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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, these terms were used to distinguish between transparent and opaque grades of the material. Fused quartz products were those produced from quartz crystal to make transparent ware, and fused silica described products manufactured from sand into opaque ware.
Today, however, advances in raw material beneficiation 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. When silicon dioxide is synthetically derived, however, the material is referred to as synthetic fused silica.
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Controlled Process
The performance of most fused quartz products is closely related to the purity of the material. Momentive 's proprietary raw material beneficiation and fusion processes are closely monitored and controlled to yield typically less than 50 ppm total elemental impurities by weight. Momentive clear fused quartz varieties have a nominal purity of 99.995 W% SiO2. The Table below summarizes the typical trace level impurity content of Momentive fused quartz products.
Structural hydroxyl (OH)- impurities are also shown. The strong IR absorption of (OH)- species in fused quartz provides a quantitative method for analysis.
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Beta Factor
The term Beta Factor is used to characterize the hydroxyl (OH)- content of fused quartz products. This term is defined by the formula shown below.
Beta Factor = β
β = (1/t)log10 (Ta/Tb) mm-1
Where:
t = Quartz thickness, mm
Ta = actual % transmission at λ
= 2.6 μm
Tb = actual % transmission at λ
= 2.73 μm
[OH-] = C, ppm
C = β x 910
910 = (M.W.(OH)- × 104)/(E x ρ(SiO2))
M.W.(OH)-
= Molecular Weight of (OH)-
g = 17 g
E
= Extinction coefficient for (OH)-
= 85 liters/mol cm
ρ
= Density of SiO2 = 2.21 g/cm3
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