Stretching modification on mesophase-pitch-based fibers during carbonization process: From laboratory batch experiments to pilot continuous production

Mesophase-pitch-based carbon fibers (MPCFs) have excellent physical performance and are widely used in many cutting-edge fields. The regulation and control of temperature-dependent crystalline structure is very vital for improving their physical properties. Herein, a stretching technique was applied in the carbonization process to produce performance-improved MPCFs, through testing initially in laboratory batch experiments, and then upgrading into a pilot continuous production. For batch carbonization stretching on a bundle of fixed-length fibers, it is found that applying an appropriate tension helps improve their crystalline structure and physical properties. Compared to the “one-stage” constant tension, the “two-stage” variable tension is more conducive to the growth and orientation of graphite microcrystals in MPCFs and the improvement of their physical properties owing to the good match of tension and temperature. For continuous carbonization stretching on a bundle of long filaments, the optimal condition is under a suitable stretching ratio of 0.965 at 500–780 °C for 40 min. The tensile strength and modulus of the 3000°C-graphitized large-diameter MPCFs rise by ∼35% and ∼10%, respectively. The axial electrical resistivity (1.66 μΩ m) decreases by ∼25%, while the thermal conductivity (759.64 W/m K) increases by ∼33%. The effect of the continuous carbonization stretching has been verified to be well accordant with that of the batch in this work, which will pave the road for the wide application of hot-stretching in industrial production of high-performance MPCFs.