应力作用下EuTiO3铁电薄膜电热效应的唯象理论研究

基于Laudau-Devonshire的热动力学模型, 计算了EuTiO₃铁电薄膜材料的电热效应. 结果显示在外加应力的调控下, 电极化、电热系数以及绝热温差都会随之变化. 外加垂直于表面的张应力加大, 薄膜的相变温度升高, 绝热温差增加, 最大绝热温差所对应的工作温度向高温区移动. 对于二维平面失配应变u_m =-0.005的薄膜, 当外加张应力σ₃ = 5 GPa时, 其最大电热系数为1.75×10^-3 C/m²·K, 电场变化200 MV/m 时室温下绝热温差ΔT 的最大值可达到14 K 以上, 绝热温差ΔT ≥13 K 的工作温区超过120 K, 表明可以通过调控外部应力来获取室温时较大的绝热温差. 此结果预示着铁电EuTiO₃ 薄膜在室温固态制冷方面可能具有较好的应用前景.

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 u_m =-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 u_m =-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 u_m, 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.