Initial stages of melting of graphene between 4000 K and 6000 K

Significance Statement

Graphene is a remarkable material with outstanding properties, and many applications have been developed based on this material. For example, freestanding graphene monolayers can be mounted on small apertures to create pressure sensors as well as in making freestanding devices and resonators. Thin films of graphene can also be used to protect surfaces.  Graphene is already being used to create bendable displays for potential use in cell phones. Graphene has been predicted to have a melting point of 4500 K.  Due to the high melting point of graphene and carbon nanotubes, further investigation of their high temperature properties is desirable.

Because of its high thermal stability, graphene can be successfully applied in high temperature electronic gadgets. Recently, a light-bulb was fabricated using a filament made of reduced graphene oxide together with single walled nanotubes. The reduced graphene oxide single-walled nanotubes can reach about 3000K, with some samples even reaching 3300K before failure.

Given the high melting points of  graphene and analogues such as carbon nanotubes, and in light of their potential high temperature applications, it is very important to have in-depth and accurate information on the initial stages of graphene melting. Therefore, a team of researchers under the guidance of Prof. Eric Ganz at the University of Minnesota in collaboration with Prof. Li-Ming Yang at Huazhong University of Science and Technology, and Ariel B. Ganz at Cornell University used an accurate ab initio density functional theory molecular dynamics simulations to analyze the initial stages of melting of freestanding graphene at temperatures ranging from 4000-6000K. Their work has now been published in Physical Chemistry Chemical Physics.

The authors performed ab initio density functional theory molecular dynamics simulations using a 10 x 10 graphene lattice with periodic boundary conditions. At 0 K the graphene unit cell would be 24.6 x 24.6 x 20.0 Å, therefore, the authors used a slightly larger unit cell in order to accommodate expansion as the temperature rose. The 4000 K simulation started from an ideal 0K graphene lattice.  For every simulation, approximately 12 ps equilibration time was used. The researchers collected the data after equilibration.

The team observed that after the 12 ps at 4000K, the surface had smooth oscillations and the lattice was maintained. Just a few 3-rings, due to neighboring atoms oscillating towards each other, were observed. No net diffusion was recorded at 4000 K. At 4500 K, the surface was displaying out of phase long-range oscillations, but the lattice was maintained over the 18ps simulation time. This may have been because the surface was not melted at that time or it was in a superheated condition. Longer simulation time might help answer the question. At 4500 K, the system appeared to be in a quasi-2D liquid state.

At 5000 K, the surface showed the initial stages of melting after 20 ps. The surface was observed to be rougher, but most atoms were still in 6-rings. Two diffusion events had occurred and one 5775 defect had nucleated. At 6000 K, the system had already melted after 7 ps, although it had not yet reached equilibrium.  At this point, the system consisted 1D double-bonded chains.  Pair correlation functions and Lindemann criteria were calculated.

The outcomes of this study sets the pace for longer and larger theoretical computations.  These results may also inspire novel experimental studies of these high temperature materials along with other high temperature bulk alloys, or carbon nanotubes.  These future experimental tests of melting could potentially be carried out in the weightless environment of space.

initial stages of melting of graphene between 4000 K and 6000 K-Advances in Engineering

Pair correlation function for graphene at five different temperatures. We observe a smooth disappearance of the pair correlation function as the temperature is raised. At 6000 K, the system shows very little correlation.

About The Author

Li-Ming Yang obtained PhD in physical chemistry at Jilin University in 2008. After then, he has been as postdoc or research scholar at University of Oslo (with Mats Tilset), Donostia International Physics Center (with Aitor Bergera), University of Georgia (with Paul Schleyer), Massachusetts Institute of Technology (with Ju Li), University of Bremen (with Thomas Frauenheim), Hanse-Wissenschafts-Kolleg (HWK), Institute for Advanced Study, Jacobs University Bremen (with Thomas Heine), Humboldt-Universität zu Berlin (with Claudia Draxl).

Since 2016, he became a full professor and group leader at School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, China. His research interest include: molecular assembly and materials design via first-principle calculations and multiscale simulations, especially 2D materials, porous materials, catalysis, CO2 capture, etc. The expected research target in his group is to provide some useful clue for the solution of challenging issues of energy, environment and sustainable development.  

About The Author

Prof. Eric Ganz is a solid-state physicist with interests in computational studies of materials.  He is an Associate Professor at the University of Minnesota in the Department of Physics.  He is currently studying new classes of porous materials called metal organic frameworks, as well as two-dimensional materials.  He uses accurate ab initio methods such as density functional theory to model properties of novel materials.  Recently, he has been studying the melting of novel high temperature materials.

About The Author

Ariel B. Ganz received her bachelor’s degree in biology from Brandeis University in 2012. She is currently a doctoral candidate in Molecular Nutrition at Cornell University. While earning her PhD, she became interested in using computational methods to study chemical and biological systems.

Reference

Eric Ganz, Ariel B. Ganz, Li-Ming Yang and Matthew Dornfeld. The initial stages of melting of graphene between 4000 K and 6000 K. Phys. Chem. Chem. Phys., 2017, 19, 3756—3762.

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