Measurement of infrared absorption of some oxides in connection with the radiative transfer in porous and fibrous materials

Abstract

The radiative heat transfer in porous and fibrous materials depends on the infrared absorption and scattering properties of the materials. The infrared absorption can be obtained by transmission measurements in single crystals. It is shown in the case of Al₂O₃, and MgO that the analysis of absorption data as a function of temperature gives the possibility of decomposing the lattice absorption coefficient in its different multiphonon contributions. The study of multiphonon components is necessary to understand as well as to predict the frequency and temperature behavior of the lattice absorption in materials. Measurements performed on TiO₂ and ZrO₂ also are reported; it is observed that electronic effects considerably enhance the absorption level at high temperatures. From diffuse reflection measurements on a scattering sample, the ratio of absorption and scattering can be obtained. So experimental data can be used to calculate the radiant flux. Several models have been proposed, based on isotropic and anisotropic approximations. They involve the absorption coefficient σ_a, the scattering coefficient σ_s, the backscattered fraction factor b, and a phase function parameter a ₁. Due to the approximations on which the models are based, the calculated values of the radiant heat flux show large differences (as large as 30%), whatever the accuracy of the values of optical parameters which are used. It appears worthy to use the experimental data on σ_a and σ_s, which can be obtained from transmission and diffuse reflection measurements.