Preparation of glass-impregnated isotropic graphite and its gas permeability

Two kinds of borosilicate glass, T1 (SiO2; 38%, B2O3. 36%, Na2O. 21%, Al2O3: 5%, softening point 620 degrees C) and T2 (SiO2: 50%, B2O: 30%, Na2O: 15%, MgO: 5%, softening point 639 degrees C), were impregnated into a high-density isotropic graphite block. Achieved impregnation depths were 50 and 24 mm for glasses T1 and T2, respectively,,vith typical impregnation conditions of 1200 degrees C and 15 MPa pressure for 1 h. Around 90% of the open pore volume of the original graphite was occupied by the glass. Gas permeability of the T1- and T2-impregnated graphites was reduced to 6.2 x 10(-5) and 1.8 x 10(-6) cm(2) s(-1), respectively, from 1.3 x 10(-1) cm(2) s(-1) for the original untreated material. Glass impregnation was shown to be more effective in reducing gas permeability than phenolic resin. It was suggested that gas permeability could be much reduced by minimizing the machining of the glass-impregnated blocks.