Silicon dioxide, also known assilica, is anoxide ofsilicon with thechemical formulaSiO2, commonly found in nature asquartz.[5][6] In many parts of the world, silica is the major constituent ofsand. Silica is one of the most complex and abundant families ofmaterials, existing as a compound of severalminerals and as a synthetic product. Examples includefused quartz,fumed silica,opal, andaerogels. It is used instructural 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 ofglass.
Structural motif found in α-quartz, but also found in almost all forms of silicon dioxideTypical subunit for low pressure silicon dioxideRelationship between refractive index and density for some SiO2 forms[7]
In the majority of silicon dioxides, the silicon atom showstetrahedral coordination, with four oxygen atoms surrounding a central Si atom (see 3-D Unit Cell). Thus, SiO2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms.[8][9] In contrast, CO2 is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of thedouble bond rule.[10]
Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In crystalline form, this substance can be found naturally occurring asquartz,tridymite (high-temperature form),cristobalite (high-temperature form),stishovite (high-pressure form), andcoesite (high-pressure form). On the other hand, amorphous silica can be found in nature asopal anddiatomaceous earth. Quartz glass is a form of intermediate state between these structures.[11]
All of thesedistinct 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. TheSi–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°.[12]
Alpha quartz is the most stable form of solid SiO2 at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz tobeta-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit.[13] The high-pressure minerals,seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz.[14] Stishovite has arutile-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.[15] 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.[16]The change in the coordination increases the ionicity of the Si–O bond.[17]
Faujasite silica, another polymorph, is obtained by thedealumination of a low-sodium, ultra-stable Yzeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, and has highcrystallinity andspecific 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 orcrystallinity even after boiling in concentratedhydrochloric acid.[18]
Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquidwater: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum.[19] Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C.[20]
The molecular SiO2 has a linear structure like CO2. It has been produced by combiningsilicon monoxide (SiO) with oxygen in anargon matrix. The dimeric silicon dioxide, (SiO2)2 has been obtained 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.[21]
SiO2 is most commonly encountered in nature asquartz, which comprises more than 10% by mass of the Earth's crust.[22] Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure formscoesite andstishovite have been found aroundimpact structures and associated witheclogites formed duringultra-high-pressure metamorphism. The high-temperature forms oftridymite andcristobalite are known from silica-richvolcanic rocks. In many parts of the world, silica is the major constituent ofsand.[23]
Even though it is poorly soluble, silica occurs in many plants such asrice. Plant materials with high silicaphytolith content appear to be of importance to grazing animals, from chewing insects toungulates. Silica accelerates tooth wear, and high levels of silica in plants frequently eaten byherbivores may have developed as a defense mechanism against predation.[24][25]
Silica is also the primary component ofrice husk ash, which is used, for example, in filtration and as supplementary cementitious material (SCM) incement andconcrete manufacturing.[26]
Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates).[27]
About 95% of the commercial use of silicon dioxide (sand) is in the construction industry, e.g. in the production of concrete (Portland cement concrete).[22]
Certain deposits of silica sand, with desirable particle size and shape and desirableclay and other mineral content, were important forsand casting of metallic products.[33] 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.
Silica is the primary ingredient in the production of mostglass. As other minerals are melted with silica, the principle offreezing point depression lowers the melting point of the mixture and increases fluidity. Theglass transition temperature of pure SiO2 is about 1475 K.[35] When molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize, but solidifies as a glass.[36] Because of this, mostceramic glazes have silica as the main ingredient.[37]
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.[38]
Fumed silica, also known as pyrogenic silica, is prepared by burningSiCl4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO2.[15]
It can also be produced by vaporizing quartz sand in a 3000 °C electric arc. Both processes result in microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles, a white powder with extremely low bulk density (0.03-0.15 g/cm3) and thus high surface area.[40] The particles act as athixotropic thickening agent, or as an anti-caking agent, and can be treated to make them hydrophilic or hydrophobic for either water or organic liquid applications.
Manufactured fumed silica with maximum surface area of 380 m2/g
Silica fume is an ultrafine powder collected as a by-product of the silicon andferrosilicon alloy production. It consists ofamorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without the branching of the pyrogenic product. The main use is aspozzolanic material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.[41]
Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti-caking agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets.[40] It canadsorb water inhygroscopic applications.Colloidal silica is used as afining agent for wine, beer, and juice, with theE number referenceE551.[22]
In cosmetics, silica is useful for its light-diffusing properties[42] and natural absorbency.[43]
Diatomaceous earth, a mined product, has been used in food and cosmetics for centuries. It consists of the silica shells of microscopicdiatoms; in a less processed form it was sold astooth powder.[44][45] Manufactured or minedhydrated silica is used as the hard abrasive intoothpaste.
Silicon dioxide is widely used in the semiconductor technology:
for the primary passivation (directly on the semiconductor surface),
as an originalgate dielectric inMOS technology. Today when scaling (dimension of the gate length of the MOS transistor) has progressed below 10 nm, silicon dioxide has been replaced by otherdielectric materials likehafnium oxide or similar with higher dielectric constant compared to silicon dioxide,
as a dielectric layer between metal (wiring) layers (sometimes up to 8–10) connecting elements and
as a second passivation layer (for protecting semiconductor elements and the metallization layers) typically today layered with some other dielectrics likesilicon nitride.
Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors likegallium arsenide orindium phosphide.
Silicon dioxide could be grown on a siliconsemiconductor surface.[46] Silicon oxide layers could protect silicon surfaces duringdiffusion processes, and could be used for diffusion masking.[47][48]
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.[49][50] The formation of athermally grown silicon dioxide layer greatly reduces the concentration ofelectronic states at the silicon surface.[50] SiO2films preserve the electrical characteristics ofp–n junctions and prevent these electrical characteristics from deteriorating by the gaseous ambient environment.[48] Silicon oxide layers could be used to electrically stabilize silicon surfaces.[47] The surface passivation process is an important method ofsemiconductor device fabrication that involves coating asilicon 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 thesurface states that otherwise prevent electricity from reaching the semiconducting layer.[49][51]
Silicon dioxide is mostly obtained by mining, includingsand mining and purification ofquartz. Quartz is suitable for many purposes, while chemical processing is required to make a purer or otherwise more suitable (e.g. more reactive or fine-grained) product.[55][56]
Precipitated silica or amorphous silica is produced by the acidification of solutions ofsodium silicate. The gelatinous precipitate orsilica gel, is first washed and then dehydrated to produce colorless microporous silica.[15] The idealized equation involving a trisilicate andsulfuric acid is:
Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.[22]
Thin films of silica grow spontaneously onsilicon wafers viathermal oxidation, producing a very shallow layer of about 1nm or 10Å of so-called native oxide.[57]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 withO2
The native oxide layer is beneficial inmicroelectronics, where it acts aselectric 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.[60]
Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO,[61] TEOS. Synthesis of silica is illustrated below usingtetraethyl orthosilicate (TEOS).[62] Simply heating TEOS at 680–730 °C results in the oxide:
Similarly TEOS combusts around 400 °C:
TEOS undergoeshydrolysis via the so-calledsol-gel process. The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:[63]
Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion ofsilane gives silicon dioxide. This reaction is analogous to the combustion of methane:
However thechemical vapor deposition of silicon dioxide onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500 °C.[64]
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.
Fluorine reacts with silicon dioxide to formSiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are unreactive.[15]
Stishovite does not react to HF to any significant degree.[65] HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide acts as aLux–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 aBrønsted–Lowry acid. While silicon dioxide is only poorly soluble in water at low or neutralpH (typically, 2 × 10−4M forquartz up to 10−3M forcryptocrystallinechalcedony), 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.[66]
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:[15]
Silicon dioxide will neutralise basic metal oxides (e.g.sodium oxide,potassium oxide,lead(II) oxide,zinc oxide, or mixtures of oxides, formingsilicates and glasses as the Si-O-Si bonds in silica are broken successively).[12] As an example the reaction of sodium oxide and SiO2 can produce sodiumorthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:[15]
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 340 °C (644 °F).[68] 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.[12] These crystals are a source of very pure quartz for use in electronic applications.[15] Above thecritical temperature of water 647.096 K (373.946 °C; 705.103 °F) and a pressure of 22.064 megapascals (3,200.1 psi) or higher, water is asupercritical fluid and solubility is once again higher than at lower temperatures.[69]
Quartz sand (silica) as main raw material for commercial glass production
Silica ingested orally is essentially nontoxic, with anLD50 of 5000 mg/kg (5 g/kg).[22] A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk ofdementia. An increase of 10 mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.[70]
Inhaling finely divided crystalline silica dust can lead tosilicosis,bronchitis, orlung cancer, as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities.[71] When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk ofsystemic autoimmune diseases such aslupus[72] andrheumatoid arthritis compared to expected rates in the general population.[73]
Silica is an occupational hazard for people who dosandblasting 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 withallergies, and the elderly (all of whom have reducedlung capacity) can be affected in less time.[74]
Crystalline silica is anoccupational hazard for those working with stonecountertops because the process of cutting and installing the countertops creates large amounts of airborne silica.[75] Crystalline silica used inhydraulic fracturing presents a health hazard to workers.[34]
In the body, crystalline silica particles do not dissolve over clinically relevant periods. Silica crystals inside the lungs can activate the NLRP3inflammasome inside macrophages and dendritic cells and thereby result in production ofinterleukin, a highlypro-inflammatory cytokine in the immune system.[76][77][78]
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.[79][80][81][82][83][84]
In 2013, the U.S.Occupational Safety and Health Administration reduced the exposure limit to 50μg/m3 of air. Prior to 2013, it had allowed 100 μg/m3 and in construction workers even 250 μg/m3.[34]In 2013, OSHA also required the "green completion" of fracked wells to reduce exposure to crystalline silica and restrict the exposure limit.[34]
This extended list enumerates synonyms for silicon dioxide; all of these values are from a single source; values in the source were presented capitalized.[101]
^abcdefgHolleman AF, Wiberg E (2001), Wiberg N (ed.),Inorganic Chemistry, translated by Eagleson M, Brewer W, San Diego/Berlin: Academic Press/De Gruyter,ISBN0-12-352651-5
^Barel AO, Paye M, Maibach HI (2014).Handbook of Cosmetic Science and Technology (4th ed.). CRC Press. p. 444.ISBN9781842145654.These soft-focus pigments, mainly composed of polymers, micas and talcs covered with rough or spherical particles of small diameters, such as silica or titanium dioxide, are used to optically reduce the appearance of wrinkles. These effects are obtained by optimizing outlines of wrinkles and reducing the difference of brightness due to diffuse reflection.
^Lee S (2006).Encyclopedia of chemical processing. CRC Press.ISBN9780824755638.
^Morgan DV, Board K (1991).An Introduction To Semiconductor Microtechnology (2nd ed.). Chichester, West Sussex, England: John Wiley & Sons. p. 72.ISBN9780471924784.
^Nandiyanto AB, Kim SG, Iskandar F, et al. (2009). "Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters".Microporous and Mesoporous Materials.120 (3):447–453.Bibcode:2009MicMM.120..447N.doi:10.1016/j.micromeso.2008.12.019.
^Morgan DV, Board K (1991).An Introduction To Semiconductor Microtechnology (2nd ed.). Chichester, West Sussex, England: John Wiley & Sons. p. 27.ISBN9780471924784.
^Fleischer M (1962)."New mineral names"(PDF).American Mineralogist.47 (2). Mineralogical Society of America:172–174.Archived(PDF) from the original on 2011-07-22.
^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.
^abKihara K, Matsumoto T, Imamura M (1986). "Structural change of orthorhombic-Itridymite with temperature: A study based on second-order thermal-vibrational parameters".Zeitschrift für Kristallographie.177 (1–2):27–38.Bibcode:1986ZK....177...27K.doi:10.1524/zkri.1986.177.1-2.27.
^Weiss A, Weiss A (1954). "Über Siliciumchalkogenide. VI. Zur Kenntnis der faserigen Siliciumdioxyd-Modifikation".Zeitschrift für Anorganische und Allgemeine Chemie.276 (1–2):95–112.doi:10.1002/zaac.19542760110.
