Nonkinetic changes in heat-treated glasslike carbons part I: Pore size and bulk dimensions

Small-angle X-ray scattering measurements have been used to characterize the radius of gyration R_g of the pores in a glasslike carbon heat treated between 1000 and 2800°C for periods up to 150 hours. Measurements were also made of the irreversible bulk dimensional changes Δl. In both cases, a nonkinetic increase occurs when the heat treatment temperature is increased stepwise to a higher value. The relative changes ΔR_g/R_g and Δl/l exhibit a similar three-region behavior: an initial increase with the annealing temperature up to 1600°C, a plateau up to about 2200°C, and thereafter an increase again. However, the maximum dilatation Δl/l is scarcely 3% whereas the increase in ΔR_g/R_g is twofold. The first region is attributed to a gas pressure mechanism, and the highest-temperature region results from strain relief due to the mismatch of the thermal expansion coefficients in the directions perpendicular and parallel to the layers in graphitic materials. The intermediate region is explained by a combination of both mechanisms.