“Rotem Industries, which was founded in 1969, provides sapphire products for defense applications. The gradient solidification method for sapphire growth was developed at Rotem to produce high quality, near-net-shape domes….

Alumina is loaded into a hemispheric molybdenum crucible containing a sapphire seed at the bottom. The crucible is heated in vacuum to produce a gradient of temperature, with the highest temperature at the top. When the seed has partially melted, the temperature is lowered in a carefully controlled manner so that crystallization proceeds out from the seed crystal….

In the heat-exchanger method, highest crystal quality is produced when growth takes place along the a-axis of the crystal. For the boule…, the a-axis is the cylindrical axis. The optical axis of sapphire (the c-axis) is perpendicular to a and comes out the curved side of the boule. To make c-axis domes, a cylindrical core is drilled from the side of the boule and domes are scooped out from the core.

In Rotem’s gradient solidification method, the thermal mass is low enough to permit the growth of good quality c-axis boules and domes, as well as a-axis material.”

D. C. Harris*, “A Century of Sapphire Crystal Growth,” Proceedings of the 10th DoD Electromagnetic Windows Symposium. Norfolk, Virginia, May 2004.

*Naval Research Laboratory

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