Experimental and theoretical analysis of transport processes in nanostructured medium aerogel.

摘要

This dissertation investigates the theoretical and experimental aspects of energy transport in nano-structured materials--aerogels, which are highly porous, open-cell foam materials produced by sol-gel processes and dried by supercritical extraction.;The pore size of aerogel is extremely fine and gas molecules can only diffuse in aerogel. Aerogel has a very high internal surface area. The mean free path of gas molecules in aerogel is shorter than that of gas molecules in a free space at standard temperature and pressure. It follows that gas in aerogel has lower conductivity than it has in a free space. The relations among the specific surface area of aerogel, mean free path and gas thermal conductivity were established.;The solid conductivity of aerogel is related to its microstructure. Three geometrical models of the microstructure of porous medium aerogel, i.e., the cubic array of intersecting square rods, cylindrical rods and spheres, were proposed, on which the calculation of the apparent conductivity was based. The parameters of the models were determined by the physical quantities of porosity, specific surface area and particle size, which are determined from BET nitrogen adsorption measurement.;Aerogel is not optically thick in the infrared over whole spectral range. The equation of transfer of radiant energy, which is an integro-differential equation, together with energy equation was solved for the calculation of heat transfer in Aerogel. An approximate closed form formulation referred to as the combined thin and thick method was developed and solved. The computed results show that the combined thin and thick method gives good agreement with the experimental results.;Silica aerogel has a very small absorption coefficient over the range 3-8 ;A thin-film-heater thermal conductivity apparatus was developed for the measurement of thermal conductivity under steady conditions. The apparatus uses a 10 nm thick gold film heater that is coated on a polyester sheet. Important advantages of this heater are spatial uniformity of the heat generation and small thickness.;The normal reflectance and transmittance of a silica aerogel slab were measured using a Fourier Transform Infrared Spectrometer. The effective optical constants n and