Electrocaloric Effect In Relaxor Ferroelectric Ceramics and Single Crystals

In the last two decades, the electrocaloric (EC) effect which is associated to the temperature (thetas) dependence of the macroscopic polarization P(E,thetas) under electric field E has been spasmodically studied in ferroelectric materials in order to find an alternative to the classical refrigeration devices using freon. Basically, large electrocaloric temperature variation Deltathetas originates from electric field-induced phase transition at the Curie temperature, but temperature changes of the sample are difficult to measure and depend on the experimental conditions. In this paper, the EC effect has been quantified directly and precisely by measuring the thermal energy exchanged under isothermal conditions using a modified Differential Scanning Calorimetry (DSC) apparatus. The DSC technique allowed comparing the EC properties in the vicinity of ferroelectric-paraelectric phase transition in both high-dielectric-constant pyroelectric ceramics and single crystals of the same formulation. It is showed that although having a much higher pyroelectric coefficient, single crystals exhibit very close ECE compared to ceramics. Indeed, the ECE is around 0.2J/g for an electric field step of 2.5kV/mm for both crystals and ceramics, but the pyroelectric coefficient of <111> single crystal is found to be around 1300 pC.m
²
.K
^-1
where it is limited to 750 pC.m
^-2
.K
^-1
for ceramic. This observation is discussed in the paper.