Silicon, Its Compounds and Alloys. Fire Safety

Проведен обзор и анализ литературных данных о физико-химических и пожароопасных свойствах кремния, его основных соединений и сплавов, а также об областях их использования. Показано, что кусковой кремний – трудногорючее вещество, не способное к самостоятельному горению. Мелкодисперсный кремний образует с воздухом взрывоопасные смеси. Из соединений кремния наиболее пожароопасны силаны и кремнийорганические соединения, а также сплавы с щелочно-земельными и переходными металлами. Silicon is a strategic raw material in every sense. Without it, it is impossible to produce silumines, the main material used in the assembly of engines and components for armoured vehicles, aircraft and other military vehicles. Silicon is also the main material in microelectronics and semiconductor devices; it is used in the manufacture of glasses transparent to IR radiation. Silicon is a component of iron and non-ferrous metal alloys (in low concentrations, silicon increases the corrosion resistance and mechanical strength of alloys, improving their casting properties; in high concentrations, it can cause brittleness); iron, copper and aluminium alloys containing silicon are the most important. Silicon is used as a starting material for the production of organosilicon compounds and silicides. Both crystalline and amorphous silicon are highly inert. They do not interact with strong oxidising agents (except fluorine), nor with strong reducing agents. A large class of silicon compounds are the organosilicon compounds (OSC). Almost all CBSs are colourless liquids, only some, such as cyclosiloxanes (R2SiO)3, are solid crystalline substances. CBS are usually thermally stable (up to ~ 600 °C), distillable at atmospheric pressure and in vacuum; soluble in hydrocarbons, chlorinated hydrocarbons, esters and other organic solvents; do not mix with water. KOS are easily hydrolysed, especially organochlorosilanes (smoke in air). Silicon hydrides (silanes) have the general formula SinH2n+2. Silanes up to octasilane Si8H18 have been obtained. The first two silicon compounds – monosilane SiH4 and disilane Si2H6 – are gaseous at room temperature, the rest are volatile liquids. All silicon compounds have an unpleasant odor and are poisonous. Silanes are much less stable than alkanes, self-ignite in air, decompose with hydrogen release by water. Silanes are not found in nature. The most practically important compound of this class is monosilane. Physical properties of monosilane: molecular weight 32.1 g/mol, melting point minus 185 °C, boiling point minus 111.9 °C, density 1.31 kg/m3, specific heat of combustion 42.5 MJ/kg. Silicon compounds with halogens are known of the following types (X-halogen): SiX4, SiHnX4-n (halogen silanes), SinX2n+2 and mixed silicon halides, for example, SiClBr3. Under normal conditions, SiF4 is a gas, SiCl4 and SiBr4are liquids (tpl 5 °C), SiI4 is a solid (tpl124 °C). SiX4 compounds are easily hydrolysed. Among silicon alloys, the most important are: alloys with iron, aluminium, magnesium, calcium, barium. All alloys are explosive in the dispersed state. Silicon carbide is a unique compound. This material is one of the most radiation resistant semiconductors, promising for use in extreme conditions of increased radiation, temperature and chemical activity. It is a non-combustible material. Silicon nitride Si3N4 is a non–combustible grey substance with a density of 3170 kg/m3. Oxidation of silicon nitride in air begins above 900 °C. It is intensively sublimated with decomposition above 1600 °C. When heated to decomposition, silicon nitride my release toxic gases. Aero-suspension of silicon powder with a dispersity of less than 50 μm is ignited only by burning a mixture of aluminium and barium peroxide powders in a ratio of 1:10 at a combustion temperature of about 3000 °C. The lower concentration limit of flame spread (LCL) is 137 g/m3. The maximum explosion pressure in a 4 litre container reaches 450 kPa. The authors did not succeed in igniting the air suspension of silicon powder of a fraction of 50–63 microns with the specified pyrotechnic composition with a concentration of up to 1300 g/m3. However, if a small amount of hydrogen is introduced into the dust cloud (below the NCPRP), the cloud begins to spread flames even at lower concentrations of silicon. It has been found that an air suspension of silicon powder with a dispersion of less than 100 microns, with a CNRP of 210 g/m3, in the presence of 1 % by volume hydrogen is capable of spreading a flame at a concentration of 152 g/m3, and at a hydrogen content of 2 % by volume – at 115 g/m3. Fine silicon powder is able to burn in a nitrogen environment under certain conditions. It should be noted that when burning silicon in nitrogen, the combustion temperature is limited by the decomposition temperature of the combustion product – silicon nitride in the whole range of nitrogen pressures (from 6 MPa to 200 MPa). Burning of silicon powder in air or nitrogen does not occur under normal conditions due to the lack of oxidant. High pressures of the reaction gas are required for such a reaction to take place. Following this statement, it can be assumed that combustion of silicon in chlorine and fluorine can be expected under conditions of increased gas pressure.