Mass transfer and formation of micro-inclusions in Ti-sapphire grown by the HDC method

The formation mechanism of micro-inclusions in titanium-sapphire crystals grown with the method of horizontal directional crystallization in a gas atmosphere was studied. It has been shown that the micro-inclusions of metallic tungsten and/or molybdenum are mainly formed at the surface of the unmelted charge. The thermodynamic analysis of the possible reactions of Mo and W among the charge components, the products of charge dissociation, and the components of the technological atmosphere was carried out. It is shown that CO2, CO, H2O, as well as suboxides of Al and Ti, can serve both as oxidizers of Mo and W and as reducing agents of their condensed oxides. The mechanism of mass transfer within which gaseous Mo and W oxides are reduced on the charge surface to form metal micro-particles was proposed. These particles further fall into the melt and then are transported with convective flows to the crystallization front and embedded into the crystal as micro-inclusions.