Fused Quartz are ultra pure, single component glasses (SiO2) with a unique combination of thermal, optical and mechanical properties, which make them the preferred materials for use in a variety of processes and applications where other materials are not suitable. The very high purity (over 99.9% ) ensures minimum contamination in process applications. These materials can routinely withstand temperatures of over 1250篊, and due to their very low coefficient of thermal expansion can be rapidly heated and cooled with virtually no risk of breakage due to thermal shock.Fused Quartz are inert to most substances, including virtually all acids, allowing their use in arduous and hostile environments. The dielectric properties and very high electrical receptivity of these materials over a wide range of temperatures, together with their low thermal conductivity allow their use as an electrical and thermal insulating material in a range of environments. Fused Quartz is manufactured using powdered quartz crystal as a feedstock and is normally transparent, the fusion process is carried out at high temperature (over 2000oC) .

fused quartz and fused silica

Vitreous silica is the generic term used to describe all types of silica glass, with producers referring to the material as either fused quartz or as fused silica. originally, those terms were used to distinguish between transparent and opaque grades of the material. Fused quartz products were those produced from quartz crystal into transparent ware, and fused silica described products manufactured from sand into opaque ware. Today, however, advances in raw material bonification permit transparent fusions from sand as well as from crystal. Consequently, if naturally occurring crystalline silica (sand or rock) is melted, the material is simply called fused quartz. If the silicon dioxide is synthetically derived, however, the material is referred to as synthetic fused silica. Controlled Process: The performance of most fused quartz products is closely related to the purity of the material. The proprietary raw material bonification and fusion processes are closely monitored and controlled to yield typically less than 50 ppm total elemental impurities by weight. Clear fused quartz varieties have a nominal purity of 99.995 W % SiO2. Structural hydroxyl (OH-) impurities are also shown. The strong IR absorption of OH- species in fused quartz provides a quantitative method for analysis. Beta Factor: The term Beta Factor is often used to characterize the hydroxyl (OH-) content of fused quartz tubing. This term is defined by the formula shown below.

Typical Physical Properties of fused quartz

Property Typical Values Density 2.2x103 kg/mm3Hardness 5.5 - 6.5 Mohs' Scale 570 KHN 100 Design Tensile Strength 4.8x107 Pa (N/mm2) (7000 psi) Design Compressive Strength Greater than 1.1 x l09 Pa (160,000 psi) Bulk Modulus 3.7x1010 Pa (5.3x106 psi) Rigidity Modulus 3.1x1010 Pa (4.5x106 psi) Young's Modulus 7.2x10-10 Pa (10.5x106 psi) Poisson's Ratio .17 Coefficient of Thermal Expansion 5.5x10 -7 cm/cm . oC (20-320oC)Thermal Conductivity 1.4 W/m . oC Specific Heat 670 J/kg . oCSoftening Point 1683oCAnnealing Point 1215oC Strain Point : 1120 oC Electrical Receptivity 7x107 ohm cm (350oC) Dielectric Properties (20oC and 1 MHz) Constant 3.75 Strength 5x107 V/m Loss Factor Less than 4x10 -4 Dissipation Factor Less than 1x10 -4 Index of Refraction 1.4585 Contingence (Nu) 67.56 Velocity of Sound-Shear Wave 3.75x103 m/s Velocity of Sound/Compression Wave 5.90x103 m/s Sonic Attenuation Less than 11 db/m MHz Permeability Constants (cm3 mm/cm2 sec cm of Hg) (700oC) Helium 210x10 -10 Hydrogen 21x10 -10 Deuterium 17x10 -10 Neon 9.5x10 -17

Electrical Properties

Since electrical conductivity in fused quartz is ionic in nature, and alkali ions exist only as trace constituents, fused quartz is the preferred glass for electrical insulation and low loss dielectric properties. In general, the electrical insulating properties of clear fused quartz are superior to those of the opaque or translucent types. Both electrical insulation and microwave transmission properties are retained at very high temperatures and over a wide range of frequencies.

Mechanical Properties

Mechanical properties of fused quartz are much the same as those of other glasses. The material is extremely strong in compression, with design compressive strength of better than 1.1 x 10 9 Pa (160,000 psi). Surface flaws can drastically reduce the inherent strength of any glass, so tensile properties are greatly influenced by these defects. The design tensile strength for fused quartz with good surface quality is in excess of 4.8 x 10 7 Pa (7,000 psi). In practice, a design stress of .68 x 10 7 Pa (1,000 psi) is generally recommended.

Thermal Properties

One of the most important properties of fused quartz is its extremely low coefficient of expansion: 5.5 x 10 -7 mm ºC (20-320ºC). Its coefficient is 1/34 that of copper and only 1/7 of borosilicate glass. This makes the material particularly useful for optical flats, mirrors, furnace windows and critical optical applications which require minimum sensitivity to thermal changes. A related property is its unusually high thermal shock resistance. For example, thin sections can be heated rapidly to above 1500 ºC and then plunged into water without cracking. The residual stress or design, depending on the application, may be in the range of 1.7 x 10 7 to 20.4 x 10 7 Pa (25 to 300 psi). As a general rule, it is possible to cool up to 100ºC /hour for sections less than 25 mm thick.

Effects Of Temperature

Fused quartz is a solid material at room temperature, but at high temperatures, it behaves like all glasses. It does not experience a distinct melting point as crystalline materials do, but softens over a fairly broad temperature range. This transition from a solid to a plastic-like behavior, called the transformation range, is distinguished by a continuous change in viscosity with temperature.

Viscosity

Viscosity is the measure of the resistance to flow of a material when exposed to a shear stress. Since the range in "flowability" is extremely wide, the viscosity scale is generally expressed logarithmically. Common glass terms for expressing viscosity include: strain point, annealing point, and softening point, which are defined as: Strain Point: The temperature at which the internal stress is substantially relieved in four hours. This corresponds to a viscosity of 10 14.5 poise, where poise = dynes/cm2 sec. Annealing Point: The temperature at which the internal stress is substantially relieved in 15 minutes, a viscosity of 10 13.2 poise. Softening Point: The temperature at which glass will deform under its own weight, a viscosity of approximately 10 7.6 poise. The softening point of fused quartz has been variously reported from 1500 ºC to 1670ºC, the range resulting from differing conditions of measurement.

Cristobalite Growth

The growth rate of cristobalite from the nucleation site depends on certain environmental factors and material characteristics. Temperature and quartz viscosity are the most significant factors, but oxygen and water vapor partial pressures also impact the crystal growth rate. Consequently, the rate of devitrification of fused quartz increases with increasing hydroxyl (OH-) content, decreasing viscosity and increasing temperature. High viscosity, low hydroxyl fused quartz materials produced , therefore, provide an advantage in devitrification resistance. The phase transformation to Beta-cristobalite generally does not occur below 1000ºC. This transformation can be detrimental to the structural integrity of fused quartz if it is thermally cycled through the crystallographic inversion temperature range (250 ºC). This inversion is accompanied by a large change in density and can result in spalling and possible mechanical failure.Thermal Properties, cont

Optical Properties

Optical transmission properties provide a means for distinguishing among various types of vitreous silica as the degree of transparency reflects material purity and the method of manufacture. Specific indicators are the UV cutoff and the presence or absence of bands at 245 nm and 2.73 um. The UV cutoff ranges from ~155 to 175 nm for a 10 mm thick specimen and for pure fused quartz is a reflection of material purity. The presence of transition metallic impurities will shift the cutoff toward longer wavelengths. When desired, intentional doping, e.g., with Ti in the case of Type 219, may be employed to increase absorption in the UV. The absorption band at 245 nm characterizes a reduced glass and typifies material made by electric fusion. If a vitreous silica is formed by a "wet" process, either flame fusion or synthetic material, for example, the fundamental vibrational band of incorporated structural hydroxyl

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