NUMERICAL MODELING OF ELASTIC MODULUS AND CONDUCTIVITY OF POROUS ALUMINA - EFFECTS OF PORE SHAPE, PORE SIZE DISTRIBUTION AND PORE DISTANCE

In this paper it is shown that the effective thermal conductivity and Young's modulus of isoporous (i.e. equally porous) alumina ceramics are significantly higher for convex pores than for concave pores, slightly higher for isolated (closed) than for overlapping (open) pores or foam cells, slightly higher for periodic than for random arrays of convex pores or foam cells, significantly higher for isometric than for anisometric pores, significantly lower for oblate pores with aspect ratio 1/R than for prolate pores with aspect ratio R, and relatively similar for convex pores and foams. The most important general finding is that differences in pore shape (convex-concave, isometric-anisometric) greatly overshadow all other effects, e.g. those of pore connectivity (open-closed) or pore arrangement (periodic-random). Oblate pore shape has a much stronger influence in decreasing the effective properties than prolate shape. Although both concave pores and oblate pores yield lower effective property values, the curvature of their porosity dependence curves is characteristically different. On the other hand, the difference between porous alumina with convex pores (isolated or overlapping) and alumina foams (closed-cell and open-cell) is not very large. This clearly shows that the common belief that porous materials with a matrix-inclusion microstructure would exhibit effective elastic and conductive properties that are principally different from cellular materials (foams) is unjustified. Also the influence of the pore size distribution and pore distance on the effective properties is usually much weaker than the influence of pore shape and should not be overestimated.