Silicon tetrachloride impurities

Sometimes it is possible to obtain some degree of elimination of impurities by controlling the condensing conditions, particularly if the chlorides of the impurities are distinctly more, or less, volatile than that of the product. This is commonly achieved in the case of silicon tetrachloride impurity, which is allowed to pass through the condenser by running the latter at a rather higher temperature than would otherwise be desirable. Ferric chloride aluminium chloride and vanadium oxychloride may be removed in a similar manner. 

Attempts by Kao and others to enhance transparency by chemically removing impurities from glass met with little success the level of purity required was indeed comparable with that needed in silicon for integrated circuits. In the event, the required purification was achieved in the same way in which semiconductor-grade silicon is now manufactured, by going through the gas phase (silicon tetrachloride), which can be separated from the halides of impurity species because of dilTerences in vapour pressures. This breakthrough was achieved by R.D. Maurer and his... 

The vapor pressure at 0° (43 mm.) is generally a sufficient criterion for purity. (The checkers also used infrared spectroscopy and noted a trace of impurity of silicon tetrachloride.) Elemental analysis is readily accomplished by heating a weighed sample at 160° for 18 hours with a slight excess of water in a sealed glass tube. A 15 to 20% excess of water over that required by the equation ... 

Thus, the direct synthesis of phenylchlorosilanes produces a complex mixture, which, apart from phenyltrichlorosilane, diphenyldichlorosilane, phenyldichlorosilane and triphenylchlorosilane, also contains silicon tetrachloride, trichlorosilane, benzene, solid products (diphenyl and carbon) and a gaseous product (hydrogen). It also forms high-boiling polyolefines, which are part of tank residue and can deposit on contact mass, reducing its activity. It should be kept in mind that the production of phenylchlorosilanes requires silicon with a minimal impurity of aluminum, because the aluminum chloride formed contributes to the detachment of the phenyl group from phenylchlorosilanes at higher temperature. The harmful effect of aluminum chloride is counteracted by the addition of metal salts to contact mass, which form a nonvolatile and nonreactive complex with aluminum chloride. 

Summary The synthesis of trichlorosilane (TCS) from silicon and HCl produces considerable amounts of less desired chlorosilanes by side reactions, especially silicon tetrachloride (STC). The results of this paper support the view that the undesired STC is formed from TCS in a consecutive reaction, which is probably catalyzed by Si impurities and which is preferred at low space velocity and high temperatures. It seems that TCS selectivity losses are due to regions or spots in the industrial reactor with such conditions. XPS surface concentrations of Si impurities dramatically change with the proceeding synthesis reaction because of the mobility of the impurity species and do not correlate with results of Si bulk analysis. 

As a gas-liquid reaction, it has been showed that the reaction between liquid silicon tetrachloride (SiCLi) and NH3 gas in dry hexane at 0 °C can be used to prepare fine Si3N4 powder with very low levels of metallic impurities (<0.03 wt%)... 

This is normally a slow process, carried out on a 60 lb and a 200 lb scale, over a period of several days, the progress being followed by determination of the loss in weight owing to evolution of chlorine. However, the rate might be considerably increased by the use of a catalyst. A final distillation from the same vessel removes the vanadium oxytrichloride and other volatile impurities such as silicon tetrachloride (b.p. S6°C) and titanium tetrachloride (b.p. 136°C), which may have remained with the product up to this point. This is performed firstly at atmospheric pressure and finally under reduced pressure. The quantity of oxytrichloride removed may be as much as 20 per cent of the total volume, and represents a loss of vanadium. 

For the manufacture of solid-state devices, it is necessary to start with extremely pure silicon (no more than 10 % impurities). You first convert the impure element to silicon tetrachloride. Si, which is a low-boiling liquid (b.p. 58°C) that can be purified by distillation. 

Silicon is a prominent material of technology. The element is the basic material used in semiconductor devices for solid-state electronics. These devices require silicon of extreme purity. The impure element is converted to silicon tetrachloride, which is purified by distillation, after which the silicon tetrachloride is reduced to pure silicon. Silica is chemically known as silicon dioxide quartz is a common form of silica. It exhibits the piezoelectric ect, which is used to control radio, television, and clock frequencies. Silicates are formed by condensation reac-... 

Brazhnikov and Sakodynskii determined the retention time of boron trichloride on various supports and stationary phases and compared its behaviour to that of boronalkyls, Zelyaev et al used a molybdenum glass column packed with poly (methylsiloxane) on Spherchrom 1 to separate in amounts down to O.lppm of various trace impurities in boron trichloride, such as chlorine, phosgene, silicon tetrachloride, chloroform, carbon tetrachloride and dichloroethane. They used a column temperature of 60°C with nitrogen as carrier gas and employed thermionic and flame ionization detectors,... 

Impurities such as methane, ethane, propane, isobutane, butane, chloromethane, chloroethane, hydrogen chloride, silicon tetrachloride and methyltrichlorosilane can be determined in trichlorosilane by separation on a glass column (4.5 metres x 4mm) packed with silanized silica gel operated at 50 C using nitrogen as carrier gas and flame ionization or thermal conductivity detectors. 

Burson and Kenner determined the purity of trichlorosilane and silicon tetrachloride with the SF-96 column. DC-LSX-3-0295 tri-fluoropropyl silicone gum was found to be the best for analysing samples of the methylchlorosilanes. Figure 57 shows a chromatogram of a sample of methyltrichlorosilane containing as impurities 0.02% silicon tetrachloride, 0.03% methyldichlorosilane, 0.04% trimethyl-chlorosilane, 0.12% dimethyldichlorosilane, and 0.07% 1,1,3,3,-tetra-chloro-1,3-dimethyldisiloxane. The concentrations of these impurities were determined by comparison of peak areas with standards prepared by adding known amounts of these impurities to methyltrichlorosilane of 99.99% purity. 

The tlrree impurities, iron, silicon and aluminium are present in the metal produced by the Kroll reduction of zirconium tetrachloride by magnesium to the extent of about 1100 ppm. After dre iodide refining process tire levels of these impurities are 350, 130 aird 700ppm respectively. The relative stabilities of the iodides of these metals compared to that of zirconium can be calculated from the exchange reactions... 

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