高温条件下Ga3PO7晶体热学及声表面波性质的理论研究

高温压电晶体是许多机电器件必需的一种多功能材料, Ga₃PO₇晶体的居里温度高达1364 ℃, 可应用于高温极限条件. 但是预测高温极限条件下晶体的结构以及物理性质的问题采用实验研究的手段非常困难, 而理论上的预测未见研究. 本文采用密度泛函-准谐振近似理论计算了温度在0-1200 ℃范围内Ga₃PO₇ 晶体的结构常数和热学性质, 结果表明Ga₃PO₇晶体的晶格常数a和c随温度的升高呈线性增大, 且c方向受温度影响更为显著; 晶体的密度随温度的升高而减小, 计算的a 和c方向平均热膨胀系数分别为1.67×10^-6 K^-1和3.58×10^-6 K^-1, 高温区定压热容为2.067 J/g·K, 与实验值一致. 计算了从常温到高温下该晶体的弹性常数以及体弹性模量的变化, 研究了高温条件下的声表面波特性, 发现随着温度的升高, 声表面波速度浮动较小, 而机电耦合系数略有增大; 在传播角为151° 时该晶体具有较好的温度稳定性且机电耦合系数达到最大值, 这表明Ga₃PO₇ 晶体是一种有望应用于高温环境下的压电晶体.

Theoretical study on thermal and acoustic surface wave properties of Ga3PO7 crystal at high temperature

The high-temperature piezoelectric crystal Ga₃PO₇is a versatile functional material widely used in many electromechanical devices. As the Curie temperature of this crystal is as high as 1346 ℃, it can break through the current temperature limitations(1200 ℃) and then be used in extremely high-temperature condition. However, it is very difficult to explore its properties in such a high-temperature environment. Moreover, the relevant theoretical research has not been reported to date. Aiming at this problem, the density function theory combined with quasi harmonic approximation theory is used to investigate the structural, thermal and surface acoustic wave (SAW) properties of Ga₃PO₇. Firstly, the Gibbs energies of Ga₃PO₇ crystal with different stains are calculated, and the equilibrium structures of Ga₃PO₇ crystal at different temperatures (from 0 ℃ to 1200 ℃) are found according to minimal energy principle. Secondly, based on the result above, we optimize Ga₃PO₇ crystal at different temperatures, and then, the thermal and elastic properties of Ga₃PO₇ crystal within 0-1200 ℃ are calculated using CASTEP package based on the density functional theory in the generalized gradient approximation. The results show that its lattice constants increase almost linearly as temperature increases while its density decreases. Owing to anisotropy, its lattice constant along the c axis increases much more greatly than along the a axis. The coefficients of thermal expansion along the a and c axis are evaluated to be 1.67×10^-6 K^-1 and 3.58×10^-6 K^-1, respectively, and the volumetric heat capacity is evaluated to be 2.067 J/g·K. These values all agree well with the experimental values. Finally, the elastic constants, bulk modulus and SAW properties of Ga₃PO₇ crystal at different temperatures (from 0 ℃ to 1200 ℃) are calculated. The results show that the bulk modulus can reach 175 GPa, and it changes very little as temperature increases. The fluctuation of elastic constants has slight influences on SAW velocity and the electric-mechanical coupling factor. When the propagation angle is 151°, it possesses the stablest SAW properties and the largest electric-mechanical coupling factor which can reach 0.7%. The comprehensive analyses of the thermal, mechanical and SAW properties show that Y-cut Ga₃PO₇ possesses a greater potential application in high temperature environment.