View source for Silicon dioxide

{{Commons|Category:Silicon_dioxide|Silicon Dioxide}}
{{Chembox_new
|ImageFile=Towering_Sand_Dunes.jpg
|OtherNames=Silica, Quartz, sand
|Section1={{Chembox_Identifiers
|CASNo=7631-86-9
}}
|Section2={{Chembox_Properties
|Formula=SiO<sub>2</sub>
|MolarMass=60.1
|Appearance=white powdery substance<br />solid (when pure)
|Density=2.2 g/cm³
|MeltingPt=1650 (±75) °C
|BoilingPt=2230 °C
|Solubility=0.012 g in 100mL
}}
|Section3={{Chembox_Structure
|MolShape=[[Tetrahedron|tetrahedral]]
}}
|Section4={{Chembox_Hazards
|FlashPt=non-flammable
|NFPA-H=0
|NFPA-F=0
|NFPA-R=0
|RPhrases=R42 R43 R49
|SPhrases=S22 S36 S37 S45 S53
}}
|Section5={{Chembox_Related
|OtherAnions=[[Silicon sulfide]]
|OtherCations=[[Carbon dioxide]]<br />[[Germanium dioxide]]<br />[[Tin(IV) oxide]]<br />[[Lead(IV) oxide]]
|OtherCpds=[[Silicic acid]]
}}
}}
{{SI}}
{{CMG}}



==Overview==
The [[chemical compound]] '''silicon dioxide''', also known as '''silica''' or '''silox''' (from the [[Latin]] "[[silex]]"), is an [[oxide]] of [[silicon]], [[chemical formula]] {{Silicon}}{{Oxygen|2}}, and has been known for its hardness since the 9th century<ref>[[Lynn Townsend White, Jr.]] (Spring, 1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition", ''Technology and Culture'' '''2''' (2), pp. 97-111 [100].
{{Quote|"[[Abbas Ibn Firnas|Ibn Firnas]] was a [[polymath]]: a [[Islamic medicine|physician]], a rather bad [[Islamic poetry|poet]], the first to make [[glass]] from stones (quartz?), a student of [[Islamic music|music]], and inventor of some sort of [[metronome]]."}}</ref>. Silica is most commonly found in nature as [[sand]] or [[quartz]], as well as in the cell walls of [[diatoms]]. It is a principal component of most types of [[glass]] and substances such as [[concrete]].

==Manufactured forms==
[[Image:Sio2_130m2_per_gram_under_blue_book_1600dpi.jpg|thumb|left|130m²/g surface area [[silica fume]]]]
Silica is manufactured in several forms including:
*[[glass]] (a colorless, high-purity form is called ''[[fused silica]]'')
*synthetic amorphous silica
*[[silica gel]] (used e.g. as [[desiccant]]s in new clothes and leather goods)

It is used in the production of various products.
*Inexpensive [[soda-lime glass]] is the most common and typically found in drinking glasses, bottles, and windows.
*A raw material for many whiteware ceramics such as [[earthenware]], [[stoneware]] and [[porcelain]].
*A raw material for the production of [[Portland cement]].
*A [[food additive]], primarily as a flow agent in powdered foods, or to absorb water (see the ingredients list for).
*The natural ("native") oxide coating that grows on [[silicon]] is hugely beneficial in [[microelectronics]]. It is a superior [[electric insulator]], possessing high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to allow small currents to flow through a device. At room temperature, however, it grows extremely slowly, and so to manufacture such oxide layers on silicon, the traditional method has been the deliberate heating of silicon in high temperature furnaces within an oxygen ambient ([[thermal oxidation]]).
*Raw material for [[aerogel]] in the [[Stardust (spacecraft)|Stardust spacecraft]]
*Used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of [[chaotropic agent|chaotropes]].
*Added to medicinal [[anti-foaming agent]], like [[Simethicone]], in a small proportion to enhance defoaming activity.
*As [[hydrated silica]] in Toothpaste (abrasive to fight away plaque.)

==Health effects==
[[Image:Kieselsaeure380m2prog.jpg|thumb|left|Manufactured silica fume at maximum surface area of 380m²/g]]
Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1mg/m<sup>3</sup>) over time can lead to [[silicosis]], [[bronchitis]] or (much more rarely) [[cancer]], as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities (silica does not dissolve over time). This effect can be an occupational hazard for people working with [[sandblasting]] equipment, products that contain powdered silica, and so on. But children, asthmatics of any age, allergy sufferers and the elderly, all of whom have reduced lung capacity, can be affected in much shorter periods of time.

In all other respects, silicon dioxide is inert and harmless. When silica is ingested orally, it passes unchanged through the gastrointestinal tract, exiting in the [[feces]], leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining. Silicon dioxide produces no fumes and is insoluble ''in vivo.'' It is indigestible, with zero nutritional value and zero toxicity.

==Chemistry==
Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 [[nanometre|nm]] or 10 [[Ångström|Å]]) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions.<br /> 
Higher temperatures and alternate environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures of 600 -1200 °C so-called "dry" or "wet" oxidation using [[oxygen|O<sub>2</sub>]] or H<sub>2</sub>O respectively.<ref name = "Sunggyu Lee">Sunggyu Lee (2006) ''Encyclopedia of chemical processing'' , CRC Press, ISBN 0824755634</ref> The thickness of the layer of silicon replaced by the dioxide is 44% of the thickness of the silicon dioxide layer produced.<ref name = "Sunggyu Lee"/><br />
Alternative methods used to deposit a layer of SiO<sub>2</sub> include:<ref>Robert Doering, Yoshio Nishi (2007), ''Handbook of Semiconductor Manufacturing Technology'', CRC Press, ISBN 1574446754</ref>
*Low temperature oxidation (LTO) of [[silane]]
:SiH<sub>4</sub> + 2O<sub>2</sub> → SiO<sub>2</sub> + 2H<sub>2</sub>O (at 400-450 °C)

*Decomposition of [[tetraethyl orthosilicate]] (TEOS) at 680 – 730°C

:Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub> → SiO<sub>2</sub> + H<sub>2</sub>O + 2C<sub>2</sub>H<sub>4</sub>

*Plasma enhanced chemical vapour deposition using TEOS at approximately 400°C
:Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub> + 12O<sub>2</sub> → SiO<sub>2</sub> + 10H<sub>2</sub>O + 8CO<sub>2</sub>

Pyrogenic silica(sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning [[silicon tetrachloride|SiCl<sub>4</sub>]] in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO<sub>2</sub>:<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref>
:SiCl<sub>4</sub> + 2H<sub>2</sub> + O<sub>2</sub>  → SiO<sub>2</sub> + 4HCl 

Quartz exhibits a maximum solubility in water at around 340 °C <ref>The solubility of amorphous silica in water at high temperatures and high pressures Fournier R.O , Rowe J.J., American Mineralogist (1977), 62, 1052-1056</ref>. 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 which is cooler at the top. Crystals of 0.5 -1 kg can be grown over a period of 1-2 months.<ref name = "Wiberg&Holleman">Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier ISBN 0123526515</ref> these crystals are a sourcer of very pure quartz for use in electronic applications.<ref name = "Greenwood"/><br />
Fluorine reacts with silicon dioxide to form SiF<sub>4</sub> and O<sub>2</sub> whereas the other halogen gases (Cl<sub>2</sub>, Br<sub>2</sub>, I<sub>2</sub>) react much less readily.<ref name = "Greenwood"/>

Silicon dioxide is attacked by [[hydrofluoric acid]] (HF) to produce "[[Hexafluorosilicic acid|hexafluorosilicic acid"]]:<ref name = "Wiberg&Holleman"/>
:SiO<sub>2</sub> + 6HF → H<sub>2</sub>SiF<sub>6</sub> + 2H<sub>2</sub>O
HF is used to remove or pattern silicon dioxide in the semiconductor industry.<br />
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide (e.g):<ref name = "Greenwood"/>
:SiO<sub>2</sub> + NaOH → [[sodium metasilicate|Na<sub>2</sub>SiO<sub>3</sub>]] + H<sub>2</sub>O
Silicon dioxide reacts with basic metal oxides (e.g. [[sodium oxide]], [[potassium oxide]], [[lead(II) oxide]], [[zinc oxide]]
or mixtures of oxides forming [[silicate|silicates]] and glasses as the Si-O-Si bonds in silica are broken successively.<ref name = "Wiberg&Holleman"/>
As an example the reaction of [[sodium oxide]] and SiO<sub>2</sub> can produce [[sodium orthosilicate]], [[sodium silicate]]  and glasses, depending on the proportions of reactants:<ref name = "Greenwood"/>
:2Na<sub>2</sub>O + SiO<sub>2</sub> → Na<sub>4</sub>SiO<sub>4</sub>

:Na<sub>2</sub>O + SiO<sub>2</sub> → Na<sub>2</sub>SiO<sub>3</sub>

:(0.25 - 0.8)Na<sub>2</sub>O + SiO<sub>2</sub> → glasses

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.<ref name = "Wiberg&Holleman"/><br />
With silicon at high temperatures gaseous [[silicon monoxide|SiO]] is produced:<ref name = "Wiberg&Holleman"/>
:SiO<sub>2</sub> + Si → 2SiO (gas)

==Structure and properties==
SiO<sub>2</sub> has a number of distinct crystalline forms in addition to amorphous forms. With the exception of [[stishovite]] and fibrous sulfur, all of the crystalline forms involve tetrahedral SiO<sub>4</sub> units linked together by shared vertices in different arrangements. Silicon-oxygen bond lengths vary between the different crystal forms, for example in α-quartz the bond length is 161 pm, whereas in α-tridymite it is in the range 154-171 pm.<ref name = "Wiberg&Holleman"/>. 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°.<br />  
Fibrous sulfur has a structure similar to that of  SiS<sub>2</sub> with chains of edge-sharing SiO<sub>4</sub> tetrahedra.<br />
[[Stishovite]], the highest pressure form, in contrast has a [[titanium dioxide|rutile]] like structure where silicon is 6 coordinate. The density of [[stishovite]] is 4.287 g/cm<sup>3</sup>, which compares to α-quartz, the densest of the low pressure forms, which has a density of 2.648 g/cm<sup>3</sup>.<ref name = "Greenwood"/> 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.<ref> Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ISBN 0-19-855370-6 </ref> The change in the coordination increases the ionicity of the Si-O bond.<ref>Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study  Kirfel A.,  Krane H.-G., Blaha P., Schwarz  K., Lippmann T., Acta Cryst. (2001). A57, 663-677 {{doi|10.1107/S0108767301010698}}</ref> <br />
Note that the only stable form under normal conditions is α-quartz and this is the form in which crystalline silicon dioxide is usually encountered.<ref name = "Greenwood"/> In nature impurities in crystalline α-quartz  can give rise to colours<ref name = "Greenwood"/> (see [[quartz]] for a list).
{| class="wikitable"
|+ Crystalline forms of SiO<sub>2</sub><ref name = "Wiberg&Holleman"/>
|-
! Form
! Space group
! Structural features
! Notes
|-
|  [[quartz|α-quartz]]
| [[rhombohedral]] <br />(trigonal) 
| Helical chains making individual single crystals optically active
| α-quartz converts to β-quartz at 573°C<br />
|-
|β-quartz
| hexagonal
| closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active
|β-quartz converts to β-tridymite at 870°C 
|-
| [[tridymite|α-tridymite]]  
| [[orthorhombic]]
|metastable form under normal pressure 
| 
|-
|β-tridymite
| [[hexagonal]]
| closely related to α-tridymite
|β-tridymite converts to β-cristobalite 1470°C
|-
|[[cristobalite|α-cristobalite]]
| [[tetragonal]]
|metastable form under normal pressure 
|
|-
|β-cristobalite
| [[cubic]]  
| closely related to α-cristobalite
|melts at 1705°C
|-
|[[keatite]]
| [[tetragonal]]  
|Si<sub>5</sub>O<sub>10</sub>, Si<sub>4</sub>O<sub>14</sub>, Si<sub>8</sub>O<sub>16</sub> rings  
|synthesised from amorphous silica and alkali at high pressure
|-
|[[coesite]]
| [[monoclinic]]
|Si<sub>4</sub>O<sub>8</sub> and Si<sub>8</sub>O<sub>16</sub> rings  
|high pressure form (higher than keatite)
|-
|[[stishovite]]
| [[tetragonal]]  
| rutile like with '''6 coordinate Si'''
|high pressure form (higher than coesite) and the densest of the polymorphs
|-
|[[melanophlogite]] 
| [[cubic]]
|Si<sub>5</sub>O<sub>10</sub>, Si<sub>6</sub>O<sub>12</sub> rings
|mineral always found with hydrocarbons in interstitial spaces-a clathrasil<ref>
Rosemarie Szostak (1998)  Molecular sieves: Principles of Synthesis and Identification, Springer,ISBN 0751404802</ref>
|-
|fibrous
| [[orthorhombic]]
| like [[silicon sulfide|SiS<sub>2</sub>]] consisting of edge sharing chains
|
|}

==Molecular forms of silicon dioxide==
When molecular [[silicon monoxide]], SiO is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge molecular SiO<sub>2</sub> is produced which has a linear structure.<ref name = "Jutzi"> Peter Jutzi, Ulrich Schubert, (2003), ''Silicon chemistry: from the atom to extended systems'', Wiley-VCH ISBN 3527306471</ref> 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.<ref name = "Jutzi"/><br />
Dimeric silicon dioxide, (SiO<sub>2</sub>)<sub>2</sub> has been prepared by reacting O<sub>2</sub> with matrix isolated dimeric [[silicon monoxide]], (Si<sub>2</sub>O<sub>2</sub>).<ref name = "Jutzi"/> 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 148.2pm.<ref name = "Jutzi"/>

==Notes==
{{Reflist}}

==References==
*R. K. Iler, ''The Chemistry of Silica'' (ISBN 0-471-02404-X)

==See also==
*[[Amorphous carbonia]]
*[[Fused silica]]
*[[Silicon carbide]]
*[[Mesoporous silica]]

==External links==
*{{ICSC|0807|08}} (Tridymite)
*{{ICSC|0808|08}} (Quartz)
*{{ICSC|0809|08}} (Cristobalite)
*[http://www.cdc.gov/niosh/npg/npgd0552.html NIOSH Pocket Guide to Chemical Hazards] (amorphous)
*[http://www.cdc.gov/niosh/npg/npgd0553.html NIOSH Pocket Guide to Chemical Hazards] (crystalline, as respirable dust)
*[http://www.piezomaterials.com/Quartz-SiO2.htm Quartz SiO2 piezolelctric properties]

<br>

[[Category:Silicon compounds]]
[[Category:Oxides]]
[[Category:Ceramic materials]]
[[Category:Refractory materials]]
[[Category:IARC Group 1 carcinogens]]

[[bn:সিলিকন ডাই অক্সাইড]]
[[bs:Silicijum dioksid]]
[[ca:Diòxid de silici]]
[[cs:Oxid křemičitý]]
[[da:Siliciumdioxid]]
[[de:Siliciumdioxid]]
[[et:Ränidioksiid]]
[[es:Óxido de silicio]]
[[eu:Silizio dioxido]]
[[fr:Silice]]
[[hr:Silicijev dioksid]]
[[it:Silice]]
[[he:צורן דו-חמצני]]
[[la:Silica]]
[[lv:Silīcija dioksīds]]
[[lt:Silicio dioksidas]]
[[hu:Szilícium-dioxid]]
[[nl:Siliciumdioxide]]
[[ja:二酸化ケイ素]]
[[pl:Ditlenek krzemu]]
[[pt:Dióxido de silício]]
[[ro:Bioxid de siliciu]]
[[qu:Ullaya]]
[[ru:Диоксид кремния]]
[[simple:Silicon dioxide]]
[[sk:Oxid kremičitý]]
[[sr:Силицијум диоксид]]
[[fi:Piidioksidi]]
[[sv:Kiseldioxid]]
[[uk:Діоксид силіцію]]
[[zh:二氧化硅]]


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