Thermal Degradation of Nitride Phosphors

Thermal stability of phosphors for solid state lighting is usually investigated by in-situ measuring the thermal quenching behavior of the sample heated up to ~ 300^oC and held at each temperature for a certain time. However, thermal quenching does not have the same meaning with thermal degradation, which thus cannot evaluate the stability of the phosphor against thermal attacks comprehensively. In this work, we attempted to study the thermal degradation of nitride phosphors (i.e., SrSi₂O₂N₂:Eu and CaAlSiN₃:Eu) by baking the powders in air or in pressued water steam at high temperatures. The degradation mechanism was discussed and clarified by carrying out the surface state analysis using various techniques.

   For SrSi₂O₂N₂:Eu, the PL intensity decreased as the baking temperature rises, degrading by 10 and 30% when the samples baked at 500 and 600^oC, respectively. In comparison with Ca-α-sialon:Eu²⁺ that retained its luminescence up to 800^oC, SrSi₂O₂N₂:Eu showed a very serious thermal degradation. By conducting the high-temperature in-situ XRD analysis and TEM observations, an impurity phase of SrSiO₃ was identified additionally, as a result of the oxidation of the host SrSi₂O₂N₂. In addition, the activator ions were also oxidized during baking in air, with divalent Eu²⁺ changed into trivalent Eu³. It indicates that both of the oxidation of host and activators leads to the thermal degradation of SrSi₂O₂N₂:Eu that is an irreversible decrease of luminescence.

   For (Sr,Ca)AlSin3:Eu (SCASN), the moisture-induced degradation was comprehensively investigated by treating it under a severe condition of high-pressure water steam. The degradation initiated at 150^oC, and the luminescence of SCASN was seen to be quenched quickly, with the powder sample being bleached after the treatment. Both of the microstructure and phase changed obviously with the oxidation, and the host turned finally into NH₃, (Sr,Ca)Al₂Si₂O₈ and Ca(OH)₂. By using a variety of spectroscopic, surface and microstructure analytic techniques, the degradation mechanism was clarified and proposed to occur viathe oxidant-gas penetration mechanism through the moisture-enhanced oxidation of both the SCASN host and divalent europium.

References:

1. C.Y. Wang, R.-J. Xie, F.Z. Li, and X. Xu, J. Mater. Chem. C. 2, 2735 (2014).

2. J. Zhu, L. Wang, T. L. Zhou, Y.J. Cho, T. Suehiro, T. Takeda, M. Lu, T. Sekiguchi, N. Hirosakia, and R.-J. Xie, J. Mater. Chem. C. 3, 3181 (2015).