Fractal surface and cluster structure of controlled-pore glasses and Vycor porous glass as revealed by small-angle x-ray and neutron scattering

Analysis of small-angle x-ray-scattering data from a series of controlled-pore glasses (nominal pore sizes from 75 to 2000 AÃÅ) and from Vycor porous glass has revealed the fractal nature of the surfaces of these materials. The controlled-pore glasses show a relatively low degree of surface irregularity, with a surface fractal dimension of D=2. 20Ǎ±0.05, regardless of pore size. Vycor glass, the fractal properties of which have been under some debate, has a rougher surface, with D=2.40Ǎ±0.10. The D value for Vycor has been verified by small-angle neutron scattering. The fractal dimensions refer to surface irregularity in the range 10aa100 AÃÅ. The scattering data suggest that the carriers of the fractal surface properties are interconnected units (particles, clusters) with an average diameter of 300 and 450 AÃÅ for the controlled-pore glasses of pore size 75 and 170 AÃÅ, respectively, and units with an average diameter of 350 AÃÅ for Vycor glass.