Helium gas permeability of low activation SiC/SiC composite

Permeability of helium gas in SiC fiber reinforced SiC composite (SiC/SiC composite) was measured by using a vacuum apparatus consisting two chambers. The SiC/SiC samples were prepared by different methods; polymer infiltration and pyrolysis (PIP), PIP and melt impregnation (MI), hot pressing (HP) and liquid phase sintering (LPS). The measurement of permeability was carried out with pressure of helium ranging from 10² to 10⁵ Pa at room temperature. The permeability of the sample made by LPS method was several orders of magnitude lower than those of the other SiC/SiC composites. The lowest permeability, 4×10^-11 m²s^-1, was observed for the SiC/SiC composite made by LPS using SiC fibers and nano powder of β-SiC. The largest permeability, 5 × 10^-3 m ²s^-1, was observed for the SiC/SiC composite made by PIP. The permeability of the SiC/SiC composite depended on the structures of fiber bundle and matrix, and roughly corresponded to the microscopic structures, i.e. pores and cracks.