Relationship between air flow through and fractal characteristic of porous ceramic materials

Abstract

Air flow through irregular porous ceramic materials was experimentally studied in the long mean free path region at low absolute pressure and resulting pressure drop across the porous medium was measured to visualize a three dimensional information of the internal pore structure. A novel method to characterize the structure characteristic of the porous medium has been proposed based on the pressure drop measurement and fractal geometry. It consists of the measurement of pressure drops at the same mass flow rate but at different absolute gas pressure to determine the effective specific surface area of the pores and to evaluate the pore structure via fractal analysis. Then the obtained results were compared with the fractal dimensions obtained from microphotographs and image analysis. The results showed that complex irregular pore structure could be characterized by the fractal dimension and the presence of stagnant air pockets and dead end caves contributed to the discrepancy in fractal dimensions obtained experimentally via pressure drop measurement and visually via microphotos and image analysis. This approach is applicable to characterize any pore structures. Pore shape and properties of porous MgO⋅Al₂O₃ refractories containing TiO₂ and Al₂O₃ as matrix with TiO₂/Al₂O₃ molar ratio of 1.0 were studied at different matrix contents. Al₂O₃⋅TiO₂ was detected in the specimens for the matrix contents less than 30% by weight. Both MgO⋅Al₂O₃-2MgO⋅ TiO₂-MgO⋅2 TiO₂ solid solution increased as the matrix contents increased. Boundary cracks in the aggregates increased as the matrix content increased, and both of the above solid solutions were observed in the boundary cracks. Bulk density decreased and apparent porosity increased as the matrix contents increased. Compressive strength showed a maximum at a matrix content of 10 wt.% and then became lower at matrix contents above 10 wt.%. Both mean pore size and permeability showed a peak at about 10-15 wt.% matrix contents. The shape of the pores was shown to have fractal nature and the fractal dimensions of the pores decreased as the matrix contents increased. It was fond that the permeability showed a maximum at a fractal dimension of 1.56 and the value agreed well with that of a magnesia refractory. Furthermore, the fractal dimension increased in proportion to the amount of MgO⋅ Al₂O₃ - 2MgO⋅ TiO₂ solid solution.