A study is presented of the effects of heat-treatment and gasification in CO2 upon the microstructure of a cellulose carbon. Techniques used include, adsorption of CO2 at 295 K, wide and small angle x-ray scattering, high resolution electron microscopy and quantitative measurement of the hydrogen evolved on heat-treatment to 1870 K. From primary carbonisation temperatures to about 1470 K the principal effects are due to loss of hetero-elements, which cause the open micropore volume to increase. The pores in such carbons are identified as gaps between, and within, small aromatic groupings. The evolution of residual hydrogen allows these groups to coalesce and form layer-planes, which stack together to form diffracting crystallites. The process of stacking re-arranges the micropore volume to become increasingly associated with the spaces between ordered layer-plane stacks. This results in closure of closely-spaced parallel planes and reduced access for molecules to the micropore volume - causing a dramatic loss in open micro-porosity at about 1670 K. The dimensions of the retained micro-porosity increase continuously to 3000 K. The effects of gasification are largely governed by accessibility of oxidant to the internal structure, with the internal ordering of layer-planes being of secondary importance. This results in low degrees of gasification having little effect on fine micropores, being restricted to burn-out of wide micropores to form mesopores. The presence of mesopores allows the latter stages of gasification to occur within the structure. Pore entrances which have closed, or are prone to closure, are sufficiently inaccessible as to be unaffected by moderate gasification (less than 50% burn off). The development of mesopores is largely governed by initial accessibility of oxidant, which is in turn governed by heat-treatment temperature. Unlike micropores, mesopores are unaffected by heat-treatment to 1470 K, and probably beyond.
|Date of Award||1979|