Silicon dioxide

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  to
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 [25 mg/m3 (cristobalite, tridymite); 50 mg/m3 (quartz)][3]
Related compounds
Related diones	Carbon dioxide Germanium dioxide Tin dioxide Lead dioxide
Related compounds	Silicon monoxide Silicon sulfide
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 [77 °F], 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 abundant as it comprises several minerals and synthetic products. All forms are white or colorless, although impure samples can be colored.

Silicon dioxide is a common fundamental constituent of glass.

Structure

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 the form of intermediate state between this structure.^[11]

All of this

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 SiO₂

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]

Natural occurrence

Geology

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]

Biology

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

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, including Equisetaceae, many 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 (

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.^[28] 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

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.[31]

The majority of

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.^[33] 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

In cosmetics, silica is useful for its light-diffusing properties[35] and natural absorbency.^[36]

• for the primary passivation (directly on the semiconductor surface),
• as an original
  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

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.^[43]

Silicon dioxide has been researched for agricultural applications as a potential insecticide.^[45][46]

Production

Silicon dioxide is mostly obtained by mining, including sand mining 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.^{[citation needed]}

Precipitated silica

Precipitated silica or amorphous silica is produced by the acidification of solutions of sodium silicate. The gelatinous precipitate or silica gel, is first washed and then dehydrated to produce colorless microporous silica.^[15] The idealized equation involving a trisilicate and sulfuric acid is:

${\displaystyle {\ce {Na2Si3O7 + H2SO4 -> 3 SiO2 + Na2SO4 + H2O}}}$

Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.^[22]

On microchips

Thin films of silica grow spontaneously on

${\displaystyle {\ce {Si + O2 -> SiO2}}}$

or wet oxidation with H₂O.^[48][49]

${\displaystyle {\ce {Si + 2 H2O -> SiO2 + 2 H2}}}$

The native oxide layer is beneficial in

Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO,[51] TEOS. Synthesis of silica is illustrated below using tetraethyl orthosilicate (TEOS).^[52] Simply heating TEOS at 680–730 °C results in the oxide:

${\displaystyle {\ce {Si(OC2H5)4 -> SiO2 + 2 O(C2H5)2}}}$

Similarly TEOS combusts around 400 °C:

${\displaystyle {\ce {Si(OC2H5)4 + 12 O2 -> SiO2 + 10 H2O + 8 CO2}}}$

TEOS undergoes

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:

${\displaystyle {\ce {SiH4 + 2 O2 -> SiO2 + 2 H2O}}}$

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.^[54]

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.

Fluorine reacts with silicon dioxide to form SiF₄ and O₂ whereas the other halogen gases (Cl₂, Br₂, I₂) are unreactive.^[15]

Most forms of silicon dioxide are attacked ("etched") by hydrofluoric acid (HF) to produce hexafluorosilicic acid:^[12]

SiO₂ + 6 HF → H₂SiF₆ + 2 H₂O

Stishovite does not react to HF to any significant degree.^[55] HF is used to remove or pattern silicon dioxide in the semiconductor industry.

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.^[56]

Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:^[15]

${\displaystyle {\ce {SiO2 + 2 NaOH -> Na2SiO3 + H2O}}}$

Silicon dioxide will neutralise basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide, or mixtures of oxides, forming silicates 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 sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:^[15]

${\displaystyle {\ce {2 Na2O + SiO2 -> Na4SiO4;}}}$

${\displaystyle {\ce {Na2O + SiO2 -> Na2SiO3;}}}$

${\displaystyle (0.25-0.8)}$ ${\displaystyle {\ce {Na2O + SiO2 -> glass}}}$.

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.^[12] 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

Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:[12]

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

Water solubility

The solubility of silicon dioxide in water strongly depends on its crystalline form and is three to four times higher for silica^{[clarification needed]} than quartz; as a function of temperature, it peaks around 340 °C (644 °F).^[58] 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 the critical 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 a supercritical fluid and solubility is once again higher than at lower temperatures.^[59]

Health effects

Silica ingested orally is essentially nontoxic, with an LD₅₀ 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 of dementia. An increase of 10 mg/day of silica in drinking water was associated with a decreased risk of dementia of 11%.^[60]

Inhaling finely divided crystalline silica dust can lead to

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.[68]^{[69][70][71][72][73]}

In 2013, the U.S.

Crystalline forms

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

Crystalline forms of SiO₂^[12]

Form	Crystal symmetry Pearson symbol, group no.	ρ (g/cm3)	Notes	Structure
α-quartz	rhombohedral (trigonal) hP9, P3121 No.152[74]	2.648	Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K
β-quartz	hexagonal hP18, P6222, No. 180[75]	2.533	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.20[76]	2.265	Metastable form under normal pressure
β-tridymite	hexagonal hP12, P63/mmc, No. 194[76]		Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K
α-cristobalite	tetragonal tP12, P41212, No. 92[77]	2.334	Metastable form under normal pressure
β-cristobalite	cubic cF104, Fd3m, No.227[78]		Closely related to α-cristobalite; melts at 1978 K
keatite	tetragonal tP36, P41212, No. 92[79]	3.011	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[80]		Si4O8 and Si6O12 rings
coesite	monoclinic mS48, C2/c, No.15[81]	2.911	Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa
stishovite	tetragonal tP6, P42/mnm, No.136[82]	4.287	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, Pbcn[83]	4.294	One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa.[84]
melanophlogite	cubic (cP*, P4232, No.208)[7] or tetragonal (P42/nbc)[85]	2.04	Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces - a clathrasil (silica clathrate)[86]
fibrous W-silica[15]	orthorhombic oI12, Ibam, No.72[87]	1.97	Like SiS2 consisting of edge sharing chains, melts at ~1700 K
2D silica[88]	hexagonal		Sheet-like bilayer structure

Safety

Inhaling finely divided crystalline silica can lead to severe inflammation of the

This extended list enumerates synonyms for silicon dioxide; all of these values are from a single source; values in the source were presented capitalized.[90]

See also

• Mesoporous silica
• Orthosilicic acid
• Silicon carbide

References

External links

Wikimedia Commons has media related to Silicon dioxide.

• Chisholm H, ed. (1911). "Silica" . Encyclopædia Britannica (11th ed.). Cambridge University Press.
• Tridymite, International Chemical Safety Card 0807
• Quartz, International Chemical Safety Card 0808
• Cristobalite, International Chemical Safety Card 0809
• Amorphous, NIOSH Pocket Guide to Chemical Hazards
• Crystalline, as respirable dust, NIOSH Pocket Guide to Chemical Hazards
• Formation of silicon oxide layers in the semiconductor industry. LPCVD and PECVD method in comparison. Stress prevention.
• Quartz (SiO₂) piezoelectric properties
• Silica (SiO₂) and water
• Epidemiological evidence on the carcinogenicity of silica: factors in scientific judgement by C. Soutar and others. Institute of Occupational Medicine Research Report TM/97/09
• Scientific opinion on the health effects of airborne silica by A Pilkington and others. Institute of Occupational Medicine Research Report TM/95/08
• The toxic effects of silica Archived 2016-04-15 at the Wayback Machine by A. Seaton and others. Institute of Occupational Medicine Research Report TM/87/13
• Structure of precipitated silica