Research On Mechanism Of The Light Emission Of Sapphire And Radiation Properties Of The Metal/Lithium Fluoride Interface Under Shock Compression

Sapphire is one of the important ceramic materials, and Alumina is believed to be one of the major constituent in the Earth’s mantle. Sapphire single crystal has excellent mechanical and optical properties at ambient condition, which could be kept under high temperatures and high pressures(HTHP). Therefore, research on the HTHP properties of sapphire is of significance for further achievements in high pressure science, the geoscience, and the materials science. Sapphire is not only an optical window material in dynamic shock-wave experiments, but also an important pressure calibration material doped with Cr in static high pressure experiments. So studying on its dynamic behaviors and thermodynamic properties at high pressures is meaningful to understand experimental data and to verify theoretical model.However, since the shock-induced light emission in sapphire was observed in recent years. The problem of optical transparency of sapphire under shock loading had attracted much attentions of the researchers, because it was extensively used as optical window during early experiments for shock-temperature measurement of opaque materials (such as metals). Due to the drawbacks of measurement methods and techniques in the earlier studies, the obtained data greatly scattered, the mechanism of light emission from sapphire under shock loading is not clear at present. It is pointed out that the light emission took place in the procces of momentum transfer and energy conversion at the microcosmic or mesoscopic level in bulk of sapphire, so the light emission or thermal radiation signal must contain some information of dynamic feathers and crystal structural transformation of the crystal material. In present work, the light-gas gun is used as the loading tool to investigate the radiation properties together with the light emission mechanism of sapphire. The experiment techniques in installing sample, surface polishing, and depressing scattered light are improved. For evaluation the reliability of the measured radiation temperature of opaque materials, the LiF single crystal, of which the optical transparent properties is considered better under HTHP (high temperature high pressure), is used as the optical window, and the shock temperature of metal material/window interface under shock loading is measured. It is found that the shock radiation characteristics of Cu/LiF and Fe/LiF interface is close to that predicted by the ideally contacted interface, the new results and conclusions are summarized in the following:1, It confirms that the diffuse reflected light from the cavity of target, the procedure of sample polishing, and the interface contacted condition will greatly contributed to the measurement results of the apparent radiation especially under lower loading pressures. The real light emission of sapphire under shock loading is obtained by improving the sample preparation technique and the target assembly method.2, Using one-stage light-gas gun and two-stage light-gas gun, we studied the light emission mechanism of sapphire under shock loading from 5 to 120GPa. The results show that the mechanism of the shock-induced light emission from sapphire can be attributed to the heterogeneous thermal radiation from the adiabatic shear bands, and the spectral distribution matches well with the typical thermal radiation feathers.3, In this work the radiation pyrometer techniques and the spectroscopy system with ICCD for spontaneous spectra measurement are combined together to observe the color temperature of light emission from shocked sapphire. It is confirmed that the color temperature of the shock-induced light emission from the thermal radiation from the shear bands is in agreement with the corresponding melting temperature. By the change trend of the radiation temperature and the characteristics of spectral distribution with pressure, it is found that the pressure of structural phase transition of alumina from Corundum phase to the Rh2O3(Ⅱ) phase near melting line is constrained between 60GPa to 87GPa.4, By improving experiment techniques in preparing metal/LiF interface, the directly contacted interface is prepared, the radiation history of the shock emission is found to be consistent with that predicted by the heat conduction model of "ideal interface", and the measured shock temperatures are in agreement with the corresponding thermodynamic calculation. The problems of experimental data scattering and disputes in heat conduction model seems to be solved by technique improvements.