EFFECT OF PARTICLE-SIZE AND MODULATION FREQUENCY ON THE PHOTOACOUSTIC SPECTRA OF SILICA POWDERS

Investigations of the infrared spectra of crystalline and amorphous silica particles of different sizes (0.05, 5, 10, 15, 30, 45, and 260 µm) in the range of 400–4000 cm−1 using Fourier transform infrared/photoacoustlc spectroscopy are reported. The change in the intensity I of the signal with porosity ϵ of powders follows the ϵ/(1 - ϵ) dependence for strong bands and ϵ dependence for weaker bands as predicted by the theory of McGovern et al. For strong bands, I also follows the empirical relation I ~ D−n, where D Is the particle diameter In cm and n = 0.34−0.42. The anomalous positive frequency dependence of I on the modulation frequency I observed for the stronger bands in the 45-μm and 260-μm particles is believed to result from saturation. Some observations on the effect of changing the background from (carbon black) powder to pellet are also made. © 1990, American Chemical Society. All rights reserved.