The purpose of the study was to increase the thermal conductivity of multilayered and
self-sealing ceramic matrix composites via the silicon melt infiltration process.
The first step of the process consisted in filling porosity using various organic xerogels by the
sol-gel route. Carbon xerogels obtained by subsequent pyrolysis may reduce and homogenize the
porous network within the composite. Cracking of the xerogels due to volumic shrinkage
occurring during air drying may be decreased by controlling the initial parameters as concerns
the gel solution and/or by operating a second impregnation/pyrolysis step.
Filling of such composites by liquid silicon revealed that a specific route and particular
conditions are necessary to eliminate porosity by controlling gas production species from pore
surface at high temperature. This may be achieved through a directional flow and using highly
viscous silicon (thanks to a localized wick), and by keeping the sides of the materials permeable
to gas.
This led to composite materials with a thermal conductivity which was four times as high as that
of those materials densified via CVI. An increase in mechanical properties was also observed.
