Thermal conductivity of the carbon felts, strongly anisotropic insulants: Modelling of heat conduction by solid phase.

Uses of carbon felts for high temperature insulation are reviewed. Effects of raw material and fabrication method on the felt properties are discussed. The connection between the thermal conductivity lambda and the other felt properties are analysed. Evidence is given for the existence of three hierarchized anisotropy levels of lambda: crystal lattice, fiber and felt. The two directional felt conductivities - the longitudinal lambda(T), and the transversal lambda(T) ones - are modelled using a definite geometry of the felt texture and a previously introduced expression for the constriction resistance of a fiber/fiber contact. lambda(T) depends on both components of fiber conductivity: axial lambda(A) (assumed to be equal to the longitudinal) and radial lambda(R). The geometrical parameters considered are: felt porosity, fiber tortuosity, aspect ratio (fiber width to heigth), fiber mean size, contact spot size a and relative frequency of direct interlayer crossing, U. The experimental validation is currently only approximate, since measurements of lambda(R), a and U are not accurate enough yet.