Mechanical control of electrocaloric response in epitaxial ferroelectric thin films

The electrocaloric (EC) effect in dielectrics has shown great potential in the next-generation solid-state refrigeration; however, most dielectrics cannot satisfy the requirement for refrigeration at various temperatures. How to control large EC response to various temperatures is a critical problem for practical cooling applications. In this work, based on entropy analysis, a nonlinear thermodynamic approach considering mechanical loading has been generalized to demonstrate mechanical control of EC response in epitaxial ferroelectric BaxSr1−xTiO3 (BST) thin films. The effects of chemical composition x, in-plane misfit strain um, and out-of-plane external uniaxial stress σ3 on the phase diagrams, phase transition temperatures, and EC response of BST thin films have been studied. The results reveal that a large EC temperature change ΔT appears in the vicinity of c-PE and r-aa phase boundaries, because the polarization component P3 becomes zero across both phase boundaries, leading to a large change of order in dipoles. It is found that the phase structures and the transition temperatures are sensitive to misfit strain and stress. Large EC response in BST thin films can be controlled and shifted to various temperatures over a wide temperature range for practical applications. It also indicates that the peak of ΔT can be shifted to higher temperature under in-plane compressive strain um, while it is shifted to lower temperature under tensile strain um or compressive stress σ3. Furthermore, the optimal combination of misfit strain and stress for the EC temperature change are identified, which further enhances EC response. These insights offer an alternative pathway to control and implement EC refrigeration over a wide range of temperature.