Technical specifications of Fused Quartz and Fused Silica
Fused quartz and silica are among the most valuable materials available to industry and science.
Semi-finished products made of fused quartz or fused silica possess a unique range of properties:
- high purity level
- low OH content
- high homogeneity
- low thermal expansion
- high chemical resistance
- excellent thermal shock resistance
- low dielectric losses
- low bubble content
- high optical transmission in the IR & UV domain
- low thermal conductivity
- high use temperatures
Originally the term “fused quartz”was used for transparent quartz glass products manufactured from quartz crystals. Fused silica was the name for opaque quartz ware, produced from fused sand.
The distinguishing feature today is the derivation of the raw material and not the appearance of the finished product. The raw materials for fused quartz both transparent and opaque, are quartz crystal or quartz sand of natural origin. Synthetic fused silica refers to quartz glass prepared from synthetic chemical precursors.
Manufacturing process
There are three different processes for manufacturing quartz glass:
This is the most commonly used melting process for manufacturing quartz glass.Two different methods of electric fusion can be used:continuous and batch(boule) fusion.
In the continuous method quartz sand is poured into the top of a vertical melter which consists of a refractory metal crusible surrounded by electric heating elements.The interior of the crucible is maintained in a neutral or slightly reducing atmosphere that keeps the silica from reacting with the refractory metal.The melted material exits the bottem orifice of the crucible which is shaped to produce rods, tubes, plates or other products of various dimensions.
In the batch fusion method, a large quantity of raw material is placed inside a refractory lined vacuum chamber which also contains graphite heating elements.Although this method has historically been used to produce large single boules of material, it can also be adapted to, produce much smaller, near net shapes.
Flame fusion was originally known as the Verneuille process. Heraeus chemist Richard Kuch first began fusing quartz rock crystal in a hydrogen/oxygen(H2/O2) flame, more than 100 years ago.Heraeus has been producing quartz glass on an industrial scale with this process ever since.
The basic process consists of trickling high purity silica sand(made of crushed quartz crystal or natural quartz sand) at a controlled rate into a high temperature H2/O2 flame.
There it melts and collects on a bait rod that is slowly withdrawn from the flame thus forming a solid round ingot.This ingot can then be shaped into desired dimension.
Synthetic fused silica is produced by the oxidation/hydrolysis of silicon tetrachloride(SiCl4) vapor in an H2/O2 flame.This process is called the soot process.
Because silicon tetrachloride is a synthetic chemical with exceptional starting purity, alkali and metal contamination in the final product is reduced to ppb levels.
Items of special precaution when handling quartz glass
Touching quartz glass with the naked hand will leave a smudge on the glass, such as fingerprints.Any smudging of quartz glass could easily cause a loss of transparency. Never touch the product directly with the naked hand.Also,when storing or setting aside the product, please be sure to place it in a polyvinyl bag, or take other precautions to ensure that no dirt, dust, or any other foreign contaminant attaches to the product.
If the product is used at high temperatures when an impurity has become attached to the product surface, the result will be a reduction of the product life.Also, use with a surface-attached impurity, means that the product could break easily; there is a fear that such breakage could cause physical injury.
When removing smudges or other impurities, in general, after immersion for several minutes in around 5ªC-10ªC hydrofluoric acid, wash and rinse sufficiently and thoroughly with pure water(demineralised water) and allow the product to dry well.
Precautions when using quartz glass at high temperatures, or for long periods of time.
Quartz glass has a high heat-resistance temperature, and it can resist temperatures as high as 1000ªC. However for long-period use at a temperature of 1100ªC or above, deformation due the products own weight or to added weight, may gradually occur.Product life can be extended by taking appropriate response measures, such as by adopting product-support methods, adding changes to usage states, etc.
Precautions when using quartz glass in rapid heating or rapid cooling situations.
Quartz glass has an extremely small coefficient of heat expansion.Therefore, compared with ordinary glass, quartz glass is several tens of times more resistant to rapid heating and rapid cooling.Nevertheless, since quartz glass also has an extremely low coefficient of thermal conductivity, in the case of rapid heating of a local portion of the quartz glass, or in the case of an extreme change in temperature, a crack or cracking may occur.Further, remember that the thicker the product, the lower the product’s heat and impact resistance characteristics.
When a temperature change occurs when quartz glass is firmly attached to an other material,respective differences in the heat expansion characteristics of the attached material and the quartz glass could be a cause of breakage.
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