Enhanced composite thermal conductivity by percolated networks of in-situ confined-grown carbon nanotubes

Despite the ever-increasing demand of nanofillers for thermal enhancement of polymer composites with higher thermal conductivity and irregular geometry,nanomaterials like carbon nanotubes(CNTs)have been constrained by the nonuniform dispersion and difficulty in constructing effective three-dimensional(3D)conduction network with low loading and desired isotropic or anisotropic(specific preferred heat conduction)performances.Herein,we illustrated the in-situ construction of CNT based 3D heat conduction networks with different directional performances.First,to in-situ construct an isotropic percolated conduction network,with spherical cores as support materials,we developed a confined-growth technique for CNT-core sea urchin(CNTSU)materials.With 21.0 wt.%CNTSU loading,the thermal conductivity of composites reached 1.43±0.13 W/(m·K).Secondly,with aligned hexagonal boron nitride(hBN)as an anisotropic support,we constructed CNT-hBN aligned networks by in-situ CNT growth,which improved the utilization efficiency of high density hBN and reduced the thermal interface resistance between matrix and fillers.With~8.5 wt.%loading,the composites possess thermal conductivity up to 0.86±0.14 W/(m·K),374%of that for neat matrix.Due to the uniformity of CNTs in hBN network,the synergistic thermal enhancement from one-dimensional(1D)+two-dimensional(2D)hybrid materials becomes more distinct.Based on the detailed experimental evidence,the importance of purposeful production of a uniformly interconnected heat conduction 3D network with desired directional performance can be observed,particularly compared with the traditional direct-mixing method.This study opens new possibilities for the preparation of high-power-density electronics packaging and interfacial materials when both directional thermal performance and complex composite geometry are simultaneously required.

Influences of graphene nanoplatelet aspect ratio and thermal treatment on dielectric performances of poly(methyl methacrylate)composites

The dielectric property and percolated behaviour of polymer matrix composites largely depend on the morphology of conductive fillers and external stimulations especially when the composites are processed by melting blending and extrusion-injection way.In this study,the poly(methyl methacrylate)(PMMA)matrix composites incorporated by two kinds of graphene nanoplatelets(GNP),G5 and G150 with different aspect ratios(the ratio of diameter and thickness)are prepared to study the influence of GNP morphology on the dielectric performances close to percolation threshold(f_(c)).After annealing at glass transition temperature(T_(g))for 1 h,the dielectric permittivities of PMMA/G5 and PMMA/G150 near fc increase 43 and 38%,respectively,while the dielectric loss change little.This improvement on the dielectric property is possibly attributed to the slight change of the distance between adjacent GNPs after annealing at T_(g)which enables to arouse stronger polarisation by tunnelling effect.

Typical synergistic effect of graphite nanoplatelets and carbon nanotubes and its influence on polymer-based dielectric composites

Graphite nanoplatelets(GNPs)with their intrinsic two-dimensional structure make an excellent compromise of advantages of both graphite and graphene.Due to their outstanding inherent properties,GNPs have been widely used as the functional fillers to improve certain performances of polymer-based composites.However,the multi-layer stacked construction of GNPs could form aggregations easily,which limits their application in polymer-based dielectric composites.Here,the authors exhibit a polydimethylsilicone(PDMS)-based nanocomposite which loading equivalent carbon nanotubes(CNTs)and GNPs as co-loading fillers,whose dielectric properties are improved significantly.The composites were fabricated through a highly-shearing mechanical mixing process.During the mixing process,both intrinsic CNTs entanglements and GNPs stacked aggregation could be ameliorated due to the interaction between these two types of fillers.Compared with the GNPs solely loaded composites,a small quantity of CNTs addition endows GNPs/CNTs/PDMS ternary composites with a significantly decreased percolation threshold(f_(c)∼1.7 vol.%).Both GNPs and CNTs,these two representative conductive carbon constructions with high aspect ratios are able to effectively enhance the establishing efficiency of internal conductive network inner composites.These results indicate that the development of internal conductive network and dielectric performance of nanocomposites are able to be optimised through the strategy of taking advantage of the synergistic effect of multi-type fillers reasonably.