The oxygen distribution in the transverse section of 30 μm diameter mesophase pitch fibres after oxidative stabilization was measured by using EPMA (electron probe X-ray microanalyser) to clarify the progress of the oxidative reaction and diffusion of the oxidant during the stabilization. Oxygen was distributed in shallow gradients regardless of the stabilization time from the surface to the centre of the mesophase pitch (MP) fibres stabilized at 230° C, suggesting sufficient diffusion of the oxidant to the centre of the fibre at this temperature. In contrast, steeper gradients of distribution were observed in the MP fibres stabilized at 270° C although oxygen up-take of the centre increased steadily with the longer stabilization time to decrease the gradient. Much steeper gradients of the oxygen distribution were observed in the cross-sectioned surface of the fibres stabilized at 300° C for 15 and 30min. The gradient became much steeper with longer stabilization, suggesting some barriers in the deeply oxidized zone which may block the oxygen diffusion. The PVC-10 fibres, whose reactivity was enhanced by blending PVC pitch of 10wt%, showed steeper distributions of oxygen after the stabilization at 270° C comparing to those of the MP fibres stabilized under the same conditions. It showed steeper gradient with the longer stabilization time. In conclusion, stabilization at a lower temperature (230° C) allows relatively rapid diffusion of the oxidant into the centre of the MP fibre during rather slow stabilization but, a higher temperature of stabilization (at 300° C) and/or higher reactivity of the mesophase pitch accelerates the oxidation much more rapidly than the diffusion, providing a blockade zone for the oxygen diffusion near the fibre surface. The extensive oxidation may cross-link three dimensionally the mesophase molecules thus allowing no diffusion of oxygen among the molecules. Such diffusion control tends to provide skin-core structure in the carbonized fibre. It should be noted that fibre thinner than 10 μm showed no skin-core structure. Diffusion within 5 μm from the surface may be rapid under any conditions.
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