Enhanced thermal degradation stability of the Sr2Si5N8:Eu2+ phosphor by ultra-thin Al2O3 coating through the atomic layer deposition technique in a fluidized bed reactor

Abstract

The red-emitting Sr₂Si₅N₈:Eu²⁺ phosphor with a superior quantum efficiency and suitable emission spectrum has been widely used as a promising down-conversion material in white light-emitting diodes. However, its thermal degradation under high temperature handicaps its large scale application, and therefore must be reduced. Here, we proposed to increase the thermal stability of Sr₂Si₅N₈:Eu²⁺ by coating a nanometer-order Al₂O₃ film on each phosphor particle using an atomic layer deposition approach in a fluidized bed reactor. The deposited Al₂O₃ layer was quite uniform and conformal when using O₃ as the oxidizer, and its thickness could be controlled by the dosage type, deposition temperature and cycle numbers, which largely affects the photoluminescence properties and thermal degradation of the title phosphor. Thermal gravimetric analysis results showed that the oxidation temperature of the coated phosphor increased from 700 to 850 °C, suggesting that the coating layer has the function of anti-oxidation. Meanwhile, the coated phosphor particle surface became hydrophobic. Consequently, the thermal degradation of phosphor powders in air at 200 °C was greatly reduced and the stability of the fabricated LEDs with coated powders was also improved. Prospectively, the proposed approach provides a new strategy to improve the thermal stability of other phosphors.