Microstructure of natural graphite flakes revealed by oxidation: Limitations of XRD and Raman techniques for crystallinity estimates

Abstract

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.

Characterization of commercial expandable graphite fire retardants

“…A recent extensive investigation on natural flake graphite, very similar to the materials under consideration in this study, concluded that the flakes are comprised of polygonized stacks of interlinked crystals as seen from an angle in Figure 1A. [36]. The morphology is the result of complex crystal growth phenomena during flake formation.…”

Untreated Natural Graphite as a Graphene Source for High-Performance Li-Ion Batteries

“…Although in the last decade NG production has grown, at current rates it is estimated that production could be maintained for over 200 years. Currently, to prepare graphene, only high-purity graphite (more than 99.9% fixed carbon) is used [15,16]. To achieve this level of purity, silicate mineral impurities in the starting graphite must be removed by vaporization at ultra-high temperature or leaching in hydrofluoric acid [17][18][19].…”

The influence of three different intercalation techniques on the microstructure of exfoliated graphite

“…The Hummers method tends to create large puffed regions during exfoliation, resembling a concertina. The reason for this may be the segmented stacking structure found in this type of graphite [31]. The gas phase material on the other hand shows a more evenly distributed expansion, due the 1 st stage intercalation, rather resulting in a lattice work structure as exemplified in Figure 10 (B).…”

“…In all cases the so-called disorder or D-peak was practically absent, indicating that the residual damage present in the material cannot be detected by this method. This is not surprising given the fairly large size of the GNPs compared to the small spot size of the Raman laser [31].…”

“…In the past this treatment has been found effective for removing even trace impurities present in graphite without damaging the sample [31], thus yielding an exceedingly pure material.…”