Characterization of Graphites by Positron Lifetimes

Abstract

Positron lifetimes have been measured in different types of graphites such as glassy carbon, nuclear graphite, pyrolytic graphite and natural graphite. An analysis based on the trapping model shows that positrons preferentially monitor structural defects such as the internal surfaces between crystallites and surfaces around pores or voids. The lifetime of free positrons delocalized in the perfect graphite lattice is estimated to be 215±10 ps. The lifetime of 400±20 ps, observed in well-crystallized pyrolytic graphite and natural graphite, is interpreted as the lifetime of positrons trapped at the internal surfaces between crystallites. It is possible to characterize different types of graphites by the positron lifetime data.