Synthesis And Luminescent Properties Of Rare-Earth Doped Oxide Phosphors For Displaying

Since Y₂O₃:Eu oxides were found to be efficient red phosphors in 1964, the rare-earth luminescent materials developed quickly and played an important role in fields of information displaying, lighting, photoconduction device and so on. Display technology has developed on the direction of high resolution, digitalized, flattening, therefore, new requirements were presented for the researches of rare-earth luminescent materials. On the other hand, the popular of high power infra-laser diode promotes the investigation of upconversion (UC) display devices. In the present thesis, sucrose combustion synthesis was proposed for nano-sized phosphors preparation. Synthetic mechanisms and factors which impact on the surface morphology and luminescent properties were discussed. Using the phosphors for displaying and lighting as investigated subjects in this thesis, the synthetic methods, ways for enhancements of luminescence intensity, color adjustments and coating techniques were investigated both in experimental and theoretical demonstration.Phosphors Y₂O₃:Eu and Gd₂O₃:Eu were prepared using sucrose combustion method. The chelating mechanisms and experimental conditions on the surface morphology and luminescent properties were discussed. The crystallization temperature was lowed after the sucrose combustion; it was found that there was a competition between the heat and gases engendered during the sucrose combustion,which would greatly affect the size, morphology and luminescent properties of particles. Experimental results showed that the surface state Eu³⁺ can be greatly decreased with high post-treated temperature and sucrose content. The results show that the sucrose combustion synthesis method can be used as a low cost and easily generalized preparation way for other kinds of luminescent materials.Stokes and Anti-stokes (upconversion) emissions of monoclinic phase Gd₂O₃:Er with low symmetry were investigated. The dependence of doping concentration on the luminescence properties and mechanisms was discussed in details. With the increasing of doping concentration, luminescence intensities were obviously enhanced. Comparing investigation with cubic Gd₂O₃:Er, the monoclinic phase provides with more lattices, which will be much more efficient for energy transfer and UC emissions. Based on the rate equations, the increasing of red and blue light can be attributed to the energy transfer, 4I11/2+4I11/2→4F9/2+4I13/2, between two Er³⁺ ions nearby. It comes to the conclusion that, the UC emissions of monoclinic phase will be more efficient than the cubic phase Gd₂O₃.UC luminescence enhancing was realized by impurity doping or sensitizer introducing into the host lattice. UC luminescence of Gd₂O₃:Zn,Er showed that the luminescence increase with the Zn2+ doping concentration elevated. Structral analysis revealed that the dopant Zn2+ changes from substitutional doping to interstitial doping and precipitated in the form of ZnO. It was found that the oxygen vacancies by Zn2+ introducing will greatly decrease the symmetry of host lattices, which will result in the luminescence increasing. UC mechanisms of Y₂O₃:Ho,Yb were studied by variable Ho³⁺ concentration. It was found that the intensity of upconverion luminescence increased with the Ho³⁺ concentration decreasing. Ratio equation analysis found that the intermediate level 5I6 and 5I7 will greatly affect the UC mechanisms. The lowing of symmetry for host material by defects introducing or proper ratio of activator and sensitizer will both improve the UC emissions.Color adjusting for UC luminescence was by the way of multi-doping or phosphors mixing. Muti-color UC emissions in Y₂O₃:Ho,Tm,Yb were investigated and the possible energy transfer between Tm³⁺ and Ho³⁺ was discussed. With the increasing of Ho³⁺ concentration, the energy transfer between 5I6 (Ho³⁺) and 3H5 (Tm³⁺) became efficient. Meanwhile, the color of luminescence was modulated. Mixed phosphors were made up of the monoclinic Gd₂O₃:6Er and Gd₂O₃:0.2Tm,3Yb phosphors. The color and correlated color temperature can be tuned by mass ratios. Silica coated on the phosphors will inhibit the re-absorption of the luminescence and also improve the paintability of the phosphors. By mixing different phosphors, the concentration quenching can be easily avoided and the resulting color can be manageable in some extent. Using the most efficiency phosphors, the highest mulit-color can be obtained. In this thesis, sucrose combustion synthesis method was tried to fabricate the luminescent materials and luminescence enhancements by impurity doping or structure changing were discussed. The luminescence color was adjusted by directly co-doping in one host or mixing different phosphors. The silica coating on the phosphors will greatly improve the paintability. The work will greatly decrease the cost of traditional synthesis and promote the application of UC phosphors, which will be an active improvement both in emission mechanisms and applications.