石墨烯基环氧树脂复合热界面材料的制备及热性能

采用氧化还原法制备的石墨烯作为改性材料,研究石墨烯的含量及其还原程度对环氧树脂热导率的影响,并进一步测定复合热界面材料的热导率在高温下的稳定性。结果表明:石墨烯可以大幅提高环氧树脂的热导率,加入质量分数为15%的石墨烯可以使环氧树脂的热导率提高2 300%。石墨烯表面的剩余官能团对产物的热导率也有显著的影响,表面官能团可以充当声子输运通道并减小界面间的Kapitza热阻,但是过多的表面官能团会减小石墨烯的本征热导率。另外,通过优化石墨烯的还原程度及尺寸,提高了复合材料产物热导率在高温下的稳定性。     

中科院纳米所开发出超高热导率石墨烯-聚合物复合材料

作为近来纳米科学领域的研究热点，新兴的石墨烯由于具有独特的二维结构、高比表面积和优异的热学性能[导热系数可高达3000～6000 W／(m · K)]，受到了广泛关注。石墨烯／聚合物导热复合材料有望在电子器件、光电子器件、消费电子及导热聚合物材料中得到重要应用。目前，石墨烯的添加一定程度上改善了聚合物复合体系的导热性能，尽管能使聚合物的导热系数提高一个数量级，但有限石墨烯添加量、无序结构以及石墨烯／聚合物高界面热阻致使石墨烯-聚合物复合体系的热导率无法实现更高突破，阻碍了其在未来热管理中的广泛应用。     

碳纳米管/聚合物基导热复合材料研究进展

碳纳米管（CNTs）由于优异的轴向导热性能,是目前制备高热导率聚合物基复合材料的一类重要填料。本文综述了近年来CNTs增强聚合物复合材料的研究进展,探讨了CNTs/聚合物复合材料的导热机理以及CNTs用量、尺寸及结构、表面改性、混杂CNTs粒子和聚合物基体结构等因素对CNTs/聚合物复合材料热导率的影响。同CNTs/聚合物的电导率相比,热导率远低于预期值,归因于CNTs/树脂界面间的声子频率失配现象导致了声子在界面的散射及很高的界面接触热阻,从而降低了体系热导率。分析和总结了改善体系热导率的方法和措施,采用特殊工艺使CNTs在基体内形成特殊的隔离结构或者取向结构是CNTs/聚合物导热复合材料的未来研究及发展方向。     

石墨烯-碳纳米管研究取得重要成果

正同济大学声子学与热能科学中心陈杰研究员与瑞士苏黎世联邦理工学院(ETH)Koumoutsakos教授研究小组合作,提出了一种显著提高石墨烯层间导热性能的新方法:用sp~2共价键(强相互作用)来代替石墨烯层间范德华力(弱相互作用),构造无缝连接的石墨烯-碳纳米管混合结构。通过计算机模拟,该团队发现相比于范德华相互作用,共价键连接极大地提高了石墨烯中的晶格振动模式(声子)在垂直方向的传输系数,使得该混合结构沿垂直方向的热导率比相     

碳填料/天然橡胶复合材料热导率及层间接触热阻对微波加热效果的影响

摘要：制备炭黑/天然橡胶（NR）、石墨烯/NR和碳纳米管/NR复合材料,采用试验与数值模拟相结合的方法研究复合材料热导率和层间接触热阻对微波加热的影响。结果表明：炭黑、石墨烯和碳纳米管自身热导率越大,复合材料的热导率越大,层间接触热阻越小;通过增大填料用量来增大复合材料的热导率和减小层间接触热阻具有一定的局限性,需考虑复合材料的配方设计适用性和经济性;复合材料的热导率对微波加热过程中高、低温区域分布规律和微波加热效率基本无影响,但影响复合材料的温度分布均匀性。为保证微波加热硫化均匀性,多层复合材料的层间接触热阻不可忽略。     

炭纤维/树脂复合材料导热性能的数值模拟

采用有限元方法对炭纤维/树脂复合材料的导热性能进行了数值模拟,分别建立一维结构和二维结构炭纤维/树脂复合材料计算分析模型,研究炭纤维含量、界面接触热阻、以及炭纤维直径对复合材料有效热导率的影响。研究结果表明炭纤维作为复合材料增强相,其含量越高复合材料的热导率越高;界面的接触热阻在10-3~10-5(m2 K)/W范围内对复合材料有效热导率有较大的影响,超出范围之后改变接触热阻对材料热导率的影响可以忽略;接触热阻比较大时,炭纤维的直径对复合材料的热导率有较大的的影响,当接触热阻比较小时,炭纤维的直径对于复合材料热导率的影响非常小。     

离子交联石墨烯相变复合材料的研究

在氧化石墨烯和膨胀石墨中掺入不同价态的金属离子,通过水热法制备了混杂三维网状石墨烯。并以石蜡为相变材料,通过浸渍法得到了石墨烯相变复合材料。对其进行扫描电镜、红外光谱仪和示差扫描量热仪等测试表征。结果表明,3种金属离子都可明显提高复合相变材料的导热性能。此外,与另外两种离子相比,二价Mg~(2+)的效果最为优异。在掺入Mg~(2+)的混杂三维网状石墨烯的微观结构发现了其明显的交联现象,显著提高相变材料热导率。     

碳系填料构筑具有隔离结构导热复合材料进展

导热途径不均匀导致导热效率较低是制备高导热聚合物复合材料的重点难题,制备具有优良导热性能的复合材料仍面临巨大的挑战。制备具有三维导热网络结构的复合材料,有效地提高了热导率,是目前导热复合材料研究的热点。隔离结构的导热复合材料具有独特的导热网络结构,为声子提供了有效的传播途径,并且能降低填料-基体、填料-填料间的界面热阻,在低负载下能获得高热导率。综述了近年来碳系填料构筑隔离结构的导热复合材料的研究进展,分析了具有隔离结构复合材料的导热机理及制备方法,对各类方法制备复合材料的性能、特点缺陷进行概括对比,并且对能提升热导率的方法进行简要分析。     

碳纳米杂化材料的分散性及分散稳定性研究

将酸化石墨烯、羟基化多壁碳纳米管通过超声离心等物理方法合成碳纳米管/石墨烯杂化材料以及用化学多步法合成碳纳米管/石墨烯杂化材料,按照0.1 mg/m L分别分散于四氢呋喃溶剂中超声72 h制备碳纳米材料的分散液,并将分散液静置24 h。通过紫外光谱证明所用碳纳米杂化材料已成功合成,同时通过紫外光谱、显微镜扫描和沉淀实验表征碳纳米材料的分散性及分散稳定性。结果表明,相比于碳纳米管、石墨烯和物理法合成碳纳米管/石墨烯杂化材料,化学多步法合成的碳纳米管/石墨烯杂化材料具备更优异的分散性及分散稳定性,这要归因于分散好的碳纳米管先与聚丙烯酰氯反应,以初步抑制碳纳米管的团聚,其次将其再与石墨烯反应,这样碳纳米管和石墨烯就通过聚丙烯酰氯连接在一起,构建出三维结构,抑制碳纳米管的重新团聚和石墨烯片层的叠加。     

介孔复合材料内界面热阻及热导率

界面广泛存在于复合材料中,对介孔复合材料热物性起着决定性的影响,研究界面的导热特性对于认识和理解介孔复合材料的导热机制十分重要。利用非平衡的分子动力学模拟方法计算介孔复合材料中基材与填充物间的界面热阻,考察界面热阻随温度、材料质量差异的变化,进一步用界面热阻修正介孔复合材料的有效热导率。结果表明,界面热阻的数量级为10~(-11) m~2·K·W~(-1),并随温度升高逐渐降低。界面两端材料质量差异越大,界面热阻越高。可通过减小孔径、减小纳米线长度、增大纳米线间距、降低纳米线填充率来降低介孔复合材料的有效热导率。界面热阻能降低材料的有效热导率。孔径越小、纳米线间距越小、纳米线长度越长、填充率越高,界面热阻降低热导率效果越显著。     

Molecular phonon couplers at carbon nanotube/substrate interface to enhance interfacial thermal transport

A novel assembling process of incorporating carbon nanotubes as thermal interface materials for heat dissipation has been developed by synthesizing vertically aligned carbon nanotubes on a copper substrate and chemically bonding the carbon nanotubes to a silicon surface. The assembling process and the copper/carbon nanotubes/silicon structure are compatible with current flip-chip technique. The carbon nanotubes are covalently bonded to the silicon surface via a thin but effective bridging layer as a “molecular phonon coupler” at the CNT-silicon interface to mitigate phonon scattering. Experimental results indicate that such an interface modification improves the effective thermal diffusivity of the carbon nanotube-mediated thermal interface by an order of magnitude and conductivity by almost two orders of magnitude. The interfacial adhesion is dramatically enhanced as well, which is significant for reliability improvement of the thermal interface materials.     

Engineering interfaces in carbon nanostructured mats for the creation of energy efficient thermal interface materials

One of the few remaining opportunities to increase heat dissipation in IC circuitry is to substantially decrease the thermal interface resistance between solid–solid contacts from source to sink. In this study, heterogeneous nanostructured mats (1–100 μm thick, randomly oriented networks of nanostructures) are synthesized for use as thermal interface materials (TIMs). Recent studies suggest that mats composed entirely of carbon nanotubes (CNTs) or graphite nanofibers (GNFs) can act as thermal insulators due to significant phonon scattering at interfaces. In this work, graphene nanoplatelets (xGnPs) with high surface areas are included in CNT and GNF mats in order to increase the contact area between nanostructures and mitigate phonon scattering. Results indicate that an increase in contact area between nanostructures increases the thermal conductance across nanostructure networks by nearly an order of magnitude. Additionally, a study of the surface topography of CNT and GNF mats using atomic force microscopy (AFM) indicates that they are able to conform well to the asperities between rough, mating surfaces. Thus, an increase in contact area between CNT junctions not only produces a thermally conductive network, but also increases the reliability of a CNT mat TIM by avoiding common issues associated with the use of wetting agents.     

Heat transport across the metal–diamond interface

Thermal conductivity of the aluminium–diamond (Al–diamond) composites, prepared by the gas pressure infiltration method, is measured by steady state technique. A detailed theoretical investigation on the heat conduction mechanism across the Al–diamond interface is presented. It was confirmed that both electrons and phonons actively take part in the flow of heat at the interface. In the Al side, electrons of Al couple with the phonons and carry the heat up to the interface. This electron–phonon pair which predominantly carries heat in the Al, breaks down at the Al–diamond interface. The coupling between phonons of both Al and diamond takes place at the interface which eventually leads the heat conduction across the interface to the diamond. The phonon–phonon coupling across the interface is discussed by scattering mediated acoustic mismatch model (SMAMM). It is shown that for Al–diamond composite, the implementation of the SMAMM yields an interface thermal resistance (ITR) value of 4.44 × 10^− 9 m²K/W, which is in fairly good agreement with values derived from experimental thermal conductivity values of this composite implemented in the Hasselman–Johnson (HJ) mean field scheme.     

介孔复合材料内界面热阻及热导率

界面广泛存在于复合材料中,对介孔复合材料热物性起着决定性的影响,研究界面的导热特性对于认识和理解介孔复合材料的导热机制十分重要。利用非平衡的分子动力学模拟方法计算介孔复合材料中基材与填充物间的界面热阻,考察界面热阻随温度、材料质量差异的变化,进一步用界面热阻修正介孔复合材料的有效热导率。结果表明,界面热阻的数量级为10^-11m²·K·W^-1,并随温度升高逐渐降低。界面两端材料质量差异越大,界面热阻越高。可通过减小孔径、减小纳米线长度、增大纳米线间距、降低纳米线填充率来降低介孔复合材料的有效热导率。界面热阻能降低材料的有效热导率。孔径越小、纳米线间距越小、纳米线长度越长、填充率越高,界面热阻降低热导率效果越显著。     

Exceptional thermal interface properties of a three-dimensional graphene foam

We have uncovered some unusual thermal interface properties of a three-dimensional, flexible and interconnected graphene foam (GF). The thermal interfacial resistance of GF at Si–Al interface is as low as 0.04 cm²K W⁻¹, which is one order of magnitude lower than conventional thermal grease and thermal paste-based thermal interfacial material (TIM). The thermal contact resistance was found to dominate the overall interfacial resistance of GF-based TIM, in as much as the bulk thermal conductivity of GF is rather high. The contact pressure-dependent thermal interfacial resistance of GF exhibits an asymptotic behavior, which converges into a plateau value at an ultralow contact pressure (∼0.1 MPa). Significantly, the GF-based TIM has shown a superior performance to vertically aligned carbon nanotubes currently held as the gold standard (at least ∼75% improvement in thermal interfacial resistance at Si–Al interface), thus providing a strong candidate for the next generation of high-performance carbon-based TIM.     

A theoretical analysis of the thermal conductivity of hydrogenated graphene

We investigate the thermal conductivity of hydrogenated graphene using non-equilibrium molecular dynamics simulations. It is found that the thermal conductivity greatly depends on the hydrogen distribution and coverage. For random hydrogenation, the thermal conductivity decreases rapidly with increasing coverage up to about 30%. Beyond this limit, however, the thermal conductivity is almost insensitive to the coverage. For patterned hydrogenation with stripes parallel to the heat flux, the thermal conductivity decreases gradually with increasing coverage from 0% to 100%. In contrast, when the stripe direction is perpendicular to the heat flux, a small (5%) coverage causes a sharp (60%) drop of thermal conductivity. The deterioration of thermal conductivity is due to the sp²-to-sp³ bonding transition upon hydrogenation, which softens the G-band phonon modes. Percolation theory can be used to explain the variation of thermal conductivity at different hydrogenation distributions and coverages. The applicability of the rule of mixtures in predicting the thermal conductivity is also discussed. The work suggests that hydrogenation is a possible route to tune graphene thermal conductivity and manage heat dissipation in graphene-based nanoelectronic devices.     

Thermal conductivity of carbon nanotubes and graphene in epoxy nanofluids and nanocomposites

We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states.PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.     

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT.This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composites.     

Influence of hot phonons on wind forces in metallic single walled carbon nanotubes

The wind force exerted on the lattice by the flux of electrons under electric loading in single walled carbon nanotubes is studied using an ensemble Monte-Carlo simulation. The momentum transfer between electrons and the lattice is treated using Quantum Mechanics. The phonon distribution and the electron distribution of the carbon nanotubes are allowed to be populated away from thermal equilibrium to study the influence of hot phonons on the wind forces. While the presence of hot phonons creates a net increase in the phonon–electron scattering rates, it appears to have a very small influence on the amount of force exerted on the lattice.