An Experimental And Theoretical Study On The Mid-high Temperature Effective Thermal Conductivity And Linear Thermal Expansion Coefficient Of Aluminum Foam

Due to the numerous special physical properties combined with light weight, metallic foams are promising to be used as multifunctional components and have drawn extensive attention from both academic and industrial fields in recent years. For instance, the closed-cell aluminum foam is expected to be used as both load-bearing and thermal-insulating material in mid-high temperature situations in view of its high specific strength and low effective thermal conductivity (ETC). The studies on the mechanical behavior of metallic foams can date back to a very early time and have comprehensively discussed the effects of diverse factors such as the pore morphology, the defects, the properties of cell wall material and the working conditions.In contrast, the study on the thermal properties of metallic foams needs to be enriched. Although quite a few experimental works have been done on measuring the ETC of metallic foams, most of them just focused on the properties at room temperature and did not analyze the effects of the structural parameters and the properties of cell wall material in depth. In order to calculate the ETC of metallic foams, models of diverse geometries were discussed via different computational methods such as finite element method (FEM), finite volume method (FVM) and Monte-Carlo method (MCM). However, the fundamental relation between the ETC and the structural parameters still remains vague in some extent. Besides, the linear thermal expansion coefficient (LTEC) significantly affects the magnitude and distribution of thermal stress and strain under heating conditions, while researches on the thermal expansion behavior were seldom reported. Therefore, the author of this dissertation has conducted following studies on the heat transfer and the thermal expansion behaviors of metallic foams.X-ray diffraction (XRD), X-ray fluorescence (XRF), pure-oxygen combustion, differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) have been used to analyze the composition, the thermal characteristics and the spectroscopic characteristics of the cell wall material in the closed-cell aluminum foams. According to the results of the tests, the cell wall material mainly consists of Al, Al4Ca and Ti2Al20Ca. In the temperature range of300～500℃, an order/disorder phase transition happens possibly due to the redistribution of some residual hydrogen in the lattice of Ti2Al20Ca Particularly, trials have been made to measure the cell wall thermal conductivity, and the experimental difficulties and the potentially feasible schemes have been discussed. The measured thermal conductivity of cell wall material tablets shows that the thermal conductivity of the cell wall material decreases with increasing temperature.The LTEC of the closed-cell aluminum foam has been measured in the temperature range of100～500℃, and the effects of relative density, temperature as well as composition have been discussed considering the measured characteristics of the cell wall material. The results show that the LTEC of the closed-cell aluminum foam is the same as that of the cell wall material, thus independent on relative density. As the residual tensile stress is released, the LTEC declines significantly and even shows negative values in the first heating process. With temperature higher than300℃, the instantaneous LTEC shows hysteresis due to lattice contraction which is probably caused by the redistribution of residual hydrogen in the Ti2Al20Ca lattice.A steady-state comparative method has been established to measure the ETC of the closed-cell aluminum foam in the temperature range of100～500℃. According to the experimental results, the ETC generally increases with increasing relative density and decreases slightly with increasing temperature. Prior to in-depth analyses of the heat transfer behavior of metallic foams, the ETC of a cubic-cell model has been calculated in wide ranges of diverse parameters so as to extract the general principles and key parameters of heat transfer in porous materials. The results of the cubic-cell model shows that with temperature lower than500℃, the radiative heat transfer accounts for less than10%in aluminum foam, while the thermal conduction of cell wall dominates. Besides, the ETC of metallic foams is significantly affected by the structural characteristics such as the cell wall tortuosity,the anisotropy of pore shape and the plateau border. Neglecting the thermal radiation and the gas conduction, a general equation of the ETC of metallic foams has been derived and can be expressed as the product of the cell wall thermal conductivity, the relative density and a redefined shape factor. The exact physical meaning of the shape factor was found to be the average of dimensionless temperature derivative weighted by volumes of cell walls. Based on the assumption of a uniform temperature gradient, the shape factor was derived as a function of the volumes (lengths) and the orientations of cell walls. Based on this function, the shape factors of real metallic foams and diverse models have been discussed and compared using an image analysis method. The results of the image analysis show that the shape factor of the closed-cell aluminum foams is macroscopically isotropic. Besides, the polyhedral geometries give obviously overestimated shape factors, while the Voronoi geometries are appropriate for simulating the closed-cell aluminum foams. Particularly, the cell wall thermal conductivity of the closed-cell aluminum foam has been reversely calculated from the measured ETC, the relative density and the shape factor based on the general equation of the ETC. The results show that obvious differences exist in the cell wall thermal conductivity among the foam samples, and the cell wall thermal conductivity slightly decreases with increasing temperature.