IR EMISSION-SPECTROSCOPY OF SILICA AEROGEL

A spectral method (infrared emission spectroscopy) to characterize the rmal properties of silica aerogel is described. It provides spectral i nformation on heat now. An emission chamber permitted measurement of t hermally emitted radiation due to an applied temperature gradient. Dif ferent boundary emissivities and different pressure conditions can be applied, so that the Knudsen effect can be observed. A rigorous numeri cal treatment of the underlying heat transport mechanism was necessary for a correct interpretation of the measured data. Internal parameter s of the aerogel sample had to be modelled in an adequate way (incorpo ration of the Knudsen effect, modelling of the spectral absorption coe fficient with the help of effective medium theories). Measurements and computer simulations of the thermally emitted radiation are presented . The influence of the Knudsen effect on the emission spectra of the a erogel was studied. Analysis of emission spectra in combination with a n integral measurement of the total heat flow in principle allows a fi t of the internal aerogel parameters, i.e., the mean pore size of the samples, the thermal conductivity of the silica skeleton and the spect ral absorption coefficient.