Silicon tetrahydride

Description

Silane is a colorless, spontaneously flammable (pyrophoric) gas. It has a choking odor and may form explosive mixtures with air. Silane will react violently with heavy metal halides and free halogens other than hydrogen chloride.

Chemical properties

Silane is a colorless, pyrophoric gas with a repulsive odor.

Chemical properties

Silicon tetrahydride is a highly toxic gas with a repulsive odor. It is spontaneously flammable in air. Trichlorosilane has an acrid odor, fumes in air and supports combustion. It is a lachrymator and is moisture sensitive. Octamethyl tetrasilane is moisture sensitive.

The Uses of Silicon tetrahydride

Source of hyperpure silicon for semiconductors.

The Uses of Silicon tetrahydride

It is used for doping of solid-state devicesand for preparing semiconducting silicon forthe electronic industry.

The Uses of Silicon tetrahydride

Silane, SiH4 is a colorless gas that is spontaneously flammable in air and slowly decomposed by water; in the presence of aqueous alkali it is completely hydrolyzed to form hydrogen and silicates. It is manufactured on a commercial scale and sold as a compressed gas in cylinders. Silane, pure or doped, is used to prepare semiconducting silicon by thermal decomposition at >600 °C. Gaseous dopants such as germane, arsine, or diborane may be added to the silane at very low concentrations in the epitaxial growing of semiconducting silicon for the electronics industry. Higher silanes, eg, Si2H6 and Si3H8, are known but are less stable than SiH4. These are analogues of lower saturated hydrocarbons.

Definition

ChEBI: The simplest silane, consisting of a single silicon atom carrying four hydrogens.

General Description

Silicon tetrahydride is a colorless, flammable and poisonous gas, with a strong repulsive odor. Silicon tetrahydride is easily ignited in air, reacts with oxidizing agents, is very toxic by inhalation, and is a strong irritant to skin, eyes and mucous membranes. Silicon tetrahydride is lighter than air. Under prolonged exposure to fire or heat the containers may rupture violently and rocket. Silicon tetrahydride is used in the production of amorphous silicon.

Air & Water Reactions

Highly flammable. Pure state ignites in air, the first seven to eight of the series 6H2n+1 ignite spontaneously in air at room temperature or slightly elevated temperatures [Mellor 1:376 1946-47]. Slowly reacts with water to form silicon hydroxides and hydrogen gas.

Reactivity Profile

Slowly reacts with water to form silicon hydroxides and hydrogen gas [Hydrides of the Elements of Main Groups I - IV 1971 p.513]. Silicon tetrahydride burns in contact with bromine, chlorine or covalent chlorides (carbonyl chloride, antimony pentachloride, tin(IV) chloride). Even traces of the free halogens may cause violent explosions, when handling Silicon tetrahydride, extreme caution should be taken, [Mellor, 1940, Vol. 6, 220]. Mixtures of Silicon tetrahydride and nitrogen oxides, nitrous oxide, detonate very easily, [Chem. Abs., 1990, 112, 121711].

Hazard

Dangerous fire risk, ignites spontaneously in air. Strong irritant to tissue, skin and upper res- piratory tract.

Health Hazard

Vapors may cause dizziness or asphyxiation without warning. Some may be toxic if inhaled at high concentrations. Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite. Fire may produce irritating and/or toxic gases.

Health Hazard

Very little information has been published onthe toxicology of this compound. The acutetoxicity of silane is much less than that ofgermane. Among the hydrides of group IVBelements, the toxicity of silane falls betweenthe nontoxic methane and moderately toxicgermane. Inhalation of the gas can causerespiratory tract irritation. A 4-hour exposureto a concentration of about 10,000 ppm in airproved fatal to rats.

Fire Hazard

EXTREMELY FLAMMABLE. Will be easily ignited by heat, sparks or flames. Will form explosive mixtures with air. Silicon tetrahydride will ignite spontaneously in air. Vapors from liquefied gas are initially heavier than air and spread along ground. Vapors may travel to source of ignition and flash back. Cylinders exposed to fire may vent and release flammable gas through pressure relief devices. Containers may explode when heated. Ruptured cylinders may rocket.

Flammability and Explosibility

Pyrophoric

Contact allergens

Various silane derivatives are used as bonding agents between glass and the resin used as a coating agent of glass filaments. Organosilanes have been implicated as sensitizers in workers at a glass filament manufactory.

Materials Uses

Piping and equipment for silane service may be of steel or stainless steel construction. Piping and equipment must be designed to withstand the pressures involved. Extreme care must be taken to avoid the contact ofsilane with materials containing heavy-metal halides or free halogens. Silane will react violently or explosively with these compounds.
Silane systems should be purged of air to prevent silane ignition and contamination with silicon dioxide. All systems to handle silane should be designed with the following factors in mind:
? Prevention of leakage, both in and out, under vacuum and pressure
? Minimum necessary internal volume
? Elimination of dead spaces
? Isolation ofsystem components in case of a leak, rupture, or other failure
? Ability to easily evacuate and purge the system and components with inert gas
? That silane should never be purged through a vacuum pump
? The use offittings preferably ofthe welded face seal gasketed type to minimize the likelihood ofleaks
? Use of diaphragm packless valves with resilient seats such as Kel-F
? The removal of backplates from gauges and rotometers where gases may collect upon leakage to allow pressure venting away from personnel if an explosion takes place
? The use ofmetal diaphragm regulators to minimize air diffusion leakage Consideration ofthe use offlow restrictions in cylinder valves.

Safety Profile

Mildly toxic by inhalation. Silanes are irritating to skin, eyes, and mucous membranes. Easily ipted in air. Explosive reaction or ignition on contact with halogens or covalent halides (e.g., bromine, chlorine, carbonyl chloride, antimony pentachloride, tin(Iv> chloride). Ignites in oxygen. Can react with oxidizers. It may self-explode. When heated to decomposition it burns or explodes.

Potential Exposure

Silane is used as a doping agent for solid-state devices; as a source of silicon for semiconductors; and in the production of amorphous silicon.

Physiological effects

The major hazards ofsilane stem from its ability to combust spontaneously and its irritating properties. Silane reacts with water to form silicic acid and therefore can cause irritation ofthe eyes, mucous membranes, and respiratory tract. Inhalation may result in headache, nausea, and irritation of the upper respiratory tract. The offensive odors of silane should be taken as a warning signal for the presence of dangerous concentrations.
ACGIH recommends a Threshold Limit Value-Time-Weighted Average (TLV-TWA) of 5 ppm (6.6 mg/m3) for silane. The TLV-TWA is the time-weighted average concentration for a normal 8-hour workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Environmental Fate

Silane is a colorless transparent gas, heavier than air. It is odorless when greatly diluted, but extremely noxious when concentrated, with a sharp, repulsive smell, somewhat similar to that of acetic acid. Its silicon–hydrogen bond is much weaker, and accordingly more reactive, than a carbon– hydrogen single bond. It is not only inflammable in air at concentrations between 1% and 96% v/v, producing particulate silicon oxides and hydrogen gas, but is pyrophoric. Above 420°C silane decomposes, releasing hydrogen and depositing high-purity silicon, which leads to some of its principal applications. It also reacts avidly with oxidants; for example, contact with even trace amounts of free halogens may cause violent explosions. It is insoluble in water, and does not react with water under normal conditions, but in the presence of even minute traces of hydroxyl ion reacts to evolve hydrogen gas and form corrosive silicic acid or hydrated silicon dioxide. The reaction with water is further accelerated by organic or inorganic bases. Silane is fully biodegradable, and will not bioaccumulate.

storage

With the proper precautions, silane and silane mixtures can be handled safely to avoid contact with air at all times. Almost all recommendations are directed at not violating this one rule. Specific precautions that should be observed in handling silane are as follows:
? Before using silane, read all equipment instructions, cylinder labels, data sheets, and other associated information pertaining to silane and its use.
? Handle silane in a well-ventilated area while avoiding the presence of combustible materials.
? Store silane at positive pressures.
? Do not condense silane by avoiding temperatures of-148°F (-100°C) or less.
Condensation ofsilane runs the risk of leaks developing with subsequent sucking back of air into the system, forming explosive mixtures.
? Do not use silane in conjunction with heavy-metal halides or free halogens, with which it will react violently. Care should be taken that all components of any silane-handling system are purged offree halogens, which might exist from degreasing agents or chlorinated hydrocarbons.
? Evacuate and thoroughly pressure check all systems, preferably with helium, for leaks at pressures two to three times the anticipated working pressure. However, do not exceed the system design pressure. In addition, a regular leak test procedure and testing schedule should be instituted and followed as part of normal preventive maintenance.
? Ground all equipment and lines using silane.
? Use an alternate vacuum and inert gas purge ofthe system to purge all air out of the system after it has been leak checked or opened for any reason.
? When pressurizing equipment with silane or a silane mixture, open the cylinder valve slowly. All other equipment adjustments of regulators, needle valves, etc. should also be made slowly.
? Before disconnecting any system that has had silane in it, thoroughly purge the system ofsilane with an inert gas. Any portion of a system that is dead-ended or allows "pocketing" ofsilane should be treated by considerable purging, on the order often times the trapped volume.
? Vent silane or silane mixtures through small diameter pipe or tubing, ending under a shallow water seal to prevent back diffusion ofair. Venting should be to an area designed for silane disposal, preferably by burning. Concentrations even in the low percentage range are dangerous and should not be exposed directly to air. Silane can also be vented by diluting with inert gas to prevent ignition upon discharge to the atmosphere. In addition, all precautions necessary for the safe handling of any toxic flammable gas must be observed.

Shipping

UN2203 Silane, Hazard Class: 2.1; Labels: 2.1-Flammable gas. Cylinders must be transported in a secure upright position, in a well-ventilated truck. Protect cylinder and labels from physical damage. The owner of the compressed gas cylinder is the only entity allowed by federal law (49CFR) to transport and refill them. It is a violation of transportation regulations to refill compressed gas cylinders without the express written permission of the owner.

Incompatibilities

An extremely flammable gas. Forms explosive mixture with air; may spontaneously ignite in air. A strong reducing agent. Reacts slowly with water forming silicon hydroxides and flammable hydrogen gas. Reacts with oxidizing agents; halogens , potassium hydroxide solution. Explodes in oxygen. Decomposes on heating or on burning producing silicon and hydrogen.

Waste Disposal

Return refillable compressed gas cylinders to supplier. Dispose by controlled burning or seal cylinders and return to suppliers.

GRADES AVAILABLE

Silane is usually sold in very high purity grades primarily for use in the electronics industry.