C-Cu

Abstract: Unraveling the deformation mechanisms at the atomistic scale of metal matrix-graphene composites is a key step toward designing and fabricating these materials with exceptional mechanical properties. While these composites can be made by embedding graphene into multiple metallic matrices, it is shown that superior mechanical properties can be obtained by combining copper and graphene. In the past few years, molecular dynamics simulations have been used to investigate the fundamental deformation mechanisms at the nanoscale of Cu-graphene composites to facilitate designing these composites with improved mechanical properties. However, in all of the reported works, Lennard-Jones potential has been used for modeling the interaction between copper and carbon atoms. The complexities in the Cu-C interaction emerges the necessity of utilizing more accurate potentials. In this work, a 2NN MEAM (second nearest-neighbor modified embedded atomic method) potential for the copper and carbon atom interaction is developed. Since crystal structures like B1 or B2 are not experimentally available for the Cu-C system, first-principle calculations are used to determine the reference structure and its elastic constants in this work. It is shown that the B1 and B2 structure of Cu-C has positive formation energy, but B1 is dynamically stable. Accordingly, the B1 crystal structure is used as the reference structure for the Cu-C system to develop the interatomic potential. It is shown that the reported potential agrees reasonably well for phonon dispersion frequencies, stacking fault energies, and the atomic forces with the available experimental data and first-principle calculations. The developed potential is utilized to study the mechanical properties of copper-graphene composites subjected to uniaxial loading. Our results show that adding graphene to a defect-free Cu crystal weakens the metallic matrix’s mechanical properties. However, when the graphene is embedded into a Cu matrix with some defects, e.g., in a polycrystalline Cu, it can significantly improve the mechanical properties.

Abstract: Carbon is the most widely studied material today because it exhibits special properties not seen in any other materials when in nano dimensions such as nanotube and graphene. Reduction of material defects created during synthesis has become critical to realize the full potential of carbon structures. Molecular dynamics (MD) simulations, in principle, allow defect formation mechanisms to be studied with high fidelity, and can, therefore, help guide experiments for defect reduction. Such MD simulations must satisfy a set of stringent requirements. First, they must employ an interatomic potential formalism that is transferable to a variety of carbon structures. Second, the potential needs to be appropriately parameterized to capture the property trends of important carbon structures, in particular, diamond, graphite, graphene, and nanotubes. Most importantly, the potential must predict the crystalline growth of the correct phases during direct MD simulations of synthesis to achieve a predictive simulation of defect formation. Because an unlimited number of structures not included in the potential parameterization are encountered, the literature carbon potentials are often not sufficient for growth simulations. We have developed an analytical bond order potential for carbon, and have made it available through the public MD simulation package LAMMPS. We demonstrate that our potential reasonably captures the property trends of important carbon phases. Stringent MD simulations convincingly show that our potential accounts not only for the crystalline growth of graphene, graphite, and carbon nanotubes but also for the transformation of graphite to diamond at high pressure.