Pitch-based carbon fibers as studied by transmission electron microscopy

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 μm, which is that of the as-spun fiber; and by controlling the diameter of the pitch fiber from 8 μm to 18 μm, 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.