Sodium Carbonate Flux Effects on the Luminescence Characteristics of (Y0.5Gd0.5)2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Na₂CO₃ flux was introduced in the preparation of phosphor particles by spray pyrolysis to improve the photoluminescence (PL) characteristics of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles. The phosphor particles directly prepared by spray pyrolysis at 1300°C from solutions with 20 wt% Na₂CO₃ flux had the highest PL intensity, which corresponded to 130% of that of particles prepared from solution without flux. On the other hand, the maximum PL intensity of the annealed particles, which were as-prepared at 900°C and posttreated at 1200°C for 3 h, was obtained from a solution with 5 wt% Na₂CO₃ flux. The maximum PL intensity of particles directly prepared by spray pyrolysis without posttreatment was 86% of that of posttreated phosphor particles. Na₂CO₃ flux was also important in control of morphology of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles.     

Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors

The phase development and luminescence of chromium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Cr or YAG:Cr) phosphors, prepared by both a chemical precipitation technique and a solid-state reaction, were studied. The YAG structure formed at a much lower temperature and by a different phase development sequence when the chemical method was employed. The light output of the chemically synthesized powders, measured by laser excitation, was discovered to increase with increasing heat treatment temperature and was found to be brightest when the YAG:Cr phosphor had excess aluminum.     

Synthesis and Characterization of Ce3+:YAG Phosphors by Heterogeneous Precipitation Using Different Alumina Sources

Ce³⁺-doped yttrium aluminum garnet (Ce:YAG) phosphor powders were synthesized by heterogeneous precipitation process using three different aluminum sources: α-phase, θ-phase, and boehmite (AlOOH). Mixtures of yttrium and cerium nitrate solutions containing various aluminum sources were precipitated by ammonia solution in normal and reverse strike methods. The influence of pH was studied in the normal strike method by maintaining the solutions at pH 7, 9, and 11 during precipitation. Dried precipitates were double calcined at 1300°C/16 h and 1300–1500°C/24 h, at a ramping rate of 10°C/min, with an intermittent wet ball milling in water. Structural evolution of the resultant phosphors was studied by powder XRD. In the normal strike method, a highly pure YAG phase was formed by α- alumina (pH 7, 11) and θ-alumina (pH 11) while boehmite source ended up with mixed phases of YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) along with YAG phase at all pH values of precipitation. However, in the reverse strike process, the θ-phase of alumina gave an extremely pure Ce:YAG phase at a relatively lower calcination temperature (1400°C/24 h) compared with the α-phase and also showed more intense emission of yellowish-green light under blue (λ=469 nm) excitation. Scanning electron microscopy revealed 1–2 μm sized particles with least agglomeration in the reverse strike method.     

Preparation and Optical Properties of Transparent Eu3+:Y3Al5(1−x)Sc5xO12 Ceramics

Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu³⁺ for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ was observed when 10 at.% Al³⁺ was substituted by Sc³⁺. The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc³⁺ doping, is the main reason for the reduced peak splitting.     

Synthesis of BaAl2S4:Eu2+ Electroluminescent Material by the Polymerizable Complex Method Combined with CS2 Sulfurization

Eu-activated BaAl₂S₄ (BaAl₂S₄:Eu) blue light emission phosphor has attracted considerable attention recently as a prospective material for full color electroluminescent display applications. Although BaAl₂S₄:Eu was discovered over 30 years ago, the solid-state reaction between metal sulfides in sealed quartz ampoules remains the main method for the synthesis of this material. In this work, a homogeneous single-phase Ba_0.95Eu_0.05Al₂S₄ (BaAl₂S₄:Eu) phosphor was obtained by the sulfurization–reduction of the multicomponent oxide precursor in a CS₂ atmosphere at 1050°C. The oxide precursor containing barium, aluminum, and europium for this process was prepared by the polymerizable complex method, which ensures a high degree of homogeneity in the final product. The BaAl₂S₄:Eu material thus obtained exhibited a single emission line at 475 nm and a fluorescence intensity of 35% compared with one of the best commercially available (Ba,Eu)MgAl₁₀O₁₇ phosphors.     

Synthesis of Blue-Emitting CaMgSi2O6:Eu2+ Phosphor Using an Electrostatic Self-Assembly Deposition Method

Eu²⁺-doped CaMgSi₂O₆ phosphor was prepared by depositing mixed hydroxides of Ca, Mg, and Eu over spherical SiO₂ particles (300 nm) pre-coated with polycations (polyethyleneimine), followed by calcination at 1200°C in a reducing atmosphere. The prepared phosphor showed intense blue emission, ascribable to the 4f⁷-4f⁶5d transition of Eu²⁺. In contrast, the luminescence intensity of the phosphor was considerably decreased when prepared without polycations. It was suggested that negatively charged hydroxides are deposited on positively charged SiO₂ surfaces pre-coated with polycations through electrostatic self-assembly interaction. On calcination, the hydroxide shells react with the SiO₂ cores to produce Eu²⁺:CaMgSi₂O₆.     

Polymer-Assisted Annealing of Spray-Pyrolyzed Powders for Formation of Luminescent Particles with Submicrometer and Nanometer Sizes

Phosphor nanoparticles (e.g., SrTiO₃:Pr,Al and (Y,Gd)₃Al₅O₁₂:Ce) were prepared using a conventional spray pyrolysis method in conjunction with a polymer-assisted heat treatment (annealing). The effects of spray/annealing temperatures, polymer type, polymer concentration, and salt addition on the morphology, crystallinity, particle/crystal size, and the photoluminescence intensity of as-prepared as well as annealed phosphor particles were investigated. The results showed that the nanocrystal growth of as-prepared-sprayed submicrometer particles was improved and that the nanocrystal networks disintegrate into nanoparticles when polymer or salt solutions are added during the heat-treatment process. The additional combustion heat and gas evolution during the thermal reaction of polymer appear to be the main reasons for this. On the contrary, only compacted agglomerated particles rather than dispersed nanoparticles were found in the case of annealing without polymer/salt addition.     

Methanol–Water System for Solvothermal Synthesis of YVO4:Eu with High Photoluminescent Intensity

A methanol–water mixed solvent was used as a reaction medium for the preparation of Eu³⁺-doped YVO₄ phosphor materials. These were synthesized by a solvothermal method at 150°–300°C using a 10 vol% solution of water in methanol as the reaction medium followed by calcination at 1000°–1200°C. The phase composition and optical properties of the products were characterized by X-ray diffraction, scanning electron microscope, and photoluminescence spectroscopy. The powders obtained were composed of spherical particles ∼0.5 μm in size, with an internal structure that was different for samples prepared under subcritical and supercritical conditions of methanol. After the calcination, the powders obtained at 240°–300°C retained the initial raspberry-like morphology, whereas the morphology of samples prepared at 150°–210°C changed significantly due to noticeable sintering. The fluorescence intensity exhibited by the prepared samples was higher than the fluorescence intensity shown by one of the best commercial YVO₄:Eu phosphors having a large particle size.     

Synthesis, Characterization, and Luminescent Properties of CaS:Eu Phosphor

Europium-doped calcium sulfide phosphor was synthesized by a co-precipitation method using calcium carbonate and ammonium sulfate as the sources for calcium and sulfide, respectively. Reduction of the precipitated CaSO₄ into CaS was carried out at 550°, 750°, 950°, and 1150°C at a flow rate of H₂ 5 mL/min and N₂ 95 mL/min. The formation of crystalline CaS phase was evident by the X-ray diffraction analysis of the fired products. The phosphor yielded red emission of wavelength 634 nm for the excitation wavelength of 465 nm. The emission intensity of the phosphors increased with increasing firing temperature from 550° to 950°C, but firing at 1150°C showed a dramatic decline in emission due to thermal quenching. The study of the effect of Eu loading level showed a decrease in emission with an increase in loading, and the optimum loading for maximum emission was found to be 0.5 mol%. And it can be possible to generate white light of light emitting devices by mixing with SrGa₂S₄:Eu green phosphor and this phosphor on the 460 nm InGaN blue chip.     

In S/Yu-Toughened Silicon Carbide-Titanium Carbide Composites

A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A1₂O₃ and Y₂O₃ by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m^1/2, approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m^1/2). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites.     

Synthesis, growth mechanism and photoluminescence of monodispersed cubic shape Ce doped YAG nanophosphor

In the present work, a novel method for the synthesis of monodispersed cubic shape Ce-doped yttrium aluminum garnet (YAG:Ce) nanophosphors is reported. Single phase Ce doped YAG nanoparticles are prepared by solvothermal processing, followed by annealing treatment. Morphological investigation by scanning electron microscopy (SEM) showed the formation of monodispersed 500 nm cubic shape Ce-doped YAG phosphor. The crystalline Ce-doped YAG showed broad emission peaks in the range of 480-640 nm with maximum intensity at 524 nm. The emission intensity increased with increase in calcination temperatures while reduced with increase in Ce³⁺ ions concentration. Detailed study was carried out to understand the formation of monodispersed cubic shape Ce doped YAG nanoparticles. It was found that the solvent, surfactant and impurity (counter ions of cerium and aluminum salt) has significant effect on the crystal growth.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Synthesis, Characterization, and Optical Properties of Pristine and Doped Yttrium Aluminum Garnet Nanopowders

Pristine, Si-doped, and Si/Nd-codoped yttrium aluminum garnet (YAG) nanoparticles were synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine. It was found that the coexistence of Si⁴⁺ and Nd³⁺ increased the solubility of both ions and promoted the formation of YAG phase. Single-phase, nanocrystalline Si/Nd:YAG powders were obtained at calcination temperatures as low as 920°C. The optical behavior of the Si/Nd:YAG nanopowders was similar to that of single-crystal Nd:YAG.     

Alumina/Epoxy Interpenetrating Phase Composite Coatings: I, Processing and Microstructural Development

Interpenetrating phase composite (IPC) coatings consisting of continuously connected Al₂O₃ and epoxy phases were fabricated. The ceramic phase was prepared by depositing an aqueous dispersion of Al₂O₃ (0.3 μm) containing orthophosphoric acid, H₃PO₄, (1–9.6 wt%, solid basis) and heating to create phosphate bonds between particles. The resulting ceramic coating was porous, which allowed the infiltration and curing of a second-phase epoxy resin. The effect of dispersion composition and thermal processing conditions on the phosphate bonding and ceramic microstructure was investigated. Reaction between Al₂O₃ and H₃PO₄ generated an aluminum phosphate layer on particle surfaces and between particles; this bonding phase was initially amorphous, but partially crystallized upon heating to 500°C. Flexural modulus measurements verified the formation of bonds between particles. The coating porosity (and hence epoxy content in the final IPC coating) decreased from ∼50% to 30% with increased H₃PO₄ loading. The addition of aluminum chloride, AlCl₃, enhanced bonding at low temperatures but did not change the porosity. Diffuse reflectance FTIR showed that a combination of UV and thermal curing steps was necessary for complete curing of the infiltrated epoxy phase. Al₂O₃/epoxy IPC coatings prepared by this method can range in thickness from 1 to 100 μm and have potential applications in wear resistance.     

Preparation of Aluminum Oxynitride by Nitridation of a Precursor Derived from Aluminum–Glycine Gel and the Effects of the Presence of Europium

Spinel-type AlON, Al_2.75□_0.25O_3.74N_0.26, was obtained by ammonia nitridation of an oxide precursor prepared by peptizing a glycine gel with aluminum nitrate. To achieve crystallization, the nitrided product had to be annealed at 1500°C for 10 min in flowing nitrogen. The use of glycine instead of citric acid was important for obtaining a white product without residual carbon. A similar preparation method was used for adding small amounts of europium below 10 mol%. A strong blue emission was observed for products ranging from 0.5 to 3.0 mol% Eu doping. The product with 0.5 mol% doping had a maximum emission intensity at 400 nm for an excitation of 254 nm. The products with 1 and 3 mol% doping showed double maxima at 475 and 520 nm. These three emissions were due to the presence of divalent europium in the EuAl₁₂O₁₉ magnetoplumbite-like aluminum oxynitride impurity mixed with the AlON spinel major phase. The 1 mol% Eu-doped product exhibited expanded hexagonal lattice parameters ( a =0.5591 and c =2.236 nm) compared with the values for EuAl₁₂O₁₉ magnetoplumbite itself, observed in the 7.7 mol%-doped product without any strong emission. The above spectrum change was discussed in relation to the coordination around Eu²⁺.     

Effect of Composition on the Oxygen Tracer Diffusion in Transparent Yttrium Aluminium Garnet (YAG) Ceramics

The grain boundary structure and oxygen tracer diffusion in transparent yttrium aluminum garnet (YAG) ceramics varying from 2% excess of Y₂O₃ to 0.5% excess of Al₂O₃ were studied. The characterization of the specimens is as follows: (i) For the Y₂O₃-excess specimen, a second phase (yttrium aluminum perovskite: YAP) containing silicon in the grain boundary was found, (ii) For the Al₂O₃-excess specimen, both aluminum-rich particles (alumina) and a silicon-rich segregant layer were observed in the grain boundary. The volume diffusion of the oxygen tracer is little influenced by the excess composition. In contrast, the grain boundary diffusion of the oxygen tracer is suppressed in the Y₂O₃-excess specimens, compared to Al₂O₃-excess specimens. These differences are thought to result from the chemical reaction between the second phase and the intergranular liquid phase during the sintering.     

The characteristics of X1 type Y2SiO5:Tb phosphor particles prepared by high temperature spray pyrolysis

The X1 type Y₂SiO₅:Tb phosphor particles with high brightness were prepared by spray pyrolysis from spray solution with NH₄F flux material. The phosphor particles prepared by spray pyrolysis at high preparation temperature had spherical shape, fine size and dense morphology. The mean sizes of the phosphor particles prepared at 900 and 1650 °C were 1.3 and 0.9 μm. The emission spectrum of the phosphor particles prepared by spray pyrolysis at 1650^oC had the characteristics of X1 type Y₂SiO₅:Tb phosphor. The photoluminescence intensity of the phosphor particles directly prepared by spray pyrolysis from spray solution with 20 wt.% NH₄F flux of the product at temperature of 1650 °C was 127 and 184% of the X1 and X2 type Y₂SiO₅:Tb phosphor particles post-treated at 1100 and 1300 °C, respectively. The Y₂SiO₅:Tb phosphor particles prepared by spray pyrolysis at 1650 °C had X1 type crystal structure because of short residence time of particles inside hot wall reactor of 0.4 s.     

Photoluminescent Properties of (Ca,Zn)TiO3:Pr,B Particles Synthesized by the Peroxide-Based Route Method

(Ca_{1− x} ,Zn _x )TiO₃:Pr, B red phosphor particles were prepared using the peroxide-based route and their photoluminescent (PL) properties were investigated by changing the sintering temperature, the concentration of the activator, the ratio of Ca to Zn, and the amount of H₃BO₃ flux. For the CaTiO₃:Pr phosphor, a pure perovksite-type CaTiO₃ phase was formed when the sintering temperature was 700°–800°C. It was found that the substitution of Zn atoms instead of Ca considerably enhanced the 614-nm red emissions. The PL intensity of (Ca_{1− x} ,Zn _x )TiO₃:Pr phosphor was also additionally improved by adding an H₃BO₃ flux. Finally, the optimized phosphor Ca_0.85Zn_0.15TiO₃:0.001Pr,0.1B showed the highest PL intensity.     

Chemical Synthesis of a Blue-Emitting NaSr1−X PO4:EuX Phosphor Powder

Pure, crystalline, blue-emitting NaSr_{1− X} PO₄:Eu _X with X =0–0.06 phosphor powder is formed by the thermal decomposition of a metal–nitrate–citrate–polyethylene glycol complex at 1300°C in a reducing atmosphere. The resulting phosphor powder emits blue luminescence with a peak intensity at 428 nm under ultraviolet excitation at 374 nm. Compared with the powder synthesized by a conventional solid-state reaction, the chemically prepared powder exhibits a higher emission intensity and requires a lower Eu content to reach the maximum intensity.     

Structural and Optical Properties of Highly Nd-Doped Yttrium Aluminum Garnet Ceramics from Alkoxide and Glycolate Precursors

Yttrium aluminum garnet (YAG) nanopowders doped with high neodymium (Nd) content (3 at.%) were synthesized by the sol–gel processing of (i) alkoxide precursors and (ii) metal chelates formed by complexing the cations with polyethylene glycol. A stoichiometric YAG composition was obtained following both procedures; however, the agglomeration of particles was significantly higher in glycolate synthesis, which shielded residual organics from oxidation (elemental analyses). Distribution of Nd³⁺ ions in the YAG matrix, as shown by the absorption of pump energy and photoluminescence spectra of Nd:YAG ceramics, was more homogeneous in alkoxide-derived powders. The segregation of Nd centers in the glycolate-derived sample was supported by the precipitation of a crystalline Nd₂O₃ phase (X-ray diffraction) during sintering. High-resolution absorption spectra (⁴I_9/2(1)→⁴F_9/2(1)) of the powders showed that a higher absorption cross-section of glycolate-derived powders is due to Nd³⁺–Nd³⁺ ion pairing, which leads to the quenching of photoluminescence. Owing to the better dispersion of optically active centers, the photoluminescence signal was found to be substantially enhanced in alkoxide-derived Nd:YAG ceramics.