Radiative heat transfer study on silica aerogel and its composite insulation materials

This paper studies the thermal radiative transfer in silica aerogel and silica aerogel composite insulation materials (a xonotlite–aerogel composite and a ceramic fiber–aerogel composite). The spectral transmittances of silica aerogel, xonotlite-type calcium silicate, and ceramic fiber insulation materials–all considered semi-transparent mediums capable of absorbing, emitting, and scattering thermal radiation–are measured with a Fourier transform infrared spectrometer (FTIR) at different infrared wavelengths ranging from 2.5 to 25 μm. The spectral transmittances are used to determine the specific spectral extinction coefficient and the specific Rosseland mean extinction coefficient of each sample. The radiative conductivity of each sample, deduced from the overall thermal conductivity measured using the transient hot-strip (THS) method, is compared against diffusion approximation predictions by using the measured spectral extinction coefficient. The results show that the spectral extinction coefficients of the samples are strongly dependent on the wavelength, particularly in the short wavelength regime (< 10 μm). The total Rosseland mean extinction coefficients of the samples all decrease with increasing temperature. The radiative conductivities are almost proportional to the cube of temperature, decreasing as sample densities increase.