OPTICAL CHARACTERIZATION OF SILICA XEROGEL WITH SPECTRAL AND ANGLE-DEPENDENT RESOLUTION

Various silica xerogels of different densities were prepared by variat ion of aging time (0-80 h) in a monomer solution, using the optimized subcritical drying conditions developed at NTNU. A silica aerogel refe rence sample was obtained by drying one sample of each batch supercrit ically from CO2. For solar-optical characterization, the normal-normal , normal-hemispherical and normal-diffuse transmittance/reflectance wa s measured in the UV/VIS/NIR region (190-2500 nm). Additionally the an gle-dependent normal-collimated transmittance/reflectance at 633 nm wa s recorded, using a He-Ne laser as the light source. In order to inter pret the measured results, different numerical approaches concerning t he modeling of scattering were tested. Starting from single particle s cattering (Mie theory), a multiple but incoherent scattering approach (N flux) and a coherent cluster scattering approach have been implemen ted with spectral and angle-dependent resolution. As input parameters, these programs require the complex index of refraction of the primary scattering centers (SiO2 particles, taken from literature), the pore size distribution function (obtained by N-2 adsorption/desorption meas urements), the thickness and the porosity (density) of the sample. Usi ng these simulations, the scattering mechanisms within the xerogel sam ples are qualitatively discussed and the different approaches are comp ared. By analyzing the angular characteristic of the scattered radiati on, it can be shown that coherent scattering is the dominant scatterin g mechanism within the aerogel/xerogel, leading to a strongly forward peaked scattering characteristic. The observed increase of scattering due to aging, especially in the short wavelength region: can be attrib uted to an increase of the mean pore size due to reduced shrinkage of the samples while drying. (C) 1998 Elsevier Science B.V. All rights re served.