Necessary conditions for thermal rectification and negative differential thermal conductance in graphene nanoribbons
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Necessary conditions for thermal rectification and negative differential thermal conductance in graphene nanoribbons Yan Wang1 and Xiulin Ruan1,2 1 School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA 2 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
ABSTRACT We have studied negative differential thermal conductance (NDTC) and thermal rectification (TR) in graphene nanoribbons (GNRs) using nonequilibrium molecular dynamics simulations. Strong ballistic transport regime and sufficient temperature gradient are found to be necessary conditions for the onset of both NDTC and TR in GNRs, while the latter also requires asymmetry in structure. Preferred direction of heat transport is also discussed for TR.
INTRODUCTION Thermal transport processes beyond the linear-response regime such as negative differential thermal conductance (NDTC) and thermal rectification (TR) may lead to revolutionary advancements in nanoscale energy managment and thermal signal manipulation[1-4]. NDTC, similar to its electrical counterpart, is a transport process where heat current decreases with increasing temperature bias. TR refers to a diode like behavior where heat propagation is faster in one direction than the opposite. Recent studies have predicted or demonstrated the existence of NDTC and TR in nanosized model systems[1, 3, 5-7]. Li and coworkers have explored several models and observed the necessity of nonlinearities for the onset of NDTC[2-4, 7]. Segal reported the observation of NDTC when the molecule is strongly coupled to the thermal baths even in the absence of nonlinearities[8]. Chang et al. used externally mass-graded carbon and boron nitride nanotubes and proved asymmetric axial thermal conductance with greater heat flow in the direction of decreasing mass density in their experiment[1]. However, Alaghemandi et al. found an opposite preferred direction of heat flow in mass-graded carbon nanotubes (CNTs) with molecular dynamics simulations[5]. These discrepancies indicate that the mechanism of NDTC and TR is quite complicated and varies with system. Graphene nanoribbons (GNRs), nanosized thin strips of monolayer graphites, have recently received much attention due to its exceptional thermal, mechanical and electrical properties[9]. Thermal conductivities in the range of 600~5000 W/m-K have been measured[9] for suspended single-layer graphene, which suggests its potential usage as high material outperforming CNTs. We reported TR behavior in geometrically asymmetric GNRs in Ref. 6 and demonstrated
the dependence of rectification ratio on the edge type and vertex angle of triangular GNRs. Yang et al. also studied similar issues in their work and reached consistent conclusions[4]. The dependence of thermal and electrical properties on edge type and topology makes GNR excellent candidate for applications in nanoelectronics and thermal management. However, only few studies[4, 6] have been done on TR in graphene systems, and conditions necessary for these nonlinear
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