Synthesis And Characteristics Of YAG:Ce³⁺Phosphor For White LED

LED is a kind of lighting equipment with a promising prospective. Recent research work mainly focus on the development of white LED. For white LED, a trivalent cerium-doped yttrium aluminum garnet fluorescent(YAG:Ce³⁺) powder with a high luminous efficiency and a chemical stability has been widely used as a yellow phosphor powder. The technology is to make a blue emitted yellow fluorescent powder coated on the blue light emitting InGaN chip for blue excitation under a yellow light and to match the rest of the blue light, leading to a white light. Therefore, the high quality of YAG:Ce³⁺ phosphor with low costs has a promising application. The existing methods of preparing YAG:Ce³⁺ phosphor include solid phase reaction method, sol-gel method, coprecipitation method, etc.. The most commonly used synthesis method in the industry is high-temperature solid state method.The first chapter of this dissertation mainly introduces the development process of modern LED, the research status as well as the preparation methods of rare-earth element doped phosphor powder. Also, the objectives and the main research contents of this dissertation are given.In the second chapter, the information regarding raw materials, equipment and apparatus used（such as laser particle size analyzer, X-ray diffractometer, scanning electron microscope,and fluorescence spectrometer） are given. The related principles of equipment, material preparation, experimental conditions and procedures are represented.In the third chapter, a lab-scale dry vibration mill was used for the soft mechanochemical treatment of the raw material and flux mixture. The effect of soft mechanochemical treatment on the particle size distribution and the crystallinity of Y2O3 crystal in the treated material at different durations was investigated. The precursor powders with and without mechanochemical treatment were sintered by a solid-state sintering method. The crystal phase compositions of sintered product were determined to analyze the reaction activity of the precursor powders treated by mechanochemical process. The results show that the efficiency of mechanochemical process can be of great effective within 1 h, and the median particle size decreases dramatically. However, for the treatment for more than 1 h, the median size changes slightly and the efficiency of grinding reduces significantly. According to the XRD patterns,the main crystals of Y2O3 in the raw materials structure are damaged to a certain degree after the raw material is ground in the vibration mill for 1 h, leading to the serious lattice damage and higher activity and improving the crystallization degree of the sintered sample in subsequent solid-state sintering. It was found that in the incomplete treatment in the vibrationmill for a certain duration, the aggregation of particles occurs due to the decreased potential energy and specific surface area, leading to the decreased reaction activity of the particles in the material treated.The forth chapter analyzes the soft mechanochemical treatment of the raw materials in a lab-scale stirred bead mill at different parameters such as filling rate of grinding media,grinding time, rotation speed of stirrer and amount of dispersion. The results show that soft mechanochemical treatment by the stirred bead mill can reduce the particle size of raw material effectively and destroy the lattice structure as well, thereby obtaining the precursor powder with a high reaction activity. According to the orthogonal experiment, the filling rate of grinding media, solid content, rotation speed of stirrer and grinding time all can have a great effect on the result of soft mechanochemical process. The effective soft mechanochemocal treatment can be obtained at optimum process parameters（i.e., filling rate of grinding media of 50 %, solid content of 20 wt. %, rotation speed of stirrer of 2600 r/min,and grinding time of 60 min）.The fifth chapter compares the efficiencies of soft mechanochemical treatments in stirred bead mill and vibration mill, respectively, as well as the corresponding sintering results obtained via subsequent solid-state reaction. Compared to vibration mill that the grinding is dominated due to impact forces, stirred bead mill mainly utilizes compression and shearing between beads for effective grinding in the initial duration,leading to the effective reduction of the particle size. The further treatment process is dominated due to friction and shear stress. The treatment by mechanical mill can damage all lattice planes to different degrees. According to the XRD patterns, the intensity of the characteristic peak decreases, and the peak width increases, giving the increase of the amorphization. It is found that stirred bead mill is more effective than vibration mill for soft mechanochemical treatment. In addition, the complete single YAG crystalline phase can be obtained with the precursor powder pre-treated by stirred bead mill after sintering at 1300 ℃,indicating that the sintering temperature of raw materials without any pretreatment（i.e.,1600 ℃） can be reduced by 300 ℃.In the sixth chapter, the solid-state reaction process of YAG:Ce³⁺ was analyzed. It is indicated that the solid-state reaction process is carried out step by step, and the particle diffusion rate is a key factor to the solid-state reaction. The results show that the soft mechanochemical process can effectively reduce the sintering temperature for the synthesis of YAG:Ce³⁺. Besides, this chapter also discussed the luminescent properties of YAG:Ce³⁺fluorescent powder. Based on the phosphor excitation spectra and emission spectra, amaximum excitation peak occurs at 340 nm in the near ultraviolet and at 460 nm in visible light（i.e., blue light）. Compared to the color coordinates of commercial phosphor, the color of YAG:Ce³⁺ fluorescent powder appears in the yellow region, showing that the powder can be used for coating.In the seventh chapter, a thin film was prepared with YAG:Ce³⁺ fluorescent powder by an electron-beam evaporation method. The annealing process as well as formation mechanism and the luminescence properties of the phosphor thin film were discussed.Finally, this dissertation also gives the major conclusions and the prospect of the future work based on the experimental results obtained.