Crystallization behaviour of Yb-doped and undoped YAG nanoceramics synthesized by microwave-assisted urea precipitation

Yb-doped and undoped Y3Al5O12 (YAG) nanometric powders were synthesized by microwave assisted liquid synthesis from nitrate precursors and calcination. The homogeneous and rapid heating of the reaction volume of this synthesis process allowed a precise control on particle nucleation and growth and the formation of nanometric primary particles with a narrow size distribution. The chemical composition of synthesized powders was checked by ICP and FTIR analysis. The morphology of the precursor and calcined powders as well as the microstructure after sintering were analysed by SEM. High temperature XRD and TG-DTA analyses, performed in the 800-1200 °C range, with two temperature ramps, made it possible for a thorough investigation of the crystallization behaviour of YAG and Yb:YAG precursors. The results show that the crystallization of Yb:YAG from the powder precursors started at 900 °C and that both the presence of Yb and a high heating rate accelerate the YAG formation through the intermediate YAH (hexagonal YAlO3 phase). The intermediate phases Y4Al2O9 (YAM) and YAlO3 (YAP) that are generally observed during the calcination step of nanometric YAG powder did not form. Pellets of pressed powders were sintered under high vacuum in clean atmosphere at 1700 °C for 16 h and a full density translucent material with grain size from 3 to 5 μm was obtained.