Particle size effects on yttrium aluminum garnet (YAG) phase formation by solid-state reaction

The solid-state reaction of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) during the heat treatment of Y₂O₃ and Al₂O₃ powder mixtures, differing in particle size and size ratio, was quantified using in situ high-temperature x-ray analysis and Rietveld refinement. Y₂O₃ particle size has the most profound effect on YAG formation. When the Y₂O₃ particle size was decreased from 5000 to 30 nm (on reaction with 270 nm Al₂O₃), the YAG formation rate increased from 20 to 48 vol% min^-1 over the temperature range of 1350-1450 °C. In this case, the final YAG content increased from 75 to 91 vol%. A simple model that includes the reactant particle coordination number, and thus particle size ratio, shows that when the size ratio (d_A/d_Y) is >1 diffusion through the alumina powder is rate controlling whereas when the ratio is <1, diffusion through the yttria, intermediate phases, and YAG is rate controlling.