Molybdenum News Center

Improved Sapphire Growth Furnace

Large diameter, sapphire crystals are grown using the heat exchanger method (HEM). In order to address current applications of sapphire 30 kg, 25 cm diameter boules have been put into production. The feasibility of growth of (0001) orientation boules has been demonstrated. This allows for larger sapphire components for zero birefringence optics applications. The HEM has been adapted as an "investment casting" technique for growing complicated sapphire components directly from the melt.

The potential for improving the crystal quality of sapphire prepared by the edge-defined, film-fed growth technique was investigate. Crystal imperfections were observed by optical microscopy and X-ray topography. Saphire ribbons were grown in single or multiple forms in an induction furnace with graphite susceptors and molybdenum reflectors. Voids, grain boundaries and dislocations appeared in the sapphire ribons in relation to the growth conditions. Voids were formed by increasing the growth rate above about 0.7 mm/min. Grain boundaries were formed by imperfect epitaxy between the seed and the melt, or by polygonization of grown-in dislocations. Stress-induced and grown-in dislocations were separately observed in the ribbons free from voids and grain boundaries. The stress-induced dislocations were mostly suppressed by cooling the ribbons at rates below 150°C/h. Grown-in dislocations were completely eliminated by realizing a convex interface by necking down or by using an inclined top die. Dislocation free sapphire ribbons were realized.


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