Evolution of microstructures and thermal properties with heat treatment for absorbing, emitting and scattering fibrous medium

The microstructure changes of a fibrous insulation for thermal protection system were examined before and after thermal exposures at different temperatures between 1000 °C and 1400 °C. The consequent thermal properties, i.e., thermal conductivity, extinction coefficient, albedo of scattering, and linear coefficient of phase function at different stages were measured by using a developed experimental device and data processing method. The effects of microstructure changes on the thermal properties degradation were discussed. It was found that the devitrification of mullite and the microstructure changes induced by heat treatment had a significant influence upon the thermal properties, and higher temperature treatment yielded a strong increase in thermal conductivity of fibrous insulation. The results also indicated that the relative contribution of conductive and radiative heat transfer would be re-regulated after high temperature thermal annealing.