Direct and indirect determination of electrocaloric effect in Na₀<sub>.</sub>₅Bi₀<sub>.</sub>₅TiO₃
Birks, E.; Dunce, M.; Peräntie, J.; Hagberg, J.; Sternberg, A. (2017-05-24)
The following article appeared in Birks, E., et.al Journal of Applied Physics, 121 (22), 224102 (2017) and may be found at http://dx.doi.org/10.1063/1.4985067.
Published by AIP Publishing.
Direct and indirect studies of the electrocaloric effect were carried out in poled and depoled Na₀.₅Bi₀.₅TiO₃. For this purpose, polarization and electrocaloric effect temperature change measurements were made at different electric field pulses as a function of temperature. The applicability of the widely used indirect electrocaloric effect determination method, using the Maxwell relation, was critically analyzed with respect to the reliable direct measurements. Quantitative differences were observed between the results obtained by both approaches in the case of the poled Na₀.₅Bi₀.₅TiO₃ sample. These differences can be explained by the temperature-dependent concentration of domains oriented in the direction of the applied electric field. Whereas in depoled Na₀.₅Bi₀.₅TiO₃, which is characterized by the electric field dependence of polar nanoregions embedded in a nonpolar matrix, the Maxwell relation is not applicable at all, as it is indicated by the obtained results. Possible mechanisms which could be responsible for the electrocaloric effect in the relaxor state were considered. The results of this study are used to evaluate the numerous results obtained and published by other authors, using the Maxwell relation to indirectly determine the electrocaloric effect. The reason for the negative values of the electrocaloric effect, obtained in such a way and widely discussed in the literature in the case of Na₀.₅Bi₀.₅TiO₃, has been explained in this study.
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