To study the influence of mass and heat transfer on the microcrystalline structure and properties of mesophase pitch and resulting carbon fiber properties, mesophase pitches were synthesized via pressurized/N₂-blowing thermal condensation with different stirring rates, with experimental conditions optimized using response surface methodology (RSM). RSM analysis confirmed that mesophase content was highly dependent on stirring rate (p < 0.05), and the influencing factors on the formation of mesophase pitch is ranked as reaction temperature > duration time > stirring rate > reaction pressure. The results demonstrated that a moderate increase in stirring rate enhanced molecular diffusion and heat transfer, improving reaction kinetics and aromatic molecule interactions. This accelerated mesophase sphere growth and coalescence while inducing molecular orientation via shear, ultimately yielding a wide-domain optical texture with 100 vol% mesophase content and an optimal softening point (294 °C). The mesophase pitch produced at 300 rpm (MP-300) exhibited a high aromatic structure content (Har = 73.24 wt%), methylene bridges (HF = 3.21 wt%), and fusible/soluble TI-PS sub-fraction (35.7 wt%). MP-300 also displayed exceptional aromaticity (fa = 0.91) and molecular stacking ((Lc = 5.4674 nm, N = 14.7536). Consequently, the resulting carbon fiber (MPCF-300) achieved optimal mechanical properties, with a tensile modulus of 168 GPa and a tensile strength of 1428 MPa. However, excessive stirring rates were found to disrupt molecular cross-linking and stacking, reducing condensation degree, disordering the orderly arrangement of the mesophase molecules, and ultimately impairing the fiber performance. These findings advance the understanding of mesophase pitch formation and provide critical insights for optimizing liquid-phase carbonization theory.

Effects of mass and heat transfer on the development of microcrystalline structure of mesophase pitch and the properties of derived carbon fibers
    Abstract
    1 Introduction
    2 Experimental section
        2.1 Materials
        2.2 Design of experiments
        2.3 Preparation of mesophase pitch
        2.4 Preparation of mesophase pitch-based carbon fibers
        2.5 Characterizations
    3 Results and discussion
        3.1 The effect of stirring rate on heat transfer
        3.2 Effect of stirring rate on the formation of mesophase pitch by RSM
        3.3 Effect of stirring rate on structure and properties of mesophase pitches
            3.3.1 Properties and optical texture analysis of mesophase pitches
            3.3.2 Molecules structure analysis of mesophase pitch
            3.3.3 Thermal stability of mesophase pitch
            3.3.4 Crystal structure of mesophase pitch
    3.4 Effect of stirring rate on structure and properties of mesophase pitch-based carbon fibers
    4 Conclusions
    References

• Guangxue Zhou(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China)
• Bin Lou(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China) Corresponding author
• Nan Shi(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China)
• Xiujie Yang(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China)
• Dong Liu(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China) Corresponding author
• Jun Li(CNOOC Institute of Chemicals & Advanced Materials, Beijing 102200, People’s Republic of China)
• Xiangdong Wang(Jining Ruicheng Aerospace Carbon Materials Co., Ltd, Jining 272000, People’s Republic of China)
• Fushan Wen(School of Materials Science and Engineering, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China)