Coalescence of mesophase spheres and microstructure of graphitic carbon revealed by scanning electron microscopy

The structural characteristics of mesophase spheres slowly coalesced in a petroleum pitch by high-temperature centrifugation were studied by high-resolution SEM, after stabilization and carbonization. The patterns of lamellae stacking as well as fine structural defects have been observed clearly below 100 nm at the level of near-single lamellar layer, for four distinct development stages, e.g., single-crystal sphere, just contacted, coalesced and bulk mesophase. Clear images from the inside and the outside and from more than one directions have confirmed the single-crystal nature before coalescence, but some defects of highly curved lamellae and cracks were found in just-contacted spheres. In highly coalesced situations, additional other nano-scaled defects were observed, including disclinations, dislocations, cracks and voids, although the defect density appeared to drop at the bulk mesophase stage. The evolution of lamellar stacking was examined schematically by bonded lamellar layers, and the microstructural defects were analyzed under different categories.