Silane
Encyclopedia
Silane is a toxic, extremely flammable chemical compound
with chemical formula
Si
H
4. In 1857, the German chemists Heinrich Buff and 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.
at about 300 °C to produce trichlorosilane
, HSiCl3, along with hydrogen
gas, according to the chemical equation
:
The trichlorosilane is then boiled on a resin
ous bed containing a catalyst which promotes the formation of silane and silicon tetrachloride
according to the chemical equation:
The most commonly used catalysts for this process are metal
halides
, particularly aluminium chloride
. This is referred to 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, even a polar covalent molecule, is ambiguous. The silicon atom could be rationalized as having the highest formal oxidation state and partial positive charge in SiCl4 and the lowest in SiH4 since Cl is far more electronegative than is H.
An alternative industrial 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:
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 SiF4 with sodium hydride (NaH) or reduction of SiCl4 with lithium aluminum hydride (LiAlH4).
with magnesium
powder to produce magnesium silicide
(Mg2Si), then pouring the mixture into a 20% dilution in non-aqueous solution of hydrochloric acid. Caution: If silane contacts water, it will react violently. The magnesium silicide reacts with the acid to produce silane gas, which burns on contact with air and produces tiny explosions. This may be classified as a heterogenous
acid-base chemical reaction since the isolated Si4 - ion in the Mg2Si antifluorite structure can serve as a Brønsted–Lowry base capable of accepting four protons. It can be written as:
In general, the alkaline-earth metals form silicides with the following stoichiometries
: MII2Si, MIISi, and MIISi2. 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 SiH4 and/or higher molecules in the homologous series SinH2n+2, a polymeric silicon hydride, or a silicic acid
. Hence, MIISi with their zigzag chains of Si2 - anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH2)x.
Yet another small-scale route for the production of silane is from the action of sodium amalgam
on dichlorosilane, SiH2Cl2, to yield monosilane along with some yellow polymerized silicon hydride
(SiH)x.
analogue
of methane
. Because of the greater electronegativity of hydrogen in comparison to silicon, in silane the hydrogen atoms have a partial negative charge
and the silicon a positive charge
. This Si-H bond polarity is the opposite of that observed in the C-H bonds of methane. However, the C-H bonds in methane are generally regarded as non-polar since carbon is only slightly more electronegative than hydrogen. At room temperature
, silane is a gas, and is pyrophoric — it undergoes spontaneous combustion
in air, without the need for external ignition. 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. Above 420 °C, silane decomposes into silicon and hydrogen
; it can therefore be used in the chemical vapor deposition
of silicon.
Silane has a repulsive smell.
Silane has recently been shown to act as a superconductor under extremely high pressures (96 and 120 GPa), with a transition temperature of 17 K.
s to a polymer
matrix, stabilizing the composite material
. In other words, silane coats the glass fibers to create better adhesion to the polymer chain. They can also be used to couple a bio-inert layer on a titanium
implant
. Other applications include water repellents, masonry
protection, control of graffiti
, applying polycrystalline silicon
layers on silicon wafers when manufacturing semiconductors, and sealants. The semiconductor industry used about 300 metric tons per year of silane in the late 1990s. More recently, a growth in low-cost solar panel manufacturing has led to substantial consumption of silane for depositing amorphous silicon
on glass and other surfaces.
Silane is also used in supersonic combustion ramjet
s to initiate combustion in the compressed air stream. As it can burn using carbon dioxide as an oxidizer it is a candidate fuel for engines operating on Mars
. Since this reaction has some byproducts which are solid (silicon dioxide and carbon) it is applicable only to liquid-fuel rockets
(with liquid carbon dioxide), ramjet
s, or other reaction engine
s.
Silane and similar compounds containing Si—H bonds are used as reducing agents in organic and organometallic chemistry.
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. Unlike methane, silane is fairly toxic: the lethal concentration in air for rats (LC50) is 0.96% (9600 ppm) over a 4-hour exposure. In addition, contact with eyes may form silicic acid
with resultant irritation.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
4. In 1857, the German chemists Heinrich Buff and Friedrich Woehler
Friedrich Wöhler
Friedrich Wöhler was a German chemist, best known for his synthesis of urea, but also the first to isolate several chemical elements.-Biography:He was born in Eschersheim, which belonged to aau...
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.
Commercial-scale routes
Industrially, silane is produced from metallurgical grade silicon in a two-step process. In the first step, powdered silicon is reacted with hydrogen chlorideHydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
at about 300 °C to produce trichlorosilane
Trichlorosilane
Trichlorosilane is a chemical compound containing silicon, hydrogen, and chlorine. At high temperatures, it decomposes to produce silicon, and therefore purified trichlorosilane is the principal source of ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to produce...
, HSiCl3, along with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas, according to the chemical equation
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
:
- Si + 3 HCl → HSiCl3 + H2
The trichlorosilane is then boiled on a resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
ous bed containing a catalyst which promotes the formation of silane and silicon tetrachloride
Silicon tetrachloride
Silicon tetrachloride is the inorganic compound with the formula SiCl4. It is a colourless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications.-Preparation:...
according to the chemical equation:
- 4 HSiCl3 → SiH4 + 3 SiCl4
The most commonly used catalysts for this process are metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
halides
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
, particularly aluminium chloride
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
. This is referred to a redistribution reaction, which is a double displacement involving the same central element. It may also be thought of as a disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
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, even a polar covalent molecule, is ambiguous. The silicon atom could be rationalized as having the highest formal oxidation state and partial positive charge in SiCl4 and the lowest in SiH4 since Cl is far more electronegative than is H.
An alternative industrial 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:
- Si + 2 H2 + 3 SiCl4 → 4 SiHCl3
- 2 SiHCl3 → SiH2Cl2 + SiCl4
- 2 SiH2Cl2 → SiHCl3 + SiH3Cl
- 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 SiF4 with sodium hydride (NaH) or reduction of SiCl4 with lithium aluminum hydride (LiAlH4).
Laboratory-scale routes
For classroom demonstrations, silane can be produced by heating sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
with magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
powder to produce magnesium silicide
Magnesium silicide
Magnesium silicide, Mg2Si, is an inorganic compound consisting of magnesium and silicon. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide, SiO2, found in sand and glass, when heated with magnesium forms magnesium oxide, and, if an excess of magnesium is...
(Mg2Si), then pouring the mixture into a 20% dilution in non-aqueous solution of hydrochloric acid. Caution: If silane contacts water, it will react violently. The magnesium silicide reacts with the acid to produce silane gas, which burns on contact with air and produces tiny explosions. This may be classified as a heterogenous
Homogeneity and heterogeneity
Homogeneity and heterogeneity are concepts relating to the uniformity or lack thereof in a substance. A material that is homogeneous is uniform in composition or character; one that is heterogeneous lacks uniformity in one of these qualities....
acid-base chemical reaction since the isolated Si4 - ion in the Mg2Si antifluorite structure can serve as a Brønsted–Lowry base capable of accepting four protons. It can be written as:
- 4 HCl + Mg2Si → SiH4 + 2 MgCl2
In general, the alkaline-earth metals form silicides with the following stoichiometries
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
: MII2Si, MIISi, and MIISi2. 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 SiH4 and/or higher molecules in the homologous series SinH2n+2, a polymeric silicon hydride, or a silicic acid
Silicic acid
Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
. Hence, MIISi with their zigzag chains of Si2 - anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH2)x.
Yet another small-scale route for the production of silane is from the action of sodium amalgam
Sodium amalgam
Sodium amalgam, commonly denoted Na, is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions involving mercury as a major component. Sodium amalgam is often used in reactions as strong reducing agents with better handling properties compared...
on dichlorosilane, SiH2Cl2, to yield monosilane along with some yellow polymerized silicon hydride
Polysilicon hydride
Polysilicon hydrides x are silicon backbone polymeric solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed and ignite readily in air.-Synthesis:The silanes are much less thermally stable than alkanes and they are...
(SiH)x.
Properties
Silane is the siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
analogue
Analog (chemistry)
In chemistry, a structural analog , also known as chemical analog or simply analog, is a compound having a structure similar to that of another one, but differing from it in respect of a certain component. It can differ in one or more atoms, functional groups, or substructures, which are replaced...
of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. Because of the greater electronegativity of hydrogen in comparison to silicon, in silane the hydrogen atoms have a partial negative charge
Partial charge
A partial charge is a charge with an absolute value of less than one elementary charge unit .-Partial atomic charges:...
and the silicon a positive charge
Partial charge
A partial charge is a charge with an absolute value of less than one elementary charge unit .-Partial atomic charges:...
. This Si-H bond polarity is the opposite of that observed in the C-H bonds of methane. However, the C-H bonds in methane are generally regarded as non-polar since carbon is only slightly more electronegative than hydrogen. At room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, silane is a gas, and is pyrophoric — it undergoes spontaneous combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
in air, without the need for external ignition. 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. Above 420 °C, silane decomposes into silicon and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
; it can therefore be used in the chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
of silicon.
Silane has a repulsive smell.
Silane has recently been shown to act as a superconductor under extremely high pressures (96 and 120 GPa), with a transition temperature of 17 K.
Applications
Several industrial and medical applications exist for silane and functionalized silanes. For instance, silanes are used as coupling agents to adhere glass fiberFiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
s to a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
matrix, stabilizing the composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
. In other words, silane coats the glass fibers to create better adhesion to the polymer chain. They can also be used to couple a bio-inert layer on a titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
implant
Prosthesis
In medicine, a prosthesis, prosthetic, or prosthetic limb is an artificial device extension that replaces a missing body part. It is part of the field of biomechatronics, the science of using mechanical devices with human muscle, skeleton, and nervous systems to assist or enhance motor control...
. Other applications include water repellents, masonry
Masonry
Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...
protection, control of graffiti
Graffiti
Graffiti is the name for images or lettering scratched, scrawled, painted or marked in any manner on property....
, applying polycrystalline silicon
Polycrystalline silicon
Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells....
layers on silicon wafers when manufacturing semiconductors, and sealants. The semiconductor industry used about 300 metric tons per year of silane in the late 1990s. More recently, a growth in low-cost solar panel manufacturing has led to substantial consumption of silane for depositing amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
on glass and other surfaces.
Silane is also used in supersonic combustion ramjet
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
s to initiate combustion in the compressed air stream. As it can burn using carbon dioxide as an oxidizer it is a candidate fuel for engines operating on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. Since this reaction has some byproducts which are solid (silicon dioxide and carbon) it is applicable only to liquid-fuel rockets
Liquid rocket
A liquid-propellant rocket or a liquid rocket is a rocket engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps to pump the propellant from...
(with liquid carbon dioxide), ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s, or other reaction engine
Reaction engine
A reaction engine is an engine or motor which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion...
s.
Silane and similar compounds containing Si—H bonds are used as reducing agents in organic and organometallic chemistry.
Safety and precautions
A number of fatal industrial accidents produced by detonation and combustion of leaked silane in air have been reported. Diluted silane mixtures with inert gases such as nitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
or argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
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. Unlike methane, silane is fairly toxic: the lethal concentration in air for rats (LC50) is 0.96% (9600 ppm) over a 4-hour exposure. In addition, contact with eyes may form silicic acid
Silicic acid
Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
with resultant irritation.