Methods And Applications For Measuring The Thermophysical Properties Of Micro/Nanowires

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...