Study On Heat Transportation And Heat Storage Properties Of Micro/Nano-Scale Thermal Functional Powders

The harmonic detection method for characterizing the heat transportation and heat storage performances of micro-and nano-scale powder is proposed in this thesis. The effective thermal conductivity, thermal diffusivity and thermal effusivity of micro-and nano-scale thermal functional powders are measured. The effects of density, size, temperature and materials type on the heat transportation and heat storage performance are discussed. The relevant heat transportation and heat storage mechanism are revealed, which provides scientific theory basis for the development, process optimization and application of micro-and nano-scale thermal functional materials.According to the relationship between the heat penetration depth and the measured frequency in harmonic detection technology, the coupling relation between ratio of length to radius of the sensor and the heat penetration depth and that between the thermophysical properties of the specimen and the heat penetration depth are analyzed. The frequency range in which the heat capacity of the sensor and the heat loss of the sensor ends can be neglected is presented. A simplified one-dimensional (1D) slope comparison method is obtained, which is verified by using the standard specimens. The1D slope comparison method can be used to measure the effective thermal conductivity and thermal diffusivity of lowly thermal-conductive materials, such as micro-and nano-scale powders.The effective thermal conductivity and thermal diffusivity of SiO2powders with different particle sizes and different densities at different temperatures are measured by using the calibrated harmonic detection systems. When the gas convection can be neglected, the coupling insulation mechanisms of the solid conduction, gas conduction and heat radiation for nano-scale SiO2powders are calculated and analyzed. The results show in the measured range, both the effective thermal conductivity and thermal diffusivity of SiO2powders increase with the increasing temperatures, and decrease with the increasing particle size. As the gas conduction is inhibited by the nano-scale pores, there exists an optimum density at which the effective thermal conductivity reaches the minimum; the thermal diffusivity reaches the maximum. The optimum density decreases with the decreasing particle diameter.The effective thermal conductivity of phase change microcapsules with different shell-core rations, temperatures, densities is measured. The phase change microcapsules with paraffin wax as core materials and urea-formaldehyde resin polymers as shell materials are produced by in-situ polymerization process. Based on the theoretical calculations, the influence laws of shell-core rations, density and temperature on the heat conduction performance of phase change microcapsules are analyzed. The theoretical model for the calculation of the effective thermal conductivity of such complex materials is presented. It is believed that when the dimensionless parameter (?)(?) relates to the longitudinal sound velocity in the material and determined by the specific heat, density and Young's modulus or called elastic modulus) ranges from4.8to6.0, the error is within10%.By using the planar source harmonic detection technology, the effective thermal effusivity of phase change microcapsules is measured. The results show the phase change core materials properties and its covering amount of the materials is the key to evaluate the heat storage capacity of the materials. The effective thermal effusivity increases with the increasing core materials content and so does the heat storage ability. The effective thermal conductivity increases with the increasing latent heat of the core materials during the phase change process, and so does the effective thermal effusivity, which suggests the increasing its heat transfer ability with the surrounding environment.