Fused quartz

Fused quartz or fused silica is glass consisting of silica in amorphous (non-crystalline) form. It differs from traditional glasses in containing no other ingredients, which are typically added to glass to lower the melt temperature. Fused silica, therefore, has high working and melting temperatures. Although the terms fused quartz and fused silica are used interchangeably, the optical and thermal properties of fused silica are superior to those of fused quartz and other types of glass due to its purity. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It transmits ultraviolet better than other glasses, so is used to make lenses and optics for the ultraviolet spectrum. The low coefficient of thermal expansion of fused quartz makes it a useful material for precision mirror substrates. Fused quartz or fused silica is glass consisting of silica in amorphous (non-crystalline) form. It differs from traditional glasses in containing no other ingredients, which are typically added to glass to lower the melt temperature. Fused silica, therefore, has high working and melting temperatures. Although the terms fused quartz and fused silica are used interchangeably, the optical and thermal properties of fused silica are superior to those of fused quartz and other types of glass due to its purity. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It transmits ultraviolet better than other glasses, so is used to make lenses and optics for the ultraviolet spectrum. The low coefficient of thermal expansion of fused quartz makes it a useful material for precision mirror substrates. Fused quartz is produced by fusing (melting) high-purity silica sand, which consists of quartz crystals. There are four basic types of commercial silica glass: Quartz contains only silicon and oxygen, although commercial quartz glass often contains impurities. The most dominant impurities are aluminium and titanium. Melting is effected at approximately 1650 °C (3000 °F) using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame-fused). Fused silica can be made from almost any silicon-rich chemical precursor, usually using a continuous process which involves flame oxidation of volatile silicon compounds to silicon dioxide, and thermal fusion of the resulting dust (although alternative processes are used). This results in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet. One common method involves adding silicon tetrachloride to a hydrogen–oxygen flame, but this precursor results in environmentally unfriendly byproducts including chlorine and hydrochloric acid. Fused quartz is normally transparent. The material can, however, become translucent if small air bubbles are allowed to be trapped within. The water content (and therefore infrared transmission of fused quartz and fused silica) is determined by the manufacturing process. Flame-fused material always has a higher water content due to the combination of the hydrocarbons and oxygen fuelling the furnace, forming hydroxyl groups within the material. An IR grade material typically has an content below 10 ppm. Most of the applications of fused silica exploit its wide transparency range, which extends from the UV to the near IR. Fused silica is the key starting material for optical fiber, used for telecommunications. Because of its strength and high melting point (compared to ordinary glass), fused silica is used as an envelope for halogen lamps and high-intensity discharge lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life. Vacuum tubes with silica envelopes allowed for radiation cooling by incandescent anodes. Because of its strength, fused silica was used in deep diving vessels such as the bathysphere and benthoscope. Fused silica is also used to form the windows of manned spacecraft, including the Space Shuttle and International Space Station. The combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks for photolithography.