Ca2B5O9Cl:Eu2+, A Suitable Blue-Emitting Phosphor for n-UV Excited Solid-State Lighting

Blue-emitting Ca₂B₅O₉Cl:Eu²⁺ phosphors have been synthesized by solid state reaction. The photoluminescence excitation (PLE) spectra show broad-band absorptions and can match the emission of near ultraviolet (n-UV) chip well. At lower Eu²⁺ concentration, the emission band can be resolved into two bands, which is assigned to the 5 d →4 f transition of Eu²⁺ ions substituting two different Ca²⁺ sites. At higher Eu²⁺ concentration, the energy transfer from Eu(1) to Eu(2) happens and is very efficient. At higher temperature the phosphor exhibits a lower temperature quenching effect. The fluorescence lifetimes are short enough for application in solid-state lighting. The electroluminescence spectrum indicates that the emission of chip can almost be absorbed by phosphor and down-converted into an intensive blue light. The chromaticity coordinates of fabricated light-emitting-diodes (LEDs) is very close to that of BaMgAl₁₀O₁₇:Eu²⁺ (BAM). Ca₂B₅O₉Cl:Eu²⁺ is a good blue component phosphor for n-UV excited solid-state lighting.     

Quaternary Compounds Prepared in a CaO–Y2O3–SnO2 System

Compounds in a CaO–Y₂O₃–SnO₂ system were prepared by a solid-state reaction at 1673 K. The phase relation in this system was investigated by powder X-ray diffraction. Besides the previously reported ternary compounds, CaSnO₃, Ca₂SnO₄, Y₂Sn₂O₇, and a quaternary compound Ca_0.4Y_1.2Sn_0.4O₃, solid-solution series of Ca_{2− x} Y_{2 x} Sn_{1− x} O₄ with 0≤ x ≤0.5, and Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ with 0≤ y ≤0.2 and 0.95≤ y ≤1.0 were found. The cell parameters of these solid-solution series were refined. The changes of rhombohedral cell parameters in the samples prepared in the range 0.565< y <0.714 of Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ suggested the existence of solid solutions of Ca_0.4Y_1.2Sn_0.4O₃, although their single phases could not be prepared, except at y =0.6.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

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.     

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.     

Rare-Earth Barium Stannates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-δ Superconductor

A new group of complex perovskites, REBa₂SnO_5.5 (where RE = Pr, Sm, Eu, and Gd) have been synthesized and sintered as single-phase materials with high sintered density and stability by the solid-state reaction method. The structure of REBa₂SnO_5.5 was studied by the X-ray diffraction technique, and all of them were found to be isostructural and have a cubic perovskite structure. The dielectric constant and loss factor values of these materials are in a range suitable for substrate applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable reaction between YBa₂Cu₃O_7-δ and REBa₂SnO_5.5, even when the substances are mixed thoroughly and sintered at 950°C for 15 h. Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline REBa₂SnO_5.5 substrates gave superconducting zero resistivity transition T _C(0)= 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-δ films.     

New Red- and Green-Emitting Phosphors, AYP2O7.5:RE3+ (A=Ca and Sr; RE=Eu and Tb) under Near-UV Irradiation

New red- and green-emitting phosphors, AY_{1− x} P₂O_7.5: x RE³⁺ (A=Ca and Sr; x =0.01, 0.03, 0.05, 0.10; RE=Eu and Tb) were synthesized by the conventional ceramic route and their photoluminescence properties under near-ultraviolet (UV) irradiation were investigated. It was found that CaYP₂O_7.5: Eu³⁺ phosphor emits strong red light when excited by a radiation of 394-nm wavelength and SrYP₂O_7.5:Eu³⁺ gives intense orange light when excited by a radiation of 396-nm wavelength. Strong green emission for AY_{1− x} P₂O_7.5:Tb³⁺ is also observed under near-UV irradiation (378 nm). When compared with emission intensity from a standard YPO₄:0.05Tb³⁺, the emission from SrYP₂O_7.5:0.05Tb³⁺ showed greater intensity values under the same excitation wavelength (378 nm). X-ray powder diffraction analysis showed that AYP₂O_7.5 has xenotime-type structure.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

The Join Ca2SiO4-CaMgSiO4

New data obtained on the join Ca₂SiO₄-CaMgSiO₄ established a limit of crystalline solubility of Mg in α-Ca₂SiO₄ corresponding to the composition Ca_1.90Mg_0.10SiO₄ at 1575°C. The α-α'Ca₂SiO₄ inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca₂SiO₄ takes Mg into its lattice up to the composition Ca_1.94Mg_0.06SiO₄ at 1400°C and to Ca_1.96Mg_0.04SiO₄ at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca_1.70Mg_0.30SiO₄, was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO₂.     

Synthesis and Luminescence Properties of Orange–Red-Emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) Light-Emitting Diode Conversion Phosphors by a Simple Nitridation of MSi2

Eu²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) orange–red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi₂, Eu₂O₃, and α-Si₃N₄ as the starting materials under N₂–H₂ (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M₂Si₅N₈ for M=Ca, Sr, and a little amount of BaSi₇N₁₀ impurity phase for M=Ba. Under the blue light excitation, M₂Si₅N₈:Eu²⁺ shows a typical broad band emission of Eu²⁺ ranging from orange to red (585–620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca2Si₅N₈:Eu²⁺ has the highest application potential as a red conversion phosphor for white light-emitting diodes.     

Photoluminescence Characteristics of Energy Transfer Between Eu3+and Bi3+in LiEu1−xBix (WO4)0.5(MoO4)1.5

A series of novel red phosphors LiEu_{1− x} Bi _x (WO₄)_0.5(MoO₄)_1.5 ( x =0, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40, and 0.50) were synthesized by the conventional solid-state reaction method. The spectrum and the crystal structure of the phosphors were characterized by Fluorescence spectrophotometry and X-ray diffraction, respectively. The photoluminescent results show that all samples can be excited efficiently by UV (396 nm) and blue (467 nm) light and that they emit red light at 615 nm with line spectra, which are coupled well with the characteristic emissions from UVLED and blue light-emitting diode (LED), respectively. There is an efficient energy transfer from Bi³⁺ to Eu³⁺ ions, leading to the emission intensity of Eu³⁺ being enhanced by 1.5 times, and even more when Bi³⁺ ions are introduced into LiEu (WO₄)_0.5(MoO₄)_1.5. The introduction of Bi³⁺ ions broadened the excitation band of the phosphor, and the optimum doping concentration is found to be 10 mol% of Bi³⁺.     

Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites

Three types of dicalcium silicate (Ca₂SiO₄–calcium zirconate (CaZrO₃) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca₂SiO₄ was added as a minor phase. The second type consisted of a 50 vol% Ca₂SiO₄-50 vol% CaZrO₃ mixture, while in the third type, CaZrO₃ constituted the minor phase. Pure CaZrO₃ was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca₂SiO₄ as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m^1/2, which was about double that of the monolithic CaZrO₃. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca₂SiO₄. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO₃ agglomerates as well as weak interfaces between the two phases.     

The System CaO–"CaCr2O4"–CaAl2O4 in Air and under Mildly Reducing Conditions

The system CaO–chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca₅Cr₃O₁₂, Ca₃(CrO₄)₂, and Ca₅(CrO₄)₃) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, δ -CaCr₂O₄, is observed. In the CaO-rich section of the CaO–Al₂O₃–Cr₂O₃ system a ternary intermediate phase, chrome-haüyne, Ca₄[(Al,Cr³⁺)₆O₁₂](Cr⁶⁺O₄), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca₆Al₄Cr₂O₁₅, coexists with CaO, CaCr₂O₄, chrome-haüyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO–Al₂O₃ system is insignificant.     

The Systems ZnO-Fe2O3-SnO2 and MgO-Fe2O3-SnO2 at 1060°C

Zn₂SnO₄, an inverse spinel, and ZnFe₂O₄, a normal spinel, form a complete series of solid solutions in the system ZnO-Fe₂O₃3nO₂. The variation of cell dimensions with composition varies from 0.8439 nm (ZnS₂nO₄) to 0.8660 nm (ZnS₂nO₄) and exhibits a positive deviation from a linear relationship. Mg₂SnO₄ and MgFe₂O₄, both predominantly inverse in nature, have only an incomplete series.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

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.     

Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites

(Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) precursor powders with large and small amounts of Ca₂PbO₄ phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca₂PbO₄ on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature ( T _m,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca₂PbO₄. Tapes with a small amount of Ca₂PbO₄ had a higher T _m,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca₂PbO₄. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current-carrying capacity of the tapes were significantly improved by reducing the Ca₂PbO₄ content of the precursor powders. These results are of practical significance for the commercialization of Bi-based high-temperature superconductors.     

Preparation and Identification of Ca2Zn4Ti15O36—A New Structure Type

A ternary oxide containing Ca, Zn, and Ti was characterized by chemical and X-ray analysis as Ca₂Zn₄Ti₁₅O₃₆. It crystallizes with rhombohedral symmetry in space group R3 , or possibly R3 . The hexagonal cell dimensions are α= 1.0371±0.0005 and c=2.0938±0.0010 nm; density is 4.18 g/cm³. Ca₂Zn₄Ti₁₅O₃₆ is the first member of this new type of structure to be synthesized; several other examples were also prepared. The mineral davidite is probably isostructural with these pure synthetic compounds and the mineral senaite appears to have a closely related structure.     

A Potential Red-Emitting Phosphor BaGd2(MoO4)4:Eu3+ for Near-UV White LED

Red-emitting phosphor BaGd_{2− x} Eu _x (MoO₄)₄ has been successfully synthesized by a simple sol–gel method. The process of phosphor formation is characterized by thermogravimetric-differential thermal analysis and X-ray diffraction. Field-emission scanning electronic microscopy is used to characterize the size and the shape of the phosphor particles. Photo-luminescent property of the phosphor is also performed at the room temperature. The effects of firing temperature and Eu³⁺ activator concentration on the photoluminescence (PL) properties are elaborated in detail. PL characterization reveals that the sample with the firing temperature at 800°C and the concentration of Eu³⁺ at 0.7 shows the most intense emission, and its intensity is about three times stronger than that of phosphor prepared by solid-state method with the same composition and firing temperature. The new red-emitting phosphor shows an intense absorption at 396 nm, which matches well with commercial near-UV light-emitting diode (LED) chips, therefore, it is a good candidate of red phosphor used for near-UV white LEDs.