Using computer modelling, researchers have predicted that amorphous graphite (yellow) can be obtained from a random initial configuration (gray) by thermal treatment at high temperature (3000K). Image: Ohio University.As the world's appetite for carbon-based materials like graphite increases, a team of researchers at Ohio University has presented evidence for a new carbon solid they name ‘amorphous graphite’.
Led by physicist David Drabold and engineer Jason Trembly, the team started with the question, "Can we make graphite from coal?"
"Graphite is an important carbon material with many uses,” the researchers write in a paper on their work in Physical Review Letters. “A burgeoning application for graphite is for battery anodes in lithium-ion batteries, and it is crucial for the electric vehicle industry – a Tesla Model S on average needs 54kg of graphite. Such electrodes are best if made with pure carbon materials, which are becoming more difficult to obtain owing to spiraling technological demand."
The researchers are pursuing novel paths to synthetic forms of graphite from naturally occurring carbonaceous material. What they have now managed to predict, using computer modelling, is a layered material that forms at very high temperatures (about 3000K).
The layers of this material stay together due to the formation of an electron gas between them. But they're not the perfect layers of hexagons that make up ideal graphene. This new material has plenty of hexagons, but also pentagons and heptagons. That ring disorder reduces the electrical conductivity of the new material compared with graphene, but the conductivity is still high in the regions dominated largely by hexagons.
"In chemistry, the process of converting carbonaceous materials to a layered graphitic structure by thermal treatment at high temperature is called graphitization,” explained Drabold, professor of physics and astronomy at Ohio University. “In this letter, we show from ab initio and machine learning molecular dynamic simulations that pure carbon networks have an overwhelming proclivity to convert to a layered structure in a significant density and temperature window, with the layering occurring even for random starting configurations. The flat layers are amorphous graphene: topologically disordered three-coordinated carbon atoms arranged in planes with pentagons, hexagons and heptagons of carbon,".
"Since this phase is topologically disordered, the usual 'stacking registry' of graphite is only statistically respected. The layering is observed without Van der Waals corrections to density functional forces, and we discuss the formation of a delocalized electron gas in the galleries (voids between planes) and show that interplane cohesion is partly due to this low-density electron gas. The in-plane electronic conductivity is dramatically reduced relative to graphene."
The researchers expect their announcement to spur experimentation and studies addressing the existence of amorphous graphite, which may be testable via exfoliation and/or experimental surface structural probes.
"The question that led us to this is whether we could make graphite from coal," said Drabold. "This paper does not fully answer that question, but it shows that carbon has an overwhelming tendency to layer – like graphite. But with many 'defects' such as pentagons and heptagons (five- and seven-member rings of carbon atoms), which fit quite naturally into the network. We present evidence that amorphous graphite exists, and we describe its process of formation. It has been suspected from experiments that graphitization occurs near 3000K, but the details of the formation process and nature of disorder in the planes was unknown."
According to Drabold, this work is also a prediction of a new phase of carbon.
"Until we did this, it was not at all obvious that layers of amorphous graphene (the planes including pentagons and heptagons) would stick together in a layered structure. I find that quite surprising, and it is likely that experimentalists will go hunting for this stuff now that its existence is predicted. Carbon is the miracle element – you can make life, diamond, graphite, Bucky Balls, nanotubes, graphene, and now this. There is a lot of interesting basic physics in this, too — for example how and why the planes bind, this by itself is quite surprising for technical reasons."
This story is adapted from material from Ohio University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.