Thermal Decomposition and Phase Evolution Behaviour of Precursors for Y₃Al₅O₁₂ Prepared by Homogeneous Precipitation

Abstract

Formation of precursors for yttrium aluminium garnet (Y₃Al₅O₁₂—YAG) composition by homogeneous precipitation method was studied. Two reactions occurring in the aqueous system—aluminium nitrate-yttrium nitrate-urea and aluminium sulphate-yttrium nitrate-urea, at 100°C were studied. The former reaction yielded a gelatinous precipitate while a granular precipitate was obtained by the second reaction. The gelatinous precipitate after washing and drying in air oven at 90°C formed a strongly agglomerated mass. However, final washing of the gel with acetone was found to yield an easily friable mass. The granular precipitate formed by the latter reaction, after washing with water and drying in an oven, was found to be quite friable. Both precursors were amorphous and contained carbonates. A detailed thermogravimety-differential thermal analysis and X-ray diffraction studies revealed that the former precursor exhibited a continuous loss of weight upto 980°C and formed an amorphous oxide compound of aluminium and yttrium which crystallized into yttrium aluminium garnet at 1040°C. No loss of weight occurred at this stage. The latter precursor lost weight in stages (endotherms at 860° and 1140°C), formed crystalline intermediates (mixture of phases Y₂O₃, Y₄Al₂0₉ and Y₃Al₅O₁₂) above 1140°C and finally formed a single phase YAG after calcining at 1400°C. This difference in the behaviour of formation of phases from the precursors has been attributed to the difference in the degree of compositional homogeneities in the respective precursor powders. The particle size distributions of the as-formed precursors were in the range of 5–10 μm while by grinding it could be reduced to the range of 0.5–1 μm. The particle size distributions of the oxides obtained by calcining the ground precursors at 1200°C remained the same as those of the precursors.