Shock-Induced Optical Emission Of Alumina And Its High-Pressure Phase Diagram

The monocrystalline alumina (sapphire) has excellent mechanical and optical transparent properties at ambient condition, and relative high impedance under shock compression, thus it is commonly used as optical window for shock temperature measurement or velocity measurement by laser interference. In static high pressure experiments, alumina doped with Cr³⁺ is also used as pressure calibrant, and in some engineering applications it is one of important ceramic materials. In addition, alumina is believed to be one of the major constituent in the Earth's mantle. Therefore, research on the high-pressure properties of sapphire is significant for further achievements in the high pressure science and technology, for the constraint of the Earth's model, and for concerned engineering applications.Optical properties of sapphire under shock compression, such as optical absorption and emission, have been especially focused by the researchers working on the subjects of dynamic high pressure science and technology, because they will directly affect the interpretation for the shock-induced radiation from the contact interface of metal/sapphire. At the low pressure range of several tens of GPa, people had observed obvious emission of shocked sapphire, however, the transparency or emission properties of sapphire under strong shock compression is still an open question. In this thesis, in virtue of the low emission property of the oxygen-free copper baseplate, the self-emission effects of monocrystalline alumina under Mega bar pressures are investigated, and some advances are made in the following aspects:1,Obvious shock-induced emission from c-cut sapphire has been found in the pressure range of 124-172GPa, the wavelength dependence of the radiance is relating to the gray body's radiation character, and the apparent temperature of the emission is fitted to be about 4000K. In this thesis, the high-temperature thermal radiations are attributed to the local melt shear banding induced by one-dimensional shock compression;2,The correlation between the observed temperature of the heterogeneous thermal radiation and the phase diagram of sapphire is founded. The temperatures of alumina in CaIrO₃ phase region at 124-172Gpa are lower by about 1000K than those inα-phase below 85GPa, this discrepancy is attributed to the phase transition induced by shock compression. Combining with the characteristics of P-T equilibrium curve for the solid-solid transformation, we supposed a three-phase point at 4000K and 80-125GPa in alumina's phase diagram;3,The intensity of apparent radiation linearly rises with the thickness of the shocked part of sapphire sample; It shows that the c-cut sapphire under shock still has a considerable optical transparent thickness or a relative small absorption coefficient. The values of absorbance at different wavelengths are determined by use of the time-dependence radiance curves, and they are found insensitive to the radiation wavelength. Therefore, the sapphire's optical emission induced by shock waves in our experiments can be treated by the gray-body model. The relationship between the absorption coefficient and shock stress is obtained by fitting the emission history curve, which is described by:α_W=0.503-0.022P+1.829*10^-4*P² (α_W in cm^-1, P in GPa)According to this formula, the effect of sapphire's optical transparency on the temperature measurement can be quantitatively accessed.In summary, the characteristics of shock-induced emission in the crystalline alumina are systemically studied in this thesis, and the obtained results are of great importance. The present work shows that sapphire gives off obvious thermal radiation from heterogeneous shear banding at 120-172GPa, with a color temperature of about 4000K, and the temperature is found to be related to the structural transition of sapphire. Therefore, when the sapphire is used as optical window to measure the shock temperature, the optical emission and the structural transformation of sapphire will affect not only the observed intensity but also the history of the radiation signal. The functional relationship between the absorption coefficient and shock pressure can be applied to make a correction to the measurement temperature.