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Correlation of Sapphire Quality With Uniformity And Optical Properties

Large sapphire boules, up to 34 cm diameter, 65 kg, are being grown by the heat exchanger method (HEM) and even larger sizes are sought to meet future requirements of advanced optical systems. These boules, especially in large sizes, exhibit lattice distortion and light scatter in a very narrow range. A qualitative grading system has been developed to characterize sapphire. Windows of five grades and different orientations were prepared and measured for refractive index homogeneity to evaluate transmitted wavefront distortion. The data showed that the refractive index homogeneity for all samples was in the 10-7 (0.1 ppm) range. The fact that lattice distortion does not affect the transmitted wavefront allows fabrication of large sapphire windows in production mode at low cost.

Examination of small euhedral crystals of α‐Al2O3 (sapphire) that were observed following oxidation of aluminum and an aluminum alloy in wet hydrogen at high temperatures showed that growth probably occurs by decomposition of AlO on an Al2O3 surface. Rapid growth occurs on the tips of needles and the edges of platelets at lattice steps formed by intersection of hollow screw dislocations with these surfaces. Screw dislocations were detected by x‐ray measurement of the lattice twist they produce in the needle‐like crystals or "whiskers".Needles were bent to elastic strains of over 2% corresponding to a strength of 1011 dyne cm-2.


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