金属纳米颗粒的热导率

本文使用统计模拟方法对金属纳米颗粒的电子平均自由程进行了计算,并考察了纳米颗粒的晶格比热和声子平均群速度,最后应用动力学理论对纳米颗粒的电子热导率和声子热导率分别进行了求解.研究结果表明:具有相同特征尺寸的方形、球形纳米颗粒的无量纲电子(或声子)平均自由程比较接近.金属纳米颗粒的电子热导率远大于声子热导率;电子、声子热导率随着直径减小呈现降低趋势,而电子热导率的颗粒尺度依赖性比声子热导率更为明显;随着颗粒直径进一步减小,声子热导率与电子热导率趋于同一数量级.当纳米颗粒特征尺寸大于4倍块材电子(或声子)平均自由程,其电子(或声子)热导率的颗粒尺度依赖性将减弱.     

碳纤维与热沉间接触热阻随温度变化的实验研究

由于微纳米材料特征尺寸小,难以测量其与热沉间的接触热阻。本文利用拉曼光谱测量不同温度下单根PAN基碳纤维的热导率及其与导电银胶、焊锡和硅油间的接触热阻。结果表明,在实验温度范围内其热导率随温度的降低而增大;接触热阻随温度的变化规律与接触物质有关。     

接触热阻的蒙特卡罗法模拟

 用蒙特卡罗随机模拟法计算了固体接触界面间的接触热阻。取服从高斯分布的随机数作为粗糙峰高度，分别对每个粗糙峰计算出单点接触热阻，再并联所有单点接触热阻得到总的接触热阻。修正了“积分法”的理论结果且与发表的文献数据相符。     

二氧化硅纳米颗粒与碳黑纳米颗粒红外漫反射光谱的测量与分析

用FTIR-FTS3000光谱仪和漫反射附件分别采集了二氧化硅纳米颗粒、炭黑纳米颗粒和二氧化硅纳米颗粒与碳黑纳米颗粒的不同配比的混合样品的红外漫反射光谱。通过对测量结果的分析,发现二氧化硅纳米颗粒的红外漫反射光谱较之体材料有蓝移和宽化现象,此现象可以用纳米粒子的小尺寸效应和量子尺寸效应来进行初步解释。而碳黑纳米颗粒因为其强吸光性的原因,实验中没有得到理想的光谱。混合样品中,碳黑纳米颗粒有一个最大吸收临界浓度,此时二氧化硅纳米颗粒与碳黑纳米颗粒的质量比是100∶20。在这个比例以内,碳黑纳米颗粒的特征峰位的F(R)函数与浓度符合朗伯-比尔定律。当碳黑纳米颗粒在体系中的含量超过这个比值,随着碳黑在体系中比例的增加,吸光度将不再增大。     

纳米悬浮液热导率测量及其预测模型的探讨

纳米颗粒悬浮液具有广泛应用于强化传热的潜在优势。本文采用加入表面活性剂的方法提高纳米悬浮液的悬浮稳定性,并用瞬态热线法测量了热导率。在分析现有理论模型和纳米悬浮液热导率影响因素研究的基础上,从弹性传动和非弹性传动两方面分析了纳米尺度效应导致纳米悬浮液热导率提高的机理。发现已有的理论公式仍然存在一定欠缺,预测值比实验值偏低,有关机理尚有待于深入探讨。     

接触热阻理论模型的简化

本文分析了机械加工表面的粗糙度曲线形貌参数的统计特征.基于单点接触热导理论模型和弹性形变理论模型,建立了总的接触热阻与总的压力之间的一般关系,简化了接触热导的计算模型,并与现有的计算方法进行了对比.研究结果表明,简化模型具有很好的精度.     

考虑界面接触热阻的一维复合结构的热整流机理

建立了考虑变截面、变热导率及界面接触热阻效应的组合热整流结构的温度场及热整流系数的理论模型和有限元解.数值算例证明了本文模型及算法的可靠性,进而通过参数影响研究确定了若干几何及材料参数对结构热整流系数的影响规律,揭示界面接触热阻对热整流效果的影响机理.研究结果表明长度比、截面半径变化率、热导率、边界条件温差和界面接触热阻等因素必须通过优化设计才能得到最大的热整流系数,同时界面接触热阻的引入也为调控热整流系数提供了一条新的途径.     

低温真空下接触热阻温差的理论分析

针对低温真空条件下 ,半无限长圆柱间的接触热阻温差进行了理论分析 ,并得到了一些有用的近似计算公式 ;同时通过与有关文献结果进行对比 ,证明文中的分析近似具有相当的准确度。该分析结果为接触热阻及其引起的温差的计算仿真提供了理论基础     

微结构接触界面热阻

微纳米结构的接触热传输是热电转换、超导冷却、集成芯片散热等高技术领域面临并必须着力解决的技术问题,它区别于宏观热传输,具有为尺度依赖效应和多个微观特征量。文中从微结构接触热传输阻力角度出发,探讨了接触热阻及界面热阻区别,阐释了微尺度的特征量,接触界面热阻实验及理论研究方法、实验参数的测量,接触热阻及界面热阻的材料选择。通过接触界面热阻这些方面的研究,为研究接触界面热阻研究提供了较全面的参考。     

低温下固体接触热阻与接触电阻的实验研究

在低温工程中,两个接触固体之间存在着接触热阻与接触电阻,将对低温实验中热量及电流的传输产生显著的影响,是进行低温下物性研究的关键。自行研制了一套可同步实现固体接触热阻和接触电阻的测量装置,该系统具有较高的精度,可实现外界力、温度等对接触热阻的测量,同时具备接触电阻的实时测量功能。在此基础上,开展了外界压力、温度、电流对接触热阻和接触电阻的实验研究。实验结果显示:随着压力的增大,接触热阻与接触电阻随之减小;低温下,随着温度的增大接触热阻与接触电阻增大,接触电阻增大的速率要比接触热阻快。温度平衡时,20mA范围内的电流变化对接触电阻的影响显著,对接触热阻影响非常小。当界面温度达到室温后,首次观测到接触热阻和接触电阻会随着温度的增大而减小。     

Characterization of Powder Beds by Thermal Conductivity: Effect of Gas Pressure on the Thermal Resistance of Particle Contact Points

The thermal conductivity of ceramic powder packed beds was measured at temperatures below 100 °C for various powder sizes and compositions and under different gas atmospheres. Measurements at low pressures (down to 10 Pa) combined with a theoretical model allowed the elucidation of geometrical and thermal resistance parameters for the contact points between granules. The gap thickness and contact point size were found to be well correlated with the mean particle size. The thermal conductivities of all powders at low pressure were found to differ at most by a factor of two, whereas the solid‐phase conductivities of the powder materials differed by more than one order of magnitude. A theoretical model accounting for the size‐dependence of contact point conductivity is incorporated to rationalize this trend.     

气凝胶单元体模型结构参数与等效热导率研究

本文基于二氧化硅气凝胶的微观结构特点,运用小球构成的立方阵列单元体纳米孔隙模型,结合固相导热和气相传热的尺度效应,计算得到了一定尺寸范围内材料的等效热导率,分析了材料密度、颗粒接触面积、材料比表面积等因素对材料等效热导率的影响。结果表明:存在使气凝胶等效热导率取最小值的最佳密度;在一定的密度范围内,孔隙率一定时,材料的等效热导率随比表面积的增大而减小;颗粒间接触界面直径与固相颗粒直径的比值越大,等效热导率越低,在该比值一定时,气孔的尺寸和分布成为影响模型等效热导率的关键因素。     

Comparison between Experimental and Theoretical Thermal Conductivity of Nanofluids Containing Multi-walled Carbon Nanotubes Decorated with TiO2 Nanoparticles

This article reports the thermal conductivity modeling of nanofluids containing decorated multi-walled carbon nanotubes with TiO₂ nanoparticles. TiO₂ nanoparticles and decorated multi-walled carbon nanotubes are synthesized with different amounts of TiO₂ nanoparticles. The experimental results show that the measured thermal conductivities of TiO₂ nanofluids and multi-walled carbon nanotube nanofluids are higher than the predicted values by theoretical models. The comparison results of multi-walled carbon nanotube nanofluids and multi-walled carbon nanotube–TiO₂ nanofluids reveal that the predicted values by the Xue model are closer to the measured values. In addition, the results show that the thermal conductivity of nanofluids containing multi-walled carbon nanotube–TiO₂ increases with respect to TiO₂ content of hybrid.     

Experimental Study of Thermal Contact Resistance in Hardened Bearing Surfaces

The significance of thermal contact resistance on a bearing steel surface and its application in the analysis of heat flow through layers has been discussed. In the present study, it is considered that the surface roughness and the interface temperature vary to estimate effective thermal conductivity of the bearing model under steady-state and insulated conditions. Surface roughness was considered in the range of 2.37–7.16 μm, and thermal contact properties of the hardened steels were studied under dry and lubricated conditions. These results help to predict temperature distributions of the bearing using SolidWorks2011 (Dassault Systèmes SolidWorks Concord, MA, USA).     

相变微胶囊的热导率测量

提出应用3ω谐波探测技术进行脲醛树脂-石蜡相变微胶囊的等效热导率测量方法。测试了跨越相变温度区间的微胶囊等效热导率,分析了等效热导率随温度的变化关系。在其相变温度区间内,热导率存在极大值,该极值点对应的温度与其相变温度峰值一致。同样温度下,降温时的等效热导率略小于升温,这主要是由降温时相变材料的过冷引起。     

Correlations of Thermal Contact Conductance for Nominally Flat Metallic Contact in Vacuum

Heat transfer experiments were performed in a vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces of copper, brass, and stainless steel with different surface roughnesses (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of thermal contact conductance for the interface of sets of similar materials has been carried out employing a steady-state approach. Results have been compared with the theoretical models based on plastic and elastic deformation based theories for nominally flat metallic contacts. It has been demonstrated that a dire need exists to develop an experimentally based generalized correlation of thermal contact conductance for nominally flat metallic contacts that can be used by design engineers as well as for research purposes in different fields. Along with the generalized correlation, a specific correlation for thermal contact conductance has been proposed for three sets of materials. The thermal contact conductance of each set of materials presented in the form of specific correlations for different roughness parameters is one of the most important outcomes of the present research.     

The Thermal Conductivity Theory of Non-uniform Granular Flow and the Mechanism Analysis

According to the fractal characteristics appearing in non-uniform granular system, we found the fractal model to study the effective thermal conductivity in the mixed system. Considering the quasi-equilibrium, we bring forward the fractal velocity probability distribution function. The equipartition of energy is employed to the non-uniform granular system, and the granular temperature is derived. We investigate the thermal conductivity in granular flow due to the movement of the particles, namely the heat transfer induced by the streaming mode only. The thermal conductivity in the mixed system changes with the fractal parameters such as the solid fraction v, structural character parameter η, and fractal dimension D of size distribution. These parameters depict the characteristics of the thermal conductivity in the actual complex granular system. Comparing our conclusion with the correlative experimental data and the theoretical conclusion of binary mixture of granular materials, the results can qualitatively confirm the generality of our prediction on the granular system.     

A Fractal Model for the Effective Thermal Conductivity of Granular Flow with Non—uniform Particles

The equipartition of energy applied in binary mixture of granular flow is extended to granular flow with non-uniform particles.Based on the fractal characteristic of granular flow with non-uniform particles as well as energy equipartition,a fractal velocity distribution function and a fractal model of effective thermal conductivity are derived.Thermal conduction resulted from motions of particles in the granular flow,as well as the effect of fractal dimension on effective thermal conductivity,is discussed.     

随机结构多孔介质等效热导率数值计算

本文对二维颗粒弥散多孔介质的辐射导热耦合等效热导率进行了数值计算研究。首先采用随机生成结构方法(Random generate-growth method,RGGM)生成实际多孔材料的复杂结构。在此基础上,采用离散坐标法及有限容积法求解了复杂结构内部辐射导热耦合换热,进而计算得到材料的等效热导率。根据建立的材料随机结构模型及等效热导率数值计算模型,分别研究了衰减系数、弥散相体积分数、温度等因素对材料等效热导率的影响,并将数值计算结果与理论计算结果进行对比,吻合较好,反映出模型用于预测实际多孔材料等效热导率的有效性。