Theoretical Study On Structure,thermal And Surface Acoustic Wave Properties Of Ga₃PO₇ Crystal Under High Temperature Conditions

With unique "mechanical-electric-mechanical" transformation function,piezoelectric crystals play an extremely important role in modern science and technology.Compared to other signal processors,the surface acoustic wave?SAW?devices enjoy many superiorities including being processed within a small volume,higher conversion efficiency and stronger ability to processing intermediate and low frequency signal.Compared to quartz,lithium niobate and LGs,Gallium phosphate?Ga₃PO₇?crystal is reported to have the best thermal stability among piezoelectric crystals so far,whose Curie temperature is so high as 1364?.Besides,Ga₃PO₇ still has excellent mechanical,optical and piezoelectric properties at high temperature.Due to the difficulty of studying and detecting the physical properties of piezoelectric crystals under high temperature condition,the material constants,such as dielectric?piezoelectric and elastic constants of Ga₃PO₇,have not been reported at the range of 120-1200? yet.Considering of that,this thesis studied the thermal and SAW properties of Ga₃PO₇ crystal under high temperature conditions using density functional and quasi resonant approximation method.The main work and results are as follows:Firstly,the structure of Ga₃PO₇ crystal at temperature of T=0K is optimized by the first principle theory,as so as the band structure and density of states of the ground state are calculated.The results show that the band gap of Ga₃PO₇ crystal is 3.455 eV in 0K,while it is 3.435 eV at 1200?.That is to say,band gap width decreases with the increasing of temperature.This variation can be attributed to the sp2 hybridization state formed by s and p orbital of phosphorus atoms and p orbitals of atomic oxygen in the valence band region?-10⁰eV?.Secondly,through a combination of quasi resonant approximation and density functional theory methods,we have studied the structural,thermal and elastic properties of Ga₃PO₇ crystal at different temperatures?0?,200?,400?,600?,800?,1000?,1200??.The results show that: the Ga₃PO₇ crystal's thermal expansion coefficients along the crystal axis a and c directions exist certain anisotropy,which is mainly caused by Ga₃PO₇ crystal's specific coordination polyhedron structure.The most important,that is agreement with theoretic results.It is found that the heat capacity of Ga₃PO₇ crystal is too large to cause heat injury.It's elastic constants fluctuation is small under different temperature.Besides,bulk modulus reach to 175 GPa,which is hardly influenced by the temperature,shown as the modulus only drop by 6.4% up to 1200?.Thirdly,by using surface acoustic wave?SAW?theory,the SAW properties of Ga₃PO₇ crystal at 0¹20? temperature were calculated.It was found that the Y cut is the best cut type of Ga₃PO₇ crystal.Based on the theoretically predicted piezoelectric parameters at high temperature condition,the surface acoustic wave properties are investigated at 400 ?,800 ?,1200 ?,respectively.The results show that the temperature coefficient of electromechanical coupling coefficient k2 for Ga₃PO₇ crystal is the lowest just in the propagation angle 151 o and it is only 1.13×10-5/K.Furthermore,electromechanical coupling coefficient reached up to 0.7%.Fourth,the filter performance of Ga₃PO₇ crystal was simulated by MATLAB program.The results predict the frequency response of Ga₃PO₇ filter is in the vicinity of 350 MHz.Both insertion loss and transmission loss are very small,and center frequency is processed higher than the lithium niobate filter,which all meet the development trend for high frequency components in modern science and technology.