The use of contrast-matching small-angle neutron-scattering techniques to monitor closed porosity in carbons

Small-angle neutron scattering has been performed on samples of phenolic resin char activated from 0 to 21% burn-off in oxygen. Comparisons of scattering intensities from samples of these chars, both dry and contrast-matched with perdeuterated toluene, were used to elucidate the behavior of this carbon with progressive reaction. It was found that the total scattering intensities from the dry samples decreased with burn-off, in contrast to those for other carbons, where they typically increase due to porosity development at low burn-offs. This behavior was shown to be due to the selective burn-off of disordered carbon material exhibiting a broad distribution of sizes up to about 300 Angstrom, with a mean size ca. 102 Angstrom, which originally blocked access to almost the entire intrinsic underlying porosity. The distribution of intrinsic porosity was found to be well fit by a bimodal Schulz distribution of fully penetrable polydisperse spheres, comprising a narrow distribution in the micropore size range with a mean of about 9.4 Angstrom, superimposed on another broad distribution of pore sizes. The primary effect of burn-off of this carbon material is progressive removal of the disordered carbon, resulting in the conversion of initially inaccessible to accessible porosity. There was no evidence of significant widening or alteration of the intrinsic underlying porosity, nor of the development of new porosity up to 21% burn-off. (C) 1998 Academic Press.