Investigation of the porous graphite structure for silication

This article presents the results of a study of the porous structure and physicomechanical properties of graphite intended for impregnation with molten silicon. The study of the porous structure of graphite has been carried out by the method of mercury porosimetry, based on the property of mercury not to moisten solid carbonaceous bodies and to fill the pores only at the corresponding hydrostatic pressure. Each pressure value corresponds to a certain pore diameter. The analysis of mechanical strength under compression, of apparent density, total, open and closed porosity of industrial graphite for siliconizing has shown unevenness of its properties and porous structure. Graphite for siliconizing is characterized by high density and microcracking. The analysis of mercury porogram of graphite samples has confirmed the unevenness of its porous structure. The average pore diameter of graphite samples widely varies from 3 to 21 micrometers. All graphite samples are distinguished by a large number of pores of less than one micrometer, into which the molten silicon penetrateswith difficulty. Of the nine graphite samples studied, five samples are distinguished by a low (2-5 %) content of large pores over 100 micrometers; in four graphite samples the number of pores over 100 micrometers is quite significant and varies within 10... 20 %. According to the research results, the presence of large pores in graphite also reduces the efficiency of impregnation and impairs the properties of siliconized graphites. Determination of the physicochemical properties of graphite impregnated with molten silicon has shown that, despite the non-uniformity of properties and porosity of the initial graphite, sufficiently high apparent density of siliconized graphite is reached, which is 2.24-2.27 g/cm3 and meets the requirements. However, the uneven porosity of graphite leads to decreasing content of silicon carbide. Only in five samples of the siliconized graphite the silicon carbide content has been reached at the minimum level of requirements of 22.23 %. In four samples the content of silicon carbide is below the permissible value and equals 20 %. The results allow us to conclude that it is necessary to reduce the density of graphite for siliconizing and reduce the number of pores less than 1 micrometer and more than 100 micrometers.