Silicon monoxide

SiO is always present in industrial processes where Si02 or silicates are in contact with reducing agents at high temperatures, or where silicon metal, or silicides react with oxidizing agents such as atmospheric oxygen. In both cases, vaporization takes place and yellow or brown SiO condenses on the colder parts of the system. 

Only the chemistry and structures will be reported here, since applications are too heterogeneous and too great in number for a complete description in this review. 

Results of work on SiO are sometimes confusing and contradictory. One must differentiate between the gaseous and solid states, as each involves a completely different stmcture. 

In this reaction, SiO is formed as a gas and then condenses on the colder parts of the apparatus in form of a brown powder. The properties of the condensate are strongly dependent on the condensation conditions, but the type of Si02 used is not important12 8.  

Other reducing agents (hydrogen or carbon, etc.) give the same results23.  

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Silicon monoxide, SiO. Formed Si02 plus C in electric furnace. The impure brown powder is used as a pigment and abrasive (Monex). Stable in vapour phase (Si plus Si02). 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Silicon Monoxide. SiO, mw 44.09, hard and abrasive, black to brn-black amorph or cubic crysts, mp > 1702°, bp 1880°, d 2.13-.20. Sol in coned aq alk and dil HF+HN03. Prepn is by subliming finely divided silicon at 1250° under high vacuum for 4 hrs. It is used as a coating for precision optical lenses... 

The USA Military Specification (Ref 14) contains the following requirements and criteria (1) a pre-production sample must meet the Spec requirements, (2) silicon monoxide shall be of a high purity grade, (3) chemical compn as detd by the powder—D-C arc semiquantitative technique... 

Silicon is generally considered to be a congener of carbon and this is also reflected in the evolution of silicon as a reducing agent for metal oxides. Silicon forms a fairly stable solid oxide silica or silicon dioxide (Si02) and also a stable gaseous oxide silicon monoxide (SiO), both of which can be useful in oxide reduction reactions. 

An extension of the reduction-chlorination technique described so far, wherein reduction and chlorination occur simultaneously, is a process in which the oxide is first reduced and then chlorinated. This technique is particularly useful for chlorinating minerals which contain silica. The chlorination of silica (Si02) by chlorine, in the presence of carbon, occurs above about 1200 °C. However, the silica present in the silicate minerals readily undergoes chlorination at 800 °C. This reaction is undesirable because large amounts of chlorine are wasted to remove silica as silicon tetrachloride. Silica is, therefore, removed by other methods, as described below, before chlorination. Zircon, a typical silicate mineral, is heated with carbon in an electric furnace to form crude zirconium carbide or carbonitride. During this treatment, the silicon in the mineral escapes as the volatile oxide, silicon monoxide. This vapor, on contact with air, oxidizes to silica, which collects as a fine powder in the furnace off-gas handling system ... 

Elemental silicon is relatively stable in most substances at ordinary temperatures. Silicon shows similarity with other elements of its group, especially with germanium in many chemical properties. It forms tetravalent compounds with tetrahedral geometry almost exclusively. However, only in silicon monoxide, SiO, is its valence +2. Also, unlike carbon, silicon does not form unsaturated double or triple bond compounds. Silicon dissolves in germanium... 

The metal is most often recovered from its principal ore, zircon. The ore is mined, crushed and preliminary segregation is by gravity, electrostatic, and magnetic separation. Separated ore mixed with carbon is charged into an arc furnace and heated to about 3,500°C. This forms zirconium carbide and silicon monoxide, and the monoxide is driven off as vapor. Zirconium carbide is then placed in a chlorinator and heated with chlorine gas at high temperatures. The carbide is converted to zirconium tetrachloride, ZrCfl. Also, small amounts of hafnium that is always associated with zirconium converts to its tetrachloride, HfCfl. 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Two principal mechanisms that may be responsible for mass loss from red giants are considered shock wave-driven winds and radiatively (dust)-dr iven winds. Effect of the periodic shocks accompanying nonlinear oscillations of red giants is most prominent in the outer layers of the stellar atmosphere where shocks are able not only to expel gas but also increase gas density so that some molecular components become supersaturated. In 0-rich stars the most abundant condensible species are silicon monoxide and iron, whereas in C-rich stars these are carbon, silicon carbide and iron. 

Device materials again may be conductive, semiconductive, dielectric, or resistive. Conductors are typically gold or aluminum, and resistors, silicon monoxide or silicon nitride. Tantalum nitride and nickel chromium are common resistor materials. 

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