25+ year journey to superstrong carbon nanotube bundles

In 1991, Sumio Iijima’s discovered multi-walled carbon nanotubes in the insoluble material of arc-burned graphite rods and Mintmire, Dunlap, and White’s independent prediction that if single-walled carbon nanotubes could be made, then they would exhibit remarkable conducting properties helped create the initial buzz that is now associated with carbon nanotubes.

Nanotube research accelerated greatly following the independent discoveries by Bethune at IBM and Iijima at NEC of single-walled carbon nanotubes and methods to specifically produce them by adding transition-metal catalysts to the carbon in an arc discharge.

It has been very difficult to make larger amounts of carbon nanotubes and to make them longer. It has been even more difficult to combine lots of carbon nanotubes and make the combined material close to the strength of individual carbon nanotubes.

In 2008, it was found individual CNT shells have strengths of up to ≈100 gigapascals (15,000,000 psi). Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa. This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ≈60 GPa for multi-walled carbon nanotubes and ≈17 GPa for double-walled carbon nanotube bundles.

2008 Cambridge University made 9 GPa ribbons that were a few centimeters long

In 2008, Alan Windle’s team at Cambridge University had created the world’s strongest ribbon. Prof. Windle’s research team at the University of Cambridge seemed to lead the way towards super-strong tethers. As Cambridge researcher Dr. Marcelo Motta pointed out they are currently able to produce almost cm long individual macroscopic CNT threads with tensile strength of up to 9 N/tex which compares to about 9 GPa at the given density of their material. Scaling up the Cambridge laboratory process to industrial production and spinning these threads, ropes and cables with 10 GPa should be soon feasible.

They were not able to scale up to industrial production.

2016 9.6 gigapascal films

In 2016, Jian Nong Wang and his colleagues made nanotubes with a process akin to glass blowing: Using a stream of nitrogen gas, they injected ethanol, with a small amount of ferrocene and thiophene added as catalysts, into a 50-mm-wide horizontal tube placed in furnace at 1,150–1,130 °C. They packed the nanotubes even more densely by pressing the film repeatedly between two rollers.

Wei Xu, Yun Chen, Hang Zhan, and Jian Nong Wang of the Nano Carbon Research Center, School of Mechanical and Power Engineering, East China University of Science and Technology and the School of Materials Science and Engineering, Shanghai Jiao Tong University.

The resulting films had an average strength of 9.6 gigapascals. By comparison, the strength of nanotube films made so far has been around 2 GPa, while that for Kevlar fibers and commercially used carbon fibers is around 3.7 and 7 GPa, respectively. The films are four times as pliable as conventional carbon fibers, and can elongate by 8% on average as opposed to 2% for carbon fibers.

2018 Tsinghau University and Beijing researchers have made 80 Gigapascal tensile strength bundles

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.

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.