| 초록 |
In recent years, there has been a lot of interest in solid-state cooling devices using the electro-caloric effect (ECE), which is the adiabatic temperature change due to the entropy change by an external electric field. High ECE has been observed at temperatures just above the Curie temperature (TC) for common ferroelectrics, or around the dielectric peak for relaxor ferroelectrics. A material with a high ECE in a broad temperature range near room temperature (RT) is favorable for efficient refrigeration. Relaxor ferroelectrics, which show a broad dielectric peak and an electric field induced phase transition (EFIPT) above a depolarization temperature (Td), are potential candidate materials for cooling devices using ECE. Lead-based relaxor ferroelectrics such as (Pb,La)(Zr,Ti)O3and Pb(Mg,Nb)O3-PbTiO3 are mostly investigated ECE materials. Recent global environment issue demands lead-free relaxor ferroelectrics. In this work, ECE were investigated in two kinds of Bi-based solid solutions which show different phase transition behaviours; (Bi,Na)TiO3-SrTiO3 (BNT-ST) and BaTiO3-Bi(Zn,Ti)O3 (BT-BZT). Solid solution samples were prepared by a conventional ceramic process. Phase transition behaviors were investigated by X-ray diffractions and temperature dependences of dielectric permittivities. Both samples showed the phase changes from a ferroelectric to a relaxor with the composition change. Polarization-Electric field (P-E) hysteresis curves were measured at the temperature range of room temperature to 180oC. Double hysteresis loops were observed near the ferroelectric-relaxor phase boundary in BNT-ST while only slim P-E loops in BT-BZT. Adiabatic temperature changes due to ECE were calculated indirectly with temperature dependences of the polarizations using Maxwell relation and were compared with the data measured directly. |
