Mapping thermal resistance around vacancy defects in graphite
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Mapping thermal resistance around vacancy defects in graphite Laura de Sousa Oliveira and P. Alex Greaney School of Mechanical, Industrial, & Manufacturing Engineering Oregon State University, Corvallis, OR 97331
ABSTRACT High purity bulk graphite is applicable in many capacities in the nuclear industry. The thermal conductivity of graphite has been found to vary as a function of how its morphology changes on the nanoscale, and the type and number of defects present. We compute thermal conductivities at the nanolevel using large scale classical molecular dynamics simulations and by employing the Green-Kubo method in a set of in silico experiments geared towards understanding the impact of defects in the thermal conductivity of graphite. We present the results obtained for systems with 1– 3 vacancies, and compile a summary of some of the methods applied and difficulties encountered.
INTRODUCTION Graphite is applicable in many capacities in the nuclear industry. It is used in gaskets, sealants, and liners, but most importantly, it is used as a moderator and a reflector, and its unique properties are being exploited in order to develop high-tech fuel elements for next-generation nuclear reactors. While graphite has been comprehensively studied since the 1950s [4], there are aspects of its thermal conductivity (κ) which have yet to be well understood. Graphite is highly anisotropic and the thermal conductivity along the basal plane (κa ) differs significantly from that along the c-axis (κc ), with an experimentally computed anisotropy ratio (κa /κc ) just below 210 at 300K in near-ideal graphite [3]. Furthermore, the thermal conductivity in bulk graphite varies as a function of how the material is manufactured and its exposure to radiation and high temperatures within a reactor. Our motivation is to establish a systematic understanding of how defect type, number and different defect-type ensembles affect thermal resistance in graphite. Defects can occur at different scales, and while grain boundaries, porosity and amorphous regions, for instance, can significantly affect thermal transport, in our first steps towards achieving our goal we examine point defects. In this Proceedings paper we report on a collection of vacancy defects. In addition, we summarize the methods we have developed for studying phonon scattering around defects and some of the difficulties that arise when computing κ in graphite.
METHODS Simulations were performed using large-scale equilibrium classical molecular dynamics. More specifically, we use the LAMMPS [6] software, distributed by Sandia National Laboratories. Molecular dynamics (MD) is a powerful tool for understanding thermal behavior and phonon scattering. However, there are several limitations to MD that make qualitative predictions of thermal conductivity unlikely. To mitigate this and to gain insight into how thermal resistance varies at the atomic-level and as a function of the different defects, we perform a comparative analysis. In this study, we compare the thermal conductivity and corre
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