While carbon nanotube fibers are known to be less durable than the nanotubes within them, a new study could help close the gap between these materials.

A team from Rice's Brown School of Engineering in Houston, Texas, led by materials theorist Boris Yakobson, generated a computational model that describes the scaling relationship between the length of a nanotube sample and the friction between the tubes in the bundle. This relationship could guide fabricators and scientists into fine-tuning the parameters for optimized strength.

Details of their report are published in the American Chemical Society journal ACS Nano, titled "Universal Strength Scaling in Carbon Nanotube Bundles with Frictional Load Transfer."

Guiding Stronger Carbon Nanotubes

The mathematical model could help develop stronger conductive fibers for different applications - from automotive, biomedical, aerospace, and textiles like those used for smart clothing. According to a news article from Rice University, carbon nanotube fibers have also been considered material for the space elevator - an Earth to space transport with a tether extending from the ground to space - a project that Yakobson has also studied in the past.

Carbon nanotubes are fundamentally tubes made up of rolled graphene sheets, one of the strongest materials known to man. According to the report, bundled, threadlike fibers are far weaker than individual tubes - about 100 times less durable. 

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"One single nanotube is about the strongest thing you can imagine, because of its very strong carbon-carbon bonds," explains Evgeni Penev, a longtime part of Yakobson's team and an assistant research professor at Rice. However, resulting materials from nanotubes are significantly weaker, prompting them to work towards resolving this gap.

Quantifying Nanotube Strength

The new computational model illustrates how nanotube length and friction best describes overall fiber strength. This also suggests possible methods to work around current limitations regarding material strength. One of which is to use longer carbon nanotubes or increase crosslinks between individual tubes. Researchers also identified the use of chemical or electron irradiation to encourage bonding between individual carbon atoms.

Researchers describe the friction between nanotubes with their coarse-grained model, focusing on how it regulates slip for the tubes under strain and how good connections between nanotubes restore themselves after breakage. Basically, the longer nanotubes are used, the fewer crosslinks are necessary.

Penev explains that lengthwise gap refers to a function of length for the nanotubes, describing them as essentially defects that allow slipping when the bundle is exposed to tension. "What became clear was that the overall strength of this interface depends a lot on the capability of these crosslinks to heal," he added.

Nitant Gupta, the lead author of the study and a graduate student from Rice, explains that their study illustrates the crosslink density and nanotube length, using the parameters to describe the entire bundle's strength.

Yakobson stressed the importance of developing material strength, calling the effort an "ongoing, uphill battle" in laboratories around the world, calling every improvement in gigapascal capacity "a great achievement."


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