The estimation of the active surface area (ASA) of various macrocrystalline graphitic materials is industrially valuable but the microstructures of these materials are still contestable. This in turn has led to difficulties in the unambiguous interpretation of crystallographic measurements with powder X-ray diffraction (pXRD) and Raman spectroscopy as well as their relationship to the ASA. To resolve this issue a systematic approach is required. As a starting point two widely accepted pXRD and Raman methodologies were utilized. Purified, oxidized, natural graphite flakes were extensively examined to elucidate the essential microstructural features. Based on this an illustrative model was formulated as grounds for interpreting the measured crystallite domain sizes. Only one of the crystallographic parameters could be linked to the observed microstructure. For macrocrystalline graphite both techniques are subject to instrumental limitations and should not be used. Due to the non-linearity of the correlations they are prone to measurement uncertainty and should not be used above acceptable limits. In addition, the current inability to distinguish between different defect types leads to ambiguous results. Despite being a single, interrelated 2 crystal the composite nature of the flakes will make it difficult to relate even an ideal, accurate domain size measurement to the ASA.
Thermal analysis and other techniques were employed to characterize two expandable graphite samples. The expansion onset temperatures of the expandable graphite's were ca.220°C and 300°C respectively. The key finding is that the commercial products are not just pure graphite intercalation compounds with sulfuric acid species intercalated as guest ions and molecules in between intact graphene layers. A more realistic model is proposed where graphite oxide-like layers are also randomly interstratified in the graphite flakes. These graphite oxide-like layers comprise highly oxidized graphene sheets which contain many different oxygen-containing functional groups. This model explains the high oxygen to sulfur atomic ratios found in both elemental analysis of the neat materials and in the gas generated during the main exfoliation event.
Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to "foam" the pitch.
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