SYNTHETIC QUARTZ CRYSTALS

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

Many properties are directionally dependent, therefore values Hsted are indicative only. 

In an attempt to stimulate onshore production of synthetic quartz and piezoelectric devices in the 1970s, Brazil imposed an embargo on exports and ultimately raised the price several-fold for small quartz crystals used as the starting material for quartz growth. However, sources of suitable pure quartz were located in the United States and Canada, including vein and pegmatic deposits (1). Synthetic processes compatible with the natural U.S. quartz starting material from a variety of sources were developed, and U.S. production became relatively independent of imports (1). 

As of 1996, synthetic quartz was produced in the United Kingdom, France, CIS, Venezuela, Canada, China, Japan, Brazil, Poland, as weU as the United States. The principal nondomestic source is Japan. Some producers in the United States are Eastiake, in Ohio Motorola, in Carlisle, Pennsylvania and Thermo Dynamics, in Shawnee Mission, Kansas. 

The result is that the weight solubihty of quartz is increased to several percent. a-Quartz is the stable sohd phase in equihbtium with such solutions and 

Silicon dioxide [7631-86-9], Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is OC-quartz (low quartz). All 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

After feldspar quartz is the most frequent mineral on the Earth. However the most important finding place of the natural quartz crystals in desired size and quality is located in Brazil. Growing demand for quartz crystals for resonators tend to the 

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Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Solubility. An important aspect of sihca chemistry concerns the sihca— water system. The interaction of the various forms of sihca with water has geological significance and is apphed in steam-power engineering where the volatilization of sihca and its deposition on turbine blades may occur (see Power generation), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble sihcates, coUoidal sihca, and sihca gel. 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

Cnthidia fasciculata - [SILICA - INTRODUCTION] (Vol21) - [CLAYS - SURVEY] (Vol 6) - [SILICA - SYNTHETIC QUARTZ CRYSTALS] (Vol 21) - [GLASS] (Vol 12) -from diatomite piATOMITE] (Vol 8) -OSHA specifications [ABRASIVES] (Vol 1) -m silica refractories [REFRACTORIES] (Vol 21) - [ANTIBIOTICS - ANSAMACROLIDES] (Vol 2)... 

Oscillators -NLO for lasers [NONLINEAR OPTICAL MATERIALS] (Vol 17) -parametric [LASERS] (Vol 15) -synthetic quartz crystals for [SILICA - SYNTHETIC QUARTZ CRYSTALS] (Vol 21)... 

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