Phonon thermal conductivity of cobalt nanofilms: role of thickness, phase, and temperature

This paper presents the results of a computational experiment aimed at determining the phonon contribution to the thermal conductivity coefficient of thin cobalt films. The study is based on molecular dynamics methods employing the Green–Kubo formalism to calculate the thermal transport properties of the systems under consideration. The simulations were performed using the LAMMPS package with the modified embedded-atom method (MEAM) potential over a temperature range from 300 to 1300 K. Verification of the model against the bulk cobalt phase showed a deviation of 2.9 % from data obtained via first-principles calculations, thereby confirming the adequacy of the chosen approach. It was established that the transition from a bulk specimen to a nanofilm with a thickness of 1.8 nm results in a twofold reduction in thermal conductivity — from 21.2 to 10.48 W / (m·K) — which is in qualitative agreement with the analytical dependence accounting for phonon mean free path confinement. The temperature evolution of the thermal conductivity coefficient was analyzed; a sharp change was revealed in the region of 700 K, attributed to a phase transition from the hexagonal close-packed (HCP) structure to the face-centered cubic (FCC) structure. It is shown that heat transfer remains efficient in the in-plane direction of the film, whereas the conductivity in the transverse direction tends to zero due to intense scattering at free surfaces. Additionally, the effect of film thickness (ranging from 0.7 to 7.1 nm) on thermal conductivity was investigated for both crystalline modifications. It was found that in the FCC phase, as the thickness decreases to sub-nanometer values, the conductivity increases, whereas in the HCP phase such an effect is weakly pronounced. The obtained data can be utilized in the design of thermal management regimes for nanoelectronic devices based on cobalt films.

1. Ministry of Science and Higher Education of the Russian Federation - 125020501412-9