Effect of Liquid Crystalline Texture of Mesophase Pitches on the Structure and Property of Large-Diameter Carbon Fibers

Abstract

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.

Constructing the Bridge from Isotropic to Anisotropic Pitches for Preparing Pitch-Based Carbon Fibers with Tunable Structures and Properties

“…As the LC content increases to ∼50 vol % ( Figure 2 d), large mesophase spheres and partial coalescence texture are present, and the isotropic pitch is still the continuous phase. On further increasing the LC content to ∼75 vol %, the optical texture changes into a coarse flow type, and some isotropic spherical or ellipsoidal domains are clearly mingled in the optically anisotropic continuous phase as shown in Figure 2 e. The prepared SNP with a high LC content of ∼98 vol % exhibits a strongly oriented fine flow texture as shown in Figure 2 f. The MP-100 pitch thermally polymerized from the NP-TS fraction also displays a 100 vol % LC content and an ideal oriented flow texture as shown in Figure 2 g. The optical texture and liquid crystalline orientation of NP-derived pitches with high mesophase contents (e.g., 95, 98, and 100 vol %) are very similar to that of the AR pitch as shown in Figure 2 h. 32 …”

“…It can be clearly seen that the optical textures at various locations (i.e., top, middle, and bottom) in the tube show no obvious differences, except for a slight textural difference of prepared SNPs, as shown in Figure 2 , owing to the thermal movement and molecular assembly of LC during the long soaking treatment. 20 , 25 , 32 This indicates that the thermal stability and phase separation of the SNPs with various liquid crystalline contents were successfully controlled by the nitrogen-bubbling treatment.…”

“…It can be seen that there is no obvious difference in the tensile strength and Young’s modulus of the carbon fibers derived from the two types of 100 vol % mesophase pitches (MP-100 and AR pitch) in spite of an obvious difference in the AC of the two. 32 The differences in the AC in the pitch precursors seem to have less effect on the mechanical properties of 1000 °C carbonized large-diameter (e.g., Φ ∼ 20 μm) fibers, than for smaller fibers (e.g., Φ ∼ 10 μm), a fact that is rarely mentioned in previous work on carbon fibers, except for the references to AC in highly purified pitch-derived carbon fibers. 2 , 15 , 44 …”

“…In contrast, AR pitch-derived carbon fibers display a good crystallite orientation and large graphenelike carbon layers notwithstanding the presence of severe shrinkage cracking as indicated in previous work. 32 , 33 …”

“…The average value of 30 sets of parallel data with an error of less than 10% was adopted. 32 The axial electrical resistivities of the carbon fibers were obtained by measuring the average value of 15–20 filaments with a BS407 precision millimicro ohmmeter using a standard four-probe method at room temperature. The microcrystalline structure of carbon fiber samples in a powder form was determined by X-ray diffraction (XRD) analysis on a Philips X′PERT MPD PRO diffractometer.…”