Simultaneous synthesis and densification of transparent, photoluminescent polycrystalline YAG by current activated pressure assisted densification (CAPAD)

We report a method for the synthesis and processing of transparent bulk polycrystalline yttrium aluminum garnet (VAG) and photoluminescent Ce-doped YAG ceramics via solid-state reactive-current activated pressure assisted densification (CAPAD). The process uses commercially available gamma-Al2O3, Y2O3, and CeO2 nanopowders. The nanopowders were reacted and densified simultaneously at temperatures between 850 degrees C and 1550 degrees C and at a maximum pressure of 105 MPa. The solid-state reaction to phase pure YAG occurs in under 4 min at processing temperatures 1100 degrees C which is significantly faster (on the order of tens of hours) and occurs at much lower temperatures (similar to 600 degrees C) compared to conventional reaction sintering. We found that the reaction significantly improves densification - the shrinkage rate of reaction-produced VAG was three times higher than that of YAG using pre-reacted powder. The Ce additions were found to retard the reaction driven shrinkage kinetics by a factor similar to 3, but are still faster (by a factor similar to 1.6) than those associated with direct densification (no synthesis). Densities > 99% were achieved in both pure YAG and Ce doped VAG (Ce:YAG). Results of optical measurements show good transparency in the visible and photoluminescence (PL) in the Ce:YAG. The PL peak is broad and appears white when excited using blue light confirming that the ceramics can be used in solid state lighting to produce white light. (c) 2012 Elsevier B.V. All rights reserved.