EXOTHERMIC EXFOLIATION OF GRAPHITE-INTERC ALATION COMPOUNDS CONTAINING PERCHLORIC-ACID

Exfoliation of graphite intercalation compounds (GIC) is usually carri ed out by an abrupt heating above a critical temperature. This method does not allow exfoliating graphite particles whose average diameter i s smaller than 200 mum. In this paper, we propose using an exothermic dissociation of the intercalate for exfoliating small particles of gra phite or carbons. Dilute perchloric acid is one intercalate that prese nts this property. Its intercalation into graphite and cokes was carri ed out by dipping the materials into the following liquids: 70% HClO4 heated at controlled temperatures (thermal process[4]) or with CrO3 as sistance (usual chemical process[1,3]); HClO4-HNO3-H2O mixtures charac terized by the molecular percentage of each species (Table 2). In this latter case, the GICs based on H.O.P.G. were characterized by X-ray d iffraction at different durations of intercalation; it appears that th e presence of a volumic concentration of commercial grade perchloric a cid lower than 75% allows synthetizing a first stage GIC, whereas a hi gher concentration leads only to second stage compounds (Table 2). Che mical analyses of these GICs (Table 3) indicate that the proportion of intercalated HClO4 increases with the perchloric acid concentration i n the liquid mixture in excess. Moreover, thermal analyses of the grap hite perchlorate (based on Madagascar flakes) indicate an exothermic p eak which is attributed to the intercalate decomposition with exfoliat ion of the intercalation compound (Figure 3). This behaviour led us to study the exothermic exfoliation of natural graphite particles of dif ferent diameters and of coke particles (Table 1). Characterization of the exfoliated material was performed by means of scanning electron mi croscopy and krypton (noted by ) or nitrogen physisorption. The study of a homogenous set of exfoliated samples based on Madagascar natural graphite flakes indicated that the specific surface areas increases w ith the HClO4 proportion in the liquid mixture (Table 4). As concerns the carbon materials intercalated by the thermal process or by dipping into a 50/50 nitric-perchloric mixture and exfoliated (Table 5), one can observe that: the nature of the intercalate influences the specifi c area of the exfoliated material: in fact, exfoliation of HClO4-GICs prepared by the thermal process leads to a specific surface area of 15 3 m2/g, whereas that of the exfoliated HClO4-HNO3-GIC is only 42 m2/g. the specific surface area of the exfoliated natural graphite decrease s with the average diameter of the pristine graphite (see Tables 1 and 5). the ratio of the specific area of exfoliated coke to that of pris tine coke indicates that the exfoliation quality is comparable to the one of the natural graphite. The S.E.M. observations (Figure 4) on the se two families of carbon materials show the vermicular aspect charact eristic of exfoliation. In the case of the exfoliated coke (HTT 1000-d egrees-C), the exfoliation seems to concern only the particle edges. I n conclusion, it seems to be possible to exfoliate all kinds of carbon materials that are able to intercalate dilute perchloric acid.