Electrocaloric Effect in Ba(Zr,Ti)O3-( Ba,Ca)TiO3 Ceramics Measured Directly

In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1 - x) Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 <= x <= 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, Delta T-direct = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at similar to 63 degrees C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.