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# Fused quartz

Fused quartz and fused silica are types of glass containing primarily silica in amorphous (non-crystalline) form. They are manufactured using several different processes.

Fused quartz is made by melting high-purity naturally occurring quartz crystal at around 2000°C using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused). Fused quartz is normally transparent.

Fused quartz can also form naturally. The naturally occurring form of fused quartz is usually referrred to as Metaquartzite and is formed under metamorphic conditions. Due to increased heat the crystals within the quartz become fused together.

Fused silica is produced using high purity silica sand as the feedstock, and is normally melted using an electric furnace, resulting in a material that is translucent or opaque. (This opacity is caused by very small air bubbles trapped within the material.)

Synthetic fused silica is made from a silicon-rich chemical precursor usually using a continuous flame hydrolysis process which involves chemical gasification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust (although there are alternative processes). 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, however use of this precursor results in environmentally unfriendly by-products including chlorine and hydrochloric acid. To eliminate these by-products, new processes have been developed using an alternative feedstock, which has also resulted in a higher purity fused silica with further improved deep ultraviolet transmission.

Fumed silica is manufactured by a similar flame hydrolysis process to synthetic fused silica, however it is in the form of a fine powder/dust and is typically used in applications such as fillers for rubbers and plastics, coatings, adhesives, cements, sealants, cosmetics, pharmaceuticals, inks and abrasives.

The optical and thermal properties are superior to those of other types of glass due to its purity (or rather, its lack of impurities). For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It has better ultraviolet transmission than most other glasses, and so is used to make lenses and other optics for the ultraviolet spectrum. Its low coefficient of thermal expansion also makes it a useful material for precision mirror substrates.

## Chemistry

Fused quartz is a noncrystalline form of silicon dioxide (SiO2), which is also called silica. (The crystalline form of this material is quartz).

## Applications

Specially prepared fused silica is also the key starting material used to make optical fiber for telecommunications.

Because of its strength and high melting point (compared to ordinary glass), fused silica is used as the envelope of halogen lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life.

The combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks for photolithography.

Due to the thermal stability and composition it is used in the semiconductor fabrication furnaces.

Fused quartz has nearly ideal properties for fabricating first surface mirrors such as those used in telescopes. The material behaves in a predictable way and allows the optical fabricator to put a very smooth polish onto the surface and produce the desired figure with fewer testing iterations.

Fused silica as an industrial raw material is used to make various refractory shapes such as crucibles, trays, shrouds, and rollers for many high temperature thermal processes including steel making, foundries, and glass manufacture. Refractory shapes made from fused silica have excellent thermal shock resistance and are chemically inert to most elements and compounds including virtually all acids, regardless of concentration. Translucent fused silica tubes are commonly used to sheathe electric elements in room heaters, industrial furnaces and other similar applications.

## Physical properties

The extremely low coefficient of thermal expansion accounts for its remarkable ability to undergo large, rapid temperature changes without cracking (see thermal shock).

"UV grade" synthetic fused silica (sold under various tradenames including "HPFS", "Spectrosil" and "Suprasil") has a very low metallic impurity content making it transparent deeper into the ultraviolet. An optic with a thickness of 1cm will have a transmittance of about 50% at a wavelength of 170 nm, which drops to only a few percent at 160 nm. However, its infrared transmission is limited by strong water absorptions at 2.2 μm and 2.7 μm.

"IR grade" fused quartz (tradenames "Infrasil", "Vitreosil IR" and others) which is electrically fused, has a greater presence of metallic impurities, limiting its UV transmittance wavelength to around 250 nm, but a much lower water content, leading to excellent infrared transmission up to 3.6 μm wavelength. All grades of transparent fused quartz/fused silica have near-identical physical properties.

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 [OH] within the material. An IR grade material typically has an [OH] content of <10 parts per million.

## Optical properties

Dispersion of fused silica can be approximated by the following Sellmeier equation (Malitson 1965):

$\varepsilon=1+\frac{a_1\lambda^2}{\lambda^2-l_1^2}+\frac{a_2\lambda^2}{\lambda^2-l_2^2}+\frac{a_3\lambda^2}{\lambda^2-l_3^2},$

where

$a_1=0.69616630, \quad l_1=0.068404300,$
$a_2=0.40794260, \quad l_2=0.11624140,$
$a_3=0.89747940, \quad l_3=9.8961610,$

and wavelength is measured in micrometers.