Formation mechanism of graphite or graphene fibers with core-shell structure from nearly stoichiometric polycrystalline SiC fibers

SiC fibers are widely used in high-temperature structural composites as an important reinforcement. The nearly stoichiometric polycrystalline SiC fibers were subjected to various high temperatures under vacuum to study the formation mechanism of graphite/graphene fibers with core-shell structure. The results indicate that the graphite/graphene fibers consist of dense graphite shell and graphene core with porous 3-dimensional network structure, and the tensile strength of porous graphite/graphene fibers is about 254 MPa. The outermost graphite shell is oriented and the {0001}_Graphite planes with large spacing are almost perpendicular to the surface of the fibers, which facilitates the release of gases. Graphite maintains a special orientation relationship of <1$\overline{1}$00 > _Graphite // <1$\overline{1}$0 > _β-SiC and {0001}_Graphite // {111}_β-SiC in the early nucleation stage. As the SiC fibers further decompose, the graphite grows and bends, and its orientation relationship with the β-SiC disappears. Instead, the graphite with the (0001)_Graphite planes perpendicular to the non-{111}_β-SiC interfaces shows a faster growth rate, higher quality and fewer defects. The volume fraction of the pores in the porous 3-dimensional network graphene core exceeds 55 %, and in some cases reaches 86 %. This characteristic not only reduces the weight of fibers but also increases the potential adsorption capacity of fibers. These findings elucidate the mechanism of decomposition of SiC fibers to form graphite/graphene fibers under high temperatures, offering valuable insights into the development of high-performance SiC fibers or porous graphite/graphene fibers.