TERMINOLOGY

Silica (SiO2) is the chief constituent of earth’s crust. It is present in various forms, the most common being called Quartz which is crystalline in character. Typical examples are silica sand and rock crystals.
Vitreous Silica is the glassy modification of Silicon di-oxide (SiO2) with a purity of atleast 99.8% by weight in the translucent form and over 99.9% (SiO2) in the transparent forms.

Depending on the Quartz used as raw material and melting process employed, the resulting material is either opaque, translucent or transparent.

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In all three cases this material is “Fused Silica”. It is also sometimes referred to as vitreous silica, quartz glass, silica glass, fused quartz or fused silica. Commercially the word fused silica is designated to the translucent variety and fused quartz to the transparent variety.

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CHEMICAL PROPERTIES

Fused Silica/Quartz is completely unaffected by all halogens and acids regardless of temperature or concentration with the exception of hydro-fluoric and phosphoric acid. The latter affects fused silica only at high temperatures and it is possible, therefore to concentrate phosphoric acid successfully in silica vessels. For all ordinary purposes they can be used with phosphoric acid.

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Sulphuric, Nitric and Hydrochloric acids or any mixture of these acids, such as acquaregia, have no action on Infusil even at temperatures upto 1000ºC.

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Strong alkalies react with Infusil. In many instances even where a certain degree of chemical action occurs, good service may be obtained by cleaning Infusil ware regularly. Salts and basic oxides react at elevated temperatures, particularly above their melting point. The extent of reaction is largely determined by the thermal diffusion of impurities into the fused silica (Vitreous silica) and in this respect the alkali metals have the highest diffusivity and are the most damaging.

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THERMAL EXPANSION

The mean value of the coefficient of thermal expansion between 0 and 800ºC is 5.5 X 10-7/ºC.
Fused silica is not sensitive to abrupt temperature changes. Red hot fused silica may be dipped in water without damage.
VISCOSITY
Vitreous silica is a highly viscous material. The viscosity drops uniformly as the temperature rises over softening point. At 1730ºC, fused silica is just as “hard” as pitch at 20ºC. At 1900º C, the material is solid enough to receive force to shape it. Vitreous silica never becomes a liquid melt since it passes directly from solid to vapour state.

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DEVITRIFICATION

The transformation of glassy form into crystalline phase of SiO2 is termed as Devitrification. This is a nucleation process and the surface condition is very important.

Fused Silica/quartz is a metastable phase of silica and no crystallisation occurs at normal temperature. Prolonged heating above 1000ºC leads to crystallisation and in the crystalline phase the formation of high crystobalites occur. The thermal expansion and specific volume of these high crystobalites will be the same as Fused Silica and no damage to material is observed at high temperature.

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At 800º C fine cracks appear due to the mismatch of the thermal expansion and at around 200-275º complete structural changes occur and the high crystobalites are converted to low Crystobalites, hence complete breakage of the material takes place. Alkali ions increase devitrification.

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OPTICAL PROPERTIES

Transmission between 200 nm to 700 nm for 2mm path length 85 to 95%. Refractive index at 5875A.....1.458.