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Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. (1979). 9780471024040, Wiley. ISBN 9780471024040 In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant families of , existing as a compound of several and as a synthetic product. Examples include fused quartz, fumed silica, opal, and . It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries. All forms are white or colorless, although impure samples can be colored.
Silicon dioxide is a common fundamental constituent of glass.
Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (Amorphous solid). In crystalline form, this substance can be found naturally occurring as quartz, tridymite (high-temperature form), cristobalite (high-temperature form), stishovite (high-pressure form), and coesite (high-pressure form). On the other hand, amorphous silica can be found in nature as opal and diatomaceous earth. Quartz glass is a form of intermediate state between these structures.
All of these distinct crystalline forms always have the same local structure around Si and O. In α-quartz the Si–O 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°.
Faujasite silica, another polymorph, is obtained by the of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, and has high crystallinity and specific 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.
is most commonly encountered in nature as [[quartz]], which comprises more than 10% by mass of the Earth's crust. Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure forms [[coesite]] and [[stishovite]] have been found around [[impact structure]]s and associated with [[eclogite]]s formed during ultra-high-pressure metamorphism. The high-temperature forms of [[tridymite]] and [[cristobalite]] are known from silica-rich [[volcanic rock]]s. In many parts of the world, silica is the major constituent of [[sand]].(2025). 9781405160544, Wiley & Sons. ISBN 9781405160544
Silica is also the primary component of rice husk ash, which is used, for example, in filtration and as supplementary cementitious material (SCM) in cement and concrete manufacturing.
Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates).
Prominent examples include:
Certain deposits of silica sand, with desirable particle size and shape and desirable clay and other mineral content, were important for sand casting of metallic products. The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons.
Crystalline silica is used in hydraulic fracturing of formations which contain tight oil and shale gas.
The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. The difference between the glass and crystalline forms arises from the connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. One example of this ordering is the preference to form rings of 6-tetrahedra.
The majority of for telecommunications are also made from silica. It is a primary raw material for many ceramics such as earthenware, stoneware, and porcelain.
Silicon dioxide is used to produce elemental silicon. The process involves carbothermic reduction in an electric arc furnace: (2025). 9780199236176, Oxford University Press. ISBN 9780199236176
Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production. It consists of amorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without the branching of the pyrogenic product. The main use is as pozzolanic material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.
In cosmetics, silica is useful for its light-diffusing properties and natural absorbency.
Diatomaceous earth, a mined product, has been used in food and cosmetics for centuries. It consists of the silica shells of microscopic diatoms; in a less processed form it was sold as tooth powder. Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.
Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors like gallium arsenide or indium phosphide.
Silicon dioxide could be grown on a silicon semiconductor surface. Silicon oxide layers could protect silicon surfaces during diffusion processes, and could be used for diffusion masking.
Surface passivation is the process by which a semiconductor surface is rendered inert, and does not change semiconductor properties as a result of interaction with air or other materials in contact with the surface or edge of the crystal. The formation of a thermally grown silicon dioxide layer greatly reduces the concentration of surface states. SiO2 Thin film preserve the electrical characteristics of p–n junctions and prevent these electrical characteristics from deteriorating by the gaseous ambient environment. Silicon oxide layers could be used to electrically stabilize silicon surfaces. The surface passivation process is an important method of semiconductor device fabrication that involves coating a silicon wafer with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below. Growing a layer of silicon dioxide on top of a silicon wafer enables it to overcome the surface states that otherwise prevent electricity from reaching the semiconducting layer.
The process of silicon surface passivation by thermal oxidation (silicon dioxide) is critical to the semiconductor industry. It is commonly used to manufacture metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon integrated circuit chips (with the planar process).
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric.
Silica is used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropic agent. (2025). 9780470010259, Wiley & Sons. ISBN 9780470010259
Silica aerogel was used in the Stardust spacecraft to collect extraterrestrial particles.
Pure silica (silicon dioxide), when cooled as fused quartz into a glass with no true melting point, can be used as a glass fibre for fibreglass.
Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.
or wet oxidation with H2O.
The native oxide layer is beneficial in microelectronics, where it acts as electric insulator with high chemical stability. It can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.
Similarly TEOS combusts around 400 °C:
TEOS undergoes hydrolysis via the so-called sol-gel process. The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:
However the chemical vapor deposition of silicon dioxide onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500 °C.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are unreactive.
Most forms of silicon dioxide are attacked ("etched") by hydrofluoric acid (HF) to produce hexafluorosilicic acid:
Silicon dioxide acts as a Lux–Flood acid, being able to react with bases under certain conditions. As it does not contain any hydrogen, non-hydrated silica cannot directly act as a Brønsted–Lowry acid. While silicon dioxide is only poorly soluble in water at low or neutral pH (typically, 2 × 10−4 M for quartz up to 10−3 M for cryptocrystalline chalcedony), strong bases react with glass and easily dissolve it. Therefore, strong bases have to be stored in plastic bottles to avoid jamming the bottle cap, to preserve the integrity of the recipient, and to avoid undesirable contamination by silicate anions.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:
Silicon dioxide will neutralise basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide, or mixtures of oxides, forming and glasses as the Si-O-Si bonds in silica are broken successively). As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:
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. The reaction is also used in to remove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag.
Silicon dioxide reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce highly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds. The silicates are essentially insoluble in all Solvent except methanol.
Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:
Inhaling finely divided crystalline silica dust can lead to silicosis, bronchitis, or lung cancer, as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities. When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus and rheumatoid arthritis compared to expected rates in the general population.
Crystalline silica is an occupational hazard for those working with stone because the process of cutting and installing the countertops creates large amounts of airborne silica. Crystalline silica used in hydraulic fracturing presents a health hazard to workers.
In 2013, the U.S. Occupational Safety and Health Administration reduced the exposure limit to 50 micrograms/m3 of air. Prior to 2013, it had allowed 100 μg/m3 and in construction workers even 250 μg/m3.
In 2013, OSHA also required the "green completion" of fracked wells to reduce exposure to crystalline silica and restrict the exposure limit.
On microchips
Thin films of silica grow spontaneously on via thermal oxidation, producing a very shallow layer of about 1 nanometre or 10 angstrom of so-called native oxide.
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 oxidation with oxygen
(1991). 9780471924784, John Wiley & Sons. ISBN 9780471924784
Laboratory or special methods
From organosilicon compounds
Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO, TEOS. Synthesis of silica is illustrated below using tetraethyl orthosilicate (TEOS). Simply heating TEOS at 680–730 °C results in the oxide:
Other methods
Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion of silane gives silicon dioxide. This reaction is analogous to the combustion of methane:
(1991). 9780471924784, John Wiley & Sons. ISBN 9780471924784
Chemical reactions
Silicon dioxide is a relatively inert material (hence its widespread occurrence as a mineral). Silica is often used as inert containers for chemical reactions. At high temperatures, it is converted to silicon by reduction with carbon.
Stishovite does not react to HF to any significant degree.
HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Water solubility
The solubility of silicon dioxide in water strongly depends on its crystalline form and is three to four times higher for amorphous silica than quartz; as a function of temperature, it peaks around . 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 that is cooler at the top. Crystals of 0.5–1 kg can be grown for 1–2 months. These crystals are a source of very pure quartz for use in electronic applications. Above the critical temperature of water and a pressure of or higher, water is a supercritical fluid and solubility is once again higher than at lower temperatures.
Health effects
Silica ingested orally is essentially nontoxic, with an of 5000 mg/kg (5 g/kg). A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. An increase of 10 mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.
Occupational hazard
Silica is an occupational hazard for people who do sandblasting or work with powdered crystalline silica products. Amorphous silica, such as fumed silica, may cause irreversible lung damage in some cases but is not associated with the development of silicosis. Children, asthmatics of any age, those with Allergy, and the elderly (all of whom have reduced Lung volumes) can be affected in less time.
Pathophysiology
In the body, crystalline silica particles do not dissolve over clinically relevant periods. Silica crystals inside the lungs can activate the NLRP3 inflammasome inside macrophages and dendritic cells and thereby result in production of interleukin, a highly pro-inflammatory cytokine in the immune system.NIOSH (2002) Hazard Review, Health Effects of Occupational Exposure to Respirable Crystalline Silica. Cincinnati, OH: U.S. Department of Health and Human Services, U.S. Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2002-129.
Regulation
Regulations restricting silica exposure 'with respect to the silicosis hazard' specify that they are concerned only with silica, which is both crystalline and dust-forming.
Crystalline forms
SiO2, more so than almost any material, exists in many crystalline forms. These forms are called polymorphs.
+ Crystalline forms of SiO2
! Form
! Crystal symmetry
Pearson symbol, group no.
! ρ
(g/cm3)
! width=350Notes
! Structure α-quartz rhombohedral (trigonal)
hP9, P3121 No.1522.648 Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K β-quartz hexagonal
hP18, P6222, No. 1802.533 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 orthorhombic
oS24, C2221, No.202.265 Metastable form under normal pressure β-tridymite hexagonal
hP12, P63/mmc, No. 194 Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K α-cristobalite tetragonal
tP12, P41212, No. 922.334 Metastable form under normal pressure β-cristobalite cubic
cF104, Fd3m, No.227 Closely related to α-cristobalite; melts at 1978 K keatite tetragonal
tP36, P41212, No. 923.011 Si5O10, Si4O8, Si8O16 rings; synthesised from glassy silica and alkali at 600–900 K and 40–400 MPa moganite monoclinic
mS46, C2/c, No.15 Si4O8 and Si6O12 rings coesite monoclinic
mS48, C2/c, No.152.911 Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa stishovite tetragonal
tP6, P42/mnm, No.1364.287 One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa seifertite orthorhombic
oP, Pbcn4.294 One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. Seifertite. Mindat.org. melanophlogite cubic (cP*, P4232, No.208) or
tetragonal (P42/nbc)2.04 Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces - a clathrasil (silica clathrate) fibrous
W-silicaorthorhombic
oI12, Ibam, No.721.97 Like silicon sulfide consisting of edge-sharing chains, melts at ~1700 K 2D silica hexagonal Sheet-like bilayer structure
Safety
Inhaling finely divided crystalline silica can lead to severe inflammation of the lung tissue, silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis. Inhalation of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.
Other names
This extended list enumerates synonyms for silicon dioxide; all of these values are from a single source; values in the source were presented capitalized.Grace Ross Lewis (1999). 9780471292128, John Wiley & Sons (Wiley-Interscience). . ISBN 9780471292128
See also
External links
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