Direct and indirect determination of electrocaloric effect in Na0.5Bi0.5TiO3

Abstract

Direct and indirect studies of electrocaloric effect were performed in poled and depoled Na 0.5 Bi 0.5 TiO 3 . For this purpose polarization and electrocaloric effect temperature change measurements were made at different electric field pulses as a function of temperature. 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 case of the poled Na 0.5 Bi 0.5 TiO 3 sample. These differences can be explained by the temperature-dependent concentration of domains oriented in the direction of the applied electric field. Whereas in the depoled Na 0.5 Bi 0.5 TiO 3 , which is characterized by 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 electrocaloric effect. The reason of the negative electrocaloric effect values, obtained in such a way and widely discussed in literature in case of Na 0.5 Bi 0.5 TiO 3 , have been explained in this study.

Origins of the Inverse Electrocaloric Effect

“…The authors assigned this discrepancy to be a result of the combination of the hysteresis loops not being fully saturated at the maximal field used (22 kV cm −1 ) and the decrease of the coercive field with increasing temperature, which leads to an increase of the induced polarization with temperature. Similarly, Birks et al . found that for unpoled NBT ceramics the indirectly estimated ECE changes the sign from negative to positive on heating whereas the directly measured effect remains positive in the full temperature range.…”

“…While in (1− x )Ba(Zr 0.2 Ti 0.8 )O 3 – x (Ba 0.7 Ca 0.3 )TiO 3 and (Ba 0.9 Ca 0.1 )(Ti 0.5 Zr 0.5 )O 3 ceramics the inverse ECE was related to the tetragonal‐rhombohedral and orthorhombic–tetragonal transition, respectively, it was actually observed at temperatures several tens of degree above the corresponding transition temperatures. For some similar materials the comparison between the directly and indirectly measured ECE shows that the negative sign of the latter is an artifact . Therefore, for NBT‐18KBT (0.92 Na 0.5 Bi 0.5 TiO 3 –0.18 K 0.5 Bi 0.5 O 3 ) ceramics, Le Goupil et al .…”

“…The applicability of the indirect approach has been discussed several times, see, for example, Refs. . Its validity relies on the Maxwell relation [Eq.…”

“…It is, therefore, very important to have a good understanding of the specific parameter ranges giving rise to large inverse EC responses, that is, factors such as doping level, strain, temperature, magnitude, and direction of the electric field, as well as process conditions, to avoid an unintentional compensation between normal and inverse effects and obtain the maximally possible response for future cooling applications. Furthermore, as recently discussed by Birks et al., an apparent inverse ECE can sometimes be an artifact of the measurement and data analysis methods. It is thus important to avoid such pitfalls in oder to obtain reliable information about the specific conditions for which a certain material exhibits a conventional or an inverse EC response.…”

Giant Negative and Positive Electrocaloric Effects Coexisting in Lead‐Free Na0.5Bi4.5Ti4O15 Films Over a Broad Temperature Range

“…In fact, negative ECEs (polarization increases with the increase of temperature) have been observed in some materials. [9,10,15,[23][24][25] The non-collinear theory is generally acceptable, which do not contradict the classic ferroelectric theory. Therefore, the occurrence of negative ECE is closely related to the phase transition, which can lead to the change of noncollinearity of NBTO films.…”