Effects of impurities on formation pores during solidification for porous alumina and its compressive strength

Abstract

Porous alumina with glassy boundary phase consisting of with silica and calcia components was fabricated by unidirectional solidification under pressurized hydrogen atmosphere using alumina feed rod with silica and calcia additives. The pores were elongated along the solidification direction. The glassy boundary phase was directly introduced in the solidified sample during the solidification. The porosity and pores size decrease with increasing total pressure. When a high purity alumina feed rod was used for the solidification, no glassy boundary phase was formed on the grain boundary and the grain boundary cracks in the porous alumina were observed. For comparison, non-porous alumina samples were fabricated by unidirectional solidification in Ar atmosphere using alumina feed rods with and without the additives. The former sample with the additives possesses a boundary phase, while the grain boundary cracks were formed in latter sample. The compressive strengths of non-porous solidified alumina samples with and without boundary glassy phase were 1679 and 267 MPa, respectively; that with boundary phase slightly increased by a heat treatment at 1600°C. The compressive strength of porous alumina with 25.9% porosity was 512 MPa which is larger than that of non-porous alumina without glassy boundary phase. The compressive strength of unidirectionally solidified alumina samples were improved by formation of glassy boundary phase at their grain boundary.