Mechanoluminescence and afterglow modulation of Zr and Mg doped strontium aluminate

Mechanoluminescent materials represent a novel category of intelligent luminescent materials capable of directly emitting light when subjected to mechanical forces such as compression, friction, torsion, and even ultrasound. Among the diverse range of mechanoluminescent materials, aluminate compounds demonstrate significant promise as elastic mechanoluminescent materials with exceptional luminescence characteristics. Notably, aluminate materials have also been extensively investigated as long afterglow phosphors during the 1960s, exhibiting broad-spectrum light emission centered around 520 nm when doped with Eu. Capitalizing on its environmentally friendly composition and remarkable long afterglow performance, strontium aluminate materials have been produced on a large scale for industrial applications in various fields. However, the impressive long afterglow and mechanoluminescence properties exhibited by strontium aluminate can be traced back to the internal 4f-5d electron transition process. Previous studies have predominantly examined these two properties in isolation, overlooking the inherent connection between them. This compartmentalized approach has hindered the comprehensive advancement of these materials. To address this limitation, our study aims to explore the intrinsic principles by employing strategies such as cation and anion substitution, matrix structure adjustment, and the use of additives to synthesize strontium aluminate materials through simple solid-state methods. Our investigation focuses on effectively regulating the afterglow, mechanoluminescence, and stress-induced afterglow properties of strontium aluminate simultaneously. In the initial phase of our study, we investigate the influence of boric acid additives and the alteration of the Al/Sr atomic ratio in the matrix on the luminescence properties of the material. Our findings reveal that the enhancement effect of boric acid varies significantly with different doping ions and concentrations, thereby partially elucidating the divergent optimal ion doping concentrations observed in previous studies. Moreover, the role of boric acid is highlighted in its pronounced enhancement of the afterglow with increasing rare earth ion doping. However, excessive rare earth doping fails to further improve the material's mechanoluminescence performance. Subsequently, our investigation revealed that the removal of 10% of Al from strontium aluminate resulted in a significant decrease in both afterglow and mechanoluminescence intensity. However, the decline in afterglow was more pronounced compared to mechanoluminescence. This can be attributed to the excessive presence of Sr atoms, leading to the formation of Sr3Al2O6, which exhibits considerably inferior luminescence performance compared to SrAl2O4. In the subsequent steps of our study, we examined the effects of Zr and Mg doping. Zr doping is proved to be instrumental in enhancing sintering uniformity and luminescence uniformity. The optimal doping concentration was determined to be 2%, resulting in a significant increase of 134% and 278% in mechanoluminescence and long afterglow intensity, respectively. However, higher doping proportions led to a slight decline in the material's mechanoluminescence performance. Building upon these findings, we investigated the impact of Mg doping, where a small quantity of Mg (0.5%) demonstrated a notable improvement in afterglow performance. However, this improvement was accompanied by a reduction of over 50% in mechanoluminescence intensity. The optimal doping ratio was observed at 8%, resulting in a remarkable enhancement of 100.76% in long afterglow intensity. Further increases in the Mg doping proportion yielded a slight decrease in afterglow intensity. Importantly, in all Mg co-doped samples, we observed a discernible deceleration of the light absorption rate. Finally, we compared the stressinduced afterglow curves of six samples and found that Zr/Dy co-doped samples exhibited exceptional stress-induced afterglow, with a duration visible for several minutes. Conversely, the removal of 10% Al atoms from the sample resulted in extremely low stress-induced afterglow, lasting approximately 3 s. The addition of boric acid yielded approximately 4 s of stress-induced afterglow alongside an exceptionally high level of non-stress-induced afterglow. Co-doping with 8% Mg led to a decay of stress-induced afterglow below the level of non-stress-induced afterglow after approximately 2.5 s. Macroscopically, this phenomenon was observed with the naked eye as a darker region in comparison to the surrounding area when subjected to rubbing.