Phenomenological theory for investigation on stress tunable electrocaloric effect in ferroelectric EuTiO3 films

Researches on electrocaloric effects of ferroelectric materials and their applications in solid-state refrigeration have attracted great interest in recent years. EuTiO3 is a new multiferroic material with many special physical properties, such as high dielectric constant, low dielectric-loss, as well as their responses to tunable external electric field and temperature. With EuTiO3 ferroelectric thin films, their polarization size and phase transition process not only can be changed by regulating external electric field and temperature applied, but also can be controlled by adjusting the external stress applied and the lattice mismatch with the substrate in a large scale. Accordingly, in this paper a phenomenological Landau-Devonshire thermodynamic theory is used to investigate the ferroelectric properties and electrocaloric effects of EuTiO3 ferroelectric films under different external tensile stresses (σ3 > 0) perpendicular to the film surface and different in-plane compressive strains. We have calculated the electric polarizations, electrocaloric coefficients and adiabatic temperature differences as a function of temperature for EuTiO3 ferroelectric films with a biaxial in-plane misfit strain um =-0.005 under different applied stresses. Results demonstrate that the changes of the electric polarization, the electrocaloric coefficient and the adiabatic temperature differences conform with the regulation of externally applied stresses. With the enhancement of applied tensile stress perpendicular to the film surface, the phase transition temperature and adiabatic temperature change of EuTiO3 thin film increase, and the operating temperature corresponding to the maximum adiabatic temperature difference moves toward high temperature region. For the thin films with a biaxial in-plane misfit compressive strain um =-0.005 and the external tensile stress σ3 = 5 GPa, when the change of electric field strength is 200 MV/m, the adiabatic temperature differences at room temperature can be over 14 K, and the maximum electrocaloric coefficient may approach 1.75×10-3 C/m2·K. In the meantime, the working temperature range, when the adiabatic temperature differences go beyond 13 K, is over 120 K. Then we investigate the effect of in-plane compressive strains on the changes of adiabatic temperature, showing that with the increase of compressive strain um, the adiabatic temperature change will also increase and the peak of the curve of adiabatic temperature change versus temperature will shift toward high temperature zone far away from room temperature. Therefore, the above results show that we can not only have relatively bigger adiabatic temperature differences in epitaxially grown EuTiO3 thin films through the regulation of external stresses and in-plane lattice misfit strain, but also a sound application prospect of ferroelectric EuTiO3 thin film in solid-state refrigeration at room temperature.