Study On Coating Technology And Properties Of Rare Earth Composite Particles

For its excellent physical and chemical properties, rare earth luminescent materials have considerable applications in almost any devices involving fluorescent lighting devices and the display devices, such as tricolor lamps, cathode ray tubes (CRTs), liquid crystal displays (LCDs), field emission displays (FEDs), and plasma display panels (PDPs), etc. With the development of nanocomposites, the composition of rare earth nanomaterials is an important issue in material science. By composition, a lot of new nano structural materials and functional materials with special morphology and size can be synthesis controllably according to the willings of peoples. In this thesis, the core-shell structure of rare earth nanomaterials had been synthesized by coating, which resolved the problem of preparing directly the spherical rare earth luminescent materials. In addition, using the inorganic barium sulfate spheres as cores, the quantities of rare earth are economized and the cost of phosphor is decreased, which is valuable for saving the rare earth resource of our country.First, the microspherical particles of barium sulfate were successfully synthesized by precipitation method, in which EDTA were used as chelating agent. The size and morphology of the obtained BaSO4 particles can be controlled during the process. X-ray diffraction (XRD), Fourier transform infrared resonance (FTIR), and scanning electron micrograph (SEM) were used on characterize the structure and morphology of BaSO4 particles. The effects of the preparation parameters, such as EDTA, pH, concentration of raw materials, reaction temperature etc, on particle distribution in size and morphology are experimentally researched. Complexation stability constant of Ba-EDTA at different pH was calculated. The results revealed that the size and morphology of BaSO4 particles can be effectively controlled by addition of EDTA in the precipitation process. Among all the operation conditions, the pH value had significant effect on the particle size. The obtained barium sulfate particles are spherical with 1μm average particle diameter and well dispersed at pH=9-10.The multiphase heat transfer and mass transfer in the process of crystals are analysed based on the theory of both crystals and complexion reactions. The mathematical model of both forming nucleus and their growth is discussed. This technology for preparing microspherical barium sulfate reveals the mechanism of complexion reaction and innate character of precipitation reaction. What is more, the mechanism and essence of nucleating (preipitation) in this reaction have primarily been opened out so that there are a little new methods in studying and thinking of preparing ultrafine through nucleating in some reactions.Microspherical BaSO4 particles have been coated with Y2O3:Eu3+ phosphor layers by the wet chemical method. The effects of the preparation parameters, such as urea concentration, PEG, reaction temperature etc, on the morphology and photoluminescence (PL) properties of BaSO4@Y2O3:Eu3+ core-shell submicrospheres were investigated. XRD, SEM, TEM, EDS, photoluminescence (PL) spectra were utilized to characterize the BaSO4@Y2O3:Eu3+ particles. The results revealed that the obtained core-shell phosphors consist of well dispersed submicron spherical particles with narrow size distribution. BaSO4 particles are well coated with the shell of Y2O3:Eu3+ and no reaction occurred between the BaSO4 cores and the Y2O3:Eu3+ shells even after annealing at 1400℃. According to the thermodynamics calculating, BaSO4@Y2O3:Eu3+ particles were preparaed by coating BaSO4 submicrospheres with nano-Y2O3:Eu3+ particles via heterogeneous nucleation processing. The BaSO4@Y2O3:Eu3+ core-shell particles show a red emission spectra under the excitation of ultraviolet, in which the strongest emission line lies in 609 nm corresponding to 5Do-7F2 of Eu3+. Comparaed with Y2O3:Eu3+ phosphors, the PL spectra of BaSO4@Y2O3:Eu3+ core-shell particles indicate that the coating process do not influence the location of emission peak. The relative photoluminescence intensity of the decline a little due to the role of reflection, scattering and absorption of the layer of Y2O3:Eu3+ nanoparticles.A novel approach for preparation of phosphors - surface diffusion method was proposed in this paper, and applied to synthesis of Y2O3:Eu3+ phosphors successfully. The structure, morphology and the component of the Y2O3:Eu3+ phosphors were characterized by XRD, SEM and EDS, respectively. During the process, Eu3+ was diffused into Y2O3 particles under heat treatment after coated with the precursor of Eu3+. SEM micrographs showed that the surface became rough and was covered with a discontinuous layer after heat treatment. EDS suggested that the concentration of Eu3+ on the particles surface was much higher than the molar ratio of the raw materials. The results show that the PL intensity of Y2O3:Eu3+ phosphors prepared by the surface diffusion method is much higher than that prepared by homogeneous co-precipitation method. For example, the intensity of Y2O3:(0.3 mol%)Eu3+ prepared by surface diffusion method are closed to that of Y2O3:(1 mol%)Eu3+ prepared by homogeneous co-precipitation method.Compared with the traditional solid phase method, these new technologies can obtain the controlled size and high luminescent intensity phosphor through wet chemical method. In addition, they are new cost-saving processes, which can save the rare earth materials, and have potential application in preparation of phosphor with excellent performance.