Processing and characterization of combustion synthesized YAG powders

Precursor powders for yttrium aluminum garnet (Al5Y3O12—YAG) were prepared by combustion of the solution of nitrates (oxidizer) of yttrium and aluminum containing lean amount of glycine (fuel–molar ratio of glycine to nitrate 0.18). The precursor formed by this reaction was a porous mass and was dry ground to a powder. Its thermal decomposition and crystallization behavior was studied using TG-DTA and XRD. The as formed precursor heated up to 880 °C was amorphous. It underwent volatile loss and crystallization into YAP and YAG at 920 °C which transformed to pure YAG at 1050 °C. The crystallization stage was characterized by two exotherms associated with loss of weight. The morphological features of the as formed and 1100 °C calcined precursor by SEM revealed that they were porous agglomerates consisting of fine particle network with fissures and cracks. The ideal conditions for grinding of the agglomerated powder into a fine one were fixed from the results of a combined study of zeta potential and viscosity evaluations. The zeta-potential variation with pH for the suspensions of the 1100 °C calcined powder exhibited a value of ∼35 mV at a pH of 3, a required condition for good dispersion stability for grinding. A study of the rheological behavior with shear rate for slurries containing various amount of solid concentration at this pH exhibited lower and strain rate independent viscosity for solid concentrations of about 20 vol.% (about 25 mPa s), showing absence of flocculates at this concentration, which is essential for effective grinding. Grinding conditions were optimized to obtain powder with size (D50)∼1.5 μm using the aqueous slurry containing 22.5 vol.% of solid concentration at a pH 3. A study of the role of calcination temperature on final sintered densities of the compacts showed that translucent bodies with a maximum sintered density > 98% of the theoretical value could be obtained at 1600 °C in 5 h for the 1300 °C calcined powder.