Principles and mechanisms of strain-dependent thermal conductivity of polycrystalline graphene with varying grain sizes and surface hydrogenation

Abstract

In this paper, the thermal conductivities (κ) of polycrystalline graphene (PG) with varying average grain size are investigated using reverse nonequilibrium molecular dynamics method. Due to the presence of grain boundary (GB), the κ of PG is found to depend on the average grain size as well as in-plane strain and hydrogenation coverage. The principles and mechanisms for the change of κ with in-plane strain and surface hydrogenation are interpreted combining the thermal transport theory and phonon density of states (PDOS) analysis. The thermal property of PG under tension is found to be related with the average stress in PG as a result of the suppression of mean free path (MFP) and the softening of phonon modes. PG with fine grains exhibits more reduction of κ than the PG with coarse grains does under the same tensile strain due to the more stress concentration at GBs. The mechanism is also revealed for the size effect on the thermal property of PG under compression. Additionally, the dependency of κ on the surface hydrogenation of PG is investigated, and an unexpected two-stage evolution of κ with hydrogenation coverage is interpreted preliminarily from the circumference and arrangement of functionalized domains. The negative effect of GB on thermal conductivity is weakened significantly under full hydrogenation. Furthermore, the coupling effect between hydrogenation and strain on the κ of PG is revealed, and the thermal conductivity of PG becomes insensitive to the in-plane strain under higher hydrogenation. Our results provide new insights into the role of GB on the thermal manipulation of PG and offer theoretical guidelines for the design of graphene-based flexible devices in thermoelectric and thermal management applications.