| Section2 = | Section3 = }} The chemical compound silicon dioxide, also known as silica (from the Latin silex), is an oxide of silicon with the chemical formula . It has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms.

Silica is manufactured in several forms including fused quartz, crystal, fumed silica (or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica, silica gel, and aerogel.

Silica is used primarily in the production of glass for windows, drinking glasses, beverage bottles, and many other uses. The majority of optical fibers for telecommunications are also made from silica. It is a primary raw material for many whiteware ceramics such as earthenware, stoneware, porcelain, as well as industrial Portland cement.

Silica is a common additive in the production of foods, where it is used primarily as a flow agent in powdered foods, or to absorb water in hygroscopic applications. It is the primary component of diatomaceous earth which has many uses ranging from filtration to insect control. It is also the primary component of rice husk ash which is used, for example, in filtration and cement manufacturing.

Thin films of silica grown on silicon wafers via thermal oxidation methods can be quite beneficial in microelectronics, where they act as electric insulators with high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.

A silica-based aerogel was used in the Stardust spacecraft to collect extraterrestrial particles. Silica is also 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. In hydrated form, it is used in toothpaste as a hard abrasive to remove tooth plaque.

In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric. In cosmetics, it is useful for its light-diffusing properties and natural absorbency. Colloidal silica is used as a wine and juice fining agent. In pharmaceutical products, silica aids powder flow when tablets are formed. Finally, it is used as a thermal enhancement compound in ground source heat pump industry.

Crystal structure

In the vast majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO₂. In each of the most thermodynamically stable crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the SiO₄ tetrahedra are shared with others, yielding the net chemical formula: SiO₂.

For example, in the unit cell of α-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered terahedra share just one of their O atoms with other SiO₄ tetrahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the 7 SiO₄ tetrahedra which are considered to be a part of the unit cell for silica (see 3-D Unit Cell).

SiO₂ has a number of distinct crystalline forms (polymorphs) 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. 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°. The change in the coordination increases the ionicity of the Si-O bond. But more important is the observation that any deviations from these standard parameters constitute microstructural differences or variations which represent an approach to an amorphous, vitreous or glassy solid.

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. In nature impurities in crystalline α-quartz can give rise to colors (see list).

Note also that both high temperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz. Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high temperature minerals. As is common with many substances, the higher the temperature the farther apart the atoms due to the increased vibration energy.

The high pressure minerals, seifertite, stishovite, and coesite, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur during their formation, resulting in a more condensed structure.

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/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order (or crystallinity) even after boiling in concentrated hydrochloric acid.

Molten silica exhibits several peculiar physical characteristics that are similar to the ones observed in liquid water: negative temperature expansion, density maximum, and a heat capacity minimum. 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. Dimeric silicon dioxide, (SiO₂)₂ has been prepared by reacting O₂ with matrix isolated dimeric silicon monoxide, (Si₂O₂). 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 150.2 pm. 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.

{|class="wikitable" |+ Crystalline forms of SiO₂ |Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K | |- |β-quartz |hexagonalhP18, P6₂22, No.180 |closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K | |- |α-tridymite |orthorhombicoS24, C222₁, No.20 |metastable form under normal pressure | |- |β-tridymite |hexagonalhP12, P6₃/mmc, No. 194 |closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K | |- |α-cristobalite |tetragonaltP12, P4₁2₁2, No. 92 |metastable form under normal pressure | |- |β-cristobalite |cubiccF104, Fd3m, No.227 |closely related to α-cristobalite; melts at 1978 K | |- |faujasite |cubiccF576, Fd3m, No.227 |sodalite cages connected by hexagonal prisms; 12-membered ring pore opening; faujasite structure. | |- |melanophlogite |cubic (cP*, P4₂32, No.208) or tetragonal (P4₂/nbc) |Si₅O₁₀, Si₆O₁₂ rings; mineral always found with hydrocarbons in interstitial spaces-a clathrasil | |- |keatite |tetragonaltP36, P4₁2₁2, No. 92 |Si₅O₁₀, Si₄O₁₄, Si₈O₁₆ rings; synthesised from glassy silica and alkali at 600–900K and 40–400 MPa | |- |moganite |monoclinicmS46, C2/c, No.15 |Si₄O₈ and Si₆O₁₂ rings | |- |coesite |monoclinicmS48, C2/c, No.15 |Si₄O₈ and Si₈O₁₆ rings; 900 K and 3–3.5 GPa | |- |stishovite |TetragonaltP6, P4₂/mnm, No.136 |One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa | |- |fibrous |orthorhombicoI12, Ibam, No.72 |like SiS₂ consisting of edge sharing chains | |- |seifertite |orthorhombicoP, Pbcn |One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. | |}

Quartz glass

When silicon dioxide SiO₂ is cooled rapidly enough, it does not crystallize but solidifies as a glass. The glass transition temperature of pure SiO₂ is about 1600 K (1330 °C or 2420 °F). Like most of the crystalline polymorphs the local atomic structure in pure silica glass is regular tetrahedra of oxygen atoms around silicon atoms. The difference between the glass and the crystals arises in the connectivity of these tetrahedral units. SiO₂ glass consists of a non-repeating network of tetrahedra, where all the oxygen corners connect two neighbouring tetrahedra. Although there is no long range periodicity in the glassy network there remains significant ordering at length scales well beyond the SiO bond length. One example of this ordering is found in the preference of the network to form rings of 6-tetrahedra.

Chemistry

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very shallow 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 between 600 and 1200 °C, using so-called dry or wet oxidation with O₂ or H₂O, respectively. The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced.

Alternative methods used to deposit a layer of SiO₂ include

Low temperature oxidation (400–450 °C) of silane

::SiH₄ + 2 O₂ → SiO₂ + 2 H₂O.

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

::Si(OC₂H₅)₄ → SiO₂ + 2 H₂O + 4 C₂H₄.

Plasma enhanced chemical vapor deposition using TEOS at about 400 °C

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

Polymerization of tetraethyl orthosilicate (TEOS) at below 100 °C using amino acid as catalyst.

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: :SiCl₄ + 2 H₂ + O₂ → SiO₂ + 4 HCl.

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.

Quartz exhibits a maximum solubility in water at temperatures about 340 °C. 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. These crystals are a source of very pure quartz for use in electronic applications.

Fluorine reacts with silicon dioxide to form SiF₄ and O₂ whereas the other halogen gases (Cl₂, Br₂, I₂) react much less readily.

Silicon dioxide is attacked by hydrofluoric acid (HF) to produce hexafluorosilicic acid:

Health effects

Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1 mg/m³) over time can lead to silicosis, bronchitis, or cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities. (In the body crystalline silica particles do not dissolve over clinically relevant periods of time.) This effect can create 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 less time. Amorphous silica, such as fumed silica is not associated with development of silicosis, but may cause irreversible lung damage in some cases. Laws restricting silica exposure with respect to the silicosis hazard specify that they are concerned only with silica that is both crystalline and dust-forming.

Note however, that plant materials with high silica phytolith content, appear to be of importance to grazing animals, from chewing insects to ungulates. It is known that it accelerates tooth wear at least, and has been doing so for hundreds of millions of years.

A 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 with an increase of 10 milligram-per-day of the intake of silica in drinking water, the risk of dementia dropped by 11%.

See also

Fused silica

Mesoporous silica

Thermal oxidation

Silicon carbide

References

External links

Tridymite, Quartz, Cristobalite,

amorphous, NIOSH Pocket Guide to Chemical Hazards

crystalline, as respirable dust, NIOSH Pocket Guide to Chemical Hazards

Formation of silicon oxide layers in the semiconductor industry. LPCVD and PECVD method in comparison. Stress prevention.

Quartz SiO₂ piezoelectric properties

Silica (SiO₂) and Water

Category:Silicon dioxide Category:Ceramic materials Category:Refractory materials Category:IARC Group 1 carcinogens Category:Common oxide glass components Category:Excipients

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