View source for Silane

{{Short description|Chemical compound (SiH4)}}
{{About|the compound with chemical formula SiH<sub>4</sub>|the broader classes of compounds|Silanes}}
{{Chembox
| Watchedfields  = changed
| verifiedrevid  = 464390173
| ImageFile1     = Silane-2D.svg
| ImageClass1 = skin-invert-image
| ImageName1     = Stereo structural formula of silane
| ImageFileL1    = Silane-3D-balls.png
| ImageClassL1 = bg-transparent
| ImageNameL1    = Ball-and-stick model of silane
| ImageFileR1    = Silane-3D-vdW.png
| ImageClassR1 = bg-transparent
| ImageNameR1    = Spacefill model of silane
| IUPACName      = Silane
| SystematicName = Silicane
| OtherNames     = {{ubl
 | Monosilane
 | Silicon(IV) hydride
 | Silicon tetrahydride
 }}
| Section1       = {{Chembox Identifiers
| CASNo = 7803-62-5
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 5J076063R1
| PubChem = 23953
| ChemSpiderID = 22393
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| UNNumber = 2203
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 29389
| RTECS = VV1400000
| Gmelin = 273
| SMILES = [SiH4]
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/SiH4/h1H4
| InChI = 1/SiH4/h1H4
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = BLRPTPMANUNPDV-UHFFFAOYSA-N
| InChIKey = BLRPTPMANUNPDV-UHFFFAOYAE}}
| Section2       = {{Chembox Properties
| Si=1 | H=4
| Appearance = Colorless gas
| Odor = Repulsive<ref name=PGCH/>
| Density = 1.313{{nbsp}}g/L<ref name=crc>Haynes, p. 4.87</ref>
| MeltingPtC = −185
| MeltingPt_ref=<ref name=crc/>
| BoilingPtC = −111.9
| BoilingPt_ref=<ref name=crc/>
| Solubility = Reacts slowly<ref name=crc/>
| VaporPressure = >1{{nbsp}}atm (20&nbsp;°C)<ref name=PGCH/>
| ConjugateAcid = Silanium (sometimes spelled silonium)
}}
| Section3       = {{Chembox Structure
| MolShape = [[Tetrahedral]] <br/>r(Si-H) = 1.4798{{nbsp}}Å<ref>Haynes, p. 9.29</ref>
| Dipole = 0{{nbsp}}[[Debye|D]]
}}
| Section4       = {{Chembox Thermochemistry
| Thermochemistry_ref = <ref>Haynes, p. 5.14</ref>
| DeltaHf = 34.31{{nbsp}}kJ/mol
| DeltaGfree = 56.91{{nbsp}}kJ/mol
| Entropy = 204.61{{nbsp}}J/mol·K
| HeatCapacity = 42.81{{nbsp}}J/mol·K
}}
| Section7 = {{Chembox Hazards
| ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0564.htm ICSC 0564]
| MainHazards = Extremely flammable, pyrophoric in air
| GHSPictograms = {{GHS02}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|220}} <ref name=LindeSDS>[https://produkte.linde-gas.at/sdb_konform/SiH4_10021753EN.pdf Linde Safety Data Sheet]</ref>
| PPhrases = {{P-phrases|P210|P222|P230|P280|P377|P381|P403|P410+P403}}
| NFPA-H = 1
| NFPA-F = 4
| NFPA-R = 3
| FlashPt = Not applicable, pyrophoric gas
| AutoignitionPt = ~
| AutoignitionPtC = 18
| ExploLimits = 1.37–100%
| PEL = None<ref name=PGCH/>
| IDLH = N.D.<ref name=PGCH>{{PGCH|0556}}</ref>
| REL = TWA 5{{nbsp}}ppm (7{{nbsp}}mg/m<sup>3</sup>)<ref name=PGCH/>
}}
| Section8       = {{Chembox Related
| OtherFunction_label = tetrahydride compounds
| OtherFunction = [[Methane]]<br />[[Germane]]<br />[[Stannane]]<br />[[Plumbane]]
| OtherCompounds = [[Phenylsilane]]<br />[[Vinylsilane]]<br />[[Disilane]]<br />[[Trisilane]]}}
}}

'''Silane''' ('''Silicane''') is an [[inorganic compound]] with [[chemical formula]] {{chem2|SiH4}}. It is a colorless, [[pyrophoric]] [[gas]] with a sharp, repulsive, [[pungent]] smell, somewhat similar to that of [[acetic acid]].<ref>{{Greenwood&Earnshaw2nd}}</ref> Silane is of practical interest as a precursor to elemental [[silicon]]. Silanes with [[alkyl group]]s are effective water repellents for mineral surfaces such as concrete and masonry. Silanes with both [[Organic compound|organic]] and [[Inorganic compound|inorganic]] attachments are used as coupling agents.  They are commonly used to apply coatings to surfaces or as an adhesion promoter.<ref>{{Cite journal |last1=London |first1=Gábor |last2=Carroll |first2=Gregory T. |last3=Feringa |first3=Ben L. |date=2013 |title=Silanization of quartz, silicon and mica surfaces with light-driven molecular motors: construction of surface-bound photo-active nanolayers |url=http://xlink.rsc.org/?DOI=c3ob40276b |journal=Organic & Biomolecular Chemistry |language=en |volume=11 |issue=21 |pages=3477–3483 |doi=10.1039/c3ob40276b |pmid=23592007 |s2cid=33920329 |issn=1477-0520}}</ref>

==Production==

===Commercial-scale routes===
Silane can be produced by several routes.<ref name=Ullmann>{{Ullmann |author=Simmler, W. |title=Silicon Compounds, Inorganic |doi=10.1002/14356007.a24_001}}</ref> Typically, it arises from the reaction of hydrogen chloride with [[magnesium silicide]]:
: {{chem2|Mg2Si + 4 HCl -> 2 MgCl2 + SiH4}}

It is also prepared from metallurgical-grade silicon in a two-step process. First, silicon is treated with [[hydrogen chloride]] at about 300&nbsp;°C to produce [[trichlorosilane]], HSiCl3, along with [[hydrogen]] gas, according to the [[chemical equation]]
: {{chem2|Si + 3 HCl -> HSiCl3 + H2}}

The trichlorosilane is then converted to a mixture of silane and [[silicon tetrachloride]]:
: {{chem2|4 HSiCl3 -> SiH4 + 3 SiCl4}}
This [[redistribution (chemistry)|redistribution reaction]] requires a catalyst.

The most commonly used catalysts for this process are [[metal]] [[halogen|halides]], particularly [[aluminium chloride]]. This is referred to as a redistribution reaction, which is a double displacement involving the same central element. It may also be thought of as a [[disproportionation]] reaction, even though there is no change in the oxidation number for silicon (Si has a nominal oxidation number IV in all three species). However, the utility of the oxidation number concept for a covalent molecule{{Vague|date={{CURRENTMONTHNAME}} {{CURRENTYEAR}}}}, even a polar covalent molecule, is ambiguous.{{Citation needed|date=December 2022}} The silicon atom could be rationalized as having the highest formal oxidation state and partial positive charge in {{chem2|SiCl4}} and the lowest formal oxidation state in {{chem2|SiH4}}, since Cl is far more electronegative than is H.{{Citation needed|date=December 2022}}

An alternative industrial process for the preparation of very high-purity silane, suitable for use in the production of semiconductor-grade silicon, starts with metallurgical-grade silicon, hydrogen, and [[silicon tetrachloride]] and involves a complex series of redistribution reactions (producing byproducts that are recycled in the process) and distillations. The reactions are summarized below:

# {{chem2|Si + 2 H2 + 3 SiCl4 -> 4 SiHCl3}}
# {{chem2|2 SiHCl3 -> SiH2Cl2 + SiCl4}}
# {{chem2|2 SiH2Cl2 -> SiHCl3 + SiH3Cl}}
# {{chem2|2 SiH3Cl -> SiH4 + SiH2Cl2}}

The silane produced by this route can be thermally decomposed to produce high-purity silicon and hydrogen in a single pass.

Still other industrial routes to silane involve reduction of [[silicon tetrafluoride]] ({{chem2|SiF4}}) with [[sodium hydride]] (NaH) or reduction of {{chem2|SiCl4}} with [[lithium aluminium hydride]] ({{chem2|LiAlH4}}).

Another commercial production of silane involves reduction of [[silicon dioxide]] ({{chem2|SiO2}}) under Al and {{chem2|H2}} gas in a mixture of [[NaCl]] and [[aluminum chloride]] ({{chem2|AlCl3}}) at high pressures:<ref>Shriver and Atkins. Inorganic Chemistry (5th edition). W.&nbsp;H. Freeman and Company, New York, 2010, p.&nbsp;358.</ref>

: {{chem2|3 SiO2 + 6 H2 + 4 Al -> 3 SiH4 + 2 Al2O3}}

===Laboratory-scale routes===
In 1857, the German chemists [[Heinrich Buff]] and [[Friedrich Wöhler|Friedrich Woehler]] discovered silane among the products formed by the action of [[hydrochloric acid]] on aluminum silicide, which they had previously prepared. They called the compound ''siliciuretted hydrogen''.<ref>Mellor, J. W. "A Comprehensive Treatise on Inorganic and Theoretical Chemistry", vol.&nbsp;VI, Longmans, Green and Co. (1947), p.&nbsp;216.</ref>

For classroom demonstrations, silane can be produced by heating [[sand]] with [[magnesium]] powder to produce [[magnesium silicide]] ({{chem2|Mg2Si}}), then pouring the mixture into hydrochloric acid. The magnesium silicide reacts with the acid to produce silane gas, which [[pyrophoricity|burns]] on contact with air and produces tiny explosions.<ref name="theodoregray">{{cite web |url=https://theodoregray.com/PeriodicTable/PopularScience/2005/10/1/index.html |title=Making Silicon from Sand |url-status=live |archive-url=https://web.archive.org/web/20101129090339/http://theodoregray.com/PeriodicTable/PopularScience/2005/10/1/index.html |work=[[Popular Science]] |via=Theodore Gray |archive-date=2010-11-29}}.</ref> This may be classified as a [[homogeneity and heterogeneity|heterogeneous]]{{Clarify|date=October 2011}} [[Acid–base reaction|acid–base]] chemical reaction, since the isolated {{chem2|Si(4-)}} ion in the {{chem2|Mg2Si}} [[antifluorite structure]] can serve as a [[Brønsted–Lowry acid–base theory|Brønsted–Lowry base]] capable of accepting four protons. It can be written as

: {{chem2|4 HCl + Mg2Si -> SiH4 + 2 MgCl2}}

In general, the alkaline-earth metals form silicides with the following [[stoichiometry|stoichiometries]]: {{chem2|M^{II}2Si}}, {{chem2|M^{II}Si}}, and {{chem2|M^{II}Si2}}. In all cases, these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include {{chem2|SiH4}} and/or higher molecules in the homologous series {{chem2|Si_{''n''}H_{2''n''+2} }}, a polymeric silicon hydride, or a [[silicic acid]]. Hence, {{chem2|M^{II}Si}} with their zigzag chains of {{chem2|Si(2-)}} anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride {{chem2|(SiH2)_{''x''} }}.

Yet another small-scale route for the production of silane is from the action of [[sodium amalgam]] on [[dichlorosilane]], {{chem2|SiH2Cl2}}, to yield monosilane along with some yellow [[polysilicon hydride|polymerized silicon hydride]] {{chem2|(SiH)_{''x''} }}.<ref>Mellor, J. W. "A Comprehensive Treatise on Inorganic and Theoretical Chemistry", vol.&nbsp;VI. Longmans, Green and Co. (1947), pp.&nbsp;970–971.</ref>

==Properties==
Silane is the [[silicon]] [[analog (chemistry)|analogue]] of [[methane]]. All four {{chem2|Si\sH}} bonds are equal and their length is 147.98 [[Picometre|pm]].<ref name=cccbdb>{{cite journal | url=https://cccbdb.nist.gov/exp2x.asp?casno=7803625&charge=0 | title=Maintenance | journal=NIST | date=17 October 2019 }}</ref> Because of the greater electronegativity of hydrogen in comparison to silicon, this Si–H bond polarity is the opposite of that in the C–H bonds of methane. One consequence of this reversed polarity is the greater tendency of silane to form complexes with transition metals.  A second consequence is that silane is [[pyrophoric]]&nbsp;— it undergoes spontaneous combustion in air, without the need for external ignition.<ref>{{
cite journal
|author1=Emeléus, H. J. |author2=Stewart, K.
 |name-list-style=amp |title = The oxidation of the silicon hydrides
| year = 1935
| journal = Journal of the Chemical Society
| pages = 1182–1189
| doi = 10.1039/JR9350001182}}</ref> However, the difficulties in explaining the available (often contradictory) combustion data are ascribed to the fact that silane itself is stable and that the natural formation of larger silanes during production, as well as the sensitivity of combustion to impurities such as moisture and to the catalytic effects of container surfaces causes its pyrophoricity.<ref>{{
cite journal
| author = Koda, S.
| title = Kinetic Aspects of Oxidation and Combustion of Silane and Related Compounds
| year = 1992
| journal = Progress in Energy and Combustion Science
| volume = 18
| issue = 6
| pages = 513–528
| doi = 10.1016/0360-1285(92)90037-2| bibcode = 1992PECS...18..513K
}}</ref><ref name=timms/> Above {{convert|420|C|F}}, silane decomposes into silicon and [[hydrogen]]; it can therefore be used in the [[chemical vapor deposition]] of silicon.

The Si–H [[bond dissociation energy|bond strength]] is around 384 kJ/mol, which is about 20% weaker than the H–H bond in {{chem2|H2}}.  Consequently, compounds containing Si–H bonds are much more reactive than is {{chem2|H2}}. The strength of the Si–H bond is modestly affected by other substituents: the Si–H bond strengths are: {{chem2|SiHF3}} 419 kJ/mol, {{chem2|SiHCl3}} 382 kJ/mol, and SiHMe<sub>3</sub> 398 kJ/mol.<ref>M. A. Brook "Silicon in Organic, Organometallic, and Polymer Chemistry" 2000, J. Wiley, New York. {{ISBN|0-471-19658-4}}.</ref><ref>{{cite web |url=https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/questions/bndenrgy.htm |title=Standard Bond Energies |publisher=Michigan State University Organic Chemistry}}</ref>

==Applications==
<!-- Please help to remove the usages of the higher silanes -->
[[File:Container 【 28T9 】 SINU 212002(1)---No,1 【 Pictures taken in Japan 】.jpg|thumb|left|Monosilane gas shipping containers in Japan.]]
While diverse applications exist for [[organosilicon chemistry|organosilanes]], silane itself has one dominant application, as a precursor to elemental silicon, particularly in the semiconductor industry. The higher silanes, such as di- and trisilane, are only of academic interest. About 300 [[metric ton]]s per year of silane were consumed in the late 1990s.{{Update inline|date=November 2023}}<ref name=timms>{{
cite journal
| author = Timms, P. L.
| title = The chemistry of volatile waste from silicon wafer processing
| year = 1999
| journal = Journal of the Chemical Society, Dalton Transactions
| issue = 6
| pages = 815–822
| doi = 10.1039/a806743k}}</ref> Low-cost [[solar photovoltaic]] module manufacturing has led to substantial consumption of silane for depositing hydrogenated [[amorphous silicon]] (a-Si:H) on glass and other substrates like metal and plastic. The [[plasma-enhanced chemical vapor deposition]] (PECVD) process is relatively inefficient at materials utilization with approximately 85% of the silane being wasted. To reduce the waste and [[ecological footprint]] of a-Si:H-based solar cells further,  several recycling efforts have been developed.<ref>Briend P, Alban B, Chevrel H, Jahan D. American Air, Liquide Inc. (2009) "Method for Recycling Silane (SiH<sub>4</sub>)". [https://patents.google.com/patent/US20110011129 US20110011129], [https://patents.google.com/patent/EP2252550A2 EP2252550A2] .</ref><ref>{{cite journal |doi=10.1016/j.resconrec.2012.10.002 |title=Life cycle analysis of silane recycling in amorphous silicon-based solar photovoltaic manufacturing |year=2013 |last1=Kreiger |first1=M.A. |last2=Shonnard |first2=D.R. |last3=Pearce |first3=J.M. |journal=Resources, Conservation and Recycling |volume=70 |pages=44–49 |bibcode=2013RCR....70...44K |s2cid=3961031 |url=https://www.academia.edu/2310926 |url-status=live |archive-url=https://web.archive.org/web/20171112225425/http://www.academia.edu/2310926/Life_Cycle_Analysis_of_Silane_Recycling_in_Amorphous_Silicon-Based_Solar_Photovoltaic_Manufacturing |archive-date=2017-11-12}}</ref>

==Safety and precautions==
A number of fatal industrial accidents produced by combustion and detonation of leaked silane in air have been reported.<ref>{{
cite journal
| author = Chen, J. R.
| title = Characteristics of fire and explosion in semiconductor fabrication processes
| year = 2002
| journal = Process Safety Progress
| volume = 21
| issue = 1
| pages = 19–25
| doi = 10.1002/prs.680210106| s2cid = 110162337
}}</ref><ref>{{
cite journal
|author1=Chen, J. R. |author2=Tsai, H. Y. |author3=Chen, S. K. |author4=Pan, H. R. |author5=Hu, S. C. |author6=Shen, C. C. |author7=Kuan, C. M. |author8=Lee, Y. C. |author9=Wu, C. C. |name-list-style=amp |title=Analysis of a silane explosion in a photovoltaic fabrication plant
| year = 2006
| journal = Process Safety Progress
| volume = 25
| issue = 3
| pages = 237–244
| doi = 10.1002/prs.10136|s2cid=111176344 }}</ref><ref>{{
cite journal
|author1=Chang, Y. Y. |author2=Peng, D. J. |author3=Wu, H. C. |author4=Tsaur, C. C. |author5=Shen, C. C. |author6=Tsai, H. Y. |author7=Chen, J. R. |name-list-style=amp |title=Revisiting of a silane explosion in a photovoltaic fabrication plant
| year = 2007
| journal = Process Safety Progress
| volume = 26
| issue = 2
| pages = 155–158
| doi = 10.1002/prs.10194|s2cid=110741985 }}</ref>

Silane is a pyrophoric gas (capable of autoignition at temperatures below {{convert|54|°C|°F|disp=or}}).<ref>[http://www.voltaix.com/images/doc/Mssi000_Silane.pdf Silane MSDS] {{webarchive|url=https://web.archive.org/web/20140519130659/http://www.voltaix.com/images/doc/Mssi000_Silane.pdf |date=2014-05-19}}</ref>
:{{chem2|SiH4 + 2 O2 -> SiO2 + 2 H2O}}{{spaces|5}}<math>\Delta H = -1517 \text{ kJ/mol } = -47.23 \text{ kJ/g}</math>
:{{chem2|SiH4 + O2 -> SiO2 + 2 H2}}
:{{chem2|SiH4 + O2 -> SiH2O + H2O}}
:{{chem2|2 SiH4 + O2 -> 2 SiH2O + 2 H2}}
:{{chem2|SiH2O + O2 -> SiO2 + H2O}}
For lean mixtures a two-stage reaction process has been proposed, which consists of a silane consumption process and a hydrogen oxidation process. The heat of {{chem2|SiO2(s)}} condensation increases the burning velocity due to thermal feedback.<ref>{{
cite journal
| author = V.I Babushok
| title = Numerical Study of Low and High Temperature Silane Combustion
| year = 1998
| journal = The Combustion Institute
| volume = 27
| issue = 2
| pages = 2431–2439
| doi = 10.1016/S0082-0784(98)80095-7
}}</ref>

Diluted silane mixtures with inert gases such as [[nitrogen]] or [[argon]] are even more likely to ignite when leaked into open air, compared to pure silane: even a 1% mixture of silane in pure nitrogen easily ignites when exposed to air.<ref>{{
cite journal
|author1=Kondo, S. |author2=Tokuhashi, K. |author3=Nagai, H. |author4=Iwasaka, M. |author5=Kaise, M. |name-list-style=amp |title=Spontaneous Ignition Limits of Silane and Phosphine
| year = 1995
| journal = Combustion and Flame
| volume = 101
| issue = 1–2
| pages = 170–174
| doi = 10.1016/0010-2180(94)00175-R|bibcode=1995CoFl..101..170K }}</ref>

In Japan, in order to reduce the danger of silane for amorphous silicon solar cell manufacturing, several companies began to dilute silane with [[hydrogen]] gas. This resulted in a symbiotic benefit of making more stable [[solar photovoltaic]] cells as it reduced the [[Staebler–Wronski effect]].{{citation needed|date=April 2022}}

Unlike methane, silane is slightly toxic: the lethal concentration in air for rats ([[median lethal dose|LC<sub>50</sub>]]) is 0.96% (9,600 ppm) over a 4-hour exposure. In addition, contact with eyes may form [[silicic acid]] with resultant irritation.<ref>{{cite web |url=http://www.vngas.com/pdf/g97.pdf |title=MSDS for silane |website=vngas.com |url-status=usurped |archive-url=https://web.archive.org/web/20090220021944/http://www.vngas.com/pdf/g97.pdf |archive-date=2009-02-20}}</ref>

In regards to occupational exposure of silane to workers, the US [[National Institute for Occupational Safety and Health]] has set a [[recommended exposure limit]] of 5 ppm (7&nbsp;mg/m<sup>3</sup>) over an eight-hour time-weighted average.<ref>{{cite web |title=Silicon tetrahydride |work=NIOSH Pocket Guide to Chemical Hazards |publisher=Centers for Disease Control and Prevention |url=https://www.cdc.gov/niosh/npg/npgd0556.html |date=April 4, 2011 |access-date=November 18, 2013 |url-status=live |archive-url=https://web.archive.org/web/20140726044247/http://www.cdc.gov/niosh/npg/npgd0556.html |archive-date=July 26, 2014}}</ref>

==See also==
*[[Binary silicon-hydrogen compounds]] (sometimes called silanes)
*[[Silanization]]
*[[Magnesium silicide]]
*[[Methane]], in which [[carbon]] (in that compound) and [[silicon]] (in this compound) are together in the [[carbon group]].

==References==
{{Reflist|30em}}

==Cited sources==
*{{cite book |editor=Haynes, William M. |year=2011 |title=[[CRC Handbook of Chemistry and Physics]] |edition=92nd |publisher=[[CRC Press]] |isbn=978-1439855119}}

==External links==
*[https://www.freepatentsonline.com/2474087.html US Patent 2474087A, Preparation of silicon halides]

{{Silicon compounds}}
{{Molecules detected in outer space}}
{{Hydrides by group}}
{{Authority control}}

[[Category:Gases]]
[[Category:Industrial gases]]
[[Category:Silanes]]
[[Category:Foul-smelling chemicals]]
[[Category:Pyrophoric materials]]