Prof. Dr. Doru Constantin Lupascu

Institute for Materials Science
University of Duisburg-Essen


  • Electronic interactions between graphene and cobaltite thin film La0.7Sr0.3CoO3 and its magnetic consequences
    Othmen, Z. and Othmen, R. and Daoudi, K. and Boudard, M. and Cavanna, A. and Madouri, A. and Gemeiner, P. and Lupascu, D.C. and Oueslati, M. and Dkhil, B.
    Surfaces and Interfaces 23 (2021)
    We have successfully synthesized and transferred graphene (Gr) monolayers on top of epitaxial mixed valence La0.7Sr0.3CoO3 (LSCO) thin films. Raman spectroscopy reveals that Jahn-Teller (JT) modes associated with the oxygen octahedral distortions usually unobserved for bare LSCO are activated by the deposited graphene. The appearance of these JT modes in the Gr/LSCO heterostructure is attributed to the electronic interactions at the interface between the graphene and the LSCO thin film promoting intermediate spin states of the Co ions. As a result, the magnetic properties of LSCO are affected. Indeed, magnetization measurements show a phase transition at ~135 K which is due to the presence of the graphene while the ferromagnetic transition of bare LSCO films is observed at~200 K. This magnetic phase is confirmed by Raman spectroscopy measurements as a function of temperature revealing a vibrational transition around the same temperature. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfin.2020.100919
  • Corrigendum to ‘Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3-NiFe2O4 multiferroics: A relationship between phase-connectivity and interface coupling’ (Acta Materialia (2018) 144 (305–313), (S1359645417309072), (10.1016/j.actamat.2017.10.048))
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Trivedi, H. and Salamon, S. and Webers, S. and Wende, H. and Hagemann, U. and Schröder, J. and Lupascu, D.C.
    Acta Materialia 187 (2020)
    The authors regret to inform the readers of Acta Materialia that Figure 10 of the article contains mistakes, none of which change the overall interpretation of the data nor the main point of the paper, but which should be rectified, nonetheless. 1. There was a calculation error during the conversion from applied voltage to the electric field and finally to the converse ME coupling coefficient. This was amended, with the correct data being shown in Table 1, to more clearly show the composition-dependent change to the reader. The overall interpretation remains unaffected, with the sample BCZTNFO30 displaying the highest ME coefficient.2. The value of the ME coefficient for the BCZTNFO50 sample was recorded at a different frequency (1.7 Hz) than the other compositions (8 Hz), which lead to a strongly deviating result for this sample. In order to obtain a dataset with consistent parameters, a single data point from the magnetic field dependent measurement regime was used, the results of which are shown in Figure 11 of the original manuscript. The slope and thus the converse ME coupling coefficient were calculated from a linear function between this point and the point of origin, which is justified by the fact that our measurement values show high stability and precision, making it highly unlikely for significant deviations to result from this approximation. The combination of parameters (magnetic field, temperature, electric field) represents the optimum for this sample, as also used for the original measurement, in addition to the now matching frequency of 8 Hz. This updated data point has also been included in Table 1 and Figure 1.3. Furthermore, the dataset of the BCZTNFO20 sample currently shown in Figure 10 of the manuscript is not the correct one, evident by the lower point density, higher amount of noise and the deviating frequency of 11 Hz. This dataset was replaced with the correct one for this sample, as shown in Figure 2, recorded at the same point density and frequency (8 Hz) as all the other samples of this set.From this updated set of data, new values of ME coefficients were calculated: 16.9(6) ps/m for BCZTNFO20, 90.1(6) ps/m for BCZTNFO30, 34.9(2) ps/m for BCZTNFO40 and 25.4 ps/m for BCZTNFO50. This does not change the conclusions that were drawn in the manuscript, as sample BCTNFO30 still has the highest value of the converse ME coupling coefficient. However, all of the values are increased at least twofold due to the corrected voltage conversion, clearly showing that this NFO-based sample set exhibits greatly improved ME response compared to previously used CFO-based materials. Furthermore, thanks to the now correct coefficient for BCZTNFO50, the composition dependent peak is much clearer, with the coupling coefficient consistently dropping when we move away from the optimum BCZTNFO30 composition to either side (see Figure 1), giving more merit to our original interpretation. The significant change of the measurement signal for BCZTNFO50 we observed when going from 1.7 Hz to 8 Hz is most probably caused by the fact that this sample has a lower resistivity of only 1 GΩ compared to the 30 GΩ of BCZTNFO30 (see Figure 3). This presumably induces a strong increase of frequency-dependence of the ME coefficient through Maxwell-Wagner relaxation due to the different dielectric permittivities of the two constituents that make up the sample, further exacerbated by the higher temperature at which this measurement was performed. These findings were discussed in greater detail in the dissertation of one of the co-authors [1]. Another minor correction needs to be made regarding Figure 1 of the original manuscript: There are typo mistakes on all four panels of Figure 1, where the phase of BCZT was incorrectly labeled as P4/mmm, which should be labeled as P4/mm. The same should be understood for Table 1 and wherever P4/mmm is mentioned in the text. The authors would like to apologise for any inconvenience caused. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.01.054
  • Effect of Mn and Ba Codoping on a Magnetic Spin Cycloid of Multiferroic Bismuth Ferrite Nanoparticles
    Dubey, A. and Escobar Castillo, M. and Landers, J. and Salamon, S. and Wende, H. and Hagemann, U. and Gemeiner, P. and Dkhil, B. and Shvartsman, V.V. and Lupascu, D.C.
    Journal of Physical Chemistry C 124 (2020)
    Bismuth ferrite (BFO) is the drosophila of research in multiferroic materials due to its simultaneous magnetic and electric ordering at room temperature. The unfortunate detail is its antiferromagnetic ordering, which practically cancels magnetization and magnetoelectric coupling of the crystals. To induce finite coupling, dopants have been introduced with a certain success so far. Nanoparticles (NPs) can additionally constrain the formation of the magnetic cycloid in BFO due to size confinement. Doping nanoparticles can thus potentially provide a sizeable magnetization of BFO, making applications in computer memories and hyperthermia cancer treatment feasible. We show that the codoping of BFO NPs by Ba and Mn balances the electrochemical equilibrium, reduces the particle size, and shifts the magnetic phase transition to lower temperatures. The ferroelectric properties are retained and the remanent magnetization is increased by 1 order of magnitude: Bi0.95Ba0.05Fe0.95Mn0.05O3 possesses a remanent magnetization of 0.277 Am2/kg. Our Mössbauer studies reveal that two effects drive this increase: partial destruction of the spin cycloid due to Mn and increased spin canting due to Ba doping inducing local stress fields. This dopant combination and particular concentration improve the effective magnetization value exceptionally well. © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c05778
  • Exchange bias effect in bulk multiferroic BiFe0.5Sc0.5O3
    Fertman, E.L. and Fedorchenko, A.V. and Desnenko, V.A. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H. and Vaisburd, A.I. and Stanulis, A. and Ramanauskas, R. and Olekhnovich, N.M. and Pushkarev, A.V. and Radyush, Y.V. and Khalyavin, D.D. and Salak, A.N.
    AIP Advances 10 (2020)
    Below the Néel temperature, TN ∼220 K, at least two nano-scale antiferromagnetic (AFM) phases coexist in the polar polymorph of the BiFe0.5Sc0.5O3 perovskite; one of these phases is a weak ferromagnetic. Non-uniform structure distortions induced by high-pressure synthesis lead to competing AFM orders and a nano-scale spontaneous magnetic phase separated state of the compound. Interface exchange coupling between the AFM domains and the weak ferromagnetic domains causes unidirectional anisotropy of magnetization, resulting in the exchange bias (EB) effect. The EB field, HEB, and the coercive field strongly depend on temperature and the strength of the cooling magnetic field. HEB increases with an increase in the cooling magnetic field and reaches a maximum value of about 1 kOe at 5 K. The exchange field vanishes above TN with the disappearance of long-range magnetic ordering. The effect is promising for applications in electronics as it is large enough and as it is tunable by temperature and the magnetic field applied during cooling. © 2020 Author(s).
    view abstract10.1063/1.5135586
  • Fine structure of the optical absorption resonance in Cs2AgBiBr6 double perovskite thin films
    Schmitz, A. and Leander Schaberg, L. and Sirotinskaya, S. and Pantaler, M. and Lupascu, D.C. and Benson, N. and Bacher, G.
    ACS Energy Letters 5 (2020)
    The lead-free double perovskite Cs2AgBiX6 (X = Br, Cl) has recently demonstrated great potential for applications in solar cells, photodetectors, and X-ray detectors. This material is characterized by a dominant resonant absorption feature in the UV-blue spectral region, which is still under controversial discussion regarding its origin. Here, we uncover an electronic fine structure of this optical feature in Cs2AgBiBr6 thin films. A detailed temperature-resolved study combining photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy reveals the existence of three optical transitions, situated approximately 100 meV around the resonance at 2.83 eV. PL measurements under pulsed excitation uncover a short-lived blue emission at the absorption resonance energy that persists up to room temperature and indicates the competition of direct emission from the resonant state and fast relaxation into the red emissive ground state. We derive a comprehensive energy scheme and suggest possible mechanisms leading to the observed fine structure splitting. © 2020 American Chemical Society
    view abstract10.1021/acsenergylett.9b02781
  • Laser fragmentation synthesis of colloidal bismuth ferrite particles
    Siebeneicher, S. and Waag, F. and Castillo, M.E. and Shvartsman, V.V. and Lupascu, D.C. and Gökce, B.
    Nanomaterials 10 (2020)
    Laser fragmentation of colloidal submicron-sized bismuth ferrite particles was performed by irradiating a liquid jet to synthesize bismuth ferrite nanoparticles. This treatment achieved a size reduction from 450 nm to below 10 nm. A circular and an elliptical fluid jet were compared to control the energy distribution within the fluid jet and thereby the product size distribution and educt decomposition. The resulting colloids were analysed via UV-VIS, XRD and TEM. All methods were used to gain information on size distribution, material morphology and composition. It was found that using an elliptical liquid jet during the laser fragmentation leads to a slightly smaller and narrower size distribution of the resulting product compared to the circular jet. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10020359
  • Lead-Free Double Perovskites for Perovskite Solar Cells
    Kung, P.-K. and Li, M.-H. and Lin, P.-Y. and Jhang, J.-Y. and Pantaler, M. and Lupascu, D.C. and Grancini, G. and Chen, P.
    Solar RRL 4 (2020)
    Perovskite solar cells (PSCs) have achieved a high power conversion efficiency (PCE) with a credible certified value over 25%. More efforts have been devoted to the development of stable and ecofriendly perovskite materials. Lead-free double perovskites (LFDPs) are a noteworthy choice as a photoactive layer because of their favorable photovoltaic (PV) properties, intrinsic chemical stability, and environmental friendliness. This Review presents various LFDP materials whose structural stability and optoelectronic properties are predicted by theoretical calculations. The synthesis and experimental properties of LFDPs and their applications in PSCs and optoelectronics in pursuing high performance, low toxicity, and functional stability are also reviewed. Perovskites active layers are critical for PSCs, and their appropriate properties are responsible for achieving a high PCE. On the other side, the stability of PSCs under working conditions is a critical requirement for their practical applications. Defect-ordered perovskites are also presented to provide another outlook on lead-free perovskite-based PVs. The introduction and interest toward LFDP in PSCs can represent a viable solution to the toxicity issue, stimulate further research, and bring a real impact to future PV technologies. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/solr.201900306
  • Multiferroic bismuth ferrite: Perturbed angular correlation studies on its ferroic α-β Phase transition
    Marschick, G. and Schell, J. and Stöger, B. and Gonçalves, J.N. and Karabasov, M.O. and Zyabkin, D. and Welker, A. and Escobar C., M. and Gärtner, D. and Efe, I. and Santos, R.A. and Laulainen, J.E.M. and Lupascu, D.C.
    Physical Review B 102 (2020)
    Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric α-phase to the high temperature β-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (BiFeO3 or BFO). Using the perturbed angular correlation (PAC) method with Cd111m as the probe nucleus, the α to β phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the γ-γ decay of the Cd111m probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to β-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency ω0 at a temperature of about Tc≈820∘C indicates the α-β phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The Pbnm (β-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.102.224110
  • Phase transitions, screening and dielectric response of CsPbBr3
    Svirskas, Š. and Balčiūnas, S. and Šimėnas, M. and Usevičius, G. and Kinka, M. and Velička, M. and Kubicki, D. and Castillo, M.E. and Karabanov, A. and Shvartsman, V.V. and De Rosário Soares, M. and Šablinskas, V. and Salak, A.N. and Lupascu, D.C. and Banys, J.
    Journal of Materials Chemistry A 8 (2020)
    Cesium-lead-bromide (CsPbBr3) is the simplest all inorganic halide perovskite. It serves as a reference material for understanding the exceptional solar cell properties of the organic-inorganic hybrid halide perovskites and is itself discussed as an alternative absorber material. Broadband dielectric spectroscopy has proven to yield an in depth understanding of charge screening mechanisms in the halide solar cell absorbers based on methylammonium and modifications hereof. For a deeper understanding of charge carrier screening, we have investigated CsPbBr3 across wide temperature (120 K-450 K) and frequency ranges. Besides the two known phase transitions at 403 K and 361 K, the dielectric data show another anomaly around 220 K, which can be interpreted as another phase transition. XRD and EPR studies confirm the presence of this anomaly, but Raman scattering spectra do not show any lattice anomalies in the vicinity of 220 K. This additional anomaly is of first order character (different transition temperatures upon cooling and heating) but hardly influences the lattice dynamics. Our broadband dielectric investigations of CsPbBr3 display the same microwave limit permittivity as for MAPbX3 (ϵr ≈ 30, X = Cl, Br, I, MA = CH3NH3+) but do not afford a second permittivity relaxation up to this frequency. Our prior assignment of the second contribution in the methylammonium compounds being due to the relaxation dynamics of the methylammonium ion as a dipole is herewith proven. Nevertheless, CsPbBr3 shows large charge carrier screening up to very high frequencies which can still play a vital role in charge carrier dynamics and exciton behaviour in this material as well. © The Royal Society of Chemistry.
    view abstract10.1039/d0ta04155f
  • Poling and annealing of piezoelectric Poly(Vinylidene fluoride) micropillar arrays
    Pariy, I.O. and Ivanova, A.A. and Shvartsman, V.V. and Lupascu, D.C. and Sukhorukov, G.B. and Surmeneva, M.A. and Surmenev, R.A.
    Materials Chemistry and Physics 239 (2020)
    This work reports on the effect of calcination and poling processes on the crystalline phase and piezoresponse of poly(vinylidene fluoride) (PVDF) micropillar arrays. PVDF micropillars were prepared by the imprinting method, heated and treated with high-voltage poling. The effect of the treatment conditions on the crystallization behaviour and the piezoelectric properties of the patterned PVDF films was investigated by piezoresponse force microscopy (PFM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC data show that poling of the PVDF micropillars increases the crystallinity of the polymer from 12% to 22.7%. FTIR measurements of PVDF films show that the calcination and poling processes affect the γ to β phase transformation. In the imprinted and annealed samples, the γ phase was predominant (58% and 46%, respectively). For the poled samples, up to 42% of the β phase was found. Piezoelectric measurements using PFM showed that the poled PVDF micropillars possess a much higher piezoelectric coefficient (29 pm/V) compared to the annealed sample (10 pm/V). The piezoresponse of the PVDF micropillar arrays is thus substantially enhanced by poling. © 2019 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2019.122035
  • Spatial Charge Separation as the Origin of Anomalous Stark Effect in Fluorous 2D Hybrid Perovskites
    Queloz, V.I.E. and Bouduban, M.E.F. and García-Benito, I. and Fedorovskiy, A. and Orlandi, S. and Cavazzini, M. and Pozzi, G. and Trivedi, H. and Lupascu, D.C. and Beljonne, D. and Moser, J.-E. and Nazeeruddin, M.K. and Quarti, C. and Grancini, G.
    Advanced Functional Materials 30 (2020)
    2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long-lived correlated electron–hole pairs. Such states have a marked charge-transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.202000228
  • Spray-flame synthesis of BaTi1-xZrxO3 nanoparticles for energy storage applications
    Tarasov, A. and Shvartsman, V.V. and Shoja, S. and Lewin, D. and Lupascu, D.C. and Wiggers, H.
    Ceramics International 46 (2020)
    BaTi1-xZrxO3 nanoparticles (x = 0, 0.05, 0.1, 0.15, 0.2) were successfully produced by the spray-flame synthesis method. The as-synthesized powders are characterized by small (~10 nm) particle sizes as shown by TEM images. The as-synthesized powders were pre-heated at 800 °C to remove organic residuals from the surface. Pellets were then pressed and sintered at 1100 °C for 3 h. XRD measurements of the sintered materials show that the crystallite size decreases with increasing Zr concentration, which was additionally confirmed by TEM. Dielectric measurements show that the Curie temperature shifts towards lower temperatures with increasing Zr concentration accompanied by a decrease in the dielectric permittivity values which is attributed to a decreasing crystallite/particle size. In addition, a frequency dispersion of the permittivity values is discovered. This is mostly ascribed to Maxwell-Wagner polarization effects typical for nanograined ceramics. © 2020 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2020.02.187
  • The phenomenon of bitumen ‘bee' structures–bulk or surface layer–a closer look
    Ganter, D. and Franzka, S. and Shvartsman, V.V. and Lupascu, D.C.
    International Journal of Pavement Engineering (2020)
    Bitumen is the primary binder of asphalt covering most of the roads in the world. The origin of the primary oil, the refinery treatment, the specific chemical structure, and the natural or induced chemical modifications determine the mechanical properties of bitumen. Aging affects the time evolution of its thermo-rheological behaviour. Numerous studies have suggested that the particular local microstructure of bitumen affects its rheological properties including ‘bee’–patterned structures. We have used scanning probe microscopy to distinguish between surface effects and bulk properties. Using scanning probe microscopy we conclude that the ‘bee’ structures exist at the surface only and do not occur in the bulk. In particular, they are not observed on freshly fractured surfaces. A special technique was used to chisel off the bitumen surface. The material relaxes and the ‘bee’ structures disappear from the surface. This proves that the ‘bees’ are formed as a thin surface layer. Improved recycling will thus have to rely on chemical treatment of aged and used bitumen rather than on intentional modifications to the surface microstructure which is merely a surface effect. The bitumen surface microstructure can nevertheless be seen as a fingerprint of the overall bitumen properties to a certain degree. © 2020 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/10298436.2020.1823390
  • Using radioactive beams to unravel local phenomena in ferroic and multiferroic materials
    Schell, J. and Hofsäss, H. and Lupascu, D.C.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 463 (2020)
    The increasing interest in using ferroic and multiferroic materials in high-tech applications requires that the underlying physical phenomena are studied at the atomic scale. Time-differential perturbed angular correlation (TDPAC) measurements have a local character and can provide important information concerning combined magnetic dipole and electric quadrupole interactions in multiferroic systems. With the application of characterization techniques and radioactive beams, this method has become very powerful, especially for the determination of the temperature dependence of hyperfine parameters, even at high temperatures. Such measurements lead to a better understanding of phase transitions, including observations of local environments in low fractions of different phases. Several facilities are in use at ISOLDE-CERN that benefit from the multitude of available ion beams that exist for the use and development of the TDPAC technique. An overview of the prior literature, combined with a discussion of measurement conditions and isotopes, is presented. Particular emphasis is given to the important case of measurements carried out at ISOLDE-CERN employing 111mCd as a probe. © 2019 Elsevier B.V.
    view abstract10.1016/j.nimb.2019.06.016
  • 3D magnetostrictive Preisach model for the analysis of magneto-electric composites
    Labusch, M. and Schröder, J. and Lupascu, D.C.
    Archive of Applied Mechanics 89 (2019)
    In the present contribution, we compare numerical simulations of magneto-electric composites with experimental measurements. The coupling between electric polarization and magnetization of such materials can improve the operation of sensors and actuators and can enable new technical devices. These composites consist of a ferroelectric and a magnetostrictive phase and generate the ME coupling as a strain-induced product property. However, the responses of the individual phases are highly nonlinear. It is important to predict the behaviors of both phases in an appropriate manner to ensure a realistic prediction of the magneto-electric coefficient. Therefore, we simulate the nonlinear ferroelectric hysteresis curves based on the ferroelectric and ferroelastic switching behavior of the spontaneous polarization directions on the submicroscopic level. Therefore, a switching criterion considering the change in the free energy is evaluated. For the simulation of the magnetostrictive behavior, we derive in this contribution a three-dimensional Preisach model. For this, the classical scalar Preisach model acts on a rotational time-dependent magnetization vector. After a homogenization approach within the finite-element (FE 2)-method, the effective macroscopic hysteresis curves are obtained. Furthermore, the magneto-electric coefficient is obtained from the homogenization and compared with experimental measurements in terms of magnitude and nonlinear behavior. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00419-019-01529-w
  • A hyperfine look at titanium dioxide
    Schell, J. and Zyabkin, D. and Lupascu, D.C. and Hofsäss, H.-C. and Karabasov, M.O. and Welker, A. and Schaaf, P.
    AIP Advances 9 (2019)
    Titanium dioxide is a commonly used material in a wide range of applications, due to its low price, and the increasing demand for it in the food- and pharmaceutical industries, and for low- and high-tech applications. Time-differential perturbed angular correlation (TDPAC) and Mössbauer spectroscopy measurements have a local character and can provide important and new information on the hyperfine interactions in titanium dioxide. With the application of characterization techniques and radioactive beams, these methods have become very powerful, especially for the determination of temperature dependence of hyperfine parameters, even at elevated temperatures. Such measurements lead to a better understanding of lattice defects and irregularities, including local environments with low fractions of particular defect configurations that affect electric quadrupole interactions. At ISOLDE-CERN, physicists benefit from the many beams available for the investigation of new doping configurations in titanium dioxide. We report the annealing study of titanium dioxide by means of the time differential perturbed γ-γangular correlation of 111mCd/111Cd in order to study the possible effects of vacancies in hyperfine parameters. This paper also provides an overview of TDPAC measurements and gives future perspectives. © 2019 Author(s).
    view abstract10.1063/1.5097459
  • A two-scale homogenization analysis of porous magneto-electric two-phase composites
    Labusch, M. and Schröder, J. and Lupascu, D.C.
    Archive of Applied Mechanics 89 (2019)
    A computational homogenization analysis for the simulation of porous magneto-electric composite materials is presented. These materials combine two or more ferroic states with each other enabling a coupling between magnetization and electric polarization. This magneto-electric coupling finds application in sensor technology or data storage devices. Since most single-phase multiferroics show coupling at very low temperatures beyond technically relevant applications, two-phase composites, consisting of a ferroelectric and a ferromagnetic phases, are manufactured. They generate a strain-induced magneto-electric coupling at room temperature. The performance and reliability of these materials is influenced by defects or pores, which can arise during the manufacturing process. We analyze the impact of pores on the magnitude of the magneto-electric coupling coefficient. In order to determine the effective properties of the composite, a two-scale finite element (FE 2) homogenization approach is performed. It combines the macroscopic and microscopic scale by direct incorporation of the microscopic morphology. We derive the basic equations for the localization and the homogenization of the individual field variables and give an algorithmic expression for the effective tangent moduli. We discuss the influence of pores on the magneto-electric coupling in two-phase composites by analyzing numerical examples. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00419-018-01500-1
  • Advanced modeling of ferroic materials
    Schröder, J. and Lupascu, D.C.
    Archive of Applied Mechanics 89 (2019)
    view abstract10.1007/s00419-019-01550-z
  • Bismuth-Antimony mixed double perovskites Cs2AgBi1-xSbxBr6in solar cells
    Pantaler, M. and Olthof, S. and Meerholz, K. and Lupascu, D.C.
    MRS Advances 4 (2019)
    Reported conversion efficiencies of lead based perovskite solar cells keep increasing steadily. But next to the demand for high efficiency, the need for analogue non-toxic material systems remains. One promising lead free absorber material is the double perovskite Cs2AgBiBr6. Interest in this and other double perovskites has been increasing in the last three years and several solar cells using different device structures have been reported. However, the efficiency of these solar cells is merely in the range of 2%. To further improve solar cell performance we prepared mixed bismuth-antimony double perovskite Cs2AgBi1-xSbxBr6 where different fractions of antimony (x=0.125, 0.25, 0.375, 0.50) are used. This was motivated by reports of lower bandgap values in these mixed system. After the optimization of preparation of these thin films, we have carefully analysed the effects on the structure, composition, electronic structure, as well as optical properties. Finally, we have fabricated Bi-Sb mixed double perovskite solar cells in a mesoscopic device architecture. © Materials Research Society 2019.
    view abstract10.1557/adv.2019.404
  • Bitumen rheology and the impact of rejuvenators
    Ganter, D. and Mielke, T. and Maier, M. and Lupascu, D.C.
    Construction and Building Materials 222 (2019)
    Bitumen is a material used in many industrial applications. It is the primary binding material for road pavements in asphalt. To improve sustainability, it is important to be aware of the finite nature of conventional oil reserves. As a cost and energy saving material, reclaimed asphalt pavement can help preserve our resources. Virgin binder ages significantly during its service life time yielding declining mechanical properties. As material properties of bitumen are responsible for the endurance of asphalt pavements, adding rejuvenators is supposed to restore the mechanical properties like the original bitumen properties. In this paper, three different rejuvenators were studied using one common polymer modified bitumen PmB 25/55-55 A. The virgin binder was aged in two different aging steps (short and long-term) and rheological properties were determined by traditional bitumen tests and dynamic shear rheometer (DSR) tests. For a better understanding of rejuvenation on the microscale, virgin, aged, and modified bitumen samples were measured using Optical Microscopy and Atomic Force Microscopy (AFM). Aging has significant influence on the macro- and micro-Rheology of bitumen. It causes changes in viscosity and at the same time clear changes in its surface structure. All Rejuvenators positively influence the rheological properties to different extents. Atomic Force Microscopy revealed considerable changes of the morphology between virgin, aged, and rejuvenated binders. The combination of rheological properties and micro structure imaging is an important tool in advancing and optimizing reclaimed asphalt pavement. © 2019 Elsevier Ltd
    view abstract10.1016/j.conbuildmat.2019.06.177
  • Dual-source evaporation of silver bismuth iodide films for planar junction solar cells
    Khazaee, M. and Sardashti, K. and Chung, C.-C. and Sun, J.-P. and Zhou, H. and Bergmann, E. and Dunlap-Shohl, W.A. and Han, Q. and Hill, I.G. and Jones, J.L. and Lupascu, D.C. and Mitzi, D.B.
    Journal of Materials Chemistry A 7 (2019)
    Non-toxic and air-stable silver bismuth iodide semiconductors are promising light absorber candidates for photovoltaic applications owing to a suitable band gap for multi- or single-junction solar cells. Recently, solution-based film fabrication approaches for several silver bismuth iodide stoichiometries have been investigated. The current work reports on a facile and reproducible two-step coevaporation/annealing approach to deposit compact and pinhole-free films of AgBi 2 I 7 , AgBiI 4 and Ag 2 BiI 5 . X-ray diffraction (XRD) in combination with scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) analysis reveals formation of pure cubic (Fd3m) phase AgBi 2 I 7 , cubic (Fd3m) or rhombohedra (R3m) phase AgBiI 4 , each with >3 μm average grain size, or the rhombohedral phase (R3m) Ag 2 BiI 5 with >200 nm average grain size. A phase transition from rhombohedral to cubic structure is investigated via temperature-dependent X-ray diffraction (TD-XRD). Planar-junction photovoltaic (PV) devices are prepared based on the coevaporated rhombohedral AgBiI 4 films, with titanium dioxide (TiO 2 ) and poly(3-hexylthiophene) (P3HT) as electron- and hole-transport layers, respectively. The best-performing device exhibited a power conversion efficiency (PCE) of as high as 0.9% with open-circuit voltage (V OC ) > 0.8 V in the reverse scan direction (with significant hysteresis). © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ta08679f
  • Effects of selenization time and temperature on the growth of Cu2ZnSnSe4 thin films on a metal substrate for flexible solar cells
    Stanchik, A.V. and Gremenok, V.F. and Juskenas, R. and Tyukhov, I.I. and Tivanov, M.S. and Fettkenhauer, C. and Shvartsman, V.V. and Giraitis, R. and Hagemann, U. and Lupascu, D.C.
    Solar Energy (2019)
    Thin film Cu2ZnSnSe4 (CZTSe) solar cells can be grown on flexible and lightweight metal substrates allowing their direct integration on bendable surfaces and where the weight of solar cell is an important criterion. Flexible substrates make it possible to use the roll-to-roll technology of solar cells, which leads to an additional reduction in the cost of production and final cost of solar cells. The CZTSe thin films were fabricated by selenization of electrodeposited metallic precursors onto tantalum (Ta) flexible substrates at different temperature and time. The results of the effect of selenization temperature and time on the morphology, structural, and optical property of the CZTSe films are presented in this work. It was found that the morphology of the CZTSe thin films depend on their elemental composition and time of selenization. Experimental data indicate that composition of the CZTSe films selenized within 10 and 20 min at 560 °C have the CZTSe basic phase and secondary phases (CuSe, SnSe and ZnSe). In contrast, the increase in selenization temperature and/or time leads to disappearing of the secondary phases (CuSe, SnSe) and better crystallization of the CZTSe films. It was found that films selenized at 560 and 580 °C within the same time have similar characteristics. Depending on selenization time and temperature of the CZTSe, thin films exhibited a shift in band gap from 1.16 to 1.19 and to 1.22 eV, respectively. The change of band gap of the CZTSe thin films is associated with changes of elemental and phase compositions, and thickness of the film. These results showed that the received CZTSe films on Ta foil can be used for fabrication of thin film solar cells. © 2018
    view abstract10.1016/j.solener.2018.12.025
  • Electrocaloric effect in P(VDF-TrFE)/ barium zirconium titanate composites
    Shanshan, G. and Castillo, M.E. and Shvartsman, V.V. and Karabasov, M. and Lupascu, D.C.
    2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings (2019)
    The electrocaloric effect in P(VDF-TrFE)/ barium zirconium titanate composites was studied by the direct method using a quasi-adiabatic ECE calorimeter. Barium zirconium titanate was synthesised using a hydrothermal method. Analytical results of the prepared material indicate the formation of phase pure particles with particle sizes between 100-200 nm. The polymer composite films with different filler content were prepared using the solution-casting method. The Curie transition temperature of barium zirconium titanate and P(VDF-TrFE) is in the range of 360-380 K. We observe an enhancement of the dielectric constant and the electrocaloric value in this temperature range due to the superposition of the polarization changes in the polymer and the filler. © 2019 IEEE.
    view abstract10.1109/ISAF43169.2019.9034944
  • Influence of synthesis route on the properties of lead iron niobate
    Bartek, N. and Shvartsman, V.V. and Lupascu, D.C. and Prah, U. and Uršič, H.
    2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings (2019)
    Pb(Fe1/2Nb1/2)O3 powders have been prepared via different synthesis routes: conventional solid state, molten salt, combustion, hydrothermal and mechanochemical activation assisted syntheses. The homogenized powders were pressed into pellets, sintered at 1000 °C in an oxygen atmosphere and characterized. The ceramic samples prepared by mechanochemical activation assisted synthesis exhibited the best functional properties. These ceramics were highly dense, with no secondary phases exhibiting sharp ferroelectric hysteresis loops with the remanent polarization 28.1 μC/cm2 and coercive field 3.1 kV/cm. In all other samples, the dielectric permittivity, dielectric losses versus temperature and polarization versus electric field measurements reveal enhanced electrical conductivity. © 2019 IEEE.
    view abstract10.1109/ISAF43169.2019.9034943
  • Magnetostriction via Magnetoelectricity: Using Magnetoelectric Response to Determine the Magnetostriction Characteristics of Composite Multiferroics
    Filippov, D.A. and Laletin, V.M. and Poddubnaya, N.N. and Shvartsman, V.V. and Lupascu, D.C. and Zhang, J. and Srinivasan, G.
    Technical Physics Letters 45 (2019)
    Abstract: We propose a new method for determining the magnetostriction characteristics of composite multiferroics by measuring the magnetoelectric (ME) response of the material structure. It is established that the integral characteristic of the ME effect coincides to within a constant factor with the magnetostriction curve. The results of an experimental investigation of the physical properties of volume composites based on lead zirconate titanate (PZT) and nickel ferrite spinel are presented. The field dependence of the ME voltage coefficient was used to determine magnetostriction curves of composite structures containing 10–70 wt % ferrospinel. © 2019, Pleiades Publishing, Ltd.
    view abstract10.1134/S1063785019110208
  • Piezoelectric response in hybrid micropillar arrays of poly(vinylidene fluoride) and reduced graphene oxide
    Pariy, I.O. and Ivanova, A.A. and Shvartsman, V.V. and Lupascu, D.C. and Sukhorukov, G.B. and Ludwig, T. and Bartasyte, A. and Mathur, S. and Surmeneva, M.A. and Surmenev, R.A.
    Polymers 11 (2019)
    This study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α,β, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from -120° to 20°-40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d33 eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response. © 2019 by the authors.
    view abstract10.3390/POLYM11061065
  • Spatially resolved investigation of the defect states in methylammonium lead iodide perovskite bicrystals
    Sirotinskaya, S. and Fettkenhauer, C. and Okada, D. and Yamamoto, Y. and Lupascu, D.C. and Schmechel, R. and Benson, N.
    Journal of Materials Chemistry C 7 (2019)
    Organic-inorganic halide perovskites are one of the most promising novel materials for photovoltaic applications. One of the most common and well-researched compounds in this material class is methylammonium lead iodide (MAPI). However, the formation of different kinds of defects in the polycrystalline MAPI thin films and their influence on the electrical performance of solar cells is still a matter of debate. In this work, we use single MAPI crystals grown by inverse temperature crystallization as a model system for the systematic experimental evaluation of defects as suggested by current theoretical work on MAPI thin films. The macroscopic nature of these crystallites enables an experimental approach that allows the separate evaluation of the grain boundary and crystallite bulk. For this purpose, we have employed a combination of μ-PL and μ-XPS measurements to probe the defect types suggested by the literature to determine the most likely defect types in the MAPI crystallites/thin film areas. © The Royal Society of Chemistry 2019.
    view abstract10.1039/c8tc06622a
  • Thermal annealing effects in polycrystalline EuTiO3 and Eu2Ti2O7
    Schell, J. and Kamba, S. and Kachlik, M. and Maca, K. and Drahokoupil, J. and Rano, B.R. and Nuno Gonçalves, J. and Thanh Dang, T. and Costa, A. and Noll, C. and Vianden, R. and Lupascu, D.C.
    AIP Advances 9 (2019)
    Using time-differential perturbed angular correlation of γrays, we investigated the electric-field gradient of polycrystalline EuTiO3 and Eu2Ti2O7, with 181Hf(181Ta) as a probe, following different thermal treatments. The measurements were performed at ISOLDE-CERN following 80 keV implantation at the Bonn Radioisotope Separator. The experimental results indicated successful induction of different phases in the implantation recovery process at 1273 and 1373 K. These observations were combined with ab initio calculations and X-ray diffraction measurements. A comparison of ab initio calculated electric-field gradients with the measured values discriminates between different structures and defects and rules out many possible cases. The Ta probe at the Ti site in the Eu2Ti2O7 phase is found to be the most probable case of site occupation after annealing at 1373 K, while annealing at 1273 K keeps EuTiO3 in the vicinity of the Ta probe. A discussion of the hyperfine interactions that promote variation in the interaction strength at the 181Ta site is presented. © 2019 Author(s).
    view abstract10.1063/1.5115466
  • Tuning the optical, structural and multiferroic properties of Bismuth Ferrite (BiFeO3) Nanoparticles by Doping with Ba
    Dubey, A. and Castillo, M.E. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H.
    2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings (2019)
    Bismuth Ferrite Nanoparticles (BiFeO3 NPs) are interesting single-phase multiferroic materials due to their notable magnetoelectric properties at room temperature. We enhance the magnetization of BFO NPs via doping 5 % Ba at the Bi site. These NPs were synthesized by a modified and reproducible sol-gel technique and further characterized by XRD (x-ray diffraction), VSM (Vibrating Sample Magnetometer), PFM (Piezoresponse Force Microscopy), DSC (Differential Scanning Calorimetry) and UV-Vis (Absorption spectroscopy) techniques. Rietveld refinement unveils a lattice distortion and shows that the average crystallite size is reduced from 43.7 to 38.4 nm after Ba incorporation. 5 % Ba enhances the ferromagnetism from 0.71 to 0.86 Am2/kg at 300 K. DSC curves confirm no significant variation in magnetic transition temperature (TN) after Ba doping. Moreover, PFM data show the ferroelectric behavior of 5 % Ba doped NPs with reduced piezoresponse. UV-Vis spectra show the presence of crystal field transition and a doubly degenerate d-d transition, whereas the band gap of 5 % Ba doped BFO NPs increases from 2.18 eV for BFO NPs to 2.23 eV. © 2019 IEEE.
    view abstract10.1109/ISAF43169.2019.9034963
  • A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides
    Khazaee, M. and Sardashti, K. and Sun, J.-P. and Zhou, H. and Clegg, C. and Hill, I.G. and Jones, J.L. and Lupascu, D.C. and Mitzi, D.B.
    Chemistry of Materials 30 (2018)
    Despite the significant progress in fabricating hybrid organic-inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and nontoxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. Here we present a versatile and facile two-step coevaporation approach to fabricate A3Bi2I9 (A = Cs, Rb) and AgBi2I7 polycrystalline films with smooth, pinhole-free morphology and average grain size of >200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI3 sources), followed by an annealing step under BiI3 vapor. The structural, optical, and electrical characteristics of the resulting thin films are studied by X-ray diffraction, optical spectroscopy, X-ray/UV photoelectron spectroscopy, and scanning electron microscopy. Copyright © 2018 American Chemical Society.
    view abstract10.1021/acs.chemmater.8b01341
  • Agglomeration-Free Preparation of Modified Silica Nanoparticles for Emulsion Polymerization - A Well Scalable Process
    Hübner, C. and Fettkenhauer, C. and Voges, K. and Lupascu, D.C.
    Langmuir 34 (2018)
    To prepare modified silica nanospheres for emulsion polymerization, a new agglomeration-free change of dispersion media has been developed. Nanosized silica spheres were synthesized by the Stöber method and directly modified with a silane coupling agent. To prepare these particles for subsequent polymerization, the dispersion medium was changed in a two-step process from ethanol to water without agglomeration of the particles. The emulsion polymerization leads to hemispherical single-core-structured silica-polystyrene composite particles. The thickness of the polymer shell can be altered by varying the amount of styrene. The developed change of dispersion media provides nonagglomerated modified silica particles for the encapsulation with polystyrene and enables the synthesis of narrowly distributed single-core composite particles. The developed process is a promising approach for the preparation of nanoparticles for subsequent polymerization and can be scaled-up for industrial applications. © 2017 American Chemical Society.
    view abstract10.1021/acs.langmuir.7b03753
  • Deposition routes of Cs2AgBiBr6 double perovskites for photovoltaic applications
    Pantaler, M. and Fettkenhauer, C. and Nguyen, H.L. and Anusca, I. and Lupascu, D.C.
    MRS Advances 3 (2018)
    The lead free double perovskite Cs2AgBiBr6 is an upcoming alternative to lead based perovskites as absorber material in perovskite solar cells. So far, the majority of investigations on this interesting material have focused on polycrystalline powders and single crystals. We present vapor and solution based approaches for the preparation of Cs2AgBiBr6 thin films. Sequential vapor deposition processes starting from different precursors are shown and their weaknesses are discussed. Single source evaporation of Cs2AgBiBr6 and sequential deposition of Cs3Bi2Br9 and AgBr result in the formation of the double perovskite phase. Additionally, we show the possibility of the preparation of planar Cs2AgBiBr6 thin films by spin coating. © 2018 Materials Research Society.
    view abstract10.1557/adv.2018.151
  • Dynamic quadrupole interactions in semiconductors
    Dang, T.T. and Schell, J. and Lupascu, D.C. and Vianden, R.
    Journal of Applied Physics 123 (2018)
    The time differential perturbed angular correlation, TDPAC, technique has been used for several decades to study electric quadrupole hyperfine interactions in semiconductors such as dynamic quadrupole interactions (DQI) resulting from after-effects of the nuclear decay as well as static quadrupole interactions originating from static defects around the probe nuclei such as interstitial ions, stresses in the crystalline structure, and impurities. Nowadays, the quality of the available semiconductor materials is much better, allowing us to study purely dynamic interactions. We present TDPAC measurements on pure Si, Ge, GaAs, and InP as a function of temperature between 12 K and 110 K. The probe 111In (111Cd) was used. Implantation damage was recovered by thermal annealing. Si experienced the strongest DQI with lifetime, τg, increasing with rising temperature, followed by Ge. In contrast, InP and GaAs, which have larger band gaps and less electron concentration than Si and Ge in the same temperature range, presented no DQI. The results obtained also allow us to conclude that indirect band gap semiconductors showed the dynamic interaction, whereas the direct band gap semiconductors, restricted to GaAs and InP, did not. © 2018 Author(s).
    view abstract10.1063/1.4993714
  • Effect of substrate orientation on local magnetoelectric coupling in bi-layered multiferroic thin films
    Naveed-Ul-Haq, M. and Webers, S. and Trivedi, H. and Salamon, S. and Wende, H. and Usman, M. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Nanoscale 10 (2018)
    In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr06041j
  • Evolution of poled state in P(VDF-TrFE)/(Pb,Ba)(Zr,Ti)O3 composites probed by temperature dependent Piezoresponse and Kelvin Probe Force Microscopy
    Shvartsman, V.V. and Kiselev, D.A. and Solnyshkin, A.V. and Lupascu, D.C. and Silibin, M.V.
    Scientific Reports 8 (2018)
    Polarized states of polymer/inorganic inclusion P(VDF-TrFE)-(Pb,Ba)(Zr,Ti)O3 composites are studied at the nanoscale using both piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). It has been shown that inorganic inclusions can be visualized using KPFM due to a discontinuity of the surface potential and polarization at the interface between the inclusions and the polymer matrix. The temperature evolution of the PFM and KPFM signal profiles is investigated. Softening of the polymer matrix on approaching the Curie temperature limits application of the contact PFM method. However non-contact KPFM can be used to probe evolution of the polarization at the phase transition. Mechanisms of the KPFM contrast formation are discussed. © 2017 The Author(s).
    view abstract10.1038/s41598-017-18838-1
  • Hysteresis-Free Lead-Free Double-Perovskite Solar Cells by Interface Engineering
    Pantaler, M. and Cho, K.T. and Queloz, V.I.E. and García Benito, I. and Fettkenhauer, C. and Anusca, I. and Nazeeruddin, M.K. and Lupascu, D.C. and Grancini, G.
    ACS Energy Letters 3 (2018)
    Hybrid perovskite solar cells have been creating considerable excitement in the photovoltaic community. However, they still rely on toxic elements, which impose severe limits on their commercialization. Lead-free double hybrid perovskites in the form of Cs2AgBiBr6 have been shown to be a promising nontoxic and highly stable alternative. Nevertheless, device development is still in its infancy, and performance is affected by severe hysteresis. Here we realize for the first time hysteresis-free mesoporous double-perovskite solar cells with no s-shape in the device characteristic and increased device open-circuit voltage. This has been realized by fine-tuning the material deposition parameters, enabling the growth of a highly uniform and compact Cs2AgBiBr6, and by engineering the device interfaces by screening different molecular and polymeric hole-transporting materials. Our work represents a crucial step forward in lead-free double perovskites with significant potential for closing the gap for their market uptake. Copyright © 2018 American Chemical Society.
    view abstract10.1021/acsenergylett.8b00871
  • Ice-Templated Poly(vinyl alcohol): Enhanced Strength and Low Thermal Conductivity
    Voges, K. and Hübner, C. and Vadalá, M. and Lupascu, D.C.
    Macromolecular Materials and Engineering 303 (2018)
    The freeze-casting technique has been used for a variety of materials to tailor their pore structure. The growing ice crystals act as a template for the subsequent voids present in the body. While it is mostly used for inorganic oxide materials, the method is also applicable to polymers like poly(vinyl alcohol) (PVA). The synthesized materials have an aligned anisotropic pore structure which significantly influences the mechanical behavior and the thermal conductivity. Glutaraldehyde (GA) can be used as a cross-linking agent to reduce the water solubility of the PVA samples. The composition-dependent thermal conductivities are measured, and found to be 0.055–0.078 W m−1 K−1 parallel to the freezing direction and 0.048–0.058 W m−1 K−1 in the transverse direction. The scaffold-like structure resembling the hydroxyapatite structures in bones yields high strength values in the axial direction well exceeding that of common unsupported building insulation materials. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mame.201800198
  • Origins of the Inverse Electrocaloric Effect
    Grünebohm, A. and Ma, Y.-B. and Marathe, M. and Xu, B.-X. and Albe, K. and Kalcher, C. and Meyer, K.-C. and Shvartsman, V.V. and Lupascu, D.C. and Ederer, C.
    Energy Technology 6 (2018)
    The occurrence of the inverse (or negative) electrocaloric effect, where the isothermal application of an electric field leads to an increase in entropy and the removal of the field decreases the entropy of the system under consideration, is discussed and analyzed. Inverse electrocaloric effects have been reported to occur in several cases, for example, at transitions between ferroelectric phases with different polarization directions, in materials with certain polar defect configurations, and in antiferroelectrics. This counterintuitive relationship between entropy and applied field is intriguing and thus of general scientific interest. The combined application of normal and inverse effects has also been suggested as a means to achieve larger temperature differences between hot and cold reservoirs in future cooling devices. A good general understanding and the possibility to engineer inverse caloric effects in terms of temperature spans, required fields, and operating temperatures are thus of fundamental as well as technological importance. Here, the known cases of inverse electrocaloric effects are reviewed, their physical origins are discussed, and the different cases are compared to identify common aspects as well as potential differences. In all cases the inverse electrocaloric effect is related to the presence of competing phases or states that are close in energy and can easily be transformed with the applied field. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ente.201800166
  • Quasi-adiabatic calorimeter for direct electrocaloric measurements
    Sanlialp, M. and Shvartsman, V.V. and Faye, R. and Karabasov, M.O. and Molin, C. and Gebhardt, S. and Defay, E. and Lupascu, D.C.
    Review of Scientific Instruments 89 (2018)
    The electrocaloric effect (ECE) in ferroelectric materials is a promising candidate for small, effective, low cost, and environmentally friendly solid state cooling applications. Instead of the commonly used indirect estimates based on Maxwell's relations, direct measurements of the ECE are required to obtain reliable values. In this work, we report on a custom-made quasi-adiabatic calorimeter for direct ECE measurements. The ECE is measured for two promising lead-free materials: Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics. Adiabatic temperature changes of ΔTEC = 0.5 K at 355 K and ΔTEC = 0.3 K at 314 K were achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The quasi-adiabatic ECE measurements reliably match other direct EC measurements using a differential scanning calorimeter or an infrared camera. The data are compared to indirect EC estimations based on Maxwell's relations and show that the indirect measurements typically underestimate the effect to a certain degree. © 2018 Author(s).
    view abstract10.1063/1.4997155
  • Semiconductor effects in ferroelectrics
    Lupascu, D.C. and Anusca, I. and Etier, M. and Gao, Y. and Lackner, G. and Nazrabi, A. and Sanlialp, M. and Trivedi, H. and Ul-Haq, N. and Schröder, J.
    CISM International Centre for Mechanical Sciences, Courses and Lectures 581 (2018)
    In this textbook ferroelectrics have so far been dealt with as insulators. External electric fields can and will induce polarization in any insulating material. This is dielectricity. On top of this, pyroelectrics exhibit a temperature dependent spontaneous electric polarization, namely a crystallographic phase transition which is polar. It disappears above the Curie-point. Below the Curie point, external electric fields can rotate or alter the direction of this spontaneous polarization. If this becomes a remanent state, the material is ferroelectric and exhibits electric hysteresis. Another aspect in these materials is the fact that electrical insulation is a stretchable term. While metals are well defined and offer conductivity down to very low temperatures, already semi-metals will turn partly insulating at low temperature. Semiconductors are typically insulating in a certain low temperature range (energetically ≲ 1/10 kT) above which thermal excitation of charge carriers into the conduction band will induce a finite conductivity. The energetic band gap determines this barrier and the exponential tail of the Fermi-Dirac distribution determines the number of charge carriers in the conduction band as well as the missing electrons (termed holes) in the valence band. Typical ferroelectrics exhibit band gaps that turn the material insulating at room temperature. This is the case for most oxides. Ferroelectric sulfides typically display much lower band gaps and turn conducting at or even below room temperature already. Another aspect enters when one considers that external electroding is always necessary to drive a ferroelectric capacitor. In the context of a semiconductor picture we deal with a classical Schottky barrier. Grain boundaries play another particular role in polycrystalline materials. This may even lead to positive temperature coefficient resistor (PTCR) characteristics. In this lecture we will draw the connection between a ferroelectric, its semiconductor character, point defects, and their overall interactions. Particularly the inner and outer boundaries of crystallites become subject to band bending, 2D-conducting planes, space charge regions, and diverse other effects. Also optical effects as well as fatigue depend on the semiconductor and defect induced properties. We intend to give the newcomer access to this complex field which has seen a peak in understanding in the late 70ies of the 20th century experiencing a certain revival recently due to a number of exciting findings associated with domain walls. Furthermore, magnetoelectric composites have recently been found to display peculiar electrical effects related to their semiconductor character rather than the magnetic part of their properties. © 2018, CISM International Centre for Mechanical Sciences.
    view abstract10.1007/978-3-319-68883-1_3
  • Sequential piezoresponse force microscopy and the 'small-data' problem
    Trivedi, H. and Shvartsman, V.V. and Medeiros, M.S.A. and Pullar, R.C. and Lupascu, D.C.
    npj Computational Materials 4 (2018)
    The term big-data in the context of materials science not only stands for the volume, but also for the heterogeneous nature of the characterization data-sets. This is a common problem in combinatorial searches in materials science, as well as chemistry. However, these data-sets may well be 'small' in terms of limited step-size of the measurement variables. Due to this limitation, application of higher-order statistics is not effective, and the choice of a suitable unsupervised learning method is restricted to those utilizing lower-order statistics. As an interesting case study, we present here variable magnetic-field Piezoresponse Force Microscopy (PFM) study of composite multiferroics, where due to experimental limitations the magnetic field dependence of piezoresponse is registered with a coarse step-size. An efficient extraction of this dependence, which corresponds to the local magnetoelectric effect, forms the central problem of this work. We evaluate the performance of Principal Component Analysis (PCA) as a simple unsupervised learning technique, by pre-labeling possible patterns in the data using Density Based Clustering (DBSCAN). Based on this combinational analysis, we highlight how PCA using non-central second-moment can be useful in such cases for extracting information about the local material response and the corresponding spatial distribution. © 2018 The Author(s).
    view abstract10.1038/s41524-018-0084-9
  • Stress induced magnetic-domain evolution in magnetoelectric composites
    Trivedi, H. and Shvartsman, V.V. and Lupascu, D.C. and Medeiros, M.S.A. and Pullar, R.C.
    Nanotechnology 29 (2018)
    Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing the surface magnetic state of barium titante-hexagonal barium ferrite (BaTiO3-BaFe12O19) ceramic composites using magnetic force microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe12O19, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO3 grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetoelastic anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An average change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetoelastic anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K1 magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aab181
  • Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3 - NiFe2O4 multiferroics: A relationship between phase-connectivity and interface coupling
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Trivedi, H. and Salamon, S. and Webers, S. and Wende, H. and Hagemann, U. and Schröder, J. and Lupascu, D.C.
    Acta Materialia 144 (2018)
    Studying multiferroic magnetoelectrics has been a focus field for the last decade and a half, and the exploration of new materials is one of the several aspects of this quest. Here we report on the synthesis and characterization of NiFe2O4-based multiferroic composites which employ (Ba,Ca)(Zr,Ti)O3 as the ferroelectric/piezoelectric component and NiFe2O4 as the magnetostrictive phase. We find that these composites show excellent magnetoelectric properties. Especially the composite with 30 vol% of NiFe2O4 has a converse ME coefficient approximately two times larger than the previously reported one for BaTiO3-CoFe2O4 composites. A relationship between the phase connectivity within these composites and the ME properties was explored by the time of flight secondary ion mass microscopy. We believe that our investigation will be helpful for the design of magnetoelectric materials as components of sensors and memory devices. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.10.048
  • Two-phase dielectric polar structures in 0.1NBT-0.6ST-0.3PT solid solutions
    Svirskas, Š. and Shvartsman, V.V. and Dunce, M. and Ignatans, R. and Birks, E. and Ostapchuk, T. and Kamba, S. and Lupascu, D.C. and Banys, J.
    Acta Materialia 153 (2018)
    In this work we address the peculiarities of the macroscopic responses in ternary 0.1Na0·5Bi0·5TiO3-0.6SrTiO3-0.3PbTiO3 (0.1NBT-0.6ST-0.3PT) solid solutions. These solid solutions exhibit a spontaneous first order relaxor to normal ferroelectric phase transition. The phase transition is accompanied by a broad dielectric relaxation which expands over 10 orders of magnitude in frequency just above the phase transition temperature. The temperature dependence of polarization shows that non-zero net polarization persists above the phase transition temperature. Below the phase transition temperature, it is not possible to describe the temperature dependence of polarization with a power law function which is valid in normal ferroelectrics. The piezoresponse force microscopy studies reveal that 0.1NBT-0.6ST-0.3PT solids solutions display several local polarization patterns which arise due to the bimodal distribution of grains in the ceramics. We associate the peculiar macroscopic responses to these complex polar structures and their different temperature behaviours. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.04.052
  • A piezoresponse force microscopy study of CaxBa1-xNb2O6 single crystals
    Shvartsman, V.V. and Gobeljic, D. and Dec, J. and Lupascu, D.C.
    Materials 10 (2017)
    Polar structures of CaxBa1-xNb2O6 (CBN100x) single crystals were investigated using piezoresponse force microscopy. Increasing Ca content results in decreasing domain size and enhancement of the polar disorder. For the composition with x = 0.32 the characteristic domain size is similar to that reported for relaxor Sr0.61Ba0.39Nb2O6 (SBN61). However, decay of an artificial macroscopic domain in CBN32 takes place below the macroscopic transition temperature, contrary to SBN61, where random fields stabilize it above the transition temperature. We can conclude that CBN with 0.26 ≤ x ≤ 0.32 does not display classical relaxor behavior and might be considered as a disordered ferroelectric. © 2017 by the authors.
    view abstract10.3390/ma10091032
  • Cd and In-doping in thin film SnO2
    Schell, J. and Lupascu, D.C. and Wilson Carbonari, A. and Domingues Mansano, R. and Freitas, R.S. and Gonçalves, J.N. and Dang, T.T. and Vianden, R.
    Journal of Applied Physics 121 (2017)
    In this paper, we investigate the effects of doping in the local structure of SnO2 by measuring the hyperfine interactions at impurity nuclei using the Time Differential Perturbed Gamma-Gamma Angular Correlation (TDPAC) method in addition to density functional theory simulations. The hyperfine field parameters have been probed as a function of the temperature in thin film samples. The experimental results reveal that 117Cd/In and 111In/Cd are incorporated and stabilized in the SnO2 lattice replacing the cationic site. Significant differences in the electric field gradient were observed from TDPAC measurements with both the probe nuclei. Furthermore, the absence of strongly damped spectra further indicates that implanted Cd atoms (for 117Cd/In probe nuclei measurements) easily occupy regular substitutional Sn sites with good stability. The simulated value for the electric field gradient obtained with the first oxygen neighbor removed is closer to the experimental value observed for 117Cd, which also indicates this configuration as stable and present in the sample. © 2017 Author(s).
    view abstract10.1063/1.4983669
  • Comparison of direct electrocaloric characterization methods exemplified by 0.92 Pb(Mg1/3Nb2/3)O3-0.08 PbTiO3 multilayer ceramics
    Molin, C. and Peräntie, J. and Le Goupil, F. and Weyland, F. and Sanlialp, M. and Stingelin, N. and Novak, N. and Lupascu, D.C. and Gebhardt, S.
    Journal of the American Ceramic Society (2017)
    Electrocaloric device structures have been developed as multilayer ceramics (MLCs) based on fundamental research carried out on PMN-8PT bulk ceramics. Two different MLC structures were prepared with nine layers each and layer thicknesses of 86 μm and 39 μm. The influence of the device design on its properties has been characterized by microstructural, dielectric, ferroelectric, and direct electrocaloric measurement. For direct characterization two different methods, ie temperature reading (thermistor and thermocouple) and heat flow measurement (differential scanning calorimetry), were used. A comparison of results revealed a highly satisfactory agreement between the different methods. This study confirms that MLCs are promising candidates for implementation into energy-efficient electrocaloric cooling systems providing large refrigerant volume and high electrocaloric effect. Due to their micron-sized active layers, they allow for the application of high electric fields under low operation voltages. We measured a maximum electrocaloric temperature change of ΔT=2.67 K under application/withdrawal of an electric field of ΔE=16 kV mm-1, which corresponds to operation voltages below 1.5 kV. © 2017 American Ceramic Society.
    view abstract10.1111/jace.14805
  • Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers
    Anusca, I. and Balčiunas, S. and Gemeiner, P. and Svirskas, S. and Sanlialp, M. and Lackner, G. and Fettkenhauer, C. and Belovickis, J. and Samulionis, V. and Ivanov, M. and Dkhil, B. and Banys, J. and Shvartsman, V.V. and Lupascu, D.C.
    Advanced Energy Materials (2017)
    Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic-inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201700600
  • Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics
    Sanlialp, M. and Luo, Z. and Shvartsman, V.V. and Wei, X. and Liu, Y. and Dkhil, B. and Lupascu, D.C.
    Applied Physics Letters 111 (2017)
    In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance. © 2017 Author(s).
    view abstract10.1063/1.5001196
  • Effect of Al3+ modification on cobalt ferrite and its impact on the magnetoelectric effect in BCZT–CFO multiferroic composites
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Constantinescu, G. and Trivedi, H. and Salamon, S. and Landers, J. and Wende, H. and Lupascu, D.C.
    Journal of Materials Science (2017)
    One of the methods to enhance the functional properties of two-phase multiferroic magnetoelectrics is to increase magnetostriction of the ferrite phase. Al3+-modified cobalt ferrite Co(Al0.5Fe1.5)O4 shows better magnetostriction than unmodified cobalt ferrite. It is used in combination with (Ba,Ca)(Zr,Ti)O3 which has very good piezoelectric properties, to form a multiferroic composite. The composite shows good magnetoelectric characteristics, both macroscopically (converse magnetoelectric coefficient of 11 ps/m) and microscopically. Al3+ proves to be the best non-magnetic dopant to enhance magnetostriction in CoFe2O4 and thus the magnetoelectric coefficient. © 2017 Springer Science+Business Media, LLC
    view abstract10.1007/s10853-017-1444-4
  • Electro-Optic Property of Mg2+/Er3+-Codoped LiNbO3 Crystal: Mg2+ Concentration Threshold Effect
    Du, W.-Y. and Zhang, Z.-B. and Ren, S. and Wong, W.-H. and Yu, D.-Y. and Pun, E.Y.-B. and Zhang, D.-L. and Lupascu, D.
    Journal of the American Ceramic Society 100 (2017)
    A series of Mg2+/Er3+-codoped congruent LiNbO3 crystals were grown by Czochralski method from the growth melts containing 0.5 mol% Er2O3 while varied MgO content from 0.0 to 7.0 mol%. The unclamped electro-optic coefficients γ13 and γ33 of these crystals were measured by Mach–Zehnder interferometry. Two different voltage-applying schemes were adopted: one is the DC voltage applied to the crystal via Al films coated onto crystal surfaces and another is via a pair of external Cu slab electrodes. The coefficients measured by the two schemes show similar strong dependence on Mg2+ concentration. The dependence is non-monotonous, dramatic, and unusual, and reveals the features of two Mg2+ concentration thresholds of optical damage: one in the Mg2+ concentration range of 1.2–2.0 mol% (in crystal) and another in 4.5–5.0 mol%. Around the threshold the electro-optic coefficient decreases abruptly at first and then recovers quickly, and the coefficient drops by >20% (12%) at the first (second) threshold, which exceeds the error 3% considerably. The dramatic behavior is qualitatively explained on the basis of the EO coefficient model of LiNbO3 and the defect structure model for Mg2+-doped LiNbO3. © 2016 The American Ceramic Society
    view abstract10.1111/jace.14574
  • Electrocaloric effect in BaTiO3 at all three ferroelectric transitions: Anisotropy and inverse caloric effects
    Marathe, M. and Renggli, D. and Sanlialp, M. and Karabasov, M.O. and Shvartsman, V.V. and Lupascu, D.C. and Grünebohm, A. and Ederer, C.
    Physical Review B 96 (2017)
    We study the electrocaloric (EC) effect in bulk BaTiO3 (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can, in principle, be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first-order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudocubic direction. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.014102
  • Energy Transfer Kinetics in Photosynthesis as an Inspiration for Improving Organic Solar Cells
    Nganou, C. and Lackner, G. and Teschome, B. and Deen, M.J. and Adir, N. and Pouhe, D. and Lupascu, D.C. and Mkandawire, M.
    ACS Applied Materials and Interfaces 9 (2017)
    Clues to designing highly efficient organic solar cells may lie in understanding the architecture of light-harvesting systems and exciton energy transfer (EET) processes in very efficient photosynthetic organisms. Here, we compare the kinetics of excitation energy tunnelling from the intact phycobilisome (PBS) light-harvesting antenna system to the reaction center in photosystem II in intact cells of the cyanobacterium Acaryochloris marina with the charge transfer after conversion of photons into photocurrent in vertically aligned carbon nanotube (va-CNT) organic solar cells with poly(3-hexyl)thiophene (P3HT) as the pigment. We find that the kinetics in electron hole creation following excitation at 600 nm in both PBS and va-CNT solar cells to be 450 and 500 fs, respectively. The EET process has a 3 and 14 ps pathway in the PBS, while in va-CNT solar cell devices, the charge trapping in the CNT takes 11 and 258 ps. We show that the main hindrance to efficiency of va-CNT organic solar cells is the slow migration of the charges after exciton formation. © 2017 American Chemical Society.
    view abstract10.1021/acsami.7b04028
  • Implantation of cobalt in SnO2 thin films studied by TDPAC
    Schell, J. and Lupascu, D.C. and Carbonari, A.W. and Mansano, R.D. and Dang, T.T. and Vianden, R.
    AIP Advances 7 (2017)
    Here we report time differential perturbed angular correlation (TDPAC) results of Co-doped SnO2 thin films. Making use of stable Co and radioactive 111In implanted at the Bonn Radioisotope Separator with energies of 80 keV and 160 keV, respectively, it was possible to study the dopant incorporation and its lattice location during annealing. The hyperfine parameters have been probed as a function of temperature in vacuum. Two quadrupole interactions were observed. At high temperatures the dominant fraction for the probe nuclei can be assigned to the Cd-incorporation at the cation substitutional site in a highly disordered structure, obtained after implantation, to high crystallinity for the measurements at 873 K and 923 K. The similarity in TDPAC spectra obtained in undoped SnO gives indirect evidence that In and Co diffuse to different depths during the annealing process. Other interpretations will be discussed. © 2017 Author(s).
    view abstract10.1063/1.4983270
  • In and Cd as defect traps in titanium dioxide
    Schell, J. and Lupascu, D.C. and Martins Correia, J.G. and Carbonari, A.W. and Deicher, M. and Barbosa, M.B. and Mansano, R.D. and Johnston, K. and Ribeiro, I.S., Jr. and ISOLDE collaboration
    Hyperfine Interactions 238 (2017)
    We present a study of TiO2 single crystals from the point of view of the dopant atom that simultaneously behaves as the probing element. We used gamma-gamma time dependent perturbed angular correlations working with selected tracer elements (111In/ 111Cd, 111mCd/ 111Cd) together to investigate the different behavior of Cd and In dopants, particularly their interaction with point defects in the TiO2 lattice. Results show that the hyperfine interactions observed at 111Cd from 111In or 111mCd decay are quite different. 111In/ 111Cd results show a single site fraction characterized by a quadrupole frequency with asymmetry parameter similar to those observed for the same probe nuclei in bulk TiO2 oxides. Results for 111mCd/ 111Cd reveal two site fractions, one characterized by the same hyperfine parameters to those measured in bulk TiO2 and another fraction characterized by a quadrupole frequency and asymmetry parameters with higher values, as observed in thin TiO2 films and correlated with point defects. The results are discussed emphasizing the differences for Cd and In as defect traps on TiO2. © 2016, Springer International Publishing Switzerland.
    view abstract10.1007/s10751-016-1373-7
  • Ion implantation in titanium dioxide thin films studied by perturbed angular correlations
    Schell, J. and Lupascu, D.C. and Carbonari, A.W. and Domingues Mansano, R. and Souza Ribeiro, I., Jr. and Dang, T.T. and Anusca, I. and Trivedi, H. and Johnston, K. and Vianden, R.
    Journal of Applied Physics 121 (2017)
    The local environment in titanium dioxide was studied by the time dependent perturbed γ-γ angular correlation of 111In/111Cd and 181Hf/181Ta at the Helmholtz-Institut für Strahlen-und Kernphysik, Bonn. An introduction to the implantation methodologies performed at the Bonn Radioisotope Separator is presented. The investigation was carried out on thin films, which were deposited by magnetic sputtering on Si. The results show two different sites for both probe nuclei with unique electric quadrupole interaction. Using 111Cd one of them has been attributed to the substitutional Ti at the rutile structure. For 181Ta, the spectra show the anatase phase, with a well-defined electric quadrupole frequency. © 2017 Author(s).
    view abstract10.1063/1.4980168
  • Magnetic properties of the Bi0.65La0.35Fe0.5Sc0.5O3 perovskite
    Fedorchenko, A.V. and Fertman, E.L. and Desnenko, V.A. and Kotlyar, O.V. and Čižmár, E. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H. and Salak, A.N. and Khalyavin, D.D. and Olekhnovich, N.M. and Pushkarev, A.V. and Radyush, Yu.V. and Feher, A.
    Acta Physica Polonica A 131 (2017)
    Magnetic properties of polycrystalline multiferroic Bi0.65La0.35Fe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions were studied using a SQUID magnetometer technique. The temperature dependent static magnetic moment M was measured in both zero-field-cooled and field-cooled modes over the temperature range of 5-300 K in low magnetic field H = 0.02 kOe. The field dependent magnetization M(H) was measured in magnetic fields up to 50 kOe at different temperatures up to 230 K after zero-field cooling procedure. A long-range magnetic ordering of the antiferromagnetic type with a weak ferromagnetic contribution takes place below TN ≈ 220 K. Magnetic hysteresis loops taken below TN show a huge coercive field up to Hc ≈ 10 kOe, while the magnetic moment does not saturate up to 50 kOe. A strong effect of magnetic field on the magnetic properties of the compound has been found. Below TN ≈ 220 K the derivatives of the initial magnetization curves demonstrate the existence of a temperature-dependent anomaly in fields of H = 15÷25 kOe. The nature of the anomaly is unknown and requires additional study.
    view abstract10.12693/APhysPolA.131.1069
  • Perturbed angular correlations at ISOLDE: A 40 years young technique
    Schell, J. and Schaaf, P. and Lupascu, D.C.
    AIP Advances 7 (2017)
    The idea that "new-is-small" is a paradigm propelling industries and research: new materials for new applications and new technologies. Precise and efficient characterization techniques are, therefore, required to make the "new" and the "small", understandable, applicable, and reliable. Within this concept, Time Differential Perturbed Angular Correlations, TDPAC, appears as one of the most exotic and efficient techniques to characterize materials and is celebrating 40 years at ISOLDE, CERN. In this overview we explore the TDPAC measurement possibilities at ISOLDE-CERN for solid state physics research with a rich potential due to the wide number of available radioactive probe elements, delivered with great purity and high yield. © 2017 Author(s).
    view abstract10.1063/1.4994249
  • State transition and electrocaloric effect of BaZrxTi1-xO3: Simulation and experiment
    Ma, Y.-B. and Molin, C. and Shvartsman, V.V. and Gebhardt, S. and Lupascu, D.C. and Albe, K. and Xu, B.-X.
    Journal of Applied Physics 121 (2017)
    We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1- xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1- xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature. © 2017 Author(s).
    view abstract10.1063/1.4973574
  • TDPAC and β-NMR applications in chemistry and biochemistry
    Jancso, A. and Correia, J.G. and Gottberg, A. and Schell, J. and Stachura, M. and Szunyogh, D. and Pallada, S. and Lupascu, D.C. and Kowalska, M. and Hemmingsen, L.
    Journal of Physics G: Nuclear and Particle Physics 44 (2017)
    Time differential perturbed angular correlation (TDPAC) of γ-rays spectroscopy has been applied in chemistry and biochemistry for decades. Herein we aim to present a comprehensive review of chemical and biochemical applications of TDPAC spectroscopy conducted at ISOLDE over the past 15 years, including elucidation of metal site structure and dynamics in proteins and model systems. β-NMR spectroscopy is well established in nuclear physics, solid state physics, and materials science, but only a limited number of applications in chemistry have appeared. Current endeavors at ISOLDE advancing applications of β-NMR towards chemistry and biochemistry are presented, including the first experiment on 31Mg2+ in an ionic liquid solution. Both techniques require the production of radioisotopes combined with advanced spectroscopic instrumentation present at ISOLDE. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6471/aa666b
  • TDPAC study of Fe-implanted titanium dioxide thin films
    Schell, J. and Schaaf, P. and Vetter, U. and Lupascu, D.C.
    AIP Advances 7 (2017)
    Fe-doping in TiO2 has been proven to improve several of its properties, including the photocatalytic activity. Time-differential perturbed angular correlation (TDPAC) as the applied spectroscopy method is particularly interesting because it can probe the electric and magnetic interactions on a local atomic scale. In this work the hyperfine interactions on 111Cd atoms substituting Ti atoms in TiO2 due to nearby Fe atoms also diluted within the TiO2 lattice were measured as a function of temperature. The results review two fractions with distinct quadrupole interaction parameters. One site, occupied by the 111Cd probes, presents the smaller quadrupole interaction frequency, namely υq1 = 45 MHz, and can be ascribed to sites that are more distant from the Fe substitutional site whereas the second site characterized with υq2 = 62 MHz is related to Cd probe atoms that are closer to the Fe defect. Additionally, the system has been characterized using electron dispersive spectroscopy. © 2017 Author(s).
    view abstract10.1063/1.4994247
  • Temperature Effect on the Stability of the Polarized State Created by Local Electric Fields in Strontium Barium Niobate Single Crystals
    Shur, V. Y. and Shikhova, V. A. and Alikin, D. O. and Lebedev, V. A. and Ivleva, L. I. and Dec, J. and Lupascu, D. C. and Shvartsman, V. V.
    Scientific Reports 7 (2017)
    The stability of ferroelectric domain patterns at the nanoscale has been a topic of much interest for many years. We investigated the relaxation of the polarized state created by application of a local electric field using a conductive tip of a scanning probe microscope for the model uniaxial relaxor system SrxBa(1-x)Nb(2)O(6) (SBN) in its pure and Ce-doped form. The temporal relaxation of the induced PFM contrast was measured at various temperatures. The average value of the induced contrast decreases during heating for all investigated crystals. Below the freezing temperature the induced state remains stable after an initial relaxation. Above the freezing temperature the induced state is unstable and gradually decays with time. The stability of the induced state is strongly affected by the measuring conditions, so continuous scanning results in a faster decay of the poled domain. The obtained effects are attributed to a decrease of the induced polarization and backswitching of the polarized area under the action of the depolarization field.
    view abstract10.1038/s41598-017-00172-1
  • The solid state physics programme at ISOLDE: Recent developments and perspectives
    Johnston, K. and Schell, J. and Correia, J.G. and Deicher, M. and Gunnlaugsson, H.P. and Fenta, A.S. and David-Bosne, E. and Costa, A.R.G. and Lupascu, D.C.
    Journal of Physics G: Nuclear and Particle Physics 44 (2017)
    Solid state physics (SSP) research at ISOLDE has been running since the mid-1970s and accounts for about 10%-15% of the overall physics programme. ISOLDE is the world flagship for the on-line production of exotic radioactive isotopes, with high yields, high elemental selectivity and isotopic purity. Consequently, it hosts a panoply of state-of-the-art nuclear techniques which apply nuclear methods to research on life sciences, material science and bio-chemical physics. The ease of detecting radioactivity - <1 ppm concentrations - is one of the features which distinguishes the use of radioisotopes for materials science research. The manner in which nuclear momenta of excited nuclear states interact with their local electronic and magnetic environment, or how charged emitted particles interact with the crystalline lattices allow the determination of the location, its action and the role of the selected impurity element at the nanoscopic state. ISOLDE offers an unrivalled range of available radioactive elements and this is attracting an increasing user community in the field of nuclear SSP research and brings together a community of materials scientists and specialists in nuclear solid state techniques. This article describes the current status of this programme along with recent illustrative results, predicting a bright future for these unique research methods and collaborations. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6471/aa81ac
  • A new (Ba, Ca) (Ti, Zr)O 3 based multiferroic composite with large magnetoelectric effect
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Salamon, S. and Wende, H. and Trivedi, H. and Mumtaz, A. and Lupascu, D.C.
    Scientific Reports 6 (2016)
    The lead-free ferroelectric 0.5Ba(Zr 0.2 Ti 0.8)O 3 - 0.5(Ba 0.7 Ca 0.3)TiO 3 (BCZT) is a promising component for multifunctional multiferroics due to its excellent room temperature piezoelectric properties. Having a composition close to the polymorphic phase boundary between the orthorhombic and tetragonal phases, it deserves a case study for analysis of its potential for modern electronics applications. To obtain magnetoelectric coupling, the piezoelectric phase needs to be combined with a suitable magnetostrictive phase. In the current article, we report on the synthesis, dielectric, magnetic, and magnetoelectric characterization of a new magnetoelectric multiferroic composite consisting of BCZT as a piezoelectric phase and CoFe 2 O 4 (CFO) as the magnetostrictive phase. We found that this material is multiferroic at room temperature and manifests a magnetoelectric effect larger than that of BaTiO 3 -CoFe 2 O 4 bulk composites with similar content of the ferrite phase.
    view abstract10.1038/srep32164
  • Dispersibility of vapor phase oxygen and nitrogen functionalized multi-walled carbon nanotubes in various organic solvents
    Khazaee, M. and Xia, W. and Lackner, G. and Mendes, R.G. and Rummeli, M. and Muhler, M. and Lupascu, D.C.
    Scientific Reports 6 (2016)
    The synthesis and characterization of gas phase oxygen-and nitrogen-functionalized multi-walled carbon nanotubes (OMWCNTs and NMWCNTs) and the dispersibility of these tubes in organic solvents were investigated. Recently, carbon nanotubes have shown supreme capacity to effectively enhance the efficiency of organic solar cells (OSCs). A critical challenge is to individualize tubes from their bundles in order to provide homogenous nano-domains in the active layer of OSCs. OMWCNTs and NMWCNTs were synthesized via HNO3 vapor and NH3 treatments, respectively. Surface functional groups and the structure of the tubes were analyzed by temperature-programmed desorption, Fourier transform infrared spectroscopy, transmission electron microscopy, and Raman spectroscopy which confirmed the formation of functional groups on the tube surface and the enhancement of surface defects. Elemental analysis demonstrated that the oxygen and nitrogen content increased with increasing treatment time of the multi-walled carbon nanotube (MWCNT) in HNO3 vapor. According to ultra-violet visible spectroscopy, modification of the MWCNT increased the extinction coefficients of the tubes owing to enhanced compatibility of the functionalized tubes with organic matrices.
    view abstract10.1038/srep26208
  • Electrocaloric Effect in Ba(Zr,Ti)O3–(Ba,Ca)TiO3 Ceramics Measured Directly
    Sanlialp, M. and Shvartsman, V.V. and Acosta, M. and Lupascu, D.C. and Alford, N.
    Journal of the American Ceramic Society 99 (2016)
    In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase. © 2016 The American Ceramic Society
    view abstract10.1111/jace.14456
  • Modified Differential Scanning Calorimeter for Direct Electrocaloric Measurements
    Sanlialp, M. and Molin, C. and Shvartsman, V.V. and Gebhardt, S. and Lupascu, D.C.
    IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 63 (2016)
    Solid-state refrigeration using the electrocaloric effect (ECE) in ferroelectric materials is a promising alternative to the conventional vapor-compression technology. In spite of growing interest to the investigation of the ECE, direct measurements of the effect are still rare. In this paper, we report on a modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the thermal correction factor and use of proper values of the heat capacitance for correct ECE measurements is discussed. The ECE measurements were performed for Ba(Zr0.2Ti0.8)O3 and Ba(Zr0.12Ti0.88)O3 bulk ceramics. Large electrocaloric temperature changes of 0.54 and 0.34 K are achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The relation between the directly measured ECE values and frequently used indirect estimation based on Maxwell's relations is discussed. © 2016 IEEE.
    view abstract10.1109/TUFFC.2016.2592542
  • Multiferroic Clusters: A New Perspective for Relaxor-Type Room-Temperature Multiferroics
    Henrichs, L. F. and Cespedes, O. and Bennett, J. and Landers, J. and Salamon, S. and Heuser, C. and Hansen, T. and Helbig, T. and Gutfleisch, O. and Lupascu, D. C. and Wende, H. and Kleemann, W. and Bell, A. J.
    Advanced Functional Materials 26 (2016)
    Multiferroics are promising for sensor and memory applications, but despite all efforts invested in their research no single-phase material displaying both ferroelectricity and large magnetization at room-temperature has hitherto been reported. This situation has substantially been improved in the novel relaxor ferroelectric single-phase (BiFe0.9Co0.1O3)(0.4)-(Bi1/2K1/2TiO3)(0.6), where polar nanoregions (PNR) transform into static-PNR as evidenced by piezoresponse force microscopy (PFM) and simultaneously enable congruent multiferroic clusters (MFC) to emerge from inherent strongly magnetic Bi(Fe,Co)O-3 rich regions as verified by magnetic force microscopy (MFM) and secondary ion mass spectrometry. The material's exceptionally large Neel temperature T-N = 670 +/- 10 K, as found by neutron diffraction, is proposed to be a consequence of ferrimagnetic order in MFC. On these MFC, exceptionally large direct and converse magnetoelectric (ME) coupling coefficients, approximate to 1.0 x 10(-5) s m(-1) at room-temperature, are measured by PFM and MFM, respectively. It is expected that the non-ergodic relaxor properties which are governed by the Bi1/2K1/2TiO3 component to play a vital role in the strong ME coupling, by providing an electrically and mechanically flexible environment to MFC. This new class of non-ergodic relaxor multiferroics bears great potential for applications. Especially the prospect of a ME nanodot storage device seems appealing.
    view abstract10.1002/adfm.201503335
  • Nanoscale mapping of heterogeneity of the polarization reversal in lead-free relaxor-ferroelectric ceramic composites
    Gobeljic, D. and Shvartsman, V.V. and Belianinov, A. and Okatan, B. and Jesse, S. and Kalinin, S.V. and Groh, C. and Rödel, J. and Lupascu, D.C.
    Nanoscale 8 (2016)
    Relaxor/ferroelectric ceramic/ceramic composites have shown to be promising in generating large electromechanical strain at moderate electric fields. Nonetheless, the mechanisms of polarization and strain coupling between grains of different nature in the composites remain unclear. To rationalize the coupling mechanisms we performed advanced piezoresponse force microscopy (PFM) studies of 0.92BNT-0.06BT-0.02KNN/0.93BNT-0.07BT (ergodic/non-ergodic relaxor) composites. PFM is able to distinguish grains of different phases by characteristic domain patterns. Polarization switching has been probed locally, on a sub-grain scale. k-Means clustering analysis applied to arrays of local hysteresis loops reveals variations of polarization switching characteristics between the ergodic and non-ergodic relaxor grains. We report a different set of switching parameters for grains in the composites as opposed to the pure phase samples. Our results confirm ceramic/ceramic composites to be a viable approach to tailor the piezoelectric properties and optimize the macroscopic electromechanical characteristics. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c5nr05032d
  • The Direct and the Converse Magnetoelectric Effect in Multiferroic Cobalt Ferrite–Barium Titanate Ceramic Composites
    Etier, M. and Shvartsman, V.V. and Salamon, S. and Gao, Y. and Wende, H. and Lupascu, D.C. and Raveau, B.
    Journal of the American Ceramic Society 99 (2016)
    We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe2O4)—barium titanate (BaTiO3) ceramic composites with (0-3) connectivity. Both the converse magnetoelectric coefficient, αC, and the direct voltage magnetoelectric coefficient, αE, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1−x) CoFe2O4–xBaTiO3 with x = 0.5 yielding values αC = 25 psm−1 and αE = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO3 content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO3 content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites. © 2016 The American Ceramic Society
    view abstract10.1111/jace.14362
  • The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO3 films
    Kiselev, D.A. and Zhukov, R.N. and Ksenich, S.V. and Kubasov, I.V. and Temirov, A.A. and Timushkin, N.G. and Bykov, A.S. and Malinkovich, M.D. and Shvartsman, V.V. and Lupascu, D.C. and Parkhomenko, Y.N.
    Journal of Surface Investigation 10 (2016)
    The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime © 2016, Pleiades Publishing, Ltd.
    view abstract10.1134/S1027451016040091
  • Thin films for photovoltaic application
    Lackner, G. and Anusca, I. and Sanlialp, M. and Escobar, M. and Iffländer, S. and Lupascu, D.C.
    Ferroelectrics 496 (2016)
    The thin film technology offers many advantages like low cost, less material consumption, and many more. Thin film technology shows the potential to open a technological route for competing with conventional power generation especially for photovoltaic application. Thin films of organic and inorganic materials like fullerenes, polymers, metal oxides, or perovskites can open the door to next generation of solar cell technology. Different processing routes and material combinations are presented and their performances are discussed. © 2016 Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2016.1155034
  • Computation of non-linear magneto-electric product properties of 0-3 composites
    Schröder, J. and Labusch, M. and Keip, M.-A. and Kiefer, B. and Brands, D. and Lupascu, D.C.
    GAMM Mitteilungen 38 (2015)
    The magneto-electric (ME) coupling of multiferroic materials is of high interest for a variety of advanced applications like in data storage or sensor technology. Since the ME coupling of single-phase multiferroics is too low for technical applications, the manufacturing of composite structures becomes relevant. These composites generate the effective ME coupling as a strain-induced product property. Several experiments on composite multiferroics showed remarkable ME coefficients that are orders of magnitudes higher than those of single-phase materials. The present paper investigates the arising effective product properties of two-phase ME composites by simulating the coupling behavior using a two-scale finite element (FE2) homogenization approach. By means of this method, microstructures with different volume fractions of the individual phases and associated macroscopic ME coupling coefficients are considered. We investigate the influence of different magnetization states by means of the non-linear dissipative magnetostriction material model originally established in [1]. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/gamm.201510002
  • Dense nanopowder composites for thermal insulation
    Voges, K. and Vadalá, M. and Lupascu, D.C.
    Physica Status Solidi (A) Applications and Materials Science 212 (2015)
    We show in this paper that composites of mechanical impedance mismatch nanopowders yield competitive thermal insulation values to aerogels. The powders are connected by a polymer matrix assuring mechanical integrity. A simple calculation for the thermal boundary resistance has been conducted based on established models. © 2014 WILEY-VCH Verlag GmbH & Co.
    view abstract10.1002/pssa.201431551
  • Direct electrocaloric measurements using a differential scanning calorimeter
    Sanlialp, M. and Shvartsman, V.V. and Lupascu, D.C. and Molin, C. and Gebhardt, S.
    Joint IEEE International Symposium on the Applications of Ferroelectric, International Symposium on Integrated Functionalities and Piezoelectric Force Microscopy Workshop (2015)
    Theelectrocaloric effect (ECE) in ferroelectric materials is a promising mechanism for the development of small, effective, low cost, and environmentally friendly solid state refrigerators. During the last decade, an increased interest has been paid to studies of this effect. Getting reliable values requires direct measurements of the ECE instead of the frequently used indirect estimates based on Maxwell's relation. In this paper, we report on the modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the heat capacitance and thermal losses for the correct ECE measurements is discussed. The ECE was measured for bulk ceramics Ba(Zr0.2Ti0.8)O3. The temperature change reaches a value of ΔTEC= 0.44 K at 305 K under an electric field of 2 kV/mm. The obtained data are compared with results of the evaluation of the indirect ECE. © 2015 IEEE.
    view abstract10.1109/ISAF.2015.7172694
  • Doping of inorganic materials in microreactors-preparation of Zn doped Fe3O4 nanoparticles
    Simmons, M.D. and Jones, N. and Evans, D.J. and Wiles, C. and Watts, P. and Salamon, S. and Escobar Castillo, M. and Wende, H. and Lupascu, D.C. and Francesconi, M.G.
    Lab on a Chip - Miniaturisation for Chemistry and Biology 15 (2015)
    Microreactor systems are now used more and more for the continuous production of metal nanoparticles and metal oxide nanoparticles owing to the controllability of the particle size, an important property in many applications. Here, for the first time, we used microreactors to prepare metal oxide nanoparticles with controlled and varying metal stoichiometry. We prepared and characterised Zn-substituted Fe3O4 nanoparticles with linear increase of Zn content (ZnxFe3-xO4 with 0 ≤ x ≤ 0.48), which causes linear increases in properties such as the saturation magnetization, relative to pure Fe3O4. The methodology is simple and low cost and has great potential to be adapted to the targeted doping of a vast array of other inorganic materials, allowing greater control on the chemical stoichiometry for nanoparticles prepared in microreactors. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5lc00287g
  • Effect of dopants on the electrocaloric effect of 0.92 Pb(Mg1/3Nb2/3)O3-0.08 PbTiO3 ceramics
    Molin, C. and Sanlialp, M. and Shvartsman, V.V. and Lupascu, D.C. and Neumeister, P. and Schönecker, A. and Gebhardt, S.
    Journal of the European Ceramic Society 35 (2015)
    This paper presents an extract of modifications of lead magnesium niobate-lead titanate in order to optimize its electrocaloric properties. Electrocaloric entropy (δS<inf>EC</inf>) and temperature changes (δT<inf>EC</inf>) are measured in a temperature range from -5°C up to 100°C using differential scanning calorimetry. The influence of dopants (Li+, Mn2+, Sr2+, Ta5+) on the electrocaloric effect as well as on microstructure, dielectric and ferroelectric behavior is investigated. The maximum of relative permittivity can be shifted to lower or higher temperatures depending on the added dopant and decreased ferroelectric hysteresis losses were observed for all altered compositions. For the undoped PMN-8PT ceramics maximum δT<inf>EC</inf> of 0.58K and δS<inf>EC</inf> of 0.51Jkg-1K-1 were measured, when applying an electric field of 2kVmm-1 at 30°C. The electrocaloric temperature change showed lower values for all doped PMN-8PT. Remarkably, this is accompanied with an increase of the entropy change for the Li-doped sample. © 2015 Elsevier Ltd.
    view abstract10.1016/j.jeurceramsoc.2015.01.016
  • 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 abstract10.1063/1.4927747
  • Local manifestations of a static magnetoelectric effect in nanostructured BaTiO3-BaFe12O9 composite multiferroics
    Trivedi, H. and Shvartsman, V.V. and Lupascu, D.C. and Medeiros, M.S.A. and Pullar, R.C. and Kholkin, A.L. and Zelenovskiy, P. and Sosnovskikh, A. and Shur, V.Y.
    Nanoscale 7 (2015)
    A study on magnetoelectric phenomena in the barium titanate-barium hexaferrite (BaTiO3-BaFe12O19) composite system, using high resolution techniques including switching spectroscopy piezoresponse force microscopy (SSPFM) and spatially resolved confocal Raman microscopy (CRM), is presented. It is found that both the local piezoelectric coefficient and polarization switching parameters change on the application of an external magnetic field. The latter effect is rationalized by the influence of magnetostrictive stress on the domain dynamics. Processing of the Raman spectral data using principal component analysis (PCA) and self-modelling curve resolution (SMCR) allowed us to achieve high resolution phase distribution maps along with separation of average and localized spectral components. A significant effect of the magnetic field on the Raman spectra of the BaTiO3 phase has been revealed. The observed changes are comparable with the classical pressure dependent studies on BaTiO3, confirming the strain mediated character of the magnetoelectric coupling in the studied composites. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c4nr05657d
  • Magnetodielectric effect in relaxor/ferrimagnetic composites
    Naveed Ul-Haq, M. and Yunus, T. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Journal of Alloys and Compounds 640 (2015)
    Abstract We report on the effect of an applied static magnetic field on the dielectric properties of ferroelectric relaxor/ferrimagnetic composites consisting of [Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf>]<inf>0.8</inf>-[CoFe<inf>2</inf>O<inf>4</inf>]<inf>0.2</inf> (BST<inf>0.8</inf>-CFO<inf>0.2</inf>). The pure Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf> (BST30) as well as the composites, were synthesized by solid state reaction method. The X-ray diffraction analysis confirmed that BST30 and CFO coexist in the composite without any secondary phase. The real and the imaginary part of the dielectric permittivity were studied as a function of temperature, with and without static magnetic field, respectively. Relaxor characteristics such as dielectric permittivity and its peak temperature are observed to vary with the magnetic field. This is explained in the context that the applied magnetic field creates magnetostriction in the ferrite phase which is transferred to the relaxor phase via the interface coupling. The strain in the relaxor phase results in the reorientation of local polarization entities, polar nano regions (PNRs), which alters the dielectric characteristics of the sample. This effect is explained in relation to local order parameter q(T) which is found to increase in a certain temperature range above the typical ferroelectric temperature regime with the application of magnetic field. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.03.215
  • Magnetoelectric coupling on multiferroic cobalt ferrite-barium titanate ceramic composites with different connectivity schemes
    Etier, M. and Schmitz-Antoniak, C. and Salamon, S. and Trivedi, H. and Gao, Y. and Nazrabi, A. and Landers, J. and Gautam, D. and Winterer, M. and Schmitz, D. and Wende, H. and Shvartsman, V.V. and Lupascu, D.C.
    Acta Materialia 90 (2015)
    In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)-barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3-0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0-3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3-0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3-0) sample compared to the traditional (0-3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.02.032
  • Measuring the magnetoelectric effect across scales
    Lupascu, D.C. and Wende, H. and Etier, M. and Nazrabi, A. and Anusca, I. and Trivedi, H. and Shvartsman, V.V. and Landers, J. and Salamon, S. and Schmitz-Antoniak, C.
    GAMM Mitteilungen 38 (2015)
    Magnetoelectric coupling is the material based coupling between electric and magnetic fields without recurrence to electrodynamics. It can arise in intrinsic multiferroics as well as in composites. Intrinsic multiferroics rely on atomistic coupling mechanisms, or coupled crystallographic order parameters, and even more complex mechanisms. They typically require operating temperatures much below T = 0C in order to exhibit their coupling effects. Room temperature applications are thus excluded. Consequently, composites have been designed to circumvent this limitation. They rely on field coupling between magnetostrictive and piezoelectric materials or in more advanced scenarios on quantum coupling in between both phases. This overview will describe experimental techniques and their particular limitations in accessing these coupling phenomena at different scales. Strain coupling is the dominant coupling mechanism at the macroscale as well as down to the micrometer. At the nanoscale more subtle effects can arise and some care has to be taken when investigating local coupling at interfaces using scanning probe techniques, e. g. due to semiconductor effects, field screening, or gradient and surface effects. At the smallest length scale atomic or molecular coupling can be tested using X-ray dichroism or probe atoms like 57Fe in Mössbauer spectroscopy. We display a selection of measuring techniques at the different scales and outline possible pitfalls for experimentalists as well as theoreticians when using material parameters extracted from such experimental work. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/gamm.201510003
  • Mechanical properties of commercial LixMn2O4 cathode under different States of Charge
    Amanieu, H.-Y. and Aramfard, M. and Rosato, D. and Batista, L. and Rabe, U. and Lupascu, D.C.
    Acta Materialia 89 (2015)
    Elastic and hardness properties of LiMn2O4 particles extracted from commercially available Li-ion batteries are investigated under different States of Charge (SoC). Instrumented indentation was used for quantitative measurements. It was found that the particles become stiffer for increasing SoC, ranging from 87 GPa (0% SoC) to 104 GPa (100% SoC). Nanoindentation could also detect dissimilar properties between particles of a same cathode. As its spatial resolution is limited, atomic force acoustic microscopy (AFAM) was used to generate stiffness maps. By combining it with micro-Raman spectroscopy as well as Electron Back-Scattered Diffraction (EBSD), elastic isotropy and homogeneity within single particles were found. On the other hand, different neighboring particles present different states of lithiation and stiffnesses. For reference, a {1 1 1} LiMn2O4 wafer was also synthesized and characterized. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.01.074
  • Strong electrocaloric effect in lead-free 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 ceramics obtained by direct measurements
    Sanlialp, M. and Shvartsman, V.V. and Acosta, M. and Dkhil, B. and Lupascu, D.C.
    Applied Physics Letters 106 (2015)
    Solid solutions of (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of ΔTEC = 0.33 K at ∼65 °C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about ∼50%. The discrepancy is attributed to the relaxor character of this material. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4907774
  • Dielectric properties of 0.9Ag0.9Li0.1NbO3 - 0.1Bi0.5K0.5TiO3 ceramics
    Pozingis, J. and Macutkevic, J. and Grigalaitis, R. and Banys, J. and Lupascu, D.C.
    Ferroelectrics 463 (2014)
    New ferrielectric 0.9Ag0.9Li0.1NbO 3-0.1Bi0.5K0.5TiO3 ceramics have been obtained. At room temperature ceramics exhibit perovskite structure, which is close to rombohedral-tetragonal phase boundary. The dielectric properties of ceramics were investigated in wide frequency range from 20 Hz to 1 GHz at temperatures 200 K-750 K. 0.9Ag0.9Li0.1NbO 3-0.1Bi0.5K0.5TiO3 ceramics exhibits a first order mixed order-disorder and displacive ferroelectric phase transition at TC = 430 K. The phase transition dynamics is caused by relaxation soft mode at frequencies below 1 GHz and soft phonon mode at higher frequencies. In ferroelectric phase an additional dielectric dispersion is observed, which could be attributed to dipolar glass relaxation. Copyright © 2014 Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2014.892358
  • Ergodicity reflected in macroscopic and microscopic field-dependent behavior of BNT-based relaxors
    Dittmer, R. and Gobeljic, D. and Jo, W. and Shvartsman, V.V. and Lupascu, D.C. and Jones, J.L. and Rödel, J.
    Journal of Applied Physics 115 (2014)
    The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 - y)(0.81Bi1/2Na 1/2TiO3-0.19Bi1/2K1/2TiO 3)-yBiZn1/2Ti1/2O3 (BNT-BKT-BZT) with y = {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV·mm-1, large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface charges. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4867157
  • Giant mechanically-mediated electrocaloric effect in ultrathin ferroelectric capacitors at room temperature
    Liu, Y. and Infante, I.C. and Lou, X. and Lupascu, D.C. and Dkhil, B.
    Applied Physics Letters 104 (2014)
    Using a phenomenological approach, we demonstrate that a giant mechanically-mediated electrocaloric effect can be obtained in ultrathin ferroelectric SrRuO3/BaTiO3/SrRuO3 capacitors at room temperature. Our results show that the electrocaloric properties of such capacitors can be systematically tuned by applying an external stress. The depolarizing field, whose effect is usually ignored in the literature, is found to be detrimental to the electrocaloric response, especially for the thinner films. Moreover, a remarkable enhancement and broadening of the electrocaloric response can be achieved in relatively thick films under compressively loaded conditions compared with the unloaded case. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4861456
  • Macroscopic and nanoscopic polarization relaxation kinetics in lead-free relaxors Bi 1/2 Na 1/2 TiO 3- Bi 1/2 K 1/2 TiO 3- BiZn 1/2 Ti 1/2 O 3
    Gobeljic, D. and Dittmer, R. and Rödel, J. and Shvartsman, V.V. and Lupascu, D.C.
    Journal of the American Ceramic Society 97 (2014)
    The stability of the field-induced ferroelectric (FE) state was studied in relaxor lead-free ceramics (1 - y)[0.81Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3]-yBiZn1/2Ti1/2O3 both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn1/2Ti1/2)O3, which are in the non-ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two-step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn1/2Ti1/2)O3 content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature. © 2014 The American Ceramic Society.
    view abstract10.1111/jace.13227
  • Mechanical property measurements of heterogeneous materials by selective nanoindentation: Application to LiMn2O4 cathode
    Amanieu, H.-Y. and Rosato, D. and Sebastiani, M. and Massimi, F. and Lupascu, D.C.
    Materials Science and Engineering A 593 (2014)
    Mechanical properties of composite materials for application as electrodes in batteries have been measured by means of selective statistical nanoindentation. The sample is strongly heterogeneous, as it consists of LiMn2O4 particles, carbon black and PVDF embedded in a soft and compliant epoxy matrix. For comparison, a similar composite sample of SiO2 particles in epoxy was prepared. The difference in terms of elastic modulus between the matrix and the particles is of one order of magnitude. Structural compliance and edge effects induce inconsistent tests which in return cause spurious measurements. A 2-step filtering method has been designed to overcome this problem. This automated method consists in identifying spurious tests and withdraw them from the final statistical analysis. First, nonquadratic load versus displacement curves are filtered out. Then, the Joslin-Oliver analysis is used to filter out tests with an apparent structural compliance. The method greatly improves the noise to signal ratio. After deconvolution, the E-modulus of the silica particles was measured as 69.8GPa (±1.2). It shows the reliability of the method. 105GPa (±7.5) was found for the LiMn2O4 particle E-modulus. After pile-up correction, the real E-modulus of LiMn2O4 particles is estimated to be 13% smaller. The developed method has been demonstrated to be an effective tool to investigate mechanical properties of composites. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.11.044
  • Mössbauer study of temperature-dependent cycloidal ordering in BiFeO3 nanoparticles
    Landers, J. and Salamon, S. and Escobar Castillo, M. and Lupascu, D.C. and Wende, H.
    Nano Letters 14 (2014)
    To study the effects of different temperatures and particle sizes on the anharmonic cycloidal spin structure in BiFeO3 nanoparticles, Mössbauer spectroscopy was applied to three sets of particles with different mean diameters in the range of 54 nm to 1.6 μm at temperatures between 4.2 and 800 K. The paramagnetic transition showed a distinct broadening upon decreasing particle size with Néel temperatures decreasing from 652 to 631 K. The anharmonicity of the long-range cycloidal structure, calculated from experimental Mössbauer spectra, is revealed to decrease upon rising temperature, starting at 150-200 K and reaching the harmonic state at about 400 K. © 2014 American Chemical Society.
    view abstract10.1021/nl5031375
  • Nanocrystalline barium strontium titanate ceramics synthesized via the "organosol" route and spark plasma sintering
    Gao, Y. and Shvartsman, V.V. and Gautam, D. and Winterer, M. and Lupascu, D.C.
    Journal of the American Ceramic Society 97 (2014)
    Dense nanocrystalline barium strontium titanate Ba0.6Sr 0.4TiO3 (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified "Organosol" process. X-ray diffraction (XRD) and dielectric measurements in the temperature range 173-313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature TC decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%-4% in the frequency range of 100 Hz-1 MHz and temperature interval 160-320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer-sized ceramics. © 2014 The American Ceramic Society.
    view abstract10.1111/jace.12933
  • Product properties of a two-phase magneto-electric composite: Synthesis and numerical modeling
    Labusch, M. and Etier, M. and Lupascu, D.C. and Schröder, J. and Keip, M.-A.
    Computational Mechanics 54 (2014)
    Magneto-electric (ME) materials are of high interest for a variety of advanced applications like in data storage and sensor technology. Due to the low ME coupling in natural materials, composite structures become relevant which generate the effective ME coupling as a strain-mediated product property. In this framework, it seems to be possible to achieve effective ME coefficients that can be exploited technologically. The present contribution investigates the realization of particulate ME composites with a focus on their experimental and computational characterization. We will show that different states of pre-polarizations of the ferroelectric material have a decisive influence on the overall obtainable ME coefficient. Details on the synthesis of two-phase composite microstructures consisting of a barium titanate matrix and cobalt ferrite inclusions will be discussed. Subsequently we will employ computational homogenization in order to determine the effective properties of the experimental composite numerically. We investigate the influence of different states of pre-polarization on the resulting ME-coefficients. For the numerical incorporation of the pre-polarization we use a heuristic method. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00466-014-1031-3
  • Structure and dielectric properties of (1-x)Ag0.9Li 0.1NbO3-(x)Bi0.5K0.5TiO3 ferroelectric ceramics
    Pozingis, J. and Macutkevic, J. and Grigalaitis, R. and Banys, J. and Lupascu, D.C.
    Ceramics International 40 (2014)
    New (1-x)Ag0.9Li0.1NbO3-xBi 0.5K0.5TiO3 ceramics were synthesized and investigated by means of broadband dielectric spectroscopy (20 Hz-37 GHz), an X-ray and an SEM. It was demonstrated that the ferroelectric phase transition temperature does not monotonically decreases with Bi0.5K 0.5TiO3 concentration and the ferroelectric phase transition completely vanishes in 0.85Ag0.9Li0.1NbO 3-0.15Bi0.5K0.5TiO3 ceramics, where the relaxor behavior is observed. In contrast the antiferroelectric phase transition disappears in 0.9Ag0.9Li0.1NbO 3-0.1Bi0.5K0.5TiO3 ceramics, at room temperature these ceramics are close to a rhombohedral-tetragonal phase boundary. The static dielectric permittivity increases with Bi 0.5K0.5TiO3 concentration due to the increase of disorder in the system and gradual appearance of new excitations in the dielectric spectra: a microwave relaxation soft mode and dynamics of polar nanoregions. © 2014 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2014.02.094
  • Temperature dependence of the local piezoresponse in (K,Na)NbO 3-based ceramics with large electromechanical strain
    Gobeljic, D. and Shvartsman, V.V. and Wang, K. and Yao, F. and Li, J.-F. and Jo, W. and Rödel, J. and Lupascu, D.C.
    Journal of Applied Physics 116 (2014)
    We report on temperature dependence of local electromechanical properties of lead-free (K,Na)NbO3-based ceramics that macroscopically manifests a large temperature-insensitive strain. Piezoresponse force microscopy reveals the particular role of the orthorhombic-tetragonal phase transition, where a reconstruction of the domain structure occurs and local piezoelectric response shows a peak value. A good quantitative agreement between temperature dependences of the local and previously reported macroscopic small-signal piezoelectric coefficients is observed. An influence of the polymorphic phase transition on polarization switching kinetics was revealed. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4891398
  • Effect of particle size on ferroelectric and magnetic properties of BiFeO3 nanopowders
    Castillo, M.E. and Shvartsman, V.V. and Gobeljic, D. and Gao, Y. and Landers, J. and Wende, H. and Lupascu, D.C.
    Nanotechnology 24 (2013)
    The ferroelectric and magnetic behaviour of multiferroic BiFeO3 nanoparticles has been studied using piezoresponse force microscopy (PFM), Mössbauer spectroscopy and SQUID magnetometry. The results of the PFM studies indicate a decay of the spontaneous polarization with decreasing particle size. Nevertheless, particles with diameter ∼50 nm still manifest ferroelectric behaviour. At the same time these particles are weakly ferromagnetic. The Mössbauer spectroscopy studies prove that the weak ferromagnetic state is due to non-compensated surface spins rather than distortions of the cycloidal spin structure characteristic for bulk BiFeO 3. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/24/35/355701
  • Ferroelectric Properties
    Lupascu, D.C. and Morozov, M.I.
    Ceramics Science and Technology 2-4 (2013)
    view abstract10.1002/9783527631940.ch30
  • Local ferroelectric properties in polyvinylidene fluoride/barium lead zirconate titanate nanocomposites: Interface effect
    Silibin, M.V. and Solnyshkin, A.V. and Kiselev, D.A. and Morozovska, A.N. and Eliseev, E.A. and Gavrilov, S.A. and Malinkovich, M.D. and Lupascu, D.C. and Shvartsman, V.V.
    Journal of Applied Physics 114 (2013)
    The local piezoelectric properties of ferroelectric composites consisting of P(VDF-TrFE) copolymer matrix with barium lead zirconate titanate ceramic inclusions were addressed both experimentally using piezoresponse force microscopy technique and theoretically applying the Landau-Ginzburg-Devonshire formalism. A transient region with a width of approximately 40 nm has been found at the interface between the two constituents. It is shown that the piezoresponse in the vicinity of the interface is strongly affected by inhomogeneous stresses originating from an incompatibility of thermal expansion coefficients of PVDF and lead zirconate titanate. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4824463
  • Magnetoelectric effect in (0-3) CoFe 2O 4-BaTiO 3 (20/80) composite ceramics prepared by the organosol route
    Etier, M. and Shvartsman, V.V. and Gao, Y. and Landers, J. and Wende, H. and Lupascu, D.C.
    Ferroelectrics 448 (2013)
    In this paper we report on a new approach to synthesize core/shell cobalt iron oxide/barium titanate composite nanoparticles combining the co-precipitation and organosol crystallization techniques. The weight fraction of CoFe2O4 and BaTiO3 was 20% and 80% respectively. The obtained core/shell powder was used to sinter (0-3) composite multiferroic ceramics. Ferroelectric, magnetic, and magnetoelectric properties of the ceramics were studied. It was found that the value of the converse magnetoelectric coefficient, c, reaches 4.4·10-12 s·m-1 at the magnetic field μ 0 H dc = 0.15 T and T = 285 K. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/00150193.2013.822292
  • Preparation of SiO2-encapsulated BaTiO3 nanoparticles with tunable shell thickness by reverse microemulsion
    Gao, Y. and Elsukova, A. and Lupascu, D.C.
    Particle and Particle Systems Characterization 30 (2013)
    A silica coating on hydrophobic BaTiO3 nanoparticles is created using a reverse microemulsion route. The process is sufficiently controllable to target a tunable silica shell thickness by choosing appropriate BaTiO 3 concentrations. Increasing the concentration of BaTiO3 nanoparticles is found to be an effective approach to decrease silica thickness. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppsc.201300104
  • Temperature-insensitive (K,Na)NbO3-based lead-free piezoactuator ceramics
    Wang, K. and Yao, F.-Z. and Jo, W. and Gobeljic, D. and Shvartsman, V.V. and Lupascu, D.C. and Li, J.-F. and Rödel, J.
    Advanced Functional Materials 23 (2013)
    The development of lead-free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead-free (K,Na)NbO3 (KNN)-based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficientd33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field-induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field-induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature-dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature-insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201203754
  • The microstructure and local piezoelectric response in polymer nanocomposites with different ferroelectric crystalline additions
    Kiselev, D.A. and Malinkovich, M.D. and Parkhomenko, Y.N. and Solnyshkin, A.V. and Bogomolov, A.A. and Silibin, M.V. and Gavrilov, S.A. and Shvartsman, V.V. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1556 (2013)
    In this work, we report on local ferroelectric and piezoelectric properties of nanostructured polymer composites P(VDF-TrFE)+x(Ba,Pb)(Zr,Ti)O3 (x = 0-50%). High-resolution imaging of ferroelectric domains, local polarization switching, and polarization relaxation dynamics were studied by piezoresponse force microscopy. In particular, we found that (Ba,Pb)(Zr,Ti)O3 inclusions usually show a strong unipolar piezoresponse signal, as compared to the polymer matrix. By scanning under high dc voltage the films can be polarized uniformly under both positive and negative electric fields. Stability of the polarized state is discussed. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.784
  • Cobalt ferrite/barium titanate core/shell nanoparticles
    Etier, M. and Gao, Y. and Shvartsman, V.V. and Elsukova, A. and Landers, J. and Wende, H. and Lupascu, D.C.
    Ferroelectrics 438 (2012)
    Cobalt ferrite/barium titanate nanoparticles with a core/shell structure were synthesized by combining co-precipitation and organosol methods. The average particle size was about 110 nm with an average shell thickness of about 40 nm. Dielectric and magnetic properties of the particles were studied using impedance and Mössbauer spectroscopy, respectively. The particles are promising for fabrication of multiferroic ceramics with the core-shell structure. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2012.743773
  • Dyes in vertically aligned carbon nanotube arrays for solar cell applications
    Lackner, G. and Endler, I. and Meissner, F. and Scholz, S. and Mayer-Uhma, T. and Liebschner, R. and Bezugly, V. and Meiss, J. and Mkandawire, M. and Boucher, R. and Michaelis, A. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1390 (2012)
    The infiltration of dissolved dyes into vertically aligned carbon nanotube arrays (va-CNT) is reported. The ultra hydrophobic surface of the CNT forest can be wetted and hence infiltrated for an appropriate choice of solvent. The dye-infiltrated CNT array forms a well ordered bulk-heterojunction structure for organic solar cells in which the CNT can act as a large electrode or, for appropriate energy levels, as an acceptor material. Derivatives of the small molecule copper phthalocyanine or the polymer poly(3-hexylthiophene) were used as dyes. Drop coating was chosen as the infiltration technique resulting in a completely embedded CNT forest. Field emission secondary electron microscopy analysis illustrates the final layer quality. Common electrical characterization under AM1.5 illumination proves photosensitivity and implies photovoltaic behavior of the composite. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.647
  • From mesoscopic to global polar order in the uniaxial relaxor ferroelectric Sr 0.8Ba 0.2Nb 2O 6
    Dec, J. and Kleemann, W. and Shvartsman, V.V. and Lupascu, D.C. and Ukasiewicz, T.
    Applied Physics Letters 100 (2012)
    In the relaxor ferroelectric Sr 0.8Ba 0.2Nb 2O 6 (SBN80) the relaxation of polar nanoregions (PNRs) reveals an anisotropic breathing mode when approaching the transition temperature, T C ≈ 289 K, on cooling. Two distinct dispersion steps and peaks, respectively, of the complex susceptibility components in the milli- and megahertz frequency range reflect their large length-to-width aspect ratio. Below T C power-law frequency dispersion characterizes the creep motion of the emerging ferroelectric domain walls and the fractal dimension D ≈ 2 of their contour lines. Piezoforce micrographs reveal nanometric jagged domains in accordance with the large disorder and random field magnitude in the extreme relaxor SBN80. © 2012 American Institute of Physics.
    view abstract10.1063/1.3680599
  • Lead-free relaxor ferroelectrics
    Shvartsman, V.V. and Lupascu, D.C.
    Journal of the American Ceramic Society 95 (2012)
    Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions. © 2011 The American Ceramic Society.
    view abstract10.1111/j.1551-2916.2011.04952.x
  • Low-temperature synthesis of crystalline BaTiO 3 nanoparticles by one-step "organosol"-precipitation
    Gao, Y. and Shvartsman, V.V. and Elsukova, A. and Lupascu, D.C.
    Journal of Materials Chemistry 22 (2012)
    The "organosol" precipitation method is proposed to produce nanosized particles of barium titanate (BaTiO 3) at temperatures as low as room temperature. The advantages of this method are a high yield, a simple but precise control of the size of the particles, low process temperature, short reaction time, as well as low cost of reagents. The particles were systematically characterized by powder X-ray diffraction (XRD), Raman scattering, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric thermal analysis (TGA/DSC), infrared spectroscopy (IR), and impedance analysis. The as-prepared BaTiO 3 nanocrystals exhibit a granular shape of around 15 nm in diameter. Oleic acid retards crystallization and thus allows generation of a uniformly small grain size and excellent dispersibility in organic solvents. The surface energy of the particles is modified and crystallization in cubes also arises. The mechanism of powder formation is discussed. The method offers an alternate low-cost route to perovskite nanopowders easily dispersed in organic media. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2jm33373b
  • Magnetoelectric properties of 0.2CoFe 2O 4-0. 8BaTiO 3 composite prepared by organic method
    Etier, M. and Gao, Y. and Shvartsman, V.V. and Lupascu, D.C. and Landers, J. and Wende, H.
    Proceedings of 2012 21st IEEE Int. Symp. on Applications of Ferroelectrics held jointly with 11th IEEE European Conference on the Applications of Polar Dielectrics and IEEE PFM, ISAF/ECAPD/PFM 2012 (2012)
    We have successfully synthesized CoFe 2O 4/BaTiO 3 composite nanoparticles with core-shell structure using a new organosol crystallization method. The weight fraction of the ferroelectric and the ferrimagnetic phase was 80% and 20%, respectively. The nanopowders were sintered to form a ceramic composite. The ceramics exhibits the magnetoelectric effect. The value of the converse magnetoelectric coefficient α c measured by a modified SQUID susceptometer reaches 4.4·10 -12 s·m -1 at the magnetic field μ 0H dc = 0.15 T and T = 285 K. © 2012 IEEE.
    view abstract10.1109/ISAF.2012.6297820
  • Reduced exciton binding energy in organic semiconductors: Tailoring the Coulomb interaction
    Engel, M. and Kunze, F. and Lupascu, D.C. and Benson, N. and Schmechel, R.
    Physica Status Solidi - Rapid Research Letters 6 (2012)
    For organic photovoltaics (OPV) the maximum in obtainable power conversion efficiency is limited by a low semiconductor permittivity and the resulting enhanced Coulomb interaction (CI). This, however, is an aspect rarely addressed in the OPV development. Here, a concept is introduced which allows a reduced CI in organic semiconductors. This is the result of a device structure, which upon exciton formation forces part of the electric field between complementary charges through a high-k material, resulting in partial field screening and as such a reduced CI. The feasibility of this concept is substantiated by an investigation on the exciton separation efficiency in pentacene devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201105488
  • Converse magnetoelectric effect in CoFe2O4-BaTiO3 composites with a core-shell structure
    Shvartsman, V.V. and Alawneh, F. and Borisov, P. and Kozodaev, D. and Lupascu, D.C.
    Smart Materials and Structures 20 (2011)
    Multiferroic composites were prepared by covering CoFe2O 4 nanoparticles with a shell of BaTiO3 using a sol-gel technique. Scanning probe microscopy confirmed the formation of a core-shell structure with a magnetic core and a piezoelectric shell. The converse magnetoelectric effect was studied at different temperatures and bias fields. The magnetoelectric coefficient peaks at approximately 270K and reaches the value αC≈(2.2 0.1)10- 11sm- 1, which surpasses those reported previously for similar structures. A change of the sign of the magnetoelectric coefficient observed for an increasing magnetic bias field is related to the non-monotonic field dependence of magnetostriction in polycrystalline CoFe2O4. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0964-1726/20/7/075006
  • Dielectric properties and conductivity of iron oxide-barium titanate composites
    Kupreviciute, A. and Banys, J. and Ramoska, T. and Sobiestianskas, R. and Alawneh, F.M.M. and Gharbi, N. and Lupascu, D.C.
    Ferroelectrics 418 (2011)
    Sintered composites of iron oxide plus barium titanate in form of core-shell structures are investigated by impedance spectroscopy. The measurements were performed in the frequency and temperature ranges 20 Hz to 1MHz and 150 K to 510 K, respectively. The composites exhibit strong low-frequency dispersion of electrical modulus and dielectric permittivity which is caused by different mechanisms of electron transfer at low and high temperatures. A hopping transport process with activation energy of 0.22 eV for exchange between Fe 2+ and Fe 3+ was tentatively assigned to reign below room temperature. It gradually changes to a conducting mechanism with activation energy of 0.45 eV above 345 K. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2011.578947
  • Investigation of dielectric and noise properties of the multiferoic composite BaTiO 3 with CoFe 2O 4
    Banys, J. and Ramoška, T. and Matukas, J. and Pralgauskaite, S. and Alawneh, F.M.M. and Shvartsman, V.V. and Lupascu, D.C.
    Ferroelectrics 417 (2011)
    Dielectric and noise characteristics of CoFe 2O 4-BaTiO 3 ceramic composites were investigated. The complex dielectric permittivity displays a broad maximum around 300 K, where a phase transition occurs. The dielectric spectra of the complex dielectric permittivity are caused by the conductivity above 300 K. Intensive almost periodic telegraphic noise with relaxation type spectra is observed in the same temperature range (around 300 K). An investigation of the noise characteristic shows that several noise intensity peaks occur during this phase transition. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2011.578464
  • Large coercivity and polarization of sol-gel derived BaTiO3 nanowires
    Boucher, R. and Renz, P. and Li, C. and Fuhrlich, T. and Bauch, J. and Yoon, K.H. and Lupascu, D.C.
    Journal of Applied Physics 110 (2011)
    BaTiO3 nanowires were grown in an amorphous aluminum oxide membrane from a sol-gel via electrophoresis. These had a ferroelectric coercive field of 5 kV/mm after undergoing a thermal treatment. This value is 12-16 times larger than the bulk value. Such enhancements are in agreement with local PFM measurements by others. An enhanced polarization of ≈80 C/cm2 was also measured, where the bulk is around 26 μC/cm2. In addition, low leakage currents were also observed. © 2011 American Institute of Physics.
    view abstract10.1063/1.3641986
  • Relaxor behaviour and soft mode in 0.85Ag0.9Li 0.1NbO3 - 0.15Bi0.5K0.5TiO 3 ceramics
    Pozingis, J. and Banys, J. and MacUtkevic, J. and Adomavicius, R. and Krotkus, A. and Lupascu, D.C.
    Ferroelectrics 416 (2011)
    New leadless ferroelectric relaxor 0.85Ag0.9Li 0.1NbO3-0.15Bi0.5K0.5TiO 3 ceramics have been obtained. The ceramics exhibits a smeared frequency-dependent maximum of permittivity, which follows the Fogel-Vulcher law with the freezing temperature T0 = 250 K. The Last mode in 0.85Al 0.9Li0.1NbO3-0.15Bi0.5K 0.5TiO3 ceramics is the ferroelectric soft mode inside polar nanoregions, with the critical temperature Tc = 437 K. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2011.577675
  • Synthesis and characterization of BaTiO3 nanopowders and BaTiO3/CoFe2O4 nanocomposites
    Gao, Y. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1397 (2011)
    Highly stable, organic-based barium titanate (BaTiO3) sols were developed by the low cost and straightforward "organosol"- precipitation and auto-combustion process of amorphous organic precursors. BaTiO3/CoFe2O4 nanocomposites with core/shell structures embedded in a BaTiO3 matrix were also obtained using this process. The particles are systematically characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric and differential thermal analyses (TGA/DTA), infrared spectroscopy (IR) and by laser granulometry for particle size distribution determination. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.1013
  • Synthesis and magnetic properties of cobalt ferrite nanoparticles
    Etier, M.F. and Shvartsman, V.V. and Stromberg, F. and Landers, J. and Wende, H. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1398 (2011)
    Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.699
  • Refatigue of ferroelectric lead zirconate titanate
    Zhang, Y. and Lupascu, D.C.
    Journal of the American Ceramic Society 93 (2010)
    Bipolar fatigue of ferroelectric lead zirconate titanate ceramics can be partly recovered by thermal annealing. For samples of initially, intermediately, or highly fatigued state, subsequent refatiguing yields a much more rapid decay in polarization and piezoelectric constant than the initially nonfatigued samples despite very similar recovery from annealing. The "material memory" of the prior fatigue treatment is discussed in the framework of microcracking and fixed polarization by crack face charges. © 2010 The American Ceramic Society.
    view abstract10.1111/j.1551-2916.2010.03883.x
  • actuator and sensors

  • composites

  • ferroelectrics

  • magnetoelectric materials

  • multiferroics

  • photocatalysis

  • photovoltaics

  • piezoelectric materials

  • piezoresponse force microscopy

  • thermal isolators

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