C-c composite or carbon-carbon composite material (C /C composite) is a carbon matrix composite reinforced by carbon fiber and its fabric. It has low density (
How to characterize them? This paper introduces several common representation methods.
Scanning Electron Microscopy (SEM)
2. Transmission Electron Microscopy
The growth process of GNs on the surface of CNTs was studied by TEM, a shows a typical TEM image of the surface of CNT, indicating that CNT has a multilayer structure and good crystallinities. In CF-CNT-GN-30, the nanogaps (yellow stars in b and c) observed in the CNT wall are the growth nuclei of GN, which grow to form covalent bonds between GN and CNT substrate. GNs of different sizes were observed in d, indicating the presence of nucleation throughout the growth phase. The graphite wall of the CNT in CF-CNT-GN-60 shows many etching pits (e, f). The CNT substrate in CF-CNT-GN-120 is severely etched and destroyed, and part of the CNT disappears (h). At the later stage of the reaction, many interwoven GNs completely cover the CNT. The hybrid CNT-GN structure (h, i) is formed in the shape of a spike bar, and CNT-GN-90 is the most likely to prepare CNT-GN-C/C composites with high mechanical properties.
Figure 2: TEM image of CF-CNT-GN hybrid preforms (a) CNT; (b, c) CF-CNT-GN-30; (def) CF-CNT-GN-60; (g) CF-CNT-GN-90; (h, i) CF-CNT-GN-120
3. Polarized Light Microscope (PLM)
PLM was used to study the polished cross sections of CNT-C/C and CNT-GN-C/C composites, and straight grain boundaries were observed for CNT-C-C composites with much larger pyrolysis carbon (PyC) grain size. PyC in phase CNT-GN-C/C composites has a smaller grain size and a shorter curved grain boundary. The PyC crystal size of CNT-C/C is significantly larger than that of the PyC grain size of CNT-GN-C/C composites, indicating that there is a refined matrix in CNT-GN-C/C composites.
4. Raman Spectroscopy
The figure shows the results of Raman spectra of onion-like carbon OLC/CNF films at different carbonation temperatures. All samples had distinct D and G peaks (characteristic peaks of sp2 hybridization, whose presence indicates the presence of a graphite structure in the material) near 1350 cm−1 and 1580 cm−1, respectively.
Figure 4: Raman spectra
5. Fourier Infrared Spectroscopy (FTIR)
OLC/CNF precursors, OLC/ CNF-intermediates, and OLC/CNF were analyzed using Fourier infrared spectroscopy. Chemical functional groups of membranes. The characteristic peaks of OLC/CNF precursors are -- CN, -- CH and -- CH2, which are located at 2243-2240 cm−1, 2940 cm−1, 2850 cm−1, 1450 cm−1 and 1360 cm−1 and 1050 cm−1 respectively in the infrared spectrum (see FIG. S7). After preoxidation, the -CN functional group located at 2243 cm−1 gradually disappeared and was replaced by a -C-C -- located at about 1660 cm−1. The -CH2 at 1450 cm−1 gradually disappears (very weakly) due to the appearance of -C-C-c.
A weak peak occurs at 800cm−1 and the carbon atoms begin to recombine. At this time, OLC/CNF precursor is transformed into OLC/CNF intermediate. During carbonization, with the increase of temperature, the functional groups are only retained at 1547cm−1 (-- C-C -- C --) and 1270 cm−1 (-- C-C -- C --), and the carbonization is basically completed. When the carbonization temperature reaches 1100℃, all the characteristic peaks of functional groups almost disappear, indicating that the carbonization has been completed. OLC/CNF intermediate is transformed into OLC/CNF membrane.
Figure 5: OLC/CNF precursor, OLC/ CNF-intermediate, and OLC/CNF. FTIR image of the membrane
6. X-ray Photoelectron Spectroscopy (XPS)
The binding energy of carbon atoms in the acid-treated sample increased by 1eV, indicating that the carbon atoms on the fiber surface formed C-O and C-H bonds after acid treatment.
Figure 6: Full-spectrum XPS and C-element XPS spectra
7. X-ray Diffraction (XRD)
The peak of G/CFF-50 is 25.8◦, and the diffraction peak at 26.88◦ is the graphite in the graphite prepreg before graphitization. The diffraction peak positions after graphitization are 26.58◦, 44.7◦, and 54.7◦, which correspond to the (0 0 2), (0 0 3), and (0 0 4) crystalline phases of graphite. High temperature graphitization leads to the crystal reconstruction of the precursors and the reduction of the lattice spacing, resulting in the synthesis of C/C composites. Samples with different CF mass fractions correspond to different diffraction peak intensities, with an overall trend of first increasing and then slightly decreasing.
Figure 7: XRD patterns of G/CFF preforms before and after graphitization
8. X-ray flaw detection
X-ray tomography analysis of pore distribution and pore size shows that the pore size of G/CFF-50 is mainly about 13µm, and the number of pores of this size accounts for about 62%. The C/C-50 pore sizes are distributed around 13, 20 and 27µm, and the number of pores of this size accounts for about 15%, 26% and 13%, respectively. The pore volume of G/CFF-50 accounts for about 1.65% of the whole sample, while that of C/C-50 accounts for about 3.47%. X-ray results show that high temperature graphitization will lead to pore expansion, resulting in a decrease in sample density, but hot isostatic pressing can effectively suppress pore expansion.
Figure 8: (a) G/CFF-50 and (b) C/C-50 aperture statistics
9. Specific Surface Area and Aperture Analysis (BET)
The adsorption and desorption curves of OLC/CNF precursors show reversible type III isotherms, and the main reason for this situation is the nonporous gas adsorption isotherms.
Figure 9: Adsorption and desorption curves of OLC/CNF intermediates
10. Nano indentation
The adsorption and desorption curves of OLC/CNF precursors show reversible type III isotherms, and the main reason for this situation is the nonporous gas adsorption isotherms.
Figure 10: Typical load-depth curves for nanoindentation
11. Conventional mechanical properties (universal testing machine)
The tensile stress and strain of OLC/CNF increased with the increase of OLC content. When the amount of OLC added was 10wt%, the tensile stress reached its highest value (3.9MPa).
Figure 11: OLC/CNF tensile test
12. Thermal Expansion coefficient (CTE)
The lowest CTE was found for HT-C/Cs and the highest for LT-C/C composite. The CTE of LT/HT-C/C composites is close to the average CTE of LT-C/C and HT-C/C.
Figure 12: CTE for C/Cs with different modification processes
13. Thermal conductivity
The highest thermal conductivity of UD-3 is 5.321W/ (m•K), and the lowest thermal conductivity of GD-1 is 3.385W/ (m·K).
Figure 13: Out-of-plane thermal conductivity of gradient and uniform C/C composites
14. Oxyacetylene ablation
The center of all samples was strongly ablated, while the edge was intact, and the maximum temperature of the ablated center surface was between 1958 ° C and 2060 ° C. The minimum mass ablation rate and linear ablation rate of GD-3 were 0.0055g/s and 0.0145mm/s, respectively. The highest mass ablation rate and linear ablation rate of UD-1 were 0.0112 g/s and 0.0350 mm/s, respectively. At the same fiber volume fraction, the ablation rate of the gradient composite is lower than that of the uniform composite.
Figure 14. Macroscopic morphology of C/C composites after ablation
