Modelling of exfoliated graphite

Abstract

Exfoliated graphite (EG) is a well known material usually produced from various intercalation compounds submitted to a thermal shock. Thus, large amounts of EG are industrially prepared from flaky particles of sulphuric acid-based complexes passing throughout the flame of a burner. Since the intercalate is suddenly volatilized, a huge unidirectional expansion of the starting intercalated flakes occurs. A kind of black snow made of pure graphite worms is recovered, with which autoconsolidated materials are easily produced by simple compression. Moderately compressed exfoliated graphite (CEG) leads to highly porous graphite “foams”, while heavily compacted (and most of the time laminated) one leads to impervious and flexible graphite foils. Both kinds of materials have numerous actual and potential applications, such as supports for various dispersed matters, and for uses in gasketing, adsorption, electromagnetic interference shielding, vibration damping, thermal insulation, electrochemical applications and stress sensing. Owing to such a number of applications, EG is a material of growing technological importance, and hence the behaviour of its properties while compacted at various densities is worth to be studied. Especially, both phases––namely carbonaceous backbone and pore space––may be investigated. In this paper, the mechanical, thermal and electrical properties of a number of CEG blocks are reviewed. It is shown that such physical properties may be accounted for on the basis of percolation and effective-medium theories. For that purpose, the main features of the latter theories are briefly recalled. While percolation deals with the quantitative behaviour of heterogeneous disordered media close to their critical point (conductivity and rigidity thresholds), effective-medium theory applies on wider ranges of composition, successfully describing the average properties of binary systems. Information about the shapes and the conductivities of the individual particles on the one hand, and about the kind of elastic forces within the materials on the other hand, may be derived on application of these theories. Moreover, simple relationships are established between apparent density, and conductivity and rigidity thresholds. Finally, the mosaicity is modelled basing on the excluded volume concept applied to disc-shaped constitutive graphite sheets. Hence, accurate calculation of disorientation angles between graphite flakes within the materials is achieved, and the results are checked with XRD experiments. Besides, the pore space of these materials is now completely characterized, basing on adsorption, gas flow, ion diffusion and pycnometry studies. Quantitative cross-property relationships are established between permeabilities, formation factors, “critical” and “dynamical” pore sizes, open porosities, particle densities and surface area. Again, percolation concepts and effective-medium theory are used in order to model the various behaviours. The surface area is also calculated basing on the predictions of the above model with which the average disorientation of the graphite sheets was already calculated. It should be emphasized that the established cross-property relationships are free of any adjustable parameter, and hence some properties may be directly and quantitatively predicted from the measurement of a few other ones. To sum up, the main physical properties of exfoliated graphite are reviewed and modelled in a completely self-consistent way, using classical concepts of disordered matter physics.