Atomistic calculations of thermal conductivity in films made from graphene sheets for electron emitter applications

S N Sami,R Islam,R P Joshi

doi:10.1063/5.0062044

Abstract

Simulations for the thermal conductivity of carbon fibers (CFs) are carried out based on a molecular dynamics (MD) approach. Our calculations are geared toward providing a general trend for thermal conductivity and its dependence on sample length and to probe the suitability of this material for high power applications from the thermal management standpoint. Since abundant data are not available for carbon fibers, calculations are first carried out in silicon to validate the predictions. The MD results yield a bulk thermal conductivity of 148 W m−1 K−1 for silicon in good agreement with published reports. However, for CF, a much lower bulk value of ∼14 W m−1 K−1 is predicted. This suggests that thermal management would likely be an issue for this material and that CF emitters of lengths below 2 µm should be avoided. The predicted increases in thermal conductivity with temperature may help alleviate the issue to a minor degree. Carbon nanotubes would likely be a better alternative in this context of field emitter arrays.

Highlights

Carbon fiber cathodes[1,2,3,4] have emerged as promising candidates for field emitters needed for large area high power microwave (HPM) devices, such as magnetically insulated transmission line oscillators (MILOs), and other pulsed power systems
The analysis was geared toward providing more general trends for thermal conductivity and its dependence on sample length
The molecular dynamics (MD) results for silicon yielded a bulk thermal conductivity of 148 W m−1 K−1 and were in good agreement with published reports.[69]

Summary

INTRODUCTION

Carbon fiber cathodes[1,2,3,4] have emerged as promising candidates for field emitters needed for large area high power microwave (HPM) devices, such as magnetically insulated transmission line oscillators (MILOs), and other pulsed power systems. The pulsed power and high power microwave arena, which is of interest here, requires high overall currents and, large-area emission at a relatively low cost From this practical standpoint carbon fibers and graphene sheets are superior alternatives at the present time for high power, large-area applications. Reductions in thermal conductivity (k) by about two orders of magnitude over bulk values have been reported in silicon nanowires[32] for lengths lower than 50 nm Such effects have been observed in carbon nanotubes,[33] which are promising candidates for cathodes, especially in the high power context.[34]. Calculations involving the size and temperature-dependent changes in thermal conductivity of graphene-based carbon fibers are presented in this contribution. Scitation.org/journal/adv the bulk value (applicable to long samples) are extracted and shown to be in reasonable agreement with the data currently available

MODEL AND SIMULATION METHOD

RESULTS AND DISCUSSION

SUMMARIZING CONCLUSIONS