Experimental Determination and Modeling of the Radiative Properties of Silica Nanoporous Matrices

Superinsulating materials are currently of interest because the heating and cooling of houses and offices are responsible for an important part of $CO2$ emissions. In this study, we aim at modeling the radiative transfer in nanoporous silica matrices that are the principal components of nanoporous superinsulating materials. We first elaborate samples from different pyrogenic amorphous silica powders that slightly differ one from another in terms of specific surface, nanoparticle diameter, and composition. The various samples are optically characterized using two spectrometers operating on the wavelength range (250 nm; $20μm$⁠). Once the hemispherical transmittance and reflectance spectra are measured, we deduce the radiative properties using a parameter identification technique. Then, as the considered media are made of packed quasispherical nanoparticles, we try to model their radiative properties using the original Mie theory. To obtain a good agreement between experiment and theory on a large part of the wavelength range, we have to consider scatterers that are up to five times larger than the primary nanoparticles; this is attributed to the fact that the scatterers are not the nanoparticles but aggregates of nanoparticles that are constituted during the fabrication process of the powders. Nevertheless, in the small wavelength range (⁠$λ$ smaller than $1μm$⁠), we can never get a satisfactory agreement using the Mie theory. This disagreement is attributed to the fact that the original Mie theory does not take into account the nanostructure of the aggregates. So we have developed a code based on the discrete dipole approximation that improves the modeling results in the small wavelength range, basing our computations on aggregates generated using the diffusion-limited cluster-cluster aggregation algorithm in order to ensure a fractal dimension close to what is usually found with aggregates of silica nanoparticles.