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350 results on '"Kroupa, J."'

1. Multiple soft-mode vibrations of lead zirconate

Author

Hlinka, J., Ostapchuk, T., Buixaderas, E., Kadlec, C., Kuzel, P., Gregora, I., Kroupa, J., Savinov, M., Drahokoupil, J., and Dec, J.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Other Condensed Matter
Abstract

Polarized Raman, IR and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibration of Pb and/or oxygen octahedra librations and their relation to particular phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. The most remarkable finding is the considerably enhanced frequency renormalization of the y-polarized polar modes, resulting in a pronounced low temperature dielectric anisotropy. Results are discussed in terms of contemporary phenomenological theory of antiferroelectricity.

Published
2014
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2. Grain Size Influence on Dynamics of Polar Nanoclusters in PMN-35%PT Ceramics: Broadband Dielectric and Infrared Spectra

Author

Bovtun, V., Kamba, S., Veljko, S., Nuzhnyy, D., Kroupa, J., Savinov, M., Vanek, P., Petzelt, J., Holc, J., Kosec, M., Amorin, H., and Alguero, M.

Subjects
Condensed Matter - Materials Science
Abstract

Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grain size ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics of PbMg_(1/3)Nb_(2/3)O3-35%PbTiO3 were investigated at temperatures 10 - 900 K. e*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below Tc, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation. On the other hand, polar phonon frequencies and their temperature dependences are almost independent of the grain size, but the selection rules for the cubic symmetry are not obeyed and all phonons are split due to a locally broken symmetry by polar nanoregions and chemical disorder. The lowest-frequency polar phonon undergoes partial softening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but the dielectric anomaly is caused predominantly by flipping and breathing of polar nanoclusters. Due to contribution of both the soft phonon mode and dielectric relaxations into the dielectric constant, the ferroelectric phase transition, which corresponds to the percolation threshold of the polar nanoregions into macroscopic domains, can be considered as a special case of crossover between the displacive and order-disorder type., Comment: subm. to PRB

Published
2008
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4. High-temperature phase transitions in SrBi_2Ta_2O_9 film: a study by THz spectroscopy

Author

Kadlec, F., Kamba, S., Kuzel, P., Kadlec, C., Kroupa, J., and Petzelt, J.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Disordered Systems and Neural Networks
Abstract

Time-domain THz transmission experiment was performed on a $\rm SrBi_2Ta_2O_9$ film deposited on sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest frequency optic phonon near 28 cm$^{-1}$ reveals a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions. We show that the dielectric anomaly near the ferroelectric phase transition can be explained by slowing down of a relaxational mode, observed in the THz spectra. A second harmonic generation signal observed in a single crystal confirms a loss of center of symmetry in the ferroelectric phase and a presence of polar clusters in the intermediate ferroelastic phase., Comment: subm. to J. Phys.: Condens. Matter

Published
2004
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10. Spin-phonon coupling in epitaxial Sr0.6Ba0.4MnO3 thin films

Author

Goian, V., Langenberg, E., Marcano, N., Bovtun, V., Maurel, L., Kempa, M., Prokscha, T., Kroupa, J., Algarabel, P.A., Pardo, J.A., and Kamba, S.

Abstract

Spin-phonon coupling is investigated in epitaxially strained Sr1-xBaxMnO3 thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the Sr0.6Ba0.4MnO3 composition grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a 17% hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with Néel temperature (TN) around 150 K. Thus, our results confirm that epitaxial Sr0.6Ba0.4MnO3 thin films display strong spin-phonon coupling below TN, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the Sr1-xBaxMnO3 system.

Published
2017

15. Spin-phonon coupling in epitaxialSr0.6Ba0.4MnO3thin films

Author

Goian, V., primary, Langenberg, E., additional, Marcano, N., additional, Bovtun, V., additional, Maurel, L., additional, Kempa, M., additional, Prokscha, T., additional, Kroupa, J., additional, Algarabel, P. A., additional, Pardo, J. A., additional, and Kamba, S., additional

Published
2017
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16. Impact damage model for ceramic materials.

Author

Rajendran, A. M. and Kroupa, J. L.

Subjects
*DYNAMIC testing of materials, *STRAINS & stresses (Mechanics), *CERAMICS
Abstract

Presents a model of the damage process in ceramics due to impact loading. Details on the Taylor-Chen-Kuszmaul model; Description of the plate-impact test; Effect of time step.

Published
1989
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23. Erfahrungen bei der Behandlung von Unterschenkelschaftbrüchen

Author

Bauer, J., primary, Andrašina, J., additional, Klíma, M., additional, Démant, A., additional, Denischi, A., additional, Dinulescu, I., additional, Antonescu, D., additional, Ionescu, Th., additional, Kroupa, J., additional, Kirschner, R., additional, Kostovski, V., additional, Rhomberg, H., additional, Häfele, H., additional, Amann, E., additional, Lippay, A., additional, Riess, J., additional, Koch, H., additional, Stanković, P., additional, Zöch, G., additional, Hiebler, W., additional, Ferlić, P., additional, Fekete, G., additional, Szántó, S., additional, Nagy, L., additional, Zolczer, L., additional, Manninger, J., additional, Nyari, T., additional, Jekic, M., additional, Letic, S., additional, Kramer, G., additional, Hranilovic, B., additional, Grujic, M., additional, Barac, M., additional, Berentey, G., additional, Strmiska, J., additional, Varga, A., additional, and Salacz, T., additional

Published
1974
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30. Multiple Soft-Mode Vibrations of Lead Zirconate

Author

Hlinka, J., primary, Ostapchuk, T., additional, Buixaderas, E., additional, Kadlec, C., additional, Kuzel, P., additional, Gregora, I., additional, Kroupa, J., additional, Savinov, M., additional, Klic, A., additional, Drahokoupil, J., additional, Etxebarria, I., additional, and Dec, J., additional

Published
2014
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44. Mechanical Properties for Advanced Engine Materials

Author

DAYTON UNIV OH RESEARCH INST, Ashbaugh, N. E., Khobaib, M., Hartman, G. A., Coker, D., Kroupa, J. L., DAYTON UNIV OH RESEARCH INST, Ashbaugh, N. E., Khobaib, M., Hartman, G. A., Coker, D., and Kroupa, J. L.

Abstract

The experimental evaluation of advanced mechanical properties that characterize the behavior of emerging materials for aerospace structures under a broad range of conditions representative of those encountered in service was emphasized in this investigation. The information guided the development of life prediction methodologies which are based on the concept of damage tolerance as a design philosophy. The research effort was subdivided into four interrelated tasks: (a) material characterization tests, (b) experimental procedure and test developments, (c) analytical and modeling developments, and (d) test support activities and data archival. Material characterization tests for fatigue, creep, and crack growth responses were conducted on a titanium matrix composite reinforced with continuous SiC fibers and fabricated from a foil-fiber-foil layup. The molybdenum wire used in the weaving of the fiber mats was a major factor in the development of damage under fatigue and creep test conditions. A model ceramic matrix composite composed of an aluminosilicate glass matrix and SiC fiber tows provided some interesting fatigue and fracture toughness responses. Behavior of monolithic materials were evaluated under creep, creep crack growth, fatigue crack growth, and oxidation conditions. Significant developments in test procedures and systems have been accomplished with emphasis on conducting thermomechanical fatigue (TMF) tests.

Published
1992

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