Giant electrocaloric effect in ferroelectric ultrathin films at room temperature mediated by flexoelectric effect and work function

摘要

In ferroelectric ultrathin films, built-in electric fields are often present due to the flexoelectric effect and the difference of work functions at asymmetric electrodes, which may change the properties of the materials. In this paper, the influence of build-in electric fields induced by flexoelectric effect and/or work function difference on the misfit strain-temperature phase diagrams, and the electro-caloric properties of epitaxial BaTiO_3 ultrathin films are investigated by using an extended nonlinear thermodynamic theory. It is found that the flexoelectric effect, i.e., the coupling of polarization and strain gradient, changes the misfit strain-temperature phase diagrams notably, in which the phases with out-of-plane polarizations increase due to the presence of a built-in field. The electro-caloric properties are remarkably enhanced when the built-in fields induced by both the flexoelectric effect and work function difference are considered. In particular, a giant adiabatic temperature change of 7.89 K in ultrathin Pt/BaTiO_3/SrRuO_3 capacitors at 460 K is predicted. Moreover, it is demonstrated that the peak of adiabatic temperature change versus working temperature is shifted from a high temperature to room temperature, suggesting that ferroelectric ultrathin films with asymmetric electrodes and strain gradient are promising candidates for room temperature refrigeration.