Porous Ceramics with Directional Pores

This chapter describes the results of the authors' previous work on the formation mechanism of porous ceramics using the solidification method. When the solidification process is adopted as a method for preparing porous ceramics, the porous structure can be controlled by the experimental conditions such as solidification rates, atmosphere, total and/or partial pressure of the atmosphere gas, etc. In this chapter, (1) the formation mechanism of porous ceramics with bi-modal pores in shape using unidirectional solidification of water will be described in detail and (2) the formation mechanism of porous alumina by unidirectional solidification under pressurized hydrogen gas atmosphere. In term (1), the following details are provided. Porous ceramics with cylindrical pores were prepared by unidirectional solidification of aqueous slurry containing CO2 gas, vacuum freeze-drying and sintering. Cylindrical pores were formed during unidirectional solidification under reduced pressure. The solid phase part is a composite of ice and ceramic powder. Porous ceramics with cylindrical pores and fine interconnected pores were obtained by removing only the ice by vacuum freeze-drying and sintering at 1250°C for 5 hours. The diameters of pores that formed by unidirectional solidification of ice from carbonated water was between from 70.97 to 37.10 µm. The pore diameter and its volume decreased with increasing total pressure according to Boyle's law. When pores are formed by the solidification of carbonate water, a very narrow pressure range is required to control the pore size. Porosity and pore size were well controlled by pressure during solidification and firing temperature and time. On the other hand, in term (2), the following details are provided. Porous alumina with a large number of cylindrical pores was prepared by a unidirectional solidification process in a pressurized hydrogen atmosphere. The porous structure was formed at the solid-liquid interface due to the solubility difference of hydrogen between the solid and liquid phases at the melting point. Hydrogen gas dissolves in the molten alumina according to Sievert's law, and insoluble gas corresponding to the solubility gap amount is generated from the solid phase near the solid-liquid interface during unidirectional solidification process, then cylindrical pores were formed. When the ambient gas is pure hydrogen or hydrogen-argon mixture gas, the porosity and pore size of the solidified samples decreased with increasing the total pressure. According to Boyle's law, there is an inverse proportional relationship between the volume of pores and total pressure. In addition, the effects of additives on pore formation were also described. The additives are intended to improve the strength of the porous alumina. For the oxides with solubility gap between solid and liquid phase at melting point, cylindrical pores were formed by the solidification process by the same mechanism.