Break | AIChE


The purpose of this project is to model the optical properties of silica aerogel by solving the 1-D radiative transfer equation (RTE). Silica aerogel is an optically transparent and thermally insulative material with a very low density. Due to its microstructure, which consists of a thing solid backbone of silica particle aggregates enveloping thousands of nanoscopic pores, the material suppresses the three modes of heat transfer. The unique combination of physical and chemical properties allows silica aerogel to be an excellent candidate for solar thermal energy applications. The forward solution to the 1-D RTE will predict the transmittance and reflectance of the aerogel as functions of both scattering albedo and extinction, which are inherent properties of the material. By compiling the numerical results of the forward solution, separate 2-D contour plots can be constructed wherein scattering albedo is plotted versus optical thickness, and the contour gradient represents the theoretical transmittance and reflectance. To determine the intrinsic properties of a specific sample, the transmittance and reflectance of that particular sample must first be determined experimentally using a UV-Vis spectrophotometer equipped with an integrating sphere to account for losses. Once values for transmittance and reflectance are available, the values may be plotted as single contour lines on the previously constructed contour plots which constitute the backward solution to the RTE. When these two plots are superimposed upon one another, it is found that the curves intersect at exactly one data point. This solution point provides the exact scattering albedo and extinction coefficient for a specific sample of silica aerogel. Repeating this process for several samples of aerogel produced by various synthesis processes and with different thicknesses will allow the team to predict the transmittance of a sample synthesized by a specific procedure and of a particular thickness. Furthermore, obtaining the intrinsic properties of a sample will allow for the calculation of the scattering coefficient of that sample, which can be used to determine the approximate pore diameter of the sample. The calculated pore diameter will be verified using Small-Angle X-Ray Scattering. Since the pore size of the resulting aerogel directly affects the degree to which light is scattered throughout the medium, it would be useful to predict the transmittance of a sample characterized by a certain pore size and vice versa; this will indeed be possible using the model formulated in this project.