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 phenol ic resin char activated from 0 to 21% burn-off in oxygen. Comparisons of scattering intensities from samples of these chars, both dry and co ntrast-matched with perdeuterated toluene, were used to elucidate the behavior of this carbon with progressive reaction. It was found that t he total scattering intensities from the dry samples decreased with bu rn-off, in contrast to those for other carbons, where they typically i ncrease due to porosity development at low burn-offs. This behavior wa s shown to be due to the selective burn-off of disordered carbon mater ial 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 distri bution of fully penetrable polydisperse spheres, comprising a narrow d istribution in the micropore size range with a mean of about 9.4 Angst rom, superimposed on another broad distribution of pore sizes. The pri mary effect of burn-off of this carbon material is progressive removal of the disordered carbon, resulting in the conversion of initially in accessible to accessible porosity. There was no evidence of significan t widening or alteration of the intrinsic underlying porosity, nor of the development of new porosity up to 21% burn-off. (C) 1998 Academic Press.