Status of Experimental Studies on the Sol-Gel Synthesis of Ceramic Powders for High-Strength Ceramic Materials Applications

摘要

Sol-gel processes are being investigated to prepare sinterable ceramic powders of alumina toughened with 10 to 30 vol % phase-stabilized zirconia and of alumina reinforced with silicon carbide whiskers. Two approaches are under investigation for the synthesis of gel powders of Al sub 2 O sub 3 -ZrO sub 2 : (1) gelation of the mixed-oxide precursor sols, and (2) direct precipitation of the mixed-oxide gels from solutions of the metal salts. Either approach yields powders of good sinterability in which Al sub 2 O sub 3 and ZrO sub 2 are homogeneously mixed. The powders sinter to near theoretical density by hot pressing to give fine-grained microstructures (approx.1 mu m). Powder sinterability and compositional uniformity are strongly influenced by the pH at which the gel powder is formed. The best sinterability is attained at pH values such that the surface charges on ZrO sub 2 and Al sub 2 O sub 3 are of the same sign, which reduces agglomeration. Methods for controlling the homogeneity of the phase stabilizer for ZrO sub 2 (usually 1 to 5 mole % Y sub 2 O sub 3 ) were also investigated. Good homogeneity of Y sub 2 O sub 3 is most readily attained by effecting the sol-gel transformation at neutral or alkaline conditions. Alumina powders reinforced with 10 to 20 vol % SiC whiskers were prepared by dispersing the SiC whiskers into an alumina sol of pH 3.5, where the surface charges of SiC and Al sub 2 O sub 3 are of opposite sign, and then gelling this dispersion. The whisker dispersions are uniform throughout the powder particles and appear uniformly coated by the alumina. (ERA citation 12:031132)