# Recycled Carbon Fibers with Improved Physical Properties Recovered from CFRP by Nitric Acid
## 3 Results
### Structure and Elemental Distribution of Carbon Fiber Cross-Section  
The results taken from the TEM observation of the carbon fiber cross-sections are shown in Figure 7. First, the vCF TEM observation results showed that voids were present in an area of 400 nm inward from the carbon fiber surface. The size of the voids ranged from 1 nm to approximately 30 nm. [PERSON] et al. confirmed the presence of voids, approximately 10 to 30 nm in size, inside the carbon fibers [20]. They showed that the voids are formed by gas generated during the manufacturing process, and the size and shape vary depending on the amount and type of gas generated [23]. The TEM observation results for vCF 8 h and rCF 12 h to 120 h confirmed the presence of voids; however, the voids located 50 nm inside from the outermost layer, were located much closer to the surface than in the vCF. The results of the void content in the observed carbon fiber cross-sections are summarized in Table 2, which were quantified using image analysis software ImageJ (ver. 1.52a). The results showed that the abundances of voids in vCF 8 h and rCF 12 h to 120 h are 1/3 or less than the abundance of those in vCF. Based on the results above, and as also indicated in the AFM observation results, the reduced abundances of voids and the movement of voids to the top surface layer was considered to be due to the removal of the voids together with the surface layer graphite structure caused by nitric acid [19; 20; 21].  
The results from the EDX mapping of target elements C, N, and O for the observed voids are shown in Figure 8. Neither C, N, nor O was found inside the voids of the vCF. On the other hand, inside the voids of vCF 8 h and rCF 12 h to 120 h, C was not found, while N and O were detected, in which a high concentration of O was confirmed.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & & **vCF** & **vCF 8 h** & **rCF 12 h** & **rCF 24 h** & **rCF 120 h** \\ \hline \multirow{5}{*}{C (\%)} & CHn & 45.1 & 38.1 & 30.7 & 35.8 & 42.3 \\  & C-OH, C-O-C & 27.8 & 23.3 & 25.6 & 23.8 & 21.1 \\  & \textgreater{}C=O & 2.8 & 3.4 & 4.2 & 3.8 & 3.3 \\  & \textgreater{}CO-O- & 0.8 & 3.0 & 2.6 & 2.7 & 4.0 \\  & \textgreater{}C=C- & 0.7 & 1.9 & 1.4 & 1.5 & 2.0 \\ \cline{2-6}  & Total & 77.2 & 69.7 & 64.5 & 67.5 & 72.7 \\ \hline \multirow{5}{*}{N (\%)} & -NH\({}_{2}\) & 0.0 & 1.4 & 1.5 & 1.1 & 1.5 \\  & -NO/NH\({}_{4}\) & 1.2 & 0.7 & 1.3 & 1.3 & 1.3 \\  & -NO\({}_{2}\) & 0.0 & 4.2 & 6.1 & 5.5 & 3.9 \\ \cline{2-6}  & Total & 1.2 & 6.2 & 8.9 & 7.9 & 6.6 \\ \hline \multicolumn{2}{c}{O (\%)} & 19.5 & 22.6 & 26.8 & 24.5 & 20.5 \\ \hline \multicolumn{2}{c}{Others (\%)} & 1.9 & 1.6 & 0.1 & 0.0 & 0.2 \\ \hline \end{tabular}
\end{table}
Table 1: Atomic concentrations of vCF and rCF measured by XPS, and polar functional groups are shown in red words.  
(1360 cm\({}^{-1}\)) indicates surface defects and the disruption of the graphite structure, and the D2 band (1230 cm\({}^{-1}\)) indicates C-H bonds [19]. The areas of the D1, D2, G1, and G2 peaks were calculated, and the results from calculating the ratios relative to the G1 band peak area are shown in Figure 9. \(I_{\text{D1}}/I_{\text{G1}}\) ratios were taken to compare the surface defects and structure disruptions in each specimen, and there was no large difference between rCF 12 h or 24 h compared with vCF. However, in vCF 8 h the ratio was 1.3 times higher than vCF, and in rCF 120 h the ratio rose to 1.7 times higher than the vCF. When the \(I_{\text{G2}}/I_{\text{G1}}\) ratios, which indicate the abundance of amorphous carbon, were compared, rCF 120 h showed a significantly higher value than the vCF, indicating an increase in the abundance of amorphous carbon. Based on the results above, this suggests that disruption occurred in the surface graphite structure in rCF 120 h, and its crystallinity was reduced.