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Using small-angle x-ray scattering, we show that porous silica aerogel has a fractal backbone structure. The observed structure is traced to the underlying chemical (polymerization) and physical (colloid aggregation) growth processes. Comparison of scattering curves for aerogel with silica aggregates confirms this interpretation.In spite of the importance of random porous media in nature and in technology, the structure of these materials has eluded characterization.In this paper we show that certain classes of porous materials can be characterized through fractal geometry and that the appropriate geometry can be determined by small-anglex-ray scattering. We report the structure of a silica aerogel prepared by critical-point drying of an alcoholic (sic) silica gel. This material is chosen because of its exceedingly low density (0.09 g/cm3) and concomitant high porosity. We show that the porosity in this material is due to a random colloid aggregation process in the solution precursor. To our knowledge, this is the first time that the structure of a porous material has been explained in terms of random growth. Classical models of porosity-dependent properties are based on highly simplified geometrical structures like packed spheres or bottlenecked bubbles. ' Recently, fractal2 structures (percolation networks, fractal surfaces) have also been postulated to explain fluorescence3 and adsorption4 5 data for porous media. Although there have been many attempts to characterize the microstructure of porous materials, modeldependent geometric assumptions, like those above, have doomed all indirect methods.A primary goal of our work is to establish the existence of a fractally porous solid. To this end, we first outline the expected scattering patterns for simple fractal structures. The key to our interpretation is based on a qualitative distinction between surface and volume fractals. We find that the low-density aerogel has purely mass-fractal character and find no evidence for fractally rough surfaces.Scattering Pom Pactals. -To interpret scattering curves it is useful to distinguish between bulk and surface scattering. In systems without distinct surfaces, such as polymers in solutions, the scattered intensity,

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