Gd2O3:Eu phosphor powders prepared using a size-controllable droplet generator

Gd₂O₃:Eu phosphor powders were prepared by a filter expansion aerosol generator (FEAG) process capable of changing the mean size of droplets. The change in the mean size of the Gd₂O₃:Eu phosphor powders according to the concentrations of polyethylene glycol added to spray solutions was caused by the difference in the mean size of the droplets produced via the FEAG process. The mean sizes of droplets produced by the FEAG process were affected by the surface tension and viscosity of the spray solutions. The mean sizes of the Gd₂O₃:Eu phosphor powders obtained from the spray solutions with the same concentration of metal salts changed from 1.5 to 4.2 μm according to the concentrations of polyethylene glycol and citric acid added to the spray solutions. The maximum photoluminescent intensity of the phosphor powders obtained from the spray solutions with polymeric precursors and boric acid flux was 144% of that of the phosphor powders obtained from the aqueous spray solutions without flux.     

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.     

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.     

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.     

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.     

Hydrothermal synthesis, structure and scintillation characterization of nanocrystalline Eu-doped Gd2O3 materials and its X-ray imaging applications

Nanocrystalline Gd₂O₃:Eu scintillators were successfully synthesized using a hydrothermal method and subsequent calcination treatment in the electrical furnace as an X-ray to visible light conversion material for an indirect X-ray image sensor. In this work, various Gd₂O₃:Eu scintillators were prepared in accordance with different synthesis conditions such as doped-Eu concentration, different calcination temperatures of 600-1400 °C and calcination time of 1-10 h. The transition of morphology from nanorods to particles was observed as the calcination temperature of Gd₂O₃:Eu scintillator increased. And the phase transformation of the sample from cubic to monoclinic structure was discovered at 1300 °C calcination temperature. In addition, scintillation properties such as luminescent spectra and light intensity under 266 nm UV illumination were measured as a function of calcination condition of as-synthesized Gd₂O₃:Eu powder. The nanocrystalline Gd₂O₃:Eu scintillator with a strong red light emission at near 611 nm wavelength under photo- and X-ray excitation will be employed for its potential X-ray image sensor applications in the future.     

Controlled synthesis and improved luminescent properties of (Gd1−x,Eux)3GaO6 phosphors fabricated via spray pyrolysis

Eu³⁺ activated trigadolinium gallate, Gd₃GaO₆:Eu, was successfully synthesized using various aqueous solutions via ultrasonic spray pyrolysis. The corresponding phosphors illuminated an efficient red emission under Hg-discharge condition with enhanced color purity, which is related to prominent peaks centered at 615 and 629 nm induced by the electric dipole transition of ⁵D₀ → ⁷F₂ of Eu³⁺. By using a proper Ga source for aqueous solution, especially gallium sulfate, uniformly distributed Gd₃GaO₆:Eu³⁺ phosphor with controlled morphology and enhanced emission efficiency was obtained.     

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.     

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.     

A novel green emitting phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb

Vacuum ultraviolet (VUV) excitation and emission properties of Tb³⁺ ion doped silico-aluminate phosphor Ca_1.5Y_1.5Al_3.5Si_1.5O₁₂:Tb³⁺ was studied. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphor showed strong green-emission peaked at 545 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 545 nm was found at a content of about 14 mol% Tb³⁺. The 4f–5d transition absorption of Tb³⁺ is in the range from 150 nm to 260 nm, and there is an energy transfer from the host to the rare earth ions. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm.     

Photoluminescence properties and the self-reduction process of CaAl2Si2O8:Eu phosphor

CaAl₂Si₂O₈:Eu anorthite phosphor was synthesized by the traditional solid state reaction. In the air-sintered phosphor, the Eu ions were partial self-reduced to Eu²⁺, which contributed to the observation of the blue (Eu²⁺) and the red (Eu³⁺) emission in CaAl₂Si₂O₈:Eu phosphor. To further investigate the photoluminescence properties and the self-reduction mechanism of CaAl₂Si₂O₈:Eu phosphor, the adjustment of the valence state of Eu ions by controllable approaches was studied. In addition, CaAl₂Si₂O₈:Eu phosphor shows a tunable emission from reddish to bluish region under ultraviolet excitation, which implies that it could be applied as potential phosphor for near ultraviolet light emitting diode.     

Conversion of green emission into white light in Gd2O3 nanophosphors

Gd₂O₃ nanophosphors were prepared by combustion synthesis with and without doping of Dy³⁺ ions. The X-ray powder diffraction patterns indicate that as-prepared Gd₂O₃ and 0.1 mol% Dy₂O₃ doped Gd₂O₃ nanophosphors have monoclinic structures. The transmission electron microscope (TEM) studies revealed that the as-prepared phosphors had an average crystallite sizes around 37 nm. The excitation and emission properties have been investigated for Dy³⁺ doped and undoped Gd₂O₃ nanophosphors. New emission bands were observed in the visible region for Gd₂O₃ nanophosphors without any rare earth ion doping under different excitations. A tentative mechanism for the origin of luminescence from Gd₂O₃ host was discussed. Emission properties also measured for 0.1 mol% Dy³⁺ doped Gd₂O₃ nanophosphors and found the characteristic Dy³⁺ visible emissions at 489 and 580 nm due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively. The chromaticity coordinates were calculated based on the emission spectra of Dy³⁺ doped and undoped Gd₂O₃ nanophosphors and analyzed with Commission Internationale de l'Eclairage (CIE) chromaticity diagram. These nanophosphors exhibit green color in undoped Gd₂O₃ and white color after adding 0.1 mol% Dy₂O₃ to Gd₂O₃ nanophosphors under UV excitation. These phosphors could be a promising phosphor for applications in flat panel displays.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Rapid synthesis of spherical-shaped green-emitting MgGa2O4:Mn phosphor via spray pyrolysis

Simple, one-step synthesis of spherical-shaped powder phosphors with aqueous precursors via a spray pyrolysis method is reported. Green-emitting MgGa₂O₄:Mn²⁺ phosphor with a controlled shape was successfully obtained by spraying under a reductive atmosphere (N₂ + H₂ carrier gas) without high-temperature post-heat treatment. In addition, the corresponding powder phosphors were well dispersed and showed a clean surface morphology compared to an existing cumbersome process using high-temperature post-annealing. The new method may help to prevent surface residual non-radiative defect sites. The result of highly luminescent and spherical morphology, non-aggregated powder phosphor by this procedure holds promise for a cost-effective and rapid synthesis process for conventional inorganic phosphors.     

Luminescence and EPR studies of Y2O3:Gd3+ phosphors prepared via solution combustion method

Gadolinium-activated Y₂O₃ phosphor has been prepared by combustion process in a short time of 5 min. The phosphors are well characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive analysis of X-ray. The as-prepared Y₂O₃:Gd powder shows that all the peaks are due to the Y₂O₃ cubic phase. Upon UV light excitation (276 nm), the phosphor exhibits a strong and sharp UV emission at 314 nm and is ascribed to ⁶ \textP _{7/ 2}  ?  ⁸ \textS _{7/ 2} ^{ 6} {\text{P}}_{ 7/ 2} \, {\to}\, ^{ 8} {\text{S}}_{ 7/ 2} transition of Gd³⁺ ions. The EPR spectrum of Y₂O₃:Gd phosphor exhibits resonance signals with effective g values at g = 1.96, g = 2.88, and g = 6.08 and are attributed to Gd³⁺ ions located at sites with weak, intermediate, and strong cubic symmetry fields, respectively. It is observed that the population of spin levels (N) and linewidth depends on temperature. The paramagnetic susceptibility (χ) is also evaluated as a function of temperature and discussed.     

Synthesis,characterization and formation mechanism of monodispersed Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S:Eu<Superscript>3+</Superscript> nanocrystals

Monodispersed Gd₂O₂S:Eu³⁺ nanostructures with tunable morphologies have been selectively fabricated by solvothermal method in the presence of stable inorganic precursors avoiding metalorganic precursors. The size and morphology of the products were controlled successfully by adjusting the reaction conditions. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). The corresponding UV absorption and photoluminescence excitation spectra show a significant blue-shift confirming the quantum confinement effect. A possible growth mechanism for the formation of monodispersed Gd₂O₂S:Eu³⁺ nanocrystals has been proposed. The luminescence mechanism and the size dependence of their fluorescence properties are also discussed.     

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.     

Vacuum ultraviolet excited photoluminescence properties of novel Na3Y9O3(BO3)8:Eu phosphor

The novel vacuum ultraviolet (VUV) excited Na₃Y₉O₃(BO₃)₈:Eu³⁺ red phosphor was synthesized and the photoluminescence (PL) properties were investigated. The phosphor showed strong VUV PL intensity, large quenching concentration (40 mol%) and good chromaticity (0.649, 0.351). The Eu³⁺-O²⁻ charge transition (CT) was observed to be at a higher energy (232 nm, 5.35 eV). The host absorption at 127-166 nm was broad and strong when monitoring the Eu³⁺ emission, which indicated that energy transfer from the host-lattice to the Eu³⁺ ions was efficient in Na₃Y₉O₃(BO₃)₈:Eu³⁺. These excellent VUV PL properties were revealed to be correlated with the unique isolated layer-type structure of Na₃Y₉O₃(BO₃)₈ host. The results showed that the Na₃Y₉O₃(BO₃)₈:Eu³⁺ would be a good candidate for VUV-excited red phosphor.     

Luminescence properties of Ba2LiB5O10:Dy phosphor

A novel warm white phosphor Ba₂LiB₅O₁₀:Dy³⁺ with various Dy³⁺ concentrations was synthesized by the conventional solid-state reaction at 800 °C. The crystal structure of the phosphor was characterized by X-ray diffraction (XRD). The photoluminescence properties of Ba₂LiB₅O₁₀:Dy³⁺ were investigated, and the critical concentration of the activator ion (Dy³⁺) was found to be 0.04 mol per formula unit. Under the ultraviolet excitation of 348 nm, the phosphor presented warm white luminescence with dominating emissions at 484.6 and 577 nm, corresponding to ⁴F_9/2-⁶H_15/2, ⁴F_9/2-⁶H_13/2 transitions, respectively. The chromatic properties of the typical sample Ba₂LiB₅O₁₀:0.04Dy³⁺ phosphor have been found to have chromaticity coordinates of x = 0.31 and y = 0.35.     

Crystallinity enhancement of flame-made LiCo0.25Ni0.75O2 nanoparticles by heat-treatment

LiCo_0.25Ni_0.75O₂ nanoparticles of 22.8 nm in geometric mean diameter were synthesized by flame spray pyrolysis from aqueous droplets of nitrate compounds of lithium, cobalt and nickel. The crystallinity of the as-prepared LiCo_0.25Ni_0.75O₂ particles was enhanced by post-heat-treatment at various conditions, such as temperature, duration, and atmospheric gas with different flow rates. The effect of heat-treatment on the particle morphology and crystallinity was investigated systematically. Higher heat-treating temperature under normal air atmosphere improved hexagonal ordering and sufficient O₂ flow during the heat-treatment reduced the cation mixing problem.