Crystal Phase and Sinterability of Wet-Chemically Derived YAG Powders.

Abstract

Four wet-chemical methods (normal strike co-precipitation, reverse strike co-precipitation, urea homogeneous precipitation, and a modified Pechini process) were adopted in the present work to synthesize the precursor for a sinterable YAG powder. The X-ray diffraction (XRD) results revealed that the phase development of the precursor during calcination and the formation temperature of pure YAG vary among the four methods. The lowest temperatures for pure YAG formation were determined by the XRD method to be 850°C, 950°C, 1200°C and>1200°C for the modified Pechini method, reverse strike co-precipitation, normal strike co-precipitation and urea homogeneous precipitation, respectively. The differences concerning YAG formation among the four methods were explained from the view point of the chemical homogeneity of the precursor. That is, higher chemical homogeneity resulted in the lower temperature of YAG formation. Except the case of modified Pechini method, the sinterability of the YAG powders increased with a decrease in the above temperatures for pure YAG formation because the YAG powder produced from the precursor with higher chemical homogeneity showed smaller particle size and weaker agglomeration. The YAG powder synthesized by the modified Pechini method showed the poorest sinterability due to its extremely hard aggregates caused by the direct pyrolysis of the mother salts. The most sinterable YAG powder, produced by the reverse strike co-precipitation method, densified up to 91.86% of its theoretical density after sintering at 1520°C with a constant heating rate of 8°C/min. While the YAG powder synthesized by the modified Pechini method only densified up to 69.2% of its theoretical density under the same sintering conditions.