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Graphene reinforced carbon fibers, Zan Gao, et al., Xiaodong Li, 2020

Carbon fibers (CFs) are of great importance for obtaining advanced composites for modern engineering. Through studying the microstructure evolution and conversion chemistry of PAN-based carbon fiber processing, 0.075 wt% addition of graphene as reinforcement lead to PAN/graphene composite CFs with 225% increase in strength and 184% enhancement in Young’s modulus compared to PAN CFs. Comprehensive characterization and simulations at different processing stages show that graphene sheets are uniformly distributed, pores are eliminated, and graphene induces favorable edge bonding and higher chain alignment. These provide a foundation for developing low-cost precursor fibers enhanced by graphene for high-performance CFs.

 

The work does not include how to compare with the properties of current industry/commercial CF. This work: carbonized PAN/graphene CFs: strength 1916 MPa, modulus 233 GPa,

Commercially CFs (from book, Hejun LiLehua Qi, Shouyang Zhang, Advanced Composite Materials Science, Northwestern Polytechnical University Press, page 80-82):

High-strength CF: 228-241 GPa, 3105-4555 MPa

High-modulus CF: 276-380 GPa, 2415-2555 MPa

 

Scientific question: how to improve CFs performance through a cost-effective manufacturing?

Key of how: adding reinforcement of graphene by wet spinning PAN; structurally obtain uniform distribution, eliminates micro and nanopores, increase alignment and local orderness/crystallinity.

Major points:

Currently, more than 90% of the CF market is dominated by the expensive polyacrylonitrile (PAN) precursors, and PAN precursors consume more than 50% of the cost of traditional PAN-derived CFs. Other processing such as interfacial control of CF-reinforced composites have been documented, while cheaper raw materials, e.g., pitch, cellulose, lignin, to replace PAN as low-cost precursors produce CFs that have poor mechanical properties. So, it needs to revisit the PAN precursor and its processing into CFs, understand the fundamental principles and the structure-processing-property relationship, especially which processing controls which type of structure thus decides which mechanical properties.

PAN-based CFs manufacturing route: wet/dry-jet spinning of polymer filaments, thermal treatment including stabilization (200-300oC, form N-containing ladder structures), carbonization (1200-1600oC, form a turbostratic carbon structure), and graphitization (2100oC, form ordered, graphitic domains).

Processing parameters (constitutes, temperature, stretching…) control structural details (alignment, defects, crystallization...), and thus affect the overall mechanical properties. Adding carbon nanotubes, graphene oxide liquid crystal, and also grapahene (this is not mentioned in the introduction) have been used to improve precursor and also CFs.

 

Adding graphene reduces the size of pores and voids, and increase tensile strength, modulus and strain of carbonized composite CFs. Through observation and Inglis solution, stress concentration of microscale elliptical and nanoscale equiaxed pores are minimized at 0.075wt% addition of graphene.

 

Mechanisms may be the addition of graphene: (1) affects spinning dope property, (2) graphene align axially in extrusion and solidification stages, (3) graphene reinforce the filament, (4) help polymer chain align, (5) facilitates local orderness in solidification.

 

To investigate the mechanisms, comprehensive characterization at different processing steps were used: (1) viscosity of spinning dope to infer the aligned graphene guiding solidification of PAN molecules, (2) TEM to observe the uniform distribution, crystal zones preserved in carbonized fibers, (3) Raman spectra to know the reduced defects, more ordered graphitic structure, and higher axial orientation, (4) TGA to know, generally similar thermal degradation between PAN and PAN/graphene CFs, (5) SEM and mechanical tests, precursor fiber (adding graphene, denser stronger, strain decrease at 0.1%), oxidized fibers (similar trend), carbonized fibers (similar trend, but marked increase in strength, modulus, decrease in strain), indicate graphene takes effects from initial spinning stage to carbonization, (6) atomistic ReaxFF simulation to study that dangling bonds at graphene edges form bonds with polymer matrix and catalyze the formation of graphitic structure (greater alignment), (7) nonreactive molecular dynamic simulation to known considerable local realignment of PAN chains along the graphene.

This can stimulate the development and processing of making high-performance carbon fibers from PAN.

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