CFD investigation of the feasibility of polymer-based microchannel heat sink as thermal solution

Microchannel heat sinks (MCHSs) are promising thermal solutions in miniaturized or compact devices. Lightweight aspect has been given huge emphasis in recent years. Metal-based materials are commonly used to fabricate MCHSs due to their high thermal conductivity. Consequently, MCHSs are heavy due to the high density of these materials albeit the small footprint of MCHSs. Polymer-based materials are interesting alternatives. Despite their poor thermal conductivity, lightweight feature attracts the interest of researchers. Heat transfer is a conjugate process of heat conduction and heat convection. Poor heat conductions aspect may be compensated through enhancement of heat convection aspects. Although polymer-based materials have been used in microscale heat transfer studies, their focus was not on their feasibility. The present study aims to evaluate the feasibility of polymer-based MCHSs as thermal solutions. The effect of thermal conductivity of fabrication materials, including polymer-based PDMS, PTFE, PDMS/MWCNT, and metal-based aluminum, on the thermal performance of MCHSs was investigated and compared at various inlet flow rate, fluid thermal conductivity, and microchannel ratio at different constant heat fluxes using three-dimensional CFD approach. Results showed that the thermal performance of MCHSs was greatly affected by the heat conduction aspect in which poor heat conduction limited the thermal performance improvement due to enhanced heat convection aspects. This suggests polymer-based materials have the potential for heat transfer applications through thermal conductivity enhancement. This was confirmed in the further analysis using a recently proposed high thermal conductivity polymer-based graphite/epoxy MCHS and a hybrid-based PDMS/aluminum MCHS.     

Multilayered nacre-mimetic inspired flexible PEI film with high thermal conductivity and reliable dielectric performances

It is critically desired to integrate high in-plane thermal conductivity (TC) and distinguished electric insulation for thermal conductive film in modern electronic devices. Herein, integration of high TC and electric insulation in sandwich-like BNNSs@MWCNTs/PEI (S-BNNSs@MWCNTs/PEI) composite film has been successfully achieved by layer-by-layer spin coating and hot pressing inspired by highly ordered structure of natural nacre. The covalently bonded connections between BNNSs and MWCNTs are beneficial to create more efficient heat conduction path, which can partly decrease the interfacial thermal resistance and phonon scattering. The resultant S-BNNSs@MWCNTs/PEI composite films possess a high in-plane TC of 6.88 W m^?1K^?1. Meantime, benefiting from the alternating multilayer structure, the composite films exhibit satisfactory reliable dielectric performances with flexibility, which shows great potential in ceramics-filled polymer composite TIMs.     

相变热界面材料导热增强及定形改善的研究进展

将相变时伴随潜热的相变材料（phase change material, PCM）特别是潜热值较大的固-液PCM引入热界面材料（TIM）领域,有望获得兼具储热和导热双功能的新型热界面材料——相变热界面材料（phase change thermal interface material, PCTIM）。然而,鉴于固-液相变材料的热导率普遍较低且存在液相流动泄漏问题,使得增强热传导并同时提升固-液相变材料的定形性成为研制高性能相变热界面材料（PCTIM）的关键。本文系统评述了国内外研究者在提升相变热界面材料热导率以及改善其定形性方面的策略及其研究进展。文中指出,目前强化PCTIM导热的手段主要有添加高导热填料、促使填料有序结构化以及使用低熔点金属等。在改善定形性方面,已运用的策略主要包括使用柔性载体负载固-液PCM以在保证一定柔性的基础上克服其液相泄漏问题,使用固-固PCM来取代固-液PCM来彻底避免液相泄漏问题的出现,以及将固-液PCM封装在微米级或纳米级胶囊内,旨在牺牲借助液相PCM增加柔性的功能,而且通过提高PCTIM的潜热值来提升其抗热流冲击性能。文章指出,当前已研制的PCTIM热导率还较低,储热和导热这两个特性对其散热性能的协同影响机制缺乏深入了解。今后,需要探索研制高性能PCTIM的新策略,以期获得定形性好、热导率高、界面热阻小且潜热值大的PCTIM,从而满足5G通信等高热流密度芯片的散热需求。     

聚合物基热界面材料与导热性能研究进展

随着电子产品的小型化、集成化和功能化发展,功率密度及热流密度急剧上升,器件内巨大的散热和温压压力使电子设备的寿命和可靠性受到影响,因此对器件在运行过程中如何有效散热提出了更为苛刻的要求。开发及使用高性能导热基复合材料（热界面材料,TIM）降低接触热阻是解决电子设备散热问题的有效途径之一,热界面材料创新与优化备受关注。本文从基本的导热机理出发,阐述聚合物基热界面材料结构及导热强化方面最新进展,讨论导热填料和聚合物基体对复合材料性能的影响。重点对微纳结构的导热强化（协同）作用、构筑3D高导热微结构、导热填料和基质间的界面微结构和导热互穿网络结构等进行讨论,为设计高性能导热结构、制备开发新型高性能TIM提供参考。     

Enhanced thermal conductivity and isotropy of polymer composites by fabricating 3D network structure from carbon-based materials

Graphene-based composite is promising as thermal interface material (TIM) due to its outstanding thermal properties. However, there are some bottlenecks to excellent performance, such as agglomeration of particles and undesirable voids between nanoplatelets. In this work, a composite with three-dimensional (3D) thermally conductive network has been assembled, which combines three kinds of nanofillers varying geometric dimensions. Thermal conductivity (TC) of composite with graphene nanoplatelets (GNPs) and carbon-nanotubes (CNTs) at a weight ratio of 3:1 is around 9% higher than that of GNP-based composite. By the introduction of carbon spheres (CSs), the TC is further increased by 28%. The enhanced thermal property further demonstrated by FLIR infrared camera is attributed to the formation of 3D heat conduction paths by GNPs, CNTs, and CSs, where the GNPs play the role of thermally conductive backbones. The other two components are introduced to attenuate the aggregation and strong thermal anisotropy. Moreover, the TC is confirmed nearly isotropic, which is different from most graphene-based TIMs because of the in-plane alignment. Our results indicate that the apparent synergy endows this 3D nanofiller great potential for heat dissipation applications requiring heat removal in two directions.     

碳纳米管填充复合材料体系的导热模型研究进展

综述了以聚合物为基体的碳纳米管填充复合材料导热模型，分析了复合材料热传导体系中碳纳米管(CNTs)填料的作用机制，归纳了均质模型、非均质模型和随机分散模型等，总结了导热模型常用的数值计算方法。这些模型丰富了导热理论，可以对复合材料导热系数进行初步预测，但在数值上都存在近似或者简化处理，对碳纳米管复合材料预测广谱性较差，若要提高预测准确性，需要更深入探究分子界面处的热阻。     

Thermal Properties of Graphene Filled Polymer Composite Thermal Interface Materials

Recent years have witnessed a staggering escalation in the power density of modern electronic devices. Because increasingly high power density accumulates heat, efficient heat removal has become a critical limitation for the performance, reliability, and further development of modern electronic devices. Thermal interface materials (TIMs) are widely employed between the two solid contact surfaces of heat sources and heat sinks to increase heat removal for electric devices. Composites of graphene and matrix materials are expected to be the most promising TIMs because of the remarkable thermal conductivity of graphene. Here, the recent research on the thermal properties of graphene filled polymer composite TIMs is reviewed. First, the composition of graphene filled polymer composite TIMs is introduced. Then, the synthetic methods for graphene filled polymer composite TIMs are primarily described. This study focuses on introducing the methods for improving and characterizing the thermal properties of graphene filled polymer composite TIMs. Furthermore, the challenges facing graphene filled poly­mer composite TIMs for thermal management applications in the modern electronic industry and the further progress required in this field are discussed.

     

Steppingstone-inspired construction of high vertical thermal conductivity material with low carbon fiber content

Thermal interface materials (TIMs) with good thermal conductivity are paramount in mitigating the heat concentration challenges encountered during the operation of highly integrated components in sophisticated electronic devices. To optimize the comprehensive performance of TIMs, a balance must be struck: minimizing the filler concentration to attenuate materials hardness while maximizing filler content to bolster the thermal conduction pathway. Drawing inspiration from the orientation of tree branches passing through steppingstones in river, a method was proposed in this study. This method exploits the shear effect of carbon fiber (CF), owing to viscosity variances during diameter extrusion, and the differential flow velocity between CF and alumina to induce a significant degree of orientation. Combined with subsequent flipping and bonding, a TIM with vertically oriented CF was prepared. The TIM was obtained with a mere 12.1 wt% CF incorporation, the composite exhibits a through-plane thermal conductivity of 21.29 W/(m·K), representing an enhancement of two orders of magnitude relative to pristine silicone rubber, while retaining its flexibility and deformability. The orientation degree and high efficiency orientation effect of CF have been characterized via scanning electron microscopy (SEM), thermal conductivity test and light-emitting diode (LED) temperature rise test.     

Unique antimicrobial/thermally conductive polymer composites for use in medical electronic devices

The common neglect of the prominent bacterial growth and accumulation on polymer-based thermal conductive materials used in medical electronic devices will hurt the functionality and lifetime of medical devices, and sometimes even lead to medical accidents. In this study, we developed a novel ternary composite with excellent antimicrobial and thermal conductive properties to solve this problem. This composite was composed of antimicrobial functionalized hexagonal boron nitride (AB@h-BN) nanoplatelets, low melt alloys (LMAs), and epoxy. Antimicrobial testing showed that the AB@h-BN/LMAs/epoxy composites were 100% against both Escherichia coli and Staphylococcus aureus; their antibacterial mechanism was contact killing and was harmless to the environment. Besides enhancing the antimicrobial property, the AB@h-BN nanoplatelets connected the mutually independent LMAs, forming the continuous network for heat conduction in the epoxy. Benefited from this distinctive structure, the thermal conductivity of AB@h-BN/LMAs/epoxy can reach 2.66 Wm⁻¹ k⁻¹, which represented an enhancement of about 1141% over the pure epoxy.     

二维纳米片基复合相变储热材料研究进展

相变材料（PCMs）作为潜热储存和释放的介质,能够解决热能供需矛盾,从而缓解能源危机。纯相变材料具有能量密度高、温度范围广、能量输出稳定性强等优点,但其热导率低和在相变过程发生渗漏的缺点阻碍了其广泛的应用和发展。通过将PCMs与二维纳米片复合,PCMs热导率低和渗漏问题被有效解决。通过在导热机理方面进行详细阐述的基础上,综述了近几年来有关碳基二维纳米片、六方氮化硼（h-BN）纳米片、二硫化钼等复合储热材料的研究进展,为高性能二维纳米片基复合PCMs的设计提供一定的研究思路。     

Critical effect and enhanced thermal conductivity of Cu-diamond composites reinforced with various diamond prepared by composite electroplating

Despite the great potential value as heat sink materials, their practical application of high thermal conductivity (TC) Cu-diamond composites is limited since high temperature and high pressure (above 1000 K and 60 MPa) were requisite in the conventional process. In this study, high TC void-free Cu-diamond composites reinforced with various diamond particles were prepared via composite electroplating. The impacts of diamond particle sizes (ranged from 10 to 400 μm) on microstructure, interface and TC of the composites were investigated. The TC of Cu-diamond composites was improved with the increase of diamond particle sizes and well-combined interface. Interestingly, a critical size for improved the TC of Cu-diamond composites was clearly observed and the critical value (22 μm) was derived from Kipitza theory. Based on the TC results and critical analysis, the Cu-diamond composite reinforced with large diamond particles (400 μm) was synthesized, which possessed the TC of 846.52 W m⁻¹ K⁻¹ and the thermal expansion coefficient of 7.2 × 10⁻⁶ K⁻¹. Such attractive thermal properties suggested that electroplating Cu-diamond composites showed the promising application as heat sink materials in microelectronic industry.     

预测聚合物基复合材料导热系数方法综述

由聚合物与高导热填料共混制得的导热聚合物基复合材料,被应用于防腐和节能要求较高的换热场合,符合换热设备新材料的要求;而聚合物基复合材料的等效导热系数预测比较复杂。总结了预测聚合物基复合材料等效导热系数的多种方法,包括最小热阻力法、热阻网络法、傅里叶定律法、均匀化方法和逾渗理论方法,归纳了这些模型和方法的特点,对应用这些模型和方法提出了建议。     

Construction of hexagonal boron nitride@polystyrene nanocomposite with high thermal conductivity for thermal management application

Hexagonal boron nitride (BN) hold great promise as emerging building blocks for thermal interface materials owing to their outstanding heat transfer performance. Herein, we report a carboxylated polystyrene-coated hydroxylated BN (BN–OH@PS-COOH) nanocomposite with highly thermal conductivity (TC) and extraordinary mechanical properties for thermal management. The exfoliated BN-OH nanosheets were obtained via molten alkali hydroxide pretreatment and sonication. Subsequently, PS-COOH nanospheres were grew on the surface of BN-OH nanosheets by in situ polymerization. Noncovalent interactions between PS-COOH and BN-OH are favor to reduce interfacial thermal resistance, which contributes to accelerate heat transport. As a result, the TC of the resultant BN-OH@PS-COOH nanocomposite with 12 wt% BN-OH addition is 1.131 W/mK, which is much higher than that of neat PS (0.186 W/mK) and BN/PS blend nanocomposite (0.312 W/mK). Moreover, the BN-OH@PS-COOH nanocomposite exhibits outstanding mechanical properties. Our study may stimulate novel perspectives on the design of high-performance polymer-based thermal interface materials.     

热界面材料热导率和接触热阻的测试

热界面材料通常用于降低电子器件中固体界面的热阻。热界面材料的性质, 如热导率、界面材料与固体表面间的接触热阻, 对于电子器件的散热分析非常重要。然而, 这些参数通常难以获得。依据ASTM D-5470测试标准, 搭建了一个热界面测试系统。通过该系统测试了硅油和导热硅脂的热导率, 以及它们与固体基板间的接触热阻。经分析, 测试热导率和接触热阻的相对误差分别小于11.3%和41.3%。     

聚合物基导热复合材料的研究进展

阐述了聚合物基导热复合材料的导热机制。综述了国内外导热胶粘剂、导热橡胶和导热塑料等研究进展。介绍了提高复合材料导热性能的途径,并在此基础上,展望了聚合物基导热复合材料的应用前景和未来的发展方向。     

High-thermal conductivities of epoxy composites via p-phenylenediamine interfacial modification and process intensification

High loadings of fillers are usually needed to achieve high-thermal conductivity (TC) of polymer-based composites, which inevitably sacrifices processability and meanwhile causes high-cost. Therefore, it is of great significance to achieve high-TC composites under low-filler loading. Here, a novel p-phenylenediamine (PPD) modified expanded graphite (EG-PPD)/epoxy (EP) composite with high TC and low-filler content was successfully prepared via pre-dispersion and vacuum assisted mixing strategy. With the improved interfacial compatibility between EG and EP by PPD, the prepared EG-PPD/EP composite exhibited excellent thermal management performance, resulting in the TC of which reached 4.00 W·m⁻¹·K⁻¹ with only 10 wt% (5.59 vol%) of EG-PPD, which is approximately 19 times higher than that of pure EP. Meantime, the interface thermal resistance of EG-PPD/EP composite between EG-PPD and EP is reduced by 33% compared with EG/EP composite. This composite with excellent TC property is expected to be used in thermal management field.     

绝缘导热高分子复合材料研究现状

高分子材料具有易加工、韧性好、绝缘性能好等优点而被广泛应用,但热导率低又是绝大多数高分子材料的固有特性,也成为很多应用环境中的瓶颈问题。于是,研究人员尝试采用不同的基体、填料和工艺来制备绝缘导热的高分子复合材料,并取得了许多显著的研究成果。     

A carbon nanotube-based composite for the thermal control of heat loads

Design optimization of an encapsulated carbon composite thermal control (TC) system is presented. The composite TC system consists of multiple phase change materials (PCM) doped with carbon nanotubes and enclosed in a casing of carbon/carbon composite sheets. Using the concept of global thermal resistance an analytical model was formulated to predict the transient temperature distribution through the composite system. The temperature data was then used to estimate maximum energy storage and heat dissipation rates. A substantial reduction in weight and size of the TC composite was observed corresponding to the optimized design. The use of carbon nanotubes both as additives with optimal loading and as a thermal interface material significantly reduced the maximum junction temperatures for different constant power loads for the multiple PCM composite as compared to its original size used for the experimental work. The optimized composite minimized the total thermal resistance through the composite sample and thereby increased its thermal response as indicated by approximately 4 times increase in the heat dissipation rate.     

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Effective thermophysical properties of ceramic materials (mainly insulating materials) with porosity (II) >30% are reviewed. Nonmonotonic pressure and temperature dependences of the effective thermal conductivity (X) are analyzed, based on the ceramic microstructure (pores, cracks, and grain boundaries present in many industrial refractories) and several heat-transfer mechanisms in composite multiphase materials. These mechanisms include heat conduction in solid and gas phases, thermal radiation, gas convection, and the mechanism originating from intrapore chemical conversion processes accompanied by gas emission. For high temperatures, λ of porous insulations is governed by thermal radiation. Contact-heat-barrier resistances play a less-important role in highly porous ceramics than in their dense counterparts. This underlies a weaker pressure dependence at low temperatures (<500°C) of λ of the majority of industrial insulating materials than in dense materials possessing microcracks and small pores in the grain-boundary region. For high gas pressure, λ of porous insulating materials is governed by free convective-gas motion. For low gas pressures (normally <1 kPa), where heat transfer in pores occurs in the free-molecular regime, X is controlled by the pressure-dependent mean free path of gas molecules in pores. A classification of the porous material structure and thermophysical properties is proposed, based on the geometric model described in Part 1 of this series.     

基于多孔支撑体的形状稳定复合相变储能材料的研究进展

固-液相变材料（PCMs）是热能储存（TES）技术发展的关键因素,然而一些固有的问题如泄漏和热导率低等严重制约了相变材料的性能。因此,选择合适的方法构建形状稳定的复合相变材料（FSCPCMs）,并有效地提高其热导率是实现相变材料实用化的重要前提。多孔载体封装相变材料为构建具有高储能密度和优异热传输性能的定形复合相变材料提供了一条有效的途径。本文对不同FSCPCMs的制备、结构热学性能、应用等方面进行了综述,详细总结和讨论了孔径和几何形状、表面改性、作用力、组成等因素对FSCPCMs相变行为的影响。重点介绍了具有高热导率、高负载率和高潜热的新型多孔复合相变材料的设计和应用。最后,基于理论、数值和实验方法,展望了FSCPCMs在约束结构中的相变和多尺度传热方面未来的研究方向及其在能源转换方面的商业化应用。