| A model of intercalation has been developed which takes into account several possible rate controlling steps for the intercalation of graphite, i.e., evolution of intercalate from a source, transport of intercalate from a source to the graphite, adsorption onto the graphite surface, insertion into the graphite, diffusion within the graphite and the reaction to produce one stage from another. Using pertinent physical constants obtained from the literature, the model has been used to estimate the rate controlling reaction steps for several intercalate species, i.e., Br(,2), lCl, HNO(,3), K, Rb, Cs, FeCl(,3), NiCl(,2), CuCl(,2), PdCl(,2), AsF(,5) and SbF(,5).;An experimental investigation was also made of the behavior of intercalation compounds during exfoliation. For bromine compounds it was observed that exfoliated graphite exhibited the same in-plane superlattice ordering as existed prior to exfoliation. (Abstract shortened with permission of author.).;Further elaboration was made of the diffusion step within graphite to take into account the presence of several stages during the intercalation process, and the model applied to the instance of the intercalation of bromine was undertaken to determine the actual parameters controlling the reaction of pyrolytic graphite with bromine. A phase diagram in log (vapor pressure) versus 1/(absolute temperature) was determined by identifying the equilibrium intercalation compound at a variety of temperatures and bromine concentrations. Enthalpies and entropies of reaction for stages 2, 3 and 4 were calculated from the phase field boundaries on the phase diagram. X-ray absorption and diffraction were used to follow the progress of intercalation at a variety of intercalate temperatures and concentrations allowing the establishment of the first time-temperature-transformation (T-T-T) and time-concentration-temperature (T-C-T) diagrams for the intercalation of graphite. The C-shape of the T-T-T curves for graphite-bromine indicates that the rate controlling step in the intercalation of bromine is principally diffusion within the sample or the staging reaction, depending on the temperature, intercalate concentration and the stage of interest. The experimental measurement of the intercalation rate was used in conjunction with the multi-stage diffusion model to calculate the activation energy and premultiplier for the diffusion coefficient of bromine within graphite. |
