Electrocaloric response modulated by misfit strain in different oriented epitaxial ferroelectric thin films

Abstract

Ferroelectric thin films have exhibited great potential in electrocaloric (EC) solid-state refrigeration due to large adiabatic temperature changes. However, current research on the EC effect of ferroelectric thin films mainly focuses on (001)-oriented thin films, while the EC response in different oriented thin films has yet to be systematically investigated. In this work, combining thermodynamic energetic and entropy analyses, a nonlinear thermodynamic approach has been generalized to investigate the EC response modulated by misfit strain in different oriented epitaxial ferroelectric (1-x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 [(1-x)PMN-xPT] thin films. The effects of the chemical composition x, the misfit strain um and the film orientation on the phase diagrams, phase transition temperatures and EC response in (1-x)PMN-xPT thin films are examined. It is found that different oriented thin films possess obviously different phase structures, phase boundaries and phase transition temperatures, resulting in different EC responses. The excellent EC response appears around the tetragonal-paraelectric (Tec-PE) phase boundary for (001)-oriented thin films, orthorhombic-paraelectric (Oaa-PE) phase boundary for (110)-oriented thin films, and rhombohedral-paraelectric (Raaa-PE) phase boundary for (111)-oriented thin films under compressive misfit strain, due to the large change in polarization. The maximum adiabatic temperature change ΔT shifts to a higher operating temperature when the composition x increases, due to the shifting of the ferroelectric–paraelectric phase boundary towards the higher temperature. Good agreement with experimental data is observed. The results also indicate that (001)-oriented thin films are beneficial to EC cooling at higher temperature, (111)-oriented thin films are beneficial to EC cooling at lower temperature, and (110)-oriented thin films are beneficial to EC cooling at medium temperature, suggesting that the EC response in ferroelectric thin films can be modulated by misfit strain via growing thin films in an appropriate orientation. These insights offer an alternative way to modulate the EC response in ferroelectric thin films.