The structure of one of mesophase pitch-based carbon fibers with random cross sectional texture was studied by various techniques;X-ray diffraction, magnetoresistance, laser Raman spectroscopy, high resolution electron microscopy, electron diffraction from periphery of the fiber, polarized light microscopy and scanning electron microscopy.
The fibers used were prepared from the mesophase pitch by spinning at370°C, oxidizing at 325°C and then heating up to 900°C (Ref.2).Their diameter was about20 μm and the density was 1.74g/cm³. The fibers were heattreated at 3000°C for 30 min in the flow of argon.They were also heattreated at different temperatures, 1600°, 2000°, 2500°, for about 10 sec in order to know the process of structural change.
The results of X-ray diffraction, magnetoresistance and laser Raman spectroscopy were summarized in Table1.The interlayer spacing d₀₀₂ decreased with the increase in heat treatment temperature, but the growth of apparent thickness of crystallite L_c was restrained.The positive value of maximum transverse magnetoresistance (Δρ/ρ) _max and small value of intensity ratio R of 1360cm^-1 line to 1580cm^-1 line indicated relatively high degree of graphitization of the fiber heat-treated at 3000°C.The value of anisotropy factor A close to 1 and small values of half-maximum width of the orientation function ψ_1/2 which were determined both from X-ray diffraction and magnetoresistance measurement revealed the remarkable orientation of crystallites along the fiber axis.On the 3000°C-treated fiber, crystallite orientation in the cross section was observed (Fig.1c, T arrangement) and it seemed to be related to the deformation of the cross section of the fiber (Fig.6).The relations between d₀₀₂ and L_c, and also between (Δρ/ρ) _max and R did not coincide with those found on various cokes, that might be due to some constraint from the fibrous shape.
Under electron microscope, well-developed graphite lamellae with wrinkles were found, their microtexture being clarified by dark-field image technique and electron diffraction (Fig .2).In the fibers heat-treated at low temperatures, the porous regions with small and randomly-oriented layers coexisted with the regions with well-developed layers (Figs .3 and 4), the former being hard-carbon-like and the latter soft-carbon-like in the microtexture and its change with heat treatment.
The different values of R were obtained for the single fiber and for its powdered sample (Table 1).It means a heterogeneity in the graphitization degree.On the 2500°C-treated fiber, the value of single fiber was 0.18 and it increased to 0.30, close to the value for the powdered, after the oxidation in air at500°C for about5h (15% weight loss). These experimental results suggest that the periphery of the fiber has higher degree of graphitization than the interior.From polarized light microscopy, the units for optical anisotropy (mosaic units) were found to be smaller in the periphery than in the interior.