Sapphire Window Materials Under High Pressure Transparent Theory

Measuring the impact temperature in the shockwave compress experiment,especially for the opaque materials, is vital for the study on high-pressure transition ofmaterials and state equation. The key to the study is the transparency of the windowunder high temperature and pressure for the technology of measuring temperature byradiation, and which is worthy of concern and wish to be solved. Because whether theheat radiation can be unscrambled correctly relate to the diaphaneity directly.Single crystals of sapphire or LiF used for window materials in the impact compressexperiment is based on that the interface temperature got from one-dimensional Fourierheat exchange model by Grover and Urtiew is time-independent. The conclusion is usedfor theory basic during measuring the impact temperature of opaque materials. In 1990,Nellis and Yoo expounded the new problems come from measuring the temperature ofwindow materials. In these problems, whether the window materials can keep thetransparency after impact compress is attentive. There are many reports on thetransparency of the window materials in experimental study, but there are few ones intheoretical study.In this paper, we calculated optical properties of sapphire for hydrostatic pressuresup to 1000GPa and hydrostatic pressures ranging from 12 to 150GPa correspondingshear stresses from 12 to 30GPa with ab initio method which employed plane wavenorm-conserving pseudopotentials. The achievements of this thesis are as follows:1. We test the different optical properties by calculating the sapphire at hydrostaticpressures up to 1000GPa. The frontier band gaps of sapphire are increasing with thehydrostatic pressures increasing. When the hydrostatic pressure exceeds 300GPa, thefrontier band gaps are gradually decreasing and sapphire has metallicity tendency. Butsapphire keeps its transparency up to 1000GPa at 0K in the range of visible lightwavelength. This is different from the result of the impact experiment. The reasons are asfollows:a. The surrounding of sapphire is absolutly ideal in the simulations, and sapphiresare only applied to hydrostatic pressures without temperature effect, but temperatureeffect is inevitable in the experiments, the temperature rapidly increases with thehydrostatic pressures. b. Shear stress is not considered in the simulations but exists in theexperiments. c. The affect of transition boundary is not considered.2. Shear stresses are applied to sapphire, the hydrostatic pressures are rangingfrom 12 to 150GPa and the shear stresses are less than or equal to hydrostatic pressures.From the band structures, it is clear that the sapphire becomes metallic when thehydrostatic pressures and stresses achieve a certain value and resumes insulator alongwith the hydrostatic pressures and the shear stresses increasing (the value of shearstress/hydrostatic pressure is decreases). The phenomenon is consistent with theexperimental observation.3. The hydrostatic pressures are destined at 34GPa according to the simulativeresults in order to affirm whether the transparency of sapphire is related to the shearstress. The shear stress is ranging from 20 to 30GPa. The band structures of the sapphirechange and the sapphire becomes opaque when the shear stress is increasing. This showsthe transparency of the sapphire is greatly related to the shear stress. It is consistent withour imagination.In conclusion, for studying on the sapphire, an advanced method of quantummechanics is applied to this article. By studying the absorption spectrum, density ofstates and band structure of sapphire in detail, a reasonable explain about thetransparent change of sapphire under high pressure is given.