The electrocaloric effect of lead-free Ba$_{1-y}$Ca$_y$Ti$_{1-x}$Hf$_x$O$_3$ from direct and indirect measurements

Abstract

We report on the dielectric and electrocaloric properties of Ba$_{1-y}$Ca$_y$Ti$_{1-x}$Hf$_x$O$_3$ for compositions $0.12<x<0.18$ and $y=0.06$, as well as $x=0.15$ and $0<y<0.15$, synthesized by the conventional solid-state reaction method. The addition of Hf/Ca broadens the ferroelectric-paraelectric phase transition, while moving it toward room temperature. Two interferroelectric transitions are seen to converge, together with the ferroelectric-paraelectric phase transition, at ca. 335 K for $0.12<x_c<0.135$ and $y=0.06$. Consistently with the dielectric properties, the electrocaloric effect maximizes closer to room temperature with increasing Hf/Ca substitutions, which promote larger temperature spans. The electrocaloric responsivity gradually decreases from 0.2 to 0.1 K$~$mm$~$kV$^{-1}$ with the addition of Hf/Ca. A homemade quasi-adiabatic calorimeter is employed to measure "directly" the electrocaloric data, which are also calculated from polarization-versus-electric-field cycles using "indirect" standard procedures. The comparison between measured and calculated values highlights the importance of having access to direct methods for a reliable determination of the electrocaloric effect.