Fused quartz is made by melting natural quartz crystals present in quartzite sand, and then cooling it to produce an amorphous substance, also known as quartz glass. This process is carried out in vacuum in order to prevent inclusion, ripple and bubble formation in the viscous melt. The temperature at which the melt is processed is 1900 degrees centigrade. The chemical composition of fused quartz is the same as that of fused silica, and its chemical representation is SiO2, or, Silicon dioxide.
Fused Quartz vs. Fused Silica
At the very outset it is important to differentiate between fused quartz and fused silica, which are often confused as the same. These materials are manufactured through dissimilar processes at disparate costs, using different components, and have different properties. Fused quartz is processed by fusing powdered crystal quartz in a hydrogen-oxygen flame and is manufactured to various purity specifications, for several specific uses. Fused silica is a synthetic amorphous substance produced using either flame or vapor phase hydrolysis of silicon halide.
Producing Fused Quartz
Fused quartz is produced at a very high cost, because of the difficulty in gathering the components and also that of the process itself. The essential properties of fused quartz, for which it is so much in demand in the semiconductor and the fiber optics industry, is because of its remarkable properties of very high purity, extremely low thermal expansion, significant chemical resistance, and extraordinary optical transmission properties. Besides, it displays excellent elasticity, remarkable compressive strength, an ability to remain stable under high temperatures, a high thermal shock resistance and low conductivity, and a high resistivity and superior dielectric strength. It is also impermeable to gases.
Fused Quartz Properties
Some of these properties of fused quartz like high purity, strength, rigidity, high electrical and thermal resistance and specific wavelength transmission make this material very useful in the semi-conductor industry. Fused quartz tubing is ideal as a furnace in the processing of silicon wafers. For the processing of silicon metal for semiconductor wafers, poly-silicon components are placed in fused quartz crucibles, melted at high temperatures and derived from the melt as a single crystal. Fused quartz crucibles are available in sizes from 8 to 24-inch diameters. Standard sidewall thickness and tolerances often range from 6 +/- 1.2mm to 10 +/- 3.5mm. Fused quartz rods in diameters of 1 to 20 mm are used in semiconductor wafer processing. With the choice of an appropriate specification, fuse quartz is able to substitute a complex subassembly of a lot of different materials used in this industry.
The performance and quality of a majority of fused quartz products is dependent on the purity level of the material and should contain not more than 50 ppm total elemental impurities by weight. The design tensile strength for fused quartz without any substantial surface flaws can be more than of 4.8 x 10 7 Pa (7,000 psi).Since surface flaws affect the tensile strength of most varieties of glass, a design stress of .68 x 10 7 Pa (1,000 psi) is often considered in practice.
The extremely low coefficient of expansion possessed by this material, 5.5 x 10 -7 mm øC (20-320øC), makes it invaluable for use in optical flats, precise scientific equipments such as telescope and microscopic lenses and several other non-demanding applications which require minimum sensitivity to thermal changes. Most optical materials are available in sizes as large as 12" x 12" square or 12" diameter. Thickness tolerances can be as less as +/-.0001", with flatness to 1/4 lightband, and parallelism to 50 millionths of an inch. It is a very good conductor of conductor of ultraviolet rays, and UV cutoff and the presence or absence of bands at 245 nm and 2.73 um is an indicator of the purity for this material.
Since fused quartz does not react to most acids and other chemicals, it is used in the manufacture of test tubes and containers used in a chemical laboratory, in order to give them a longer service life. It is used to manufacture optical fiber in telecommunications, and due to its stable behavior at higher temperatures, fused quartz is used in lighting applications and in ovens and heaters in many industrial machines.
Fused quartz has been a blessing to semiconductor, optical, telecommunication and the glass chemical container industries, among many others. More and more uses of this expensive material are certain to be found in the next few years in various applications in need of its significant properties.