Thermal Conductivity of Vapor-Liquid-Solid and Vapor-Solid Silicon Carbide Whisker-Reinforced Lithium Aluminosilicate Glass-Ceramic Composites

The thermal conductivities of two lithium aluminosilicate glass-ceramic matrix composites reinforced with 30 vol% of either SiC VS (rice hull) whiskers or SiC VLS (vapor-liquid-solid) whiskers were determined from room temperature to 500°C. Because of the preferred alignment of the whiskers, the thermal conductivity values normal to the hot-pressing direction were found to be significantly higher than those in the parallel direction. The composites with the VLS whiskers exhibited higher thermal conductivity values than those with the VS whiskers. An analysis of the room-temperature data showed that the thermal conductivity values parallel to the hot-pressing direction were higher than those predicted from theory, even for whiskers with infinite thermal conductivity and perfect interfacial thermal contact. This effect was attributed to a significant contribution of percolation to the total heat flow as a result of direct whisker-to-whisker contact. For both types of whiskers, the interfacial thermal conductance and thermal conductivity values (at ∼6.5 × 10⁵ W/(m²-K) and 200 W/(m·K), respectively) inferred from the composite thermal conductivity values perpendicular to the hot-pressing direction were essentially the same. It was concluded that the order of magnitude difference in thickness for the two whisker types was primarily responsible for the differences in thermal conductivity measured for these two composites.     

Influence of Thermal Conductivity and Fracture Toughness on the Thermal Shock Resistance of Alumina—Silicon–Carbide–Whisker Composites

The thermal shock resistance (indentation–quench method), fracture toughness, and thermal conductivity of three alumina–silicon–carbide–whisker composites and alumina have been investigated. A new procedure for the evaluation of thermal conductivity data is suggested, and higher room-temperature thermal conductivity than that reported in the literature is determined for silicon carbide whiskers. The ranking of the materials according to thermal shock resistance is consistent with the ranking according to fracture toughness but disagrees with the ranking according to thermal conductivity. This finding supports the analytically obtained result that, in defining thermal shock resistance, fracture toughness is more important than thermal conductivity.     

Interpreting Impedance Response of Silicon Carbide Whisker/Alumina Composites Through Microstructural Simulation

A three-dimensional, object-defined Monte Carlo simulation is applied to alumina-silicon carbide whisker ceramic matrix composites. The simulation takes whisker orientation and size distributions into account simultaneously, and calculates a connectivity factor that relates whisker conductivity to macroscopic conductivity. Simulation results are compared with electrical measurements taken on real samples via impedance spectroscopy. Results show that the effect of whisker clumping can be seen in the impedance response as a decrease in the overall measured conductivity. Results also show that interfacial resistance influences the overall resistivity strongly relative to connectivity at volume fractions far above the percolation threshold. The possible mechanisms for interfacial resistance in the composite and their effect on the impedance response are discussed.     

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.     

Effects of Interfacial Films on Thermal Stresses in Whisker-Reinforced Ceramics

Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al₂O₃, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al₂O₃), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.     

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT.This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composites.     

Effect of Silicon Carbide Fiber or Whisker Reinforcement on the Thermal Diffusivity/Conductivity of an Osumilite Glass-Ceramic

The thermal diffusivity of silicon carbide fiber- and whisker-reinforced osumilite glass-ceramic was determined by the laser-flash method. The thermal conductivity was calculated from the data of thermal diffusivity, density, and specific heat measured by differential scanning calorimetry. Because of the much higher thermal conductivity of the crystalline silicon carbide whiskers compared to the corresponding value for the amorphous fibers, the whisker-reinforced composites exhibited significantly higher thermal diffusivity and thermal conductivity than the fiber-reinforced composites.     

Thermal Diffusivity/Conductivity of Alumina—Silicon Carbide Composites

Thermal diffusivity and conductivity values for several Al₂O₃-SiC whisker composites were determined. The thermal diffusivity values spanned the range from 373 to 1473 K, and thermal conductivity data wre obtained between 305 and 365 K. The thermal diffusivity decreased with increasing temperature and increased with SiC-whisker content. An estimate of the thermal conductivity of the whiskers was obtained from the direct thermal conductivity measurements, but attempts to derive whisker conductivity values from the thermal diffusivity data were not successful because the laser flash method lacks the required accuracy and precision. Specimens were subjected to two different thermal quench experiments to investigate the effect of thermal history on diffusivity. In the most severe case, multiple 1073- to 373-K quenches, radial cracks were observed in the test specimens; however, there was no change in diffusivity. The lack of sensitivity to thermal cycling appears to be related to the sample size.     

Thermal conductivity of Al2O3/SiC platelet composites

The thermal conductivity of hot-pressed Al₂O₃/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al₂O₃/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselman's approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.     

Thermal Diffusivity/Conductivity of Magnesium Oxide/Silicon Carbide Composites

SiC-particle-reinforced MgO composites have been fabricated by hot pressing, and the thermal diffusivities of the composites measured in the temperature range 200–1000°C using a laser flash technique. The thermal conductivity of the composites was calculated by multiplying the diffusivity with density and with heat capacity. The Eshelby inclusion model has been examined, and an equation suitable for particulate composites with porosity has been derived using the multiphase Eshelby model. The model also considers the interfacial thermal condition. Good agreement was obtained between the predictions and the experimental results of the thermal conductivity of the composites, even for various levels of porosity in the composites. Crystal defects, observed in the composites, influenced the thermal conductivity, resulting in a deviation from isothermal interfacial condition. This was reflected in the interfacial thermal parameter,β used in the modeling, and the predicted value of β was in the range of 3–10, depending on the thermal conductivity of SiC used for the calculations.     

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。     

Effect of Reinforcement Particle Size on the Thermal Conductivity of a Particulate-Silicon Carbide-Reinforced Aluminum Matrix Composite

The thermal conductivity of a 40 vol% silicon carbide-particulate-reinforced aluminum matrix composite was determined as a function of silicon carbide mean particle size ranging from 0.7 to 28 μm. A size dependence was found consisting of a decrease in thermal conductivity with decreasing SiC particle size. This effect is in accordance with theoretical expectations for composites with an interfacial thermal barrier at the dispersion–matrix interface. At the finest particle size of the silicon carbide, the composite thermal conductivity approached the value for the matrix with pores, as expected from theory. Only at the largest SiC particle size did the composite thermal conductivity exceed the value for the matrix. These results suggest that in maximizing the thermal conductivity of composites with an interfacial thermal barrier, the reinforcement particle size should be as large as practically possible.     

Effect of Crystallization on Thermal Shock Behavior of a Chain-Silicate Canasite Glass-Ceramic

The resistance of a canasite glass-ceramic to the initiation of thermal stress fracture due to a water quench was found to be higher than for the original glass, due to higher values of strength and thermal conductivity which offset increases in thermal expansion and Young's modulus. Relative strength retention behavior of the glass-ceramic was also higher than for the glass, attributed to its crack-size-dependent fracture toughness.     

Studies on the Preparation of Polystyrene Thermal Conductivity Composites

Various thermally conductive fillers including aluminum oxide(Al₂O₃), magnesium oxide(MgO), β-silicon carbide particle(β-SiCp) and β-silicon carbide whisker(β-SiCw) were used to prepare polystyrene thermal conductivity composites. Experimental results showed that, for given filler loading, the thermal conductivity of the composites was higher for PS flake than that of PS particle, and the thermal conductivity was optimal by powder blending method. The SiCw filler was more favorable to improve the thermal conductivity of the composites; a much higher thermal conductivity of 1.18 W/mK could be achieved for the composite with 40 vol% SiCw, about six times higher than that of native polystyrene. The experimental thermal conductivity values were in agreement with those predicted by lower bound of Maxwell-Eueken model. For given SiC loading, the thermal conductivity increased with the increasing shape parameter of n. The SiCw was much easier to form the thermal conductivity chains and network than that of SiCp.     

Thermal Properties of Carbon Nanotube–Copper Composites for Thermal Management Applications

Carbon nanotube–copper (CNT/Cu) composites have been successfully synthesized by means of a novel particles-compositing process followed by spark plasma sintering (SPS) technique. The thermal conductivity of the composites was measured by a laser flash technique and theoretical analyzed using an effective medium approach. The experimental results showed that the thermal conductivity unusually decreased after the incorporation of CNTs. Theoretical analyses revealed that the interfacial thermal resistance between the CNTs and the Cu matrix plays a crucial role in determining the thermal conductivity of bulk composites, and only small interfacial thermal resistance can induce a significant degradation in thermal conductivity for CNT/Cu composites. The influence of sintering condition on the thermal conductivity depended on the combined effects of multiple factors, i.e. porosity, CNTs distribution and CNT kinks or twists. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. CNT/Cu composites are considered to be a promising material for thermal management applications.     

环氧树脂／碳化硅晶须导热复合材料的制备研究

用共混复合-浇注成型法制备环氧树脂／碳化硅晶须（EP／SiCw）导热复合材料,研究了导热填料种类、形状、用量和表面处理对复合材料的导热性能、力学性能和热性能的影响。结果表明,SiCw较SiCp更易改善材料的导热性能,热导率随SiCw用量的增加而增大,当SiCw体积分数为42．1％时,复合材料热导率为0．9611W／（m·K）;力学性能随SiCw用量的增加先增加后降低。表面处理有利于提高复合材料的导热性能和力学性能。SiCw的加入使环氧树脂的耐热性提高、Tg降低。     

Thermal conductivity of semicrystalline polymers — a model

The thermal conductivity of semicrystalline polymers, regarded as two-phase materials, is discussed in terms of the Maxwell model generalized to the case where the inclusions are thermally anisotropic. The predicted effect of orientation agrees well with the large anisotropy observed in oriented polymers. The conductivity of the crystallites normal to the chain axes has also been extracted using this model. A recently proposed model for composites which incorporates interfacial boundary resistance has been applied to the low temperature data for poly(ethylene terephthalate), not only explaining the decrease of conductivity with crystallinity, but also allowing the effective crystallite shape and the boundary resistance to be determined. The latter is found to vary as T^?2.     

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。     

Fabrication of Oriented SiC-Whisker-Reinforced Mullite Matrix Composites by Tape Casting

Dense SiC-whisker-reinforced mullite composites with up to 50 vol% whiskers can be obtained by tape casting and hot pressing. The tape casting process results in high degrees of SiC whisker orientation as determined visually and by X-ray diffraction. The ability to achieve dense composites with as much as 50 vol % whiskers is attributed to the higher percolation threshold of aligned whiskers. The factors affecting the degree of whisker orientation during tape casting are described using a fluid dynamics model derived from Jeffery's equations and show that the orientation of anisometric particles is enhanced primarily by the casting rate and particle aspect ratio.     

剑麻纤维与晶须混杂增强聚丙烯复合材料

采用熔融共混和注塑成型方法制得了剑麻短纤维(SF)和CaSO4晶须混杂增强聚丙烯(PP)复合材料，研究了复合材料的热性能、微观结构和力学性能。结果表明，晶须提高了复合材料的热稳定性，阻碍了PP的结晶，降低了复合材料中PP相的结晶度和结晶速率；SF和晶须提高了复合材料的模量和韧性，但由于混杂增强复合材料弱界面键合的制约，晶须的高强性能并没有在复合材料中充分表现出来。