HR-TEM study of the silicon segregation at grain boundaries in Yb:YAG ceramics

Segregation of selected elements at the grain boundaries of YAG based ceramic materials for laser applications influences the optical properties and consequently the laser efficiency. Silicon used in the production of Yb3+:YAG to promote the sintering may segregate at the grain boundaries during post-sintering air annealing cycles, and may lead to the formation of amorphous or crystalline secondary phases (Y2SiO5) that decrease the optical properties and compromise the use of the material as a laser source. [1] On the other hand, silicon is an effective sintering aid and after vacuum sintering the annealing cycle is a necessary step, since it promotes the Yb2+ to Yb3+ conversion, required for the laser emission. The segregation coefficient of silicon in YAG decreases with temperature, thus through a proper annealing cycle the formation of undesired secondary phases can be avoided. The present work illustrates a way to control the silicon segregation through the balancing of the temperature and soaking time during the annealing treatment. The Si segregation at the grain boundaries of 5 and 10 at % Yb3+: YAG doped with 1.425 at % Si was studied by Transmission Electron Microscopy (High Resolution TEM and EDX nano-probe chemical analysis) after selected annealing cycles. Under optimized annealing conditions the silicon amount at the grain boundaries was similar to that in the YAG grains, and consequently the laser efficiency of the material improved. Samples obtained with different processes are tested in the same laser cavity set-up. [2] 1. T. Epicier, G. Boulon, W. Zhao, M. Guzik, B. Jiang, A. Ikesue and L. Esposito, "Spatial distribution of the Yb3+ rare earth ions in Y3Al5O12 and Y2O3 optical ceramics as analyzed by TEM", J. Mat. Chem., 2012, 22, 18221-18229. 2. This research program is supported by a bilateral joined project CNRS-France / CNR-Italy. Thanks are due to the CLYM (www.clym.fr) for the TEM observations.