NMR DETERMINATION OF THE FRACTAL DIMENSION IN SILICA AEROGELS

Si29 nuclear relaxation experiments were performed in different forms of silica (alcogel, aerogel, amorphous, and crystallized) doped with paramagnetic impurities. Magic-angle spinning is used to quench the nuclear-spin diffusion. Under this condition, the saturation recovery of the Si29 magnetization follows a power law in a very large time range (up to 5 orders of magnitude). This nonexponential relaxation m(t)t reflects the mass distribution in the sample: M(r)rD with =D/6. In aerogels, the measured fractal dimension D2.2 is in agreement with that determined by small-angle x-ray scattering. In densified materials, a dimension D3 is actually observed. © 1990 The American Physical Society.