Attenuation Mechanism Of Sr₂Si₅N₈ And Strategy For Improving Structural Stability

In recent years,wLED（White Light Emitting Diode）are considered to be the new generation of green lighting sources,and these light sources have become an integral part of our life,owing to their low energy consumption,high efficiency,high brightness,long lifetime,and environmentally-friendly features.According to the principle of the three primary colours of light,people usually prepare wLED with a combination of red,green and blue phosphors,of which the red emission component is an important factor in achieving warm LED with high color rendering index,so the nitride red phosphors of Sr₂Si₅N₈:Eu²⁺has attracted much attention due to its mild synthesis conditions,excellent luminescence efficiency and suitable emission wavelength.However,the occurrence of thermal degradation results in a decrease of their luminous intensity and quantum efficiency at the high working temperature of high-power wLED.In this paper,the researches were carried out by investigating the thermal degradation mechanism of Sr₂Si₅N₈:Eu²⁺and enhancing its thermal stability.At first,Sr₂Si₅N₈:Eu²⁺phosphors were prepared by a high-temperature solid-state reaction,undergo severe thermal degradation,not only in air atmosphere,but even in highly pure N₂ gas.After heat treatment in nitrogen,there was no evidence of alterations in the crystalline regime or in the Eu-oxidation state after the heat-treatment,which was different from the thermal degradation mechanism in air.Moreover,the analyses of the results revealed that after heat-treatment in N₂,the bonds between Si atom and O impurity inside the crystal lattice become unstable,resulting in their detachment and in formation of local nano-defects.Then it leads to the destruction of the original lattice structure and to the formation of deep defect energy levels.The electrons frozen in the deep defect energy levels are unable to recombine with holes in the ground state to produce luminescence,which causes a decrease in the luminous intensity of the phosphor.Based on the novel mechanism for the thermal degradation,a Sr₂Si₅N₈:Eu²⁺phosphor was coated by a nano carbon layer by chemical vapor deposition and the carbon-coated powders were further annealed in N₂ atmosphere at a high temperature to trigger the carbothermal reaction.On the one hand,the carbothermal reaction resulted in the removal of oxygen in the phosphor particle,which reduced the formation of the crystal defects and the oxidation of the host lattice and luminescent ions.On the other hand,the nano carbon layer crystallized,forming a graphene-like multilayer that protected the phosphor particle from the penetration of the external oxygen.This eventually resulted in a significant improvement of the thermal stability and oxidation resistance of the phosphor.The wLED prepared from the modified powders has excellent performance indicators and meet the needs of solid state lighting.In addition,the carbon-coated and annealed phosphor also exhibits resistance to hydrolysis,with essentially no change in luminous intensity after temperature and humidity tests.Therefore,present study provides a new insight into the thermal degradation mechanism and proposes an effective method to improve the thermal stability,which favors solving the shortcomings of Sr₂Si₅N₈:Eu²⁺phosphor in the process of industrialization and has great potential for application in the wLED field.