Modeling of enhanced electrocaloric effect above the Curie temperature in relaxor ferroelectrics

The electrocaloric (EC) effect offers promise as a means to realize solid-state refrigeration, which requires EC materials possessing a pronounced pyroelectric effect over a broad temperature range. Pauli's master equation is adopted to investigate the recently observed phenomenon of enhanced EC effect above the Curie temperature in relaxor ferroelectrics. The proposed approach allows the EC coefficient to be determined within the framework of classic Landau-Ginzburg-Devonshire thermodynamics and the Maxwell relation, taking into account both the depolarization effect and dielectric permittivity dispersion based on the concept of superparaelectricity and the nanopolar region. We analyze three contributions of the EC effect: temperature-dependent dielectric dispersion, intrinsic pyroelectric effect and enhanced dielectric stiffness. The maximum EC coefficient is determined through the derivatives of the three components with respect to temperature. The proposed approach, in which the evolution of polarization correlation length is accounted for, cannot only provide a microscopic explanation for the thermally driven enhancement of EC responses, but also improves upon the existing models for estimating the EC effect in paraelectric phase of relaxors. Finally, some potential approaches for engineering the enhancement of EC coefficient are also suggested. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.