Transformation-assisted consolidation of $Y_{2}O_{3}:Eu^{3+}$ nanospheres as a concept to optical nanograined ceramics

A concept is proposed to produce Y2O3:Eu3+ optical nanograined ceramics by the transformation-assisted consolidation of nanospheres under 8 GPa pressure. The nanoceramics were prepared by high-pressure low-temperature sintering (~0.04–0.2Tm, where Tm is the melting temperature) accompanied by the cubic-to-monoclinic phase transition. The effects of sintering conditions upon phase composition, grain size evolution, density, morphology, optical and luminescent properties of sintered ceramics have been studied. It has been shown that Y2O3:Eu3+ nanograined ceramics consisting of individual (cubic or monoclinic) phases or their mixture can be obtained by variation of the sintering temperature. The use of transformation-assisted consolidation makes it possible to prepare Y2O3:Eu3+ nanoceramics with average grain size three times smaller (12 nm) than that of the starting nanopowders (37 nm), which corresponds to extremely low grain growth factor of 0.3. The grain size refinement is related to numerous nucleation events in the parent phase of cubic yttrium oxide. The preparation conditions of translucent (T=50%) composite Y2O3:Eu3+ nanograined ceramics with a relative density of 99±1 % have been determined. The obtained two-phase ceramics show high optical transparency due to negligible birefringence at extremely small (~12 nm) average grain size