The high-strengthening mechanism of carbon nanotube (CNT) yarns subjected to Joule heating was investigated by nanostructural characterization. CNT yarns produced at two drawing speeds (40 and 120 mm/min) with a spindle rotation of 1000 min−1 were used. The mechanical properties of the CNT yarns were improved by Joule heating under a vacuum of 10−6 Pa. The mean tensile strength and Young's modulus of both yarns heated at 2273 K were approximately 1.1–1.4 GPa and 44–50 GPa, respectively; these values are approximately more than two times those of the as-spun samples. The high-strengthening of the Joule-heated CNT yarns is attributed to the synergistic effect of structural changes such as improvement in the crystallinity of the CNTs, formation of the bundle structure of individual CNTs possessing a polygonal structure, and entwining of these bundles. Meanwhile, the samples in which the high-strengthening mechanism by Joule heating did not initiate, were included in the CNT yarns produced at a drawing speed of 40 mm/min. Cross-sectional observation of the lower-strength CNT yarns revealed the presence of numerous voids in the outer portion of the yarn. The structural defects formed during the spinning of the CNT yarn strongly affected the tensile strength of the yarn.
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