Engineering microstructure toward split-free mesophase pitch-based carbon fibers

High alignment of graphene in mesophase pitch-based carbon fibers endows them with excellent properties such as high thermal conductivity and ultrahigh modulus. However, the weak interlayer interaction between large planar molecules leads to unwanted open crack in circular carbon fiber, leading to low strain-to-failure and low utilization rate in composite manufacturing. It remains a challenge to produce split-free circular carbon fiber while keeping a high degree of alignment of graphite layers. Herein, a die swell manipulating strategy is employed to address the problem through elaborating spinneret design. By applying an exit angle of 30° at the spinneret, the stored stress and shear-induced orientation of crystallites during the capillary flow were relaxed at the exit to enhance the die swell. The disoriented texture of fibers prevents the carbon fibers from splitting while still maintaining high degree of molecular alignment. The obtained carbon fibers present 20% higher of tensile strength, and more than twice of the compressive strength than the split ones, without sacrificing their superior thermal properties, which is up to 512 W·m⁻¹·K⁻¹. The work provides valuable insights into the design of carbon fiber for structural and functional integration materials in aerospace and beyond.