Naoto Kijima

Synthetic Method and Luminescence Properties of Sr x Ca1 − x AlSiN3 : Eu2 + Mixed Nitride Phosphors

Single-phase Eu 2+ -doped Sr x Ca 1-x AlSiN 3 (0.2 ≤ x ≤ 0.8) red phosphor was obtained by nitridation of Ca 1-x-y Sr x Eu y (Al 0.5 ,Si 0.5 ) 2 alloy powder under the high pressure of 190 MPa in a hot isostatic pressing apparatus at 2173 K. The unit cell volumes of the single phase expanded linearly with the increase in value of x from 0.2 to 0.8. A blueshift of the red emission peak of Eu 2+ from 650 to 620 nm was observed by weakening the crystal field strength around the Eu 2+ ion. Sr x Ca 1-x AlSiN 3 :0.8 mol % Eu 2+ , with the optimum Eu concentration, showed high photoluminescence intensity over the whole range of x value from 0.2 to 0.8. Unlike nitrides of alkaline-earth or rare-earth metals, the intermetallic alloy powder was stable under ambient conditions and was thus suitable as starting material for red phosphors for industrial production. In x = 0.8 samples synthesized at 190 MPa, the same structure as CaAlSiN 3 , space group 36 (Cmc2 1 ), was retained at unit cell volumes up to 289.92 A 3 , exhibiting the maximum solubility of Sr 2+ in the solid solution, Sr x Ca 1-x AlSiN 3 (0 ≤ x ≤ 0.8).

Photoluminescence and Crystal Structure of Green-Emitting Phosphor CaSc2O4 : Ce3 +

A new green phosphor, Ce 3+ -activated CaSc 2 O 4 , was developed, which shows green luminescence with a peak wavelength of 515 nm under excitation with blue light. Ce 3+ -activated CaSc 2 O 4 can be used as a material for color conversion of white-light-emitting diodes (LEDs), which consist of a blue LED, a green phosphor, and a red phosphor, because the luminescence efficiency of this phosphor is comparable to those of commercial phosphors such as Y 3 Al 5 O 12 :Ce 3+ . The host crystal of this phosphor has an orthorhombic CaFe 2 O 4 structure, and the Ce ion probably exists in an eight-coordinated Ca position. We investigated the dependency of the firing temperature and dopant concentration for luminescence intensity and found that the optimum temperature and concentration were 1600°C and 1 mol % of Ce substituted to the Ca position. The luminescence peak wavelength was shifted toward longer wavelengths by replacing Ca with Mg. In contrast, replacing Ca with Sr resulted in a shift toward shorter wavelengths.

New phosphors for white LEDs: Material Design Concepts

Efficient phosphors for white LEDs have been successfully developed, wherein some Material Design Concepts were utilized to promote our research and development effectively and efficiently. Useful ideas for the development of our red and green phosphors, Sr-rich (Sr,Ca,Eu)AlSiN3, (Ba,Eu)3Si6O12N2 and (Ca,Ce)3(Sc,Mg)2Si3O12, are reviewed.

Redshift of Green Photoluminescence of Ca3Sc2Si3O12 : Ce3 + Phosphor by Charge Compensatory Additives

The luminescence wavelength of Ca 3 Sc 2 Si 3 O 12 :Ce 3+ , which is a green-emitting phosphor under blue light excitation, was moved toward longer wavelength by addition of Mg. This modification is very valuable for commercial applications because it increases the brightness of the phosphor by increasing the overlap with the visibility curve. Magnesium is likely incorporated into the Sc position of the host crystal because the ionic radius of Mg is close to that of Sc. The substitution probably compensates for the excess positive charge induced by incorporation of Ce into the Ca position. The content of Ce in the host crystal was increased by increasing the amount of Mg in the raw material mixture; the inverse relationship was also observed. The spectral shift was also observed by adding Na or Li, which probably substituted the Ca position and compensated the excess charge induced by Ce in the Ca position. The change in luminescence spectra occurred irrespective of the type of charge compensatory additive used. Therefore, the spectral modification was mainly due to the increase in Ce 3+ activator. Modification of the luminescence decay characteristics by the additives was also examined. The decay curve of the phosphor containing Mg showed a nonexponential profile.

Synthesis of Y(P,V)O4:Eu3+ red phosphor by spray pyrolysis without postheating

We synthesized europium-activated yttrium phosphovanadate (Y(P,V)O4:Eu3+) red phosphor, which is expected to be applicable in plasma display panels, by spray pyrolysis. The effect of pyrolysis temperature on luminescence characteristics was clarified and the mechanism of this effect is discussed. A precursor solution containing sodium, which would act as a flux, was used in the first experiments. The obtained phosphor showed high luminance under UV excitation but the luminance under vacuum UV (VUV) excitation was very low. The luminance of the phosphor was improved using precursors without sodium. The phosphor synthesized at 1600°C had the same luminance as that synthesized by solid-state reaction under UV excitation. Its luminance under VUV excitation was 93% of the solid-state phosphor. Sodium ions accelerated the crystal growth of phosphor particles, but they decreased the luminance of the phosphor, particularly under VUV excitation.

Nitridoaluminosilicate CaAlSiN3 and its Derivatives - Theory and Experiment

Nitridoaluminosilicate MAlSiN 3 (M: alkaline-earth element) and its derivatives have attracted more and more attention owing to the fact that the material doped with rare-earth element has intense body color and exhibit efficient luminescence under InGaN diode irradiation. In particular, red phosphor, Eu-doped CaAlSiN 3 (CASN), has good thermal property of luminescence and sufficient chemical durability for white LED use. Still, for the lineup of various kinds of white color, it is hoped to tune the red luminescence with other physical/chemical properties kept as possible. Thus the derivatives with different chemical compositions have been intensively explored so far. For the feasibility of such chemical composition change, it is necessary to understand its atomic/electronic structure of the unique crystal, which is a distorted AlN-based wurtzite superstructure (Cmc21, No.36) with Al and Si disordered on 8b site and Ca occupying 4a site. Recently, we have performed first-principles band calculation of CASN and clarified the origin of the Al/Si disorder configuration as well as the feasibility of the virtual crystal approximation of heterovalent cations (Al/Si) for the reproducibility of atomic/electronic structure of CASN.[1] As a natural extension of this study, we have investigated some CASN-derivatives to confirm/predict the crystal structure. The VCA allows us to model the superstructure with various chemical compositions quite easily. In this work, we will present two examples of solid-solution, (Ca,Sr)AlSiN 3 and CaAlSiN 3 -Si 2 N 2 O. The agreement between experiment and theory appears quite satisfactory. It is emphasized that the crystal structure of SrAlSiN 3 has been successfully predicted by first-principles calculation prior to experimental result. The collaboration of experiment and theory promises us ‘gcrystal-engineering’ to develop new nitrides/oxynitrides effectively and efficiently.

Oxidation of doped europium in BaMgAl10O17 by annealing studied by x-ray-absorption fine-structure measurements

We studied the influence of annealing in air on doped europium in BaMgAl 10 O 17 by performing x-ray absorption fine-structure measurements. We determined the oxidation of doped divalent europium by annealing in air at over 500°C. The interatomic distance between the europium and the surrounding oxygen atoms was compressed by oxidation. It also appears that the oxidation process of europium is determined by the diffusion of oxygen into BaMgAl 10 O 17 .

Improved Optical Degradation Characteristics of Eu Complex Encapsulated by High-Pressure Annealing

The instability of Eu complexes against ultraviolet (UV) light irradiation is an important problem to solve before they can be practically applied in white light-emitting diodes. A novel technique of encapsulating tris(2-thenoyltrifluoroacetonato)(1,10-phenanthroline)europium(III) [Eu(TTA)3phen] was investigated using high-pressure annealing (solvothermal process) as a final process in the sol–gel synthesis. The photoluminescence and excitation spectra of encapsulated Eu(TTA)3phen samples synthesized by solvothermal and conventional annealing processes were almost the same. A half brightness time of 589 min was achieved while irradiating with UV light of 360 nm and 5 mW/cm2 by optimization of ammonia concentration and annealing temperature. The longest half brightness time was longer than that of encapsulated Eu(TTA)3phen synthesized by conventional thermal treatment. One possible reason for this result is that the chemical reaction of the sol–gel based glass network occurs more efficiently with high-pressure annealing. As a result, a high encapsulating efficiency was achieved owing to the small amount of organic component in the sol–gel derived glass network.

XAFS Analysis of Local Structure around Ce in Ca3Sc2Si3O12:Ce Phosphor for White LEDs

We have studied the local structure around Ce atom in Ca3Sc2Si3O12 host crystal, which has been developed as a new green phosphor for white light emitting diodes (LEDs). As the local structure and chemical environment of the dopant atom are very important to improve the performance of the phosphor, we have used XAFS to get chemical and structural information around the Ce dopant. The XANES spectrum of the Ce LIII‐edge reveals that the Ce atom is trivalent in Ca3Sc2Si3O12. There are two kinds of possible Ce substitution sites, Ca site and Sc site, in garnet type Ca3Sc2Si3O12 crystal structure. The Ce atom is found to be at the Ca site in the host crystal by the comparison of the Fourier transform of Ce K‐edge EXAFS spectrum with those of Ca and Sc K‐edge EXAFS spectra. The theoretical analysis with FEFF also clarified the Ce substitution at the Ca site. Furthermore, the result of the analysis indicates the structural disorder around Ca and Si atoms at 3.75 A. It is possible that there are some defects arou...

Synthetic Conditions and Structural Properties of the High-Tc Phase in the Superconducting Bi-Sr-Ca-Cu-(Pb)-O System

Synthetic conditions, structural and thermal properties of the high-Tc phase (Tc > 100 K) in the Bi-Sr-Ca-Cu-O system have been investigated. Three methods have been found to be effective in increasing the volume fraction of the high-Tc phase in the Bi-Sr-Ca-Cu-0 system: (1)starting from a nominal composition with more Ca and Cu than Bi2Sr2Ca2Cu3O (2) the addition of Pb to Bi-Sr-Ca-Cu-0 system (3) annealing at 870°C under higher oxygen partial pressures than 0.2 atm. Sample with the nominal composition of Bi2Sr2Ca6Cu8Ox fired at 87O°C for 120 hours was found that it consisted of mainly the high-Tc phase. Unit cell dimensions of the high-Tc phase determined by X-ray powder diffraction and transmission electron diffraction measurement are a = 5.40 A, b = 27.0 A and c = 36.8 A, indicating a pseudotetragonal symmetry. Transmission electron microscopy showed that there were stacking faults in the high-Tc phase. The high-Tc phase and the low-Tc phase (T ~ 80 K) decompose into Bi2Sr2CuOx by annealing under oxygen partial pressure of 0.02 and 0.1 atm at 87O°C. Under higher oxygen partial pressures than 0.2 atm at 870°C, the high-Tc phase was remarkably formed from the possible disproportionation reaction of the low-Tc phase.