Carbon nanotubes share the load

With a tensile strength exceeding 100 GPa, individual carbon nanotubes (CNTs) are among the strongest known materials. Realizing that ideal strength is challenging. When CNTs are fabricated in bulk, the end product is often a jumbled mass of tubes shorter than a few hundred micrometers in length and filled with structural defects and impurities. Those features reduce the nanotubes’ collective strength to a small fraction of the strength of an individual CNT.

Recently Fei Wei, Rufan Zhang, and their colleagues at China’s Tsinghua University developed a method to synthesize CNTs into bundles several centimeters long, with each containing up to 15 nearly defect-free CNTs. Despite the near-perfect crystalline growth, the researchers found that the CNTs in each bundle still differed in orientation and length. The larger the number of CNTs in a bundle, the further the bundle’s tensile strength fell below the ideal value. That’s because the load would be concentrated in just one or two CNTs instead of being evenly shared by them all.

To shore up the CNTs’ collective strength, Wei and colleagues fixed one end of the bundle and then repeatedly stretched and trimmed the different CNTs at the opposite end until they had equalized the initial strain on each nanotube. (The researchers determined spectroscopically how much to trim for a uniform strain distribution.) The process thereby increased the maximum load the bundle could collectively support. Accounting for the nanotubes’ cross-sectional area, the team was able to produce ultralong CNT bundles with a tensile strength of about 43 GPa. That’s 5–24 times the strength of any other type of engineering fiber, including graphitic fiber, Kevlar, and steel. (Y. Bai et al., Nat. Nanotechnol., 2018, doi:10.1038/s41565-018-0141-z .)