Sintering, spectral and laser properties of Nd:YSAG transparent ceramics (Nd3+:Y3ScAl4O12, 2%at)
Résumé
Rare-earth doped garnets (YAG:Nd, YAG:Yb, YSAG:Nd, GGG:Nd) are usually used materials in the field of high energy femtosecondes lasers. In one hand, the as-mentioned garnet phases present sufficiently high thermo-mechanical properties to withstand solicitations created at high working power. In other hand, the rare-earth ion introduced in solid solution in garnet matrix presents the required luminescent properties to produce laser amplification. Generally, amplifier medium pieces are manufactured in the single-crystal form by crystal growth methods (such as Czochralski method for example). Nevertheless, such materials present very long time and high costs with manufacturing by this route. By using the promising ceramic route, the transparent polycrystalline materials so-obtained can show similar optical properties to the single-crystal ones. To reach optical properties necessary to a laser application, numerous problems related to the elaboration of rare-earth doped garnet ceramic pieces must be previously solved. More particularly, experimental parameters such as the average grain size, volume fraction of residual secondary phases or porosity, must be controlled to obtain expected transparency and transmittance. The aim of the present work is to understand well the microstructural evolution of YSAG (Y3ScxAl5 xO12) ceramics by taking into account the presence of silica as a sintering aid. If the YAG-based ceramics production starts to be well controlled, sintering mechanisms remain to be explored for YSAG ceramics. In the present study, the role of addition agents (SiO2) on the densification and grain growth processes for neodymium-doped YSAG matrix has been studied. The samples have been prepared by ball milling of pure oxides, shaped by cold uniaxial pressing and sintered under vacuum between 1200°C and 1700°C. After treatment, the samples were characterized by different techniques: EPMA, SEM, TEM and thermal analyses. Optical properties of Nd:YSAG were finally correlated to microstructural characteristics and laser emission with broadband absorption was demonstrated.