A New Technology For Preparation Of Y₂O₃: Eu³⁺ And YAG: Ce³⁺ Phosphors By Molten Salt Method

Combined co-precipitation with the molten salt method,a new technology for preparation of Y₂O₃:Eu³⁺and YAG:Ce³⁺phosphors was proposed with the controlled size and higher luminescent intensity.It was investigated systematically for the whole preparation process,including the selection of molten salts,preparation of precursors and optimization of molten salt sintering.The samples were characterized through XRD, SEM,emission spectra,luminescent intensity,color purity,etc.In addition,the crystal constant and structure of samples were also concerned.With rare earths oxide as raw materials,the molten salt method was compared with solid phase method.Some main principles for the selection of molten salt system were obtained:the melting point should be lower the temperature of phosphor preparation;the difference of boiling point and melting point should be as wide as possible;and the molten salt must be not hazardous to luminescent intensity.It was found that the best multiple molten salt system for the preparation of Y₂O₃:Eu³⁺ and YAG:Ce³⁺were(NaCl+S+Na₂CO₃)and(Na₂SO₄+BaF₂) respectively.The results showed that molten salt sintering could improve the crystal degree and configuration of phosphors,resulting in higher luminescent intensity.Using YCl₃ and EuCl₃ as raw material,the preparation of Y₂O₃:Eu³⁺ precursor was investigated concerning some factors,such as temperature, complexing agent,precipitation agent and the dripping mode.The results showed that the size of precursor was the smallest as pH=7 and the complexing agent could control the release velocity of rare ionic effectively to gain fine precursor.The smaller precursor size brought about the smaller sintering sample size.With the citric acid as a complexing agent,the size of precursor and sintering sample was the smallest and the luminescent intensity of sintering sample was the highest. Probably,the citric acid could complex effectively the earth ionic and buffer the pH during the precipitation process in the presence of ammonia and therefore enhanced the precursor density and activity.For the preparation of Y_3-xCe_x)Al₅O₁₂(YAG:Ce³⁺)precursor by co-precipitation,the optimal process condition were:the concentration of salt was as low as about 0.05M,precipitation agent was NH₄HCO₃,pH=8, temperature=90℃,and adverse dripping mode was preferred.Because the precursor was a sol mixture of Y₂(CO₃).nH₂O and (NH₄AlO(OH)HCO₃),it was easy to agglomerate after drying.It was found that the agglomeration problem could be solved by adding active carbon before precipitation.For active carbon,its numerous capillary frameworks might disconnect the sol effectively and its incomplete combustion in sintering was helpful for the deoxidization from Ce⁴⁺to Ce³⁺.Through adjusting components of multiple molten salt and mol ratio of the molten salt to precursor,the optimal sintering conditions for preparation of Y₂O₃:Eu³⁺and YAG:Ce³⁺were obtained.The samples were sphere-like particles whose average size were 1～3μm and 3～5μm and luminescent intensity were 11%and 8%better than commercial phosphor respectively.The results for different sintering temperature indicated that molten salt could reduce activation energy of phosphor like a kind of catalyst,leading to lower sintering temperature than solid phase method.The formation of sphere-like particles might be owing to the surface tension difference between liquid molten salt and phosphor,and the existence of double layer insured the dispersion of particles.The liquid molten salt provided the stable high temperature field and liquid environment and promoted the crystal degree,resulted in the increased luminescent intensity.In addition,the mole ratio of Y:Al:Ce was investigated for increasing luminescent intensity of YAG:Ce³⁺.The results presented that luminescent intensity of sample was enhanced evidently when the mole ratio of Y:Al:Ce was reduced from 2.94:5:0.06 to 2.90:5:0.06 in (Y_3-xCe_x)Al₅O₁₂.It was shown that a little lack of Y in crystal lattice might help to increase luminescent intensity,which coincided with the theory of radiation from crystal lattice defect. At last,the crystal lattice analysis and structure calculation were carried out for the two samples.The structure of Y₂O₃:Eu³⁺and YAG:Ce^{3 +} was body-centered cubic structure and yttrium aluminitum garnet structure respectively.It revealed that the diffraction peak coincided with the standard diffraction card and there was no mixed other impurity, showing that the molten salt would not enter into the crystal lattice of phosphor.Compared with the traditional solid phase method,the new technology can obtain the controlled size and higher luminescent intensity phosphor through only one sintering process,avoiding comminution process required in solid phase method.In addition,it is a new energy-saving process with lower sintering temperature and has a potential application in preparation of phosphor with excellent performance...