Raw Materials for Ceramics, Refractories and Glasses

Quartz exhibits a very low coefficient of thermal expansion (0.5 pm/m.K) and an elevated Mohs hardness of seven. Large and pure single crystals of quartz of gem quality called lascas are used due to their high purity in the preparation of elemental silicon for semiconductors (see Section 5.8.1). 

At a temperature of 870°C, a-quartz transforms irreversibly into alpha-tridymite (a-tri-dymite, orthorhombic) [15468-32-3] with an important volume change of 14.4 vol.% as follows  

But in practice, the kinetic of the above reaction is too slow, and tridymite never forms below 1250°C, and hence at 1250°C or 1050°C in the presence of impurities, a-quartz transforms irreversibly into alpha-cristoballite (a-cristoballite, tetragonal) [14464-46-1] with an important volume change (17.4 vol.%) as follows  

However, if the temperature is raised to 1470°C, a-tridymite transforms also irreversibly into alpha-cristoballite (a-cristoballite) without any change in volume as follows  

On cooling a-cristoballite transforms reversibly into beta-cristoballite (P-cristoballite, cubic) at 260°C with a volume change 0 2.0 to 2.8 vol.%  

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Crystalline Silica. Quartz sand is of course the principal raw material for the production of glass (qv). Cristobalite and 3-quartz are used in glass ceramics (qv), ie, ceramics produced by the controlled crystallization of glass. Silica is a main constituent of ceramics (qv). For example, refractory silica brick containing small amounts of A O is used as roof brick for open-hearth furnaces at temperatures >1600° C (see Refractories). Silica sand or flour (ground quartz) is the raw material for soluble silicates, such as sodium silicate, which is consistently ranked as one of the top 50 U.S. industrial chemicals (98) (see Silicon compounds, synthetic inorganic silicates). 

Zircon is used in the manufacture of refractories, as opacifier in glazes and enamels, as nucleant in glass-ceramics and also as a raw material for electrotechnical ceramics, chemically resistant glasses and enamels, etc. 

Uses Electron tubes resistor cores windows in klystron tubes transistor mountings high-temperature reactor systems additive to glass and plastics beryllium alloy ingred. prep, of beryllium compds. catalyst for organic reactions electronic oxide ceramics ingred. refractory ceramics raw material Manuf./Distrib. Alfa Aesar http //www.alfa.com. 

Obviously, all these structure-sensitive factors influence the reaction kinetics in practical applications to such systems as refractories, ceramics, cement, glass, luminescent materials, semiconductors, catalysts, and pigments. The reactivity of the raw mixes of the constituents, the effects of calcination or burning conditions, and the various types of diffusing species in the course of solid reactions influence the quality of the final product. From an industrial viewpoint the reactions in ionic solid systems have a universal importance. This is so vast a field that for an illustration of this significance an arbitrary selection is necessary. It is hoped, however, that the examples discussed will convey the impact of the reactivity of ionic solids on modern industrial processes and products. 

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