Forming mechanism of surface nitriding of high strength and high modulus carbon fiber by electrochemical modification
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摘要: 高温石墨化使高强高模碳纤维(CF)表面光滑，反应活性低，导致其复合材料界面粘接性能差。杂原子改性是改善CF表面反应活性的有效手段之一。采用循环伏安(CV)方法在有机复合电解液中对高强高模CF进行了表面氧化和氮化改性，采用CV优选的复合电解液进行恒流电化学氧化处理，研究了CV扫描次数和电解液中含氮有机物对CF表面化学组成的影响。电化学处理前后CF表面化学元素组成和微观形态变化通过XPS、SEM及拉曼光谱表征。基于实验数据探讨了CF表面含氮官能团的生成及转变机制。研究结果显示，有机溶剂、有机氮源和含硫铵盐的协同作用使CF表面N含量从0.28at%增至4.77at%。适量的水存在，可以使CF表面O含量显著提高。CF表面的含氧官能团可以与CO(NH2)2中的—NH2及电解液中的NH4+反应形成酰胺基团，随着反应时间延长，CF表面的酰胺N会先转变成氧化氮，随后转变成吡啶和吡咯N，并最终转换成石墨N。恒流电化学处理后CF/环氧树脂复合材料的层间剪切强度(ILSS)较未处理的提高了132%，同时CF拉伸强度略有提高，表明有机复合电解液是一种温和、有效的CF表面电化学处理液。Abstract: CF has smooth surface and low reactivity due to high temperature graphitization, resulting in poor interfacial adhesion of CF composites. Heteroatom modification is one of the effective methods to improve the surface reactivity of CF. In the organic composite electrolyte solution, the surface oxidation and nitridation treatment of the high-strength and high-modulus carbon fiber (CF) were carried out by cyclic voltammetry (CV). The surface element composition and microscopic morphology of CF were characterized by XPS, SEM and Raman spectroscopy. A comparative analysis based on the obtained data reveals the nitriding mechanism of the CF surface and the source of the substance introducing the nitrogen-containing functional group. The results show that under the synergistic action of organic solvent, organic nitrogen source and S-containing ammonium salt, the surface nitrogen content of CF increases from 0.28at% to 4.77at%. When there is an appropriate amount of water in the solution, the number of oxygen-containing functional groups is significantly increased. The reaction between the acidic oxygen-containing functional groups formed during oxidation, the amino group of urea, and the ammonium ions in the solution is the key to forming C—N bonds on the surface of CFs. As the reaction time prolongs, the nitrogen-containing functional groups on the surface of the CF gradually transform from amide nitrogen to nitrogen oxides, then to pyridine and pyrrole, and finally to the graphitized nitrogen. The interlayer shear strength (ILSS) of CF/epoxy composites after constant current electrochemical treatment is 132% higher than that of untreated, and the tensile strength of CF increased slightly. The results show that the organic composite electrolyte is a mild and effective solution for electrochemical treatment of CF surface.
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图 2 不同含氮无机物电解液中CF的CV曲线及XPS图谱：(a) E/U/S-2；(b) E/U/C-2；(c) E/U/P-2；(d) XPS曲线
Figure 2. CV curves of CF in different nitrogen-containing inorganic electrolytes: (a) E/U/S-2; (b) E/U/C-2; (c) E/U/P-2; (d) XPS survey spectra of CF
图 3 不同含氮有机物电解液中CF的CV曲线及XPS图谱：(a) E/U/S；(b) E/S；(c) XPS曲线
Figure 3. CV curves of CF in different nitrogen-containing organic electrolyte solutions: (a) E/U/S; (b) E/S; (c) XPS spectra of CF
图 5 CF表面氮化反应历程
Figure 5. Schematic diagram of the CF surface nitridation reaction path
图 6 不同水含量溶液处理后CF的SEM图像
Figure 6. SEM images of CF after treatment with different water content solutions
图 7 电解液E/U/S-50中处理前后CF的拉曼图谱
Figure 7. Raman spectra of CF before and after treatment with E/U/S-50 electrolyte
R—Ratio of relative intensity of D peak to G peak
表 1 循环伏安(CV)实验参数
Table 1. Cyclic voltammetry (CV) experimental conditions
Electrolyte Sample Scanning
speed/(mV·s−1)EG/NH4HSO4 E/S 150 EG/urea/NH4HSO4 E/U/S 150 EG/urea/NH4HSO4/2wt%H2O E/U/S-2 150 EG/urea/NH4HCO3/2wt%H2O E/U/C-2 150 EG/urea/NH4H2PO3/2wt%H2O E/U/P-2 150 EG/urea/NH4HSO4 E/U/S 20 EG/urea/NH4HSO4/1wt%H2O E/U/S-1 20 EG/urea/NH4HSO4/2wt%H2O E/U/S-2 20 EG/urea/NH4HSO4/5wt%H2O E/U/S-5 20 EG/urea/NH4HSO4/20wt%H2O E/U/S-20 20 EG/urea/NH4HSO4/50wt%H2O E/U/S-50 20 H2O/urea/NH4HSO4 H2O/U/S 20 Note: EG—Ethylene glycol. 
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表 2 不同CV循环次数下CF表面元素含量
Table 2. Element contents on CF surface after treatment with different CV cycles of E/U/S electrolyte
Cycles 0 1 8 15 20 50 70 O/at% 4.97 6.59 8.52 9.33 13.00 7.80 10.30 N/at% 0.22 1.90 2.59 3.22 4.77 2.38 3.78
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表 3 不同CV循环次数下CF表面的含氧官能团含量
Table 3. Contents of oxygen-containing functional groups on CF surface with different CV cycles
Cycle Atomic fraction of functional groups/at% C—C C—OH, C—N,
C—O—RC=O COOH
COOR0 88.4 8.81 0.36 2.43 1 76.3 12.64 0.23 10.83 8 66.1 21.10 — 12.71 15 68.6 19.33 — 12.07 20 82.8 11.79 1.23 4.18 50 90.3 6.87 1.72 1.02 70 76.0 12.05 0.49 11.46
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表 4 不同水含量下CV处理的CF表面元素组成
Table 4. Element contents on CF surface after CV treatment with different water content electrolytes
Sample C/at% O/at% N/at% Si/at% E/U/S 88.14 7.80 2.38 1.68 E/U/S-1 86.93 9.85 3.05 0.17 E/U/S-2 78.55 16.59 4.74 0.12 E/U/S-5 82.82 13.52 3.66 0.00 E/U/S-20 79.98 15.59 4.31 0.13 E/U/S-50 88.79 8.72 2.06 0.44 H2O/U/S 95.06 4.26 0.49 0.19
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表 5 电解液E/U/S-50中处理后CF力学性能变化
Table 5. Changes of mechanical properties of CF before and after treatment with E/U/S-50 electrolyte
Sample σ/MPa E/GPa ε/% ILSS/MPa Untreated CF1 4052 541 0.7 22 Treated-CF1 4478 567 0.8 51 Untreated CF2 4082 507 0.8 40 Treated-CF2 4280 499 0.8 60 Notes: σ—Tensile strength; E—Tensile modulus; ε—Elongation at break; ILSS—Interlaminar shear strength.
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