Carbon Nanotube Thermal Pastes for Improving Thermal Contacts

The use of 0.6 vol.% single-walled carbon nanotubes in a poly(ethylene glycol)-based dispersion gave a thermal paste that was as effective as solder for improving thermal contacts. A thermal contact conductance of 20 × 10⁴ W m⁻² K⁻¹ was attained. An excessive amount of nanotubes (e.g. 1.8 vol.%) degraded the performance, because of conformability loss. The nanotubes were more effective than hexagonal boron nitride particles but were less effective than carbon black, which gave a thermal contact conductance of 30 × 10⁴ W m⁻² K⁻¹.     

Effects of Thermal Cycling on Material Properties of Nonconductive Pastes (NCPs) and the Relationship Between Material Properties and Warpage Behavior During Thermal Cycling

In this paper, the effects of thermal cycling on material properties such as coefficient of thermal expansion (CTE), modulus, and glass transition temperature $({rm T}_{rm g})$ of nonconductive pastes (NCPs) for flip chip applications were investigated. Using a thermomechanical analyzer, the dimensional changes of NCPs and an underfill material were measured. The dimensional changes of all materials during the first cycle rapidly increased near ${rm T}_{rm g}$. However, the rapid increase of dimensional change near ${rm T}_{rm g}$ was not observed during the second and third cycles. Furthermore, using a dynamic mechanical analyzer, the modulus and ${rm T}_{rm g}$ were measured. The modulus in the first cycle was smaller than that in the second cycle for all materials. After the first cycle, the modulus curves followed the second cycle curve. Next, the warpage behavior of the flip chip assemblies was observed using the Twyman–Green interferometry method to investigate how material property changes affect warpage behavior during thermal cycling (T/C) and it was found that the warpage of the flip chip assembly decreased after the first cycle. However, after the first cycle, the amount of warpage was constant for the following five cycles. As a result, it was verified that the material properties of NCPs and the underfill material change after the first thermal cycle, and the material property changes are closely related to the warpage hysteresis behavior during T/C. Finally, warpage hysteresis was understood as shear strain.      

Electrically Nonconductive Thermal Pastes with Carbon as the Thermally Conductive Component

Electrically nonconductive thermal pastes have been attained using carbon (carbon black or graphite) as the conductive component and ceramic (fumed alumina or exfoliated clay) as the nonconductive component. For graphite particles (5 μm), both clay and alumina are effective in breaking up the electrical connectivity, resulting in pastes with electrical resistivity up to 10¹³Ω·cm and thermal contact conductance (between copper surfaces of roughness 15 μm) up to 9 × 10⁴ W/m²·°C. For carbon black (30 nm), clay is more effective than alumina, providing a paste with resistivity 10¹¹ Ω·cm and thermal contact conductance 7 × 10⁴ W/m²·°C. Carbon black increases the thermal stability, whereas either graphite or alumina decreases the thermal stability. The antioxidation effect of carbon black is further increased by the presence of clay up to 1.5 vol.%. The addition of clay (up to 0.6 vol.%) or alumina (up to 2.5 vol.%) to graphite paste enhances the thermal stability.     

Antioxidant-Based Phase-Change Thermal Interface Materials with High Thermal Stability

This work provides phase-change thermal interface materials (TIMs) with high thermal stability and high heat of fusion. They are based on antioxidants mainly in the form of hydrocarbons with linear segments. The thermal stability is superior to paraffin wax and four commercial phase-change materials (PCMs). The use of 98.0 wt.% thiopropionate antioxidant (SUMILIZER TP-D) with 2.0 wt.% sterically half-hindered phenolic antioxidant (GA80) as the matrix and the use of 16 vol.% boron nitride particles as the solid component give a PCM with a 100°C lifetime indicator of 5.3 years, in contrast to 0.95 year or less for the commercial PCMs. The heat of fusion is much higher than those of commercial PCMs; the values for antioxidants with nonbranched molecular structures exceed that of wax; the value for one with a branched structure is slightly below that of wax. The phase-change properties are degraded by heating at 150°C much less than those of the commercial PCMs. The stability of the heat of fusion upon phase-change cycling is also superior. The viscosity is essentially unaffected by heating at 150°C. Commercial PCMs give slightly lower values of the thermal contact conductance for the case of rough (12 μm) mating surfaces, in spite of the lower values of the bond-line thickness.     

矿渣-粉煤灰基地质聚合物性能与微观结构的研究

本文重点研究了不同矿渣掺量对循环流化床粉煤灰基地质聚合物流变性能和强度的影响,以及矿渣-粉煤灰复合地质聚合的反应机理。矿渣的加入增加了地质聚合物浆体的粘度,且浆体粘度随着矿渣掺量的增加而增大。矿渣的加入明显增加了地质聚合物的强度,其中对早期强度的提高尤为明显,当矿渣掺量为30％时28d强度提高最为明显。 X射线衍射,扫描电子显微镜和红外光谱分析证明矿渣的反应产物中有水化硅酸钙（ C-S-H）生成,矿渣-粉煤灰基地质聚合物性能的提高来源于C-S-H和水化硅铝酸钠（ N-A-S-H）的共同作用。     

改性氧化铝黑瓷粉体制备高固相含量浆料的研究

通过对氧化铝黑瓷粉料进行预烧技术处理,增大颗粒尺寸、降低粉体比表面能、改善其团聚现象,制备出固相体积分数达55%的氧化铝黑瓷稳定料浆,并研究了分散剂用量、体系固相体积分数、R值(塑性剂/粘结剂)对料浆流变性的影响.研究结果表明:浆料的粘度随剪切速率增加而下降,呈剪切稀化的流变学特性,随着固相体积分数的增加,浆料的最佳分散剂用量也相应增加;浆料的粘度随着R值的增加而急剧减小,流变性能得到明显改善.     

Joint-Inspired Liquid and Thermal Conductive Interface for Designing Thermal Interface Materials with High Solid Filling yet Excellent Thixotropy

For advanced thermal interface materials (TIMs), massive inorganic addition for high isotropic thermal conductivities conflicts with suitable rheological viscosity for low contact thermal resistance. Traditional strategies rarely resolve such a contradiction, and it remains an academic and industrial challenge. Herein, inspired by the structure and function of the bone joint, a best-of-both-worlds approach is reported that endows a standard polydimethylsiloxane/alumina (PDMS/Al₂O₃) TIM with simultaneously enhanced rheological mobility and thermal conductivity. It is conducted by employing morphology-controllable gallium-based liquid metal (LM) to the surface of Al₂O₃ by a scalable mechanochemical process. At the typical polymer-LM-Al₂O₃ interface, LM droplets with low cohesive energy can release the freedom for macromolecular chain relaxation and reduce the viscosity, successfully allowing the high-loading TIMs (79 vol.%) to keep the thixotropic state and effectively reducing its contact thermal resistance with a copper substrate by 65%. At the same time, adjacent LMs merge to thermally bridge separate Al₂O₃ particles, which facilitates the interfacial thermal conduction and enhances the thermal conductivity from 5.9 to 6.7 W m⁻¹ K⁻¹. Along with additional electrical insulation, this filler modification strategy is believed to inspire others to develop high-performance polymer-based TIMs for future advanced electronics.     

Dow Corning新出三款散热介面材料

Dow Corning 公司推出三款新型散热介面材料(Thermal Interface Material,TIM),扩充了电子工业中热管理解决方案的品种和范围。其中两款材料——TP-1600 薄膜系列和TP-2400 垫片系列——是Dow Corning 公司自去年并购Tyco Electronics' Raychem 能源材料业务部之后,首次推出的TIM 产品。热传导薄膜和垫片特别为使用方便和工艺灵活而设计,能帮助用户改善电子组件和生产线的散热功效。由于已被预先固化,制成的TIM 不再需要专门的应用工具,具有可修复性,在很广的厚度范围内都可以使用。它们适用于汽车制造、显示设备、计算机和电力行…     

The Effect of Polydispersivity on the Thermal Conductivity of Particulate Thermal Interface Materials

A critical need in developing thermal interface materials (TIMs) is an understanding of the effect of particle/matrix conductivities, volume loading of the particles, the size distribution, and the random arrangement of the particles in the matrix on the homogenized thermal conductivity. Commonly, TIM systems contain random spatial distributions of particles of a polydisperse (usually bimodal) nature. A detailed analysis of the microstructural characteristics that influence the effective thermal conductivity of TIMs is the goal of this paper. Random microstructural arrangements consisting of lognormal size-distributions of alumina particles in silicone matrix were generated using a drop-fall-shake algorithm. The generated microstructures were statistically characterized using the matrix-exclusion probability function. The filler particle volume loading was varied over a range of 40%–55%. For a given filler volume loading, the effect of polydispersivity in the microstructures was captured by varying the standard deviation(s) of the filler particle size distribution function. For each particle arrangement, the effective thermal conductivity of the microstructures was evaluated through numerical simulations using a network model previously developed by the authors. Counter to expectation, increased polydispersivity was observed to increase the effective conductivity up to a volume loading of 50%. However, at a volume loading of 55%, beyond a limiting standard deviation of 0.9, the effective thermal conductivity decreased with increased standard deviation suggesting that the observed effects are a tradeoff between resistance to transport through the particles versus transport through the interparticle matrix gap in a percolation chain.      

Impact of Thermal Interface Materials for Thermoelectric Generator Systems

A primary challenge still exists in the field of thermoelectric generators (TEG) for practical applications in which a thermal system of the TEG is a crucial factor in TEG power generation. The material development for TEG has contributed significantly towards advancement in TEG applications over a decade, the need for a thermal system configuration is inevitable considering the applications. The thermal efficiency of TEG depends upon the temperature difference across its modules (between the hot and cold surfaces). Thermal design of the thermoelectric system is important to ensure that there exists a maximum temperature difference across the hot and cold surfaces of the TEG. Thermal Interface Material (TIM) in thermoelectric systems plays a main role in improving the efficiency of thermoelectric systems by reducing the temperature difference between the heat source and the hot surface of the TEG and similarly, the temperature difference between the cold surface of TEG and the heat sink. This review paper predominantly focuses on the thermal interfaces between the TEG modules which reduces the performance of a thermoelectric system. The characteristics of TIM in a TEG system (contact pressure, surface roughness and thermal conductivity) were analyzed with a mathematical model to emphasize the importance of TIM in a TEG system. This paper also highlights the existing challenges for Thermal Interface Materials in TEG applications and concludes with a brief discussion on future directions of TIM in TEG thermal systems.     

Factors That Govern the Performance of Thermal Interface Materials

Finite element modeling is conducted to understand the factors that govern the performance of thermal interface pastes of controlled thickness between copper surfaces of controlled roughness. Carbon black paste is lower in thickness than metal particle paste, so it shows better performance. The performance of both pastes is more influenced by the paste-copper interfacial conductance than by the paste thermal conductivity. The effects of pressure, paste thickness, and copper surface roughness on performance are mainly due to the change in fractional filling of the valleys in the copper surface topography. Reasonable agreement is found between modeling and experimental results.     

LED导热胶片性能研究与评估

大功率LED器件的基座与散热基板之间的接触热阻会阻碍其芯片PN结与散热基板之间的热传导,从而影响到器件的光、色、电性能及寿命。导热胶片用于填充材料界面之间接合或接触时产生的微空隙及表面凹凸不平的孔洞,是减小接触热阻、降低芯片PN结温度的有效手段。文章主要通过测定热阻值来开展对导热胶片在实际应用中导热性能的研究。通过阻容...     

Thermal Behavior Investigation of Terahertz Quantum-Cascade Lasers

This paper investigates the heat conduction behavior of a terahertz (THz) quantum-cascade laser (QCL) active region by measuring its temperature using in-situ microprobe band-to-band photoluminescence (PL) technique. The heat resistance of different regions in QCL structure is derived from the temperature measurement. Experimental results show that thinning the substrate from 300 $mu{hbox {m}}$ thick to 140 $mu{hbox {m}}$ lowers thermal resistance of the device by 21%, which helps achieving continuous-wave operation. A thermodynamic differential equation was numerically solved and the temperature profiles and thermal behavior of various regions within actively biased QCL devices under various conditions were obtained. The simulation confirms the measured results that the substrate accounts for 59% of the total device thermal resistance.      

陶瓷复合FR4结构界面形貌与导热性能

基于热电分离式设计理念,将AlN陶瓷片金属化后作为微散热器嵌入FR4材料内形成了复合散热基板.采用电镜扫描、光学显微,通过冷热循环冲击试验对FR4与AlN两相界面处在高低温突变情况下的界面形貌进行了分析.利用ANSYS软件对基板进行了仿真热模拟,研究了AlN嵌入后FR4导热性能的变化规律.利用结温测试仪、功率计和半导体制冷温控台等仪器设备,通过结温测试对比研究了该复合散热结构与金属芯印刷电路板(MCPCB)对大功率LED封装散热效果的影响.结果表明,该复合散热基板在经低温-55℃,高温125℃,1 000个冷热循环后,FR4和AlN界面无剥离现象发生,在环境温度急剧变化的条件下结合力良好.同时,FR4在嵌入AlN之后,导热性能得到了明显改善,且与MCPCB相比,能更有效降低LED芯片结温.     

大功率LED封装界面材料的热分析

基于简单的大功率LED器件的封装结构,利用ANSYS有限元分析软件进行了热分析,比较了四种不同界面材料LED封装结构的温度场分布。同时对纳米银焊膏低温烧结和Sn63Pb37连接时的热应力分布进行了对比,得出纳米银焊膏低温烧结粘接有着更好的热机械性能。     

Pastes for electromagnetic interference shielding

A paste has been developed for electromagnetic interference shielding by mixing stainless steel fibers (8-μm diameter) and graphite flakes (5 μm) in a water-based graphite colloid (containing submicron graphite flakes and a binder) in a volume ratio 0.5:20:100. The resulting coating of thickness 0.1–0.2 mm after drying exhibits shielding effectiveness of 34 dB at 1 GHz, primarily due to reflection. The paste is superior to those containing graphite flakes or carbon filaments (0.1-μm diameter), due to the higher shielding effectiveness of the steel fibers. The graphite flakes in the paste serves to help suspend the steel fibers, in addition to contributing to shielding.     

LEC不掺杂半绝缘GaAs的热处理特性

本文对LEC不掺杂SI-GaAs的热处理特性进行了实验研究和分析。结果表明,As、Ga原子从表面挥发及缺陷的内扩散引起表面热蚀和热致导电层的形成。文章讨论了它们对注入层激活行为的影响。指出,采用本文设计的高温快速热处理结合压盖、InAs作过砷压源技术,可以获得满足注硅材料退火要求的SI-GaAs热处理特性。     

体声波谐振器热行为仿真

为了预测体声波谐振器(BAWR)的热行为并评估其功率容量,提出一种BAWR热行为仿真方法。首先提取BAWR电磁模型中的导体表面损耗,并以此作为热仿真的热源,得到谐振器的温度分布。然后就信号馈送边、有源区形状、功率容量这3方面进行了研究。仿真结果表明,为了减弱BAWR的自热效应,信号馈送宜遵循“长边馈入、长边馈出”原则;有源区形状对自热效应影响甚小;所设计的BAWR功率容量可达3 W。