Large Negative Electrocaloric Effect in BiFeO₃-BaTiO₃ Solid Solution Ceramics

The electrocaloric effect (ECE) has attracted considerable interest due to the possible application for new environmental-friendly solid-state cooling devices. The ECE is an adiabatic temperature change or isothermal entropy change induced by an external electric field in (anti)ferroelectric materials. A material with a high ECE in a broad temperature range near room temperature (RT) is favorable for efficient refrigeration. The cooling devices with EC materials have not been realized for many decades due to small ECEs in most ferroelectric materials. In recent years, very large adiabatic temperature changes over 10 ^oC have been reported in several (anti)ferroelectric and relaxor thin films such as Pb(Zr,Ti)O₃, (Pb,La)(Zr,Ti)O₃, and (Bi,Na)TiO₃-BaTiO₃. Most ferroelectric materials show positive ECE that the heat is released and absorbed by applying and removing electric field alternately. The performance of an ECE-based cooling device has been reported to be improved by adopting a positive and a negative EC material together. However, the negative ECE has been reported only in a few EC materials. Giant negative ECE over wide temperature range has been observed in a few antiferroelectric and relaxor ferroelectric thin films recently. In most reports, the giant positive and/or negative ECEs have been observed in thin film EC materials by applying much larger electric fields than those for saturated polarizations. The practical application of a thin film EC material for cooling devices is, however, restricted due to its small thermal mass. Lately, very large negative ECE with an adiabatic temperature change of 11.5 ^oC has been reported in (Pb,La)(Zr,Sn,Ti)O₃ antiferroelectric bulk ceramics. And the negative ECE with an adiabatic temperature change of 2 ^oC was reported in eco-friendly lead-free 0.68BiFeO₃-0.32BaTiO₃ ceramics with coexisting ferroelectric rhombohedral and non-polar cubic phases. Several interpretations for the negative ECE in (anti)ferroelectrics and relaxor ferroelectrics have been suggested: non-collinearity between electric dipoles and an applied electric field, electric field-induced phase transition to a high entropy phase and so on., but the origin is still controversial.<br/>In this work, ECE were investigated in lead-free (1-x)BiFeO₃-xBaTiO₃ (BF-xBT) (x = 0.1-0.5) solid solution ceramics. The samples were fabricated by a conventional ceramic process. The crystal structure changed from a ferroelectric rhombohedral (R3c) to relaxor-like pseudo-cubic as the BT content increased. Two phases were coexisted in samples with the composition range of x = 0.25 to 0.4. Polarization-electric field (P-E) hysteresis curves were measured at temperatures from RT to 150 ^oC. The temperature changes due to ECE were calculated indirectly with the Maxwell relation, using the temperature dependence of polarization. Polarization increase with increasing temperature and very large negative ECE with an adiabatic temperature change over 10 ^oC were observed in the sample with a rhombohedral ferroelectric phase. The origin for polarization change and the large ECE will be discussed.