Comparison of pyrolytic graphite spheres from propylene with spheroidal graphite nodules in steel

Abstract Two types of the spherical growth form of graphite from different precursors, pyrolytic graphite spheres (PGSs) formed by pyrolysis of propylene and spheroidal graphite nodules (SGNs) formed by heat treatment of the medium carbon steel, are investigated by scanning electron microscopy and transmission electron microscopy. Microstructural similarities and differences of these graphite spheres are drawn. The results show that PGSs and SGNs have a similar core-shell microstructure, which comprises a low graphitic core in the center and conical grains growing outward radially from the core with the cone axis along the c-axis direction of graphite. The role of hydrogen on pyrolytic graphite sphere formation is discussed, and we suggest layer-by-layer growth of the spheroidal graphite nodules by two-dimensional nucleation on the basal plane based on the parallel arrangement of graphene layers in the sectors. It is shown that the state of the precursor, gas, liquid, or solid, is not a determining factor on governing the resultant microstructure of graphite spheres.