| Property* | Value |
|---|---|
| General | |
| Chemical Formula | α-Al2O3 |
| Mechanical | |
| Density | 3.97 gm/cc |
| Hardness | 2200 Knoop perpendicular to the c-axis 1900 Knoop parallel to the c-Axis |
| Tensile Strength | 58 kpsi |
| Modulus of Elasticity | 68 x 106 psi Young's |
| Flexural Strength | 100 kpsi |
| Compressive Strength | 425 kpsi |
| Poisson's Ratio | ~ 0.28 Varies with orientation |
| Electrical | |
| Dielectric Strength | 1200 ac V/mil |
| Dielectric Constant | 9.3 - 11.5 (@ 1 MHz) parallel to the c-axis |
| Volume Resistivity | > 1014 ohm-cm |
| Thermal | |
| Coefficient of Thermal Expansion | 4.3 x 10-6/°C perpendicular to the c-axis 5.4 x 10-6/°C parallel to the c-axis |
| Thermal Conductivity | 46.0 W/mK |
| Specific Heat | 0.16 cal/g °C |
| Maximum Working Temperature | 2000 °C |
| Optical | |
| Index of Refraction | 1.768 Ordinary ray, No, c-axis 1.760 Extraordinary ray, Ne, c-axis |
| Birefringence | 0.008 (No-Ne) |
| Transmission Band | 0.3 - 5.0 Wavelength (microns) > 80% transmission |
| All properties are at room temperature unless otherwise noted. Engineering data are representative, and are not intended as absolute nor warrantable. Manufacturer’s Data shown is blended from multiple sources and therefore illustrates the marketplace. | |
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. Insaco has more than 60 years of 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 useable 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.
Wikipedia Reference for Sapphire
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