Combinatorial synthesis of phosphors using arc-imaging furnace

Abstract

We have applied a novel ‘melt synthesis technique’ rather than a conventional solid-state reaction to rapidly synthesize phosphor materials. During a synthesis, the mixture of oxides or their precursors is melted by light pulses (10–60 s) in an arc-imaging furnace on a water-cooled copper hearth to form a globule of 1–5 mm diameter, which is then rapidly cooled by turning off the light. Using this method, we synthesized several phosphor compounds including Y₃Al₅O₁₂:Ce(YAG) and SrAl₂O₄:Eu,Dy. Complex phosphor oxides are difficult to produce by conventional solid-state reaction techniques because of the slow reaction rates among solid oxides; as a result, the oxides form homogeneous compounds or solid solutions. On the other hand, melt reactions are very fast (10–60 s) and result in homogeneous compounds owing to rapid diffusion and mixing in the liquid phase. Therefore, melt synthesis techniques are suitable for preparing multi component homogeneous compounds and solid solutions.     

Synthesis and luminescent properties of SrAl2O4:Eu green-emitting phosphor for white LEDs

SrAl₂O₄:Eu²⁺ phosphor was prepared by a solid-state reaction in CO-reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of SrAl₂O₄:Eu²⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 7–8 μm. Photoluminescence measurements showed that the phosphor can be efficiently excited by UV–visible light from 350 to 430 nm, and exhibited bright green emission peaked at about 516 nm. Bright green LEDs were fabricated by incorporating the phosphor with an InGaN-based UV chip. All the characteristics indicated that SrAl₂O₄:Eu²⁺ is a good candidate phosphor applied in white LEDs.     

Luminescent properties of Eu doped α-Gd2(MoO4)3 phosphor for white light emitting diodes

A novel red emitting phosphor α-Gd₂(MoO₄)₃:Eu³⁺ was developed for white light emitting diodes (LEDs). The phosphor was prepared by solid-state reaction. The effects of the flux content and the activator concentration on the crystal structure, morphology and luminescent properties were investigated by using XRD, SEM, and fluorescent spectra. These results showed that this phosphor can be effectively excited by ultraviolet (UV) (395 nm) and blue (465 nm) light, matching the output wavelengths of ultraviolet or blue LED chips. The α-Gd₂ (MoO₄)₃ phosphor may be a better candidate for solid state lighting application.     

Synthesis and Properties of Zn2TiO4–Zn2SnO4–ZnFe2O4 Solid Solutions

Zn₂TiO₄–Zn₂SnO₄–ZnFe₂O₄ mixed oxides are prepared by low-temperature hydrogen–oxygen plasma synthesis and solid-state reactions. Zn_{2 – x} (Ti _a Sn _b )_{1 – x} Fe_2x O₄ (a+ b = 1, x = 0–1.0) solid solutions are shown to have an inverted cubic spinel structure (sp. gr. Fd3m). The lattice parameter, electrical conductivity, band gap, and molar polarizability of about 100 samples are determined. With increasing Fe content, the lattice parameter, degree of spinel inversion, density, activation energy, and polarizability of the solid solutions decrease, while their conductivity rises.     

Coprecipitation synthesis and optical absorption property of Zn2TixSn1–xO4 (0?≤?x?≤?1) solid solutions

We report a coprecipitation method for the preparation of solid solutions in the Zn₂Ti _x Sn_1–x O₄ (0 ≤ x ≤ 1) series. The precipitates obtained from the coprecipitation were calcined using different temperatures and then characterized with X-ray diffraction (XRD), Raman scattering (RS), scanning electron microscopy (SEM), and surface area measurements to gain insights into the solid-state reaction and phase transformation during the calcinations. Formation of the Zn₂Ti _x Sn_1–x O₄ solid solutions was observed after the calcination up to 1000 °C, which is much lower than the temperature (1300 °C) required in the conventional solid-state reaction method. The optical absorption property of the Zn₂Ti _x Sn_1–x O₄ solid solutions, measured by ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS), was shown to change according to the composition of the solid solutions.     

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.     

Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles

Eu²⁺,Dy³⁺ co-doped strontium aluminate (SrAl₂O₄) phosphor nanoparticles with high brightness and long afterglow were prepared by glycine–nitrate solution combustion synthesis at 500 °C, followed by heating the resultant combustion ash at 1100 °C in a weak reductive atmosphere of active carbon. The average particle size of the SrAl₂O₄:Eu,Dy phosphor nanoparticles ranges from 15 to 45 nm as indicated by transmission electron microscopy (TEM). The broad-band UV-excited luminescence of the SrAl₂O₄:Eu,Dy phosphor nanoparticles was observed at λ_max=513 nm due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The results indicated that the main peaks in the emission and excitation spectrum of phosphor nanoparticles shifted to the short wavelength compared with the phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for phosphor nanoparticles was faster than that obtained by the solid-state reaction method.     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Use of LiF flux in the preparation of Ca3Sc2Si3O12:Ce3+ phosphor by sol-combustion method

LiF flux was added when prepare Ca₃Sc₂Si₃O₁₂:Ce³⁺ phosphors with sol-combustion method using urea as a fuel and chelating agent. Investigation of phase, morphology and photoluminescence of the phosphors reveals that this kind of method can decrease the phase-forming temperature by 400–500 °C and the phosphors show more uniform morphology and more excellent photoluminescence properties compared to conventional solid-state method. As one of the most important parameters for phosphors used in white light emitting diodes, the thermal quenching properties of the prepared samples are compared and discussed. The fabrication of single-color light emitting diodes show the Ca₃Sc₂Si₃O₁₂:Ce³⁺ phosphor prepared by sol-combustion method with LiF flux can be considered as a more promising high efficiency candidate used for white light emitting diodes than the phosphor made by solid-state method.     

Luminescent properties of Eu3+ doped α-Gd2(MoO4)3 phosphor for white light emitting diodes

A novel red emitting phosphor α-Gd₂(MoO₄)₃:Eu³⁺ was developed for white light emitting diodes (LEDs). The phosphor was prepared by solid-state reaction. The effects of the flux content and the activator concentration on the crystal structure, morphology and luminescent properties were investigated by using XRD, SEM, and fluorescent spectra. These results showed that this phosphor can be effectively excited by ultraviolet (UV) (395 nm) and blue (465 nm) light, matching the output wavelengths of ultraviolet or blue LED chips. The α-Gd₂ (MoO₄)₃ phosphor may be a better candidate for solid state lighting application.     

Simultaneous synthesis and densification of transparent,photoluminescent polycrystalline YAG by current activated pressure assisted densification (CAPAD)

We report a method for the synthesis and processing of transparent bulk polycrystalline yttrium aluminum garnet (YAG) and photoluminescent Ce-doped YAG ceramics via solid-state reactive-current activated pressure assisted densification (CAPAD). The process uses commercially available γ-Al₂O₃, Y₂O₃, and CeO₂ nanopowders. The nanopowders were reacted and densified simultaneously at temperatures between 850 °C and 1550 °C and at a maximum pressure of 105 MPa. The solid-state reaction to phase pure YAG occurs in under 4 min at processing temperatures 1100 °C which is significantly faster (on the order of tens of hours) and occurs at much lower temperatures (∼600 °C) compared to conventional reaction sintering. We found that the reaction significantly improves densification – the shrinkage rate of reaction-produced YAG was three times higher than that of YAG using pre-reacted powder. The Ce additions were found to retard the reaction driven shrinkage kinetics by a factor ∼3, but are still faster (by a factor ∼1.6) than those associated with direct densification (no synthesis). Densities >99% were achieved in both pure YAG and Ce doped YAG (Ce:YAG). Results of optical measurements show good transparency in the visible and photoluminescence (PL) in the Ce:YAG. The PL peak is broad and appears white when excited using blue light confirming that the ceramics can be used in solid state lighting to produce white light.     

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.     

PZT Material by Spray-Dried Precursors and Solid-State Reaction: Cold Consolidation and Sintering

The Nb-doped Pb(Zr_0.52 Ti_0.48)O₃ (PZT) powder, synthesized by spray drying of the solution followed by calcination, was cold consolidated and sintered under different process conditions. The microstructure and final properties were compared with the material produced by the conventional solid-state reaction of the oxides. The spray-dried powder undergoes a complete reaction in the perovskitic phase at 550°C, while the mixed oxides are converted at temperatures not lower than 800–850°C. Because of the hollow spherical structure of the spray-dried powder, low green densities are obtained. Consequently, a grinding process, as well as high-pressure cold isostatic compaction, were applied. The high reactivity of the powder results in the reduction of the densification temperature of 100°C. The final microstructure differs substantially from that developed with mixed-oxide processing and a different sintering mechanism is proposed.     

Fine yellow α-SiAlON:Eu phosphors for white LEDs prepared by the gas-reduction–nitridation method

Yellow-emitting α-SiAlON:Eu²⁺ phosphors were synthesized by the gas reduction and nitridation of a homogeneous oxide precursor in a CaO–Al₂O₃–SiO₂–Eu₂O₃ system at 1400–1450 °C using an NH₃–CH₄ mixture gas as a reduction–nitridation agent. The precursor was prepared by a sol–gel process using a low-cost nitrate, tetraethyl orthosilicate and citric acid as the starting materials. The effects of reaction parameters such as heating rate, temperature, holding time and CH₄ content on the composition, microstructure and photoluminescence of the prepared powders were investigated. Nearly single-phase α-SiAlON was successfully synthesized by the one-step gas reduction and nitridation without the need for post-annealing at a higher temperature. The prepared powders consisted of relatively well-dispersed and uniform crystals with a hexagonal shape. The photoluminescence spectra of Eu-doped Ca-α-SiAlON phosphors excited by near-ultraviolet or blue light showed a broad, yellow emission band at 500–700 nm, which agrees well with that obtained from phosphors prepared by the conventional solid-state reaction.     

Preparation and properties of Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se (M = Ti,V, Fe,Co) solid solutions

Mn_{1 – x} M_x Se (M = Ti, V, Fe, Co) solid solutions are prepared by solid-state reactions and high-pressure synthesis (7 GPa, 1600 K). All of the solid solutions have a cubic (NaCl) structure, and their lattice parameter decreases with increasing solute concentration. The magnetic susceptibility of the solid solutions is measured from 77 to 900 K by the Faraday method. Below 130–150 K, the solid solutions undergo antiferromagnetic ordering.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1431–1434.Original Russian Text Copyright © 2004 by Makovetskii, Galyas, Demidenko     

A red-emitting phosphor of fully concentrated Eu-based molybdenum borate Eu2MoB2O9

The fully concentrated Eu³⁺-based molybdenum borate Eu₂MoB₂O₉ was synthesized by the solid-state reaction method. The photoluminescence excitation and emission spectra, the temperature dependent luminescence intensities and the decay curve were investigated. Photoluminescence spectra show that the phosphor can be efficiently excited by near-UV light and exhibits an intense red luminescence corresponding to the electric dipole transition ⁵D₀ → ⁷F₂ at 615 nm. The luminescence intensities and color purity were investigated by increasing the fired temperatures. The phosphor shows the stable luminescence and color purity at high temperature.     

Synthesis and Physicochemical Properties of Crystalline and Glassy La14 – x – y Gd x Eu y (BO3 )6(GeO4)2O8 Solid Solutions

Using solid-state reactions, La_{14 – x – y} Gd _x Eu _y (BO₃)₆(GeO₄)₂O₈ solid solutions isostructural with Ln₁₄(BO₃)₆(GeO₄)₂O₈ (Ln = Pr, Nd, Sm, Eu, Gd) were prepared for the first time, and their melting points and melting behavior were established. Liquid quenching of the solid solutions was found to yield thermally stable glasses containing as much as 60 mol % rare-earth oxides. The composition limits of the glass-forming region were determined, and the luminescence spectra of both crystalline and glassy solid solutions were studied.     

Mechanochemical synthesis of indium complex oxides (InAO4; A = P, V, Nb, Ta, Sb) and their solid solutions

Indium complex oxides, InAO₄ (A = P, V, Nb, Ta, Sb), were synthesized by milling of the In₂O₃ and A ₂O₅ mixture using planetary ball mill at room temperature. Formation of these compounds was monitored by powder X-ray diffraction (XRD) and was found to be complete with 240 or 480 min. This method has been successfully extended to the synthesis of solid solutions, InP_1−x V _x O₄ and InTa_1−x Nb _x O₄, between these indium complex oxides.     

Wurtzite-derived ternary I–III–O2 semiconductors

Ternary zincblende-derived I–III–VI₂ chalcogenide and II–IV–V₂ pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I–III–O₂ oxide semiconductors with a wurtzite-derived β-NaFeO₂ structure are limited. Wurtzite-derived β-LiGaO₂ and β-AgGaO₂ form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. β-CuGaO₂, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I–III–O₂ semiconductors is still limited. In this paper we review previous studies on β-LiGaO₂, β-AgGaO₂ and β-CuGaO₂ to determine guiding principles for the development of wurtzite-derived I–III–O₂ semiconductors.     

The effect of addition of alcohol and calcination methods on the synthesis of layered Li[Li1/5Ni1/10Co1/5Mn1/2]O2 using solid-state reaction method

Li[Li_1/5Ni_1/10Co_1/5Mn_1/2]O₂ solid solutions were prepared by four different routes using a solid-state reaction method. All the prepared samples were of a layered manganese oxide structure. The prepared samples were subjected to XRD, Raman and charge–discharge studies. The addition of ethyl alcohol in the starting materials during the synthesis process has exhibited superior electro-chemical properties and offered higher discharge capacity than the samples calcinated in the form of pellets without solvent. The added ethyl alcohol not only assists to get a homogeneous mixing of reacting species, but also influences the chemical reaction during the synthesis process. The materials are found to be superior in terms of minimum capacity fading per cycle than the materials so far reported. The other electro-chemical properties like irreversible capacity and discharge capacity of the synthesized materials have also been discussed based on the experimental observations.