ANALYSIS OF GAS PERMEABILITY FOR LIQUID PHASE-SINTERED POROUS SiC COMPACT

A submicrometer−sized SiC powder was densified with Al2O3−Y2O3 additives by liquid phase sintering during hot-pressing at 1400−1900° C. Gas permeability of N2, CO2, and O2 gases in SiC with 18−36% open porosity was measured at room temperature. The transportation of gas introduced occurred above a critical minimum pressure. The gas flux increased linearly with an increase of the applied pressure gradient. The Knudsen numbers (Kn = λ/r, where λ and r are the molecular mean-free path of gas molecules and the pore radius of SiC compact, respectively) for the porous SiC compacts were in the range of 0.1−0.3, suggesting the transition range between the viscous flow and the Knudsen flow. A theoretical gas permeability coefficient was derived from the Poiseuille equation which describes a viscous flow. The measured permeability coefficient was in the same order of magnitude by the theoretical prediction. The permeability of gas depends basically on pore structure (pore size, porosity, deviated angle of pore channels against ideal straight pores), viscosity of gas molecules, average pressure in a porous ceramics, and temperature. The permeability coefficient calculated by the Knudsen equation was two orders of magnitude smaller than that of the measured permeability coefficient, reflecting a small contribution of Knudsen flow to the flux of gas permeation.