Regulating the radial structure during pre-oxidation of polyacrylonitrile fibers and its effect on the mechanical properties of carbon fibers
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摘要: 预氧纤维结构及其分布直接影响碳纤维的性能，优化预氧结构径向分布，构建预氧结构与碳纤维力学性能的关系，对制备高性能炭纤维具有指导意义。本文采用固体核磁、光学显微镜、热失重等表征方法，通过温度控制预氧化反应速率，研究预氧化反应速率对纤维结构和径向分布及碳纤维性能的影响。结果表明，整体提高预氧化反应速率，在促进预氧结构向纤维更深区域扩展的同时，也导致含氧结构增加较多，热稳定性降低，影响碳纤维的性能；而提高预氧化初期反应速率，有效改善预氧结构径向分布的同时纤维含氧结构增加较少，热稳定性提高，最终碳纤维的石墨化程度和致密性较高，力学性能显著提升，成功获得一种具有高强中模特征、直径相对较大的炭纤维新品种。Abstract: The radial structure of pre-oxidized fibers and its distribution directly affect the performance of the resulting carbon fibers. Optimizing the radial distribution of pre-oxidized structure and establishing the relationship between the pre-oxidized structure of polyacrylonitrile fibers and the mechanical properties of the final carbon fibers will help to optimize the pre-oxidation conditions in the preparation of high-performance carbon fibers. Herein, solid-state nuclear magnetic resonance spectroscopy, optical microscopy, thermogravimetric analysis, and mechanical tests were used to investigate the effect of the pre-oxidation reaction rate on the radial structural distribution of pre-oxidized fibers and the mechanical properties of the resulting carbon fibers. The pre-oxidation reaction rates were controlled by regulating the pre-oxidation temperature gradient. The results showed that the pre-oxidation degree of pre-oxidized fibers increased with both the overall and initial rates of pre-oxidation. With increasing the overall pre-oxidation reaction rate, the pre-oxidized structure was deepened into the core region of the fibers, the content of oxygen-containing functional groups increased, the thermal stability of the fibers decreased, the graphitization degree of the corresponding carbon fibers increased, but the density of the carbon fibers decreased and the mechanical properties of the carbon fibers were degraded. With increasing the initial reaction rate of pre-oxidation, the radial distribution of the pre-oxidation structure was effectively improved, the content of oxygen-containing functional groups of the pre-oxidized fibers increased slightly, their thermal stability was improved, the degree of graphitization and density of the final carbon fibers increased, and the tensile strength and tensile modulus of the final carbon fibers were markedly increased. A new type of carbon fibers with high strength, medium modulus and a relatively large diameter was obtained under the optimized pre-oxidation conditions.
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Figure 1. Schematic diagram of the positions where the optical density of a pre-oxidized fiber was measured.
Figure 2. Radial distribution of the pre-oxidation degree in sample 1 fibers: (a) Micrograph of the fiber cross section and (b) radial optical density distribution of the fiber cross section.
Figure 3. Micrographs of the cross sections of pre-oxidized fibers prepared with different pre-oxidation rates: (a) Sample 1, (b) 2, and (c) 3.
Figure 4. Radial optical density distribution of pre-oxidized fibers subjected to different pre-oxidation reaction rates
Figure 5. Effect of the pre-oxidation reaction rate on thermal stability of pre-oxidized fibers.
Figure 6. Radial distribution of the pre-oxidation degree in fibers: (a) Micrograph of the fiber cross section of sample 4 and (b) optical density distribution of the fiber cross section for sample 1 (black) and 4 (red).
Figure 7. Effect of the initial reaction rate on the thermal stability of pre-oxidized fibers.
Figure 8. Radial distribution of the g values of carbon fiber samples determined from Raman spectra.
Table 1. Pre-oxidation furnace temperatures (°C) used to prepare Samples 1-4.
Sample 1 2 3 4 5 6 1# 200 215 238 255 260 265 2# 200 213 235 250 255 260 3# 200 220 243 255 265 270 4# 210 225 238 255 260 265 Note: processing time: 1 h, atmosphere: air. 
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Table 2. 13C ssNMR analysis results for pre-oxidized fibers subjected to different pre-oxidation reaction rates.
Sample C=
C115
ppmC=
CH139
ppmC=
N153
ppm-C=
O176
ppmRCI/
(%)Gh/
(%)RC=O/C=N/
(%)△RC=O/C=N/
(%)1# 14.21 8.86 20.41 4.61 55.3 65.3 22.6 0.0 2# 14.24 8.84 20.11 4.51 54.5 65.0 22.2 -0.4 3# 15.60 9.69 21.10 5.21 56.7 78.0 24.7 +2.1
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Table 3. Optical density analysis results of PAN fibers pre-oxidized at different reaction rates.
Sample OD0 OD±1 OD±2 OD±3 OD±4 OD±5 ODm 1# 0.555 0.546 0.531 0.505 0.500 0.494 0.508 2# 0.559 0.553 0.540 0.525 0.519 0.512 0.522 3# 0.543 0.530 0.478 0.418 0.412 0.408 0.436
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Table 4. Characteristic structural parameters of PAN fibers pre-oxidized at different initial reaction rates.
Sample RCI
/(%)Gh
/(%)RC＝O/C＝N
/(%)△RC＝O/C＝N
/(%)ODm 1# 55.3 65.3 22.6 0.0 0.508 4# 56.6 69.8 22.7 +0.1 0.492
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Table 5. Structures of different pre-oxidized fibers and properties of corresponding carbon fibers.
Samples Structures of pre-oxidized fibers Properties of carbon fibers RCI/(%) Gh/(%) RC＝O/C＝N/(%) Linear density/(g·m-1) ρ/(g·m-3) σ/(GPa) E/(GPa) 1# 55.3 65.3 22.6 0.0697 1.7399 5.28 273 2# 54.5 65.0 22.2 0.0682 1.7524 5.02 268 3# 56.7 78.0 24.7 0.0717 1.7384 5.21 265 4# 56.6 69.8 22.7 0.0700 1.7549 5.50 279
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