Carbon nanotubes help create reinforced graphene "rebar"

Rice University researchers have found that fracture-resistant rebar graphene is more than twice as tough as pristine graphene. While on the two-dimensional scale, graphene is stronger than steel, its extremely thin nature makes it subject to ripping and tearing. Rebar graphene is the nanoscale analog of rebar (reinforcement bars) in concrete, in which embedded steel bars enhance the material’s strength and durability. Rebar graphene, developed by the Rice lab of chemist James Tour in 2014, uses carbon nanotubes for reinforcement.

In a new study, Rice materials scientist Jun Lou, graduate student and lead author Emily Hacopian and collaborators, including Prof. James Tour, stress-tested rebar graphene and found that nanotube rebar diverted and bridged cracks that would otherwise propagate in unreinforced graphene.

The experiments showed that the CNTs help graphene stay "stretchy" and also reduce the effects of cracks. That could be useful not only for flexible electronics but also electrically active wearables or other devices where stress tolerance, flexibility, transparency and mechanical stability are desired, Lou said.

The team reports that rebar didn’t keep graphene from eventual failure, but the nanotubes slowed the process by forcing cracks to zig and zag as they propagated. When the force was too weak to completely break the graphene, the nanotubes effectively bridged cracks and in some cases even preserved the material’s conductivity.

In earlier tests, Lou’s lab showed graphene has a native fracture toughness of 4 megapascals. In contrast, rebar graphene has an average toughness of 10.7 megapascals, he said.

Simulations by study co-author Huajian Gao and his team at Brown confirmed results from the physical experiments. Gao’s team found the same effects in simulations with orderly rows of rebar in graphene as those measured in the physical samples with rebar pointing every which way. The simulations are important because they let us see the process on a time scale that isn’t available to us with microscopy techniques, which only give us snapshots, Lou said. The Brown team really helped us understand what’s happening behind the numbers.

He said the rebar graphene results are a first step toward the characterization of many new materials. We hope this opens a direction people can pursue to engineer 2D material features for applications, Lou said.