Silicon dioxide

The chemical compound silicon dioxide, also known as silica or silox (from the Latin "silex"), is the oxide of silicon, chemical formula SiO₂, and has been known for its hardness since the 9th century^[1]. Silica is most commonly found in nature as sand or quartz. It is a principal component of most types of glass and substances such as concrete.

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Manufactured forms

  Silica is manufactured in several forms including:

• glass (a colorless, high-purity form is called fused silica)
• synthetic amorphous silica
• silica gel (used e.g. as desiccants in new clothes and leather goods)

It is used in the production of various products.

• Inexpensive soda-lime glass is the most common and typically found in drinking glasses, bottles, and windows.
• A raw material for many whiteware ceramics such as earthenware, stoneware and porcelain.
• A raw material for the production of Portland cement.
• A food additive, primarily as a flow agent in powdered foods, or to absorb water (see the ingredients list for).
• The natural ("native") oxide coating that grows on silicon is hugely beneficial in microelectronics. It is a superior electric insulator, possessing high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to allow small currents to flow through a device. At room temperature, however, it grows extremely slowly, and so to manufacture such oxide layers on silicon, the traditional method has been the deliberate heating of silicon in high temperature furnaces within an oxygen ambient (thermal oxidation).
• Raw material for aerogel in the Stardust spacecraft
• Used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
• Added to medicinal anti-foaming agent, like Simethicone, in a small proportion to enhance defoaming activity.
• As hydrated silica in Toothpaste (abrasive to fight away plaque.)

Health effects

  Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1mg/m³) over time can lead to silicosis, bronchitis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered silica, and so on. But children, asthmatics of any age, allergy sufferers and the elderly, all of whom have reduced lung capacity, can be affected in much shorter periods of time.

In all other respects, silicon dioxide is inert and harmless. When silica is ingested orally, it passes unchanged through the gastrointestinal tract, exiting in the feces, leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining. Silicon dioxide produces no fumes and is insoluble in vivo. It is indigestible, with zero nutritional value and zero toxicity.

Chemistry

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nm or 10 Å) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternate environments are used to grow well-controlled layers of silicon dioxide on silicon.

Silicon dioxide has covalent bonding and forms a network structure (also known as lattice or continuous).

Silicon dioxide is attacked by hydrofluoric acid (HF). HF is used to remove or pattern silicon dioxide in the semiconductor industry.

Notes

References

• R. K. Iler, The Chemistry of Silica (ISBN 0-471-02404-X)

See also

• Amorphous carbonia
• Fused silica
• Silicon carbide
• Mesoporous silica