Structures and physical properties of carbon fibers from coal tar mesophase pitch
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M. Endo Faculty of Engineering, Shinshu University, 500, Wakasato, Nagano 380, Japan (Received 20 May 1987; accepted 10 August 1987) Carbon fibers having various types of structures were prepared by spinning coal tar mesophase pitch, followed by thermosetting and heat treatment at high temperature. Two kinds of spinning—spinning with stirring the pitch above a capillary and without stirring— have been tried to form pitch fibers from coal tar mesophase pitch. Carbon fibers obtained from mesophase pitch and spun without stirring have a radial transverse structure where the graphite layers are arranged radially in the transverse cross section of the fibers. Carbon fibers made with a stirring system can have random, onion, and a novel "quasionion structure" by changing the spinning conditions. Carbon fibers spun with stirring are less graphitizable than those spun without stirring. No separation of the ten diffraction bands into 100 and 101 peaks and no appearance of a 112 peak were observed by x-ray diffraction when the fibers were heat treated at 2700 °C, whereas carbon fibers spun without stirring show clear evidence of graphitization by heat treatment at 2700 °C. Transverse magnetoresistance effects at 77 K, (Ap/p)t have been measured to characterize the structure of the carbon fibers. The carbon fibers spun with stirring and heat treated at 2500 °C generally exhibit a negative transverse magnetoresistance effect, whereas the carbon fibers spun without stirring exhibit a positive magnetoresistance. Good correlations are found among d002, Lc (002), transverse magnetoresistance, and resistivity at room temperature of carbon fibers spun under various conditions and heat treated at 2500 °C. The tensile strengths (TS) of carbon fibers that are less graphitized are higher than those of carbon fibers with a higher degree of graphitization if tensile moduli (TM) are almost constant.
I. INTRODUCTION The structures and physical properties of carbon fibers from mesophase pitch have been studied using electron spin resonance (ESR), scanning electron microscopy (SEM), x-ray diffraction techniques, etc.1"6 Carbon fibers made from mesophase pitch are more easily graphitized than conventional PAN(polyacrylonitrile)-based carbon fibers. Furthermore, the high degree of orientation of the carbon layers from mesophase pitch-based fibers can give rise to higher tensile modulus (TM) by heat treatment at high temperature than that of PAN-based carbon fibers. The next significant feature of mesophase pitchbased carbon fibers is that they exhibit radial transverse structures in almost all cases where the graphite layers are aligned radially in the cross sections of the fibers. These fibers sometimes have cracks along their filament axes because of their radial structure.5 Some studies have investigated how these radial transverse structures are constructed,7"9 but the mechanism is not exactly understood. Bright and Singer2 have studied the structures and physical properties of two kinds of carbon fibers from 850
J. Mater. Res. 2 (6), Nov
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