New process could make coal-to-graphite conversions more efficient

The Ohio University team performed a series of simulations that showed how coal can be turned into valuable, carbon-neutral materials such as graphite and carbon nanotubes.

Using the Bridges-2 system at the Pittsburgh Supercomputing Center, the researchers simulated carbon and graphite in computer software and virtually recreated the carbon-to-graphite conversion. Generations of scientists have known that, at least in theory, it is possible to turn coal into graphite by applying enough pressure to the fossil fuel at high enough temperatures.

Pure graphite is a line of plates consisting of six carbon rings. A special chemical bond called an “aromatic bond” holds these carbons together.
In aromatic bonds, pi electrons float above and below the rings. These “slippery” electron clouds make the sheets easily slide past each other. The pencil, a low-grade form of graphite, leaves a mark on the paper as the leaves slide off each other and stick to it.

Aromatic bonds have another advantage that is important in electronic engineering. Pi electrons move easily from ring to ring and from disk to disk. This makes graphite conduct electricity even though it is not a metal.
Carbon is chemically impure. Unlike the strictly two-dimensional nature of graphite sheets, it has three-dimensional bonds. It also contains hydrogen, oxygen, nitrogen, sulfur, and other atoms that can prevent the formation of graphite.

Simplified Carbon 

To begin their research, David Drabold and his team created a simplified “carbon” consisting only of carbon atoms in random positions. By subjecting this simplified carbon to pressure and high temperatures—about 3,000 Kelvin, or nearly 5,000 Fahrenheit—they can take the first step in researching how to transform it into graphite.

“We had to do a lot of serious analysis to get rid of the amorphous graphite paper,” said Chinonso Ugwumadu, a doctoral student in Drabold’s group. “Compared to our other systems, Bridges is the fastest and most accurate.” Our home system takes about two weeks to simulate 160 atoms. The bridges allow us to work on 400 atoms in six to seven days using density functional theory.

Initially, the Ohio researchers performed simulations using basic physical and chemical principles through density functional theory. This precise but computationally intensive approach required many parallel calculations. They then ported their calculations to a new software tool called Gaussian Approximation Potential (GAP), which uses machine learning to perform essentially the same calculations much faster.
Their results were more complicated than the team expected. The leaves have formed. However, carbon atoms have not fully developed simple six-carbon rings. Some rings had five carbons; others had seven.

The non-six-carbon tires created an interesting wrinkle in a number of ways. While the six-carbon rings are flat, the five- and seven-membered carbon rings bend, but in the opposite sense of “positive and negative curvature.”

Scientists might assume that these wrinkles would destroy the formation of graphite sheets. But the plates formed anyway, perhaps because the pentagons and heptagons balanced each other out in the simulations. The sheets were technically amorphous graphite because they were not exclusively six-ringed. But again, they formed layers.


Carbon nanotubes 

In another set of simulations, Ugwumadu continued with Rajendra Thapa to study molecules instead of solids. The conditions in these sims have caused the sheets to buckle. Instead of sheets, they formed nested amorphous carbon nanotubes (CNTs), a series of tubes with a single atomic layer.
CNTs have been hot in materials science recently because they are actually tiny wires that can conduct electricity on an incredibly small scale. Other promising applications of CNTs include catalysis in fuel cells, production of supercapacitors and lithium-ion batteries, electromagnetic interference shielding, biomedicine, and nanoneuroscience. One important aspect of the
In his CNT work, Ugwumadu studied how amorphous grooves in the tube walls affected the flow of electricity through the structure. In materials science, every “defect” is also a “property”—engineers can use such irregularities to tune the behavior of a particular CNT to precisely match the requirements of a new electronic device.
The group continues to investigate the transformation of carbon atoms into graphite and related materials.

Source:Mining weekly

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