Sapphire materials and sapphire components consist of a single-crystal or monocrystalline form of aluminum oxide (Al2O3). As ceramic materials, they have superior hardness, mechanical and optical properties. Applications include jewel bearings, lasing rods, wear parts, wafer substrates and optics.

Manufacture and Growth

Single crystals of sapphire materials and sapphire components are manufactured or grown in melting and deposition processes. Melt-grown sapphire is used more commonly than sapphire produced through chemical vapor deposition (CVD) or epitaxial deposition processes, techniques which produce thin sheets or monocrystalline films on other substrates.

Single-crystal growing processes also include Czochralski (CZ), float zone (FZ), Bridgeman, Kyropoulos, and edge defined film-fed growth (EFG), as well as other crystal-pulling methods. Many of these processes immerse a seed crystal in the melt and then remove it slowly, forming a single-crystal boule or ingot. Crystallization begins at the surface of the monocrystal seed, but additional atoms arrange themselves along the same crystal orientation or plane.

Of these single-crystal methods, the Czochralski crystal growth process is the most common. Edge-defined film-fed growth (EFG), a newer method, allows continuous sapphire ribbons, tubes, rods, profiles or fibers to be pulled from the sapphire melt with controlled crystal orientation.

Dopants

Sapphire monocrystal can be doped with additional elements to modify properties and appearance.

  • Chromium and titanium dopants are common modifiers for lasers or lasing rod applications. Chromium-doped sapphire (Cr: Sapphire or Cr: Al2O3) produces a reddish, synthetic, ruby material.
  • Titanium-doped alumina and titanium-doped sapphire can produce a blue colored material at sufficient concentrations after heat treatment.

The GlobalSpec SpecSearch database provides information about these and other sapphire materials.

Properties

With some materials, grain boundaries and flaws in the manufacturing process can significantly alter physical and electrical properties. Sapphire can have superior properties compared polycrystalline alumina, however, because no grain boundaries are present. As a result, sapphire has superior mechanical properties, chemical stability and light transmission. Single-crystal materials are anisotropic.

Properties values vary depending on the orientation or crystal plane under test. For example, sapphire’s coefficient of thermal expansion (CTE) is 5.3x10-6 along the c-axis and 5.3x10-6 perpendicular to the c-axis.  Sapphire’s hardness is 1800 Knoop parallel to c-axis and 2200 Knoop perpendicular to c-axis.

Resistivity and dielectric constant also vary with crystal orientation. As a rule, the orientation of sapphire materials and components should be taken into consideration when designing dielectric substrates and wafers. Sapphire is a transparent electrical insulator with high thermal conductivity.

Applications

Sapphire materials and components have exceptional properties that are useful in many electronic, photonic or optical applications. End-uses include optical windows, laser optics, lasing rods, lenses, micro-optics, reflectors, dummy wafers, silicon-on-sapphire (SOS) substrates, fixtures for high-temperature or high-pressure equipment, carrier plates or substrates for blue light-emitting diodes (LEDs) and laser diodes, high speed integrated circuit (IC) chip substrates, microwave plasma tubes, flash lamp tubes, infrared (IR) detectors, and fiber optic lenses.

Because of their high hardness and wear characteristics, jewel bearings, bearing balls, valve balls, gage or styli points, wear parts and jewel pivots are often made from sapphire and ruby. Sapphire’s optical transparency and wear resistance also makes it very useful for optical windows, point-of-sale (POS) scanner windows, liquid crystal display (LCD) projector windows, and watch faces.

References

MolTech GmbH Molecular Technology - Sapphire

Saint-Gobain Crystals - Sapphire Components