Silica aerogels represent a new class of open-pore materials with many new applications, such as insulated windows for solar pore dimensions on a scale of tens of nanometers, and are thus applications, catalysts, gas separation media, and Cherenkov classified as mesoporous materials. In this work, we show that the counters (1-5). Silica aerogels have been synthesized using combination of NMR spectroscopy and chemical-shift selective conventional sol-gel processing techniques from an ''alcomagnetic resonance imaging (MRI) can resolve some of the imgel'' precursor followed by supercritical solvent extraction, portant aspects of the structure of silica aerogels. The use of xenon which leaves the original gel structure virtually intact (6, 7).
as a gaseous probe in combination with spatially resolved NMR
Understanding the processes that control the pore structure techniques is demonstrated to be a powerful, new approach for is paramount to developing better approaches for producing characterizing the average pore structure and steady-state spatial more uniform microstructured materials.
distributions of xenon atoms in different physicochemical environments. Furthermore, dynamic NMR magnetization transfer exper-Numerous techniques have been used to study the pore iments and pulsed-field gradient (PFG ) measurements have been structure of silica aerogels. These materials have been diffiused to characterize exchange processes and diffusive motion of cult to characterize with certainty, largely because available xenon in samples at equilibrium. In particular, this new NMR measurement techniques for determining porosity characterapproach offers unique information and insights into the nanosistics can give drastically differing results on the same samcopic pore structure and microscopic morphology of aerogels and ple. The volume of gas adsorption determined by the method the dynamical behavior of occluded adsorbates. MRI provides of Brunauer, Emmit, and Teller (BET) has been used to spatially resolved information on the nature of the flaw regions determine specific surface areas of aerogels, and the hysterefound in these materials. Pseudo-first-order rate constants for sis in the adsorption/desorption curves has provided poremagnetization transfer among the bulk and occluded xenon phases indicate xenon-exchange rate constants on the order of 1 s 01 for size distributions via the Kelvin equation (8)(9)(10)(11). While specimens having volumes of 0.03 cm 3 . PFG diffusion measure-BET measurements have generally been valid in the determiments show evidence of anisotropic diffusion for xenon occluded nation of voids in the mesopore regime, there are significant within aerogels, with nominal self-diffusivity coefficients on the limitations associated with the use of conventional gas adorder of D Å 10 03 cm 2 /s. ᭧ 1998 Academic Press sorption methods to evaluate the total porosity of aerogels (12). It has been assumed that inconsistencies in the data result from the inability of BET measurements to adequately