First-principles study of carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers

Abstract

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.