Carbyne Predicted To Be Strongest Known Material

According to theoretical calculations, one-dimensional strings of carbon atoms called carbyne should be stronger than any known material—if experimentalists can figure out how to make it in bulk (ACS Nano 2013, DOI: 10.1021/nn404177r). The researchers predict that the carbon allotrope also could have novel electrical and magnetic properties that would be useful in computing systems.

The concept of carbyne is not new, but the new computational predictions are the most comprehensive yet, says Rik R. Tykwinski, a chemist at the Friedrich-Alexander University, in Erlangen-Nuremberg, Germany, who was not involved with the work.

Chemists have been toying with these forms of carbon since the 1950s. Researchers have tried synthesizing polyynes, chains of carbon atoms linked by alternating single and triple bonds, and cumulenes, chains of double-bonded carbons. Both are difficult to make and not very stable, so carbyne research has remained something of a backwater. But the success of graphene, carbon nanotubes, and fullerenes—carbon forms once thought to be difficult to synthesize and potentially unstable—has spurred a small resurgence of interest in carbyne. In 2010, Tykwinski made 44-carbon-long polyyne chains in solution (Nature Chem. 2010, DOI: 10.1038/nchem.828).

Boris I. Yakobson, a chemist at Rice University, was inspired by Tykwinski’s experimental success. Seeing that carbyne could be made in the lab, Yakobson says, he and his colleagues “wanted to get a clear, comprehensive picture of its properties.”

Using first-principles modeling, the Rice group predicted various properties of carbyne by calculating what would happen to the energy state of the carbon chain under certain conditions. For example, they predicted carbyne’s tensile strength by examining what happens when the distance between two carbon atoms increases, as would happen if a real chain were stretched. The chemists calculated how the material’s energy would change to determine how much force it could endure before becoming unstable.

In general, carbyne should have even better mechanical properties than other carbon materials. Its tensile stiffness, for example, should be twice that of graphene and carbon nanotubes. Carbyne also should have a novel magnetic property. The researchers predict that twisting the carbon chain 90º from its normal state would transform it into a magnetic semiconductor. The researchers think this property would be of interest to materials scientists developing digital memory devices.

And it should have tremendous surface area, Yakobson says. “If someone can produce a small cube of these filaments, it would be very light, with high porosity,” he says. Such a high-surface-area material might find use in energy-storage devices like battery electrodes or chemical sensors.

Yakobson acknowledges that their predictions are somewhat removed from experimental realities. For example, they have not predicted how oxygen in the environment might destabilize the chain’s structure. The chemists are working to predict carbyne’s electrical properties in greater detail.

Of course, the biggest challenge for carbyne is finding a large-scale synthesis method, Tykwinski says. Making carbyne in small quantities in the lab is so difficult that it’s really a “labor of love,” he says: Most attempts to synthesize the allotrope yield a layer of difficult-to-characterize black goo at the bottom of a beaker.

But Tykwinski takes inspiration from graphene. Chemists theoretically modeled graphene years before anyone knew how to make it or knew if it would be stable in open air. Then in 2004, researchers made graphene with common desk supplies by crushing graphite and peeling it with tape. He hopes a similar leap may happen in carbyne research.