Carbyne Predicted To Be Strongest Known Material | Chemical & Engineering News
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Web Date: October 17, 2013

Carbyne Predicted To Be Strongest Known Material

Materials: The one-dimensional chain of carbon atoms could be stronger than diamond, graphene, and carbon nanotubes
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE
Keywords: carbon allotropes, computational chemistry, nanomaterials, carbyne
The Strongest Chain?
Strings of carbon atoms called carbyne are predicted to be stronger than any known material, including graphene. This illustration shows a chain of the material and its highest-occupied molecular orbital (yellow and green).
Credit: Vasilii I. Artyukhov
Strings of carbon atoms
The Strongest Chain?
Strings of carbon atoms called carbyne are predicted to be stronger than any known material, including graphene. This illustration shows a chain of the material and its highest-occupied molecular orbital (yellow and green).
Credit: Vasilii I. Artyukhov

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.

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Sergey Evsyukov (November 8, 2013 10:43 AM)
The recent frenzy in media about theoretical findings by Yakobson et al. [1] on predicted mechanical properties of carbyne is really both amazing and amusing. As it frequently happens, new things are well-forgotten old ones. As far back as 1975, Perepelkin published theoretical calculations predicting defect-free filamentous carbyne crystals to be strongest of all known and even feasible materials [2]. On the other hand, the reason for euphoria was, still is, and will for unforeseeable time be rather phantasmal. In fact, Sir Harold Kroto said it all in his short essay, having argued that "The existence of carbyne is myth based on bad science and perhaps even wishful thinking" [3]. And Sir Harold is certainly right, particularly if we think of carbyne as a wisp of 'endless', one-dimensionally ordered polyyine and/or cumulene carbon chains ('pencils in a box'). Unfortunately, the most of carbon and carbonaceous materials reported so far and interpreted to be carbyne have in my opinion nothing to do with this structural model, but can rather be assigned to a group of carbynoid materials, a comfortable term coined to combine all that poorly defined stuff, routinely coming from the labs. Carbynoids are generally defined as carbon-rich chain-like poly(or oligo)mers with both polyyne- and cumulene-type moieties, extended interchain cross-linking as well as end-capping and/or pendant side groups. Naturally, carbonaceous materials containing short polyyne or cumulene sequences embedded in whatever as a matrix cannot be considered as an individual allotropic form of carbon.
From mid 80’s until late 90’s I was closely involved in the carbyne story for some fifteen years, although I had never been a rabid fan of the 'pencils in a box' concept. In an attempt of critically summing up all what was known at that time and to understand the ways to go, we published a review [4] followed by a book [5] on carbyne and carbynoid structures. Having analyzed a lot of publications, I have to admit that wishful thinking did and probably still does play a special role, even in our own early studies. In the preface to the book we wrote in 1998: "Despite a host of papers dealing with different aspects of the physics and chemistry of carbyne, many questions remain to be solved. We believe that a critical assessment of these papers is required but we also emphasize that continuing efforts will eventually result in carbyne single crystals large enough to permit in-depth structural analysis. It is thus not the intention of this text to reveal completely new and stunning research results on carbyne but to provide a rather complete, ordered and concise summary of what is known with certainty today and what is still ambiguous or downright wrong. Hence this book should be considered an important stepping stone towards an understanding of carbyne, an aspect many researchers around the world have attempted hitherto. It will show the research community what has been confirmed, what is still doubtful, what is or was wrong, and most importantly, where do we go from here." Now I have to admit that today, fifteen years later, we could write the same once again. In spite of significant advances made recently in both physical [6] and chemical syntheses [7], no real breakthrough has happened in these years, while other scientists kept baking the same stuff, obstinately calling it carbyne. On the other hand, the notorious reactivity of sp-carbon chains has unambiguously been confirmed [8].
The main problem with carbyne (whatsoever we mean by using this term, excluding a homonymous herbicide) was and still is the embarrassing lack of reasonably large single crystals for a clear-cut X-ray structural analysis. Various structural models of carbyne have been proposed to date, and some of them are based on a layered lattice, wherein the layers are oriented transversely to the chain axis [6a-e]. Such a structure seems to be incompatible with extreme strength predicted by theoreticians. So, suming up, we have to wait for arrival of carbyne-anchored space elevators for quite a while [9].
1. M. Liu, et al.: Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope? // a). ACS Nano, 2013, Ahead of print, Publication date (Web) October 5, 2013. DOI: 10.1021/nn404177r; b)., e-Print Archive, Condensed Matter, (2013), 19 pp., arXiv:1308.2258 [cond-mat.mtrl-sci].
2. a). K.E. Perepelkin, et al.: Evaluation of the ultimate mechanical properties for carbyne, a carbon chain polymer // Dokl. Akad. Nauk SSSR, 1975, 220 (6), 1376-1379 (Russ.); b). Fibres and fibrous materials for reinforcing composites with extreme characteristics // Mechanics of Composite Materials, 1992, 28 (3), 195-208 [Transl. from Mekhanika Kompozitnykh Materialov, 1992, (3), 291-306 (Russ.)]; c). Theory of extremal mechanical and thermal properties of fibres and needle crystals. Comparison with experimental data // Fibre Chemistry, 2004, 36 (4), 237-248 [Transl. from Khim. Volokna, 2004, (4), 3-11 (Russ.)].
3. H. Kroto: Carbyne and other myths about carbon // RSC Chemistry World, 2010, November.
4. Yu.P. Kudryavtsev, et al.: Carbynes: Advances in the field of linear carbon chain compounds // J. Mater. Sci., 1996, 31 (21), 5557-5571.
5. Carbyne and Carbynoid Structures, ed. by R.B. Heimann, S.E. Evsyukov, and L. Kavan, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1999, 452 pp.
6. a). J.G. Korobova, et al.: The structural properties of the sp1-carbon based materials: Linear carbon chains, carbyne crystals and a new carbon material - two dimentional ordered linear-chain carbon // in: Carbon Nanomaterials in Clean Energy Hydrogen Systems - II, NATO Science for Peace and Security Series C: Environmental Security, 2011 (©2008), Volume 2, Springer, Dordrecht, pp. 469-485; b). V.G. Babaev, et al.: Laser-assisted synthesis of carbyne from graphite and amorphous carbon // Nanotekhnologii: Razrabotka, Primenenie [Nanotechnologies: Development, Applications], 2010, (1), 88-94 (Russ.); c). Films of linear chain-like carbon - ordered assemblies of quantum wires - material for nanoelectronics // Ibid., 53-68; d). Yu.P. Kudryavtsev, et al.: Carbyne - the third allotropic form of nanocarbon // Ibid., 37-52; e). V.G. Babaev, et al.: Carbon material with a highly ordered linear-chain structure // in: Polyynes: Synthesis, Properties, and Applications, ed. by F. Cataldo, CRC Press, Boca Raton, 2006, pp. 219-252; f). L. Ravagnan, et al.: Synthesis and characterization of carbynoid structures in cluster-assembled carbon films // Ibid., pp. 15-35; g). G. Bongiorno, et al.: Electronic properties and applications of cluster-assembled carbon films // J. Mater. Sci.: Materials in Electronics, 2006, 17 (6), 427-441.
7. a). W.A. Chalifoux and R.R. Tykwinski: Synthesis of extended polyynes: Toward carbyne // Comptes Rendus Chimie, 2009, 12 (3-4), 341-358; b). Synthesis of polyynes to model the sp-carbon allotrope carbyne // Nature Chemistry, 2010, 2 (11), 967-971.
8. a). L. Ravagnan, et al.: sp Hybridization in free carbon nanoparticles - presence and stability observed by near edge X-ray absorption fine structure spectroscopy // Chem. Commun., 2011, 47 (10), 2952–2954; b). C.S. Casari, et al.: Gas exposure and thermal stability of linear carbon chains in nanostructured carbon films investigated by in situ Raman spectroscopy // Carbon, 2004, 42 (5-6), 1103-1106.
9. T. Shelley: Fifth element makes the ultimate material // Eureka, 1999, 19 (8), 24-25.

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