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The superior effect of edge functionalization relative to basal plane functionalization of graphene in enhancing the thermal conductivity of polymer–graphene nanocomposites – a combined molecular dynamics and Green's functions studyTo achieve polymer–graphene nanocomposites with high thermal conductivity ( k ), it is critically important to achieve efficient thermal coupling between graphene and the surrounding polymer matrix through effective functionalization schemes. In this work, we demonstrate that edge-functionalization of graphene nanoplatelets (GnPs) can enable a larger enhancement of effective thermal conductivity in polymer–graphene nanocomposites relative to basal plane functionalization. Effective thermal conductivity for the edge case is predicted, through molecular dynamics simulations, to be up to 48% higher relative to basal plane bonding for 35 wt% graphene loading with 10 layer thick nanoplatelets. The beneficial effect of edge bonding is related to the anisotropy of thermal transport in graphene, involving very high in-plane thermal conductivity (∼2000 W m −1 K −1 ) compared to the low out-of-plane thermal conductivity (∼10 W m −1 K −1 ). Likewise, in multilayer graphene nanoplatelets (GnPs), the thermal conductivity across the layers is even lower due to the weak van der Waals bonding between each pair of layers. Edge functionalization couples the polymer chains to the high in-plane thermal conduction pathway of graphene, thus leading to overall high thermal conductivity of the composite. Basal-plane functionalization, however, lowers the thermal resistance between the polymer andmore »
Thermodynamically‐driven exfoliation and self‐assembly of pristine graphene sheets is shown to provide thermally and electrically functional polymer composites. The spreading of graphene sheets at a high energy liquid/liquid interface is driven by lowering the overall energy of the system, and provides for the formation of water‐in‐oil emulsions stabilized by overlapping graphene sheets. Polymerization of the oil phase, followed by removal of the dispersed water phase, produces inexpensive and porous composite foams. Contact between the graphene‐stabilized water droplets provides a pathway for electrical and thermal transport through the composite. Unlike other graphene foams, the graphite used to synthesize these composites is natural flake material, with no oxidation, reduction, sonication, high temperature thermal treatment, addition of surfactants, or high shear mixing required. The result is an inexpensive, low‐density material that exhibits Joule heating and displays increasing electrical conductivity with decreasing thermal conductivity.
Effect of Alignment on Enhancement of Thermal Conductivity of Polyethylene–Graphene Nanocomposites and Comparison with Effective Medium TheoryThermal conductivity (k) of polymers is usually limited to low values of ~0.5 Wm−1K−1 in comparison to metals (>20 Wm−1K−1). The goal of this work is to enhance thermal conductivity (k) of polyethylene–graphene nanocomposites through simultaneous alignment of polyethylene (PE) lamellae and graphene nanoplatelets (GnP). Alignment is achieved through the application of strain. Measured values are compared with predictions from effective medium theory. A twin conical screw micro compounder is used to prepare polyethylene–graphene nanoplatelet (PE-GnP) composites. Enhancement in k value is studied for two different compositions with GnP content of 9 wt% and 13 wt% and for applied strains ranging from 0% to 300%. Aligned PE-GnP composites with 13 wt% GnP displays ~1000% enhancement in k at an applied strain of 300%, relative to k of pristine unstrained polymer. Laser Scanning Confocal Microscopy (LSCM) is used to quantitatively characterize the alignment of GnP flakes in strained composites; this measured orientation is used as an input for effective medium predictions. These results have important implications for thermal management applications.
Liquid Metal Composites with Enhanced Thermal Conductivity and Stability Using Molecular Thermal Linker
Gallium‐based liquid metal (LM) composite with metallic fillers is an emerging class of thermal interface materials (TIMs), which are widely applied in electronics and power systems to improve their performance. In situ alloying between gallium and many metallic fillers like copper and silver, however, leads to a deteriorated composite stability. This paper presents an interfacial engineering approach using 3‐chloropropyltriethoxysilane (CPTES) to serve as effective thermal linkers and diffusion barriers at the copper‐gallium oxide interfaces in the LM matrix, achieving an enhancement in both thermal conductivity and stability of the composite. By mixing LM with copper particles modified by CPTES, a thermal conductivity (κ) as high as 65.9 W m−1K−1is achieved. In addition, κ can be tuned by altering the terminal groups of silane molecules, demonstrating the flexibility of this approach. The potential use of such composite as a TIM is also shown in the heat dissipation of a computer central processing unit. While most studies on LM‐based composites enhance the material performance via direct mixing of various fillers, this work provides a different approach to fabricate high‐performance LM‐based composites and may further advance their applications in various areas including thermal management systems, flexible electronics, consumer electronics, and biomedical systems.
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