Reducing thermal contact resistance using a bilayer aligned CNT thermal interface material

A thermal interface material (TIM) was fabricated by synthesizing aligned carbon nanotubes (CNT) on both sides of a thin copper foil. The Hot Disk^® method was applied to measure the thermal conductivity of these CNT-TIMs. Results showed that a thicker copper foil substrate or CNT layer led to a lower overall thermal resistance. The laser flash method was used to study the performance of the bilayer aligned CNT-TIMs using two copper plates as heat source and sink. An enhancement in thermal conductivity of more than 290% could be obtained under an applied contact pressure of 0.01 MPa, as compared with two copper plates in direct contact. By filling in the space between the CNTs in the CNT layer with a conventional thermal conductive elastomer, Sylgard 160, the thermal resistance of the TIM was reduced to 8.78 mm² K/W, a value that is better than similar devices in the literature.     

接触式机械密封界面泄漏机理分析

旋转机械装备的密封性能对生产过程的能耗、效率和环保具有重要影响。基于逾渗理论,探讨了接触式机械密封界面空隙状态随动、静环表面分形参数和接触压力变化的规律。研究建立了密封界面单层网格微通道结构模型,指出了密封界面在接触压力作用下表现的逾渗、逾渗点和非逾渗3种状态。根据液体毛细管力和气体Knudsen数,提出了微通道内流体流动判据,分析了密封界面流体的流动阻力和泄漏流量,阐释了接触式机械密封界面泄漏机理。研究结果为接触机械密封优化设计和泄漏控制提供依据。     

Effect of interface thermal resistance on thermoelectric properties of acoustically mismatched composite

Composite materials can be promising to decouple electron and phonon transport for improved thermoelectric properties. Herein, the synergistic effect of the particle size of the dispersed phase and the interface thermal resistance (R_int) between the phases on the phonon thermal conductivity (κ_ph) of the (1-x)La_0.95Sr_0.05Co_0.95Mn_0.05O₃/(x)WC thermoelectric composite is demonstrated. The embedded WC, owing to its high electrical conductivity (σ), does improve σ of the composite. A high acoustic impedance mismatch between the phases results in a large R_int and lowers κ_ph. Change in κ_ph is further explained using theoretical analysis of Bruggeman asymmetrical model. The correlation between R_int and Kapitza radius is discussed to analyze the reduced κ_ph. Synergistic effect of improved σ and lowered κ_ph result in improved thermoelectric figure of merit (zT) of 0.20 at 463 K for (1-x)La_0.95Sr_0.05Co_0.95Mn_0.05O₃/(x)WC composite. This study shows promise to design thermoelectric composites with the desired κ_ph considering the elastic properties between the phases.     

Effective conductivity of dispersions: The effect of resistance at the particle surfaces

The effective thermal or electrical conductivity of a suspension of monodisperse spherical particles embedded in a matrix of different conductivity is computed for the case when there exists a finite resistance at the particle-matrix interface. The calculation is carried out to second order in the volume fraction θ of the particle phase for a dispersion having the structure of equilibrium hard spheres. Both the first-order (Maxwell) and the second-order approximations are found to represent experimental data reasonably well for dispersions of low to moderate concentration. To first order in θ, the calculation of the conductivity of a dispersion with surface resistance is equivalent to the calculation of the conductivity of another dispersion with no surface resistance, but with a different particle conductivity. In the case of polydisperse systems, the effective conductivity is found to be sensitive to the particle size distribution.     

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%。     

The effect of organofunctional nanosilica on the cross-linking process and thermal resistance of UF resin

The paper investigates the effect of surface modification of fumed nanosilica with (3-aminopropyl)triethoxysilane (APTES) on the kinetics and thermal stability of urea-formaldehyde (UF) resin. In the course of the investigation, nanoparticles were modified with APTES in the ratio 1, 2, 3, 4 and 5 part by weight (PBW) per 100 PBW of SiO₂. The parameters of curing kinetics of the resin, the conversion degree and its thermal stability were determined with use of differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). The effect of nanosilica silanization on the curing process of resin was evaluated by determining the gel time at 100 °C and the activation energy (E_a) of the cross-linking process, the initial and final temperature of the reaction (T_onset, T_endset), the maximum value of the exothermic peak (T_p), the amount of emitted heat (ΔH_Tp) and the conversion degree (α_Tp) that responds to T_p. With the maximum level of silica modification, we have noted a decrease in the reactivity of the resin, which is manifested by a slightly longer gel time of the resin as well as an increase in the value of activation energy of the cross-linking process. It is accompanied by a slight decrease of resin conversion degree α_Tp. The modification of silica, regardless of the amount of silane inoculated on its surface, results in the increase in the thermal stability of UF resin.     

CNT/CB填充PE材料正电温度系数特性的研究

研究了炭黑（CB）和碳纳米管（CNT）／CB复合填充聚乙烯（PE）体系的正电温度系数（PTC）特征，初步考察了CNT含量、CNT与CB配比对体系PTC性能的影响，并与CB／PE体系进行了比较。结果表明：添加10％（w）的CNT时，室温电阻保持不变，而体系的PTC强度发生明显的变化；改变CNT与CB的配比，体系的PTC强度和室温电阻都发生了明显的变化，均随CNT含量的增加而增加。     

Investigation on CNT/alumina interface properties using molecular mechanics simulations

The pull-out of a carbon nanotube (CNT) from an alumina (α-Al₂O₃) matrix was investigated using molecular mechanics simulations to study the interfacial properties due to van der Waals and electrostatic Coulombic interactions. The pull-out force of the CNT was found to be proportional to its diameter, but independent of its length and alumina grain boundary type. A theory was proposed to predict the force for an arbitrary pull-out of a CNT from the alumina matrix using the outermost wall diameter of CNT.     

The gas–liquid contacting effect on the operation of small scale upflow hydrotreaters

The effect of reactor geometry and bed dilution on the extent of gas oil hydrodesulfurization was tested by conducting hydrodesulfurization experiments in two laboratory reactors of different scale with non-diluted and diluted beds in ascending flow. The superficial gas and liquid velocities and the catalyst bed height were kept constant while the main difference between the two reactor scales was the reactor diameter. The diluted bed of the mini-reactor showed the best performance and its results were identical in upflow and downflow mode. The differences between the performance of the mini- and the bench-scale reactor operating in upflow mode have been investigated. Reactor performance simulation was attempted by a mathematical model that takes into account axial dispersion of the liquid phase and gas–liquid mass transfer. Bench-scale reactor operation was characterized by lower mass transfer rates than the corresponding mini-scale one. Combining model predictions and mock up operation it is concluded that the stronger mass transfer resistances calculated for the bench-scale reactor are associated with poorer gas distribution through the catalyst bed. Reduction of the bed diameter results in better gas–liquid contact by forcing the gas bubbles to distribute more effectively into the liquid phase.     

Steady melting of rectangular thermoplastic bars induced by hot contacting surfaces

The steady melting of rectangular thermoplastic bars in contact with hot surfaces is analyzed by solving a simplified set of the momentum and energy balance equations, assuming a temperature and shear-rate dependent melt viscosity. A numerical model is developed for predicting the flow field and the temperature distribution in the solid and molten regions of the bar and the location of the solid/melt interface. Computer simulations show that the steady melting rate of the thermoplastic solid is mainly affected by the temperature sensitivity of the melt viscosity, by the pressure applied on the end of the bar, and by a balance between heat conduction and the convection of colder material into the molten region. For the amorphous and semicrystalline polymers considered, heat convection in the outflow direction of the molten material, viscous dissipation, and shear-thinning of the melt viscosity have a much smaller effect on the melting process. These results provide an insight into conduction-induced melting with forced melt removal caused by pressure-induced flow; they also provide a basis for developing a transient model for the hot-tool welding process.     

接触式机械密封摩擦界面温度分布分形模型

为揭示接触式机械密封摩擦界面的温度分布规律,用分形参数表征机械密封端面形貌特性,根据重新建立的微凸体接触变形方式,结合热传导和概率理论建立了机械密封摩擦界面最大温度以及温度分布的分形模型并用数值方法对其最大温度、温度分布规律以及影响因素进行了分析.研究结果表明,当分形维数一定时,随着转速的增大,密封界面最大温度呈线性增大;当转速一定时,随着分形维数的增大,密封界面最大温度呈非线性减小;随着量纲1特征尺度的增大,量纲1最大接触温度也在增大.当已知润滑膜汽化温度时,由温度分布密度函数,可以求出处于非正常润滑部分的真实接触面积,为进一步研究磨损、热破坏提供基础,这对接触式机械密封的实际运行和密封端面的设计具有重要的意义.     

The effect of interface characteristics on the morphology,rheology and thermal behavior of three‐component polymer alloys

Immiscible polymer blends are interesting multiphase host systems for fillers. Such systems exhibit, within a certain composition limits, either a separate dispersion of the two minor phases or a dispersion of encapsulated filler particles within the minor polymer phase. Both thermodynamic (e.g. interfacial tension) and kinetic (e.g. relative viscosity) considerations determine the morphology developed during the blending process. The effect of interfacial characteristics on the structure‐property relationships of ternary polymer alloys and blends comprising polypropylene (PP), ethylene‐vinyl alcohol copolymer (EVOH) and glass beads (GB), or fibers (GF), was investigated. The system studied was based on a binary PP/EVOH immiscible blend, representing a blend of a semi‐crystalline apolar polymer with a semicrystalline highly polar copolymer. Modification of the interfacial properties was obtained through using silane coupling agents for the EVOH/glass interface and compatibilization using a maleic anhydride grafted PP (MA‐g‐PP) for the PP/EVOH interface. The compatibilizer was added in a procedure aimed to preserves the encapsulated EVOH/glass structure. Blends were prepared by melt extrusion compounding and specimens by injection molding. The morphology was characterized using scanning electron microscopy (SEM) and high resolution SEM (HRSEM), the shear viscosity by capillary rheometry and the thermal behavior using differential scanning calorimetry (DSC). The system studied consisted of filler particles encapsulated by EVOH, with some of the minor EVOH component separately dispersed within the PP matrix. Modification of the interfaces resulted in unique morphologies. The aminosilane glass surface treatment enhanced the encapsulation in the ternary [PP/EVOH]GB blends, resulting in an encapsulated morphology with no separtely dispersed EVOH particles. The addition of a MA‐g‐PP compatibilizer preserves the encapsulated morphology in the ternary blends with some finely dispersed EVOH particles and enhanced PP/EVOH interphase interactions. The viscosity of the binary and ternary blends was closely related to the blend's morphology and the level of shear rate. The treated glass surfaces showed increased viscosity compared to the cleaned glass surfaces in both GB and GF containing ternary blends. Both EVOH and glass serve as nucleating agents for the PP matrix, affecting its crystallization process but not its crystalline structure. The aminosilane glass surface treatment completely inhibited the EVOH crystallization process in the ternary blend. In summary, the structure of the multicomponent blends studied has a significant effect on their behavior as depicted by the rheological and thermal behavior. The structure‐performance relationships in the three‐component blends can be controlled and varied.     

The effect of electrostatic phenomena on the cleaning of surfaces

The objective of this paper is to determine the nature of adsorption of different components by measuring the surface potential in order to reduce fouling and to improve cleaning. In the discussion of adsorption, electrostatic and van der Waals forces are of paramount importance. van der Waals forces are often responsible for the adherence of hydrophobic groups. Electrical forces should be considered, since stainless steel becomes electrically charged in contact with ionic liquids. The electrical charge of stainless steel pipes is described by the Galvani potential, measured by means of special electrodes. This potential is mainly influenced by the pH and the oxidizing power of cleaning solutions and by the kind of surface finish.It was found that electrically charged globular or nearly globular particles show a tendency to adsorb by electrical forces.In contrast, particles which can be distinguished in hydrophilic and hydrophobic regions are mostly adsorbed by their hydrophobic groups by means of van der Waals forces. According to these results it is possible to reduce fouling and to get a better idea of the effects of cleaning and disinfectant solutions.     

The preparation of carbon nanotube (CNT)/copper composites and the effect of the number of CNT walls on their hardness,friction and wear properties

Carbon nanotubes with 2, 3, 8 and 20 walls are mixed with a copper powder (micrometer sized) and consolidated by spark plasma sintering. The microhardness of resulting composites is found to be over 50% higher than that for Cu and the friction coefficient against a steel ball is decreased by a factor of 3–4 while the wear and wear rate are reduced by a factor of 10–20. Raman maps of selected specimens outside and inside the worn surface show that double-wall carbon nanotubes remain intact. The reasons for the effect of the number of walls and carbon content are discussed.     

氯磺化聚乙烯橡胶的耐热性能

该文研究了矿物质填料SiO2、Sb2O3及氯化石蜡对氟磺化聚乙烯橡胶可燃性的影响。在以CSM为基础的耐火包覆材料组成物中将上述3种物质配合加入,有明显阻燃效果。在对这种材料在试验装置上进行热冲击法试验时发现,在试样表面与热至1100℃的点火器接触时立刻发生燃烧。接着发生聚合材料的碳化,碳化层底下是矿物质填充剂的白色粉末,其下面却是丝毫未发生变化的原始材料。     

Studies on synergistic effect of CNT and CB nanoparticles on PVDF

Poly(vinylidene fluoride) (PVDF) nanocomposites with different loading of carbon nanotubes (CNT) and carbon black (CB) were prepared by melt blending method. The conductivity and mechanical properties of the nanocomposites were investigated. The results showed that percolation threshold of CNT/CB/PVDF nanocomposites appeared at a lower concentration (1.25 vol% CNT) than that of CNT/PVDF (>2.08 vol% CNT). The tensile strength of CNT/CB/PVDF nanocomposites was also improved, with 32.1% increase compared to PVDF and 18.0% increase compared to CNT/PVDF at loading of 1.25 vol% CNT/0.96 vol% CB. To explore the synergistic effect of CNT and CB, nonisothermal crystallization and isothermal crystallization behaviors of PVDF and its nanocomposites were studied by differential scanning calorimetry, and the crystallization morphology of them was observed under the three dimensional digital microscope with the polarized model. The crystallization rate of PVDF was speeded up markedly because of heterogeneous nucleation effect of nanoparticles, and CNT and CB nanoparticles had a synergistic effect on nucleation. Polarized microscope observation confirmed that spherulite size of PVDF became smaller owing to the accelerating of crystallization, which influenced the distribution of nanoparticles. The dispersion of nanofillers in matrix was observed by scanning electron microscope. It was revealed that CB could make CNT disperse more evenly in the PVDF matrix. The synergies network of CNT and CB is suggested to build in matrix, which improved conductivity and mechanical properties of PVDF nanocomposites. POLYM. COMPOS., 36:2248–2254, 2015. © 2014 Society of Plastics Engineers     

The effect of the interface/interphase on fiber composite properties

The interfacial region between fibers and matrix in fiber composites governs the transfer of forces between the relatively weak and compliant matrix and the reinforcing fibers. An effective interphase can ensure that the mechanical properties of the composite reflect the high strength and modulus of the fibers. Although composites can be made with the expected strengths and moduli, it is not entirely clear why this is achieved: Tests with critical composites, i.e., those containing very short aligned fibers, do not show the expected stress-strain behavior. This paper examines the effect of an interphase having a shear modulus that is less than that of the matrix. It is found that to explain the Young's moduli of the short fiber composites, the interphase must have a very low modulus indeed; i.e., a few kPa at most. In addition, the strength results can be accounted for only if we assume that the short lengths of fiber used in the experiments had higher strengths than anticipated. Although agreement between experiments and theory is thus not very good, the small amount of experimental evidence available indicates a need for further systematic experiments on critical (i.e. short aligned fiber) composites before firm conclusions are drawn.     

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.     

The effect of interface reaction on the thermal and mechanical properties of Mn3.2Zn0.5Sn0.3N/Al composites

The salient feature of low-expansion metal matrix composites is their dimensional stability. Due to the limitation of the performance of the reinforcement, the current low-expansion metal matrix composite materials cannot achieve both low expansion and high mechanical strength at the same time. With its extremely negative thermal expansion behavior and metallic characteristics, the anti-perovskite manganese nitrogen compound can be used as an ideal thermal expansion inhibitor to prepare high mechanical strength low-expansion composite materials. In the present work, fully-dense Mn_3.2Zn_0.5Sn_0.3N/Al composites with low thermal expansion and high strength have been successfully fabricated by pressure infiltration. The effects of fabricating temperature on the microstructure, thermal expansion and mechanical properties of the Mn_3.2Zn_0.5Sn_0.3N/Al composites have been discussed. Several interfacial reactions were caused by the high fabrication temperature (750 °C and 800 °C). Lower fabrication temperature (700 °C) was used to obtain a well-controlled interface composite with low thermal expansion (α = 6.5 × 10^?6 K^?1), high compression strength (481 MPa) and hardness (612 HV). A modified theoretical model has also been used to analyze the thermal expansion behavior of the composites.     

The effect of downstream plasma treatments on graphene surfaces

This paper reports on the effects of growth, transfer and annealing procedures on graphene grown by chemical vapour deposition. A combination of Raman spectroscopy, electrical measurements, atomic force microscopy, and X-ray photoemission spectroscopy allowed for the study of inherent characteristics and electronic structure of graphene films. Contributions from contaminants and surface inhomogeneities such as ripples were also examined. A new cleaning and reconstruction process for graphene, based on plasma treatments and annealing is presented, opening a new pathway for control over the surface chemistry of graphene films. The method has been successfully used on contacted graphene samples, demonstrating its potential for in situ cleaning, passivation and interface engineering of graphene devices.