Purity is crucial for most industrial applications and processes. Fused silica has an outstanding high purity and therefore is an indispensable material in the fabrication of high-tech products.
Despite existing at very low levels, contaminants have subtle yet significant effects. Purity is mostly determined by the raw material, the manufacturing method and subsequent handling procedures. Special precautions must be taken at all stages of manufacture to maintain high purity. Additionally, Heraeus has different purification steps to improve the quality of the quartz sand as raw material even further.
The most common impurities are metals (such as Al, Na and Fe among others), water (present as OH groups) and chlorine. These contaminants not only affect the viscosity, optical absorption and electrical properties of the quartz glass. They can also influence the properties of material processed in contact with the quartz glass during the final use application.
The purities of fused quartz and fused silica are outstandingly high. Synthetic fused silica from Heraeus contains total metallic contamination below 1ppm. For fused quartz the amount is approximately 20 ppm and consists primarily of Al2O3 with much smaller amounts of alkalis, Fe2O3, TiO2, MgO and ZrO2.
In addition to metallic impurities, fused quartz and fused silica also contain water present as OH units. OH content influences the physical properties like attenuation and viscosity. General, high OH contents means lower use temperature. Typical values are given in the table. Electrically fused quartz has the lowest hydroxyl content (< 1 – 30 ppm) since it is normally made in vacuum or a dry atmosphere. Hydroxyl content in this range is not fixed in the glass structure. It can go up or down depending on the thermal treatment and amount of moisture to which the quartz glass is exposed at elevated temperature. Flame fused quartz has significantly more hydroxyl (150 – 200 ppm) since fusion occurs in a hydrogen/ oxygen flame. Due to the production method synthetic fused silica has similar high OH contents of up to 1000 ppm.