Properties Of Sapphire Under High Temperature And High Pressure

High temperature and high pressure is an essential condition often faced by the field of national defense and military industry,and it is also involved in the field of geophysics.Sapphire is often used as optical window material in the field of high temperature and high pressure physics because of its high melting point,hardness and special optical properties.Its optical properties in high temperature and high pressure directly determine the reliability and accuracy of experimental signal analysis.However,there are still limitations and divergences in this field.First,there is no unified view on whether sapphire is opaque or not under shock compression,and its application scope as an optical window for shock experiment is not clear;Second,the study of optical properties of sapphire under uniaxial stress is not enough--According to the fluid approximation theory,the one-dimensional stress is applied to the sample in the shock compression experiment.If the refractive index of sapphire under hydrostatic pressure is used to correct the experimental signal,it may bring in errors due to the different physical conditions.In view of the problems,we researched the properties of sapphire under high temperature and high pressure based on first principles calculation and preheating shock experiment.At the same time,the theoretical calculation of sample properties in the next stage research is carried out,which provides basic data for the property analysis of sapphire in subsequent experimental research.The works and results of this paper as follows:1.The first-principle calculation is used to simulate the uniaxial stress of sapphire with eight orientations.The results show that the band gap of sapphire under uniaxial stress is significantly different from that under hydrostatic pressure;the optical properties of sapphire with eight crystal orientations under uniaxial stress are obtained by simulation.It is found that sapphire with a and m orientations can also be used as window for shock experiments.2.Optical properties of sapphire at initial high temperature.Using the preheating shock system and the experimental technologies developed based on this system,the interface velocity profile of Fe-Si alloy in shock experiment at preheating temperature of ～1100℃were measured with sapphire as optical window;By comparing the calculated results at the critical times of the experiments with the interface velocity profile in the experiments,it shows that sapphire has good transparency in the range of 80-200 GPa at ～1100℃.It is found that the corrected signals are in good agreement by comparing the refractive index simulated in this paper with the refractive index of sapphire in the literature.3.Study on sample properties in sapphire optical properties experiment.In order to research the optical properties of sapphire under high temperature and high pressure and expand the application further more,the disordered Fe-Ni alloys with SQS method were simulated by the first-principles calculation.The elastic constants and sound velocity of disordered Fe-Ni alloys with different Ni contents at 0-400 GPa were obtained.The results provide a parameter reserve for the subsequent experimental study of sapphire optical properties and provides a reference for the application of sapphire.Based on the results,we have a new understanding on the optical properties of sapphire under high temperature and high pressure.The calculation of disordered Fe-Ni alloys provides basic data for the subsequent study of sapphire optical properties and further development of application under high temperature and high pressure.