Defect structure and properties of pyrolytic carbons

Abstract

Defects have been studied systematically in graphites prepared by the pyrolysis of methane at temperatures in the range 1600 to 2200 $^\circ$C. Physical methods used included measurements of the bulk density and studies of X-ray diffraction photographs. Electronic properties examined were the electrical resistivity and its degree of anisotropy, the magneto-resistance, the Hall effect, and the thermoelectric power. The uptake of bromine to saturation at room temperature was used to characterize the structural disorder by chemical means. One main conclusion is that a striking change occurs in pyrolytic carbons as their deposition temperature passes through a critical region, around 1900 $^\circ$C. Specimens prepared below this temperature have low bulk densities, show only a comparatively small degree of preferred orientation, and contain appreciable concentrations of residual hydrogen. Specimens deposited above this temperature have bulk densities and other properties which tend towards those of perfect graphite. The crystal orientation improves progressively in specimens deposited up to 2200 $^\circ$C and can be brought still closer to ideal graphite by subsequent recrystallisation at around 2700 $^\circ$C. A second main finding is that the a-axis thermoelectric power, whose values at room temperature lead to the same general conclusions about crystal defects as the other methods used, shows remarkable anomalies as the temperature is lowered to values of reduced temperature T/$\theta$ around 0.03. A preliminary examination has been made of changes in the anomalies resulting from crystal compound formation and from neutron bombardment. Tentative attribution of these anomalies can be made to the interaction of charge carriers with lattice vibrations.