The Mechanism of Formation of Light-Scattering Centers in Sapphire Crystals Grown in Gas Atmospheres

The mechanism of the formation of light-scattering centers in sapphire crystals grown by horizontal directed crystallization in gas atmospheres reducing with respect to the Al2O3 melt was studied. The experimental regularities here significantly differ from those observed upon formation of other conventional defects in sapphire crystals (vacancy pores, gas bubbles, and so on). It is shown that the known formation mechanisms of macroscopic ≥1 mm) inclusions in crystals are not acceptable in this case. Using the model of bulk crystallization is proposed to describe the obtained regularities.

The micro-pulling-down method has been adapted to grow fibers with diameters as small as 150 μm. We describe the use of this method to grow fibers of the oxide eutectic materials Al2O3/Y3Al5O12 and Al2O3/GdAlO3, which have exceptional mechanical properties at high temperature. The “Chinese script” microstructure of Al2O3/Y3Al5O12 is stable and uniform, and varies with the pulling rate in the same way as the conventional lamellar eutectics. The microstructure of Al2O3/GdAlO3 is finer, but less stable and homogeneous, particularly at high pulling rate. Fibers produced at low growth rate exhibited self-cladding by the sapphire phase.


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