Influence of HIP post-treatment on the microstructure and luminescent properties of SPSed Al2O3−Ce:YAG composite ceramics

Fine-grained Al₂O₃−Ce:YAG composite ceramics with 0.05-0.3 at% Ce³⁺ doping were prepared by single reactive spark plasma sintering (SPS) and a two-step approach combining SPS and hot isostatic pressing (HIP) techniques. It was revealed that application of HIP provided additional homogenization of the Ce:YAG solid solution throughout the material volume, while changes in the recrystallization kinetics of constituent phases contributed to the stabilization of garnet/alumina grains sizes. An increase in external quantum efficiency in the series up to 72% and 76% was observed with saturation occurring at 0.2 at% Ce³⁺. The synchrotron radiation luminescence (SRL) kinetics allowed distinguishing two modes. In comparison with PL kinetics, the underestimation of decay time τ_slow values is explained by the SR effect, which similarly to the thermal effect can also induces the formation of radiation defects and the ionization of Ce³⁺ 5d electrons. Both PL and SRL studies revealed the presence of concentration quenching effect in phosphors with 0.3 at% Ce³⁺. Measurements under excitation by blue laser diodes (LDs) in the reflection mode showed that 0.1-0.2 at% Ce³⁺-doped samples after HIP treatment exhibited the most favorable characteristics for application in high-power white laser lighting. White LDs equipped with 0.4 mm-thick Al₂O₃–0.2 at% Ce:YAG showed an optimum correlated color temperature of 6499-6602 K, a high luminous flux of 222-2500 lm, and luminous efficiency of 242-232 lm·W^-1 as LD power increased from 0.8 to 9.5 W·mm^-2.