Breakthrough Carbon nanotube bundles are over 20 times stronger than Kevlar

Finally ultralong (several centimeter) carbon nanotube fibers have been made into stronger bundles. The tensile strength of CNTBs (Carbon nanotube bundles) is at least 9–45 times that of other materials. If a more rigorous engineering definition is used, the tensile strength of macroscale CNTBs is still 5–24 times that of any other types of engineering fiber, indicating the extraordinary advantages of ultralong Carbon nanotubes in fabricating superstrong fibers.

The work was done at Tsinghua University and other facilities in Beijing. Researchers were Yunxiang Bai, Rufan Zhang, Xuan Ye, Zhenxing Zhu, Huanhuan Xie, Boyuan Shen, Dali Cai, Bofei Liu, Chenxi Zhang, Zhao Jia, Shenli Zhang, Xide Li & Fei Wei.

A synchronous tightening and relaxing (STR) strategy further improves the alignment of the carbon nanotubes to increase the strength.

Superstrong fibers are in great demand in many high-end fields such as sports equipment, ballistic armour, aeronautics, astronautics and even space elevators. In 2005, the US National Aeronautics and Space Administration (NASA) launched a ‘Strong Tether Challenge’, aiming to find a tether with a specific strength up to 7.5GPa cm3 per gram for the dream of making space elevators. Unfortunately, there is still no winner for this challenge. The specific strength of existing fibres such as steel wire ropes (about 0.05–0.33 GPa cm3 per gram), carbon fibres (about 0.5–3.5GPa cm3 per gram) and polymer fibers (about 0.28–4.14GPa cm3 per gram) is far lower than 7.5GPa cm3 per gram). Carbon nanotubes, with inherent tensile strength higher than 100GPa and Young’s modulus over 1TPa, are considered one of the strongest known materials.

Generally, there are three types of CNT:
agglomerated CNTs
vertically aligned CNT (VACNT) arrays
ultralong horizontally aligned CNT (HACNT) arrays (‘ultralong CNTs’ for short).

Almost all the reported CNT fibers are fabricated using agglomerated CNTs or VACNT arrays with lengths less than a few hundred micrometres and with plenty of structural defects and impurities, giving those CNT fibers a tensile strength ranging from about 0.5 to 8.8GPa which is much lower than that of single CNTs.

Ultralong CNTs should have great advantages in fabricating fibers because of their macroscale lengths (ranging from centimeters to decimeters), neat surface, perfect structures and super-parallel alignments. But because the production of ultralong CNTs is extremely low, there have been no reports of fibers fabricated using ultralong CNTs, so the question of whether ultralong CNTBs possess equivalent strength to single CNTs has remained open.

Fabrication of ultralong Carbon Nanotubes into superstrong bundles

Researchers have fabricated CNTBs that are several centimeters long, using ultralong CNTs with defined number and parallel alignment, to quantitatively investigate the relationship between the tensile strength of ultralong-CNT-based fibers and their components. Generally, the ultralong CNTs are synthesized through a gas-flow directed chemical vapor deposition (CVD) method with parallel orientations and large intertube distance on flat substrates. The resulting CNTs usually have one to three walls with perfect structures.

Nature Nanotechnology – Carbon nanotubes (CNTs) are one of the strongest known materials. When assembled into fibers, however, their strength becomes impaired by defects, impurities, random orientations and discontinuous lengths. Fabricating CNT fibers with strength reaching that of a single CNT has been an enduring challenge. Here, researchers demonstrate the fabrication of CNT bundles (CNTBs) that are centimeters long with tensile strength over 80 GPa using ultralong defect-free CNTs. The tensile strength of CNTBs is controlled by the Daniels effect owing to the non-uniformity of the initial strains in the components. We propose a synchronous tightening and relaxing strategy to release these non-uniform initial strains. The fabricated CNTBs, consisting of a large number of components with parallel alignment, defect-free structures, continuous lengths and uniform initial strains, exhibit a tensile strength of 80 GPa (corresponding to an engineering tensile strength of 43 GPa), which is far higher than that of any other strong fiber.