Effects of ZrO 2 ultrafine particle additive on the formation of Al 2O 3 from sulfate

Abstract

An aqueous solution of aluminum sulfate containing ultrafine ZrO2 particles was dehydrated at 300°C to prepare the starting material of ZrO2-Al2O3 composite. The effects of ZrO2 additive on the decomposition of anhydrous sulfate into η-Al2O3, on the η→α phase transformation and on the sinterability of produced ZrO2-Al2O3 composite powder were studied. The apparent crystallite size of the anhydrous sulfate decreased with increasing ZrO2 content from 42nm (no additive) to 18nm (20vol% ZrO2). The composite oxide powder, in which aggregated particle (50-100nm in diameter) of tetragonal ZrO2 dispersed in the secondary grains of η-Al2O3, was obtained by decomposing the anhydrous sulfate. In the composite powder containing 20vol% ZrO2, α-Al2O3 was found above 1100°C, while it formed above 1200°C for the single component Al2O3 powder. The grain growth of Al2O3 was prevented by ZrO2 particles dispersed in the η-Al2O3 grains; that is, the primary particle size of Al2O3 decreased with increasing ZrO2 content from 120nm (no additive) to 60nm (20vol% ZrO2) after calcination at 1250°C for 1h. The composite powders had very low green densities and they were difficult to sinter. But ball-milling of the composite powders yielded high densities up to 94% for 10vol% ZrO2 bodies fired at 1600°C for 1h in air. The ZrO2 particles were located at the boundary of Al2O3, and the grain size of Al2O3 decreased from 3.0μm (no additive) to 1.2μm (10vol% ZrO2) as the content of ZrO2 increased.