Fused quartz is a man-made material manufactured principally from sands. It is non-crystalline, and in a high purity state is a useful material for high performance fluid and gas delivery. Its mechanical and thermal properties are superior to that 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.

The name quartz comes from the mineral of the same basic chemical composition. The natural material is crystalline.

The chemical name of fused quartz is silicon dioxide, which is also called silica. This material was originally called fumed silica because the high purity manufacturing process involves chemical gassification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust. From this final melting process it also gets the name fused silica.

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

Fused silica is transparent to ultraviolet light (down to around 170nm where it has a transmittance of about 50% and a transmittance of only a few percent at 160nm. for an optic with thickness of about 1 cm) and near infrared light, and has many uses in technical optics as a consequence.

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.

See also: quartz, list of minerals

Properties of Clear Fused Quartz

(Based on information in Fused Quartz Catalogue Q-7A, General Electric Company)

• Density: 2.203 g/cm³
• Hardness: 7 (Modified Scale); 5.3-6.5 (Mohs Scale)
• Tensile strength: 48.3 MPa
• Compressive strength: >1.1 GPa
• Bulk modulus: ~37 GPa
• Rigidity modulus : 31 GPa
• Young's modulus: 71.7 GPa
• Poisson's ratio: 0.16
• Coefficient of thermal expansion: 5.5E-7 cm/(cm*K) (average from 20°C to 320°C)
• Thermal conductivity: 1.3 W/(m×K)
• Heat capacity: 45.3 J/mol
• Softening point : ~1665°C
• Annealing point : ~1140°C
• Strain point: 1070°C
• Electrical resistivity: >10¹⁸ Ω×m
• Dielectric constant: 3.75 at 20°C 1 MHz
• Dielectric loss factor : less than 0.0004 at 20°C 1 MHz
• Index of refraction: 1.4585