Intercalation into carbons with varying degree of graphitisation

Abstract

In their beginnings, researches on physical properties of lamellar intercalation compounds of graphite indicated parallels with charge transfer compounds between the same electron donor or electron acceptor partners, and smaller aromatic molecules. Charge transfer bonds in lamellar compounds of graphite are often only weak. However, a large increase in electrical conductivity is found in all cases, compared with the component substances. Graphite charge transfer compounds can be likened in their electrical properties to semi-conductors with zero energy gap. Modern band theories of graphite which aim at precise calculations of properties (including those of the lamellar compounds) assume fractional transfer of charge of the partner molecules to or from the carbon hexagon layers. This fraction may be quite small, though unit charge transfer has been assumed in the class of lamellar compounds which contain macro-anions formed by electrochemical oxidation. Recent work, e.g., on optical properties of lamellar compounds with intercalated halogens such as bromine, indicates a charge transfer (in this case) of only a few tenths of one percent. Various considerations suggest that the very high electrical conductivity parallel to the layers, as is found for so many lamellar charge transfer compounds, must be associated with an exceptionally high mobility parallel to the layer planes of the “charges transferred”. To achieve optimum conductivity of lamellar synthetic metals, factors which reduce mobility must be suppressed or reduced in various ways. These include thermal scattering as well as scattering at diverse defects in the graphite. Increasing the separation between the carbon hexagon layers by binary intercalation may be useful device. Use of well-oriented graphites of high quality seems essential. Properties of ‘low scatter’ layer conductors which urgently call for further research have been discussed. They include evaluation of relationships between electrical and thermal conductivities (the Wiedemann-Franz ratio). At right angles to the layers little systematic information is yet available about significant properties such as thermal expansion. We need to know, too, the anisotropy of electrical resistance and its temperature coefficient, and of the thermo-electric power. Within each layer of many lamellar compounds the ‘ripple potential’ which restricts lateral displacements of intercalate molecules is surprisingly smooth. Two-dimensional mobilities are accordingly often high compared with three-dimensional mobilities in ordinary solids. Under favorable circumstances, synthetic metals whose electrical conductivities surpass the best natural conductors such as defect-free copper or silver are now readily prepared. In order to measure significant electrical properties, some of which are novel, of lamellar synthetic metals it is important to extend measurements to high (possibly to critical) current densities. To meet this need a Pulse Meter has been devised and constructed whose present performance gives us current densities up to 40 000 A/cm 2 with pulse durations and rest periods between pulses variable at will. This is being used to help in elucidating some of the remarkable properties of lamellar synthetic metals.