Two-phase zone at Аl2О3 melting by laser radiation and at melt solidification under the conditions of free cooling
V.K. Bityukov1, V.A. Petrov2, I.V. Smirnov3
1 Moscow State University of Radio Engineering, Electronics, and Automatics (Technical University)
2 Moscow State University of Radio Engineering, Electronics, and Automatics (Technical University)
3 Moscow State University of Radio Engineering, Electronics, and Automatics (Technical University)
Keywords: alumina, two-phase zone, laser heating, free cooling, brightness temperature, plateau, crystallization, structure
Pages: 507-518
Abstract
Experiments and numerical simulation have shown that the two-phase zone of liquid and solid phases concentration, variable over the coordinate, is formed both at alumina heating by laser radiation and at free melt cooling. During heating this zone is formed only at the initial stage of heating. Its width is not large and is lesser than the depth of the heating laser beam penetration due to the effect of intense heat losses through the crystal by thermal radiation and thermal conductivity. Extension of the two-phase zone formed at crystallization is much more under the conditions of free cooling than at melting, and it can cover almost the whole thickness of the melt. Temperature plateau of the brightness (effective) temperature registered at solidification by the pyrometer with the wavelength of 0.65 μm is explained by formation of the extensive two-phase zone. However, this plateau is realized not for all wavelengths. The importance of the melt absorption coefficient influence on probable appearance of temperature plateau is shown. As a result of crystallization with the two-phase zone formation, porous structure consisting of the crystals with small size can be formed. It may cause the diffuse surface reflectivity.
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