The monocrystalline form of Al2O3 is called sapphire, which is one of the hardest materials and ranks ninth under the Mohs hardness. Sapphire is the second hard crystal after diamond. Due to its structural strength, the sapphire optical window can be made thinner than other common dielectric optical windows, thus increasing transmittance.
The sapphire optical window and other sapphire components such as the Sapphire lens can be used in a wide wavelength range from 0.15 to 5.5 μm and can resist the attenuation of ultraviolet radiation. Sapphire windows and corrugated sapphire plates are environmentally stable and resistant to common chemical acids and alkalis due to sapphire’s high dielectric constant.
The other properties that make sapphire are high compressive strength, high melting point, high thermal stability, and high thermal conductivity. Sapphire’s birefringence makes it an ideal material for environmentally stable high-quality wave plates operating from ultraviolet to mid-infrared.
Sapphire windows and sapphire waveplates are often used in research, medical, space, and military applications due to their excellent performance.
Qualities of Sapphire
Natural sapphire is a gem variety of corundum and alumina (Al2O3). Sapphire is a form of alumina, which is one of the most abundant compounds in nature. In its natural state, aluminum oxide (Al2O3) is a white powder material widely used as an industrial abrasive. When heated to about 2050 ° C (about 4000 ° F), the powder melts, and then any of several crystal growth methods can be used to form a single crystal.
How is Artificial Sapphire Made?
Today, many monocrystalline sapphire manufacturing methods are variants of the Czochralski process, invented in 1916. During this process, a small piece of the sapphire seed crystal is dipped into a crucible filled with molten alumina and then slowly pulled upward at a rate of 1 to 100 millimeters per hour. The alumina crystallizes at the end to form large carrot-like granules with a large size up to 400 mm in diameter and 500 kg in weight.
Because it is a single crystal, sapphire cannot be molded or stretched. It must be “grown” in a specific way according to the selected growing process. Artificial sapphire has the same monocrystalline rhombohedral structure as natural sapphire, but it has greater purity and transparency. The beauty and color of sapphire gemstones are due to impurities received when the crust solidifies.
Qualities of Artificial Sapphire
The artificial sapphire has a lot of robust properties that make it an ideal choice for many optical applications. It is chemically, physically, and optically, it is a replica of natural sapphire, but it does not contain contaminants or natural impurities. Blue, purple, and green sapphire contain impurities of titanium and iron.
Sapphire has a variety of properties that make it very attractive for various optical applications, including windows and corrugated plates. It is the second-largest hard crystal after diamond. Due to its structural strength, it can make the window outside the sapphire much thinner than other common dielectric windows, thus increasing the transmittance (lower loss). The sapphire window is useful in the 0.15-5.5μm optical transmission range, and certain qualities of sapphire can resist darkening by ultraviolet radiation.
Qualities of Sapphire Optical Windows
The sapphire window can be made very thin to maintain strict optical specifications, thus achieving high transmittance. In high power applications, due to their high thermal stability and thermal conductivity, compared to standard windows, losses in sapphire windows have received less attention. The standard window will heat up and start to warp, while the sapphire window will not. Many standard windows will darken when exposed to high-intensity UV rays, rendering the windows unusable after absorption and requiring replacement. The UV quality sapphire window does not darken, making it suitable for applications where there is UV radiation.
Several custom Sapphire shapes like Sapphire lens and Sapphire rods can also be created from artificial Sapphire that is used in many applications.