16 August 2013

Carbyne: A new form of carbon that’s stronger than graphene

A new form of carbon, dubbed carbyne, is stronger and stiffer than any known material. In fact, carbyne is about two times stronger than graphene and carbon nanotubes, which until now were the strongest materials by some margin. Carbyne has a long list of unusual and highly desirable properties that make it an interesting material for a wide range of applications, from nanoelectronic/spintronic devices to hydrogen storage to higher-density batteries.

Also known as linear acetylenic carbon, carbyne (not pictured above; that’s carbon nanotubes) is an indefinitely long chain of carbon atoms that are joined together by sequential double bonds or alternating single and triple bonds (a polyyne). Until now, very little was known about carbyne. Astronomers believe that they’ve detected carbyne in meteorites and interstellar dust, and a couple of years ago a carbyne chain of 44 atoms was synthesized in a lab, but we have very little practical knowledge of what carbyne is, how it forms, and its properties.

To rectify this, Mingjie Liu and fellow researchers at Rice University set out to calculate the properties of carbyne from first principles — a mathematical approach that, given our intimate understanding of carbon atoms, should give us a sound jumping off point for more empirical, experimental work. The researchers found that carbyne is massively strong (6.0–7.5×107N∙m/kg, vs. 4.7–5.5×107 N∙m/ kg for graphene), very high tensile stiffness (it’s almost impossible to stretch), fairly chemically stable, and yet surprisingly flexible.

While carbyne cannot be stretched, it can be bent into an arc or circle — and by doing so, the additional strain between the carbon atoms alters the electrical bandgap. This property could lead to some interesting uses in microelectromechanical systems (MEMS). By adding different molecules to the end of a carbyne chain, such as a methylene (CH2) group, carbyne can also be twisted — much like a strand of DNA — again adding strain and modifying the electrical bandgap. By “decorating” carbyne chains with different molecules, other properties can be added, too: Tack some calcium atoms on the end, which like to mop up spare hydrogen molecules, and suddenly you have a high-density, reversible hydrogen storage sponge.

It’s also important to note that, just like graphene, carbyne is just one atom thick. This means that, for a given mass of carbyne, its surface area is relatively massive. A single gram of graphene, for example, has a surface area of about five tennis courts. This could be very important in areas such as energy storage (batteries, supercapacitors), where the surface area of the electrode is directly proportional to the energy density of the device. In the aforementioned hydrogen storage sponge, too, the huge surface area of carbyne could result in a lot of hydrogen being mopped up relative to the size of the device.

Moving forward, then, carbyne appears to have lots of desirable properties that warrant further investigation. Thanks to some previous studies, we know how to synthesize small amounts of carbyne, but we’ll have to refine our processes so that it can be studied properly. Given its odd and desirable properties, we wouldn’t be surprised if carbyne quickly becomes the target of as much research as graphene and carbon nanotubes.

Research paper: arXiv:1308.2258 - “Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope?”


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