Silicon dioxide

A sample of silicon dioxide
Names
IUPAC name Silicon dioxide
Other names Quartz Silica Silicic oxide Silicon(IV) oxide Crystalline silica Pure Silica Silicea Silica sand
Identifiers
CAS Number	7631-86-9 Y
ChEBI	CHEBI:30563 Y
ChemSpider	22683 Y
ECHA InfoCard	100.028.678
EC Number	231-545-4
E number	E551 (acidity regulators, ...)
Gmelin Reference	200274
KEGG	C16459 Y
MeSH	Silicon+dioxide
PubChem CID	24261
RTECS number	VV7565000
UNII	ETJ7Z6XBU4 Y
CompTox Dashboard (EPA)	DTXSID1029677
InChI InChI=1S/O2Si/c1-3-2 Y Key: VYPSYNLAJGMNEJ-UHFFFAOYSA-N Y
Properties
Chemical formula	SiO2
Molar mass	60.08 g/mol
Appearance	Transparent or white
Density	2.648 (α-quartz), 2.196 (amorphous) g·cm−3[1]
Melting point	1,713 °C (3,115 °F; 1,986 K) (amorphous)[1]: 4.88
Boiling point	2,950 °C (5,340 °F; 3,220 K)[1]
Magnetic susceptibility (χ)	−29.6·10−6 cm3/mol
Thermal conductivity	12 (|| c-axis), 6.8 (⊥ c-axis), 1.4 (am.) W/(m⋅K)[1]: 12.213
Refractive index (nD)	1.544 (o), 1.553 (e)[1]: 4.143
Hazards
NFPA 704 (fire diamond)	0 0 0
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 20 mppcf (80 mg/m3/%SiO2) (amorphous)[2]
REL (Recommended)	TWA 6 mg/m3 (amorphous)[2] Ca TWA 0.05 mg/m3[3]
IDLH (Immediate danger)	3000 mg/m3 (amorphous)[2] Ca [3]
Related compounds
Related diones	Carbon dioxide Germanium dioxide Tin dioxide Lead dioxide
Related compounds	Silicon monoxide Silicon disulfide
Thermochemistry
Std molar entropy (S⦵298)	42 J·mol−1·K−1[4]
Std enthalpy of formation (ΔfH⦵298)	−911 kJ·mol−1[4]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is YN ?) Infobox references

Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO₂, commonly found in nature as quartz.^[5][6] In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant families of materials, existing as a compound of several minerals and as a synthetic product. Examples include fused quartz, fumed silica, opal, and aerogels. 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.

In the majority of silicon dioxides, the silicon atom shows tetrahedral 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 the double 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 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.^[11]

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°.^[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 to beta-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 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.^[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 the dealumination 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.^[18]

Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: 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 combining silicon monoxide (SiO) with oxygen in an argon 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 as quartz, 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 forms coesite and stishovite have been found around impact structures and associated with eclogites formed during ultra-high-pressure metamorphism. The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks. In many parts of the world, silica is the major constituent of sand.^[23]

Even though it is poorly soluble, silica occurs in many plants such as rice. Plant materials with high silica phytolith content appear to be of importance to grazing animals, from chewing insects to ungulates. Silica accelerates tooth wear, and high levels of silica in plants frequently eaten by herbivores may have developed as a defense mechanism against predation.^[24][25]

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.^[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]

Prominent examples include:

• Tests or frustules (i.e. shells) of diatoms, Radiolaria, and testate amoebae.^[6]
• Silica phytoliths in the cells of many plants^[28] including Equisetaceae,^[29] many grasses, and a wide range of dicotyledons.^[30][31]
• The spicules forming the skeleton of many sponges.^[32]

About 95% of the commercial use of silicon dioxide (sand) occurs in the construction industry, e.g. for the production of concrete (Portland cement concrete).^[22]

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.^[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.

Crystalline silica is used in hydraulic fracturing of formations which contain tight oil and shale gas.^[34]

Silica is the primary ingredient in the production of most glass. As other minerals are melted with silica, the principle of freezing point depression lowers the melting point of the mixture and increases fluidity. The glass 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, most ceramic 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]

The majority of optical fibers 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:^[39]

${\displaystyle {\ce {SiO2 + 2 C -> Si + 2 CO}}}$

Fumed silica, also known as pyrogenic silica, is prepared by burning SiCl₄ in an oxygen-rich hydrogen flame to produce a "smoke" of SiO₂.^[15]

${\displaystyle {\ce {SiCl4 + 2 H2 + O2 -> SiO2 + 4 HCl}}}$

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 a thixotropic 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.

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.^[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 can adsorb water in hygroscopic applications. Colloidal silica is used as a fining agent for wine, beer, and juice, with the E number reference E551.^[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 microscopic diatoms; in a less processed form it was sold as "tooth powder".^[44][45] Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.

Silicon dioxide is widely used in the semiconductor technology:

• for the primary passivation (directly on the semiconductor surface),
• as an original gate dielectric in MOS technology. Today when scaling (dimension of the gate length of the MOS transistor) has progressed below 10 nm, silicon dioxide has been replaced by other dielectric materials like hafnium 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 like silicon nitride.

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.^[46] Silicon oxide layers could protect silicon surfaces during diffusion 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 a thermally grown silicon dioxide layer greatly reduces the concentration of electronic states at the silicon surface.^[50] SiO₂ films preserve the electrical characteristics of p–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 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.^[49][51]

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).^[49][51]

Hydrophobic silica is used as a defoamer component. Zdroj:https://en.wikipedia.org?pojem=Silicon_dioxide
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