and Schmid, M. and Krüger, J. and Boeck, T.
    Applied Surface Science 418 548-553 (2017)
    Indium islands on molybdenum coated glass can be grown in ordered arrays by surface structuring using a femtosecond laser. The effect of varying the molybdenum coated glass substrate temperature and the indium deposition rate on island areal density, volume and geometry is investigated and evaluated in a physical vapor deposition (PVD) process. The joined impact of growth conditions and spacing of the femtosecond laser structured spots on the arrangement and morphology of indium islands is demonstrated. The results yield a deeper understanding of the island growth and its precise adjustment to industrial requirements, which is indispensable for a technological application of such structures at a high throughput, for instance as precursors for the preparation of Cu(In,Ga)Se2 micro concentrator solar cells. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2016.11.135
  • 2017 • 121 A kinetic Monte Carlo approach to diffusion-controlled thermal desorption spectroscopy
    Schablitzki, T. and Rogal, J. and Drautz, R.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2017)
    Atomistic simulations of thermal desorption spectra for effusion from bulk materials to characterize binding or trapping sites are a challenging task as large system sizes as well as extended time scales are required. Here, we introduce an approach where we combine kinetic Monte Carlo with an analytic approximation of the superbasins within the framework of absorbing Markov chains. We apply our approach to the effusion of hydrogen from BCC iron, where the diffusion within bulk grains is coarse grained using absorbingMarkov chains, which provide an exact solution of the dynamics within a superbasin. Our analytic approximation to the superbasin is transferable with respect to grain size and elliptical shapes and can be applied in simulations with constant temperature as well as constant heating rate. The resulting thermal desorption spectra are in close agreement with direct kinetic Monte Carlo simulations, but the calculations are computationally much more efficient. Our approach is thus applicable to much larger system sizes and provides a first step towards an atomistic understanding of the influence of structural features on the position and shape of peaks in thermal desorption spectra. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2016.0404
  • 2017 • 120 Multi-phase-field model for surface and phase-boundary diffusion
    Schiedung, R. and Kamachali, R.D. and Steinbach, I. and Varnik, F.
    Physical Review E 96 (2017)
    The multi-phase-field approach is generalized to treat capillarity-driven diffusion parallel to the surfaces and phase boundaries, i.e., the boundaries between a condensed phase and its vapor and the boundaries between two or multiple condensed phases. The effect of capillarity is modeled via curvature dependence of the chemical potential whose gradient gives rise to diffusion. The model is used to study thermal grooving on the surface of a polycrystalline body. Decaying oscillations of the surface profile during thermal grooving, postulated by Hillert long ago but reported only in few studies so far, are observed and discussed. Furthermore, annealing of multi-nanoclusters on a deformable free surface is investigated using the proposed model. Results of these simulations suggest that the characteristic craterlike structure with an elevated perimeter, observed in recent experiments, is a transient nonequilibrium state during the annealing process. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.96.012801
  • 2017 • 119 Tailored protein encapsulation into a DNA host using geometrically organized supramolecular interactions
    Sprengel, A. and Lill, P. and Stegemann, P. and Bravo-Rodriguez, K. and Schöneweiß, E.-C. and Merdanovic, M. and Gudnason, D. and Aznauryan, M. and Gamrad, L. and Barcikowski, S. and Sanchez-Garcia, E. and Birkedal, V. and Gatso...
    Nature Communications 8 (2017)
    The self-organizational properties of DNA have been used to realize synthetic hosts for protein encapsulation. However, current strategies of DNA-protein conjugation still limit true emulation of natural host-guest systems, whose formation relies on non-covalent bonds between geometrically matching interfaces. Here we report one of the largest DNA-protein complexes of semisynthetic origin held in place exclusively by spatially defined supramolecular interactions. Our approach is based on the decoration of the inner surface of a DNA origami hollow structure with multiple ligands converging to their corresponding binding sites on the protein surface with programmable symmetry and range-of-action. Our results demonstrate specific host-guest recognition in a 1:1 stoichiometry and selectivity for the guest whose size guarantees sufficient molecular diffusion preserving short intermolecular distances. DNA nanocontainers can be thus rationally designed to trap single guest molecules in their native form, mimicking natural strategies of molecular recognition and anticipating a new method of protein caging. © 2017 The Author(s).
    view abstractdoi: 10.1038/ncomms14472
  • 2017 • 118 Mechanisms of oxygen permeation through plastic films and barrier coatings
    Wilski, S. and Wipperfürth, J. and Jaritz, M. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R. and Hopmann, C.
    Journal of Physics D: Applied Physics 50 (2017)
    Oxygen and water vapour permeation through plastic films in food packaging or other applications with high demands on permeation are prevented by inorganic barrier films. Most of the permeation occurs through small defects (&lt;3 μm) in the barrier coating. The defects were visualized by etching with reactive oxygen in a capacitively coupled plasma and subsequent SEM imaging. In this work, defects in SiOx-coatings deposited by plasma-enhanced chemical vapour deposition on polyethylene terephthalate (PET) are investigated and the mass transport through the polymer is simulated in a 3D approach. Calculations of single defects showed that there is no linear correlation between the defect area and the resulting permeability. The influence of adjacent defects in different distances was observed and led to flow reduction functions depending on the defect spacing and defect area. A critical defect spacing where no interaction between defects occurs was found and compared to other findings. According to the superposition principle, the permeability of single defects was added up and compared to experimentally determined oxygen permeation. The results showed the same trend of decreasing permeability with decreasing defect densities. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8525
  • 2016 • 117 An experimental study of ultrafiltration for sub-10nm quantum dots and sub-150 nm nanoparticles through PTFE membrane and Nuclepore filters
    Chen, S.-C. and Segets, D. and Ling, T.-Y. and Peukert, W. and Pui, D.Y.H.
    Journal of Membrane Science 497 153-161 (2016)
    Ultrafiltration techniques (pore size of membrane below 100nm) are widely used in chemical engineering, semiconductor, pharmaceutical, food and beverage industries. However, for small particles, which are more and more attracting interests, the pore size often does not correlate well with sieving characteristics of the ultra-membranes. This may cause serious issues during modeling and prediction of retention efficiencies. Herein, a series of liquid filtration experiments with unfavorable conditions were performed. PTFE membranes (50, 100nm) and Nuclepore filters (50, 400nm) were challenged with 1.7nm manganese doped ZnS and 6.6 nm ZnO quantum dots (QDs), 12.4, 34.4 and 50 nm Au and 150 nm SiO2 nanoparticles. For larger and medium sized particles, sieving and eventually pore blockage phenomena were observed. In comparison, for small QDs, a high initial retention efficiency (&gt;0.4) in both filters was monitored, followed by a reduced efficiency with ongoing particle loading. This high initial retention of small nanoparticles was attributed to diffusion deposition rather than to sieving since the ratio of pore size to particle size was significantly high (up to 58). Our experimental results allow a basic understanding of the deposition mechanism of small nanoparticles (diffusion vs. sieving) in different filter structures. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.09.022
  • 2016 • 116 Improved thermodynamic treatment of vacancy-mediated diffusion and creep
    Fischer, F.D. and Hackl, K. and Svoboda, J.
    Acta Materialia 108 347-354 (2016)
    Approximately a decade ago a new concept to describe the kinetics of one-phase solid state systems evolving by diffusion and activity of vacancies has been published by the authors. The concept is based on the Onsager-Ziegler Thermodynamic Extremal Principle (TEP). In course of the last decade several improvements and corrections have been performed, which justify an overworking of the concept. A short introduction of the TEP is followed by a detail investigation of the Gibbs energy and its rate as well as of dissipation and dissipation function due to multicomponent diffusion process coupled with vacancy activity provoking swelling/shrinkage and creep and thus internal stress state development. The application of TEP allows an exact derivation of driving forces for the coupled processes. The Manning theory of diffusion is applied and the derivation of evolution equations for all system parameters (site fractions, swelling/shrinkage and creep strain tensor) is provided. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.017
  • 2016 • 115 Analysis of liquid water formation in polymer electrolyte membrane (PEM) fuel cell flow fields with a dry cathode supply
    Gößling, S. and Klages, M. and Haußmann, J. and Beckhaus, P. and Messerschmidt, M. and Arlt, T. and Kardjilov, N. and Manke, I. and Scholta, J. and Heinzel, A.
    Journal of Power Sources 306 658-665 (2016)
    PEM fuel cells can be operated within a wide range of different operating conditions. In this paper, the special case of operating a PEM fuel cell with a dry cathode supply and without external humidification of the cathode, is considered. A deeper understanding of the water management in the cells is essential for choosing the optimal operation strategy for a specific system. In this study a theoretical model is presented which aims to predict the location in the flow field at which liquid water forms at the cathode. It is validated with neutron images of a PEM fuel cell visualizing the locations at which liquid water forms in the fuel cell flow field channels. It is shown that the inclusion of the GDL diffusion resistance in the model is essential to describe the liquid water formation process inside the fuel cell. Good agreement of model predictions and measurement results has been achieved. While the model has been developed and validated especially for the operation with a dry cathode supply, the model is also applicable to fuel cells with a humidified cathode stream. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2015.12.060
  • 2016 • 114 Bayesian prediction of crack growth based on a hierarchical diffusion model
    Hermann, S. and Ickstadt, K. and Müller, C.H.
    Applied Stochastic Models in Business and Industry 32 494-510 (2016)
    A general Bayesian approach for stochastic versions of deterministic growth models is presented to provide predictions for crack propagation in an early stage of the growth process. To improve the prediction, the information of other crack growth processes is used in a hierarchical (mixed-effects) model. Two stochastic versions of a deterministic growth model are compared. One is a nonlinear regression setup where the trajectory is assumed to be the solution of an ordinary differential equation with additive errors. The other is a diffusion model defined by a stochastic differential equation where increments have additive errors. While Bayesian prediction is known for hierarchical models based on nonlinear regression, we propose a new Bayesian prediction method for hierarchical diffusion models. Six growth models for each of the two approaches are compared with respect to their ability to predict the crack propagation in a large data example. Surprisingly, the stochastic differential equation approach has no advantage concerning the prediction compared with the nonlinear regression setup, although the diffusion model seems more appropriate for crack growth. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/asmb.2175
  • 2016 • 113 Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy
    Laplanche, G. and Volkert, U.F. and Eggeler, G. and George, E.P.
    Oxidation of Metals 85 629-645 (2016)
    Oxidation of the Cr20Mn20Fe20Co20Ni20 (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn2O3 with a thin inner Cr2O3 layer; at 700 and 800 °C it was mainly Mn2O3 with some Cr enrichment; at 900 °C it was Mn3O4. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11085-016-9616-1
  • 2016 • 112 Diffusion of small molecules in a shape memory polymer
    Marquardt, A. and Mogharebi, S. and Neuking, K. and Varnik, F. and Eggeler, G.
    Journal of Materials Science 51 9792-9804 (2016)
    The present work studies the diffusion of small molecules (acetone, ethanol, and water) in a shape memory polymer (SMP) of type Estane ETE 75DT3 (SMP-E), which represents a thermoplastic polyurethane. The work aims at providing background information on the chemical reaction between SMPs and small molecules which can limit the service life of SMP actuators operating in harsh chemical environments. Weight gain studies after immersion of plate specimens in liquid acetone, ethanol, and water yield data which can be assessed on the basis of analytical and numerical solutions of Fick’s second law. The diffusion coefficients which are obtained for 21, 30, and 40 °C in the present study scale as Dacetone &gt; Dethanol &gt; Dwater. The diffusion coefficients show Arrhenius types of temperature dependencies with apparent activation energies of 33 (acetone), 59 (ethanol), and 58 (water) kJ mol−1. The diffusion coefficients and the apparent activation energies obtained in the present work are in reasonable agreement with data which were reported for the reaction of the three small molecules with similar polymers in the literature. It is not straightforward to correlate differences in molecular mobility with individual physical properties. The Hansen solubility parameter (originally derived to explain solubility not mobility) qualitatively rationalizes the observed differences. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-016-0213-0
  • 2016 • 111 Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires
    Nematollahi, Gh.A. and Grabowski, B. and Raabe, D. and Neugebauer, J.
    Acta Materialia 111 321-334 (2016)
    A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.052
  • 2016 • 110 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 109 A micromechanics model for molecular diffusion in materials with complex pore structure
    Timothy, J.J. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 40 686-712 (2016)
    Molecular diffusion in fully saturated porous materials is strongly influenced by the pore space, which, in general, is characterized by a complex topological structure. Hence, information on macroscopic diffusion properties requires up-scaling of transport processes within nano-pores and micro-pores over several spatial scales. A new model in the framework of continuum micromechanics is proposed for predicting the effective molecular diffusivity in porous materials. Considering a representative volume element, characterizing a porous material without any information about the pore space microstructure complexity, the uniform flux is perturbed by recursively embedding shape information hierarchically in the form of the ESHELBY matrix-inclusion morphology to obtain the effective diffusivity as a function of the recurrence level and the porosity. The model predicts a threshold value for the porosity, below which no molecular diffusion can occur because of the presence of isolated pore clusters that are not connected and unavailable for transport. The maximum porosity, below which no molecular transport is possible, is predicted as one-third for spherical inclusions. The model allows for extensions to more complex morphologies of the inclusions. We also identify, that the effects of the micro-structure on molecular transport are characterized by porosity dependent long-range and short-range interactions. The developed framework is extended to incorporate realistic pore size distributions across several spatial scales by means of a distribution function within the hierarchical homogenization scheme. Available experimental results assert the model predictions. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nag.2423
  • 2016 • 108 Hydrogen diffusion and trapping in Ti-modified advanced high strength steels
    Winzer, N. and Rott, O. and Thiessen, R. and Thomas, I. and Mraczek, K. and Höche, T. and Wright, L. and Mrovec, M.
    Materials and Design 92 450-461 (2016)
    The influence of Ti on hydrogen diffusion and trapping in various advanced high strength steels was investigated. Electrochemical hydrogen permeation tests were performed on various model steels, with and without Ti, with benchmark tests performed using a commercial steel variant. The hydrogen trapping parameters for each steel were determined by fitting the permeation curves with a finite element model based on the McNabb and Foster equations using least squares minimisation. The influence of Ti on the hydrogen trapping parameters was greatly dependent on microstructure, with ferrite-containing grades being most affected. The results are inconsistent with hydrogen trapping by TiC particles, but consistent with trapping by boundaries between neighbouring ferrite and martensite grains. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.12.060
  • 2016 • 107 Experimental Evidence for a Three-Body Interaction between Diffusing CO Molecules
    Zaum, C. and Morgenstern, K.
    Nano Letters 16 3001-3004 (2016)
    The diffusion of carbon monoxide molecules on Cu(111) is investigated in time-lapsed scanning tunneling microscopy in a temperature range from 30 to 38 K. An asymptotic theory of adsorbate diffusion predicted a trio interaction that changes the diffusion barrier of three particles diffusing in close proximity beyond the change induced by the long-range interaction between three pairs of molecules. Distance-dependent variations in the diffusion energy confirm this theoretical prediction. In future, the theory can better assist experiments for a broader exploration, not only for diffusion, but also for nucleation and reaction. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.5b05212
  • 2016 • 106 Temperature calibration for diffusion experiments to sub-Kelvin precision
    Zaum, C. and Bertram, C. and Meyer Auf Der Heide, K.M. and Mehlhorn, M. and Morgenstern, K.
    Review of Scientific Instruments 87 (2016)
    Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4949484
  • 2016 • 105 Phase field modeling of intercalation kinetics: A finite interface dissipation approach
    Zerihun, N.A. and Steinbach, I.
    MRS Communications 6 270-282 (2016)
    When two materials interact, the processes between the phases determine the functional properties of the compound. Pivotal interface phenomena are diffusion and redistribution of atoms (molecules). This is especially of interest in Lithium ion batteries where the interfacial kinetics determines the battery performance and impact cycling stability. A new phase field model, which links the atomistic processes at the interface to the mesoscale transport by a redistribution flux controlled by the so called 'interface permeability' was developed. The model was validated with experimental data from diffusion couples. Calculations of the concentration profiles of the species at the electrode-electrolyte interface are reported. Active particle size, morphology and spatial arrangement were put in correlation with diffusion behavior for use in reverse engineering. © Materials Research Society 2016.
    view abstractdoi: 10.1557/mrc.2016.31
  • 2015 • 104 Electrochemical strain microscopy time spectroscopy: Model and experiment on LiMn2O4
    Amanieu, H.-Y. and Thai, H.N.M. and Luchkin, S.Yu. and Rosato, D. and Lupascu, D.C. and Keip, M.-A. and Schröder, J. and Kholkin, A.L.
    Journal of Applied Physics 118 (2015)
    Electrochemical Strain Microscopy (ESM) can provide useful information on ionic diffusion in solids at the local scale. In this work, a finite element model of ESM measurements was developed and applied to commercial lithium manganese (III,IV) oxide (LiMn<inf>2</inf>O<inf>4</inf>) particles. ESM time spectroscopy was used, where a direct current (DC) voltage pulse locally disturbs the spatial distribution of mobile ions. After the pulse is off, the ions return to equilibrium at a rate which depends on the Li diffusivity in the material. At each stage, Li diffusivity is monitored by measuring the ESM response to a small alternative current (AC) voltage simultaneously applied to the tip. The model separates two different mechanisms, one linked to the response to DC bias and another one related to the AC excitation. It is argued that the second one is not diffusion-driven but is rather a contribution of the sum of several mechanisms with at least one depending on the lithium ion concentration explaining the relaxation process. With proper fitting of this decay, diffusion coefficients of lithium hosts could be extracted. Additionally, the effect of phase transition in LiMn<inf>2</inf>O<inf>4</inf> is taken into account, explaining some experimental observations. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4927747
  • 2015 • 103 Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100)
    Antczak, G. and Kamiński, W. and Sabik, A. and Zaum, C. and Morgenstern, K.
    Journal of the American Chemical Society 137 14920-14929 (2015)
    We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b08001
  • 2015 • 102 Bioconjugated gold nanoparticles penetrate into spermatozoa depending on plasma membrane status
    Barchanski, A. and Taylor, U. and Sajti, C.L. and Gamrad, L. and Kues, W.A. and Rath, D. and Barcikowski, S.
    Journal of Biomedical Nanotechnology 11 1597-1607 (2015)
    Spermatozoa are not only essential for animal reproduction they also represent important tools for the manipulation of animal genetics. For instance, the genetic labeling and analysis of spermatozoa could provide a prospective complementation of pre-fertilization diagnosis and could help to prevent the inheritance of defective alleles during artificial insemination or to select beneficial traits in livestock. Spermatozoa feature extremely specialized membrane organization and restricted transport mechanisms making the labeling of genetically interesting DNA-sequences, e.g., with gold nanoparticles, a particular challenge. Here, we present a systematic study on the size-related internalization of ligand-free, monovalent and bivalent polydisperse gold nanoparticles, depending on spermatozoa membrane status. While monovalent conjugates were coupled solely to either negatively-charged oligonucleotides or positively-charged cell-penetrating peptides, bivalent conjugates were functionalized with both molecules simultaneously. The results clearly indicate that the cell membrane of acrosome-intact, bovine spermatozoa was neither permeable to ligand-free or oligonucleotide-conjugated nanoparticles, nor responsive to the mechanisms of cell-penetrating peptides. Interestingly, after acrosome reaction, which comprises major changes in sperm membrane composition, fluidity and charge, high numbers of monovalent and bivalent nanoparticles were found in the postequatorial segment, depicting a close and complex correlation between particle internalization and membrane organization. Additionally, depending on the applied peptide and for nanoparticle sizes <10 nm even a successive nuclear penetration was observed, making the bivalent conjugates promising for future genetic delivery and sorting issues. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jbn.2015.2094
  • 2015 • 101 First-principles investigation of hydrogen trapping and diffusion at grain boundaries in nickel
    Di Stefano, D. and Mrovec, M. and Elsässer, C.
    Acta Materialia 98 306-312 (2015)
    Abstract In this work, the interaction of hydrogen with high-angle GBs in nickel has been investigated by means of density functional theory simulations. Two distinct types of GBs have been considered: the Σ3(111)[1¯10] with a close-packed interface structure and the Σ5(210)[001] with a less dense interface structure consisting of open structural units. Our calculations reveal that these two GBs have a markedly different interaction behavior with atomic hydrogen. The close-packed Σ3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the Σ5 GB provides numerous trapping sites for H within the open structural units as well as easy migration pathways for H diffusion along the GB plane that can enhance the H diffusivity by about two orders of magnitude compared to bulk Ni. The obtained results are analysed in detail and compared with available experimental and other theoretical data. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.031
  • 2015 • 100 Modelling and evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride
    Drozdov, I.V. and Vaßen, R. and Stöver, D.
    RSC Advances 5 5363-5371 (2015)
    The 'shrinking core' model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for microand nano-powders. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ra08089k
  • 2015 • 99 Interplay of strain and interdiffusion in Heusler alloy bilayers
    Dutta, B. and Hickel, T. and Neugebauer, J. and Behler, C. and Fähler, S. and Behler, A. and Waske, A. and Teichert, N. and Schmalhorst, J.-M. and Hütten, A.
    Physica Status Solidi - Rapid Research Letters 9 321-325 (2015)
    Combining conventional and inverse magnetocaloric materials promises to enhance solid state refrigeration. As a first step here we present epitaxial Ni-Mn-Ga/Ni-Mn-Sn bilayer films. We examine the dependence of the lateral and normal lattice constants on the deposition sequence by combining experimental and ab initio techniques. Structural properties are determined with X-ray diffraction as well as highresolution transmission electron microscopy, while ab initio calculations explain the interplay of strain, local relaxations and the interdiffusion of atoms. The latter is confirmed by Auger electron spectroscopy and is expected to have a noticeable impact on the functional properties of the Heusler materials. ( © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510070
  • 2015 • 98 Diffusional nanoimpacts: The stochastic limit
    Eloul, S. and Kätelhön, E. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
    Journal of Physical Chemistry C 119 14400-14410 (2015)
    The probability expressions for the average number of diffusional impact events on a surface are established using Fick's diffusion in the limit of a continuum flux. The number and the corresponding variance are calculated for the case of nanoparticles impacting on an electrode at which they are annihilated. The calculations show the dependency on concentration in the limit of noncontinuous media and small electrode sizes for the cases of linear diffusion to a macroelectrode and of convergent diffusion to a small sphere. Using random walk simulations, we confirm that the variance follows a Poisson distribution for ultradilute and dilute solutions. We also present an average "first passage time" for the ultradilute solutions expression that directly relates to the lower limit of detection in ultradilute solutions as a function of the electrode size. The analytical expressions provide a straightforward way to predict the stochastics of impacts in a "nanoimpact" experiment by using Fick's second law and assuming a continuum dilute flux. Therefore, the study's results are applicable to practical electrochemical systems where the number of particles is very small but much larger than one. Moreover, the presented analytical expression for the variance can be utilized to identify effects of particle inhomogeneity in the solution and is of general interest in all studies of diffusion processes toward an absorbing wall in the stochastic limit. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b03210
  • 2015 • 97 Near conductive cooling rates in the upper-plutonic section of crust formed at the East Pacific Rise
    Faak, K. and Coogan, L.A. and Chakraborty, S.
    Earth and Planetary Science Letters 423 36-47 (2015)
    A new geospeedometer, based on diffusion modeling of Mg in plagioclase, is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast-spreading mid-ocean ridges. The investigated natural sample suites include gabbroic rocks formed at three different locations along the fast-spreading East Pacific Rise. These samples cover a depth interval of 0-840 m below the sheeted dike/gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined. We demonstrate that the cooling rates we obtained are robust (reproducible and consistent across different vertical sections at fast spreading ridges) and decrease significantly with increasing sample depth (covering almost 4 orders of magnitude, ranging from ~1 °C y-1 for the shallowest samples to 0.0003 °C y-1 for the deepest samples). Both the absolute cooling rates, and the rate of decrease of cooling rate with depth, are consistent with conductive thermal models. In contrast, the absolute cooling rates determined from the deeper samples (>300 m below DGB), and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off-axis hydrothermal circulation within the upper plutonic section of the crust. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.epsl.2015.04.025
  • 2015 • 96 Hydrogen diffusion and segregation in α iron ∑ 3 (111) grain boundaries
    Hamza, M. and Hatem, T.M. and Raabe, D. and El-Awady, J.A.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 9-2015 (2015)
    Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/IMECE2015-53118
  • 2015 • 95 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 • 94 Impact of bacterial endotoxin on the structure of DMPC membranes
    Nagel, M. and Brauckmann, S. and Moegle-Hofacker, F. and Effenberger-Neidnicht, K. and Hartmann, M. and De Groot, H. and Mayer, C.
    Biochimica et Biophysica Acta - Biomembranes 1848 2271-2276 (2015)
    Abstract Bacterial lipopolysaccharides are believed to have a toxic effect on human cell membranes. In this study, the influence of a lipopolysaccharide (LPS) from Escherichia coli on the structure, the dynamics and the mechanical strength of phospholipid membranes are monitored by nuclear magnetic resonance spectroscopy (NMR) and by atomic force microscopy (AFM). Model membranes are formed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and are either prepared as multilamellar bulk samples or multilamellar vesicles. Field gradient NMR data directly prove the rapid integration of LPS into DMPC membranes. Solid state NMR experiments primarily detect decreasing molecular order parameters with increasing LPS content. This is accompanied by a mechanical softening of the membrane bilayers as is shown by AFM indentation measurements. Altogether, the data prove that lipopolysaccharide molecules quickly insert into phospholipid bilayers, increase membrane fluctuation amplitudes and significantly weaken their mechanical stiffness. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.bbamem.2015.06.008
  • 2015 • 93 Single graphene nanoplatelets: Capacitance, potential of zero charge and diffusion coefficient
    Poon, J. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
    Chemical Science 6 2869-2876 (2015)
    Nano-impact chronoamperometric experiments are a powerful technique for simultaneously probing both the potential of zero charge (PZC) and the diffusion coefficient (D<inf>0</inf>) of graphene nanoplatelets (GNPs). The method provides an efficient general approach to material characterisation. Using nano-impact experiments, capacitative impacts can be seen for graphene nanoplatelets of 15 μm width and 6-8 nm thickness. The current transient features seen allow the determination of the PZC of the graphene nanoplatelet in PBS buffer as -0.14 ± 0.03 V (vs. saturated calomel electrode). The diffusion coefficient in the same aqueous medium, isotonic with many biological conditions, for the graphene nanoplatelets is experimentally found to be 2 ± 0.8 × 10-13 m2 s-1. This quick characterisation technique may significantly assist the application of graphene nanoplatelets, or similar nano-materials, in electronic, sensor, and clinical medicinal technologies. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5sc00623f
  • 2015 • 92 Modeling function–perfusion behavior in liver lobules including tissue, blood, glucose, lactate and glycogen by use of a coupled two-scale PDE–ODE approach
    Ricken, T. and Werner, D. and Holzhütter, H.G. and König, M. and Dahmen, U. and Dirsch, O.
    Biomechanics and Modeling in Mechanobiology 14 515-536 (2015)
    This study focuses on a two-scale, continuum multicomponent model for the description of blood perfusion and cell metabolism in the liver. The model accounts for a spatial and time depending hydro-diffusion–advection–reaction description. We consider a solid-phase (tissue) containing glycogen and a fluid-phase (blood) containing glucose as well as lactate. The five-component model is enhanced by a two-scale approach including a macroscale (sinusoidal level) and a microscale (cell level). The perfusion on the macroscale within the lobules is described by a homogenized multiphasic approach based on the theory of porous media (mixture theory combined with the concept of volume fraction). On macro level, we recall the basic mixture model, the governing equations as well as the constitutive framework including the solid (tissue) stress, blood pressure and solutes chemical potential. In view of the transport phenomena, we discuss the blood flow including transverse isotropic permeability, as well as the transport of solute concentrations including diffusion and advection. The continuum multicomponent model on the macroscale finally leads to a coupled system of partial differential equations (PDE). In contrast, the hepatic metabolism on the microscale (cell level) was modeled via a coupled system of ordinary differential equations (ODE). Again, we recall the constitutive relations for cell metabolism level. A finite element implementation of this framework is used to provide an illustrative example, describing the spatial and time-depending perfusion–metabolism processes in liver lobules that integrates perfusion and metabolism of the liver. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10237-014-0619-z
  • 2015 • 91 Comprehensive investigation of phase transformation during diffusion alloying of Nb-rich powder metallurgical tool steels
    Weddeling, A. and Krell, J. and Huth, S. and Theisen, W.
    Powder Metallurgy 58 142-151 (2015)
    Stainless tool steels highly alloyed in niobium can be produced by powder metallurgy using diffusion alloying. Steel powder atomised without carbon is subsequently mixed with graphite and hot isostatically pressed. The atomised powder contains the intermetallic Laves phase NbFe<inf>2</inf>that transforms into MC-type carbides during HIP when graphite has been added. The obtained structure features a fine distribution of carbides to increase wear resistance and chromium fully dissolved in the matrix to provide corrosion resistance. X-ray diffraction (XRD) measurements and reflection position analysis with additional scanning electron microscopy (SEM) have been conducted to study the phase transition of NbFe<inf>2</inf>Laves phase into NbC carbides in two high Nb alloyed stainless tool steels. The results show that carburisation starts at 1000-1050°C and also confirm the correlation between oxide reduction and carburisation. The formed carbides are distinctly understoichiometric, which leads to an overestimation of the suitable quantitiy of added carbon in the thermodynamic calculations. © 2015 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstractdoi: 10.1179/1743290115Y.0000000001
  • 2015 • 90 On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys
    Wollgramm, P. and Buck, H. and Neuking, K. and Parsa, A.B. and Schuwalow, S. and Rogal, J. and Drautz, R. and Eggeler, G.
    Materials Science and Engineering A 628 382-395 (2015)
    In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni-Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.010
  • 2015 • 89 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 • 88 Dynamic strain-induced transformation: An atomic scale investigation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Springer, H. and Raabe, D.
    Scripta Materialia 109 23-27 (2015)
    Phase transformations provide the most versatile access to the design of complex nanostructured alloys in terms of grain size, morphology, local chemical constitution etc. Here we study a special case of deformation induced phase transformation. More specifically, we investigate the atomistic mechanisms associated with dynamic strain-induced transformation (DSIT) in a dual-phased multicomponent iron-based alloy at high temperatures. DSIT phenomena and the associated secondary phase nucleation were observed at atomic scale using atom probe tomography. The obtained local chemical composition was used for simulating the nucleation process which revealed that DSIT, occurring during load exertion, proceeds by a diffusion-controlled nucleation process. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.07.010
  • 2014 • 87 Molecular basis for preventing α-synuclein aggregation by a molecular tweezer
    Acharya, S. and Safaie, B.M. and Wongkongkathep, P. and Ivanova, M.I. and Attar, A. and Klärner, F.-G. and Schrader, T. and Loo, J.A. and Bitan, G. and Lapidus, L.J.
    Journal of Biological Chemistry 289 10727-10737 (2014)
    Recent work on α-synuclein has shown that aggregation is controlled kinetically by the rate of reconfiguration of the unstructured chain, such that the faster the reconfiguration, the slower the aggregation. In this work we investigate this relationship by examining α-synuclein in the presence of a small molecular tweezer, CLR01, which binds selectively to Lys side chains. We find strong binding to multiple Lys within the chain as measured by fluorescence and mass-spectrometry and a linear increase in the reconfiguration rate with concentration of the inhibitor. Top-down mass-spectrometric analysis shows that the main binding of CLR01 to α-synuclein occurs at the N-terminal Lys-10/Lys- 12. Photo-induced cross-linking of unmodified proteins (PICUP) analysis shows that under the conditions used for the fluorescence analysis, α-synuclein ispredominantlymonomeric.Theresultscan be successfully modeled using a kineticschemein which two aggregation- pronemonomerscanformanencountercomplexthat leads to further oligomerization but can also dissociate back to monomers if the reconfiguration rate is sufficiently high.Takentogether, the data provide important insights into the preferred binding site of CLR01 on α-synuclein and the mechanism by which the molecular tweezer prevents self-assembly into neurotoxic aggregates by α-synuclein and presumably other amyloidogenic proteins. © 2014 by The American Society for Biochemistry and Molecular Biology, Inc.
    view abstractdoi: 10.1074/jbc.M113.524520
  • 2014 • 86 Bulk mixed ion electron conduction in amorphous gallium oxide causes memristive behaviour
    Aoki, Y. and Wiemann, C. and Feyer, V. and Kim, H.-S. and Schneider, C.M. and Ill-Yoo, H. and Martin, M.
    Nature Communications 5 (2014)
    In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x~1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms4473
  • 2014 • 85 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 • 84 Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Thin Solid Films 558 37-46 (2014)
    We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti island-atom ascent onto the island surface. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.02.053
  • 2014 • 83 A new Mg-in-plagioclase geospeedometer for the determination of cooling rates of mafic rocks
    Faak, K. and Coogan, L.A. and Chakraborty, S.
    Geochimica et Cosmochimica Acta 140 691-707 (2014)
    We present a new Mg-in-plagioclase geospeedometer based on the diffusive exchange of Mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined. The major element composition of plagioclase is shown to play an important role in driving the flux of Mg as well as the partitioning of Mg between plagioclase and clinopyroxene. Therefore, commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed. Practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the Mg concentration profile shapes are discussed. Propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates (e.g., log[dT/dt°Cyr-1]=-3±0.3 in one example). The approach is illustrated by application to two samples from the lower oceanic crust. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.gca.2014.06.005
  • 2014 • 82 Ab initio based understanding of the segregation and diffusion mechanisms of hydrogen in steels
    Hickel, T. and Nazarov, R. and McEniry, E.J. and Leyson, G. and Grabowski, B. and Neugebauer, J.
    JOM 66 1399-1405 (2014)
    A microscopic understanding of the processes that lead to hydrogen embrittlement is of critical importance for developing new generations of high-strength steels. With this article, we provide an overview of insights that can be gained from ab initio based methods when investigating the segregation and diffusion mechanisms of hydrogen in steels. We first discuss the solubility and diffusion behavior of hydrogen in the ferrite, austenite, and martensite phases. We consider not only defect-free bulk phases but also the influence of alloying elements and geometric defects such as vacancies and grain boundaries. In the second part, the behavior of hydrogen in the presence of precipitates, the solubility, the surface absorption, and the influence of hydrogen on the interface cohesion are studied. Finally, we provide simulation results for the interaction of hydrogen with dislocations. For all these applications, we will comment on advantages and shortcomings of ab initio methods and will demonstrate how the obtained data and insights can complement experimental approaches to extract general trends and to identify causes of hydrogen embrittlement. © 2014 The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1055-3
  • 2014 • 81 Constraints on the nature and evolution of the magma plumbing system of Mt. Etna volcano (1991-2008) from a combined thermodynamic and kinetic modelling of the compositional record of minerals
    Kahl, M. and Chakraborty, S. and Pompilio, M. and Costa, F.
    Journal of Petrology 56 2025-2068 (2014)
    Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive such dynamics are major goals of modern volcanology. Here we present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the nature and evolution of the plumbing system of Mt. Etna and the processes governing its internal dynamics. We investigated the compositional and temporal record preserved in 180 olivine crystals that were erupted between 1991 and 2008. Detailed systemization of the information stored in the sequential zoning record of the olivines reveals the existence of at least five compositionally different magmatic environments (MEs), characterized by different olivine compositions: M0 (Fo79-83), M1 (Fo75-78), M2 (Fo70-72), M3 (Fo65-69) and mm1 (Fo73-75). Several routes of magma transfer connect these environments. We identified three prominent magma passageways between the environments M0:M1, M2:mm1 and M1:M2 that were active during the entire period of observation between 1991 and 2008. Modelling the diffusive relaxation of the olivine zoning patterns reveals that the transfer of magma along such routes can occur over fairly heterogeneous timescales ranging from days to 2 years. Although some of the passageways have been sporadically active in the months and sometimes years before an eruption, the magma migration activity increases clearly in the weeks and days prior to an eruptive event. In this context, major transfer routes such as M2:mm1 might represent temporary passageways that are activated only shortly before eruptive events. A forward modelling approach was developed using thermodynamic calculations with the MELTS software to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79-83), rather than single compositions, are associated with thermodynamic parameters [pressure, temperature, water content, oxygen fugacity (fO2) and bulk composition of the melt] to identify the most plausible set corresponding to each ME. We found that temperature, water content and possibly oxidation state are the main distinguishing features of the different magmatic environments. The most primitive olivine population M0 (Fo79-83) and some of its associated clinopyroxenes formed at high melt water contents (3·5-5·2 wt %), at fO2 conditions buffered at quartz-fayalite-magnetite (QFM) or Ni-NiO (NNO), at temperatures ≥ 1110°C and at pressures ranging between 1·5 and 3·0 kbar (or higher). The intermediate population M1 (Fo75-78) can be produced over a broad spectrum of conditions, but all require similar temperatures and lower water contents (0·1-1·4 wt %). The most evolved, more Fe-rich olivines of M2 and M3 are products of melts with much lower water contants (0·2-1·1 wt % H2O for M2, < 0·5 wt % H2O for M3), and at probably somewhat more reducing (QFM) conditions at somewhat lower temperatures (~1080°C). The M3 environment characterized by very low water contents and reducing conditions could be related to an enhanced flux of CO2. Combination of the characteristics of the various magmatic environments with temporal information (residence time in different environments and timing of magma transfer between these environments) allows a dynamic model of the plumbing system beneath Mt. Etna to be constructed. © The Author 2015.
    view abstractdoi: 10.1093/petrology/egv063
  • 2014 • 80 DFT-supported phase-field study on the effect of mechanically driven fluxes in Ni4Ti3 precipitation
    Kamachali, R.D. and Borukhovich, E. and Hatcher, N. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Formation of the Ni4Ti3 precipitate has a strong effect on the shape memory properties of NiTi alloys. In this work, growth of this precipitate is studied using phase-field modelling and density functional theory (DFT) calculations. Using first-principles calculations, the composition-dependent stability and elastic properties of the B2 phase are obtained. Composition-dependent elastic constants are incorporated into our phase-field model to investigate the interplay between stress and concentration fields around the precipitate. The model introduces a source of diffusion due to mechanical relaxation which is accompanied by local softening/hardening of the B2 phase. The results are discussed in light of previous experimental and simulation studies. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034003
  • 2014 • 79 Anisotropy of electromigration-induced void and island drift
    Latz, A. and Sindermann, S.P. and Brendel, L. and Dumpich, G. and Meyer zu Heringdorf, F.-J. and Wolf, D.E.
    Journal of Physics Condensed Matter 26 (2014)
    By means of our novel self-learning kinetic Monte Carlo model (Latz et al 2012 J. Phys.: Condens. Matter 24 485005) we study the electromigration-induced drift of monolayer voids and islands on unpassivated surfaces of single crystalline Ag(111) and Ag(001) films at the atomic scale. Regarding the drift velocity, we find a non-monotonic size dependence for small voids. The drift direction is aligned with the electromigration force only along high symmetry directions, while halfway between, the angle enclosed by them is maximal. The magnitude of these directional deviations strongly depends on the system parameter, which are investigated in detail. The simulation results are in accordance with void motion observed in experiments performed on Ag(111). © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/5/055005
  • 2014 • 78 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
  • 2014 • 77 Timescales of disequilibrium melting in the crust: Constraints from modelling the distribution of multiple trace elements and a case study from the Lesser Himalayan rocks of Sikkim
    Neogi, S. and Bolton, E.W. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 168 1-22 (2014)
    A numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting. The model handles diffusion with moving boundaries and accounts for the processes of diffusion, dissolution and precipitation in a partially molten system. Among the various input parameters for modelling, source composition (i.e. modal abundance) variations, diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry. The other inputs such as melt reaction stoichiometry, kinetics of melting and grain size of protolith have lesser influence. Exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs, trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting, depending on the specific circumstances. After exploring some general features of melting in a pelitic system, the model is applied to calculate trace element distributions in migmatites from the Lesser Himalayan rocks in Sikkim, India. We focus on the distribution of trace elements during the initial stages of melt formation. These partially molten rocks show disequilibrium distribution of trace elements, and the numerical code is capable of quantitatively reproducing many of the observed patterns. The results of the modelling indicate that the observed melts in this zone were formed within 50,000 years and that segregation of melts (into leucosome and restite) was complete between 50,000 and 250,000 years. These short timescales may point to deformation-enhanced melt segregation at least on a hand specimen scale. It is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons-the latter, requiring higher degrees of melting and larger distances of migration, take longer. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00410-014-1020-8
  • 2014 • 76 Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces
    Raabe, D. and Herbig, M. and Sandlöbes, S. and Li, Y. and Tytko, D. and Kuzmina, M. and Ponge, D. and Choi, P.-P.
    Current Opinion in Solid State and Materials Science 18 253-261 (2014)
    Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as 'segregation engineering' solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cossms.2014.06.002
  • 2014 • 75 Growth mechanism of Al2Cu precipitates during in situ TEM heating of a HPT deformed Al-3wt.%Cu alloy
    Rashkova, B. and Faller, M. and Pippan, R. and Dehm, G.
    Journal of Alloys and Compounds 600 43-50 (2014)
    The microstructural evolution of Al2Cu precipitates in an ultrafine-grained Al-3wt.% Cu model alloy produced by high-pressure torsion (HPT) was studied by in situ transmission electron microscopy (TEM). The precipitation growth was systematically investigated by isothermal heating experiments in the temperature range of 120 C to 170 C. The experimental data is analysed with respect of the diffusion kinetics and activation energy to determine the most prominent diffusion path: lattice or grain boundary diffusion. The results imply that grain boundary diffusion is the relevant mechanism for Al2Cu growth in the HPT deformed material. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2014.02.090
  • 2014 • 74 Rheological properties of colloidal systems
    Rehage, H. and Willenbacher, N.
    Current Opinion in Colloid and Interface Science 19 501-502 (2014)
    doi: 10.1016/j.cocis.2014.10.005
  • 2014 • 73 Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations
    Sangiovanni, D.G. and Edströma, D. and Hultmana, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Surface Science 627 34-41 (2014)
    Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane b100N channels. b100N and b110N single jumps, as well as b100N double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea/ (kBT)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients Ds(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures ≤600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower Ds values (Ds corr) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D s corr (T)=(4.5 × 10-4 cm2 s-1) exp[.0.55 eV/ (kBT)]. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2014.04.007
  • 2014 • 72 Explanation of the local galactic cosmic ray energy spectra measured by Voyager 1. I. Protons
    Schlickeiser, R. and Webber, W.R. and Kempf, A.
    Astrophysical Journal 787 (2014)
    Almost exactly 100 yr after the original discovery of cosmic rays, the V1 spacecraft has observed, for the first time, the local interstellar medium energy spectra of cosmic ray H, He, C/O nuclei at nonrelativistic kinetic energies, after leaving the heliosphere modulation region on 2012 August 25. We explain these observations by modeling the propagation of these particles in the local Galactic environment with an updated steady-state spatial diffusion model including all particle momentum losses with the local interstellar gas (Coulomb/ionization, pion production, adiabatic deceleration, and fragmentation interactions). Excellent agreement with the V1 cosmic ray H observations is obtained if the solar system resides within a spatially homogeneous layer of distributed cosmic ray sources injecting the same momentum power law p -s with s = 2.24 ± 0.12. The best fit to the V1 H observations also provides an estimate of the characteristic break kinetic energy TC = 116 ± 27 MeV, representing the transition from ionization/Coulomb energy losses at low energies to pion production and adiabatic deceleration losses in a Galactic wind at high energies. As the determined value is substantially smaller than 217 MeV in the absence of adiabatic deceleration, our results prove the existence of a Galactic wind in the local Galactic environment. © 2014. The American Astronomical Society. All rights reserved..
    view abstractdoi: 10.1088/0004-637X/787/1/35
  • 2014 • 71 Vacancy mobility and interaction with transition metal solutes in Ni
    Schuwalow, S. and Rogal, J. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Interaction of Re, Ta, W and Mo solutes with vacancies and their diffusion in fcc Ni is investigated by density-functional theory in combination with kinetic Monte Carlo simulations. Interaction energies are calculated for the first six neighbor shells around the solutes and a complete set of diffusion barriers for these shells is provided. Further, diffusion coefficients for the four elements in Ni as well as for vacancies in the presence of these elements are calculated. The calculated solute diffusion coefficients based on our ab initio data are found to compare favorably to experimental values. The mobility of the vacancies as a key factor in dislocation climb is only minimally influenced by the solute atoms within the dilute limit. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/48/485014
  • 2014 • 70 High-temperature cooling histories of migmatites from the High Himalayan Crystallines in Sikkim, India: Rapid cooling unrelated to exhumation?
    Sorcar, N. and Hoppe, U. and Dasgupta, S. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 167 1-34 (2014)
    The High Himalayan Crystallines (HHCs) provide an excellent natural laboratory to study processes related to crustal melting, crustal differentiation, and the tectonic evolution of mountain belts because partial melting in these rocks occurred under well-defined tectonic boundary conditions (N-S collision of the Indian and the Eurasian plates) and the rocks have not been modified by subsequent metamorphic overprinting. We have used petrogenetic grids, kinetically constrained individual thermobarometry, pseudosection calculations, and reaction histories constrained by textural evidence to determine that the migmatites in the HHC of Sikkim attained peak P-T conditions of 750-800 °C, 9-12 kbar, followed by steep isothermal decompression to 3-5 kbar, and then isobaric cooling to ~600 °C. There may be a trend where rocks to the north [closer to the South Tibetan detachment system (STDS)] attained somewhat higher maximum pressures. The decompression may have been triggered by a reduction in density due to the production of melt (~20 vol%); minor amounts of additional melt may have been produced in individual packages of rock during decompression itself, depending on the exact geometry of the P-T path and the bulk composition of the rock. The stalling of rapid, isothermal exhumation at depths of 10-18 km (3-5 kbar) is related to metamorphic reactions that occur in these rocks. Geospeedometry indicates that at least a two-stage cooling history is required to describe the compositional zoning in all garnets. Both of these stages are rapid (several 100's °C/my between 800 and 600 °C, followed by several 10's °C/my between 600 and 500 °C), but there appears to be a spatial discontinuity in cooling history: Rocks to the south (closer to main central thrust) cooled more slowly than rocks to the north (closer to STDS). The boundary between these domains coincides with the discontinuity in age found in the same area by Rubatto et al. (Contrib Mineral Petrol 165:349-372, 2013). Combined with the information on petrologic phase relations, the data reveal the remarkable aspect that the rapid cooling and change of cooling rates all occurred after, rather than during, the rapid exhumation. This result underscores that high-temperature (e.g., >550 °C) cooling is a result of several processes in addition to exhumation and a one-to-one correlation of cooling and exhumation may sometimes be misplaced. Moreover, average cooling rates inferred from the closure temperatures of two isotopic systems should be interpreted judiciously in such nonlinearly cooling systems. While many aspects (e.g., isothermal decompression, isobaric cooling, duration of metamorphism, and cooling rates) of the pressure-temperature history inferred by us are consistent with the predictions of thermomechanical models that produce midcrustal channel flow, the occurrence of blocks with two different cooling histories within the HHC is not explained by currently available models. It is found that while exhumation may be initiated by surface processes such as erosion, the course of exhumation and its rate, at least below depths of ~15 km, is mostly controlled by a coupling between mechanical (density gain/loss) and chemical (metamorphic reactions) processes at depth. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00410-013-0957-3
  • 2014 • 69 Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 246 17-25 (2014)
    AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS=920°C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.02.056
  • 2013 • 68 Long-range segregation phenomena in shape-selected bimetallic nanoparticles: Chemical state effects
    Ahmadi, M. and Behafarid, F. and Cui, C. and Strasser, P. and Cuenya, B.R.
    ACS Nano 7 9195-9204 (2013)
    A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (< 200-270 C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn403793a
  • 2013 • 67 Solutal gradients in strained equilibrium
    Darvishi Kamachali, R. and Borukhovich, E. and Shchyglo, O. and Steinbach, I.
    Philosophical Magazine Letters 93 680-687 (2013)
    We demonstrate that the distortion of a crystal, caused by secondary phase precipitates, can stabilize a solutal gradient around the precipitate. The gradient persists in the quasi-static state stabilized by the gradient of the elastic energy around the precipitate. The peak concentration at the interface between precipitate and matrix hereby is independent of the radius of the precipitate and no mechanism of ripening is active in an arrangement of precipitates of different size. The model offers an explanation of experimental observations of the anomalous stability of nano-precipitates in Al-Cu. © 2013 Taylor & Francis.
    view abstractdoi: 10.1080/09500839.2013.847288
  • 2013 • 66 Mg in plagioclase: Experimental calibration of a new geothermometer and diffusion coefficients
    Faak, K. and Chakraborty, S. and Coogan, L.A.
    Geochimica et Cosmochimica Acta 123 195-217 (2013)
    The temperature-sensitive exchange of Mg between plagioclase (Pl) and clinopyroxene (Cpx) has been studied experimentally, accounting for different anorthite-contents in plagioclase (XAn) and various silica activities (aSiO2) in the system. The partitioning of Mg between plagioclase and clinopyroxene was determined over a temperature range of 1100-1200°C, using plagioclase single crystals of different compositions (XAn=0.5-0.8), surrounded by different clinopyroxene-bearing matrix powders to account for different silica activities from 0.55 to 1.0. The experimental design also allows the diffusivity (DMgPl) of Mg in plagioclase under these conditions to be determined. Both KMgPl/Cpx (defined as KMgPl/Cpx=CMgPl/CMgCpx) and DMgPl decrease with temperature and increase with aSiO2. Isothermal data for different XAn in plagioclase show a linear increase of lnKMgPl/Cpx with increasing XAn, but DMgPl appears to be insensitive to XAn. The partitioning data allow a new geothermometer to be calibrated, which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist:T[K]=(-9219+2034XAn)/(lnKMgPl/Cpx-1.6-lnaSiO2).Application of this geothermometer to experimental data from this study reproduces the experimental temperatures within ±20. °C. Diffusion of Mg in plagioclase is described by. DMgPl[m2s-1]=1.25×10-4[m2s-1]exp(-320,924[J mol-1]/(RT))(aSiO2)2.6. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.gca.2013.05.009
  • 2013 • 65 Electrochemical deposition of Co(Cu)/Cu multilayered nanowires
    Fedorov, F.S. and Mönch, I. and Mickel, C. and Tschulik, K. and Zhao, B. and Uhlemann, M. and Gebert, A. and Eckert, J.
    Journal of the Electrochemical Society 160 D13-D19 (2013)
    Electrochemical deposition in alumina templates is proved as a promising method for production of Co(Cu)/Cu nanowires showing high giant magnetoresistance (GMR). This study discusses the deposition of multilayered structures in highly ordered alumina templates in dependence on diffusion (Cu) or kinetically controlled (Co) behavior. Results show a high impact of spherical diffusion on the enhancement ofcurrent density of the diffusion controlled component compared todepositionof thin films on planar electrodes. To achieve a separation of the layers and to decrease the amount of Cu in the Co layer the deposition potential of Co was shifted toward more negative potentials. Lithographic structuring of the template surface was carried out to allow a 4-point measurement of the resistance. A high GMR of about 12% was obtained for [Co(Cu) (9 nm)/Cu (11 nm)]380 multilayered nanowires with high accuracy and reproducibility. © 2012 The Electrochemical Society.
    view abstractdoi: 10.1149/2.006302jes
  • 2013 • 64 H-chondrite parent asteroid: A multistage cooling, fragmentation and re-accretion history constrained by thermometric studies, diffusion kinetic modeling and geochronological data
    Ganguly, J. and Tirone, M. and Chakraborty, S. and Domanik, K.
    Geochimica et Cosmochimica Acta 105 206-220 (2013)
    We present a detailed thermometric study and cooling history analysis of selected H-chondrites from the petrologic types 4-6 on the basis of their mineralogical properties, and integrate these data with other available constraints on the cooling rates to develop a comprehensive model for the cooling, fragmentation and re-accretion history of the parent asteroid. Temperatures have been determined on the basis of two-pyroxene (2-Px) and spinel (Spnl)-orthopyroxene (Opx)/olivine (Ol) thermometers using the average of line scans and distributed spot analysis of coexisting pairs in each set. All of these minerals have been found to be compositionally homogeneous from ∼1 to 2. μm from the interface within the resolution of microprobe analysis. The thermometric results for the H5 (Allegan and Richardton) and H6 (Guarena and Kernouvé) samples are very similar. Also, while the 2-Px temperature increases by ∼90. °C from H4 to H5/6, a reverse trend is observed for the Spnl-Opx/Ol temperatures implying compositional resetting of these pairs during cooling. For the H4 sample (Forest Vale) all thermometric results are similar. The cooling rates calculated from numerical modeling of the compositional profiles in Opx-Cpx pairs in H5 and H6, corrected for the spatial averaging or convolution effect in microprobe analysis, are ∼25-100. °C/ky, which are 3-4 orders of magnitude higher than the cooling rates implied by in situ cooling in an onion-shell parent body model. Similar numerical simulation of the compositional profile in Opx-Spnl pair in H4 yields a cooling rate ∼50. °C/ky, which is in very good agreement with recent metallographic cooling rate of this sample and geochronological constraints on the cooling T-t path. Numerical simulation suggests that the slow cooling of the H5/6 samples at a rate of ∼15. °C/My, as deduced by recent metallographic study, could not have commenced at a temperature above ∼700. °C since, otherwise, the simulated compositional profile fails to match the observed profile. For the H5 samples, the T-t path constructed on the basis of Hf-W age of peak metamorphism and two stage cooling model satisfies the constraints imposed by the Pb-Pb ages of the phosphates and Ar-Ar ages of the feldspars vs. their respective closure temperatures, whereas that for H6 samples constructed using the same approach fails to satisfy these geochronological data. A second stage of even slower cooling at ∼3. °C/My needs to be invoked to satisfy the geochronological age vs. closure temperature relations. We present a model of fragmentation and re-accretion history of the parent body that could lead to the reconstructed T-t paths of the H-chondrite samples studied in this work, and discuss some of their broader implications. The cooling rates retrieved from the available data on the Fe-Mg ordering states of orthopyroxenes of some other H5 and H6 samples are orders of magnitude faster than the metallographic cooling rates that are applicable to similar low temperature domain. It is, thus, likely that all samples of the same petrologic type did not share a common cooling history. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.gca.2012.11.024
  • 2013 • 63 Modelling of organic-solvent flux through a polyimide membrane
    Hesse, L. and Mićović, J. and Schmidt, P. and Górak, A. and Sadowski, G.
    Journal of Membrane Science 428 554-561 (2013)
    The tremendous experimental effort to find a suitable organic solvent nanofiltration (OSN) membrane for a specific separation problem can be minimised by using model-based simulation tools to predict organic-solvents fluxes. A new predictive model, based on the Maxwell-Stefan (MS) approach, for prediction of fluxes through dense polymeric OSN membranes was developed to aid the experimental effort. Model parameters were determined from independent experiments to model solubilities and diffusivities. The flux measurements were performed using a cross-flow nanofiltration filtration unit LSta60LM (SIMA-tec) and the OSN membrane STARMEM240™. The calculated organic-solvents fluxes of toluene, ethyl acetate, ethanol, and 2-propanol show an excellent agreement with the measured fluxes at 25°C and different transmembrane pressures. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2012.10.052
  • 2013 • 62 Comparability of mobility particle sizers and diffusion chargers
    Kaminski, H. and Kuhlbusch, T.A.J. and Rath, S. and Götz, U. and Sprenger, M. and Wels, D. and Polloczek, J. and Bachmann, V. and Dziurowitz, N. and Kiesling, H.-J. and Schwiegelshohn, A. and Monz, C. and Dahmann, D. and Asbach, C.
    Journal of Aerosol Science 57 156-178 (2013)
    A large study on the comparability of various aerosol instruments was conducted. The study involved altogether 24 instruments, including eleven scanning, sequential and fast mobility particle sizers (five Grimm SMPS+C, three TSI SMPS and three FMPS) with different settings and differential mobility analyzers (DMAs), twelve instruments based on unipolar diffusion charging to determine size integrated concentrations and in some cases mean particle size (five miniDiSCs of the University of Applied Sciences and Arts Northwestern Switzerland, four Philips Aerasense nanoTracers, two TSI Nanoparticle Surface Area Monitors and one Grimm nanoCheck) and one TSI ultrafine condensation particle counter (UCPC). All instruments were simultaneously challenged with particles of various sizes, concentrations and morphologies. All measurement results were compared with those from a freshly calibrated SMPS for size distributions and the UCPC for number concentration. In general, all SMPSs showed good comparability with particularly the sizing agreeing to within a few percent. Differences in the determined number concentration were somewhat more pronounced, but the largest deviations could be tracked back to the use of an older software version. The comparability of the FMPSs was shown to be lower, with discrepancies on the order of ±25% for sizing and ±30% for total concentrations. The discrepancies between FMPSs and the internal reference SMPS seemed to be influenced by particle size and morphology. Total number and/or lung deposited surface area concentrations measured with unipolar diffusion charger based instruments generally agreed to within ±30% with the internal references (CPC for number concentrations; lung deposited surface area derived from SMPS measurements), as long as the particle size distributions of the test aerosols were within the specified limits for the instruments. When the upper size limit was exceeded, deviations of up to several hundred percent were detected. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jaerosci.2012.10.008
  • 2013 • 61 Influence of supersaturated carbon on the diffusion of Ni in ferrite determined by atom probe tomography
    Kresse, T. and Li, Y.J. and Boll, T. and Borchers, C. and Choi, P. and Al-Kassab, T. and Raabe, D. and Kirchheim, R.
    Scripta Materialia 69 424-427 (2013)
    In patented and cold-drawn pearlitic steel wires dissociation of cementite occurs during mechanical deformation. In this study the influence of the carbon decomposition on the diffusion of nickel in ferrite is investigated by means of atom probe tomography. In the temperature range 423-523 K we observed a much smaller activation energy of Ni diffusion than for self-diffusion in body-centered cubic iron, indicating an increased vacancy density owing to enhanced formation of vacancy-carbon complexes. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2013.05.039
  • 2013 • 60 Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W. and Fockenberg, T. and Chakraborty, S.
    Journal of Materials Science 48 348-358 (2013)
    The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-012-6752-0
  • 2013 • 59 Time-monitoring sensor based on oxygen diffusion in an indicator/polymer matrix
    Marek, P. and Velasco-Veléz, J.J. and Haas, T. and Doll, T. and Sadowski, G.
    Sensors and Actuators, B: Chemical 178 254-262 (2013)
    A time-monitoring sensor based on the oxidation of leuco methylene blue (LMB) to methylene blue (MB) was developed. The sensor changes its color from yellow to green in the presence of oxygen and was integrated into a poly(vinyl alcohol) matrix. The diffusion of the oxygen in the polymer matrix as well as the oxygen uptake due to the oxidation reaction determines the time monitoring of the sensor. A physical model has been developed that accounts for both the diffusion as well as the oxidation reaction. For this purpose, the reaction kinetics was determined experimentally. Moreover, the diffusion coefficient of oxygen was determined and concentration profiles in the polymer matrix modeled. Based on these modeling, the time sensor could be calibrated very precisely. This widely applicable, low-cost visual sensor is compatible with current technologies for the processing of plastics and can be integrated into different types of packaging, e.g. for application in freshness monitoring of consumer goods. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2012.12.028
  • 2013 • 58 Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source
    Matthiesen, C. and Geller, M. and Schulte, C.H.H. and Le Gall, C. and Hansom, J. and Li, Z. and Hugues, M. and Clarke, E. and Atatüre, M.
    Nature Communications 4 (2013)
    Resonance fluorescence in the Heitler regime provides access to single photons with coherence well beyond the Fourier transform limit of the transition, and holds the promise to circumvent environment-induced dephasing common to all solid-state systems. Here we demonstrate that the coherently generated single photons from a single self-assembled InAs quantum dot display mutual coherence with the excitation laser on a timescale exceeding 3 s. Exploiting this degree of mutual coherence, we synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field. In contrast to post-emission filtering, our technique avoids both photon loss and degradation of the single-photon nature for all synthesized waveforms. By engineering pulsed waveforms of single photons, we further demonstrate that separate photons generated coherently by the same laser field are fundamentally indistinguishable, lending themselves to the creation of distant entanglement through quantum interference. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2601
  • 2013 • 57 Toluene sorption in poly(styrene) and poly(vinyl acetate) near the glass transition
    Mueller, F. and Naeem, S. and Sadowski, G.
    Industrial and Engineering Chemistry Research 52 8917-8927 (2013)
    The sorption of vaporized toluene into poly(styrene) (PS) at 30 C and into poly(vinyl acetate) (PVAc) at 25 C were investigated. A Maxwell-Stefan (MS) diffusion model combined with a PC-SAFT equation of state and a mechanical spring-dashpot model was used to describe the experimental sorption profiles. The mechanical parameters of the glassy polymers, such as the Young's modulus of a neat glassy polymer and the reference viscosity at the glass transition, were determined via independent tensile creep measurements. The volatile organic compound (VOC)-concentration dependence of the viscosity was predicted using the William, Landel, and Ferry (WLF) and the Kelley-Bueche (KB) equations. The mechanical model successfully predicted the glass-transition temperature of the polymer/toluene system and the viscosity near and above the glass transition. The sorption isotherms for the pressure steps in the region of the glass transition were modeled by adjusting the MS diffusion coefficients and exhibited good agreement, both qualitative and quantitative, with the diffusion coefficients predicted by the free-volume theory. Thus, the developed diffusion model can be utilized to calculate the sorption profiles of VOCs in glassy polymers above, below, and, especially, near the glass transition. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ie302322t
  • 2013 • 56 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 • 55 Salty Water in KOH-Doped Hexagonal Ice: A Proton and Deuteron NMR Study
    Schildmann, S.a and Schöler, A.a and Nowaczyk, A.a and Geil, B.b and Böhmer, R.a
    Applied Magnetic Resonance 44 203-215 (2013)
    Water doped with 10-2 mol of KOH was cooled to temperatures at which most of the solution freezes to form hexagonal ice. Using proton and deuteron spin-lattice relaxometry as well as static field gradient diffusometry, it was found that a liquid-like phase coexists with the crystal down to below 200 K. The ionic dopants are expelled from the crystalline phase and form a KOH-enriched aqueous solution probably in the form of inclusions within the ice crystal. Its self-diffusion coefficient is only slightly smaller than that of nominally pure water. Motional correlation times were determined on the basis of spin-lattice relaxation times and compared with previous electrical conductivity and calorimetry results. © 2012 Springer-Verlag Wien.
    view abstractdoi: 10.1007/s00723-012-0414-x
  • 2013 • 54 Investigations on diffusion limitations of biocatalyzed reactions in amphiphilic polymer conetworks in organic solvents
    Schoenfeld, I. and Dech, S. and Ryabenky, B. and Daniel, B. and Glowacki, B. and Ladisch, R. and Tiller, J.C.
    Biotechnology and Bioengineering 110 2333-2342 (2013)
    The use of enzymes as biocatalysts in organic media is an important issue in modern white biotechnology. However, their low activity and stability in those media often limits their full-scale application. Amphiphilic polymer conetworks (APCNs) have been shown to greatly activate entrapped enzymes in organic solvents. Since these nanostructured materials are not porous, the bioactivity of the conetworks is strongly limited by diffusion of substrate and product. The present manuscript describes two different APCNs as nanostructured microparticles, which showed greatly increased activities of entrapped enzymes compared to those of the already activating membranes and larger particles. We demonstrated this on the example of APCN particles based on PHEA-l-PDMS loaded with α-Chymotrypsin, which resulted in an up to 28,000-fold higher activity of the enzyme compared to the enzyme powder. Furthermore, lipase from Rhizomucor miehei entrapped in particles based on PHEA-l-PEtOx was tested in n-heptane, chloroform, and substrate. Specific activities in smaller particles were 10- to 100-fold higher in comparison to the native enzyme. The carrier activity of PHEA-l-PEtOx microparticles was tenfold higher with some 25-50-fold lower enzyme content compared to a commercial product. Biotechnol. Bioeng. 2013; 110:2333-2342. © 2013 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.24906
  • 2013 • 53 Phase-field model for microstructure evolution at the mesoscopic scale
    Steinbach, I.
    Annual Review of Materials Research 43 89-107 (2013)
    This review presents a phase-field model that is generally applicable to homogeneous and heterogeneous systems at the mesoscopic scale. Reviewed first are general aspects about first- and second-order phase transitions that need to be considered to understand the theoretical background of a phase field. The mesoscopic model equations are defined by a coarse-graining procedure from a microscopic model in the continuum limit on the atomic scale. Special emphasis is given to the question of how to separate the interface and bulk contributions to the generalized thermodynamic functional, which forms the basis of all phase-field models. Numerical aspects of the discretization are discussed at the lower scale of applicability. The model is applied to spinodal decomposition and ripening in Ag-Cu with realistic thermodynamic and kinetic data from a database. © Copyright © 2013 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-071312-121703
  • 2013 • 52 Diffusion in fracturing porous materials: Characterizing topological effects using cascade micromechanics and phase-field models
    Timothy, J.J. and Meschke, G.
    Poromechanics V - Proceedings of the 5th Biot Conference on Poromechanics 2250-2259 (2013)
    The diffusion properties of fracturing porous materials, such as concrete or geological materials, are strongly influenced by the complex and random topological structure of the pore space, the state of distributed micro-cracks inevitably caused by processes such as autogenous and drying shrinkage of concrete, and finally by propagating cracks caused by various loading conditions. Information on macroscopic diffusion properties of the porous material requires up-scaling of transport processes within nano- and micro-pores over several spatial scales. The macroscopic transport coefficients are computed using a cascade continuum micromechanics model recently proposed by the authors. The cascade continuum micromechanics model recursively embeds shape information in the form of the ESHELBY matrix-inclusion problem to obtain the homogenized effective diffusion coefficient. The model is able to predict mathematically and physically consistent percolation thresholds. To consider the effects of oriented, diffusely distributed micro-cracks on the diffusion properties, the homogenization scheme for in intact concrete is enhanced by representing micro-cracks as additional ellipsoidal inclusions within the aforementioned homogenized porous matrix. Finally, the effect of propagating macro-cracks on the diffusion process is taken into consideration by weakly coupling the diffusion model and a fracture energy based staggered phase-field model to simulate brittle fracture. © 2013 American Society of Civil Engineers.
    view abstractdoi: 10.1061/9780784412992.264
  • 2013 • 51 Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 226 130-139 (2013)
    Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. The coated steel substrates were austenitized at 920. °C for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth; thus having a negative effect on the mechanical properties of the steel. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.03.045
  • 2012 • 50 Coarsening phenomena of metal nanoparticles and the influence of the support pre-treatment: Pt/TiO 2(110)
    Behafarid, F. and Roldan Cuenya, B.
    Surface Science 606 908-918 (2012)
    One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO 2(110). The TiO 2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO 2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~ 3 nm after annealing in UHV at 1060 °C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO 2, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO 2 after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.01.022
  • 2012 • 49 Multicomponent diffusion in garnets I: General theoretical considerations and experimental data for Fe-Mg systems
    Borinski, S.A. and Hoppe, U. and Chakraborty, S. and Ganguly, J. and Bhowmik, S.K.
    Contributions to Mineralogy and Petrology 164 571-586 (2012)
    We have carried out a combined theoretical and experimental study of multicomponent diffusion in garnets to address some unresolved issues and to better constrain the diffusion behavior of Fe and Mg in almandine-pyrope-rich garnets. We have (1) improved the convolution correction of concentration profiles measured using electron microprobes, (2) studied the effect of thermodynamic non-ideality on diffusion and (3) explored the use of a mathematical error minimization routine (the Nelder-Mead downhill simplex method) compared to the visual fitting of concentration profiles used in earlier studies. We conclude that incorporation of thermodynamic non-ideality alters the shapes of calculated profiles, resulting in better fits to measured shapes, but retrieved diffusion coefficients do not differ from those retrieved using ideal models by more than a factor of 1.2 for most natural garnet compositions. Diffusion coefficients retrieved using the two kinds of models differ only significantly for some unusual Mg-Mn-Ca-rich garnets. We found that when one of the diffusion coefficients becomes much faster or slower than the rest, or when the diffusion couple has a composition that is dominated by one component (&gt;75 %), then profile shapes become insensitive to one or more tracer diffusion coefficients. Visual fitting and numerical fitting using the Nelder-Mead algorithm give identical results for idealized profile shapes, but for data with strong analytical noise or asymmetric profile shapes, visual fitting returns values closer to the known inputs. Finally, we have carried out four additional diffusion couple experiments (25-35 kbar, 1,260-1,400 °C) in a piston-cylinder apparatus using natural pyrope- and almandine-rich garnets. We have combined our results with a reanalysis of the profiles from Ganguly et al. (1998) using the tools developed in this work to obtain the following Arrhenius parameters in D = D 0 exp{-[Q 1bar + (P-1)ΔV +]/RT} for D Mg * and D Fe *: Mg: Q 1bar = 228.3 ± 20.3 kJ/mol, D 0 = 2.72 (±4.52) × 10 -10 m 2/s, Fe: Q 1bar = 226.9 ± 18.6 kJ/mol, D 0 = 1.64 (±2.54) × 10 -10 m 2/s. ΔV + values were assumed to be the same as those obtained by Chakraborty and Ganguly (1992). © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s00410-012-0758-0
  • 2012 • 48 In-Situ TEM Straining Experiments: Recent Progress in Stages and Small-Scale Mechanics
    Dehm, G. and Legros, M. and Kiener, D.
    In-Situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science 227-254 (2012)
    doi: 10.1002/9783527652167.ch10
  • 2012 • 47 Antiferromagnetic coupling in combined Fe/Si/MgO/Fe structures with controlled interface diffusion
    Gareev, R. and Stromberg, F. and Buchmeier, M. and Keune, W. and Back, C. and Wende, H.
    Applied Physics Express 5 (2012)
    We study antiferromagnetic coupling and interface diffusion in Fe/Si/MgO/Fe structures grown by molecular beam epitaxy. The Fe/Si/Fe samples with a 1.2-nm-thick Si spacer demonstrate antiferromagnetic coupling J 1 ∼- 1:5 mJ/m 2 and prevailing interdiffusion at the top Si/Fe interface, as revealed by conversion electron Mössbauer spectroscopy. For combined Si/MgO spacers with 0.9-nm-thick Si, interdiffusion continuously reduces upon changing the MgO thickness from 0.3 to 0.5nm accompanied by a decrease of antiferromagnetic coupling from |J 1| ∼ 1 mJ/m2 to |J 1| ∼ 0:002mJ/m 2. We emphasize that monolayer-scaled engineering of insulating spacers is a promising tool for the precise control of antiferromagnetic coupling and interface diffusion. © 2012 The Japan Society of Applied Physics.
    view abstractdoi: 10.1143/APEX.5.033003
  • 2012 • 46 Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions
    Gutsche, C. and Niepelt, R. and Gnauck, M. and Lysov, A. and Prost, W. and Ronning, C. and Tegude, F.-J.
    Nano Letters 12 1453-1458 (2012)
    Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204126n
  • 2012 • 45 Characterizing permeability and stability of microcapsules for controlled drug delivery by dynamic NMR microscopy
    Henning, S. and Edelhoff, D. and Ernst, B. and Leick, S. and Rehage, H. and Suter, D.
    Journal of Magnetic Resonance 221 11-18 (2012)
    Microscopic capsules made from polysaccharides are used as carriers for drugs and food additives. Here, we use NMR microscopy to assess the permeability of capsule membranes and their stability under different environmental conditions. The results allow us to determine the suitability of different capsules for controlled drug delivery. As a measure of the membrane permeability, we monitor the diffusion of paramagnetic molecules into the microcapsules by dynamic NMR microimaging. We obtained the diffusion coefficients of the probe molecules in the membranes and in the capsule core by comparing the measured time dependent concentration maps with numerical solutions of the diffusion equation. The results reveal that external coatings strongly decrease the permeability of the capsules. In addition, we also visualized that the capsules are stable under gastric conditions but dissolve under simulated colonic conditions, as required for targeted drug delivery. Depending on the capsule, the timescales for these processes range from 1 to 28 h. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jmr.2012.05.009
  • 2012 • 44 VOC sorption in glassy polyimides-Measurements and modeling
    Hesse, L. and Naeem, S. and Sadowski, G.
    Journal of Membrane Science 415-416 596-607 (2012)
    The sorption behavior of volatile organic compounds (VOCs) in polymeric membrane materials such as polyimide, has a strong effect on the separation efficiency of dense polymeric nanofiltration membranes. To investigate the sorption behavior, this work presents gravimetrically measured and modeled sorption profiles for a series of VOCs (like n-hexane, ethanol, toluene, 2-propanol, and ethyl acetate) in two polyimides (P84 and Matrimid) at 25 °C.The experimental results show significant differences in the sorption speed of the VOCs in the two polyimides. The sorption of toluene, ethyl acetate, and ethanol in Matrimid is very fast compared to the sorption of 2-propanol, but the sorption in P84 is very slow for all the VOCs studied. The sorption behavior of the polyimide/VOC systems has been modeled using an improved one-dimensional Maxwell-Stefan diffusion model. The measured and modeled results provide detailed insight into the specific sorption behavior of the VOCs in the glassy polyimides P84 and Matrimid. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2012.05.054
  • 2012 • 43 Current induced surface diffusion on a single-crystalline silver nanowire
    Kaspers, M.R. and Bernhart, A.M. and Bobisch, C.A. and Möller, R.
    Nanotechnology 23 (2012)
    Scanning tunnelling microscopy was used to study the morphological changes of the surface of a single-crystalline silver nanowire caused by a lateral electron current. At current densities of about 1.5×10 7 A cm -2, surface atoms are extracted from step edges, resulting in the motion of surface steps, islands and holes with a thickness or depth of one monolayer. Upon current reversal the direction of the material transport can be altered. The findings are interpreted in terms of the wind force. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/23/20/205706
  • 2012 • 42 Computational fluid dynamics based stochastic aerosol modeling: Combination of a cell-based weighted random walk method and a constant-number Monte-Carlo method for aerosol dynamics
    Kruis, F.E. and Wei, J. and van der Zwaag, T. and Haep, S.
    Chemical Engineering Science 70 109-120 (2012)
    No method is currently available to combine stochastic, particle-based PBE modeling by means of Monte-Carlo simulation of individual particles and CFD. CFD is based on solving numerically partial differential equations, whereas Monte-Carlo simulation of the PBE bases on converting kinetic rate equations into probabilities and selecting the relevant events by means of random numbers. A joint mathematical framework is thus missing. The goal of this work is to develop a method which allows combining Monte-Carlo based PBE modeling with a CFD model. As a first step towards this goal, a Weighted Random Walk (WRW) method to simulate the particle transport due to convection and diffusion is developed. The simulation particles have no exact position as in Lagrangian particle tracking methods but belong to a CFD cell. The movement of the simulation particles in space is performed by calculation of the transition probability into the neighboring cells and the use of random numbers to simulate the particle transport into these cells. As the particle number concentration can be very different in different regions of the simulated reactor volume, we introduce here also a weighting method which allows fixing the number of simulation particles per cell. The WRW method is combined with a relatively simple constant-number MC method allowing to simulate stochastically the dynamic evolution of the particle population. Four different validative case studies of increasing complexity are performed, comparing the simulation results with those of a CFD-based moment model. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ces.2011.10.040
  • 2012 • 41 Pulsed-field gradient NMR measurements on hydrogels from phosphocholine
    Linders, J. and Mayer, C. and Sekine, T. and Hoffmann, H.
    Journal of Physical Chemistry B 116 11459-11465 (2012)
    Gels from diacylphosphatidylcholine in glycerol/butylene glycol mixtures were investigated by pulsed-field gradient NMR measurements. Previous measurements had shown that the gels are formed by networks from crystalline multilamellar vesicles (MLV). The obtained self-diffusion coefficients for water and butylene glycol molecules indicate that both molecules occur in two different environments, even at temperatures above the phase transition T m where the system is still in a liquid crystalline state. While the larger fraction of the molecules shows a free self-diffusion process like in a homogeneous phase, the smaller fraction seems to be encapsulated in closed domains and undergoes only hindered self-diffusion. It is concluded that the hindered diffusions are due to the solvent molecules trapped between the bilayers of the multilamellar vesicles, while the free diffusion is assigned to the solvent molecules outside of the MLV. Since the fraction of the entrapped molecules does not change during phase transition, we assume that the structure of the network in the samples remains the same when gelation occurs. The gelation process is simply due to the transformation of the vesicle bilayers from the liquid crystalline to the crystalline state. The permeability of the bilayer for the solvent molecules is drastically changed by this transition. The exchange of water molecules through the bilayers slows down significantly below Tm: while the average residence time of water molecules inside the vesicles is smaller than 50 ms in the liquid crystalline state, this value increases to more than 1 s for the gel state. In the case of pure butylene glycol, no vesicles are present, and it is likely that these gels are formed from crystalline fibers. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp3046565
  • 2012 • 40 Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
    Rudolf, D. and La-O-Vorakiat, C. and Battiato, M. and Adam, R. and Shaw, J.M. and Turgut, E. and Maldonado, P. and Mathias, S. and Grychtol, P. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Kapteyn, H.C. and Murnane, M...
    Nature Communications 3 (2012)
    Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro-or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2029
  • 2012 • 39 Diffusion coefficients for some organic and organometallic compounds using quartz crystal microbalance
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Thermochimica Acta 532 152-158 (2012)
    The diffusion coefficient data for organometallic substances are scarce in the literature. A recently developed quartz crystal microbalance (QCM) system has been used to measure the binary diffusion coefficients. The system consists of a QCM with a driving oscillator circuit, a closed Stefan tube, a temperature controlled unit, a data logger, and a frequency counter. The QCM crystal is placed on the top of a Stefan tube and its active surface is coated with a thin layer of the substance. On the other end of the Stefan tube highly adsorptive charcoal powder is placed. Thus a mass concentration gradient is established in the diffusion tube. The rate of mass loss from the QCM was then used to determine the binary diffusion coefficient in air. Diffusion coefficients of polyaromatic hydrocarbons anthracene and phenanthrene and some organometallic compounds (e.g. ferrocene, aluminium acetylacetonate and chromium acetylacetonate) in air at various temperatures are reported. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2011.05.009
  • 2012 • 38 Pearlite revisited
    Steinbach, I. and Plapp, M.
    Continuum Mechanics and Thermodynamics 24 665-673 (2012)
    Zener's model of pearlite transformation in steels can be viewed as the prototype of many microstructure evolution models in materials science. It links principles of thermodynamics and kinetics to the scale of the microstructure. In addition it solves a very practical problem: How the hardness of steel is correlated to the conditions of processing. Although the model is well established since the 1950s, quantitative explanation of growth kinetics was missing until very recently. The present paper will shortly review the classical model of pearlite transformation. Zener's conjecture of maximum entropy production will be annotated by modern theoretical and experimental considerations of a band of stable (sometimes oscillating) states around the state of maximum entropy production. Finally, an explanation of the growth kinetics observed in experiments is proposed based on diffusion fluxes driven by stress gradients due to large transformation strain. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0204-y
  • 2012 • 37 Phase-field model with finite interface dissipation
    Steinbach, I. and Zhang, L. and Plapp, M.
    Acta Materialia 60 2689-2701 (2012)
    In rapid phase transformations, interfaces are often driven far from equilibrium, and the chemical potential may exhibit a jump across the interface. We develop a model for the description of such situations in the framework of the phase-field formalism, with separate concentration fields in each phase. The key novel feature of this model is that the two concentration fields are linked by a kinetic equation which describes the exchange of components between the phases, instead of an equilibrium partitioning condition. The associated rate constant influences the interface dissipation. For rapid exchange between the phases, the chemical potentials are equal in both coexisting phases at the interface as in previous models, whereas in the opposite limit strong non-equilibrium behavior can be modeled. This is illustrated by simulations of a diffusion couple and of solute trapping during rapid solidification. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.01.035
  • 2012 • 36 Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel
    Yuan, L. and Ponge, D. and Wittig, J. and Choi, P. and Jiménez, J.A. and Raabe, D.
    Acta Materialia 60 2790-2804 (2012)
    Austenite reversion during tempering of a Fe-13.6 Cr-0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400 °C for 1 min produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min at 400 °C results in a UTS of ∼1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure-property relationships that control the material's strength and ductility. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.01.045
  • 2011 • 35 Mechanisms of grain boundary softening and strain-rate sensitivity in deformation of ultrafine-grained metals at high temperatures
    Ahmed, N. and Hartmaier, A.
    Acta Materialia 59 4323-4334 (2011)
    Two-dimensional dislocation dynamics and diffusion kinetics simulations are employed to study the different mechanisms of plastic deformation of ultrafine-grained (UFG) metals at different temperatures. Besides conventional plastic deformation by dislocation glide within the grains, we also consider grain boundary (GB)-mediated deformation and recovery mechanisms based on the absorption of dislocations into GBs. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a pre-existing dislocation population, dislocation sources and GBs. The mechanical response of the material to an external load is calculated with this model over a wide range of temperatures. We find that at low homologous temperatures, the model material behaves in agreement with the classical Hall-Petch law. At high homologous temperatures, however, a pronounced GB softening and, moreover, a high strain-rate sensitivity of the model material is found. Qualitatively, these numerical results agree well with experimental results known from the literature. Thus, we conclude that dynamic recovery processes at GBs and GB diffusion are the rate-limiting processes during plastic deformation of UFG metals. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.056
  • 2011 • 34 A low pressure drop preseparator for elimination of particles larger than 450 nm
    Asbach, C. and Fissan, H. and Kaminski, H. and Kuhlbusch, T.A.J. and Pui, D.Y.H. and Shin, H. and Horn, H.G. and Hase, T.
    Aerosol and Air Quality Research 11 487-496 (2011)
    Measurement techniques which allow the detection of airborne nanoparticles are of great interest for e.g. exposure monitoring and quality control during nanoparticle production. An increasing number of commercial devices use a unipolar diffusion charger to charge the particles and determine the nanoparticle concentration and sometimes size. The analysis however may be biased by the presence of large particles. We therefore developed a preseparator that removes particles larger than 450 nm, i.e the minimum in the range of particle lung deposition curves, but only causes a low pressure drop. The preseparator uses a total flow rate of 2.5 L/min and consists of two stages. The first stage is a virtual impactor that removes particles larger than approximately 1 μm with a minor flow of 1 L/min. Particles above 450 nm are removed from the remaining 1.5 L/min in the cyclone of the second stage. The combination of a cyclone with a virtual impactor was shown to reduce the pressure drop of the preseparator from 8.1 to 5.6 kPa compared with a cyclone alone and improve the sharpness of the separation curve for cut-off diameters around 450 nm. Furthermore the virtual impactor extends the cleaning intervals of the preseparator, because large particles are no longer deposited in the cyclone. Eventually the preseparator was tested with an opposed flow diffusion charger and it was shown that particle charging is not affected by the pressure drop. © Taiwan Association for Aerosol Research.
    view abstractdoi: 10.4209/aaqr.2011.05.0057
  • 2011 • 33 Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 518-523 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.012
  • 2011 • 32 Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation
    Dmitrieva, O. and Ponge, D. and Inden, G. and Millán, J. and Choi, P. and Sietsma, J. and Raabe, D.
    Acta Materialia 59 364-374 (2011)
    Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility). © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.09.042
  • 2011 • 31 NMR studies of benzene mobility in metal-organic framework MOF-5
    Hertel, S. and Wehring, M. and Amirjalayer, S. and Gratz, M. and Lincke, J. and Krautscheid, H. and Schmid, R. and Stallmach, F.
    EPJ Applied Physics 55 (2011)
    The concentration and temperature dependence of the self-diffusion of benzene adsorbed in the metal-organic framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PFG) NMR spectroscopy. When increasing the loading from 10 to 20 molecules per unit cell of MOF-5, the experimental diffusion data drop by a factor of about 3 while current molecular dynamic (MD) simulations predict slightly increasing diffusion coefficients for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate molecules in microporous systems for temperatures well below the adsorbate critical temperature. Necessary improvements of molecular simulation models for predicting diffusivities under such conditions are discussed. © EDP Sciences, 2011.
    view abstractdoi: 10.1051/epjap/2011100370
  • 2011 • 30 Mechanistic studies of Fc-PNA(·DNA) surface dynamics based on the kinetics of electron-transfer processes
    Hüsken, N. and Gȩbala, M. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    Chemistry - A European Journal 17 9678-9690 (2011)
    N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(·DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(·DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA·DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k 0. An in-depth understanding of the dynamics of surface-bound PNA and PNA·DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201003764
  • 2011 • 29 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 • 28 Characterization of oxidation and reduction of a platinum-rhodium alloy by atom-probe tomography
    Li, T. and Marquis, E.A. and Bagot, P.A.J. and Tsang, S.C. and Smith, G.D.W.
    Catalysis Today 175 552-557 (2011)
    An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh2O3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2011.03.046
  • 2011 • 27 Morphology and photoluminescence study of titania nanoparticles
    Memesa, M. and Lenz, S. and Emmerling, S.G.J. and Nett, S. and Perlich, J. and Müller-Buschbaum, P. and Gutmann, J.S.
    Colloid and Polymer Science 289 943-953 (2011)
    Titania nanoparticles are prepared by sol-gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)- block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol-gel components-hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer-are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol-gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13-20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies. © 2011 The Author(s).
    view abstractdoi: 10.1007/s00396-011-2421-0
  • 2011 • 26 Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 26-31 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000219
  • 2011 • 25 Effect of shot-peening on the oxidation behaviour of boiler steels
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Oxidation of Metals 76 233-245 (2011)
    The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s11085-011-9250-x
  • 2011 • 24 Diffusion of poly(ethylene glycol) and ectoine in NIPAAm hydrogels with confocal Raman spectroscopy
    Poggendorf, S. and Adama Mba, G. and Engel, D. and Sadowski, G.
    Colloid and Polymer Science 289 545-559 (2011)
    The diffusion behavior of poly(ethylene glycol) (PEG) in N-isopropylacrylamide (NIPAAm) hydrogels was investigated using confocal Raman spectroscopy with regard to temperature (25°C, 30°C and 35°C), PEG concentration (10 and 40 wt.%), PEG molecular weight (2,000 and 12,000 g/mol) and addition of the compatible solute ectoine (0.1 and 2 wt.%). Swelling and shrinking of the gels was observed by means of confocal Raman spectroscopy. The swelling behavior of NIPAAm gels in aqueous solutions of PEG and ectoine was found to resemble the swelling behavior in pure water with regard to temperature, i.e., the gel shrinks with increasing temperature. However, the presence and concentration of PEG and ectoine influence the swelling behavior by lowering the volume phase-transition temperature of the gel and facilitating shrinking. In some cases, a re-swelling of the gel was observed after the initial shrinking at the onset of PEG diffusion, which can be explained by PEG changing the chemical potential in the gel phase as it diffuses into the sample allowing the water to re-enter. The expulsion of water from the gel during shrinking and the so-caused increase of PNIPAAm and PEG concentrations in some cases led to the PEG diffusion seemingly being faster in more shrunken gels despite of their higher diffusion resistance. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00396-011-2399-7
  • 2011 • 23 Diffusion and phase transformation at the interface between an austenitic substrate and a thermally sprayed coating of ledeburitic cold-work tool steel
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vaßen, R.
    Steel Research International 82 671-682 (2011)
    An overlay coating against wear or corrosion on components is required for various technical applications. Thermal spraying is a well-established and near-net-shape deposition method. In this work, high-velocity oxy-fuel spraying of two different ledeburitic cold-work tool steels was employed to produce wear-resistant Fe-base coatings on a stainless steel substrate. This work focuses on the investigation of diffusion processes across the coating/substrate interface. Specimens were heat-treated for different dwell times and then analyzed by means of EBSD, XRD, OM, as well as SEM. Results of phase formation and diffusion profiles were compared with equilibrium and diffusion calculations obtained with ThermoCALC® and DICTRA®. The influence of diffusion processes across the coating/substrate interface on the mechanical properties, such as adhesive bond strength and hardness, was investigated by shear tests and microhardness profiles. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000199
  • 2011 • 22 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 • 21 Hot Direct Extrusion of Abrasion Resistant Fe-Base Metal Matrix Composites-Interface Characterization and Mechanical Properties of Co-Extruded Layered Structures
    Silva, P.A. and Weber, S. and Karlsohn, M. and Röttger, A. and Theisen, W. and Reimers, W. and Pyzalla, A.R.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany 690-695 (2011)
    doi: 10.1002/9783527628513.ch90
  • 2011 • 20 Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
    Xu, J. and Yamazaki, D. and Katsura, T. and Wu, X. and Remmert, P. and Yurimoto, H. and Chakraborty, S.
    Journal of Geophysical Research: Solid Earth 116 (2011)
    Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO<inf>3</inf> perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D=D<inf>0</inf> exp (-H/RT) at 25GPa, with D <inf>0</inf>=5.10×10-11m2/s for Si and 4.99×10-11m2/s for Mg, H=308kJ/mol for Si, and 305kJ/mol for Mg. Mg diffusivity in MgSiO<inf>3</inf> perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect-based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO<inf>3</inf> perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle. Copyright 2011 by the American Geophysical Union.
    view abstractdoi: 10.1029/2011JB008444
  • 2011 • 19 Phase-field simulation of diffusion couples in the Ni-Al system
    Zhang, L. and Steinbach, I. and Du, Y.
    International Journal of Materials Research 102 371-380 (2011)
    By linking thermodynamic and atomic mobility databases with two-dimensional phase-field simulation, the evolution of interdiffusion microstructures in a series of Ni-Al diffusion couples associated with the γ, γ', and b-phases was studied. The formation and subsequent growth of the γ'- phase layer in β/γ and γ' + β/γ diffusion couples reproduced the experimental observations well. Moreover, the effect of coherent strain on the γ - γ' microstructural evolution, as well as that of an external compressive force on the γ + γ'/γ + γ' diffusion couple, was investigated. The phase-field simulated concentration profiles of some of the Ni-Al diffusion couples were also compared with the corresponding experimental data and the results of one-dimensional DICTRA (DIffusion Controlled TRAnsformations) simulations. A discussion of the rafting direction was also made by comprehensively comparing the phase-field simulations with the predicted results from an elastic model. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110493
  • 2010 • 18 Dissolution kinetics of Si into Ge (111) substrate on the nanoscale
    Balogh, Z. and Erdélyi, Z. and Beke, D.L. and Wiedwald, U. and Pfeiffer, H. and Tschetschetkin, A. and Ziemann, P.
    Thin Solid Films 519 952-955 (2010)
    In this paper we present experiments and simulations on the dissolution of Si into single crystalline Ge(111) substrates. The interface shift during the dissolution was tracked by X-ray Photoelectron Spectroscopy. It was obtained that the interface remained sharp and shifted according to anomalous kinetics similarly to our previous measurement in the Si/amorphous-Ge system. The interface shift, x, can be described by a power function of time x ∞ t kc with a kinetic exponent, kc, of 0.85 ± 0.1, larger than the one measured for the amorphous system (0.7 ± 0.1). Both exponents, however, are different from the kc = 0.5 Fickian (parabolic) value and it is interpreted as a nanoscale diffusional anomaly caused by the strong composition dependence of the diffusion coefficients. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.08.146
  • 2010 • 17 A local high-order doubly asymptotic open boundary for diffusion in a semi-infinite layer
    Birk, C. and Song, C.
    Journal of Computational Physics 229 6156-6179 (2010)
    A high-order open boundary for transient diffusion in a semi-infinite homogeneous layer is developed. The method of separation of variables is used to derive a relationship between the modal function and the flux at the near field/far field boundary in the Fourier domain. The resulting equation in terms of the modal impedance coefficient is solved by expanding the latter into a doubly asymptotic series of continued fractions. As a result, the open boundary condition in the Fourier domain is represented by a system of algebraic equations in terms of . iω. This corresponds to a system of fractional differential equations of degree . α=. 0.5 in the time-domain. This temporally global formulation is transformed into a local description by introducing internal variables. The resulting local high-order open boundary condition is highly accurate, as is demonstrated by a number of heat transfer examples. A significant gain in accuracy is obtained in comparison with existing singly-asymptotic formulations at no additional computational cost. © 2010 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcp.2010.04.046
  • 2010 • 16 The time scales of magma mixing and mingling involving primitive melts and melt-mush interaction at mid-ocean ridges
    Costa, F. and Coogan, L.A. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 159 371-387 (2010)
    We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~ 11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were < 140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slowspreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts. © Springer-Verlag 2009.
    view abstractdoi: 10.1007/s00410-009-0432-3
  • 2010 • 15 Diffusion of Li in olivine. Part I: Experimental observations and a multi species diffusion model
    Dohmen, R. and Kasemann, S.A. and Coogan, L. and Chakraborty, S.
    Geochimica et Cosmochimica Acta 74 274-292 (2010)
    There are an increasing number of studies that focus on the systematics of the distribution of Li and its isotopes among different geochemical reservoirs. These studies have found that Li is relatively mobile compared to many other elements (e.g., Fe, Mg), and diffusion has been considered as a mechanism to generate large isotopic fractionations even at high temperatures. In order to quantify some of these aspects, we have measured Li diffusion rates experimentally along [0 0 1] of single crystals of olivines from San Carlos, Arizona and Pakistan, at 800-1200 °C at a total pressure of 100 kPa and fO2 ≈ WM buffer. A complex diffusion behavior of Li is observed, indicating that two mechanisms of diffusion (a fast and a slower one) operate simultaneously. The behavior is well described by a model that partitions Li between two different sites in olivine - an octahedral site (LiMe) and an interstitial site (Lii). Transport of Li is a combination of hopping within and between each of these kinds of sites involving also vacancies on the octahedral site (VMe). It is assumed that the homogeneous reaction (LiMe = VMe + Lii) that maintains equilibrium distribution of Li between the sites is instantaneous compared to the timescales of all other processes associated with diffusive transport. One consequence of this mode of transport of Li in olivine is that the shape and length of diffusion profiles depend on the boundary conditions imposed at the surface of a crystal; i.e., the chemical environment (e.g., fO2, aLi4SiO4), in addition to temperature and pressure. Our model describes the variable experimentally determined Li-profile shapes produced at different temperatures and with different boundary conditions, as well as their time evolution, quantitatively. Modeling the observed isotopic fractionation shows that 6Li diffuses about 5% faster than 7Li on the interstitial site. Inspection of published data on Li distribution in natural olivines that are available until now indicates that the fast (interstitial) mechanism of Li diffusion is unlikely to be dominant in most natural systems; Li rich, oxidizing environments (e.g., fluids?) may be exceptions. However, when it operates it can decouple the equilibration of Li isotopic gradients from the time scale of equilibration of overall Li concentrations. Diffusion dominated by the slower mechanism will occur on the average at a rate that is about an order of magnitude faster than diffusion of Fe, Mg and most other divalent cations in olivine; such diffusion of Li in olivine will be much slower than the rates of diffusion in clinopyroxene and plagioclase crystals at the same conditions. Fractionation of isotopes of Li by diffusion is likely to be a transient phenomenon and is more likely to be observed in crystals showing zoning of Li concentrations. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.gca.2009.10.016
  • 2010 • 14 Penetration of thin C 60 films by metal nanoparticles
    Duffe, S. and Grönhagen, N. and Patryarcha, L. and Sieben, B. and Yin, C. and Von Issendorff, B. and Moseler, M. and Hövel, H.
    Nature Nanotechnology 5 335-339 (2010)
    Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C 60 supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C 60 supported on gold, or on one monolayer of C 60 supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer. © 2010 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nnano.2010.45
  • 2010 • 13 Time reversal of some stationary jump diffusion processes from population genetics
    Hutzenthaler, M. and Taylor, J.E.
    Advances in Applied Probability 42 1147-1171 (2010)
    We describe the processes obtained by time reversal of a class of stationary jump diffusion processes that model the dynamics of genetic variation in populations subject to repeated bottlenecks. Assuming that only one lineage survives each bottleneck, the forward process is a diffusion on [0, 1] that jumps to the boundary before diffusing back into the interior. We showthat the behavior of the time-reversed process depends on whether the boundaries are accessible to the diffusive motion of the forward process. If a boundary point is inaccessible to the forward diffusion then time reversal leads to a jump diffusion that jumps immediately into the interior whenever it arrives at that point. If, instead, a boundary point is accessible then the jumps off of that point are governed by a weighted local time of the time-reversed process. © Applied Probability Trust 2010.
    view abstractdoi: 10.1239/aap/1293113155
  • 2010 • 12 Multiscale simulations on the grain growth process in nanostructured materials
    Kamachali, R.D. and Hua, J. and Steinbach, I. and Hartmaier, A.
    International Journal of Materials Research 101 1332-1338 (2010)
    In this work, multi-phase field and molecular dynamics simulations have been used to investigate nanoscale grain growth mechanisms. Based on experimental observations, the combination of grain boundary expansion and vacancy diffusion has been considered in the multi-phase field model. The atomistic mechanism of boundary movement and the free volume redistribution during the growth process have been investigated using molecular dynamics simulations. According to the multi-phase field results, linear grain growth in nanostructured materials at low temperature can be explained by vacancy diffusion in the stress field around the grain boundaries. Molecular dynamics simulations confirm the observation of linear grain growth for nanometresized grains. The activation energy of grain boundary motion in this regime has been determined to be of the order of onetenth of the self-diffusion activation energy, which is consistent with experimental data. Based on the simulation results, the transition from linear to normal grain growth is discussed in detail and a criterion for this transition is proposed. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110419
  • 2010 • 11 Effective reaction rates of a thin catalyst layer
    Lenzinger, M. and Schweizer, B.
    Mathematical Methods in the Applied Sciences 33 974-984 (2010)
    The catalyst layer in a fuel cell can be described with a system of reaction diffusion equations for the oxygen concentration and the protonic overpotential. The Tafel law gives an exponential expression for the reaction rate, and the Tafel slope is a coefficient in this law. We present a rigorous thin layer analysis for two reaction regimes. In the case of thin catalyst layers and bounded potentials, the original Tafel law enters as an effective boundary condition. Instead, in the case of large protonic overpotentials, we derive an exponential law that contains the doubled Tafel slope. Copyright © 2009 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/mma.1223
  • 2010 • 10 4D shearforce-based constant-distance mode scanning electrochemical microscopy
    Nebel, M. and Eckhard, K. and Erichsen, T. and Schulte, A. and Schuhmann, W.
    Analytical Chemistry 82 7842-7848 (2010)
    4D shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) is designed to assess SECM tip currents at several but constant distances to the sample topography at each point of the x,y-scanning grid. The distance dependent signal is achieved by a shearforce interaction between the in-resonance vibrating SECM tip and the sample surface. A 4D SF/CD-SECM measuring cycle at each grid point involves a shearforce controlled SECM tip z-approach to a point of closest distance and subsequent stepwise tip retractions. At the point of closest approach and during the retraction steps, pairs of tip current (I) and position are acquired for various distances above the sample surface. Such a sequence provides x,y,I maps, that can be compiled and displayed for each selected data acquisition distance. Thus, multiple SECM images are obtained at known and constant distances above the sample topography. 4D SF/CD-SECM supports distance-controlled tip operation while continuous scanning of the SECM tip in the shear-force distance is avoided. In this way, constant-distance mode SECM imaging can be performed at user-defined, large tip-to-sample distances. The feasibility and the potential of the proposed 4D SF/CD-SECM imaging is demonstrated using on the one hand amperometric feedback mode imaging of a Pt band electrode array and on the other hand the visualization of the diffusion zone of a redox active species above a microelectrode in a generator/collector arrangement. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ac1008805
  • 2010 • 9 Development of Mn-Cr-(C-N) corrosion resistant twinning induced plasticity steels: Thermodynamic and diffusion calculations, production, and characterization
    Roncery, L.M. and Weber, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 2471-2479 (2010)
    In this work, the development of corrosion-resistant twinning induced plasticity steels is presented, supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations, ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium is used to increase nitrogen solubility and provide corrosion resistance, and carbon and nitrogen are used as interstitial elements to provide mechanical strength. Isopleths of the different elements vs temperature as well as isothermal sections were calculated to determine the proper amount of Mn, Cr, total interstitial content, and the C/N ratio. Scheil and diffusion calculations were used to predict the extent of microsegregations and additionally to evaluate the effect of diffusion annealing treatments. The materials were produced in laboratory scale, being followed by thermomechanical processing and the characterization of the microstructure. Tensile tests were performed with three different alloys, exhibiting yield strengths of 460 Mpa to 480 MPa and elongations to fracture between 85 pct and 100 pct. © The Minerals, Metals & Materials Society and ASM International 2010.
    view abstractdoi: 10.1007/s11661-010-0334-z
  • 2010 • 8 Effects of wetting layer structure on surface phase stability and on indium surface diffusion
    Rosini, M. and Kratzer, P. and Magri, R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 181-184 (2010)
    We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2×4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.200982501
  • 2010 • 7 Accessing ultrashort reaction times in particle formation with SAXS experiments: ZnS precipitation on the microsecond time scale
    Schmidt, W. and Bussian, P. and Lindén, M. and Amenitsch, H. and Agren, P. and Tiemann, M. and Schüth, F.
    Journal of the American Chemical Society 132 6822-6826 (2010)
    Precipitation of zinc sulfide particles is a very rapid process, and monitoring of the particle growth is experimentally very demanding. Applying a liquid jet flow cell, we were able to follow zinc sulfide particle formation on time scales down to 10 -5 s. The flow cell was designed in such a way that data acquisition on the microsecond time scale was possible under steady-state conditions along a liquid jet (tubular reactor concept), allowing SAXS data accumulation over a time scale of minutes. We were able to monitor the growth of zinc sulfide particles and found experimental evidence for very rapid particle aggregation processes within the liquid jet. Under the experimental conditions the particle growth is controlled by mass transfer: i.e., the diffusion of the hydrogen sulfide into the liquid jet. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101519z
  • 2010 • 6 Thermal stability, vapor pressures, and diffusion coefficients of some metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Journal of Chemical and Engineering Data 55 2149-2154 (2010)
    Many metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds are volatile enough to be useful as precursors of the metals in vapor-phase deposition processes, for example, metal organic chemical vapor deposition (MOCVD). The thermal stability, vapor pressures, and gaseous diffusion coefficients of these compounds are, therefore, of fundamental importance for achieving reproducible and effective depositions. The present communication reports the thermal stability, vapor pressures, enthalpies of sublimation, and diffusion coefficients (in nitrogen and/or helium) for some metal 2,2,6,6-tetramethyl-3,5-heptandionate compounds [M(tmhd)n], namely, [Al(tmhd)3], [Cr(tmhd)3], [Cu(tmhd)2], [Fe(tmhd)3], [Mn(tmhd)3], and [Ni(tmhd)2] at temperatures between (341 and 412) K at ambient pressure. All of these are found to be stable under the investigated experimental conditions and thus are suitable precursors for CVD. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/je9006822
  • 2010 • 5 Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling
    Sprodowski, C. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physics Condensed Matter 22 (2010)
    Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10-9 per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I -t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/26/264005
  • 2010 • 4 Synthesis and characterization of chiral and thermo responsive amphiphilic conetworks
    Tobis, J. and Thomann, Y. and Tiller, J.C.
    Polymer 51 35-45 (2010)
    Amphiphilic polymer conetworks (APCN) combine the properties of different polymers on the nanoscale affording advanced materials with unique properties. Here, we present the first APCN with a chiral hydrophilic phase. The conetworks were prepared by copolymerizing the tailored chiral monomer (R)-N-(1-hydroxybutan-2-yl)acrylamide (R-HBA) with two different crosslinkers that consist of bitelechelic methacrylate-terminated poly(dimethylsiloxane) (PDMS) of a molecular weight of 1100 g/mol and 5620 g/mol, respectively. The resulting polymer conetworks P-R-HBA-l-PDMS exhibited both two different Tg values, indicating nanophase separation. However, the conetwork with PDMS1.1 did not show nanophases in the AFM and did not swell the phases separately in orthogonal solvents. On the other hand the materials with PDMS5.6 acted like a typical APCN. The APCN P-R-HBA-l-PDMS5.6 was found to be temperature sensitive in water, decreasing its degree of swelling linearly with increasing temperature. Additionally, the conetwork is increasing its degree of swelling in n-heptane in the region of the Tg of the P-R-HBA phase. The impact of the chiral polymer on the release of cinchona alkaloids was examined. For example, (-)-cinchonine diffuses four times faster off the P-R-HBA-l-PDMS networks than off the P-S-HBA-l-PDMS conetworks. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.polymer.2009.10.055
  • 2010 • 3 Imaging diffusion fields on a surface with multiple reconstructions: Ag/Si(111)
    Wall, D. and Lohmar, I. and Roos, K.R. and Krug, J. and Horn-Von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    New Journal of Physics 12 (2010)
    Photoemission electron microscopy is used for studying the thermal decay of Ag islands grown epitaxially on a Si(111) surface. During the decay, the islands feed adatoms to the surrounding surface. The adatoms diffuse and eventually desorb, resulting in a radial coverage gradient that induces the formation of two concentric reconstructed zones, namely (√3 × √3)-R30 o and (3 × 1), around each island. We have developed a diffusion model to describe this multizone formation and demonstrate how diffusion constants can be determined for different reconstructed phases in a simple experiment. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/10/103019
  • 2010 • 2 Atomic mobilities and diffusivities in the fcc, L12 and B2 phases of the Ni-Al system
    Zhang, L. and Du, Y. and Chen, Q. and Steinbach, I. and Huang, B.
    International Journal of Materials Research 101 1461-1475 (2010)
    A phenomenological model was utilized to describe diffusivities in the γ (fcc)/γ′ (L12) and A2/B2 phases of the Ni-Al system. An effective strategy, which takes the homogeneity range and defect concentration into account, was developed in the present work to optimize the atomic mobilities of γ′ phase. Such a strategy results in a dramatic decrease in the number of atomic mobility parameters to be evaluated for the L12 phase. The measured composition-and temperature-dependent diffusivities in the Ni-Al system have been well replicated by the present mobility descriptions. For the L12 phase, comprehensive comparisons show that with fewer model parameters the presently obtained mobilities yield a better fit to experimental diffusivities, compared with previous assessments. The mobility descriptions are further validated by comparing calculated and measured concentration profiles for various diffusion couples. The time-dependent Al composition profile for the annealed vapor Al/γ couple is accurately described for the first time. © Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282.
    view abstractdoi: 10.3139/146.110428
  • 2010 • 1 Diffusivities of an Al-Fe-Ni melt and their effects on the microstructure during solidification
    Zhang, L. and Du, Y. and Steinbach, I. and Chen, Q. and Huang, B.
    Acta Materialia 58 3664-3675 (2010)
    A systematical investigation of the diffusivities in an Al-Fe-Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al-Ni, Al-Fe, Fe-Ni and Al-Fe-Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al-Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.03.002