Carbon porosity characterization via small angle neutron scattering

Abstract

Small angle neutron scattering (SANS) employing contrast matching can provide considerable information concerning the details of porosity development, activation mechanisms, and the nature of carbon materials. SANS with contrast matching (SANS-CM) is sensitive to both ‘closed’ (inaccessible) and ‘open’ (accessible) pores, which can provide a more complete description of porosity. The application of these techniques is demonstrated here with two different carbons. In the case of Wyodak coal char, it was found that total scattering intensity generally increased with activation. The unactivated sample exhibited a significant amount of closed porosity, especially in the micropore size range at high q. Results at increasing levels of activation show the relative contributions from unblocking this closed porosity and developing new porosity. Finally, the effects of pore wall collapse were quite evident in the high burn-off data. The activation behavior of a glassy phenolic resin char (PRC) was also investigated. It was found that the total scattering intensities from the dry samples generally decreased with burn-off, unlike the behavior observed for the Wyodak coal char. It was shown that this glassy carbon evolves during initial activation primarily by preferential consumption of disordered carbon which initially blocks the underlying intrinsic porosity. The nature of the intrinsic porosity did not appear to be affected very much in this regime of behavior, other than being progressively exposed by the activation process. 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. Although the two different chars behaved differently with respect to total scattering from the dry samples with activation, the same mechanisms were observed for both. Thus, the difference in overall behavior between the two chars was primarily due to the relative importance of the same mechanisms.