Sol-gel processing and sintering of yttrium aluminum garnet (YAG) powders

Gels of yttrium aluminum garnet (YAG) with the stoichiometric composition 3Y₂O₃·5AI₂O₃, were prepared by a sol-gel technique and dried by supercritical extraction with CO₂. Powders were produced by lightly grinding the dried gels. Crystallization of the powder occurred at 900°C and within the limits of detection, the X-ray diffraction pattern of the crystallized material was identical to that of the stoichiometric composition. Powder compacts with a green density of 0.50 of the theoretical were sintered to nearly full density in O₂ during constant heating rate sintering at 5 °C min^–1 to 1600 °C. This is better than the density obtained with powders from a similar gel dried conventionally (by evaporation of the liquid) and considerably better than that obtained with powders prepared by solid state reaction. The room temperature flexural strength and fracture toughness of the material fabricated from the supercritically dried gels were 190 MPa and 2.2 MPa.m^1/2, respectively. These strength and fracture toughness values are higher than those reported in other studies for YAG produced by the sintering route.