Theoretical Study On Surface Acoustic Wave Properties Of Doped KTP Crystal At Different Temperature Conditions

KTP crystal is a kind of piezoelectric crystal material with high Curie temperature,good thermal stability and excellent optical properties.It is expected to be widely used in high temperature and high pressure environment.However,it is difficult to measure its piezoelectric parameters in high temperature environment.Theoretical studies and first-principles calculations are applied to the prediction of the structure and mechanical properties at the absolute temperature of 0K.Therefore,.There has been no major progress in experimental and theoretical research from the room temperature to high temperature.In this dissertation,density functional theory and quasi-harmonic approximation methods were used to systematically calculate the lattice structure,thermal expansion coefficient and elastic constants of KTP crystal under different temperature conditions.The surface acoustic wave theory was used to calculate the crystal under different temperature conditions.We calculate the parameters such as surface acoustic wave velocity,electromechanical coupling coefficient,and PFA.We discuss the influence of iron doped KTP on the piezoelectric performance.Finally,an acoustic surface filter is designed for high temperature environments.The main contents and results are as follows.Firstly,the structure optimization of KTP crystal was optimized,and the band structure of the ground state was calculated.The forbidden band width of KTP crystal was 3.206 eV at 0 K.The quasi resonant approximation theory was used to calculate the forbidden band width of 3.207 eV at 800℃.It was found that the forbidden band width is almost unchanged with increased temperature.Secondly,the lattice constants and elastic constants of KTP crystals under the temperature of 0 K,273 K,673 K and 1073 K were calculated by the combination of quasi resonant approximation and density functional theory.It was found that the KTP crystal swelled more in the directiom of a and b and less in the c direction.The elastic constants varied by about 12% in the temperture range from 0 K to 1073 K.Thirdly,we calculated the SAW velocity of Z cut KTP crystal by using the surface acoustic wave(SAW)theory and the material constants of different temperatures.Based on the elastic constants calculated before,the surface acoustic wave properties at 20℃ and 800℃degrees were compared.We obtained that the largestelectromechanical coupling coefficient k~2 of KTP crystal,had reached 0.72% at the propagation angle of 90°.Fourth,a single-electrode filter based on KTP crystal was designed and its filter performance was simulated.When the temperature was 25℃,the response center frequency was 398 MHz and the insertion loss was 4.8 dB.When the temperature was at 800℃,the response frequency of the center was 393 MHz and the insertion loss was 5.8 dB,which has the advantages of high central frequency and small temperature drift.