Composition-dependent thermal degradation of red-emitting (Ca1−xSrx)AlSiN3:Eu2+ phosphors for high color rendering white LEDs

Abstract

Luminescence degradation under moisture and thermal attacks during the term of service cannot be ignored for phosphors used in solid state lighting. In this work, the moisture-assisted thermal degradation of two commercially available red phosphors (Ca_1−xSr_x)AlSiN₃:Eu²⁺ with different Sr/Ca ratios (RP625: Sr/Ca = 0.9/0.1; RP660: Sr/Ca = 0/1) was investigated by treating phosphor powders at 170–230 °C for 8–96 h in pressurized water steam. The experimental results show that the chemical composition has a great impact on the thermal stability of (Ca_1−xSr_x)AlSiN₃:Eu²⁺, and a lower Sr/Ca ratio results in a smaller thermal degradation. The activation energies for thermal degradation are 66.26 and 72.13 kJ mol⁻¹ for RP625 and RP660, respectively. The phase transformation, microstructure evolutions, diffuse reflectance spectra, photoluminescence and cathodoluminescence spectra of samples before and after thermal treatment were discussed to reveal the thermal degradation mechanism. The serious degradation observed in RP625 is caused by the decomposition of the phosphor host and the oxidation of the Eu²⁺ dopant, which is closely related to the increased bond length of Sr/Ca/Eu–N chemical bonds.