Fused quartz

Fused quartz, also called fused silica or quartz glass, is a type of glass made from nearly pure amorphous (non-crystalline) silicon dioxide (SiO₂). Unlike common glasses such as soda-lime, lead, or borosilicate varieties, it contains no added ingredients to alter its properties. This absence of additives gives it a very high working and melting temperature—around 2200 °C (4000 °F)—making it difficult to shape and less practical for everyday uses, but it also provides superior strength, chemical resistance, and a very low coefficient of thermal expansion. These traits make it valuable for specialized applications in lighting, science, semiconductor manufacturing, and lab equipment.

The terms "fused quartz" and "fused silica" are often used interchangeably, though they can refer to different production methods that yield different levels of trace impurities. Fused quartz is in a glassy state, which gives it physical properties quite unlike those of crystalline quartz, even though both are made of the same substance. Fused silica, by contrast, is entirely synthetic. Its optical transmission extends well into the ultraviolet and infrared ranges, far beyond that of other common glasses, so it is used for lenses and optics designed for those wavelengths. Impurities introduced during manufacturing can limit this transmission, leading to commercial grades optimized for either infrared or ultraviolet use. The material’s low thermal expansion also makes it ideal for precision mirror substrates and optical flats.

**Manufacture** Fused quartz is made by melting high-purity silica sand (quartz crystals). There are four main commercial types: - Type I: produced by electrically melting natural quartz in a vacuum or inert atmosphere. - Type II: made by fusing quartz crystal powder in a high-temperature flame. - Type III: created by burning silicon tetrachloride (SiCl₄) in a hydrogen-oxygen flame. - Type IV: produced by burning SiCl₄ in a water vapor-free plasma flame.

Although quartz contains only silicon and oxygen, commercial quartz glass often includes impurities. The most common are aluminum and titanium, which affect ultraviolet transmission. If water is present during manufacturing, hydroxyl (OH) groups can become embedded, reducing infrared transmission.

**Fusion** Melting occurs at about 2200 °C (4000 °F) using either an electrically heated furnace (electrically fused) or a gas/oxygen-fueled furnace (flame-fused). Fused silica can also be made from nearly any silicon-rich chemical precursor, typically through a continuous process where volatile silicon compounds are flame-oxidized into silicon dioxide dust, which is then thermally fused. This method yields an ultra-high-purity transparent glass with improved deep-ultraviolet transmission. A common approach involves adding silicon tetrachloride to a hydrogen-oxygen flame.

**Product Quality** Fused quartz is normally transparent but can become translucent if small air bubbles are trapped inside. Its water content—and thus its infrared transmission—depends on the manufacturing process. Flame-fused material always has a higher water content because hydrocarbons and oxygen in the furnace form hydroxyl groups within the glass. An infrared-grade material typically has an OH content below 10 parts per million.

**Applications** Many optical uses of fused quartz rely on its wide transparency range, which extends well into the ultraviolet and near-mid infrared. It is a key starting material for optical fiber used in telecommunications. Its strength and high melting point make it suitable for envelopes in halogen lamps and high-intensity discharge lamps, which must operate at high temperatures for brightness and long life. Some high-power vacuum tubes also use silica envelopes, as their good infrared transmission helps cool incandescent anodes through radiation.

Because of its physical strength, fused quartz was used in deep-diving vessels like the bathysphere and benthoscope, as well as in windows for crewed spacecraft such as the Space Shuttle and the International Space Station. It has also been used in composite armor development. In the semiconductor industry, its combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks in photolithography. Its UV transparency is also exploited in windows for EPROMs (erasable programmable read-only memory), which are erased by exposure to strong ultraviolet light—these chips are recognizable by the transparent fused quartz (or, in later models, UV-transparent resin) window on top of the package. Fused quartz is also used in 5D optical data storage and in semiconductor fabrication furnaces.

For telescope mirrors and other first-surface mirrors, fused quartz offers nearly ideal properties: it behaves predictably, takes a very smooth polish, and allows the optical fabricator to achieve the desired figure with fewer testing iterations. In some cases, high-purity UV-grade fused quartz has been used for individual uncoated lens elements in special-purpose lenses, such as the Zeiss 10.