Molecular Dynamics Study of Iron Cluster Coalescence at Sub-Melting Point Temperatures

Abstract

The coalescence of two iron clusters (Fe300+Fe300 → Fe600) at temperatures below the cluster melting points has been studied by molecular dynamics (MD). At temperatures ≈200°C below the melting point phase change from the icosahedral Fe300 structure to the face centered cubic (fcc) Fe600 structure occurs even though the clusters are not molten. Moreover, surface melting is not required for the coalescence or the phase change. At lower temperatures elongated Fe600 clusters that are not fcc, and that may be metastable, are formed.

Molecular dynamics study of the catalyst particle size dependence on carbon nanotube growth

“…Simulations based on this PES 15 show that SWNTs nucleate on a Fe 50 C surface between 800 and 1400 K, which is similar to the temperature range needed to nucleate CNTs in catalyzed chemical vapor deposition (CCVD) experiments. The simulations also revealed atomistic details of the nucleation mechanism.…”

“…The nucleation of the graphitic cap was thus not simulated in these studies. Simulations that are based on empirical potential energy surfaces (PESs) 14,15 cannot capture the time dependence of the electron density, and the catalytic role of the transition metal cluster has thus not been studied. However, the role of the metal particle as a solvent and a template for CNT growth can be studied using these PESs.…”

“…The PES described above has been used to study the thermal properties of FeC cluster systems 15 . MD simulations based on this PES yielded the correct trends of the FeC phase diagram, where a eutectic was found at 20% carbon content by weight.…”

The role of the catalytic particle temperature gradient for SWNT growth from small particles

“…Instead, the edges of the graphitic cap -or the ends of the SWNT -act as a C sink during the growth process. This reduces the carbon concentration in the regions of the particle that are near the cap edge (or 12 SWNT end), which leads to the diffusion of C atoms from regions of high C concentration to the cap or SWNT edge, where they incorporate into the growing structure.…”

“…The MD simulations discussed here are based on an analytic potential energy surface (PES) that has been used to study the iron (Fe) catalysed nucleation of SWNTs [12]. Details of the PES and the SWNT nucleation mechanism appear elsewhere [12].…”