Research On Elastic Properties And Thermal Conductivity Of Cellular Ceramic Materials

Porous/cellular materials are of significant interest due to their wide applications in industry. They are playing important roles in science, technology and civil economics. Because of the peculiar properties showing on these materials, the study of the porous material becomes a recent research focus. In this dissertation, the elastic and thermal transfer properties of porous ceramic materials have been systematically studied and analyzed.In the first part of this dissertation, the current research status of the cellular ceramic preparation are reviewed. The current research status for determining the elastic properties of cellular ceramic materials are introduced. Especially the numerical, empirical and semi-emperical methods for the prediction of the effective thermal conductivity of cellular ceramic materials have been extensively summarized.Based on the principle of pore formation, geometric models to describe the closed-cell cellular materials were constructed. The models are rhombus dodecahedron ones and tetrakaidecahedron cell shapes with all thick edges. The dependence of relative density on microstructure of these models has been analyzed. By using finite elements method, the relative elastic modulus and Poisson's ratio of these models were calculated. The influence of microstructure and relative density on the elastic modulus and Poisson's ratio were obtained. Wall-thickness and wall-face connective curvature radius were found to be the two independent factors affecting the relative density. The wall-face connective curvature radius is the primary affecting factor and the wall-thickness is less significant. For rhombus dodecahedron with thick edge and tetrakaidecahedron with thick edge, the material added at the intersecting part of the wall-faces bring different influence on elastic modulus.Based on the principle of pore formation, geometrical models to describe open-cell cellular materials were also constructed. The models are based on rhombus dodecahedron and tetrakaidecahedron cell shapes with circle-strut and transitional-junction. The dependence of relative density on the microstructure of these models have been analyzed. By using finite elements method, the relative elastic modulus and Poisson's ratio of these models were calculated. The influence of microstructure and relative density on the elastic modulus and Poisson's ratio were obtained. Circle-strut radius and transitional-junction curvature radius were the primary factors on relative density increment. The solid materials added at the intersecting part of the circle-struts bring different influence on elastic modulus for the two models. For low density cellular materials under loading, strut flection has greater influence on Poisson's ratio through volume change. For cellular materials with thick strut, the strut flection does not exist, and the influence of cell volume is gradually diminished. Poisson's ratio of cellular materials with thick strut is close to that of the matrix materials. The transitional-junction curvature radius of the circle-strut has little influence on Poisson's ratio.The effective thermal conductivity model using rhombus dodecahedron geometry which can satisfactorily describe microstructure of cellular materials have been established. The dependence of microstructure parameters on the strut-length was obtained. By using parallel/series and layered/steped effective thermal conductivity calculation method, two formulae to compute the effective thermal conductivity were derived. The dependence of effective thermal conductivity on porosity and material compositions was analyzed. The results can provide theoretical support in the optimization design of the cellular material structures.Elastic properties and thermal conductivity of cellular ceramic materials were experimentally tested. The samples were made of two different materials and three different aperture specifications. The cellular ceramic materials used were prepared using organ sponge impregnation method. Density and porosity of raw materials were tested by waxed method. Properties of the base materials were tested by high temperature and high pressure sintering technique. Based on the two kinds of models, elastic modulus of the cellular ceramic materials was predicted. After careful comparison of the predicted values with experimental values, a rational prediction method has been proposed. Based on the micrograph of the cellular ceramic materials, The two main factors, solid surface and inhomogeneity of strut section-area, were wiped off. Results also indicate that elastic modulus of cellular materials increases remarkably with reducing of block pore and inhomogeneity of strut cross-section area.The experimental environment for determining the thermal diffusion properties of the cellular ceramic materials was constructed. By using six samples testing instrument of thermal conductivities, under Ar condition, the effective thermal conductivity of cellular ceramic was tested. Results indicate that thermal diffusion coefficients of open cellular ceramic increase with increasing temperature. It is interesting to notice that this behaviour is quite different from that in compact sintered body. It was found that the trend is opposite. The reason for this is due to the heat radiation in cellular ceramic structures. By using the rhombus dodecahedron model, the effective thermal conductivity of cellular ceramic materials was predicted and compared with the measured results. The results have shown that the pore radius bring little influence on the thermal diffusion coefficients, and the thermal diffusion coefficients turn bigger when relative density increases.The conclusion drawn from this research work can hopefully provide theoretical support for the optimization design, performance evaluation, and practical guidance for engineering design of the cellular ceramic materials.