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Understanding Fused Quartz and Fused Silica

Quartz crystals can be seen in new age shops in their raw state, which isn't surprising, as it is the most common solid minerai on earth. Some varieties of quartz, such as amethyst and citrine, are worn as gems. Quartz crystals are used in radios and watches, and it has been involved in technological advances since it was first discovered to have the power to generate electricity. This effect, called piezoelectricity, and known in the 18th century, led to the early use of quartz crystals in the World War I, in sonar devices.

Fused quartz, or silicon dioxide, to give its chemical name, was originally produced from pure quartz crystal, melted at extremely high temperatures. But while quartz crystals continue to be used for fused quartz, it has been found that when crystalline silica occurs in sand or rock, it can also produce fused quartz, or synthetic fused silica, as this man-made fused quartz is sometimes called.

To most of us, the desert sands would have as much to do with the modem technology we use every day as dinosaurs with fast food. Yet sand is now the basic material used to create fused quartz. This process begins 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 Si02, 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.

Fused Quartz Properties

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 and remarkable properties of fused quartz, for which it is so much in demand in the semiconductor and the fiber optics industries, are very high purity, extremely low thermal expansion, significant chemical resistance, and extraordinary optical transmission properties. Fused quartz also displays excellent elasticity, remarkable compressive strength, an ability to remain stable under high temperatures, a high thermal shock resistance and low conductivity, a high resistivity and superior dielectric strength. It is also impermeable to gases.

Its high purity, strength, rigidity, high electrical and thermal resistance, and specific wavelength transmission make this material very useful in the semi-conductor 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 107 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 (I ,000 psi) is often considered in practice.

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