GB1328249A - Process for manufacturing porous beta silicon-carbide articles - Google Patents

Abstract

1328249 Silicon carbide COMMISSARIAT A L'ENERGIE ATOMIQUE 26 Nov 1970 [28 Nov 1969] 56282/70 Heading C1A A porous #-silicon carbide article is prepared by a process comprising preparing a porous graphite carbon matrix having the shape of the required article and a density of from 1À4 to 1À5 g./cm.<SP>3</SP>, heating the matrix in vacuo to a temperature at which #-silicon carbide formation will occur, immersing the matrix in vacuo in a liquid silicon bath at substantially the same temperature for impregnation with consequential formation of silicon carbide, removing the matrix from the bath after impregnation and bringing it into contact with a porous graphite member to remove excess free liquid silicon by capillary attraction and then destroying the excess free carbon in the matrix by oxidation at a temperature of from 600‹ to 800‹ C. The carbon matrix may be prepared by agglomerating graphite grains of size between 1 and 100 Á by means of a binder, shaping by moulding and compacting and then subjecting it to a thermal cracking treatment with a hydrocarbon gas at about 900‹ C., the conditions being adjusted to provide the required density; such a matrix has a porosity of about 30%, and comprises graphite grains bonded by pyrocarbon. The bath temperature for impregnation is suitably 1500-1700‹ C., an amount of silicon equal to about 90% of the weight of the carbon is absorbed and most is converted into #-silicon carbide. Contacting of the impregnated matrix with porous graphite may be effected at 1500- 1600‹ C., during which some of the free silicon in the matrix pores is converted into silicate and that on the surface is removed by capillary attraction. The final oxidation at 600-800‹ C. may be effected with air for 200 hours yielding an article comprising 97% #-silicon carbide, density of about 1À7 g./cm.<SP>3</SP>, and 43% porosity, the carbide being in skeletonal form. Articles which may be so made include high temperature reactor fuel elements, filter elements, thermal shield elements and, in spherical form, catalyst supports. Special refractories may be obtained by impregnating the porous material with silicides. Specifications 1,328,250 1,050,233 and 1,013,056 are referred to.