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Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as a synthetic product. Notable examples include
fused quartz Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...
, fumed silica,
silica gel Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other l ...
, opal and aerogels. It is used in structural materials, microelectronics (as an
electrical insulator An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
), and as components in the food and pharmaceutical industries.


Structure

In the majority of silicates, 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. 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. SiO2 has several distinct crystalline forms, but they almost 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°. ; Polymorphism
Alpha quartz The room-temperature form of quartz, α-quartz, undergoes a reversible change in crystal structure at 573 °C to form β-quartz. This phenomenon is called an inversion, and for the α to β quartz inversion is accompanied by a linear expansion ...
is the most stable form of solid SiO2 at room temperature. The high-temperature minerals, cristobalite and
tridymite Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first describe ...
, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to
beta-quartz The room-temperature form of quartz, α-quartz, undergoes a reversible change in crystal structure at 573 °C to form β-quartz. This phenomenon is called an inversion, and for the α to β quartz inversion is accompanied by a linear expansion ...
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. The high-pressure minerals,
seifertite Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of silica. It has only been found in Martian and lunar meteorites, where it is presumably formed from either tridymite or cristobalite – other polymo ...
, stishovite, and
coesite Coesite is a form ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...
, though, have higher densities and indices of refraction than quartz. Stishovite has a
rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at visib ...
-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. 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. The change in the coordination increases the ionicity of the Si–O bond. More importantly, any deviations from these standard parameters constitute microstructural differences or variations, which represent an approach to an amorphous, vitreous, or glassy solid.
Faujasite Faujasite (FAU-type zeolite) is a mineral group in the zeolite family of silicate minerals. The group consists of faujasite-Na, faujasite-Mg and faujasite-Ca. They all share the same basic formula by varying the amounts of sodium, magnesium and ...
silica, another polymorph, is obtained by 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 Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to dete ...
(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 Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...
. ;Molten SiO2 Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum. Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C. ;Molecular SiO2 The molecular SiO2 has a linear structure like . It has been produced by combining
silicon monoxide Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase, it is a diatomic molecule. It has been detected in stellar objects and has been described as the most common o ...
(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.


Natural occurrence


Geology

is most commonly found 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 Coesite is a form ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...
and stishovite have been found around impact structures and associated with
eclogite Eclogite () is a metamorphic rock containing garnet (almandine- pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, ...
s formed during
ultra-high-pressure metamorphism Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic r ...
. The high-temperature forms of
tridymite Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first describe ...
and cristobalite are known from silica-rich
volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
s. In many parts of the world, silica is the major constituent of sand.


Biology

Even though it is poorly soluble, silica occurs in many plants such as
rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...
. Plant materials with high silica
phytolith Phytoliths (from Greek, "plant stone") are rigid, microscopic structures made of silica, found in some plant tissues and persisting after the decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different ...
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. 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. For well over a billion years,
silicification In geology, silicification is a petrification process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and ...
in and by cells has been common in the biological world. In the modern world, it occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates). Prominent examples include: * Tests or
frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on ...
s (i.e. shells) of diatoms,
Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...
, and testate amoebae. *Silica
phytolith Phytoliths (from Greek, "plant stone") are rigid, microscopic structures made of silica, found in some plant tissues and persisting after the decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different ...
s in the cells of many plants, including
Equisetaceae Equisetaceae, sometimes called the horsetail family, is the only extant family of the order Equisetales, with one surviving genus, ''Equisetum'', which comprises about twenty species. Evolution and systematics Equisetaceae is the only survivin ...
, practically all grasses, and a wide range of dicotyledons. *The spicules forming the skeleton of many sponges. Crystalline minerals formed in the physiological environment often show exceptional physical properties (e.g., strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of scales. The minerals are crystallized from an environment that is undersaturated concerning silicon, and under conditions of neutral pH and low temperature (0–40 °C).


Uses


Structural use

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 Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th cen ...
). 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 Tight oil (also known as shale oil, shale-hosted oil or light tight oil, abbreviated LTO) is light crude oil contained in unconventional petroleum-bearing formations of low permeability, often shale or tight sandstone. Economic production from ...
and
shale gas Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and some ...
.


Precursor to glass and silicon

Silica is the primary ingredient in the production of most
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
. 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. When molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize, but solidifies as a glass. Because of this, most
ceramic glaze Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a pottery body through firing. Glaze can serve to color, decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding ...
s have silica as the main ingredient. 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 optical fibers for telecommunication are also made from silica. It is a primary raw material for many ceramics such as earthenware,
stoneware Stoneware is a rather broad term for pottery or other ceramics fired at a relatively high temperature. A modern technical definition is a vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire clay. Whether vi ...
, and porcelain. Silicon dioxide is used to produce elemental silicon. The process involves carbothermic reduction in an
electric arc furnace An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to ...
: :SiO2 + 2 C -> Si + 2 CO


Fumed silica

Fumed silica, also known as pyrogenic silica, is prepared by burning SiCl4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO2. :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-.15 g/cm3) and thus high surface area. The particles act as a
thixotropic Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed ( ...
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 Silica fume, also known as microsilica, (CAS number 69012-64-2, EINECS number 273-761-1) is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon a ...
is an ultrafine powder collected as a by-product of the silicon and
ferrosilicon Ferrosilicon is an alloy of iron and silicon with a typical silicon content by weight of 15–90%. It contains a high proportion of iron silicides. Production and reactions Ferrosilicon is produced by reduction of silica or sand with coke in t ...
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 Pozzolans are a broad class of siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide (Ca(OH)2) ...
material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.


Food, cosmetic, and pharmaceutical applications

Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti-
caking Caking is a powder's tendency to form lumps or masses. The formation of lumps interferes with packaging, transport, flowability, and consumption. Usually caking is undesirable, but it is useful when pressing powdered substances into pills or briq ...
agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets. It can adsorb water in hygroscopic applications.
Colloidal silica {{Unreferenced, date=November 2021Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS). Properties U ...
is used as a fining agent for wine, beer, and juice, with the E number reference E551. 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 A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
; in a less processed form it was sold as "tooth powder". Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.


Semiconductors

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 MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor), later known as CSG (Commodore Semiconductor Group) and GMT Microelectronics, was a semiconductor design and fabrication company based in Audubon, Pennsylvania. It is mos ...
. 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 Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
. 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 A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
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 electronic states at the silicon surface. SiO2 films 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 In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...
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 Surface states are electronic states found at the surface of materials. They are formed due to the sharp transition from solid material that ends with a surface and are found only at the atom layers closest to the surface. The termination of a mate ...
that otherwise prevent electricity from reaching the semiconducting layer. The process of silicon surface passivation by
thermal oxidation In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The ra ...
(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).


Other

Hydrophobic silica Hydrophobic silica is a form of silicon dioxide (commonly known as silica) that has hydrophobic groups chemically bonded to the surface. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains. Hydrophobic silica can be processed ...
is used as a defoamer component. 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 chaotropes. 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.


Insecticide

Silicon dioxide has been researched for agricultural applications as a potential insecticide.


Production

Silicon dioxide is mostly obtained by mining, including
sand mining Sand mining is the extraction of sand, mainly through an open pit (or sand pit) but sometimes mined from beaches and inland dunes or dredged from ocean and river beds. Sand is often used in manufacturing, for example as an abrasive or in conc ...
and purification of quartz. 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.


Precipitated silica

Precipitated silica or amorphous silica is produced by the acidification of solutions of sodium silicate. The gelatinous precipitate or
silica gel Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other l ...
, is first washed and then dehydrated to produce colorless microporous silica. The idealized equation involving a trisilicate and sulfuric acid is: :Na2Si3O7 + H2SO4 -> 3 SiO2 + Na2SO4 + H2O Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.


On microchips

Thin films of silica grow spontaneously on
silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...
s via
thermal oxidation In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The ra ...
, producing a very shallow layer of about 1 nm or 10 Å 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 O2 :Si + O2 -> SiO2 or wet oxidation with H2O. :Si + 2 H2O -> SiO2 + 2 H2 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.


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 Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the of orthosilicic acid, Si(OH)4. It i ...
(TEOS). Simply heating TEOS at 680–730 °C results in the oxide: :Si(OC2H5)4 -> SiO2 + 2 O(C2H5)2 Similarly TEOS combusts around 400 °C: :Si(OC2H5)4 + 12 O2 -> SiO2 + 10 H2O + 8 CO2 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: :Si(OC2H5)4 + 2 H2O -> SiO2 + 4 HOCH2CH3


Other methods

Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion of
silane Silane is an inorganic compound with chemical formula, . It is a colourless, pyrophoric, toxic gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Sila ...
gives silicon dioxide. This reaction is analogous to the combustion of methane: :SiH4 + 2 O2 -> SiO2 + 2 H2O 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.


Chemical reactions

Silica is converted to silicon by reduction with carbon. Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are essentially unreactive. Most forms of silicon dioxide (except for stishovite, which does not react to any significant degree) are attacked by hydrofluoric acid (HF) to produce
hexafluorosilicic acid Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexafluo ...
: :SiO2 + 6 HF -> H2SiF6 + 2 H2O HF is used to remove or pattern silicon dioxide in the semiconductor industry. Under normal conditions, silicon does not react with most acids but is dissolved by hydrofluoric acid. :Si(s) + 6HF(aq) -> iF6(aq) + 2H+(aq) + 2H2(g) Silicon is attacked by bases such as aqueous sodium hydroxide to give silicates. :Si(s) + 4NaOH(aq) -> iO4(aq) + 4Na+(aq) + 2H2(g) 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 Chalcedony ( , or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monocli ...
), 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: :SiO2 + 2 NaOH -> Na2SiO3 + H2O Silicon dioxide will neutralise basic metal oxides (e.g. sodium oxide,
potassium oxide Potassium oxide ( K O) is an ionic compound of potassium and oxygen. It is a base. This pale yellow solid is the simplest oxide of potassium. It is a highly reactive compound that is rarely encountered. Some industrial materials, such as fertili ...
, lead(II) oxide,
zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...
, or mixtures of oxides, forming silicates 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: :2 Na2O + SiO2 -> Na4SiO4; :Na2O + SiO2 -> Na2SiO3; :(0.25-0.8) Na2O + SiO2 -> glass. Examples of such glasses have commercial significance, e.g. soda-lime glass,
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
, lead glass. In these glasses, silica is termed the network former or lattice former. The reaction is also used in blast furnaces to remove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag. Silicon dioxide reacts in heated
reflux Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions ...
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 polar solvent except methanol. Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO: :SiO2 + Si -> 2 SiO


Water solubility

The solubility of silicon dioxide in water strongly depends on its crystalline form and is three-four times higher for 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 decreased risk of dementia of 11%. Inhaling finely divided crystalline silica dust can lead to silicosis,
bronchitis Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...
, 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.


Occupational hazard

Silica is an occupational hazard for people who do
sandblasting Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove s ...
or work with products that contain powdered crystalline silica. 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
allergies Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic dermat ...
, and the elderly (all of whom have reduced
lung capacity Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting breathin ...
) can be affected in less time. Crystalline silica is an
occupational hazard An occupational hazard is a hazard experienced in the workplace. This encompasses many types of hazards, including chemical hazards, biological hazards (biohazards), psychosocial hazards, and physical hazards. In the United States, the Nation ...
for those working with stone
countertop A countertop, also counter top, counter, benchtop, worktop (British English) or kitchen bench ( Australian or New Zealand English), bunker ( Scottish English) is a raised, firm, flat, and horizontal surface. They are built for work in kitchens ...
s, 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.


Pathophysiology

In the body, crystalline silica particles do not dissolve over clinically relevant periods. Silica crystals inside the lungs can activate the NLRP3
inflammasome Inflammasomes are cytosolic multiprotein oligomers of the innate immune system responsible for the activation of inflammatory responses. Activation and assembly of the inflammasome promotes proteolytic cleavage, maturation and secretion of pro-in ...
inside macrophages and dendritic cells and thereby result in production of
interleukin Interleukins (ILs) are a group of cytokines (secreted proteins and signal molecules) that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. The human genome encodes more than 50 interleukins and related ...
, a highly pro-inflammatory cytokine in the immune system.


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. In 2013, the U.S. Occupational Safety and Health Administration reduced the exposure limit to 50
µg In the metric system, a microgram or microgramme is a unit of mass equal to one millionth () of a gram. The unit symbol is μg according to the International System of Units (SI); the recommended symbol in the United States and United Kingdom whe ...
/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 "green completion" of fracked wells to reduce exposure to crystalline silica besides restricting the limit of exposure.


Crystalline forms

SiO2, more so than almost any material, exists in many crystalline forms. These forms are called polymorphs.


Safety

Inhaling finely divided crystalline silica can lead to severe inflammation of the lung tissue, silicosis,
bronchitis Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...
, 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.


See also

*
Mesoporous silica Mesoporous silica is a form of silica that is characterised by its mesoporous structure, that is, having pores that range from 2 nm to 50 nm in diameter. According to IUPAC's terminology, mesoporosity sits between microporous (50  ...
* Orthosilicic acid *
Silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...


References


External links

* *Tridymite, *Quartz, *Cristobalite, *amorphous
NIOSH Pocket Guide to Chemical Hazards
*crystalline, as respirable dust

LPCVD and PECVD method in comparison. Stress prevention.
Epidemiological evidence on the carcinogenicity of silica: factors in scientific judgement
by C. Soutar and others.
Institute of Occupational Medicine The Institute of Occupational Medicine (IOM) was founded in 1969 by the National Coal Board (NCB) as an independent charity in the UK and retains this charitable purpose and status today. The "Institute" has a subsidiary, IOM Consulting Limited, whi ...
Research Report TM/97/09
Scientific opinion on the health effects of airborne silica
by A Pilkington and others.
Institute of Occupational Medicine The Institute of Occupational Medicine (IOM) was founded in 1969 by the National Coal Board (NCB) as an independent charity in the UK and retains this charitable purpose and status today. The "Institute" has a subsidiary, IOM Consulting Limited, whi ...
Research Report TM/95/08
The toxic effects of silica
by A Seaton and others.
Institute of Occupational Medicine The Institute of Occupational Medicine (IOM) was founded in 1969 by the National Coal Board (NCB) as an independent charity in the UK and retains this charitable purpose and status today. The "Institute" has a subsidiary, IOM Consulting Limited, whi ...
Research Report TM/87/13
Structure of precipitated silica
{{Authority control Ceramic materials Refractory materials IARC Group 1 carcinogens Excipients E-number additives Oxides