Preparation of exfoliated graphite by microwave irradiation

Abstract

Exfoliated graphite (EG) is an important raw material for the production of flexible graphite sheets, which have been widely used as gaskets, thermal insulators, fire-resistant composites, etc. [1–3]. Recently this material was shown to have excellent capabilities for oil sorption and recovery [4,5]. The preparation of EG was usually done by rapid heating of either graphite residue or intercalation compounds, but also conducted by different heating systems, including inductively coupled plasma, laser irradiation and flame heating [1]. The exfoliation of graphite intercalation compounds by means of microwave irradiation was also reported, which gave no significant difference in resultant EG from those prepared by rapid heating [6]. The same conclusion was obtained from the analysis of pore structure inside the worm-like particles of EG prepared by microwave irradiation [7]. Exfoliation under microwave irradiation seems to be interesting process because it can be performed at room temperature. However an accumulation of the experimental data through the fundamental studies on the influences of the microwave irradiation power, the precursors of graphite and intercalate, for example, is required for the practical application of this process. In the present work, the exfoliation of two residue compounds with H2SO4 under microwave of different powers was carried out. The residue compound GIC1 was prepared through electrochemical intercalation (anodic oxidation) of H2SO4 with an electric power consumption of 7.7 A h/ kg, followed by water rinsing [8,9]. The residue compound GIC2 was prepared through washing the intercalation compound synthesized in concentrated H2SO4 with H2O2 at room temperature (chemical intercalation) [10]. SEM photos of these two residue compounds are shown in Fig. 1. The residue compound GIC2 was partially exfoliated along the c-axis of the original graphite flakes. Two precursors were exfoliated under microwave irradiation, applying different powers from 500 to 1000 W for 40 s. For each experiment, the precursor powder was placed at the bottom of the beaker in a layer of about 1 mm thickness, ensuring that all particles were in mutual contact. EG samples thus prepared were characterized by the mass loss during the exfoliation, BET surface area, bulk density, SEM observation and thermoexfoliation coefficient. Bulk density was measured from mass and volume of the samples after exfoliation. The thermoexfoliation coefficient defined as the volume ratio of exfoliated graphite to its precursor, VEG/VGIC, was calculated by measuring the change in sample volume before and after exfoliation. For comparison, two precursors were also exfoliated by a rapid heating to 1000 C. Under microwave irradiation, a marked exfoliation of the precursors was observed, accompanied by fuming and lightening. The characteristics of EGs obtained are summarized in Table 1. After microwave irradiation, a marked volume increase of the lump of both residue compounds after microwave irradiation were observed, just like in the case of rapid heating to 1000 C. EG obtained was confirmed to be composed from so-called worm-like particles as shown in Fig. 2. The pore structure in these worm-like particles was analyzed by an aid of image processing in our previous paper [7]. The apparent distance between neighbouring balloons on the worm-like particles looks different, depending on the precursor residue compound and also on exfoliation conditions. These distances in EG prepared from the precursor GIC1 under 1000 W power were not homogeneous, as shown in Fig. 2(a), but that