Experiments and modeling for thermal conductivity of graphite nanoplatelets/carbon composites

In this work, the graphite nanoplatelets/carbon composites were fabricated from graphite nanoplatelets and pitch powders by a hot-pressing technology followed by carbonization and graphitization. The XRD and pole figure results show that the incorporation of pitch induces the decrease of size (L_a) and orientation degree of graphitic crystallites, while the in-plane thermal conductivity of graphitized sample is increased with the increasing pitch content up to 6 wt.%, achieving a maximum value of 405 W/m K. The pitch binders are filled into the voids to bridge two or more graphite nanoplatelets particles together to form extra thermal paths, which makes a great contribution to the enhancement of thermal conductivity. A thermal conductivity model for the graphitized composites is constructed based on a bridging mechanism, and the predicted results fit well with the experimental results.     

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m⁻¹ K⁻¹ compared to 0.1795 W m⁻¹ K⁻¹ for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.     

流延法制备高热导率定向石墨/高分子复合片层材料(英文)

以天然鳞片石墨为原料,PVB为黏结剂,PEG和DBP混合物为增塑剂,通过流延工艺在室温下制备了定向排列的石墨/聚合物片层复合材料。系统分析了不同黏结剂用量和流延刀口高度下复合片层材料的定向排列状况,并探讨了定向排列程度对其热导率的影响。XRD和SEM的结果表明,石墨/聚合物复合片层材料显示了不同程度的定向排列。热导率测试结果表明,片层复合材料的热导率随着定向排列程度的提高而增大。通过优化黏结剂的用量和流延刀口高度制备了具有较高热导率的片层复合材料,其热导率最高可达490 W/(m.K)。     

Fabrication and thermal conductivity of copper matrix composites reinforced with Mo2C or TiC coated graphite fibers

Graphite fiber reinforced Cu-based composites have good thermal conductivity, low coefficient of thermal expansion for heat sink applications. In these composites, the quality of interfacial bonding between the copper matrix and the graphite fibers has significant influence on the thermal properties of composites. In this study, two different carbide coatings (Mo₂C or TiC) were synthesized on graphite fiber to promote the interfacial bonding in composites. Fibers/Cu composites had been produced by spark plasma sintering process. The results showed that the densification, interfacial bonding and thermal conductivity of coated composites were improved distinctly compared to that of uncoated ones. The enhanced composites present 16–44% increase of thermal conductivity in X–Y plane. An original theoretical model was proposed to estimate the interface thermal resistance. The result showed that the interfacial thermal resistance was largely reduced by one order of magnitude with the introduction of carbide interlayer.     

鳞片石墨取向对鳞片石墨/聚丙烯、鳞片石墨/尼龙66复合材料导热性能的影响

为了研究鳞片石墨在基体中的取向对复合材料导热性能特别是不同方向导热性能的影响,通过双螺杆挤出混合及注射成型制备了鳞片石墨/聚丙烯（PP）、鳞片石墨/尼龙66（PA66）导热复合材料,并利用扫描电子显微镜和超声波测试对制备的样品进行了分析。结果表明：鳞片石墨的粒径越小,平面取向度越高,平面与垂直方向的热导率差值越大。加工中双螺杆挤出机的过度剪切会破坏鳞片石墨的片层结构,影响鳞片石墨导热网络的形成,降低复合材料的热导率,但提高了材料导热的各向均匀性。适度的剪切可以打开鳞片石墨的片层结构,提高复合材料的热导率,注射成型更多影响到制品导热的各向异性。     

In-plane thermal enhancement behaviors of Al matrix composites with oriented graphite flake alignment

Oriented graphite flakes (G_f)/Si/Al composites were fabricated to study their thermal enhancement behaviors. The in-plane thermal conductivity (TC) of the composites increases with the increase of G_f volume fraction. At a given volume fraction, a larger G_f size can achieve a higher in-plane TC of the composites. Microstructural characterization revealed a clean and Al₄C₃-free interface between the side surface of G_f and the Al matrix. Based on the observed microstructures, an analytical model was presented to predict the in-plane TC of the composites with oriented G_f alignment by incorporating interfacial thermal resistance within the framework of effective medium approach (EMA). Comparisons of the present model predictions with the experimental data of the as-fabricated G_f/Si/Al and previously reported G_f/Al and G_f/polymer (polyvinyl butyral, PVB) composites show good agreement. The results indicate that our model can well predict the in-plane thermal enhancement behaviors of the composites at different effective phase contrasts (i.e. the ratio between effective TC of the G_f and TC of the matrix).     

Effect of carbon nanofibers on the infiltration and thermal conductivity of carbon/carbon composites

Preforms containing 0, 5, 10, 15 and 20 wt.% carbon nanofibers (CNFs) were fabricated by spreading layers of carbon cloth, and infiltrated using the electrified preform heating chemical vapor infiltration method (ECVI) under atmospheric pressure. Initial thermal gradients were determined. Resistivity and density evolutions with infiltration time have been recorded. Scanning electron microscopy, polarized light micrograph and X-ray diffraction technique were used to analyze the experiment results. The results showed that the infiltration rate increased with the rising of CNF content, and after 120 h of infiltration, the density was the highest when the CNF content was 5 wt.%, but the composite could not be densified efficiently as the CNF content ranged from 10 wt.% to 20 wt.%. CNF-reinforced C/C composites have enhanced thermal conductivity, the values at 5 wt.% were increased by nearly 5.5-24.1% in the X-Y direction and 153.8-251.3% in the Z direction compared to those with no CNFs. When the additive content was increased to 20 wt.%, due to the holes and cavities in the CNF web and between carbon cloth and matrix, the thermal conductivities in the X-Y and Z directions decreased from their maximum values at 5 wt.%.     

Tensile properties and thermal stability of ZnO-coated aluminum borate whiskers reinforced 2024Al composite

ZnO-coated aluminum borate whiskers reinforced 2024Al composite was fabricated by squeeze casting. Interfacial microstructures and tensile properties of the composite in as-cast and after thermal exposure were investigated. Fracture mechanisms of the composite in as-cast and after thermal exposure were also investigated. The results show that ZnO coating of the whiskers reacts with molten 2024Al and MgAl₂O₄ forms at the interface during squeeze casting. On the one hand, the interfacial reaction between ZnO and 2024Al can improve the wettability of the whiskers by molten 2024Al, increasing the tensile properties of as-cast composite. On the other hand, during thermal exposure, MgAl₂O₄ at the interface can effectively hinder serious interfacial reactions between the whiskers and magnesium in the matrix of 2024Al, improving the thermal stability of the composite at high temperatures.     

新型定向碳纳米管/炭复合材料的制备与表征

采用化学气相沉积（CVD）方法制备了厚度超过4mm的定向碳纳米管（ACNTs）阵列，并以此为骨架，利用化学气相渗（CVI）工艺制备了新型的定向碳纳米管／炭（ACNT／C）复合材料。利用SEM、PLM、XRD、TGA和Raman光谱对ACNT／C复合材料进行了表征。结果表明：ACNT／C复合材料中的热解炭主要为类粗糙层（RL）结构，而在相同工艺条件下制备的炭／炭（C／C）复合材料的热解炭为典型的光滑层（SL）结构。ACNT／C复合材料晶化程度明显优于相同工艺条件下的C／C复合材料。同时，ACNT／C复合材料在空气中的热失重转变温度比相同工艺条件下制备的C／C复合材料提高了约50℃左右。     

纳米石墨微片/聚氯乙烯复合材料的制备与性能

利用超声作用制备纳米石墨微片(nano-Gs), 并采用混酸对其进行表面活化, 最后通过熔融共混法制备nano-Gs/聚氯乙烯(PVC)复合材料。通过FTIR、 SEM对nano-Gs的结构进行表征, 并研究了nano-Gs对nano-Gs/PVC复合材料导电性能和力学性能的影响。FTIR分析表明: nano-Gs经混酸处理后表面活性官能团含量明显升高, 并与PVC 分子链发生一定程度的氢键作用; SEM图片显示: nano-Gs 厚度为30～80 nm, 其微片宽度为4～20 μm, 在PVC 树脂基体中呈无规状均匀分布; 导电性能测试表明: 随着nano-Gs 含量升高, nano-Gs/PVC复合材料的体积电阻率呈非线性降低趋势, 最低为103 Ω·cm, nano-Gs 的逾渗阈值为10%(质量分数); 力学性能测试表明, 随着nano-Gs含量升高, nano-Gs/PVC复合材料的拉伸强度及缺口冲击强度均先升高后降低, nano-Gs质量分数为1%时, 复合材料的拉伸强度及缺口冲击强度均达到最大值, 相比纯PVC分别升高约14%和38%。      

Effect of expanded graphite and modified graphite flakes on the physical and thermo-mechanical properties of styrene butadiene rubber/polybutadiene rubber (SBR/BR) blends

The aim of the present research work is to develop expanded graphite (EG) and isocyanate modified graphite nanoplatelets (i-MG) filled SBR/BR blends, which can substitute natural rubber (NR) in some application areas. The present study investigated the effect of i-MG on the physical, mechanical and thermo-mechanical properties of polybutadiene rubber (BR), styrene butadiene rubber (SBR) and SBR/BR blends in the presence of carbon black (CB). Graphite sheets were modified to enhance its dispersion in the rubber matrices, which resulting in an improvement in the overall physical and mechanical properties of the rubber vulcanizates. Compounds based on 50:50 of BR and SBR with ∼3 wt% nanofillers with CB were fabricated by melt mixing. The morphology of the filled rubber blends was investigated by wide angle X-ray diffraction (WAXD) and high resolution transmission electron microscopic (HR-TEM) analyses. The intercalated and delaminated structures of the nanofiller loaded rubber blends were observed. Scanning electron microscopic (SEM) analysis of the cryo-fractured surfaces of the rubber compounds showed more rough and tortuous pathway of the fractured surfaces compared to the fractured surfaces of the only CB loaded rubber composites. Filled rubber compounds exhibit increase in the ΔS (torque difference) value, reduced scorch and cure time compared to their respective controls. Dynamic mechanical thermal analysis (DMTA) of the filled rubber compounds shows an increase in the storage modulus compared to the controls. Isocyanate modified graphite nanoplatelets (i-MG) containing rubber compounds in the presence of CB showed an increase in the mechanical, dynamic mechanical, hardness, abrasion resistance and thermal properties compared to the alone CB filled rubber vulcanizates.     

Effect of tungsten addition on thermal conductivity of graphite/copper composites

Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%.     

Direct measurements of the thermal conductivity of various pyrolytic graphite samples (PG,TPG) used as thermal dissipation agents in detector applications

We performed model measurements on heat conduction in graphite-based structures, using several configurations. We describe our method for the direct measurement of thermal conductivity both in-plane and out-of-plane, for TPG and PG samples. Our results for the in-plane thermal conductivity coefficient, K_ab were obtained with two different sets of boundary conditions; they are in good mutual agreement. Those for the transverse coefficient, K_c, differ by a significant factor from the values published by the producers of the material.     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Synthesis, characterization and a.c. conductivity of polypyrrole/Y2O3 composites

Conducting polymer composites of polypyrrole/yttrium oxide (PPy/Y₂O₃) were synthesized byin situ polymerization of pyrrole with Y₂O₃ using FeCl₃ as an oxidant. The Y₂O₃ is varied in five different weight percentages of PPy in PPy/Y₂O₃ composites. The synthesized polymer composites are characterized by infrared and X-ray diffraction techniques. The surface morphology of the composite is studied by scanning electron microscopy. The glass transition temperature of the polymer and its composite is discussed by DSC. Electrical conductivity of the compressed pellets depends on the concentration of Y₂O₃ in PPy. The frequency dependent a.c. conductivity reveals that the Y₂O₃ concentration in PPy is responsible for the variation of conductivity of the composites. Frequency dependent dielectric constant at room temperature for different composites are due to interfacial space charge (Maxwell Wagner) polarization leading to the large value of dielectric constant. Frequency dependent dielectric loss, as well as variation of dielectric loss as a function of mass percentage of Y₂O₃ is also presented and discussed.     

Design and preparation of gradient graphite/cermets self-lubricating composites

Based on the functionally graded materials (FGMs) design concept, the laminated-graded graphite/cermets self-lubricating composite was prepared to achieve the integration of mechanical properties and lubrication performance of the cermet. The effects of the layer number and thickness of graded structure on residual stresses in the gradient composites were investigated by finite element method (FEM). From the FEM analyses, the optimal gradient structure design was obtained corresponding to the following parameters: the number of graded layers n = 2 and the thickness of graded structure t = 1 mm. According to the optimum design, a graded graphite/cermets self-lubricating material with two layers was fabricated by a typical powder metallurgy technique. Compared with the homogenous graphite/cermets composite, the surface hardness and indentation fracture toughness of graded composite were increased by approximately 15.9% and 6.3%, respectively. The results of X-ray diffraction (XRD) stress measurement identified the existence of residual compressive stress on the surface of graded composite. Additionally, the friction and wear tests revealed that the wear resistance of the graphite/cermets self-lubricating composite was improved significantly via the graded structural design, whereas the coefficient of friction changed slightly.     

Thermal conductivity of diamond/Ag composites with chromium carbide coated diamonds for the building materials of high power modules

Abstract

This paper reports on a study of the preparation and characterisation of diamond/Ag composites for the building materials of high power modules. The Cr₇C₃ coated diamond particles are utilised to improve the interfacial bonding between the Ag and diamond and composites are prepared by hot pressing technique. The characteristics of Cr₇C₃ coating layers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the Cr₇C₃ coatings on the diamonds result in a strong interfacial bonding and a greatly enhanced thermal conductivity of the composites. A largely enhanced thermal conductivity of 768 W m^?1 K^?1 is obtained in Cr₇C₃ coated composites, which increases 168% relative to that of uncoated composites at 65% diamond volume fraction. The measured thermal conductivity agrees reasonably well with the predictions by a differential effective-medium (DEM) model.