UCLA

Mesoporous materials could serve as promising thermally insulating materials for window applications due to their low thermal conductivity at ambient conditions. However, they tend to be translucent and hazy due to their nanoscale porous architecture controlling their optical and radiative properties. This dissertation aims to (i) computationally generate and characterize realistic mesoporous materials, (ii) explore how their nanoscale structure affect their light scattering characteristics, and (iii) apply the knowledge gained to understand light transfer through drying mesoporous monoliths and optical characterization of thin films.

First, three-dimensional mesoporous materials consisting of a network of touching or overlapping spheres were computationally generated using the diffusion-limited cluster-cluster aggregation (DLCCA) method. A new algorithm was developed to reproduce nitrogen adsorption porosimetry and retrieve their pore size distributions. The numerically computed specific surface areas and pore size distributions were in good agreement with experimental data reported for mesoporous silica. Second, unpolarized electromagnetic wave scattering by the computer-generated mesoporous structures, described as porous fractal aggregates, were investigated using the T-matrix and discrete-dipole approximation (DDA) methods. The transition between the independent and dependent scattering regimes was studied in systems with up to 8 particles. Independent scattering refers to situations when particles are sufficiently distant that some radiation characteristics of the ensemble can be determined by adding the contributions of each particle. When particles are in close proximity, however, dependent scattering prevails and is affected by near-field interactions and far-field interferences among scattered waves from nearby particles. Here, the dimensionless parameters governing the scattering cross-section and asymmetry factor of non-absorbing particle suspensions and aggregates were found to be (i) the particle size parameter, (ii) the relative index of refraction, (iii) the interparticle distance-to-wavelength ratio, and (iv) the number of particles. Different transition criteria were derived for the scattering cross-section and the asymmetry factor. Dependent scattering effects prevailed in all aggregates and increased with decreasing particle size parameter. In addition, particle overlapping had a negligible effect on the scattering cross-section and asymmetry factor. Furthermore, predictions of the integral scattering characteristics of non-absorbing aggregates with relatively small particle size parameter were found to be accurately predicted by the equivalent effective property (EEP) approximation treating the aggregates as homogeneous spheres with the same volume and an effective refractive index. The EEP approximation was then combined with the Monte Carlo method to predict the transmittance and haze of ambiently drying mesoporous monoliths. The temporary decrease in transmittance and increase in haze observed experimentally during drying of the monoliths could be explained by light scattering by growing dry domains forming within the monolith. Finally, optical interferometry was demonstrated as a robust and simple alternative to ellipsometry for measuring the film thickness, effective refractive index, and total porosity of non-absorbing multicomponent mesoporous thin films.