The chemical compound silicon dioxide, also known as silica (from the Latin silex), is an oxide of silicon with a chemical formula of SiO₂ and has been known for its hardness since antiquity.^[1] Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms. It is a principal component of most types of glass and substances such as concrete. Silica is the most abundant mineral in the earth's crust.

Structure and properties

SiO₂ has a number of distinct crystalline forms in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO₄ units linked together by shared vertices in different arrangements. Silicon-oxygen bond lengths vary between the different crystal forms, for example in α-quartz the bond length is 161 pm, whereas in α-tridymite it is in the range 154-171 pm.^[2] The Si-O-Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz the Si-O-Si angle is 144°.
Fibrous sulfur has a structure similar to that of SiS₂ with chains of edge-sharing SiO₄ tetrahedra.
Stishovite, the highest pressure form, in contrast has a rutile like structure where silicon is 6 coordinate. The density of stishovite is 4.287 g/cm³, which compares to α-quartz, the densest of the low pressure forms, which has a density of 2.648 g/cm³.^[3] The difference in density can be ascribed to the increase in coordination as the six shortest Si-O bond lengths in stishovite (four Si-O bond lengths of 176 pm and two others of 181 pm) are greater than the Si-O bond length (161 pm) in α-quartz.^[4] The change in the coordination increases the ionicity of the Si-O bond.^[5]
Note that the only stable form under normal conditions is α-quartz and this is the form in which crystalline silicon dioxide is usually encountered.^[3] In nature impurities in crystalline α-quartz can give rise to colours^[3] (see quartz for a list).

Faujasite-silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and high surface area (over 800 m²/gr). Faujasite-silica has very high thermal and acid stability (it maintains high crystallinity even after boiling in concentrated hydrochloric acid).^[6]

Molecular forms of silicon dioxide

When molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO₂ is produced which has a linear structure.^[8] The Si-O bond length is 148.3 pm which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.^[8]
Dimeric silicon dioxide, (SiO₂)₂ has been prepared by reacting O₂ with matrix isolated dimeric silicon monoxide, (Si₂O₂).^[8] In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94° and bond length of 164.6 pm and the terminal Si-O bond length is 148.2 pm.^[8]

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 alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures of 600 -1200 °C so-called "dry" or "wet" oxidation using O₂ or H₂O respectively.^[9] The thickness of the layer of silicon replaced by the dioxide is 44% of the thickness of the silicon dioxide layer produced.^[9]
Alternative methods used to deposit a layer of SiO₂ include:^[10]

• Low temperature oxidation (LTO) of silane

SiH₄ + 2O₂ → SiO₂ + 2H₂O (at 400-450 °C)

• Decomposition of tetraethyl orthosilicate (TEOS) at 680 – 730°C

Si(OC₂H₅)₄ → SiO₂ + H₂O + 2C₂H₄

• Plasma enhanced chemical vapour deposition using TEOS at approximately 400°C

Si(OC₂H₅)₄ + 12O₂ → SiO₂ + 10H₂O + 8CO₂

• Polymerization of tetraethyl orthosilicate (TEOS) at less than 100°C using amino acid as catalyst.^[11]

Pyrogenic silica (sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning SiCl₄ in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO₂:^[3]

SiCl₄ + 2H₂ + O₂ → SiO₂ + 4HCl

Amorphous silica, silica gel, is produced by the acidification of solutions of sodium silicate to produce a gelatinous precipitate that is then washed and then dehydrated to produce colorless microporous silica.^[3]

Quartz exhibits a maximum solubility in water at around 340 °C.^[12] This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel which is cooler at the top. Crystals of 0.5–1 kg can be grown over a period of 1–2 months.^[2] These crystals are a source of very pure quartz for use in electronic applications.^[3]
Fluorine reacts with silicon dioxide to form SiF₄ and O₂ whereas the other halogen gases (Cl₂, Br₂, I₂) react much less readily.^[3]

Silicon dioxide is attacked by hydrofluoric acid (HF) to produce "hexafluorosilicic acid":^[2]

SiO₂ + 6HF → H₂SiF₆ + 2H₂O

HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide (e.g):^[3]

SiO₂ + NaOH → Na₂SiO₃ + H₂O

Silicon dioxide reacts with basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide or mixtures of oxides forming silicates and glasses as the Si-O-Si bonds in silica are broken successively).^[2] As an example the reaction of sodium oxide and SiO₂ can produce sodium orthosilicate, sodium silicate and glasses, depending on the proportions of reactants:^[3]

2Na₂O + SiO₂ → Na₄SiO₄

Na₂O + SiO₂ → Na₂SiO₃

(0.25 - 0.8)Na₂O + SiO₂ → glasses

Examples of such glasses have commercial significance e.g. soda lime glass,borosilicate glass, lead glass. In these glasses silica is termed the network former or lattice former.^[2]
With silicon at high temperatures gaseous SiO is produced:^[2]

SiO₂ + Si → 2SiO (gas)

Manufactured forms

Silica is manufactured in several forms including:

• Glass (a colorless, high-purity form is called fused silica)
• Synthetic amorphous silica, silica gel
• Fumed silica (also known as pyrogenic silica, colloidal silica, or under the genericized trademark, Cab-o-Sil)
• Precipitated silica is produced by precipitation from a waterglass solution by acidification.
• Silica aerogel.

Applications

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, with 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, the traditional method has been 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.
• As hydrophobic silica it is used as a defoamer component.
• As hydrated silica in toothpaste (abrasive to remove plaque.)
• As a high-temperature thermal protection fabric.
• In cosmetics for its light-diffusing properties and its absorbency.
• Liquid silicon dioxide (colloidal silica) is used as a wine and juice fining agent.
• As a glidant in pharmaceutical products silicon dioxide aids powder flow when tablets are formed.
• In the production of tires
• Thermal enhancement compound used in thermal grouts for the ground source heat pump industry.

Inhalation health effects

Inhaling finely divided crystalline silica dust or fumed silica in very small quantities (OSHA allows 0.1 mg/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 crystalline silica and so on. 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.

Other health effects

In respects other than inhalation, pure silicon dioxide is inert and harmless.

Pure silicon dioxide produces no fumes and is insoluble in vivo. It is indigestible, with zero nutritional value and zero toxicity. 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.

Because some silicas take on water, extended exposure may cause local drying of the skin or other tissue.

According to an article in the American Journal of Epidemiology on February 15, 2009, a French study which followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. The study found that for every 10 miligram-per-day intake of silica in drinking water, the risk of dementia dropped by 11%.

Taken from Wikimedia.org.