Graphene reinforced carbon fibers

Abstract

The superlative strength-to-weight ratio of carbon fibers (CFs) can substantially reduce vehicle weight and improve energy efficiency. However, most CFs are derived from costly polyacrylonitrile (PAN), which limits their widespread adoption in the automotive industry. Extensive efforts to produce CFs from low cost, alternative precursor materials have failed to yield a commercially viable product. Here, we revisit PAN to study its conversion chemistry and microstructure evolution, which might provide clues for the design of low-cost CFs. We demonstrate that a small amount of graphene can minimize porosity/defects and reinforce PAN-based CFs. Our experimental results show that 0.075 weight % graphene-reinforced PAN/graphene composite CFs exhibits 225% increase in strength and 184% enhancement in Young's modulus compared to PAN CFs. Atomistic ReaxFF and large-scale molecular dynamics simulations jointly elucidate the ability of graphene to modify the microstructure by promoting favorable edge chemistry and polymer chain alignment.

Fig. 1. Illustration of the wet spinning process, microstructure, and mechanical properties of the PAN/graphene composite CFs.

(A) Illustration of the fabrication process of PAN/graphene precursor fibers. DI, dionized. (B to G) SEM images of the carbonized PAN/graphene composite fibers with different weight percentage of graphene, (B) 0.00 wt %, (C) 0.01 wt %, (D) 0.025 wt %, (E) 0.05 wt %, (F) 0.075 wt %, and (G) 0.1 wt %. (H and I) Mechanical properties of the carbonized PAN/graphene CFs with different graphene concentration.

Fig. 2. Nanotomography measurement of PAN/graphene composite CFs.

Images are shown for different levels for graphene content: (A) 0.00 wt %, (B) 0.025 wt %, (C) 0.075 wt %, and (D) 0.1 wt %. (E) Each panel shows two axial cross sections located at different positions on the fiber length. (F) 2D model of elliptical hole in infinite plate.

Fig. 3. TEM images and Raman spectra of the PAN/graphene composite CFs.

(A) TEM images of the added graphene obtained from shear exfoliation. (B) HRTEM image of the graphene, with inset showing the corresponding FFT pattern. (C) TEM image of PAN/graphene (0.075 wt %) precursor fiber, with inset showing the FFT pattern of the selected area (red square). (D and E) TEM images of the carbonized PAN/graphene fiber (0.075 wt % graphene) at different magnifications. (F) Raman spectra of the carbonized PAN/graphene fibers with different concentrations of graphene. a.u., arbitrary unit. (G) The possible flow-induced graphene alignment mechanism of the PAN/graphene dope (33).

Fig. 4. Atomistic ReaxFF simulations of the initial stage of carbonization process for oxidized PAN and oxidized PAN/graphene precursors.

Production of (A) N₂, (B) H₂, (C) H₂O molecules. Formation of (D) 5-, (E) 6-, and (F) 7-membered carbon rings. (G) Carbon content of fibers at different simulation times for the oxidized PAN and oxidized PAN/graphene precursors. (H and I) Snapshots of the oxidized PAN/graphene taken during the carbonization process to show the formation of 5/6/7-membered carbon-only rings at the graphene edges. Carbon, nitrogen, oxygen, and hydrogen are represented black, blue, red, and white, respectively. Purple spheres represent the initial graphene structure. Dark lines and shadows in (A) to (F) are the average and SD over eight different samples, respectively. Insets (A to F) show simulation results for the last 100 ps.

Fig. 5. Nonreactive MD simulations of structural self-organization of PAN chains with and without the presence of a graphene sheet.

Snapshots (A and B), ring orientation distributions (C to E), and HOF distributions (F) of PAN/graphene and PAN structures. In the snapshots of PAN/graphene and PAN structures, individual PAN chains are shown by lines joining carbon ring centers. For PAN/graphene structure, the horizontal gap in the middle of the system corresponds to the graphene sheet, which is not shown for visualization purposes. The ring centers are colored according to the cosine angles of ring normal vectors with respect to the vertical z axis. The color scheme is shown in (D) and (E).