Tape-shaped pitch fibers with a transverse cross-sectional size of 400 m width and ~30 m thickness, melt-spun from mesophase pitch, were adopted as a model for treatment in oxygen using various temperatures and durations to investigate their stabilization behavior. Several characterization techniques were used to systematically analyze the functional group species, oxygen content and distribution, local composition, thermal pyrolysis behavior and micro-structural changes in the various stabilized tapes. After oxidative stabilization treatment, the tape-shaped fiber exhibits uniform shrinkage behavior during subsequent heat treatments thereby maintaining its * Corresponding author. E-mail address: xkli8524@sina.com (X. Li) 2 tape shape and structural integrity. The ~30 m thick tapes can be stabilized completely by treatment in oxygen at 220 °C for ~10 h and this indicates a high efficiency of stabilization, which is, perhaps unexpectedly, higher than that of corresponding ~30 m diameter round-shaped fibers. Thermal decomposition pathways varied with the degree of stabilization and have obvious effects on the microstructure of the resulted tapes, which in turn strongly influences their final physical properties. Pitch tapes oxidized under mild conditions offered relatively higher mechanical performance. Tensile strength and Young's modulus of 2500 °C graphitized tapes, previously oxidatively stabilized at 220 °C for 20 h, were measured to be about 2 and 250 GPa, respectively.
Two
types of carbon fibers with a large diameter of ∼22
μm, derived from unstirred and vigorously stirred mesophase
pitch melts with different liquid crystalline mesophase textures,
were prepared by melt-spinning, stabilization, carbonization, and
graphitization treatments. The morphology, microstructure, and physical
properties of the carbon fibers derived from the two kinds of mesophase
precursors after various processes were characterized in detail. The
results show that the optical texture (i.e., size and orientation)
of the liquid crystalline mesophase in the molten pitch is obviously
modified by thermomechanical stirring treatment, which has a significant
effect on the texture of as-spun pitch fibers, and finally dominates
the microstructure and physical properties of the resulting carbon
and graphite fibers. These large-diameter fibers expectedly maintain
their morphological and structural integrity and effectively avoid
shrinkage cracking during subsequent high-temperature heat treatment
processes, in contrast to those derived from the unstirred pitch.
This is due to the smaller crystallite sizes and lower orientation
of graphene layers in the former. The tensile strength and axial electrical
resistivity of the 3000 °C-graphitized large fibers derived from
the unstirred pitch are about 1.8 GPa and 1.18 μΩ m, respectively.
In contrast, upon melt stirring treatment of the pitch before spinning,
the resulting large-diameter graphite fibers possess the corresponding
values of 1.3 GPa and 1.86 μΩ m. Despite the acceptable
decrease of mechanical properties and axial electrical and thermal
conduction performance, the latter possesses relatively high mechanical
stability (i.e., low strength deviation) and ideal morphological and
structural integrity, which is beneficial for the wide applications
in composites.
Spinnable mesophase pitches C-MP, P-MP and C/P-MP were synthesized from coal tar pitch, petroleum pitch and their co-carbonized pitches, respectively. The molecular structures of these mesophase pitches and their effect on the microcrystalline sizes of the mesophase and the properties of carbon fibers derived from them were comparatively investigated. The molecular structures and orientation of the prepared mesophase pitches have significant influence on the performance of resultant carbon fibers. In comparison with P-MP and C/P-MP, C-MP possessing the highest aromaticity, a rigid molecular structure and a very small amount of methyl groups makes C-MP-CFs with smaller crystal size and lower decomposition during the preparation process, thus results in best mechanical properties of their carbon fibers, consequently. The prepared P-MP, however, containing abundant methyl groups
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