Low oxygen content β-SiAlON ceramics derived from polysilazane as host materials for blue-emitting Ce3+-based phosphors

Abstract

In this study, we exploit a precursor route to form low oxygen content β-SiAlON and host Ce³⁺ centres for phosphor applications. Our approach involves the chemical modification of a polysilazane (PSZ) to form a series of compounds with different Si : Al ratios before pyrolysis in flowing ammonia at 1000 °C, time-controlled exposure to air at room temperature and then heat-treatment up to 1800 °C under flowing nitrogen. The NH₃ pyrolysis of the as-synthesized precursors leads to carbon-free, amorphous compounds highly sensitive to air as discussed based on NMR spectroscopy coupled with elemental analysis. The subsequent air exposure and then heat-treatment in flowing nitrogen allows crystal growth with a dominant phase evolving from α-Si₃N₄ for high Si : Al ratios (Si : Al = 15, 30) to the targeted β-SiAlON phase (Si : Al = 3, 5) as identified by X-ray diffraction at a temperature as low as 1600 °C (β-SiAlON phase content of 92% for Si : Al = 3). The introduction of CeCl₃ (Si : Al = 3; Ce : Si = 0.05) in the early stage of this process significantly affects the high-temperature behaviour of ammonia-treated samples since the β-SiAlON phase content decreases to 54.7%. Thus, the sample exhibits a blue-green emission at around 488 nm when excited at 400 nm and the larger crystal field splitting suggests the positioning of Ce³⁺ ions in the β-SiAlON crystal. In contrast, the investigation of Gas Pressure Sintering (GPS) of ammonia-treated samples at 1800 °C under a N₂ gas pressure at 980 kPa allows forming nearly-pure β-SiAlON-based phosphors – with Ce³⁺ ions aggregated and positioned in the amorphous phase at the grain boundary – which exhibit significant blue emission at around 476 nm when excited at 400 nm.