Effect of silica on formation of porous alumina during unidirectional solidification in hydrogen atmosphere

Abstract

A porous alumina with cylindrical pores was fabricated by unidirectional solidification in pressurized H2 or H2-Ar mixed gases using alumina feed rod with 15%SiO2-Al2O3 composition. The effect of silica additive on the formation of pores during the solidification was investigated. The porosity of the samples increases with increasing hydrogen partial pressure under a fixed total pressure and decreases with increasing total pressure under a fixed hydrogen partial pressure. No cylindrical pores are formed under 10%H2-90%Ar atmosphere. On the other hand, when the solidification is performed under 50%H2-50%Ar or 100%H2 atmosphere, the porosity and pore size for porous alumina fabricated using 15%Si02-Al2O3 feed rod are larger than those for the samples fabricated using 99.99%A12O 3 feed rod. Many small facet shape pores are formed due to vaporization of silica component on the cooling step during the solidification. When a solidification is performed under 10%H2-90%Ar atmosphere using 15%SiO2-Al203feed rod, an excess hydrogen atom in the solid phase is lack to make the cylindrical pores due to decreasing the hydrogen solubility in solid phases by silica addition then, non-porous alumina is formed. As a result, the porosity for porous alumina fabricated using 15%SiO2-Al2O3 feed rod is not proportional to square root of hydrogen partial pressure under a fixed total pressure. © 2009 The Japan Institute of Metals.