Metallurgical-grade silicon

Different techniques are currently used for the production of large scale electronic grade silicon. Metallurgical grade silicon powder at 300°C in the presence of a catalyst is reacted with anhydrous HC1 ... 

FIGURE 4.4 The production of polycrystalline silicon for the eleetronics industry involves several ehemieal steps aimed at the reduetion of impurities. These inelude (1) reaction of metallurgical grade silicon to produce a mixture of chlorosilanes, (2) distillation of trichlorosilane, and (3) reduction of trichlorosilane to polycrystalline silicon. Excerpted by special permission from Chemical Engineering, June 10, 1985. Copyright 1985 by McGraw-Hill, Inc., New York, NY 10020. 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

Purified polycrystalline CVD silicon from this reaction is then melted and a single-crystal boule weighing as much or more than 50 kg, and having a diameter up to 20 cm, is pulled from the melt by Czochralski growth (8). Metallurgical-grade silicon is not sufficiendy pure for applications in electronics... 

S. Bade, U. Hoffmann, K. Schonert, Mechano-chemical reaction of metallurgical grade silicon with gaseous hydrogen chloride in a vibration mill, Int. J. Miner. Process, 1996, 44-AS, 167. 

The purity of silicon in this first step is only ca. 98%, and is referred to as metallurgical grade silicon (MG-Si). In order for the silicon to be used for electronics applications, additional steps are necessary to decrease the number of impurities. Reaction of MG-Si with hydrogen chloride gas at a moderate temperature converts the silicon to trichlorosilane gas (Eq. 3). When SiHCl3 is heated to a temperature of ca. 1,150°C, it decomposes into high-purity silicon and gaseous by-products (Eq.4). This reaction is typically performed in a bell-shaped Siemens-type reactor, where Si is deposited onto heated electrodes. 

P. Menna, Y. S. Tsuo, M. M. Al-Jassim, S. E. Asher, F. J. Pern, and T. F. Ciszek, Light emitting porous silicon from cast metallurgical-grade silicon, J. Electrochem. Soc. 144, LI 15, 1997. 

Manufacture of metallurgical grade silicon by reduction of quartzite with coke in an electric-arc furnace... 

Metallurgical grade silicon is marketed in a coarsely crushed form or as a finely ground powder in different particle sizes. Powders with increased purity due to acid washing, particularly for the removal of metallic impurities, are specialty products. They are utilized, for example, in the manufacture of silicon nitride powder or reaction-bonded silicon nitride ceramic components and are therefore the starting materials for engineering ceramic specialties. 

Metallurgical grade silicon plays an important role as an alloy constituent in aluminum alloys. Addition of 2-25% improve the casting properties of aluminum in the manufacture of castings for example for engine blocks or cylinder heads. The utilization of metallurgical grade silicon in the manufacture of methylchlorosi lanes and the silicones produced therewith by direct synthesis (Rochow process) is covered in Chapter 4. 

Brochures of the Firms Elkem and SKW over Fcrrostlicon and Metallurgical Grade Silicon. 

A distillation process is shown in Fig. P2.59. You are asked to solve for all the values of the stream flows and compositions. How many unknowns are there in the system How many independent material balance equations can you write Explain each answer and show all details whereby you reached your decision. For each stream, the only components that occur are shown below the stream. Metallurgical-grade silicon is purified to electronic grade for use in the semiconductor industry by chemically separating it from its impurities. Si metal reacts in varying degrees with hydrogen chloride gas at 300°C to form several polychlorinated... 

Finally, meltback of the substrate is also suitable, when the surface of the substrate is not flat (for instance when using multicrystalline or metallurgical grade silicon). The substrate is contacted with the melt at a temperature above liquidus one. Then, return to equilibrium is achieved after dissolution of a thin region of the substrate, leaving the surface smoother and more accurate for the following growth. 

For solar cell applications, thin film LPE is economically viable only if it is combined with a low-cost multicrystalline Si substrate (high-throughput silicon ribbons, upgraded metallurgical grade silicon MG-Si) or with a foreign substrate (glass, ceramic, metallic sheet... see section 9-7). 

Silicon of exceptional purity for semiconductor and photovoltaic industries is obtained from metallurgical-grade silicon or liquid silicon tetrachloride. At least four of six steps involve fluidized beds. In Osaka Titanium s process, step 1 (obtaining SiHCls from Si and HCl) takes place at 300°C, whereas in the Union Carbide process, SiHCls is obtained by hydrogenating gaseous SiCU over CUCI2 catalyst at 500°C. Other... 

Silicon is produced in large quantities as metallurgical grade (MG) material for use in the metals and chemicals industries and some is further refined to produce ultrapure electronics grade (EG) silicon. MG silicon is made from lumps of quartz (10-100 mm) and carbon in the form of MG coal, woodchips, charcoal, or coke. They are mixed and heated by an electric arc between carbon electrodes and the furnace body. The reduction of quartz to silicon proceeds according to the (principal) reaction,... 

Now let s apply what we have learned from the biqrcle example to a calculation that deals with a chemical reaction. Electronic grade (EG) silicon used in the electronics industry is a purified form of metallurgical grade silicon, which is made from the reaction of silica, Si02, with carbon in the form of coke at 2000 °C. (Silica is found in nature as quartz or quartz sand.)... 

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