The identification and some physico-chemical consequences of non-basal edge and screw dislocations in graphite

Abstract

An optical microscope study of the basal faces of partially oxidized (by oxygen) single-crystal flakes of natural graphite has been carried out. It has proved possible to distinguish the sites of emergence of non-basal edge and non-basal screw dislocations by utilizing both the extreme reactivity of such sites in the presence of added molybdenum or boron, and the fact that cleavage, under certain conditions, can reveal screw dislocations. Non-basal dislocations often form small-angle boundaries (and possibly walls arising from polygonization) thus converting a single crystal into a mosaic. A method of analysing the dislocations constituting such boundaries is described. Values of the ratio of the rate of oxidation along thecaxis to that along the basal planes,R_c/a(a measure of the kinetic anisotropy), are estimated. At dislocation cores containing added molybdenum,R_c/acan be greater by a factor of at least 10¹⁵than at ‘ordinary' sites in the basal planes. The actual rate of oxidation at the deliberately contaminated cores is approximately 5 x 10^-3g cm^-2s^-1at 800°C. From cleavage experiments, two main conclusions are drawn. First, internal oxidation may occur at dislocation cores within the bulk of a so-called single crystal, it being postulated that access by the oxidizing gas is gained via microcracks and voids running parallel to the basal plane and via basal-plane intercrystallite boundaries. Secondly, when stresses are applied along thecaxis, extensive deformation (resulting in lack of correspondence of cleavage steps on pairs of separated cleavage faces) may occur during the act of cleavage. It is also concluded that the fracture surface energies for {101̄l} and {112̄l} faces are approximately equal.