Structure of round-shaped methylnaphthalene-derived mesophase pitch-based carbon fibres prepared by spinning through a Y-shaped die hole

Structures of as-spun, stabilized, carbonized and graphitized fibres prepared by spinning a methylnaphthalene-derived mesophase pitch through a Y-shaped die hole at 295 °C, was examined by combining optical, scanning electron and transmission electron microscopy from the macro- and microscopic view points. The prepared round-shaped fibre spun through a Y-shaped spinning die hole at 295 °C exhibited excellent tensile and compressive strengths of 410 and 70 Kg mm^-2, respectively, after graphitization at 2500 °C. The stabilized fibre consisted of densely packed anisotropic domains in very random alignment, of which transverse domains and longitudinal features appeared as bent, multi-bent and looped, and endless thin stripes, respectively. The size of domain in the transverse section ranged above 100 nm in length and below 100 nm in thickness, respectively. Further heat treatment (carbonization and graphitization) slightly reduced the dimension and deformed the shape of domains to shrink and to have more sharp edges at their bends according to the graphitic growth; however, the shapes and distribution of domains in transverse section were basically unchanged. High-resolution SEM and TEM observations of the domain confirmed the existence of smaller units of graphitic layers in their assemblies which were more closely arranged in the domain. Such a sub-unit was defined as a micro-domain. TEM revealed that the micro-domain was composed of more than one unit of graphitic layers in the graphitized fibre. Most of them were around 10 nm thick and 10-100 nm long. The thickness of micro-domains was observed to be smaller than the value of L_c(002), 23 nm, in the same graphitized fibre. Micro-domains have not yet been identified in the stabilized fibre, while TEM suggested some stackings of hexagonal planes. A number of voids (micro- and meso-voids) up to 40 nm diameter were formed at the intra- or inter-domain locations, due to the graphitic shrinkage and evolution of volatile matter by the heat treatments. Micro-voids of around 5 nm diameter were formed within a domain. The better mechanical performances of the present fibre spun through a Y-shaped die hole were ascribed to the homogeneous distribution of looped or bent domains in the transverse section (random nature of transverse alignment). Such a random alignment may also lead to the least number of macro-voids and cracks in the fibril.