Development of mesoscopic textures in transverse cross-section of mesophase pitch-based carbon fibers

摘要

Development of the transversal texture in the mesophase pitch-based as-spun at 310 deg C, stabilized, carbonized,calcined and graphitized fibers was examined using a high resolution scanning electron microscope (HR-SEM). The cross-sectional surface of the as-spun fiber showed randomly oriented microdomains of sphericla or ellipsoidal shape, ca. 50 nm long in their longer axis. Such shape and orientation of the microdomain stayed almost the same up to 500 deg C, where the microdomain took a plate shape with some curvatures. The heat treatment up to 700 deg C induced the connection and orientation of microdomains through the shrinkage due to the evolution of volatile gas, forming the definite domain. Some voids were also observed among the microdomains. The heat treatment at 700 deg C enhanced the growth of carbon planes and their layer stacking in the microdomain. The cross-sectional domain showed gentle curvatures in the carbonized fiber at 700 to 1500 deg C owing to smaller hexagons. The domain shape was basically not changed up to graphitization. The curved domain became straight durin graphitization up to 2000 deg C, owing to the growth of graphitic planes. The as-spun fiber was extracted with pyridine to observe its microtexture. The extracted as-spun fiber spun 310 deg C showed macroscopically the same random transversal texture at low magnigication to that of the carbonized fiber. The microdomain in the extracted as-spun fiber was already straight and oriented in the same direction as that observed in the graphitized fiber, no curvature being observable. Two other fibers spun at 300 and 340 deg C to show the radial and onion transverse textures, respectively, exhibited the same trend of as-spun and graphitized fibers. Such a series of observaton suggests that the insoluble fraction frames the shape, orientation and arrangement of microdomains in the graphitized fiber. The soluble fraction in the as-spun fiber formed the curvature of the mesophase microdomain to be carbonized to follow the shape of the oriented insoluble fraction. The texture in the extracted as-spun fiber was basically maintained after the carbonization, although bright spurs appeared in the insoluble microdomain after the carbonization of extracted as-spun fiber. The carbonization of extracted as-spun fiber induced the very straight periphery of the shorter domain.Less bending was observable in the heat-treated fibers after extraction of as-spun fiber, suggesting the connecting roles of the soluble fraction in the growth of domain. The stacking of carbon layers observed as spurs in the microdomain is basically directed along the longer axis of the shape and alignment of microdomains.