What Is Fused Quartz?

What Is Fused Quartz?

Quartz, fused silica, quartz glass, and even silica sand are all terms that can get confused because they all refer to the same material: pure, crystal-free silicon dioxide. Fused quartz, however, can be made from either natural quartz crystal or synthetic fused silica (which may also be referred to as man-made fused silica). Unlike crystalline silicon dioxide, fused quartz has very low metallic impurities. This makes it very different from silica sand and other types of glass because it has superior optical transmission in the ultraviolet and infrared. Fused quartz can be used to make windows, lenses, prisms, and wedges for a variety of optical applications.

Fused quartz is made by melting highly pure crushed quartz in an electric or gas/oxygen-fuelled furnace. The result is an amorphous glass with excellent thermal and chemical properties. It is normally transparent and is used to manufacture a variety of specialty glass items including precision mirror substrates, semiconductor fabrication, and laboratory equipment. It is particularly well suited for use as an envelope for halogen lamps because it can withstand high temperatures, has very good UV transmission and a very low coefficient of thermal expansion.

The tensile strength of fused quartz, without substantial surface flaws, is up to 4.8 x 107 Pa (7,000 psi). This strength is comparable with that of ordinary glass, but the ductility and thermal conductivity are significantly better than those of other glasses. This combination of strength, ductility and low thermal expansion makes it very useful in the semiconductor industry where many processes require extremely close tolerances.

Fused quartz also has excellent resistance to chemicals and a wide range of temperature extremes. It is very resistant to acids (except hydrofluoric acid and phosphoric acid) and is not degraded by ionising radiation. It is therefore commonly used in laboratory equipment and in the manufacture of scientific instruments. It is the preferred material for new builds of historical glass instruments such as the verrophone and glass harp, as it gives a much more consistent and vibrant sound than the historically used lead crystal.

Because of its superior properties compared with ordinary glass, fused quartz is used to fabricate first surface mirrors for telescopes and other optical instruments. It has the advantage of exhibiting more predictable behaviour than crystalline silicon dioxide which allows optical fabricators to put a very smooth polish on the surface and achieve a desired figure with fewer testing iterations. It is also used in special UV photographic lenses such as the Zeiss 105 mm f/4.3 UV Sonnar and the Nikon UV-Nikkor 105 mm f/4.5. Lastly, it is one of the key starting materials for optical fiber used in telecommunications. This is because it can be made to be transparent at very short wavelengths, including the ultraviolet and infrared. This is important because many telecommunications waveforms are transmitted over this region. It is also used for UV sterilisation of medical products such as surgical masks and trays.