Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage (T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ⩽ T ⩽ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage (T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation.

中文翻译：

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聚丙烯腈基碳纤维高温轴向应力演化机制

温度和拉伸是碳纤维高温处理的重要因素。碳纤维高温处理过程中的轴向应力会影响其拉伸能力。通过原位拉伸试验、拉曼光谱、X射线衍射、元素分析、X射线光电子能谱、高分辨透射电子显微镜、热膨胀系数测试，研究了聚丙烯腈基碳纤维的高温轴向应力演化机制, 和密度方法。聚丙烯腈基碳纤维的高温轴向应力演化包括三个阶段：快速增加、快速减少和松弛。聚丙烯腈基碳纤维的最高应力和松弛温度分别为 1600°C 和 1950°C。影响纤维轴向应力的主要因素包括碳结构重排和热胀冷缩对纤维长度的影响。在第一阶段（T < 1600°C），去除氮原子后的碳结构重排增加了纤维的轴向应力。在第二阶段（1600 ⩽ T ⩽ 1950°C），进入石墨炉的纤维热膨胀与离开石墨炉的纤维冷收缩差异逐渐增大，导致纤维轴向应力减小直到 1950°C，纤维松弛，第三阶段（T > 1950°C）开始。膨胀和收缩之间的差异显着增加，从而增加了纤维松弛。氮原子较少、结构较规则的碳纤维在高温处理过程中轴向应力较低，但趋势和特征温度保持不变。相应的纤维高温最大拉伸比和轴向应力在1950℃以下呈现相反的趋势。拉伸碳纤维的能力在 1950°C 以上增加，这与轴向应力松弛不同。