| With the increase of white light emitting diode (LED) indoor lighting penetration, the performance requirements for white LED have been converted from high luminous efficiency to high color rendering index (CRI), low color temperature and high efficiency. For white LED fabricated with the combination of blue chips and yellow phosphors, red phosphors play an important role in increasing CRI and lowering color temperature. Nitride red phosphors have attracted much attention because of their good physical and chemical stability, excellent spectral characteristics and so on. However, due to high melting point and chemical inertness of raw materials, the synthesis conditions of nitride phosphors are usually very harsh. To obtain nitride phosphors with high phase purity and excellent luminescent properties by low cost becomes the common scientific and technical challenges worldwide. In the commercialized nitride red phosphors, M2Si5Ng:Eu2+(M=Ca, Sr, Ba) phosphor has high quantum efficiency and excellent spectral characteristics. Moreover, it can be synthesized under the condition of relatively low temperature (1500-1700℃) and atmospheric pressure. However, the thermal quenching properties of M2Si5N8:Eu2+red phosphors are poorer than that of CaAlSiN3:Eu2+phosphor, which limits its application in high-performance white LED. To improve the thermal quenching properties of M2Si5N8:Eu2+phosphors are very important for the application of the materials.In this thesis, the structure, spectra and thermal quenching properties of M2Si5N8:Eu2+ phosphors are studied in detail with the purpose of improving the thermal quenching properties firstly. Then, Eu2+activated SrSiN2, SrAlSi4N7 and CaAlSiN3 phosphors are prepared by using M2Si5N8 as raw materials. The structure, luminescence and the synthesis mechanism of these phosphors are investigated in detail.The preparation, structure and luminescence properties of M2-xSi5N8:xEu2+phosphors are systematically studied. As increasing x value, the thermal quenching properties of Sr2-xSi5N8:xEu2+phosphors are improved, reaching an optimal value when x=0.05, then decreased with further increasing of x value. This may be ascribed to the variation of probability for Eu2+occupying different Sr sites (Sr1 and Sr2). As increasing Eu2+ concentrations, the probability for Eu2+occupying Sr2 sites increases and average Eu-N bond length is shortened, which improves the thermal quenching properties of the samples. However, when Eu2+concentration exceeds quenching concentration, the thermal quenching properties are reduced due to the enhanced interaction between the Eu2+ions.The Sr1.95-xCaxSi5N8:0.05Eu2+and Sr1.95-xBaxSi5N8:0.05Eu2+phosphors have been successfully synthesized by Sr/Ca and Sr/Ba solid solution, respectively. The structure, spectra and thermal quenching properties of these phosphors are studied in detail. For the Sr1.95-xCaxSi5N8:Eu2+phosphors, the thermal quenching properties are reduced with increasing x values. When x<1.2, the samples crystallize in Sr2Si5N8 phase. With increasing x value, the Stokes shift increases, which reduces the thermal quenching properties of the samples. When 1.2≤x<≤1.6, there coexist Sr2Si5N8 and Ca2Si5N8 two phases. As x values continue to increase, the samples crystallize in Ca2Si5N8 phase. Due to the poor structure stability of Ca2Si5N8 phase, the thermal quenching property of the samples is greatly reduced. For the Sr1.95-xBaxSi5N8:0.05Eu2+phosphors, complete solid solutions are formed between Sr2Si5N8 and Ba2Si5N8. It is noting that the thermal quenching properties are enhanced when 1.0<x<1.5. The possible reasons are proposed. When x<1.0, Ba2+ions preferentially occupy the Sr1 site, due to the long average Sr-N bond length. When 1.0≤x≤1.5, the Sr1 sites are completely occupied by Ba2+ions. As the radius of Eu2+ions is closer to that of Sr2+ions, Eu2+ions gradually occupy the Sr2 sites. In this case, the average Eu-N bond length is shortened, which improves the thermal quenching properties of the samples.The Sr2AlxSi5-xN8-δ:Eu2+phosphors are successfully prepared by partial Al-N substituting Si-N. The structure, spectra and thermal quenching properties are investigated in detail. It is found that the optimal Al doping content is less than x=0.7. The crystal structure is studied by using X-ray powder diffraction, transmission electron microscope and nuclear magnetic resonance spectroscopy. The results indicate that Al ions are successfully incorporated into the host lattice and exist in the form of [AIN4] tetrahedron. The spectral data show that partial Al substituting Si has little influence on the emission peak-wavelength, color coordinates and the brightness of the samples. The introduced Al ions change the microenvironment surrounding Eu2+ions, which enhances the covalent of Eu-N. Therefore, the thermal quenching properties of the samples are improved. However, the impurity phase is detected when the Al content is too high, which reduces the thermal quenching properties.Base on the special structural characteristics of M2Si5N8, Eu2+doped SrSiN2 and SrAlSi4N7 red phosphors are successfully synthesized by using N2Si5N8 as raw materials. The synthetic mechanism and the luminescent properties are studied in detail. It is noting that the external quantum efficiency of phosphors prepared by this method is higher than that of samples synthesized by conventional method. This may be mainly ascribed to better crystallization and moderate preferential orientation of the crystal structure. The luminous efficiency of SrAlSi4N7:Eu2+phosphor is greatly improved by the co-doping of Ce3+ions. The mechanism of energy transfer should be the electric dipole-dipole interaction. For the study of CaAlSiN3 synthesized by Ca2Si5N8, the reaction mechanism is confirmed to be Ca2Si5Ng8â†'CaSiN2â†'CaAlSiN3. Finally, we propose a new idea to develop novel nitride phosphors in Sr2Si5Ng-AlN-Si3N4 ternary system based on the above results. |
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