Inelastic neutron spectra of polyacrylonitrile-based carbon fibers

Z. E. Brubaker, A. Miskowiec, Y. Q. Cheng, L. Daemen, and J. L. Niedziela

Phys. Rev. Materials 6, 013609 – Published 26 January 2022

Abstract

We present the vibrational spectra of polyacrylonitrile-based carbon fibers collected using inelastic neutron scattering. We ascertain the behavior of a broad range of vibrational spectra that are optically silent, and demonstrate a direct connection between these modes and thermomechanical properties of the fibers. We show directionally dependent coupling of hydrogen in the carbon fiber matrix, which is directly linked to mechanical properties. Further, we show hydrogen preferentially couples to the midband of the vibrational spectrum, and that there are higher overall mode populations in the traditional Raman D–G intervalley region, suggesting involvement of these modes in tensile strength reduction and transport properties.

Received 16 July 2021
Revised 2 November 2021
Accepted 7 January 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.013609

Physics Subject Headings (PhySH)

Defects Lattice dynamics

Carbon-based materials

Inelastic neutron scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Z. E. Brubaker ^*, A. Miskowiec, Y. Q. Cheng, L. Daemen, and J. L. Niedziela ^†

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*brubakerze@ornl.gov
^†niedzielajl@ornl.gov

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Vol. 6, Iss. 1 — January 2022

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Images

Figure 1
Raman spectra of each carbon fiber acquired with a 100X objective and 532 nm excitation wavelength. The width and amplitude of the D1 peak near 1350 ${cm}^{- 1}$ decreases substantially with increasing modulus; see [21] for a more detailed discussion. The changes of the D1 peak may be correlated with hydrogen defects observed in the present work.
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Figure 2
Raw diffraction intensity overplotted with the background from the aluminum sample holder for the (a) $k_{i}^{75}$ and (b) $k_{i}^{⊥}$ configuration. The black dashed line shows the position of the peak that was used to normalize the inelastic neutron scattering data. Inset: background-subtracted data near 2 Å. All INS spectra were normalized to the peak height near 2 Å.
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Figure 3
INS spectra of carbon fibers collected in the (a) $k_{i}^{75} and$ (b) $k_{i}^{⊥} configurations$ . The data are background subtracted, corrected for the thermal occupation factor, and normalized to the diffraction peak near 2 Å. The constant background is not subtracted to highlight the incoherent scattering arising from hydrogen. The solid black lines correspond to the total fit for each spectrum and the dashed curves correspond to the fit components of the IM10 spectra. The vertical black dashed lines denote peaks discussed qualitatively, and the gray dot-dashed lines denote peaks discussed quantitatively in Fig. 4. The peaks highlighted near 800–900 ${cm}^{- 1}$ and 1200 ${cm}^{- 1}$ correspond to C–H bending modes. The solid red line corresponds to data obtained from polycrystalline graphite and is reproduced from [34].
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Figure 4
Analysis of several peak amplitudes compared against thermomechanical properties for (a)–(c) $k_{i}^{75}$ and (d)–(f) $k_{i}^{⊥}$ configurations. The 3000 ${cm}^{- 1}$ peak corresponds to the C–H stretch mode and the background likely corresponds to incoherent scattering from hydrogen. The peak amplitude of the C–H stretch mode decreases with increasing modulus and strength and increases with increasing conductivity, suggesting that hydrogen-based defects critically affect the thermomechanical properties of fibers. Other peaks show similar trends and highlight strong coupling observed in the 800–1800 ${cm}^{- 1}$ spectral region.
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Figure 5
Background subtracted data for all fibers from the (a) $k_{i}^{75}$ and (b) $k_{i}^{⊥}$ configuration. The relative positions of major crystallographic peaks are well aligned for the graphitic portions of the system. Substantial structural information is retained below $d = 1$ Å. Sharp dips and spikes correspond to residuals from the background subtraction. All INS spectra were normalized to the peak height near 2 Å.
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Figure 6
INS spectra of T700 fibers at 5 K (black) and 300 K (gray) in the (a) $k_{i}^{⊥}$ and (b) $k_{i}^{75}$ configurations. Data at 300 K were acquired for less total time, resulting in a poorer counting statistic. The intensity mismatch is possibly due to a background subtraction issue, so we focus on the relative peak locations and shapes. The low energy acoustic band is broadened and a new low energy peak appears in the $k_{i}^{75}$ configuration near 120 ${cm}^{- 1}$ . The peaks near 800 ${cm}^{- 1}$ appear to merge and the ratio of peak intensities for the high energy peaks shifts.
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