Characterizing phonon thermal conduction in polycrystalline graphene
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Phonon thermal conduction was explored and discussed through a combined theoretical and simulation approach in this work. The thermal conductivity j of polycrystalline graphene was calculated by molecular dynamics simulations based on a hexagonal patch model in close consistency with microstructural characterization in experiments. The effects of grain size, alignment, and temperature were identified with discussion on the microscopic phonon scattering mechanisms. The effective thermal conductivity was found to increase with the grain size and decrease with the mismatch angle and dislocation density at the grain boundaries (GBs). The ;T1 temperature dependence of j is significantly weakened in the polycrystals. The effect of GBs in modifying thermal transport properties of graphene was characterized by their effective width and thermal conductivity as an individual phase, which was later included in a predictive effective medium model that showed degraded reduction in thermal conductivity for grains larger than a few micrometers.
I. INTRODUCTION
Graphene, according to its unique thermal and mechanical properties,1 is a promising material for applications such as heat spreading material in very large-scale integration, 2 thermal interface materials, 3,4 and so on. In experiments, the room-temperature thermal conductivity j was measured up to 5300 W/mK for a monolayer graphene by inspecting the dependence of the frequency of Raman G peak on the excitation laser power.1 This value is much higher than that of the best bulk thermal conductor, diamond (with j in the range of 1000–2200 W/mK at room temperature),5 and even higher than carbon nanotubes with j 5 3500 W/mK at room temperature. 6 However, large-scale single-crystalline graphene is very difficult to be produced.7 Although 30-inch continuous graphene films can be developed by the chemical vapor deposition (CVD) technique,8 the polycrystalline nature9 of these films poses a question as to how the thermal transport properties of graphene is modified by the presence of lattice imperfections, especially the grain boundaries (GBs). The thermal conductivity of graphene at room temperature was predicted theoretically with values ranging from a few hundreds to 8000 W/mK, depending on the interatomic potentials and simulation methods adopted.10–14 On the other hand, experimental measurements suggested values approximately from 2000 to 5000 W/mK, 15,16 where a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2013.380 362
J. Mater. Res., Vol. 29, No. 3, Feb 14, 2014
http://journals.cambridge.org
Downloaded: 01 Aug 2014
the material actually features a polycrystalline nature with grain size of a few to hundreds of micrometers.1,9,17 It is not yet clear how the polygrain nature could modify the intrinsic thermal transport properties of graphene. In the literature, the GB was known to play a key role in determining the j for semiconductors. Superlattice structures could reduce
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