ANALYSIS OF PHOTOACOUSTIC IR-SPECTRA OF AEROGEL AND SILICA POWDER

Especially for solid-gas systems with a small volume fraction of the solid material, photoacoustic spectroscopy (PAS) is a very useful tool for the determination of the optical properties. For such materials, the photoacoustic signal is extremely large because of their low thermal conductivity. With other methods, the measurement of reflectance and transmittance is very difficult for such samples. The expansion of the interstitial gas in porous samples is responsible for a significant part of the photoacoustic signal. It is important to include this contribution in the quantitative interpretation of the PAS experiments. In addition to the optical properties of the sample, the interstitial gas expansion yields information about the structure of the pores. A model calculation is presented for the analysis of photoacoustic spectra in the infrared regime (nu = 400... 2000 cm-1; lambda = 5... 25-mu-m) of silica powder, monolithic aerogel and opacified aerogel powder.