The severization of requirements at the consuming end (producers of organic silicon compounds and semiconductor products) to the quality of silicon obtained at silica-containing batch materials melting process in arc furnaces makes it necessary to investigate the principles of impurity elements distribution both on melt products and silicon pool.
At silica carbothermic reduction the impurity elements from silica-containing crude ore (quarts), carbonic reducing agents, carbon electrodes, furnace lining and auxiliary process materials are reduced and transform into the melted silicon. To determine the principles of impurity distribution between the products of melting process (silicon, mixed dust-gas and slag) the data on physicochemical properties of the substances entering the process are used: the temperature and heat of melting, vapors dissociation tension, constituents´ activity, formation heat, Gibbs thermodynamic potential, etc. And for the obtained crystalline silicon properties forecasting (as in the case of various alloys obtaining) the knowledge of binary and ternary diagrams is of great importance. The liquid melt silicon obtained in an electric arc furnace represents a multicomponent system with 0,1-2% ballast content. At the solidification the impurity elements form various intermetallic compounds, non-metallic compounds with silicon (slag enclosures), dissolved oxygen, silicon carbide and unreacted hydrogen of reducing substances and crystal silica [2, 3]. For the multicomponent systems of any type the forecast of properties on the basis of binary and ternary components systems data is possible if their image is used according to the method of optimal projections [4]. Geometrical methods have an important advantage, which makes possible to express the quality and quantity dependence of the properties on the composition also in the case, when the algebraic expression of this function in unknown to us. More over it is common knowledge that properties vary continuously within the limits of one and the same phase existence fields and undergo sudden alternations (kinking, bending) at the given phase conversion into another one. That is why, if it is possible to restrict the crystallization region of any phase, the general regularity detection in properties alteration of the whole region is possible when oriented to comparatively inconsiderable number of datum points. Thus, it is sufficient to study experimentally the properties of three-four compositions of the system, which correspond to the specified limits, to forecast the system´s properties values with some degree of certainty within the whole region [5].
The construction of binary and more complex constitutional diagrams by experimental methods or with the help of rigorous thermodynamic calculations is a labour intensive, time-taking and very often - a stubborn problem. In this connection the approximate calculation methods of constitutional diagrams construction [6] are extremely productive. Model representations of a melt composition and repeating units´ interaction energy in solution and solid phases lie in their basis. The initial calculations data are comparatively easy to obtain for ideal and regular solutions models. The necessity to use approximate calculation methods is connected with the fact that a production manager, in conditions of ultimate product requirements change, needs to have the computation data forecasting the melt emerging temperature, crystallization path, comparative crystallization rate, silicon phase composition.
For the liquidus and solidus curves construction for a predetermined composition of the n-component system we accepted as the main assumption (initial approximation) the statement that these systems quite adequately satisfy the model of regular solutions. Indirectly it is proved out by the author´s multicomponent oxide systems´ data [4], which testify that with the complication of the system´s composition, it approaches ideal solutions in its properties.
For the calculation of liquidus temperature, the solution and solid phases´ composition in the field of silicon crystallization the following data are necessary: the amount and concentration of the introduced components; values of mixing energies within the binary system, eutectic temperature and also values of distribution coefficients of impurity elements in silicon melt. In the initial stage of our research the separate flux components´ thermodynamic properties and silicon-based binary systems data base is formed. The data base will allow passing directly to modeling on «DIATRIS 1.2» and «MULTICOMDIA 2.0» programs´ flows [7].
References:
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- Investigations of refined metallurgical silicon impurities´ phase composition // IrSTU Bulletin. Irkutsk: Publishing House of IrSTU, 2007, №2 (30), V.1, pp. 30-35.
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- Berezhnoy A.S. Multicomponent oxide systems and their value for silicates technology and petrology // Experiment in the area of technological mineral formation. - M.: Nauka (Science), 1975, pp. 8-12.
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- Suvorov S.A., Semin Ye.G., Gusarov V.V. Phase diagrams and oxide solid solutions´ thermodynamics. - L.: Publishing House of LSU, 1986, p. 140.
- Nemchinova N.V., Udalov Yu.P., Klyots V.E. Silicon-based multicomponent system crystallization field investigation // Materials of International Council "Present-day methods of complex ore processing and alternative mineral raw material" ("Plaksin Readings-2007"), Apatity, October, 1-7, 2007. - Apatity: Publishing House of Mining Institute of Kola Scientific Center RAS, 2007. - Ch.2, pp. 468-471.
The investigations are supported by the project of The Ministry of Education and Science of Russian Federation № RNP 2.1.2.2382 according to the analytic department programme «The development of Higher School Scientific potential (2006-2008)»
The article is admitted to the International Scientific Conference «Innovation Technologies», USA, New-York, December 19-27, 2007, came to the editorial office on 19.10.07