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Sapphire is an anisotropic, rhombohedral crystal form of Aluminum Oxide.
Anisotropic single crystal materials exhibit some properties such as thermal expansion and hardness which vary significantly by orientation. For most applications this is unimportant, however it should be considered. Since 1947, INSACO has had experience with this material and we can provide considerable insight as to how orientation might affect your application.
Sapphire is also “birefringent” which is an optical refractive property that offsets wave transmission up to 0.8% at right angles to the optic axis. Birefringence is eliminated along the optic or C-axis of the crystal. Therefore, for certain optical applications, C-axis sapphire should be specified to avoid this effect.
Industrial sapphire is created by melting aluminum oxide (Al2O3) at 2040°C and then encouraging crystal growth with a seed and careful control of the environment. Growers have developed several unique methods for growth, with varying levels of resultant quality, size, and cost. The EFG or Stephanov methods allow the directed growth of shapes like ribbon, or even tubes, however there are many limitations to what can be done. The Czochralski, HEM, or Kiropolous methods allow the highest optical quality sapphire, but the result is a rod like “blob” of crystal called a boule, that must be entirely machined into usable shapes and sizes.
Sapphire and Ruby are actually the same material with small amounts of chromium (typically ≤ 0.05% by weight) added which affects color and optical properties, while not affecting mechanical, thermal and electrical properties significantly.