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Silicon hydride

SiH4

CAS:7803-62-5 

   Silane (or monosilane) is a toxic, extremely flammable chemical compound with chemical formula SiH4. 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.

Production
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 chloride at about 300°C to produce trichlorosilane, HSiCl3, along with hydrogen gas, according to the chemical equation:

Si + 3 HCl → HSiCl3 + H2

    The trichlorosilane is then boiled on a resinous bed containing a catalyst which promotes the formation of silane and silicon tetrachloride according to the chemical equation:

4 HSiCl3 → SiH4 + 3 SiCl4

    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 tetravalent IV oxidation number 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 sand 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 an 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: 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.

Properties:
    Silane is the silicon 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 partial 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. Confusingly, there was briefly an EE Times article that grossly exaggerated this achievement and claimed that room-temperature superconductivity had been achieved.

Applications:
   Several industrial and medical applications exist for silane and functionalized silanes. For instance, silanes are used as coupling agents to adhere glass fibers 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 ramjets 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), ramjets, or other reaction engines.

    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 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. Unlike methane, silane is fairly toxic: the lethal concentration in air for rats (LC50) is 0.96% (9,600ppm) over a 4-hour exposure. In addition, contact with eyes may form silicic acid with resultant irritation.



SILANE ICSC: 0564
Date of Peer Review: July 1997
Monosilane 
Silicon tetrahydride 
Silicane 
CAS # 7803-62-5 SiH4
RTECS # VV1400000 Molecular mass: 32.1
UN # 2203
EC/EINECS # 232-263-4
TYPES OF HAZARD / EXPOSURE ACUTE HAZARDS / SYMPTOMS PREVENTION FIRST AID / FIRE FIGHTING
FIRE Extremely flammable. 
NO open flames, NO sparks, and NO smoking. 
Shut off supply; if not possible and no risk to surroundings, let the fire burn itself out; in other cases extinguish with powder, carbon dioxide. 
EXPLOSION Gas/air mixtures are explosive. 
Closed system, ventilation, explosion-proof electrical equipment and lighting. 
Combat fire from a sheltered position. 
EXPOSURE
STRICT HYGIENE! 

Inhalation Cough. Headache. Nausea. Sore throat. 
Ventilation, local exhaust, or breathing protection. 
Fresh air, rest. Refer for medical attention. 
Skin Redness. ON CONTACT WITH LIQUID: FROSTBITE. 
Cold-insulating gloves. 
ON FROSTBITE: rinse with plenty of water, do NOT remove clothes. Rinse skin with plenty of water or shower. 
Eyes Redness. Pain. 
Safety goggles, or eye protection in combination with breathing protection. 
First rinse with plenty of water for several minutes (remove contact lenses if easily possible), then take to a doctor. 
Ingestion


SPILLAGE DISPOSAL PACKAGING & LABELLING
Evacuate danger area! Consult an expert! Ventilation. Remove gas with fine water spray. (Extra personal protection: self-contained breathing apparatus). 

EMERGENCY RESPONSE STORAGE
NFPA Code: H2; F4; R3; 
Fireproof. 
IPCS 
International 
Programme on 
Chemical Safety
Prepared in the context of cooperation between the International Programme on Chemical Safety and the Commission of the European Communities � IPCS, CEC 2005 

SEE IMPORTANT INFORMATION ON BACK
SILANE ICSC: 0564
IMPORTANT DATA
PHYSICAL STATE; APPEARANCE: 
COLOURLESS GAS , WITH CHARACTERISTIC ODOUR.

PHYSICAL DANGERS: 
The gas is heavier than air.

CHEMICAL DANGERS: 
The substance may spontaneously ignite on contact with air. The substance decomposes on heating or on burning producing silicon and hydrogen , causing fire and explosion hazard. The substance is a strong reducing agent and reacts with oxidants. Reacts slowly with water. Reacts with potassium hydroxide solution and halogens.

OCCUPATIONAL EXPOSURE LIMITS: 
TLV (as TWA): 5 ppm; 6.6 mg/m� (ACGIH 1996).
MAK not established.
ROUTES OF EXPOSURE: 
The substance can be absorbed into the body by inhalation.

INHALATION RISK: 
A harmful concentration of this gas in the air will be reached very quickly on loss of containment.

EFFECTS OF SHORT-TERM EXPOSURE: 
The substance irritates the eyes, the skin and the respiratory tract. Rapid evaporation of the liquid may cause frostbite.

PHYSICAL PROPERTIES
Boiling point: -112癈
Melting point: -185癈
Solubility in water: slow reaction 
Relative vapour density (air = 1): 1.3
Explosive limits, vol% in air: 1.37-100
ENVIRONMENTAL DATA

NOTES

ADDITIONAL INFORMATION


LEGAL NOTICE Neither the CEC nor the IPCS nor any person acting on behalf of the CEC or the IPCS is responsible for the use which might be made of this information
� IPCS, CEC 2005
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