【作者】 王建立；

【导师】 张兴；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2010， 博士

【摘要】 微纳米纤维广泛应用于航天航空、能源转换等领域,其热性能已成为研究的热点之一。由于空间分辨率的限制及接触热阻等因素的影响,宏观尺度下的热测试方法已无法满足微纳米线的测量要求,因此开发微纳米尺度下的热物性测量方法具有重要意义。本文研发了变长度稳态T形法和3ω-T形法,测量了微米线热导率、热扩散率等热物性参数,进而探讨了界面材料对测量结果的影响。基于稳态T形法,通过改变热沉的位置,得到了同一根待测线不同长度对应的总热阻,实现了待测线热导率、铂黑胶节点接触热阻以及待测线表面发射率的同时测量。热阻分析表明,热线热阻是待测线总热阻的4倍时,测量灵敏度最大。室温下得到不同金属线的表面发射率均与文献值吻合;碳纤维热导率为400 W m-1 K-1,不确定度小于25 W m-1 K-1;铂黑胶节点的接触热阻约为103~104 K W-1,且随待测线尺寸及热导率的增大而减小。研发了3ω-T形法及其虚拟锁相实验系统,进一步测量了待测线吸热系数及节点热阻抗。结果表明:节点热阻抗可以简化为界面材料的稳态热阻与比例系数的乘积,当节点热阻抗小于待测线时,由于界面材料提高了待测线与热线间的热量交换能力,导致热线温度振荡量减小;当节点热阻抗远大于待测线时,节点热阻抗与接触热阻相当;采用不同界面材料得到的待测线吸热系数相同。采用3ω-T形法测量了交叉铂线间干节点接触热阻随温度的变化,并揭示了T形结构中交叉圆柱节点的变形机理。考虑热线弯曲变形以及微观接触点塑性变形的影响,能够解释接触热阻随温度升高而下降的实验结果,拟合得到的载荷指数与文献值吻合很好。结果表明:微观接触热阻占总节点热阻的比重随着温度下降及表观接触面积的增大而增大,在室温下约占30%。研究了三种功能材料的热输运特性,结果表明:纳米多孔金线孔壁内部缺陷随着退火温度的升高而增加,导致热导率减小;随着热处理温度升高,碳纤维内部声子与缺陷间的散射减弱,热导率峰值位置将向低温偏移,对应的比热减小;由于受到界面热阻、长径比等因素限制,多壁碳纳米管并不能明显提高复合材料的热导率。更多还原

【Abstract】 Micro/nanofibers have wide applications in areas such as aerospace, energy conversion and etc. The researches on their thermophysical properties have become hot topics. Due to the limitation of the spatial resolution and the presence of the thermal contact resistance, the thermometry applied in the macroscale can no longer meet the requirement of the measurement of the micro/nanowires, therefore, it is critical importance to develop more reliable experimental methods. In this dissertation, the changing length T type probe and the 3ω-T type method are developed to measure the thermal properties, including the thermal conductivity and the thermal diffusivity, of the microwires, and the effect of the interstitial material on the experiemental results is also clarified.Based on the steady-state T type probe, in the same contact condition of the junction between the heating wire and the test wire, by measuring the total thermal resistances with various lengths of the same test wire, the thermal conductivity and the surface emissivity of the fine wire, and the thermal contact resistance of the platinum black junction are simultenously obtained. Thermal resistance analysis shows that for best sensitivity, the thermal resistance of the heating wire should be four times that of the test wire. This method is verified by measuring the surface emissivities of the metallic wires, and the results agree well with the reference values. The thermal conductivity of a carbon fiber is obtained to be 400 W m-1 K-1 at room temperature, and the uncertainty is less than 25 W m-1 K-1. The thermal contact resistance of platinum black junction is obtained to be about 103~104 K W-1, which decreases with increasing diameter and thermal conductivity of the test wire.The 3ω-T type method is proposed and the experimental system based on virtual lock-in is established to simultaneously measure the thermal effusivity of the test wire and the thermal impedance of the junction. It is found that the thermal impedance of the junction can be simplified to be the product of the steady-state thermal resistance and a ratio function. The thermal impedance of the interstitial material is similar to the thermal contact resistance, if the former is much larger than the thermal impedance of the test wire, while the interstitial material with relative smaller thermal impedance gives a decrease of the temperature oscillation of the heating wire, because the corresponding interstitial material will enhance the ability to absorb energy from the heating wire. However, the same value of the thermal effusivity of the test wire is obtained with different interstitial materials.Using the 3ω-T type method, the temperature dependence of the thermal resistance of a bare junction between two crossed platinum wires is measured, and the deformation mechanism of the microscopic contacts is confirmed. The thermal contact resistance is found to decrease as the temperature increases from 100 to 200K, then the trend slows down, which can be explained by considering both the bending effect of the heating wire and the plastic deformation of the microscopic contacts, and the fitted conductance-load exponent also agrees well with the reference value. The microscopic thermal resistance covers about 30% of the total thermal contact resistance of the junction at room temperature, and the ratio is found to increase as the nominal contact area increases and the temperature decreases.The characteristics of the thermal transport in three functional materials are inverstigated. As the annealing temperature increases, new defects in the ligament of the nanoporous Au will be introduced, resulting in a decrease of the thermal conductivity. The phonon scattering against defects in carbon fibers will be depressed as the heat treatment temperature increases, so the peak of the thermal conductivity shifts to lower temperature, and the corresponding specific heat is smaller. Due to the effects of the interfacial thermal resistance, the apect ratio of multiwalled carbon nanotube and etc, no noticeable enhancement in the thermal conductivity of the composite material is observed. Key words: micro/nanowire; 3ω-T type method; thermal contact resistance; thermal conductivity; thermal diffusivity更多还原