Graphene is an atom-thick sheet of carbon. On a two-dimensional scale, this material is stronger than steel, but because graphene is very thin, it can still be torn and ripped. “Rebar graphene” is a nanoscale analog of rebar in concrete, where the embedded “rebar” enhances the material’s strength and durability. Developed by chemist James Tour in 2014, “rebar graphene” uses carbon nanotubes for reinforcement.
In a new study published in the American Chemical Society journal ACS Nano (“Toughening Graphene with Carbon Nanotube Reinforcement”), scientists subjected “rebar graphene” to pressure tests and found that the nanotube reinforcement could transfer and bridge cracks within the graphene, which would otherwise propagate in non-reinforced graphene.
These nanotubes help the graphene maintain its tensile strength and reduce the impact of cracks. This is useful not only for flexible electronics but also for electroactive wearable devices or other applications requiring pressure resistance, flexibility, transparency, and mechanical stability. Mechanical tests and molecular dynamics simulations in the laboratory revealed the toughness of this material.
Researchers note that the excellent conductivity of graphene makes it a strong candidate for devices, but its brittleness is a drawback. Two years ago, lab reports indicated that the strength of pristine graphene was “far below its reported intrinsic strength.” In a subsequent study, the lab found that another 2D material of interest, molybdenum diselenide, was also fragile.
An image depicted scientists testing “rebar graphene” samples under an electron microscope. It showed how cracks propagated in a jagged pattern rather than a straight line.
The team conducted similar tests on “rebar graphene” by spin-coating single-walled nanotubes onto a copper substrate and growing graphene on its surface via chemical vapor deposition.
To stress-test “rebar graphene,” researchers pulled it to the point of fragmentation and measured the applied force. Through repeated trials, the lab developed a method to cut tiny portions of the material and mount them on a test bed for use with scanning and transmission electron microscopes.
In the experiments, the “rebar” did not prevent the graphene from eventual destruction, but the nanotubes slowed this process by causing cracks to bend and become jagged during propagation. When the force was too weak to completely destroy the graphene, the nanotubes effectively bridged the cracks, in some cases maintaining the material’s conductivity.
In earlier tests, the lab showed that the intrinsic fracture toughness of pristine graphene was 4 megapascals. By contrast, the average toughness of “rebar graphene” was 10.7 megapascals. Researchers believe, “These simulations are very important because they allow us to see processes on a timescale that we cannot capture with microscopy techniques. The results for ‘rebar graphene’ are the first step in characterizing many new materials. We hope this opens up the direction for designing 2D material characteristics for applications.”