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。     

Enhanced mechanical and anisotropic thermal conductive properties of polyimide nanocomposite films reinforced with hexagonal boron nitride nanosheets

To attain thermally conductive but electrically insulating polymer films, in this study, polyimide (PI) nanocomposite films with 1–30 wt% functionalized hexagonal boron nitride nanosheets (BNNSs) were fabricated via solution casting and following imidization. The microstructures, mechanical and thermal conductive properties of PI/BNNS nanocomposite films were examined by taking account of the relative content, anisotropic orientation, and interfacial interaction of BNNS and PI matrix. The scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry data revealed that BNNSs with hydroxy and amino functional groups have specific molecular interactions with PI matrix and they form stacked aggregates in the nanocomposite films with high BNNS loadings of 10–30 wt%. The tensile mechanical strength/modulus, thermal degradation temperatures, and thermal conductivity of the nanocomposite films were found to be significantly enhanced with increasing the BNNS loadings. For the nanocomposite films with 1–30 wt% BNNS loadings, the in-plane thermal conductivity was measured to be 1.82–2.38 W/mK, which were much higher than the out-of-plane values of 0.35–1.14 W/mK. The significant anisotropic thermal conductivity of the nanocomposite films was found to be owing to the synergistic anisotropic orientation effects of both BNNS and PI matrix. It is noticeable that the in-plane and out-of-plane thermal conductivity values of the nanocomposite film with 30 wt% BNNS were ~1.31 and ~3.35 times higher than those of neat PI film, respectively.     

Determination of the Thermal Resistance of the Polymer–Ceramic Interface of Alumina-Filled Polymer Composites

To model the thermal conductivity of polymer composites that are filled with ceramic powders, the conductivity of each component as well as the interfacial resistance at each ceramic–polymer boundary must be known. An indirect method to determine this interfacial boundary resistance is proposed by preparing large-scale "macromodel" simulations of the polymer–ceramic interface. Macromodels, prepared by spin-coating a polymer layer onto sapphire wafers, were formed in a sapphire–polymer–sapphire sandwich type structure. The interfacial boundary thermal resistance was calculated from thermal resistance measurements made on the macromodels.     

Thermal contact resistance and adhesion studies on thin copper films on alumina substrates

A new photothermal method for measuring the thermal contact resistance in the interfacial area is presented. Copper thin films were prepared on alumina substrates by physical vapour deposition. On the basis of a mathematical model developed here, thermal contact resistance was determined in samples of various thicknesses and processed under various argon pressures. The effects of these parameters on the films and interface properties are discussed. A correlation between the thermal contact resistance and the adhesion, as determined by the scratch test, is found. In order to understand the origin of the mean critical load and the thermal contact resistance evolution, observations were made by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results obtained have shown that the change in stress level in the copper film and the formation of a new compound in the interfacial area seem to be the main reasons for the enhancement of heat transfer.     

Substrate vs. free surface: Competing effects on the glass transition of polymer thin films

The glass-transition temperature (T_g) of polymer thin films can be strongly influenced by the combined effects of the supporting solid substrate and the free surface. The relative importance of these two effects, which often compete with each other, depends on the strength of the substrate–film interactions. Utilizing an atomistically informed coarse-grained model for poly(methyl methacrylate) (PMMA), here we uncover the relationship between the substrate–film interfacial energy and the spatial distribution of T_g across thin films. We find that above a critical interfacial energy, the linear dependence of film T_g on the interfacial energy breaks down and film T_g attains an asymptotic value. Analyses on the spatial variation of T_g across the thin film reveal that the short-range interface near the cohesive surface generates a long-range interphase that leads a spatially uniform appreciation of T_g throughout the film, unlike weakly cohesive surfaces that show sharp gradients along the depth of film. These findings explain recent experiments and reveal a versatile approach for tuning film T_g via engineered substrate-film interactions.     

Carbon black thin films with tunable resistance and optical transparency

Layer-by-layer assembly was used to produce highly conductive thin films of carbon black and polymer. Positively and negatively-charged polyelectrolytes, polyethylenimine (PEI) and poly(acrylic acid) (PAA), were used to stabilize carbon black in aqueous mixtures that were then deposited onto a PET substrate. The effects of sonication and pH adjustment of deposition mixtures on the conductivity and transparency of deposited films was studied, along with drying temperature. Sonication and oven drying at 70 °C produced films with the lowest sheet resistance (∼1500 Ω/sq), which is a bulk resistivity below 0.2 Ω cm for a 14-bilayer film that is 1.3 μm thick. These two variables improve packing and connectivity amongst carbon black particles that results in increased electrical conductivity. Increasing the pH of the PAA-stabilized mixture and decreasing the pH of the PEI-stabilized mixture resulted in transparent films due to increased polymer charge density. These pH-adjusted films have much higher sheet resistance values than their non-adjusted counterparts due to their reduced thickness and patchy deposition. Varying the number of bilayers allows both sheet resistance and optical transparency to be tailored over a broad range. Carbon black-filled thin films able to achieve these levels of resistivity and transparency may find application in a variety of optoelectronic applications.     

Thermal conductivity of polyimide/boron nitride nanocomposite films

The thermal conductivity of polyimide/boron nitride (PI/BN) nanocomposite thin films has been studied for two sizes of BN nanofillers (40 and 120 nm) and for a wide range of content. A strong influence of BN particle size on the thermal conduction of PI has been identified. In the case of the largest nanoparticles (hexagonal‐BN), the thermal conductivity of PI/h‐BN (120 nm) increases from 0.21 W/mK (neat PI) up to 0.56 W/mK for 29.2 vol %. For the smaller nanoparticles (wurtzite‐BN), PI/w‐BN (40 nm), we observed two different behaviors. First, we see a decrease until 0.12 W/mK for 20 vol % before increasing for higher filler content. The initial phenomenon can be explained by the Kapitza theory describing the presence of an interfacial thermal resistance barrier between the nanoparticles and the polymer matrix. This is induced by the reduction in size of the nanoparticles. Modeling of the experimental results allowed us to determine the Kapitza radius a_K for both PI/h‐BN and PI/w‐BN nanocomposites. Values of a_K of 7 nm and >500 nm have been obtained for PI/h‐BN and PI/w‐BN nanocomposite films, respectively. The value obtained matches the Kapitza theory, particularly for PI/w‐BN, for which the thermal conductivity is expected to decrease compared to that of neat PI. The present work shows that it seems difficult to enhance the thermal conductivity of PI films with BN nanoparticles with a diameter <100 nm due to the presence of high interfacial thermal resistance at the BN/PI interfaces. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42461.     

Direct synthesis and characterization of optically transparent conformal zinc oxide nanocrystalline thin films by rapid thermal plasma CVD

We report a rapid, self-catalyzed, solid precursor-based thermal plasma chemical vapor deposition process for depositing a conformal, nonporous, and optically transparent nanocrystalline ZnO thin film at 130 Torr (0.17 atm). Pure solid zinc is inductively heated and melted, followed by ionization by thermal induction argon/oxygen plasma to produce conformal, nonporous nanocrystalline ZnO films at a growth rate of up to 50 nm/min on amorphous and crystalline substrates including Si (100), fused quartz, glass, muscovite, c- and a-plane sapphire (Al2O3), gold, titanium, and polyimide. X-ray diffraction indicates the grains of as-deposited ZnO to be highly textured, with the fastest growth occurring along the c-axis. The individual grains are observed to be faceted by (103) planes which are the slowest growth planes. ZnO nanocrystalline films of nominal thicknesses of 200 nm are deposited at substrate temperatures of 330°C and 160°C on metal/ceramic substrates and polymer substrates, respectively. In addition, 20-nm- and 200-nm-thick films are also deposited on quartz substrates for optical characterization. At optical spectra above 375 nm, the measured optical transmittance of a 200-nm-thick ZnO film is greater than 80%, while that of a 20-nm-thick film is close to 100%. For a 200-nm-thick ZnO film with an average grain size of 100 nm, a four-point probe measurement shows electrical conductivity of up to 910 S/m. Annealing of 200-nm-thick ZnO films in 300 sccm pure argon at temperatures ranging from 750°C to 950°C (at homologous temperatures between 0.46 and 0.54) alters the textures and morphologies of the thin film. Based on scanning electron microscope images, higher annealing temperatures appear to restructure the ZnO nanocrystalline films to form nanorods of ZnO due to a combination of grain boundary diffusion and bulk diffusion.PACS: films and coatings, 81.15.-z; nanocrystalline materials, 81.07.Bc; II-VI semiconductors, 81.05.Dz.     

Assembly and benign step-by-step post-treatment of oppositely charged reduced graphene oxides for transparent conductive thin films with multiple applications

We report a new approach for the fabrication of flexible and transparent conducting thin films via the layer-by-layer (LbL) assembly of oppositely charged reduced graphene oxide (RGO) and the benign step-by-step post-treatment on substrates with a low glass-transition temperature, such as glass and poly(ethylene terephthalate) (PET). The RGO dispersions and films were characterized by means of atomic force microscopy, UV-visible absorption spectrophotometery, Raman spectroscopy, transmission electron microscopy, contact angle/interface systems and a four-point probe. It was found that the graphene thin films exhibited a significant increase in electrical conductivity after the step-by-step post-treatments. The graphene thin film on the PET substrate had a good conductivity retainability after multiple cycles (30 cycles) of excessively bending (bending angle: 180°), while tin-doped indium oxide (ITO) thin films on PET showed a significant decrease in electrical conductivity. In addition, the graphene thin film had a smooth surface with tunable wettability.     

CURING STUDY AND OPTIMIZATION OF A POLYURETHANE-BASED MODEL PAINT COATED ON SHEET MOLDING COMPOUND PART II: DRYING DEFECTS RELATED TO CURING CONDITIONS

This study deals with the bubbling phenomenon which appears in some drying conditions on paint films applied as thin layers on SMC substrates. The bubbles and craters origin can be due either to reactions which may occur at the interface paint / SMC either to inadequate drying conditions which affect the polymerization reaction (gelation, viscosity) and the solvent vaporization.

Firstly, we reported data describing the solvent interaction with the SMC substrate - namely the contact angle, the interfacial tension, the absorption kinetics and the sorption isotherms - and the relationship between the bubbling defects and the numerous process variable (operating conditions, paint formulation and support properties).

Then, we gathered experimental data concerning the influence of different parameters (state and type of substrate, temperature, heating mode, film thickness. paint formulation, etc.) on the bubbling phenomenon and we tried to establish some quantitative interpretation. It proved that this defect was mainly due to the SMC heterogeneity expressed by the porosity of this composite material.     

基于热成像的背胶石墨膜面向热导率测试方法

导热石墨膜广泛用于电子设备中的发热器件散热,面向热导率是反映其传热性能的关键参数,但目前行业中一般只提供裸材数据,无法对背胶石墨膜进行测试,给石墨膜用户的热设计和产品质量管理带来不便。提出一种基于热成像的背胶石墨膜面向热导率稳态测试方法,样品直接黏附于平整台面测试,通过温度梯度环路积分消除热流不均匀的影响,通过热损失标定减小表面换热和旁路传热带来的测试误差。基于多种规格石墨膜产品和具有参考数据的金属薄片进行实验,结果证明了方法的有效性。背胶石墨膜测试数据和裸材参数的比较表明二者有很大偏差,表明石墨膜产品标称参数和实际参数可能有较大偏差,应用中需直接测试背胶石墨膜参数。     

Thermal,optical, and electrical characterization of thin film coated RTV 655 bilayer system

Adhesion of thin films to hydrophobic elastomeric substrates is of particular interest in the area of flexible electronics and nano‐sensor technology. Here, nanometer‐thick Au films were deposited directly onto hydrophobic RTV 655 substrates by means of sputtering, thermal evaporation, and electroless techniques without an adhesion‐promoting layer. The bilayer system was exposed to repeat thermal cycling and changes to the surface morphology of the thin film were monitored electrically and optically. Buckle formation in the as‐deposited film was attributed to stress in the film and substrate stiffness rather than thermal coefficient mismatch between films. The Au‐RTV 655 interface was water tight and maintained a strong adhesion despite repeated thermal cycles. Sputtered and thermally evaporated carbon‐coated RTV 655 substrates were also studied in parallel for comparison. Periodic arrays of buckles formed in pre‐strained RTV 655 samples showed reproducibility in their optical properties demonstrating good adhesion between the two layers without an interfacial layer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41396.     

高填充改性复合材料导热预测模型的建立及应用

在对高填充改性复合材料导热过程进行研究的基础上,建立了基于串并联/并串联模型并考虑了界面热阻作用的高填充改性复合材料导热预测模型。借助于双转子连续混炼机制备两种氧化铝粒径不同填充量的聚丙烯/氧化铝（PP/Al₂O₃）复合材料,运用激光导热仪对其导热性能进行了表征,并与模型预测结果进行了对比。结果表明,所建立的模型对高填充复合材料的导热性能的预测具有较高的准确性;当氧化铝填充量较低时,模型中的界面热阻因子最高;随着氧化铝填充量增加,界面热阻因子显著降低;当氧化铝填充量继续增加时,界面热阻因子逐渐降低并趋于稳定;高填充量下相同制备工艺下同种填充改性复合材料的界面热阻近似相同。     

Boron Nitride-Based Overcoat Thick Films for MoSi2 Planar Heating Elements

This work first reports a boron nitride-based dielectric system that is designed for MoSi₂–based planar heating elements patterned on a regular 96% alumina substrate. The dielectric system is expected to function as an overcoat layer mainly to protect the printed heating elements from environments and to reduce thermal stress induced during thermal heating through improved heat dissipation. The boron nitride (BN) pastes mixed with a low softening glass of calcium barium aluminoborosilicate were screen printed onto MoSi₂ thick films and then fired at a temperature of 900°C. The addition of BN was found to increase the thermal conductivity considerably without detrimental chemical reactions with glass constituents. For instance, the thick film containing 30 wt% BN was regarded as a promising composition from the supporting evidences of good adhesion with MoSi₂, an increased thermal conductivity of ∼31 W·(m·K)⁻¹, and a high electrical resistance of 4.7 × 10¹⁰Ω.     

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.     

Thickness-dependent thermal resistance of a transparent glass heater with a single-walled carbon nanotube coating

A single-walled carbon nanotube (SWCNT) film was formed on a glass substrate by using the dip-coating method and the resulting material used as a transparent glass heater. The heaters have an optical transparency above 95% in visible light. Their performance was investigated by measuring their heating/cooling characteristics and thermal resistance. They have a heating performance that is ∼70% higher than that of Pt film heaters, which are widely used. The thickness dependence of the thermal resistance is observed, and can be understood in relation to heat-transfer losses that are related to the porous structure of the SWCNT film. The stability and reliability of the heater were also investigated.     

Thermal Conductivity of Diamond Films: 0.5 μm to 0.5 mm

A review of the thermal properties of chemical-vapor-deposited (CVD) diamond ranging in thickness from 0.5 μm to 0.5 mm is presented. The typical columnar microstructure of the material has a strong influence on the thermal properties, causing a steep gradient in both the in-plane (κ_‖) and normal (κ_⊥) conductivities, as well as considerable anisotropy. Data for κ_‖ from above room temperature down to liquid helium temperatures for high-quality thick samples has revealed several types of phonon scattering centers preferentially located along grain boundaries. This model of dirty grain boundaries provides a framework for understanding the conductivity of thinner, lower-quality material. The general difficulty of identifying microscopic sources of thermal resistance in CVD diamond is discussed, especially in view of the tendency for the concentrations of many types of defects to be highly correlated with each other. Finally, recent work on interfacial resistance between CVD diamond and Si substrate shows that the columnar microstructure has a strong influence for high-quality films as thin as 2 μm.     

Thermal ageing of electrical conductivity in carbon nanotube/polyaniline composite films

The influence of carbon nanotubes on the thermal ageing effect of the electrical conductivity of composite thin films is presented. The composite thin films comprise carbon nanotube/polyaniline nanofibers. When subject to thermal treatment, the presence of nanotubes retards the loss of dopants from the polyaniline and enhances the thermal stability in electrical conductivity of the composite thin films. Specifically, an increase in temperature for the conductivity degradation and a significant reduction in the rate of the conductivity degradation of the composite thin films are observed. Upon prolonged heating, the composite thin films exhibit relative large conductivity at high nanotube content, while the polyaniline thin films become insulating.     

低界面热阻改性氮化铝和多壁碳纳米管充填PVDF构建杂化三维网络及其导热性能强化

通过对多壁碳纳米管(MWCNT)和氮化铝(AlN)颗粒进行不同的改性,提高其分散性以及与聚偏氟乙烯(PVDF)基体的界面相容性,降低界面热阻。通过溶液共混后再用热压法将填料与基体的混合物压制成致密的薄膜,提高PVDF的导热性能。TEM测试证明改性后的填料分散性能提高,SEM证明两种填料成功地在PVDF基体中构成三维杂化网络结构。当填料含量50%时,a-AlN-PVDF复合薄膜的热导率达到原膜的300%、断裂强度变为原膜的92%。当MWCNT与AlN的体积比为1∶1、改性混合填料的质量分数为50%时,热导率变为原膜的565%,断裂强度变为原来的51%。     

陶瓷涂层三明治板的抗热震性

采用解析法研究了第3类边界条件下双面陶瓷涂层三明治板的瞬态温度场及瞬态热应力场.对不同Biot模数的热冲击过程中,Al2O3涂层/硬质合金(WC-8%Co,质量分数)基体/Al2O3涂层三明治板的瞬态热应力进行了数值计算.分析了涂层/基体厚度比、涂层与基体热-物理性能匹配对陶瓷涂层三明治板表面热应力峰值的影响.结果表明:陶瓷涂层三明治板的基体的热导率、线膨胀系数和弹性模量应高于涂层,这样可以降低其表面热应力,获得高抗热震性陶瓷涂层三明治板.此外,涂层厚度应尽可能小,以利于改善涂层的抗热震性.