Aluminium oxide crystals that do not contain chromium are called sapphires; they can be found in a range hues including blue, yellow, green, pink, purple, and colourless. One of the hardest materials in existence, sapphire can stand up to the rigors of almost any environment, and it can be polished to a very good surface finish. Sapphire is chemically inert. In addition, sapphire can transmit ultraviolet, visible and infrared light, as well as microwaves— a range broader than most materials.
Synthetically produced sapphire single crystals have a high thermal conductivity, volume resistivity, hardness and wear resistance. With a melting point of over 2000°C, it is much stronger than glass and due to its transparency it can be used as viewing window in high pressure and vacuum applications. Sapphire is considered to be a superior alternative to quartz, alumina, and silicon carbide for semiconductor processing applications.
In particular, sapphire tubes are used in various analytical instruments in lamp applications that transmit visible, infrared or ultraviolet light, often with high intensity at high temperatures and pressures. Sapphire continues to be the engineers’ material of choice for applications that call for reduced contamination, lower particle generation and increased productivity.