From the stone ages to modern history, new materials have often been the enablers of revolutionary technologies.[1] For a wide variety of envisioned applications in space exploration, energy-efficient aircraft, and armor, materials must be significantly stronger, stiffer, and lighter than what is currently available. Carbon nanotubes (CNTs) have extremely high strength, [2][3][4][5] very high stiffness, [6,7] low density, good chemical stability, and high thermal and electrical conductivities.[8]These superior properties make CNTs very attractive for many structural applications and technologies. Here we report CNT fibers that are many times stronger and stiffer per weight than the best existing engineering fibers and over twenty times better than other reported CNT fibers. Additionally, our CNT fibers are nonbrittle and tough, making them far superior to existing materials for preventing catastrophic failure. These new CNT fibers will not only make tens of thousands of products stronger, lighter, safer, and more energy efficient, but they will also bring to fruition many envisioned technologies that have been to date unavailable because of material restrictions. Strong, stiff, and lightweight are critical property requirements for materials that are used in the construction of space shuttles, airplanes, and space structures. These properties are assessed by a material's specific strength and specific stiffness, which are defined as the strength or stiffness (Young's modulus) of a material divided by its density.[9] The combination of high strength, high stiffness, and low density affords CNTs with extremely high values for specific strength and specific stiffness. The most effective way to utilize these properties is to assemble CNTs into fibers. However, despite extensive worldwide efforts to date, the specific strength and specific stiffness of CNT fibers that have been reported by various research groups are much lower than currently available commercial fibers. [10][11][12][13][14][15][16][17][18][19][20][21][22] In early studies, researchers attempted to reinforce polymer fibers with short CNTs, but the reinforcement was limited by several issues, including poor dispersion, poor alignment, poor load transfer, and a low CNT volume fraction. [10][11][12][13][14][15] Recently, pure CNT fibers (also called yarns)were reported with and without twisting. [16][17][18][19][20][21][22] For example, Zhang et al. [20] demonstrated that spinning from aligned CNT arrays could significantly improve the strength of CNT fibers by twisting them. However, to date no breakthrough has been reported in the specific strength and specific stiffness of CNT fibers.Here we report CNT fibers with values for specific strength and specific stiffness that are much higher than values reported for any current engineering fibers as well as previously reported CNT fibers. As shown in Figure 1, the specific strength COMMUNICATION 4198

Carbon nanotubes (CNTs) are much stronger than any existing material. To fully utilize their extremely high strength, carbon-nanotubes must be spun into continuous fibers. [1][2][3] The most efficient way to produce commercial-scale CNT fibers is by the five-thousand-years-old cotton-based spinning technology.[4] Therefore, it is technologically attractive to produce CNT materials that have spinning properties similar to cotton. Here we report a new form of CNT material, CNT cotton, that is made of ultralong individual CNTs. This CNT cotton is analogous to conventional cotton in many aspects including the color and fluffiness, and is found favorable for spinning. It is found that the CNT cotton is hydrophobic, and is composed of low spatial density and ultra-long individual CNTs. Figure 1a is a photograph of fluffy and gray CNT cotton on a quartz support (15 mm × 35 mm). The CNT cotton has very low density with large space between individual CNTs, making it very similar to conventional cotton in terms of structural form. The gray color is surprising because all other CNT materials are black. This gray color is caused by the unique low spatial density of nanotubes in the CNT cotton, as well as a light-scattering effect. From the side view (Fig. 1b), we can see a 2 mm thick CNT cotton above the support, indicating that the CNT cotton is composed of long (at least millimeters) individual CNTs.The CNT cotton is also found to be hydrophobic. As shown in Figure 2a, a water droplet with a diameter of 2.5 mm was suspended on the CNT cotton. It sank into the CNT cotton because of its weight, but maintained a quasi-spherical shape. After the water-droplet was completely evaporated, the morphology of the CNT cotton was examined by scanning electron microscopy (SEM). The CNTs in the area that supported the water droplet were rearranged into a web-like mesh (see Fig. 2b), but the CNTs underneath the web still retained the cotton form (Fig. 2c), further confirming the hydrophobic behavior of CNT cotton.Catalytic chemical vapor deposition (CVD) [5] was used to synthesize the CNT cotton.

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