YAG has excellent optical, mechanical, thermal and high temperature stability. Rare earth doped YAG transparent materials are the best performance, most versatile and most productive solid-state laser gain media materials for China's national defense. Science and technology have important strategic significance and have important applications in many fields such as radiation detection scintillators and LEDs. At present, YAG transparent materials are mainly composed of single crystal and transparent ceramics. The former is mainly prepared by directional solidification technology, and has problems such as long crystal growth period, high cost, difficulty in increasing the size and doping concentration, and the like. YAG transparent ceramics mainly adopt powder sintering preparation method. To achieve high transparency, it is necessary to completely exclude pores and impurity phases in ceramics, which are completely dense and extremely thin grain boundaries, and have extremely demanding requirements on raw materials, equipment, molding and sintering processes, such as High-purity, highly dispersible nano-powder raw materials, hot isostatic pressing and vacuum sintering are required, and the crystal grains are easily grown in a long-time high-temperature sintering process.
The team led by Li Jianqiang, a researcher at the Institute of Process Engineering of the Chinese Academy of Sciences, proposed the innovative idea of ​​preparing YAG transparent ceramics by solidification-amorphous crystallization, and carried out systematic research. Firstly, in view of the problem of poor melt amorphous formation ability of YAG component, the nucleation under deep subcooling condition of the melt was suppressed by the containerless solidification technique, and the influence of cooling rate on the phase and structure of the solidified product of the melt was obtained. Furthermore, the amorphous forming ability of Al2O3-Y2O3 melt was strengthened by adjusting the Al2O3/Y2O3 ratio, and Al2O3-26mol% Y2O3 bulk amorphous alloy with complete amorphous state and uniform composition distribution was successfully prepared. Further, in collaboration with the French National Science Center (CNRS) researcher Mathieu Allix, the crystallization behavior of amorphous matrix under pressureless conditions was studied, and the influence of crystallization temperature and time on the phase and microstructure of the product was obtained. Heat treatment (~1100 ° C) one step to obtain a fully crystallized YAG-based transparent ceramic. The obtained YAG-based transparent ceramic is composed entirely of nanocrystals, the YAG phase mass fraction is as high as 77%, and the rest is Al2O3 phase, and the chemical composition and microstructure distribution are highly uniform. It is found that the rare phase of Al2O3 around the YAG grains in the nanocomplex structure has a good effect of alleviating the crystallization shrinkage stress and inhibiting the growth of YAG grains. The YAG grains can be maintained at the nanometer level by heating to 1400 °C. YAG-based transparent ceramics have extremely high transmittance, comparable to YAG single crystals in the near-infrared and mid-infrared bands, and 10% higher in hardness than YAG single crystals and transparent ceramics. Excellent fluorescence performance after Ce doping, quantum efficiency up to 87.5 % (pictured). The excellent optical and mechanical properties make this new nanocrystalline YAG-based transparent ceramics have important application potential in high-power LED, LD and other fields.
This new method of preparation of transparent ceramics combines the advantages of solidification and amorphous crystallization methods. On the one hand, the solidification of the bulk amorphous matrix is ​​not only fast, can form complex shapes and various sizes, but also can be used as a conventional raw material powder, avoiding the dependence of the conventional powder sintering method on high quality powder. On the other hand, the amorphous crystallization process has the characteristics of mild conditions, simple process, uniform product, high doping concentration, and easy control of grain size. The related results were published on March 21st in "Pressureless glass crystallization of transparent yttrium aluminum garnet-based nanoceramics, Nature Communications, 9, 1175 (2018), DOI: 10.1038/s41467-018-03467-7). . Li Jianqiang and Mathieu Allix are co-authors. Dr. Ma Xiaoguang, a research engineer, and Li Xiaolan, an associate researcher, are co-first authors. The research was supported by the National Natural Science Foundation (Grant Nos. 51471158, 51674232, etc.), the Beijing Natural Resources Fund (No.2152032) and the Chinese Academy of Sciences International Talent Program (Grant No. 2017VEA0010), and has applied for international patents ( PCT/CN2018/078307).
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