Strengthened PAN-based carbon fibers obtained by slow heating rate carbonization.

Extensive research of PAN-based carbon fibers (CFs) has focused on developing new precursors and modifying the process of stabilization and carbonization in order to control and to maximize their mechanical performance1,2,3,4. Particularly, a number of steps and temperature variations implemented to reach the final carbonization and graphitization temperatures have been recognized as important variables5. It is well-known that increasing the final carbonization temperature up to ~1200 °C increases both the tensile strength and the modulus of CFs, but a further temperature increase can affect the tensile strength while still increasing the tensile modulus because the carbon microstructure changes to a more graphitic one6. Therefore, during the production of high-strength grade CFs, the maximum temperature for carbonization is generally below 1200 °C. Another important feature of the CFs production process is the two-step carbonization process of the PAN precursor; the first step is a low-temperature treatment (LTT) at 600–800 °C, while the second step is a high-temperature treatment (HTT) at ~1200 °C (5–10 min for each step)7. It should be noticed that the evolution of gas during the carbonization process must be taken into account in order to optimize the tensile strength. Indeed, the carbonization stage is a process where elements such as nitrogen, oxygen, and hydrogen are removed from the precursor fibers, leaving a material with at least 93 wt% of carbon content7,8. We herein report the effect of the microstructure in PAN-based CFs on their mechanical properties. Commercially available PAN fibers were stabilized by oxidation in air using a tailor-made apparatus. The resulting fibers were infusible and could be carbonized in a tubular furnace at temperatures up to 1000, 1050, 1100, and 1200 °C with heating rates of 0.5–10 °C/min, respectively. The chemical compositions of the CFs, particularly the carbon and nitrogen content, and the chemical environment as a function of the carbonization temperature and heating rate were correlated with mechanical properties. We also carried out MD simulations to understand the role of nitrogen species and modeled the carbon sp3 structure. We found that both the sp3 hybridized carbon and especially the quaternary nitrogen atoms have a strong effect on the tensile strength of the fibers. We have proposed a microstructure model where two adjacently stacked hexagonal carbon networks strongly bond by interstitial sp3 carbons with the aid of the interlayer forces, which are enhanced by the neighboring quaternary nitrogen at the bonding site. This model of carbon microstructure is a new concept in the high-strength CF research field. This research is expected to contribute significantly to the production of carbon fiber for high performance products.