Synthesis,Luminescence Properties And Application Research Of Rare-Earth Ion Doped CaGdAlO₄ Micro/Nanophosphors

Rare-earth ion doped inorganic luminescence materials have been widely used in the fields of optical temperature sensing and space solar cells and become a hot research topic.To meet the requirements of the increased sorts and improved performance of oxide-based luminescence materials with good thermal stability and radiation resistance（i.e.to aim to resolve the issues on realizing a wide-range of optical temperature sensing using rare-earth ion doped phosphors and expanding the functional application based on difficultly synthesized gadolinium aluminate nanophosphors）,Tm³⁺/Yb³⁺or Tb³⁺/Yb³⁺codoped CaGdAlO₄micro-and nano-phosphors were prepared by self-propagating combustion and Pechini-type sol-gel methods,respectively.We studied the effects of preparation parameters,doping concentration and Ga³⁺doping on particle size distribution and optical properties of the samples,and we also demonstrated these phosphors the applications in temperature sensing and silicon-based space solar cells.The main results are listed as follows:Firstly,CaGdAlO₄:Tm_x,Yb_y（0.0005≤x≤0.005,0.06≤y≤0.14）micro-sized phosphors were synthesized by a combustion process at 720℃and then heating treatment at 900℃for 3h in air.It is found that the particle size distribution is in the range of 0.60 to 1.74 um.Under980 nm excitation,four emission peaks at 474,651 and 693,801 nm can be observed.They can be attributed to the three-and two-photon processes related to the ¹G₄→³H₆,¹G₄→³F₄and³F_2,3→³H₆,³H₄→³H₆transitions of Tm³⁺ions,respectively.In particular,it can be found that CaGdAlO₄:Tm_0.001,Yb_0.1phosphors exhibit the strongest upconverted luminescence.Based on the combination of fluorescence intensity ratio(FIR_693/801)and full width at half maximum(FWHM₈₀₁)of emission peaks,the optical temperature sensing in the 173-823 K region has been realized using CaGdAlO₄:Tm_0.001,Yb_0.1phosphors.In the temperature range of 300-823K,based on FIR_693/801parameters,the absolute and relative sensitivities are（0.19-6.55）×10^-4K^-1and（2.93-0.38）×10^-2K^-1,respectively.In the temperature range of 173-823 K,based on FWHM₈₀₁parameters,the absolute and relative sensitivities are（4458-2044）×10^-4K^-1and（0.20-0.05）×10^-2K^-1,respectively.In addition,the phosphors exhibited good thermal stability,temperature uncertainty（～1 K）and repeatability（>98%）of optical temperature sensing.Our results also show that CaGdAlO₄:Tm_0.001,Yb_0.1phosphors can be applied in wide-range optical temperature sensing under reduction and/or oxidation extreme conditions.Secondly,CaGdAlO₄:Tm_0.001,Yb_0.1nanophosphors were prepared by Pechini-type sol-gel method.The average particle size is approximately 114 nm.It is found that the preparation parameters,such as calcination temperature and p H value,greatly influence the crystal structure,particle morphology and upconverted luminescence of the target products.Meanwhile,CaGdAlO₄:Tm_0.001,Yb_0.1,Ga_z（0≤z≤0.1）nanophosphors with strong upconverted luminescence were prepared using Ga³⁺doping strategy.The maximum Ga³⁺doping concentration is lower than z=0.06 due to the limitation for maintaining pure phase CaGdAlO₄structure.Under 980 nm excitation,the upconverted luminescence intensities of the Tm³⁺/Yb³⁺/Ga³⁺codoped phosphors at 474 and 801 nm are about 1.44 and 1.09 times of that of the phosphors without Ga³⁺,respectively.It is found that Ga³⁺doping can effectively improve the stability of nanoparticle suspension and the uniform distribution of the composite films.The CaGdAlO₄:Tm_0.001,Yb_0.1,Ga_0.06nanophosphors/epoxy resin composite films show high transmittance of UV（>380 nm）-visible-NIR light and excellent upconverted luminescence due to the uniform distribution of nanophosphors in the composite films.Finally,Tb³⁺/Yb³⁺codoped CaGdAlO₄nanophosphors were prepared by using Pechini-type sol-gel method.It is found that the cross-relaxation processes between[⁵D₃(Tb³⁺)+⁷F₆(Tb³⁺)→⁵D₄(Tb³⁺)+⁷F₀(Tb³⁺)]caused by high Tb³⁺doping concentration under254 nm excitation could be beneficial for the NIR quantum cutting luminescence related to the Tb³⁺→Yb³⁺transition.For CaGdAlO₄:Tb_0.06,Yb_0.04nanophosphor,the strongest NIR quantum cutting luminescence can be obtained.The energy transfer efficiency and quantum cutting efficiency upon 265 nm excitation are 62.6%and 162.6%,respectively.Meanwhile,for Ga³⁺doped CaGdAlO₄:Tb_0.06,Yb_0.04nanophosphors under 254 nm excitation,the intensities of the emission peaks at 545 and 980 nm are increased by about 1.26 and 1.13times,respectively.This can be explained by the increased average particle size leading to the enhanced luminescence.The CaGdAlO₄:Tb_0.06,Yb_0.04,Ga_0.06nanophosphor/epoxy resin composite film exhibited strong deep UV to NIR quantum cutting luminescence.Under 254nm irradiation,the surface temperature of the composite film containing CaGdAlO₄:Tb_0.06,Yb_0.04,Ga_0.06nanophosphors was reduced by～2℃compared to that containing CaGdAlO₄:Tb_0.06nanophosphors.Our results suggest that CaGdAlO₄:Tb_0.06,Yb_0.04,Ga_0.06nanophosphor/epoxy resin composite film is used not only as a surface protective layer but also as a deep violet to NIR quantum cutting layer for silicon-based space solar cells.