Compounds of silicon free#

When they come into contact with water they are decomposed (hydrolysis), giving off hydrogen chloride. Both tetrachlorosilane and trichlorosilane, however, are highly reactive compounds that can emit a highly irritant asphyxiating vapour. If chlorine, nitrogen and so on are added, however, they can be used for chemical synthesis. Most of them are very stable, oily substances which in themselves find but little practical application. There are three types of silicon compounds considered here: silanes, siloxanes and heterosiloxanes. Hazards may arise from the chemicals which are used in conjunction with silicon in various manufacturing processes. When silicon is prepared and purified in the absence of air, the process takes place in a sealed, gas-tight enclosure which should limit exposures. For most regulatory purposes it is considered a nuisance dust. The toxicological hazards of metallic silicon are not known. The effects of silicon carbides are also discussed elsewhere. The reader is referred elsewhere in the Encyclopaedia for discussions of the important health effects of exposure to silicates, particularly crystalline silicates. This section discusses the hazards of organosilicon compounds. Ethyl silicate is used as a binding agent for making moulds in special metal-founding processes or as a starting point in chemical synthesis. When it is decomposed in water, a transparent layer of silicic acid results, which secures the screen to the glass wall. Methyl silicate is a fairly volatile liquid used in the manufacture of television screens. Silicon resins are used in paints, varnishes, moulding compounds (plastics), electrical insulation, pressure-sensitive and release coatings, and laminates. Silicon resins have numerous applications because of their heat stability and water repellency, and their resistance to solvents, high temperatures and sunlight. Other resins are used as so-called foils (thin layers applied in the electronics industry) because of their low combustibility and good electrical insulating properties even in a damp environment. Among other things, these resins are also used as moulding compounds (plastics), and in the manufacture of foams which display good resistance to high temperatures and are useful thermal insulators. In addition they are fairly resistant to momentary heating or attack by oxygen, and do not fade much in sunlight. Resins are organosilicon compounds which, when mixed with a number of other substances used in the paint industry (hardeners, accelerators and so on), form very stable layers and are readily applicable even on bases to which other paints generally do not adhere well (such as metal surfaces). Organosilicon compounds are used in the form of resins, elastomers (rubbery compounds) or oils. It is also widely used in semiconductor manufacture and in the production of silanes and organosilicon compounds. It is used as an alloying agent for steel, aluminium, copper, bronze and iron.

Siloxanes exist in the form of resins, elastomers (rubbery compounds) or oils.

Starting from small molecules, they can be built up into large units (polymers), to which various properties (liquidity, elasticity, stability and so on) can be imparted. Siloxanes are compounds which contain oxygen in addition to hydrogen, silicon and, usually, carbon (although there are some inorganic siloxanes). This is shaped into rod form, and the last impurities are “floated” out from the rod by successively heating small portions of it to melting point, in an atmosphere of inert gas, such as argon, compounded with any trace elements to be added, which become dissolved in the liquid silicon. If this liquid is heated together with hydrogen, pure silicon is released. During this process the volatile compound SiCl 4 occurs and is separated by distillation. Pure silicon is prepared by heating raw silicon in chlorine. This grade is sufficiently pure for incorporation in alloys-for example, of aluminium and iron-in order to make them harder or less brittle. One method of preparation is by heating quartz (SiO 2) with carbon during this process carbon monoxide is emitted and raw silicon (98% pure) remains.

Compounds of silicon free#

It does not occur free in nature, but as an oxide (silica) or silicate (feldspar, kaolinite and so on) in sand, rock and clay. After oxygen, silicon is the element most frequently found on earth.