Hiromasa Hanzawa

Characterization and luminescence properties of Sr2Si5N8:Eu2+ phosphor for white light-emitting-diode illumination

Eu2+-doped ternary nitride phosphor, Sr2Si5N8:Eu2+, was prepared by the carbothermal reduction and nitridation method. The Rietveld refinement analysis showed that the single phase products were obtained. Two main absorption bands were observed on the diffuse reflection spectra peaking at about 330 and 420nm, so that the resultant phosphor can be effectively excited by InGaN light-emitting diodes. The emission peak position of (Sr1−xEux)2Si5N8:Eu2+ series varied from 618to690nm with increasing Eu2+ ion concentration. The redshift behavior of the emission band was discussed on the basis of the configuration coordination model.

Preparation of ( Sr1 − x Ca x ) 2Si5N8 ∕ Eu2 + Solid Solutions and Their Luminescence Properties

A series of ternary nitride solid solutions with a general formula of (Sr 1-x Ca x ) 2 Si 5 N 8 /Eu 2+ (2 atom %) were synthesized by the carbothermal reduction and nitridation (CRN) method. The structure and luminescence properties were characterized for practical applications as a potential red phosphor for the phosphor-converted white light-emitting diodes (LEDs). The solid solutions were formed as the uniformly single phases with orthorhombic (Pmn2 1 ) and monoclinic (Cc) symmetry at each end of (Sr 1-x Ca x ) 2 Si 5 N 8 , while in the range of 0.5 < x < 0.75, such solid solution phases coexisted. All of the obtained phosphors were well crystallized and the grains grew from 5 to 20 μm with increasing Ca 2+ content. These phosphors showed broadened excitation spectra originated from the 4f 7 → 4f 6 5d transition of Eu 2+ ions, so that the intense orange-red emission bands were observed under the excitation of 380-470 nm corresponding to the output lights of near-UV or blue LEDs.

Self-propagating high temperature synthesis of yellow-emitting Ba2Si5N8:Eu2+ phosphors for white light-emitting diodes

High-performance nitride phosphors, Ba2Si5N8:Eu2+, were prepared by the self-propagating high temperature synthesis method. The broadband absorption in the range of 380–500nm indicated that the resultant phosphors were effectively excited by an InGaN blue light-emitting diode (400∕470nm). The emission peak position of Ba2−xEuxSi5N8 (x=0–0.3) varied from 572to650nm with increasing Eu2+ concentration, and the intensity was maximized at x=0.04. The optimized sample gave a strong yellow emission peak at 580nm. The redshifting behavior of the emission band was attributed to three factors: energy migration, crystal field strengthening, and reabsorption.

Reddish-Orange Long-Lasting Phosphorescence of Ca2Si5N8 : Eu2 + , Tm3 + Phosphor

Reddish-orange long-lasting phosphorescence materials, various amounts of Eu 2+ - and Tm 3 +-co-doped phosphors (Ca 2 Si 5 N 8 :Eu 2+ ,Tm 3+ ), were prepared by the conventional high temperature solid-state reaction method, and their luminescence properties were systematically investigated by photoluminescence spectra, afterglow spectra, afterglow decay curves, and thermoluminescence spectra. The Ca 2 Si 5 N 8 :Eu 2+ ,Tm 3+ materials exhibited strong reddish-orange emission bands in a wavelength range of 500-750 nm with peak positions at about 600 nm due to the 5d → 4f transition of the Eu 2+ ion. Furthermore, these phosphors emitted strong reddish-orange long-lasting phosphorescence with an afterglow time of more than 1 h after turning off the activating lamp in the light perception of the dark-adapted human eye (0.32 mcd/m 2 ). Such afterglow of Ca 2 Si 5 N 8 :Eu 2+ ,Tm 3+ phosphors was attributed to the recombination of holes and electrons that were trapped within the lattice defect centers. The results of the thermoluminescence spectra indicated that the increase in the predominating band at 350 K, which was associated with the charge-trapping centers, was responsible for the enhancement of the afterglow properties of Ca 2 Si 5 N 8 :Eu 2+ Tm 3+ .

Preparation and Luminescence Properties of Single-Phase BaSi2O2N2 : Eu2 + , a Bluish-Green Phosphor for White Light-Emitting Diodes

BaSi 2 O 2 N 2 :Eu 2+ (Ba 1―x Eu x Si 2 O 2 N 2 ) phosphor was prepared by a method using Ba 2 SiO 4 :Eu 2+ as a precursor with a starting composition of (Ba 1―x Eu x ) 2 Si l―u O 4―2u :Si 3 N 4 = 1/2:(3 + u)v/6 (x = 0―0.1, u = 0―0.05, v = 1-1.05). The formation behavior of BaSi 2 O 2 N 2 :Eu 2+ and secondary phases was investigated in detail by means of X-ray diffraction analysis of the samples prepared under various conditions. The formation of secondary phases was suppressed by the method employed in this work and a single phase of BaSi 2 O 2 N 2 :Eu 2+ was successfully obtained when the starting compositions with x = 0-0.7, u = 0.05, and u = 1.05 were used. BaSi 2 O 2 N 2 :Eu 2+ phosphors showed the bluish-green emission peaking at 490-500 nm along with varying concentrations of Eu 2+ from 1 to 10 mol % (x = 0―0.1); particularly, the BaSi 2 O 2 N 2 :Eu 2+ (5 mol %, x = 0.05) phosphor showed an exceedingly strong emission peak intensity, which was about 3.7 times higher than that of the commercial yellow phosphor, (Y,Gd) 3 Al 5 O 12 :Ce 3+ (P46-Y3), under 460 nm excitation light. Furthermore, this phosphor maintained luminescence intensity at 150°C by 75% of the value measured at room temperature. These excellent luminescence properties and small thermal quenching suggested this phosphor to be applied as a bluish-green component for phosphor conversion white light-emitting diodes.

Synthesis of Nitridosilicate CaSr1-xEuxSi5N8 (x = 0-1) Phosphor by Calcium Cyanamide Reduction for White Light-Emitting Diode Applications

A convenient calcium cyanamide (CaCN 2 ) reduction route was developed to synthesize the Eu 2+ -doped nitridosilicate phosphors, CaSr 1-x Eu x Si 5 N 8 (x = 0-1), and the photoluminescence properties of the obtained phosphors were characterized for solid-state lighting applications [white light-emitting diodes (LEDs)]. The Rietveld refinement analysis for undoped CaSrSi 5 N 8 was carried out in space group Pmn2 1 (no. 31) with a = 569.0(4) pm, b = 673.0(1) pm, and c = 932.0(6) pm. The Eu 2+ ions formed a complete solid solution by occupying the Sr 2+ sites in CaSrSi 5 N 8 host lattice. A typical sample doped with an optimized amount of Eu 2+ (2 atom % vs Ca/Sr) showed a broadband absorption covering the range from UV to visible region (380-500 nm) and strong red emission peaking at 629 nm, revealing that this phosphor was a potential candidate for the phosphor-converted white LEDs. Under excitation of 460 nm, this phosphor gave the intensity comparable (about 120%) to that of yttrium aluminum garnet YAG:Ce 3+ (P46-Y3) standard. The emission peaks of CaSr 1-x Eu x Si 5 N 8 :Eu 2+ varied from 625 to 680 nm with increasing Eu 2+ ion concentration. The redshifting behavior of the emission band dominantly contributed to the energy migration among Eu 2+ ions.

Facile Combustion Route for Low-Temperature Preparation of Sr2SiO4:Eu2+ Phosphor and Its Photoluminescence Properties

Uniform and nanosized phosphors of Sr2SiO4:Eu2+ were successfully synthesized by a simple, inexpensive, and relatively low-temperature synthesis route using the combustion method. Through the chemical reaction of Sr(NO3)2, Eu(NO3)3, and fumed silica, the pure phase of nanosized Sr2SiO4:Eu2+ was obtained by sintering at 800 °C for 3 min. The crystallinity and morphology of the as-synthesized Sr2SiO4:Eu2+ powders were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The particle size of the Sr2SiO4:Eu2+ powders was distributed in a range of 30–50 nm. Furthermore, their photoluminescence and luminescence decay properties were also systemically discussed and compared with those of Sr2SiO4:Eu2+ phosphor prepared by a conventional high-temperature solid-state reaction method. The luminescence decay curves indicated that the distribution of Eu2+ ions in the nanosized Sr2SiO4 powders was in a nearly homogeneous average local environment.

Synthesis and luminescent properties of low oxygen contained Eu2+-doped Ca-α-SiAlON phosphor from calcium cyanamide reduction

Abstract A new convenient calcium cyanamide (CaCN2) reduction route was developed to synthesize the Eu2+ activated Ca-α-SiAlON phosphors containing low oxygen content. The luminescence properties of the obtained products were investigated for white LEDs application. The critical Eu2+ concentration in various hosts and its effect on the photoluminescence properties were studied. The optimized sample (10at.% Eu2+ vs. Ca2+) could be efficiently excited by the current GaN/InGaN blue LED chips and provided emission intensity competitive with that of YAG:Ce3+ (P46-Y3) standard, revealing that this phosphor was a potential candidate for phosphor-converted white LEDs.

Preparation of Sr2Si5N8:Eu2+ phosphors using various novel reducing agents and their luminescent properties

Divalent europium-doped metal nitride phosphors, Sr2Si5N8:Eu2+, were synthesized using three novel reducing agents, C3H6N6, C2H4N4, and Sr(CH3COO)2. The samples obtained showed higher emission intensity and lower residual oxygen/carbon content as compared with the sample prepared by the conventional carbothermal reduction and nitridation (CRN) method. A maximum emission intensity of 155% relative to that of the commercially available YAG:Ce3+ (P46-Y3) standard phosphor was achieved using the Sr(CH3COO)2 route. White LEDs fabricated using synthesized phosphors emitted white light with a correlated color temperature in the range 2900–6300 K. Moreover, a general color rendering index (Ra) of 80 was obtained. The reducing methods in this study were shown to be more effective than the conventional CRN route; these phosphors can be used to compensate for a red color deficiency of the current white LED.

Luminescence properties of CaAlSiN3:Eu2+ mixed nitrides prepared by carbothermal process

Mixed metal nitride (CaAlSiN3:Eu2+) phosphor was synthesized from an appropriate mixture of CaCN2, CaCO3, AlN, Si3N4, Eu2O3 and C by the carbothermal nitriding method in a N2 gas flow: 1/2 CaCN2 + 1/2(1-2x)CaCO3 + AlN + 1/3Si3N4 + xEu2O3 + C → (Ca1-xEux)AlSiN3 + CO2↑. The resultant phosphor gave an intense red emission and good high-temperature durability for LED solid illuminations by optimizing some synthesis conditions.