Exfoliation exothermique des composés d'insertion du carbone contenant de l'acide perchlorique

Abstract

Exfoliation of graphite intercalation compounds (GIC) is usually carried out by an abrupt heating above a critical temperature. This method does not allow exfoliating graphite particles whose average diameter is smaller than 200 μm. In this paper, we propose using an exothermic dissociation of the intercalate for exfoliating small particles of graphite or carbons. Dilute perchloric acid is one intercalate that presents this property. Its intercalation into graphite and cokes was carried out by dipping the materials into the following liquids: • 70% HClO 4 heated at controlled temperatures (thermal process[4]) or with CrO 3 assistance (usual chemical process[1,3]); • HClO 4-HNO 3-H 2O mixtures characterized 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 diffraction at different durations of intercalation; it appears that the presence of a volumic concentration of commercial grade perchloric acid lower than 75% allows synthetizing a first stage GIC, whereas a higher concentration leads only to second stage compounds (Table 2). Chemical analyses of these GICs (Table 3) indicate that the proportion of intercalated HClO 4 increases with the perchloric acid concentration in the liquid mixture in excess. Moreover, thermal analyses of the graphite perchlorate (based on Madagascar flakes) indicate an exothermic peak which is attributed to the intercalate decomposition with exfoliation of the intercalation compound (Figure 3). This behaviour led us to study the exothermic exfoliation of natural graphite particles of different diameters and of coke particles (Table 1). Characterization of the exfoliated material was performed by means of scanning electron microscopy 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 with the HClO 4 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 specific area of the exfoliated material: in fact, exfoliation of HClO 4-GICs prepared by the thermal process leads to a specific surface area of 153 m 2/g, whereas that of the exfoliated HClO 4-HNO 3-GIC is only 42 nr/g. • the specific surface area of the exfoliated natural graphite decreases 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 pristine coke indicates that the exfoliation quality is comparable to the one of the natural graphite. The S.E.M. observations (Figure 4) on these two families of carbon materials show the vermicular aspect characteristic of exfoliation. In the case of the exfoliated coke (HTT 1000°C), the exfoliation seems to concern only the particle edges. In conclusion, it seems to be possible to exfoliate all kinds of carbon materials that are able to intercalate dilute perchloric acid.