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Solid State Thermal Conversion of Polycrystalline Alumina to Sapphire Using a Seed Crystal

A solid state seed crystal process for bulk conversion of a polycrystalline ceramic body to a single crystal body (of the same chemical composition) having the same crystal orientation as the seed crystal. The process comprises heating said body to form a monolithic join between the body and the seed crystal, heating the joined structure to reduce grain growth inhibitors and further heating the joined structure above the minimum temperature required for crystallite growth of the crystalline material, but not hot enough to melt and distort the original shape of the polycrystalline ceramic body during its conversion to a single crystal. This process has been used to convert polycrystalline alumina (PCA) bodies to sapphire having the same crystal orientation as the seed crystal by heating the PCA body, monolithically joined to a sapphire seed crystal, at a temperature above 1700 DEG C without melting the body.

A solid state process for bulk conversion of a solid PCA body comprising partially sintering the PCA body, placing a solid sapphire seed crystal in contact with the partially sintered PCA body in a position in which further sintering of the PCA body will cause pressure between said PCA body and said seed crystal so as to monolithically join said seed crystal to said PCA body, further sintering said PCA body until said PCA body has a density no less than 3.9g/cc and thereafter heating said PCA body with said seed crystal to a temperature above the temperature for alumina crystal grain boundary mobility but without melting said PCA body for a time sufficient to convert said PCA body to a sapphire body.


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