UV and VUV characteristics of (YGd)2O3:Eu phosphor particles prepared by spray pyrolysis from polymeric precursors

Red-emitting (YGd)₂O₃:Eu phosphor particles, with high luminescence efficiency under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation, were prepared by a large-scale spray pyrolysis process. To control the morphology of phosphor particles under severe preparation conditions, spray solution with polymeric precursors were introduced in spray pyrolysis. The prepared (YGd)₂O₃:Eu phosphor particles had spherical shape and filled morphology even after post-treatment irrespective of Gd/Y ratio. In the case of solution with polymeric precursors, long polymeric chains formed by esterification reaction in a hot tubular reactor; the droplets turned into viscous gel, which retarded the precipitation of nitrate salts and promoted the volume precipitation of droplets. The brightness of (YGd)₂O₃:Eu phosphor particles increased with increasing gadolinium content, and the Gd₂O₃:Eu phosphor had the highest luminescence intensity under UV and VUV excitation. The maximum peak intensity of Gd₂O₃:Eu phosphor particles under UV and VUV were 118 and 110% of the commercial Y₂O₃:Eu phosphor particles, respectively.     

Morphology control and luminescence properties of YAG:Eu phosphors prepared by spray pyrolysis

Starting from nitrate aqueous solutions with citric acid and polyethylene glycol (PEG) as additives, Y₃Al₅O₁₂:Eu (YAG:Eu) phosphors were prepared by a two-step spray pyrolysis (SP) method. The obtained YAG:Eu phosphor particles have spherical shape, submicron size and smooth surface. The effects of process conditions of the spray pyrolysis on the crystallinity, morphology and luminescence properties of phosphor particles were investigated. The emission intensity of the phosphors increased with increasing of sintering temperature and solution concentration due to the increase of the crystallinity and particles size, respectively. Adequate amount of PEG was necessary for obtaining spherical particles, and the optimum emission intensity could be obtained when the concentration of PEG was 0.10 g/ml in the precursor solution. Compared with the YAG:Eu phosphor prepared by citrate-gel (CG) method with non-spherical morphology, spherical YAG:Eu phosphor particles showed a higher emission intensity.     

Effect of surface area and crystallite size on luminescent intensity of Y2O3:Eu phosphor prepared by spray pyrolysis

The luminescent intensity of Y₂O₃:Eu phosphor prepared by spray pyrolysis was monitored as a function of the surface area and crystallite size. In order to control the surface area or the crystallite size, some organic additives and flux materials were introduced in the spray solution. The reduction of surface area, which was achieved by only controlling the quantity of the organic additive, was found to be helpful for the improvement of luminescent intensity of Y₂O₃:Eu particles. The addition of Li₂CO₃ flux to the spray solution with the organic additive of 0.3 M enlarged the surface area of Y₂O₃:Eu particles as a consequence of the reduction of particle size. Then, the increase of surface area did not lower the luminescent intensity of Y₂O₃:Eu phosphor particles since the added Li₂CO₃ flux increased the crystallinity, simultaneously. It was found that the Y₂O₃ particles with larger crystallite size had higher photoluminescence regardless of the surface area. Therefore, we concluded that the crystallinity is more important factor than the surface area and increasing the crystallite size is essentially needed to enhance the luminescent intensity of Y₂O₃ phosphor prepared by spray pyrolysis.     

Effect of precursory solution anion and non-stoichiometry on the properties of yttrium gadolinium iron garnet

Experimental data for the Y_2.01 Gd_0.99Fe₅O₁₂ system indicates that addition of sulphate anion to the Fe(NO₃)₃ solutions increases sintered density of the garnet made by spray drying precursory solutions followed by rotary calcining. A smaller particle size and a higher green density of calcined powders obtained from solutions with sulphate added, provide a higher sinterability compared to the powders obtained from nitrate solutions. Conventional ball-milling of calcined powders is eliminated. Sintered density of more than 99% theoretical can be obtained. Data for the Y_2.01(1–x) Gd_0.99(1–x) Fe_5(1–y)O₁₂ system indicates that a second phase of (Y, Gd) FeO₃ decreases the sintered density of iron deficient garnet. A second phase of (Y, Gd) FeO₃ also accounts for lower dielectric loss tangent for these compositions.     

Y3Al5O12:Tb phosphor particles prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux

High brightness Y₃Al₅O₁₂:Tb phosphor particles with spherical shape and fine size were prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux. The effect of ammonium fluoride flux on the characteristics of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis was investigated. Ammonium fluoride flux dissolved into spray solution improved the photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis. The optimum addition amount of ammonium fluoride flux to produce the maximum photoluminescence intensity was 1 wt.% of the Y₃Al₅O₁₂:Tb phosphor particles. The spherical shape of the as-prepared particles obtained by spray pyrolysis from spray solution with polymeric precursors and small amount of ammonium fluoride flux had maintained after post-treatment below 1300 °C. The optimum photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared from polymeric precursors and ammonium fluoride flux was 156% of that of the phosphor particles prepared from pure aqueous solution.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA·NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with x=0.12 for Y_2−xO₃:Eu_x.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA$NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with xZ0.12 for Y_2KxO₃:Eu_x.     

Synthesis and photoluminescence properties of (Y,Gd)BO3 : Eu phosphor prepared by ultrasonic spray

The mixed solution of acetate of Y, Gd, Eu and boric acid diluted in water was used as the precursor for the ultrasonic spray for the synthesis of (Y,Gd)BO₃ : Eu phosphor. It was found that (Y,Gd)BO₃ : Eu phosphor made by ultrasonic spray pyrolysis had a spherical shape and a narrow size distribution having a mean size of 1.3 m, while it had irregular, coarse and non-uniform size distribution for the phosphor formed by solid-state reaction. The as-sprayed particles was amorphous but they converted into the same polycrystalline phase of solid state reaction after post heat treatment at 1100 °C for 2 hr. The emitting intensity under 147 nm VUV excitations for the spray-formed (Y,Gd)BO₃ : Eu phosphor, however, was inferior to the later one. It was found that the optimum concentrations of Gd and Eu were 30% and 5%, respectively in (Y_1–x Gd _x )_1–y BO₃ : Eu _y phosphors prepared both by spray and solid state reaction.     

Characteristics of Eu-doped Ca-α-SiAlON phosphor powders prepared by spray pyrolysis process

Eu²⁺-doped Ca-α-SiAlON phosphor powders with fine size and regular morphology were prepared by combining spray pyrolysis and the carbothermal reduction and nitridation processes. The precursor powders were prepared by spray pyrolysis from the spray solution with ethylenediaminetetraacetic acid, citric acid, and sucrose; they had large sizes, were hollow, and had thin wall structures. The precursor powders containing a carbon component turned into Ca-α-SiAlON phosphor powders after firing at 1450 °C under a H₂/N₂ mixture gas. The mean size of the phosphor powders was 5.1 μm. The phosphor powders had a broad excitation spectra range of 250-500 nm; this consisted of two broadbands centered at 305 and 400 nm. When excited by a 455-nm blue light, the emission spectra of the phosphor powders displayed a broadband in the range of 500-700 nm, which resulted in a yellow emission. The wavelength of the emission spectrum showing the maximum intensity was 576 nm.     

Synthesis and characterisation of rare earth oxysulphide phosphors. I. Studies on the preparation of Gd2O2S:Tb phosphor by the flux method

Terbium activated gadolinium oxysulphide phosphor (Gd₂O₂S:Tb) shows bright green luminescence and high efficiency under X-ray excitation. Phosphor utilisation depends on powder characteristics and luminescence properties that are regulated during the synthesis stage. The paper presents some of our new results on the synthesis of Gd₂O₂S:Tb phosphor by solid-state reaction route from oxide precursors. Efficient luminescent powders utilisable in the manufacture of X-ray intensifying screens for medical diagnosis were prepared from optimised synthesis mixtures containing oxide precursors, alkaline carbonate based flux, alkaline phosphate based mineralising additives and sulphur suppliers.     

PL and EL properties of Tm-activated vanadium oxide-based phosphor thin films

The preparation of high-luminance blue emitting Tm-activated multicomponent oxide phosphor thin films is described. The phosphor thin films were deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets. A very high photoluminescence (PL) intensity in blue emission could be observed in postannealed Tm-activated gadolinium oxide-vanadium oxide Gd_0.5-V_0.5-O:Tm thin films prepared using Tm₂O₃-doped (Gd₂O₃)_0.5-(V₂O₅)_0.5 targets (V₂O₅ content of 50 mol.%). It should be noted that thin films of this phosphor postannealed at 1100 °C in air exhibited higher PL intensity than those from commercially available blue BaMgAl₁₄O₂₃:Eu (BAM) phosphor powder and Gd_0.5-V_0.5-O:Tm powders. In addition, a Gd_0.5-V_0.5-O:Tm thin-film electroluminescent device exhibited a blue emission that was the same as the PL emission.     

The Effect of flux types on the formation of green light emitting phosphor particles with spherical shape and filled morphology

(CeTb)MgAl₁₁O₁₉ (CTMA) phosphor particles were prepared by high-temperature spray pyrolysis from spray solutions with various types of flux materials. The particles prepared from spray solutions with ammonium dihydrogen phosphate and lithium carbonate fluxes had spherical shape and filled morphology at temperatures of 900^∘C and 1650^∘C. On the other hand, the particles prepared from spray solutions without flux material had hollow and fractured morphology at temperatures of 900^∘C and 1650^∘C. The melting of flux material formed the spherical intermediate particles with filled morphology. These spherical intermediate particles were formed from spray solutions with flux material that transformed into spherical CTMA phosphor particles with filled morphology at a high-preparation temperature. The phosphor particles prepared by spray pyrolysis from the spray solution with appropriate flux materials at 1650^∘C had high photoluminescence intensities, spherical shape, and filled morphology.     

Preparation and characterization of rod-like Eu:Gd2O3 phosphor through a hydrothermal routine

Rod-like Gd(OH)₃ precursor was successfully synthesized through a hydrothermal routine. Eu:Gd₂O₃ red phosphor was obtained by a subsequent calcination of the precursor at 600 °C for 4 h. The dehydration of Gd(OH)₃ precursor took two steps and an intermediate phase, monoclinic GdOOH, was produced. The rod-like phosphor will emit a strong visible light peaking at 610 nm upon 258 nm UV illumination, implying its potential application in the field of luminescent devices.     

Towards carbon-free flame spray synthesis of homogeneous oxide nanoparticles from aqueous solutions

Flame-assisted spray pyrolysis (FASP) is a versatile process for synthesis of nanoparticles from a broad choice of precursors and solvents. Water is an attractive solvent particularly for inexpensive inorganic precursors (e.g. metal nitrates) as it can effectively reduce the process cost. Furthermore when water usage is combined with a carbon-free fuel (e.g. H₂), nanoparticles can be made without forming CO₂. Here such a FASP process is explored for synthesis of Bi₂O₃ and other oxide nanoparticles from aqueous precursor solutions. The flame temperature was measured by FTIR emission–transmission spectroscopy while powders were characterized by X-ray diffraction and N₂ adsorption. At low FASP fuel gas (H₂ or C₂H₂) flow rates or process temperatures, product powders had a bimodal crystal size distribution. Its large and small modes were made by droplet- and gas-to-particle conversion, respectively. Homogeneous Bi₂O₃ and CeO₂ powders were obtained for sufficiently high flow rates of either C₂H₂ or H₂. Prolonged high temperature residence times promoted precursor evaporation from the spray droplets and yielded homogeneous nanostructured powders by gas-to-particle conversion. In contrast, FASP of aqueous solutions of aluminum nitrate yielded rather large particles by droplet-to-particle conversion at all fuel flows investigated.     

Nano-sized manganese oxide particles prepared by low-pressure spray pyrolysis using FEAG process

Nano-sized manganese oxide particles were prepared by low-pressure spray pyrolysis using the new type of liquid aerosol generator called as FEAG process. The particles prepared from polymeric precursors spray solution with organic additives had large size, hollow and fractured morphologies due to the gas evolution from the decomposition of the organic additives. The precursor particles with micron size, hollow and thin wall structures turned to nano-sized manganese oxide particles after post-treatment at temperatures of 700 and 800 °C. The optimum concentrations of citric acid (CA) and ethylene glycol (EG) to prepare the nano-sized manganese oxide particles by spray pyrolysis using the FEAG process were 0.3 M each. The manganese oxide particles prepared from the polymeric precursors spray solutions with high concentrations of CA and EG had spherical shape and porous morphologies.     

Effect of preparation temperature on the characteristics of Eu-doped borate phosphor particles in the spray pyrolysis

Eu-doped gadolinium borate (GdBO₃:Eu) phosphor particles with fine size and uniform morphology were prepared by spray pyrolysis. The effects of the preparation temperatures on the characteristics of the GdBO₃:Eu phosphor particles prepared by spray pyrolysis were analyzed. The precursor particles obtained at preparation temperatures below 1400 °C had hollow and porous inner structures. On the other hand, the precursor particles obtained at preparation temperature of 1600 °C had dense structure and uniform morphologies. The precursor particles with glass phases were formed at high-preparation temperatures above 1400 °C. The precursor particles prepared at preparation temperature of 1400 °C turned into GdBO₃:Eu phosphor particles with fine size and regular morphology after post-treatment. The GdBO₃:Eu phosphor particles prepared from the precursor particles obtained at a preparation temperature of 1400 °C had the maximum photoluminescence intensity, which was similar to that of the commercial (Y,Gd)BO₃:Eu phosphor particles.     

Low-temperature sintering characteristics of nano-sized BaNd2Ti5O14 and Bi2O3–B2O3–ZnO–SiO2 glass powders prepared by gas-phase reactions

Nano-sized BaNd₂Ti₅O₁₄ (BNT) powders were prepared by spray pyrolysis from solutions containing ethylenediaminetetraacetic acid and citric acid. Treatment at temperatures ≥900 °C and subsequent milling resulted in nanoparticle powders with orthorhombic crystal structures. The mean particle size of the powder post-treated at 1000 °C was 160 nm. Nano-sized Bi₂O₃–B₂O₃–ZnO–SiO₂ glass powder with 33 nm average particle size was prepared by flame spray pyrolysis and used as a sintering agent for the BNT. BNT pellets sintered at 1100 °C without the glass had porous structures and fine grain sizes. Those similarly sintered with the glass had denser structures and larger grains.     

A simple sol–gel technique for preparing lanthanum oxide nanopowders

Nanosized lanthanum oxide powders have been prepared by a simple sol–gel technique using commercial lanthanum oxide, nitrate acid and polyethylene glycol (PEG) as the starting materials. The decomposition process of dried gel powders were investigated by differential thermal and thermogravimetric analysis(TG-DSC). The crystalline structures and morphologies of the powders were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The effects of calcination temperature, calcination time, stirring and the concentration of PEG on the particle size were studied. The results show that the calcination temperature and concentration of PEG have obvious influence on the La₂O₃ particle size and agglomeration tendency. At lower calcination temperature the prolongation of calcination time has slight influence on the particle size. Stirring will promote the growth of particles. The weak-agglomerated nanosized powders with the mean particle size less than 40 nm could be obtained by controlling the process condition.     

Controlled processing of (Gd,Ln)2O3:Eu (Ln = Y,Lu) red phosphor particles and compositional effects on photoluminescence

Synthesis of (Gd_0.95−xLn_xEu_0.05)₂O₃ (Ln = Y and Lu, x = 0–0.95) powders via ammonium hydrogen carbonate (AHC) precipitation has been systematically studied. The best synthesis parameters are found to be an AHC/total cation molar ratio of 4.5 and an ageing time of 3 h. The effects of Y³⁺ and Lu³⁺ substitution for Gd³⁺, on the nucleation kinetics of the precursors and structural features and optical properties of the oxides, have been investigated. The results show that (i) different nucleation kinetics exist in the Gd–Y–Eu and Gd–Lu–Eu ternary systems, which lead to various morphologies and particle sizes of the precipitated precursors. The (Gd,Y)₂O₃:Eu precursors display spherical particle morphologies and the particle sizes increase along with more Y³⁺ addition. The (Gd,Lu)₂O₃:Eu precursors, on the other hand, are hollow spheres and the particle sizes increase with increasing Lu³⁺ incorporation, (ii) the resultant oxide powders are ultrafine, narrow in size distribution, well dispersed and rounded in particle shape, (iii) lattice parameters of the two kinds of oxide solid solutions linearly decrease at a higher Y³⁺ or Lu³⁺ content. Their theoretical densities linearly decrease with increasing Y³⁺ incorporation, but increase along with more Lu³⁺ addition and (iv) the two kinds of phosphors exhibit typical red emissions at ∼613 nm and their charge-transfer bands blue shift at a higher Y³⁺ or Lu³⁺ content. Photoluminescence/photoluminescence excitation intensities and external quantum efficiency are found to decrease with increasing value of x, and the fluorescence lifetime mainly depends on the specific surface areas of the powders.     

Fine-sized LiCoO2 particles prepared by spray pyrolysis from polymeric precursor solution

Fine-sized LiCoO₂ particles with high discharge capacities and good cycle properties were prepared by spray pyrolysis from a spray solution with citric acid and ethylene glycol. The precursor particles obtained from the spray solution with citric acid and ethylene glycol transformed into fine-sized LiCoO₂ particles with regular morphology after post-treatment at temperatures between 700 and 900 °C. The mean size and aggregation degree of the primary particles were affected by the concentrations of polymeric precursors added into spray solutions. The LiCoO₂ particles obtained from the spray solution without polymeric precursors had irregular and aggregated morphology. The discharge capacities of the LiCoO₂ particles changed from 132 to 151 mAh/g when the concentrations of the citric acid and ethylene glycol changed from 0 to 1 M. The particles prepared from the spray solution containing 0.3 M each of citric acid and ethylene glycol had the maximum discharge intensity.