Pitch-based carbon fibers as studied by transmission electron microscopy
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M. Endo Faculty of Engineering, Shinshu University 500, Wakasato, Nagano 380, Japan (Received 13 September 1988; accepted 14 March 1989)
The microstructures of carbon fibers prepared from coal tar mesophase pitch and heat-treated at 2500 °C were studied by transmission electron microscopy (TEM). The degree of preferred orientation of the carbon fibers along the fiber axis was not significantly changed: by spinning, while stirring the pitch above the capillary; by changing the viscosity of the pitch in spinning from 60 poise to 250 poise, keeping the diameter of the pitch precursor fiber about 11.5 fxm, which is that of the as-spun fiber; and by controlling the diameter of the pitch fiber from 8 /u,m to 18 jitm, keeping the viscosity of the pitch at about 250 poise. Such a weak dependency of the degree of preferred orientation of the carbon fibers on the spinning conditions is qualitatively consistent with the relationships between the degree of preferred orientation of a pitch precursor fiber and the spinning conditions. The graphite layers of the carbon fibers heat-treated at 2500 °C showed shrinkage in the transverse fiber cross section. The degree of shrinkage was significantly increased by stirring the pitch above the capillary while spinning, and the shrinkage also increased when the viscosity of the pitch was increased for spinning, and when the diameter of the fibers was decreased. Roughly speaking, the degree of shrinkage was proportional to the mean size of the mesophase domains in the transverse section of pitch precursor fibers.
I. INTRODUCTION Structures and physical properties of mesophase pitchbased carbon fibers have been studied using ESR, SEM, and x-ray diffraction analysis.1'2 PAN-based carbon fibers and benzene-derived carbon fibers have also been studied by TEM in fair detail.3"6 However, carbon fibers from coal tar mesophase pitch have been studied by TEM in only a few papers.7'8 It had been found that a fine transverse microstructure makes the tensile strength higher.9 Therefore, the relation between process conditions and the microstructure of carbon fibers are very important for applications of the fibers. X-ray diffraction profiles and electrical properties measurements indicated that the degree of graphitization of pitch-based carbon fibers was lowered by spinning while stirring the pitch above the capillary, by spinning at higher viscosity of the pitch, and by reducing the diameters of the fibers.9"12 Matsumura found that spinning high viscosity pitch or reducing the fiber diameter lowers the preferred orientation of the fiber.13'14 In this article, TEM observations directly yield the microstructures of the carbon fibers, and the results show that the microstructures were changed by controlling the spinning conditions of the coal tar pitch. Two parameters, the mean radius of curvature of the graphite layers in the fiber cross section and the degree of preferred orientation along the fiber axis, were used to characterize the microstructures of the coal tar mesophase J. Mater. Res., Vol.
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