Influence of mechanical boundary conditions on the electrocaloric properties of ferroelectric thin films

Abstract

A thermodynamic analysis of the electrocaloric (EC) effect in BaTiO3 ferroelectric thin films has been carried out under differing mechanical boundary conditions. It is shown that both the magnitude of the electrocaloric effect and temperature at which it is maximized depend not only on the extent of the applied field change but also on the value of the initial field. For initial fields smaller than a critical value the EC effect is largest at the phase transition temperature but the effect is a strong function of temperature. For external electrical fields larger than this value, conversely, the EC effect is the largest at a higher temperature and is a weak function of temperature. Perfect lateral clamping transforms the first-order phase transition into a second-order transition, lowering the magnitude of the electrocaloric effect and dependence on temperature. Compressive and tensile misfit strains also alter the nature of the phase transition and affect the electrocaloric properties in an analogous way. A compressive misfit strain shifts the maximum in the EC effect to higher temperatures, reduces its magnitude, and reduces its dependence on temperature, while tensile misfit strain results in the opposite effects. Control of the misfit strain by appropriate choice of substrate provides potential means to vary both the magnitude and the temperature sensitivity of the EC effect for use in cooling or thermodielectric power conversion devices.