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

Abstract

Superinsulating materials are currently of interest because the heating and cooling of houses and offices are responsible for an important part of CO 2 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; 20 m). 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 ( smaller than 1 m), 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.

Thermal transport in nano-porous insulation of aerogel: Factors, models and outlook

“…The specific extinction coefficient (defined as the ratio of spectral extinction coefficient to the density) is presented in Fig. 11 compared with other experimental data from Lallich et al [92], Wei et al [70], Zeng et al [42] and Lu et al [15]. We can see that the trends of all the curves from both simulations and experiments are in good agreement in the range of long-wavelength.…”

“…It is especially suitable for the characteristic size comparable to the wavelength. Lallich et al [92] developed a code based on the DDA to compute the radiative properties of silica nanoparticle aggregates whose structure was generated by the diffusion-limited clusterecluster aggregation algorithm. In comparison of Mie scattering theory, the results from the DDA were improved in the small wavelength range.…”

Modeling silica aerogel optical performance by determining its radiative properties

“…The size discrepancy between the Rayleigh scattering center and real silica particles can be attributed to the fact that in the real material system, particle aggregates form effective scattering centers of larger size. 7,24 One of the benefits of knowing the intrinsic radiative properties of the aerogel is the capability to predict the transmittance as well as reflectance of a sample with any thickness. The experimentally measured transmittance at 300 nm, as well as the prediction by the B-L law and by the RTE are plotted in Figure 4(A).…”

“…In this regard, the RTE is useful since it accounts for the absorption and scattering simultaneously. 24,[28][29][30] In our model, we treated the aerogel as a homogeneous absorbing and scattering medium. Our main goal was to decouple the extinction contribution from absorption and scattering by identifying the absorption (σ a ) and scattering (σ s ) coefficient at each wavelength.…”

“…The RTE has also been applied to silica nanoporous matrices, a material made by pressure packing off-the-shelf silica nanoparticles, to study its radiative properties in the solar and infrared spectrum. 24 In this study, we applied our RTE model to in-house synthesized silica aerogels and focused on the optical properties in the solar spectrum. Two sets of wavelength dependent radiative properties, the absorption and scattering coefficient, were deduced from the model based on experimentally measured transmittance and reflectance spectra.…”