Preparation, Sintering And Optical Properties Of Rare Earth Doped Nano YAG And CePO₄ Powders

Rare-earth doped YAG and CePO4 powders are two kind of important luminescent phosphors, which are widely used in many fields of color display and lighting. Besides, rare earth doped YAG transparent ceramics are important solid-state laser materials, and are widely in various fields of laser weapons, materials processing, and biomedical. The optical properties of both transparent ceramics and luminescent phosphors are closely related to the preparation process, activator, phase composition, macro-size, microstructures, surface and interface state. In this thesis, pristine and rare-earth doped YAG nano-powders and CePO4 with different architectures are prepared. The growth mechanism of nano-powders under calcination conditions, the formation mechanism of different architectures, and the influence of powder properties on luminescent performance are investigated.YAG precursors were synthesized by the urea method in aqueous solution using conventional drying method and supercritical carbon dioxide and ethanol fluid drying technique respectively, and YAG powders were obtained by calcining the precursors at different temperatures. The homogenous precipitation process was analyzed by using the solubility product principle. The composition of the precursors, the phase formation process and the properties of the calcined powders, and the growth mechanism of YAG nano-powders during calicination process were investigated by means of XRD, IR, TG/DSC, BET, TEM, SEM and HRTEM. The results indicate that, the difference in solubility products cause a sequential precipitation process in which Al3+ precipitates first, followed by the precipitation of Y3+ onto aluminum precipitates to give a core-shell structure with homogenous atomic mixing. The composition of precursor precipitate is determined by the competition between OH" and the carbonate species generated by the hydrolysis reactions of urea during combining with metal cations. Aluminum precipitates are mainly hydrated aluminum hydroxides, while yttrium precipitates may consist of hydrated carbonates, hydrated basic carbonates, and hydrated hydroxides. The general formula of the precursor precipitate may be expressed as Y3Al5(OH)24-2x·(CO3)x·nH2O. Compared with the classically prepared powders, the amorphous precursor dried by supercritical CO2 fluid was loosely agglomerated and directly converted to pure YAG at about 800℃. The resultant YAG powders obtained at 1000℃were highly dispersed spherical-like particles with an average crystallite size about 23nm and specific surface area of 30m·g-1. The precursor dried by supercritical ethanol fluid was also loosely agglomerated but crystalline, consisting of monoclinic Y(OH)3 and pseudoboehmite. YAM and YAP phases appeared in the calcination process and phase pure YAG was not detected until 1200℃.Particle growth of YAG nano-powders occurred during the calcination process. At temperatures from 900 to 1300℃, the particle growth is attributed to the crystallographically specific oriented attachment along the (112) planes. As the growth process proceeds above 1300℃, oriented attachment based growth becomes less important because of the increase on particle size, and the self-integration assisted by the Ostwald ripening becomes dominant, accompanied by severe agglomerations which finally lead to the formation of irregular shaped particles and aggregates.Rare earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) doped YAG nano-powders were synthesized by urea method combined with supercritical CO2 drying technique. The influence of doping rare earth cations on the homogenous precipitation process was analyzed. Phase composition, morphology, and luminescence properties of samples doped with different rare earth cations were investigated. The results indicate that, the sequential precipitation process was not changed due to small doping quantities of rare earth cations. Pure YAG was obtained after calcining at 1100℃, and the average crystallite size is between 28 and 40nm. The emission intensities of the samples prepared by supercritical drying technique were much stronger than those of samples prepared by conventional drying method.CePO4 powders with different size and crystal structures were prepared by solvothermal method in water-ethanol system using Ce(SO4)2·4H2O as cerium source and H3PO4 as mineralizer. The phase composition, morphology, and luminescence properties of products obtained at different ratios of ethanol to water, reaction times, temperatures, Ce4+ concentrations, and calcination temperatures were analyzed by means of XRD, XPS, TEM, SEM and PL. The results indicate that, hexagonal CePO4 spheres with different size of 20nm, 50nm,200nm, 1～3μm, and 6～10μm were obtained by simply changing the ethanol to water ratios from 7 to 0. For CePO4 nano-particles, the size of diameter decreased when the concentration of Ce4+ was reduced, the crystallinity increased along with the prolonging of reaction time and raising of temperature, and the hexagonal phase transformed to monoclinic phase after 96h reaction, accompanied by morphology change from sphere-like to rod-like. For CePO4 microspheres, the morphology undergoes evolution from spheres with smooth surface to lamellar spheres and then to lamellar star-shaped particles, accompanied by the phase transition from hexagonal to mixture of hexagonal and monoclinic. Hexagonal CePO4 nanospheres are low-temperature stable phase, which are transformed to monoclinic phase without morphology change after calcination at high temperatures, corresponding to a displacement type phase transition. While the crystal structure and morphology of monoclinic CePO4 nanorods are not changed by calcining, just leading to an improvement of crystallinity. Besides, oxidation of Ce3+to Ce4+ occurred for both of the two kinds of CePO4 samples, resulting in the formation of CeO2.Photoluminescence analysis of CePO4 samples with different morphology and compositions indicate that the emission intensity is directly affected by the crystal structure, crystallinity, oxidation state of Ce, and activator concentration. The emission intensity of monoclinic phase is stronger than that of hexagonal phase. For the uncalcined samples, the emission intensity is mainly determined by the crystallinity regardless of the particle size and shape, higher crystallinity, stronger emission intensity. After calciantion, Ce3+ was oxidized to Ce4+, and the emission intensity decreased. Rare earth doped CePO4 samples exhibit energy transfer including Ce3+→Dy3+, Ce3+→Dy3+→Eu, and Ce3+→Tb3+, emitting corresponding special light. The energy transfer effiency of Tb3+ doped CePO4 sample is higher than those of others, emitting stronger light. The emission intensity is affected by activator concentrations, and samples doped with 5% of activator present stronger emission due to larger quantity of active centers.Investigations on the formation mechanism of CePO4 particles with different sizes and morphologies indicate that, with low ratios of ethanol to water, the precursor precipitates are cerium (IV) phosphate nanorods that crystallize in the monoclinic system with a composition of Ce(H2O)(PO4)3/2(H3O)1/2(H2O)1/2, which was reduced by ethanol to form Ce3+ after hydrothermal reactions, resulting in the dissolution of nanorods and the crystallization of hexagonal CePO4 nuclei. With high ratios of ethanol to water, the precursor precipitates are of amorphous, which was also reduced to form Ce3+after hydrothermal reaction and leads to the crystallization of hexagonal CePO4 nuclei. The newly formed nuclei subsequently adsorb organic products to decrease their surface energy, and then they assembled to form hexagonal CePO4 particles with different sizes. With higher contents of ethanol in the reaction solution, the nuclei adsorb more amounts of organic products, resulting in the decrease of assemble driving force, and the higher saturated vapor pressure also inhibits the aggregation of small particles, thus nano-particles were obtained. Conversely, lower contents of ethanol in the reaction solution are in favor of the formation of microspheres. During the formation process of CePO4 samples with different morphologies, the redox reaction between Ce4+ and ethanol has played a key role.The morphology evolution and phase transition process of CePO4 nano-particles and microspheres along with the increase of reaction time occurs through dissolution-recrystallization mechanism. Due to the energy difference between hexagonal and monoclinic phase, the surfaces of hexagonal particles were gradually dissolved to generate Ce3+, which then spontaneously nucleate onto the surface again to form monoclinic CePO4. After 96h reaction, hexagonal nano-particles has transformed to pure monoclinic rod-like structures. While the hexagonal smooth microspheres transformed to star-shaped particles with mixed phases of hexagonal and monoclinic. During the whole transformation process, CePO4 nano-particles exhibit a much faster speed.