AN EFFECT OF " SCATTERING BY ABSORPTION " OBSERVED IN THE NEAR-INFRARED PROPERTIES OF NANOPOROUS SILICA

Abstract : The obtained spectroscopic data for absorption and scattering characteristics of nanoporous silica in the wavelength range from 0.25 to 7 µm are analyzed on the basis of the Mie theory. It is shown that aggregates of primary nanoparticles are responsible for relatively high scattering in the shortwave range from 0.25 to 1.4 µm. But the presence of the aggregates and micron-size cracks in the samples cannot explain unusual behavior of scattering in the long-wave part of the spectral range. The work is focused on understanding of strong scattering peaks observed at exactly the same wavelengths as the infrared absorption peaks of bulk silica: at 9. 2   , 5.3, and 6.1 µm. The known physical effect of " scattering by absorption " is considered as a source of these scattering peaks. It means that absorption centers are assumed to be not uniformly distributed in the nanoporous matrix but collected in some micron-size regions. The estimates based on this hypothesis are supported by the fact that near-infrared absorption peaks are produced by silanol groups which may be nonuniformly distributed in the hydroxylated nanoporous silica. INTRODUCTION Various porous materials are widely used as thermal insulations in many engineering problems [1]. The majority of these materials are made of substances characterized by low thermal conductivity. Thus, the resulting thermal conductivity of the insulation decreases. But the low-conductivity substances are usually semi-transparent in the visible and near-infrared spectral ranges. The latter makes especially important the correct account for thermal radiation mode of heat transfer in the porous material with a specific attention to spectral radiative characteristics in the near-infrared [2]. One can formulate the following typical features of scattering and absorption characteristics of semi-transparent porous materials: 1. The absorption coefficient is independent on the material morphology and can be calculated as  
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Leonid Dombrovsky, Sylvain Lallich, Franck Enguehard, Dominique Baillis. AN EFFECT OF " SCATTERING BY ABSORPTION " OBSERVED IN THE NEAR-INFRARED PROPERTIES OF NANOPOROUS SILICA. 6th International Symposium on Radiative Transfer, Jun 2010, Antalya, Turkey. ⟨hal-01287478⟩

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