Die mechanische Nachgiebigkeit bei Phasenübergängen in bleifreien Ferroelektrika
Zusammenfassung der Projektergebnisse
In this project, the influence of uniaxial compressive stress on the phase transitions of perovskite ferroelectrics was investigated. A novel experimental setup was developed and built, capable of measuring the piezoelectric coefficient and the dielectric permittivity of a sample as a function of frequency, temperature, and mechanical stress. The overall control of the measurement was realized by writing a custom-built LabVIEW program, which also processed and evaluated all measurement data. First results on single crystal and polycrystalline barium titanate showed an increase in the ferroelectric-paraelectric phase transition temperature with increasing stress. In contrast, the Curie-Weiss temperature was found to decrease with increasing stress, indicating an increased first order character of the phase transition. A thermodynamic analysis done with the Landau-Ginsburg-Devonshire theory was able to describe the results qualitatively by introducing a stress-dependent Landau parameter in the Gibbs free energy function. Ongoing work investigates also the influence of a uniaxial mechanical compression on the lower temperature phase transitions. The tetragonal-orthorhombic transition temperature was found to increase with stress, whereas the orthorhombic-rhombohedral transition temperature remained nearly constant. Measurements of the piezoelectric coefficient and the dielectric permittivity of poled soft PZT as a function of temperature at constant mechanical stresses showed an increase in the depolarization temperature with increasing mechanical load. At the depolarization temperature the piezoelectric coefficient was found to drop sharply, but a small piezoelectric response was still measureable up to 25 °C above the depolarization temperature. The drop in the piezoelectric coefficient did not coincide with the temperature at maximum permittivity, which was only broadened with increasing stress but not remarkably shifted. Quenching experiments with hard PZT showed that a mechanical load likely reorients defect diploes present in hard doped ferroelectric materials. This was found by measuring the stress-dependent piezoelectric coefficient after different cooling and poling scenarios. Since current research in lead-containing relaxor ferroelectrics also focuses on single crystal materials, characterization of <001>c-PIN- PMN-PT single crystals was performed, showing the influence of mechanical loads on the electromechanical properties. The compressive stress caused reversible stress-induced phase transitions in the crystals, which limits the applications in e.g. underwater sound projectors. Various compositions of the lead-free solid solution BNT-xBT were investigated regarding their properties under combined mechanical and thermal load. It was possible to determine a stress-dependent phase diagram of relaxor-type BNT-6BT, revealing the load dependence of the ferroelectric-relaxor transition. In addition, it was found that a mechanical load was able to induce a long-range ferroelectric order in BNT-6BT, similar to the effect of an electric field. Finally, the stress- and temperature-dependent electromechanical response of iron doped BNT- 6BT was investigated, which resulted in a behavior similar to the undoped lead-free material, but with significant lower values of the piezoelectric coefficient.
Projektbezogene Publikationen (Auswahl)
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“Influence of uniaxial stress on the ferroelectric-to-paraelectric phase change in barium titanate”, Journal of Applied Physics, 113(17) 174103 (2013)
F. H. Schader, E. Aulbach, K. G. Webber, and G. A. Rossetti
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“Stress, temperature and electric field effects in the lead-free (Ba,Ca)(Ti,Zr)O3 piezoelectric system”, Acta Materialia, 78 37-45 (2014)
M. C. Ehmke, F. H. Schader, K. G. Webber, J. Rödel, J. E. Blendell, and K. J. Bowman
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“Chemical and structural effects on the high-temperature mechanical behavior of (1−x)(Na1/2Bi1/2)TiO3-xBaTiO3 ceramics”, Journal of Applied Physics, 117(13) 134110 (2015)
M. Deluca, G. Picht, M. J. Hoffmann, A. Rechtenbach, J. Töpfer, F. H. Schader, and K. G. Webber
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“Enhancing the operational range of piezoelectric actuators by uniaxial compressive preloading”, Journal of Physics D: Applied Physics, 48(21) 215302 (2015)
J. Koruza, D. J. Franzbach, F. H. Schader, V. Rojas, and K. G. Webber
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“Mechanical stability of piezoelectric properties in ferroelectric perovskites”, Journal of Applied Physics, 117(19) 194101 (2015)
F. H. Schader, M. Morozov, E. T. Wefring, T. Grande, and K. G. Webber
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“Temperature Stability of Lead-Free Niobate Piezoceramics with Engineered Morphotropic Phase Boundary”, Journal of the American Ceramic Society, 98(7) 2177-2182 (2015)
R. Wang, K. Wang, F. Yao, J.-F. Li, F. H. Schader, K. G. Webber, W. Jo, J. Rödel, and S. Zhang
