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Researchers Determine Real-World Limitations of Graphene

Posted on: 05/01/2014
Research published by TMS members at Rice University and the Georgia Institute of Technology should prompt manufacturers to look a little deeper as they consider using graphene for applications.

Jun Lou at Rice and Ting Zhu at Georgia Tech have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. While it's still very useful, graphene is really only as strong as its weakest link, which they determined to be "substantially lower" than the intrinsic strength of graphene.

“Graphene has exceptional physical properties, but to use it in real applications, we have to understand the useful strength of large-area graphene, which is controlled by the fracture toughness,” said Zhu.

In their study, Lou and Zhu physically pulled graphene apart to see how much force it would take. Specifically, they wanted to see if graphene follows the century-old Griffith theory that quantifies the useful strength of brittle materials.

It does, said Lou. "Remarkably, in this case, thermodynamic energy still rules," he said. Imperfections in graphene drastically lessen its strength—with an upper limit of about 100 gigapascals (GPa) for perfect graphene previously measured by nanoindentation—according to physical testing at Rice and molecular dynamics simulations at Georgia Tech. That's important for engineers to understand as they think about using graphene for flexible electronics, composite material and other applications in which stresses on microscopic flaws could lead to failure.

The Griffith criterion developed by a British engineer during World War I describes the relationship between the size of a crack in a material and the force required to make that crack grow. Ultimately, A.A. Griffith hoped to understand why brittle materials fail. Graphene, it turns out, is no different from the glass fibers Griffith tested.

"Everybody thinks the carbon-carbon bond is the strongest bond in nature, so the material must be very good," Lou said. "But that's not true anymore, once you have those defects. The larger the sheet, the higher the probability of defects. That's well known in the ceramic community."

While the Rice team was working on the experiment, Zhu and his team performed computer simulations to understand the entire fracture process.

“We can directly simulate the whole deformation process by tracking the motion and displacement with atomic-scale resolution in fairly large samples so our results can be directly correlated with the experiment,” said Zhu. “The modeling is tightly coupled with the experiments.”

The combination of modeling and experiment provides a level of detail that allowed the researchers to better understand the fracture process—and the tradeoff between toughness and strength in the graphene. What the scientists have learned in the research points out the importance of fabricating high quality graphene sheets without defects—which could set the stage for fracture.

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