Properties of Fused Silica

A number of unique optical, mechanical and thermal properties of fused silica have made quartz glass an indispensable material in the fabrication of high-tech products.

Among these are:

  • high chemical purity and resistance,
  • high softening temperature and thermal resistance,
  • low thermal expansion with high resistance to thermal shocks,
  • high transparency from the ultraviolet to the infrared spectral range,
  • high radiation resistance.

About SiO2 and Other Glass Types

Silicon Dioxide – Glass – Quartz – Fused Silica
Silicon Dioxide (SiO2) is the most simple chemical composition of glass. Quartz is the most stable crystal modification at normal temperature and pressure conditions. Quartz is one of the most common minerals in the earths crust. Glass (from “glasa”, Germanic for amber, the shiny or shimmery) also consists of silicon and oxide, but is a uniform amorphous solid material. Many glass varieties are clear and transparent respectively. This means transmissibility for the visible spectrum of light. In general such glasses are associated with glass. Transparent materials allow light to pass through them without diffusing (scattering) the light.

Most common types of glass
At least 2000 years ago humans learned how to lower the glass softening temperatures by adding lime an soda before heating, which resulted in a glass containing sodium and calcium oxide.

Glass – Additives and the industrial use of glass
The use of glass as one of the oldest, but also very important materials for the industry is linked with the application of additives. Chemical like soda (Sodium carbonate, Na2CO3) and in the past also potash (potassium carbonate, K2CO3), manganese oxide and metal oxides influence the properties of glass. Manufactured glass is a material formed when a mixture of sand, soda and lime is heated to a high temperature and stays in a molten, liquid state. Glass can be made from pure silica, but quartz glass (also referred as quartz) has a high glass transition point at around 1200°C, which makes it difficult to form into panes or bottles.

Quartz glass is the purest form of SiO2 and therefore the most valuable and sophisticated variety. Extremely clear glass can be used for optical fibers. Therefore synthetic quartz glass is used to transmit light across many kilometers. Lots of glasses are impermeable to ultraviolet radiation, but only pure fused silica (only SiO2) is transparent for wavelengths < 350 nm (UV). Quartz glass also exists as an opaque variety and with different coloration to change the physical and chemical properties like transmission or absorption for specific wavelength (filter glass). The opaque material at Heraeus, OM® 100 , is also used as a heat barrier or for diffuse scattering of IR radiation.

Raw Materials

At first glance, quartz glass appears very simple, both chemically and structurally, since it is made from a single oxide component (silicon dioxide – SiO2).

Chemical structure:
Silica, as it is also known, is found throughout the earth’s crust. However, only a small fraction has sufficient purity (> 99.98 %) to be suitable as raw material for quartz glass. Sand at the beach is also SiO2, but isn’t suitable for use in the semiconductor industry.

Structure of quartz and fused silica
In the quartz glass structure all atoms are bonded t at least two others. Together with the strength of the siliconoxygen (Si-O) chemical bond gives quartz glass high temperature stability and chemical resistance. But the structure is also rather open with wide spaces (interstices) between the structural units. This accounts for higher gas permeability and much lower thermal expansion coefficient of quartz glass relative to other materials.