Enhanced photoluminescence property of Dy co-doped BaAl2O4:Eu green phosphors

Eu²⁺-doped BaAl₂O₄ green phosphors were prepared by a conventional solid-state reaction and the effects of Dy³⁺ co-doping on the photoluminescence property were investigated. The phosphors were characterized by X-ray powder diffraction (XRD), fluorescence spectroscopy, field-emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). XRD showed that all prepared samples exhibited a hexagonal BaAl₂O₄ phase. Fluorescence spectroscopy showed that the photoluminescence efficiency increased with increasing Eu²⁺ concentration until 3 mol% then decreased at higher concentrations due to concentration quenching effect. Moreover, Dy³⁺ co-doping increased the photoluminescence efficiency of the Eu²⁺-doped BaAl₂O₄ phosphor.     

Polyacrylamide gel synthesis and sintering of Mg2SiO4:Eunanopowder

A Mg₂SiO₄:Eu³⁺ nanopowder was synthesized by a polyacrylamide gel method. In this route, the gelation of the solution is achieved by the formation of a polymer network which provides a structural framework for the growth of particles. The densification of the powders was also studied. An amorphous nanopowder was synthesized and crystallized to Mg₂SiO₄ after heat-treatment via a solid-state reaction at a relatively low temperature of about 700 °C. The powders prepared by the polyacrylamide gel method showed better sinterability than the powders synthesized by the conventional sol–gel method. The relative density of the sample was 97% at 1500 °C.     

Enhanced luminescence of novel Ca3B2O6:Dy phosphors by Li-codoping for LED applications

The Ca_3−xB₂O₆:xDy³⁺ (0.0 ≤ x ≤ 0.105) and Ca_2.95−yDy_0.05B₂O₆:yLi⁺ (0 ≤ y ≤ 0.34) phosphors were synthesized at 1100 °C in air by solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation (PLE) and photoluminescence (PL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm is due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions upon 350 nm excitation is observed at 480 nm (blue) due to the ⁴F_9/2 → ⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2 → ⁶H_13/2 transitions and a weak 660 nm (red) due to ⁴F_9/2 → ⁶H_11/2 emissions, respectively. The optimal PL intensity of the Ca_3−xB₂O₆:xDy³⁺ phosphors is found to be x = 0.05. Moreover, the PL results from Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors show that Dy³⁺ emissions can be enhanced with the increasing codopant Li⁺ content till y = 0.22. By simulation of white light, the CIE of the investigated phosphors can be tuned by varying the content of Li⁺ ions, and the optimal CIE value (0.300, 0.298) is realized when the content of Li⁺ ions is y = 0.22. All the results imply that the Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors could be potentially used as white LEDs.     

Soluble silica assisted synthesis and luminescent characteristics of yellow emitting CaSrSiO4:Eu2+ phosphors for warm white light production

To meet the challenges and additional requirements towards the development of white LED׳s with utmost efficacy, a sol–gel approach is adopted wherein a water soluble silicon compound is used as a silica source. The phosphor material obtained is subjected to detailed structural, morphological and luminescent studies. The results obtained show that the XRD patterns of Eu²⁺ doped CaSrSiO₄ phosphors are in good agreement with the CaSrSiO₄ (ICSD no. 49660) whose structure is orthorhombic. The output of the luminescence studies clearly portrays a broad yellow emission between 450 and 750 nm with a peak at ~600 nm under the broad excitation range. This confirms its efficient emission towards the development of a warm white light using blue LEDs. A red shift in the peak emission wavelength was observed for the prepared samples. This longer shift in wavelength can be credited to the sol–gel method adopted and is not offered by the conventional solid state reaction method. A warm white emitting LED was fabricated by combining near ultraviolet LED (400 nm) chip with our sol–gel synthesized CaSrSiO₄:Eu²⁺ phosphor. The CIE chromaticity coordinates (0.44 and 0.41), color correlated temperature (CCT) <4000 K, color rendering index (CRI) >80 provide their emission potentiality in the warm white light region.     

Fluorescence improvement of Ba1.3Ca0.7SiO4:Eu,Mn phosphors via Dy addition and their color-tunable properties

A series of novel single-phase white phosphors Ba_1.3Ca_0.69−x−ySiO₄:0.01Eu²⁺,xMn²⁺, yDy³⁺ were synthesized by the solid-state method. The excitation spectra of these phosphors exhibit a broad band in the range of 260–410 nm, which can meet the application requirements for near-UV LED chips (excited at 350–410 nm). The emission spectra consist of two broad bands positioned around 455 nm and 596 nm, which are assigned to 5d→4f transition of Eu²⁺, and ⁴T₁→⁶A₁ transition of Mn²⁺, respectively. The luminescence intensity of phosphors enhances obviously by doping Dy³⁺ ions, and the intensity of two bands reaches an optimum when Dy³⁺ amounts to 2 mol%. In addition, thermoluminescence investigation of phosphor was conducted, getting two shallow trap defects with activation energy of 0.43 eV and 0.45 eV, which demonstrates the energy transfer mechanism of Dy–Eu through the process of hole and electron traps. By precisely tuning the Mn²⁺ content, an optimized white light with color rendering index (CRI) of R_a=84.3%, correlated color temperature (CCT) of T_c=8416 K and CIE chromaticity coordinates of (0.2941, 0.2937) is generated. The phosphor could be a potential white phosphors for near-UV light emitting diodes.     

Effect of strontium and phosphorus source on the structure,morphology and luminescence of Sr5(PO4)2SiO4:Euphosphor prepared by solid-state reaction

Sr₅(PO₄)₂SiO₄:Eu²⁺ phosphosilicate phosphor was prepared by high temperature solid-state reaction. Effects of strontium sources (strontium oxide, strontium nitrate and strontium carbonate) and of phosphorus sources (diammonium phosphate, strontium monophosphate) on the reactivity of their mixture during heating and on phase composition, morphology and photoluminescence excitation and emission properties of the phosphors were investigated by TG–DTG–DSC, XRD, SEM and photoluminescence spectroscopy. The sequence of the solid-state reactions when using the different starting reagents was discussed based on the TG–DTG–DSC results. It was found that it is hard to prepare pure Sr₅(PO₄)₂SiO₄:Eu²⁺ phosphor with either of strontium sources studied when stoichiometric (NH₄)₂HPO₄ was used as a phosphorus source. Minor Sr₂SiO₄ impurity phase was present in the phosphors. The content of impurity phase, the morphology and resultant photoluminescence properties of the phosphors were markedly influenced by the strontium source employed. When SrCO₃ was used as the strontium source, the phase purity of the phosphor was improved with the addition of excess (NH₄)₂HPO₄. When (NH₄)₂HPO₄with 5% excess or SrHPO₄ in stoichiometric ratio was used as the phosphorus source a pure phase phosphor was obtained. In addition, the morphology and photoluminescence of the phosphor were also influenced by phosphorus source. The possible reasons causing different properties of the phosphors prepared using different raw materials were discussed based on reaction schemes.     

Yellowish green-emitting KSrPO4:Tb phosphors with various doping concentrations prepared by using microwave assisted sintering

KSr_1−xPO₄:xTb³⁺ phosphors with various concentrations (x = 0.05, 0.06, 0.07, 0.08) of Tb³⁺ ions were synthesized in succession by using microwave assisted sintering. The sintering condition was set at 1200 °C for 1 h in air. The microstructural and luminescent characteristics of KSrPO₄:Tb³⁺ phosphors were investigated and are discussed here. The XRD result shows that the prepared KSr_1−xPO₄:xTb³⁺ phosphors would have an impure phase as the Tb³⁺ ion increases to more than x = 0.06. The photoluminescence measurement shows that the series of the emission-state ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃, corresponding to the typical 4f → 4f intra-configuration forbidden transitions of Tb³⁺, are appeared and the major emission peak is around at 542 nm. Moreover, the maximum photoluminescence intensity is appeared when the molar concentration of Tb³⁺ is 0.06. The decay time value of the KSr_1−xPO₄:xTb³⁺ phosphors with x = 0.06 is about 0.27 ms.     

Luminescence properties of Eu activated Ba2Si5N8 red phosphors with various Eu contents

This study was carried out to characterize the crystal structure and luminescence properties of Eu²⁺ doped red-emitting Ba₂Si₅N₈ phosphor. In this research, Ba₂Si₅N₈ phosphors with various Eu compositions were prepared by normal pressure sintering (NPS). Ba₃N₂, Si₃N₄ and Eu₂O₃ were sintered at a high temperature in a mixture of N₂ and H₂. The crystal structure was analyzed by X-ray diffraction(XRD), and the photoluminescence(PL) properties of the Eu²⁺ - activated Ba₂Si₅N₈ phosphors were evaluated as a function of the Eu²⁺ activator concentration. The red-emitting Ba₂Si₅N₈ phosphors showed a broad excitation band range as well as high quantum output.     

Crystalline morphology and photoluminescent properties of YInGe2O7:Eu phosphors prepared from microwave and conventional sintering

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇:Eu³⁺ powders using microwave assisted sintering. For comparison, the properties of YInGe₂O₇:Eu³⁺ powders sintered at 1200 °C in conventional furnace for 10 h were also investigated. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ without second phase or phases of starting materials as YInGe₂O₇:50 mol% Eu powders sintered at 1200 °C in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave assisted sintering. In the PL studies, both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 620 nm under excitation of 393 nm with similar luminescent intensity. The results show that microwave processing has the potential to reduce the time and required energy input for the production of YInGe₂O₇:Eu³⁺ phosphors without sacrificing the photoluminescence.     

Fuel mixture approach for solution combustion synthesis of Ca3Al2O6 powders

Single-phase 3CaO·Al₂O₃ powders were prepared via solution combustion synthesis using a fuel mixture of urea and β-alanine. The concept of using this fuel mixture comes from the individual reactivity of calcium nitrate and aluminum nitrate with respect to urea and β-alanine. It was proved that urea is the optimum fuel for Al(NO₃)₃ whereas β-alanine is the most suitable fuel for Ca(NO₃)₂. X-ray diffraction and thermal analysis investigations revealed that heating at 300 °C the precursor mixture containing the desired metal nitrates, urea and β-alanine triggers a vigorous combustion reaction, which yields single-phase nanocrystalline 3CaO·Al₂O₃ powder (33.3 nm). In this case additional annealing was no longer required. The use of a single fuel failed to ensure the formation of 3CaO·Al₂O₃ directly from the combustion reaction. After annealing at 900 °C for 1 h, the powders obtained by using a single fuel (urea or β-alanine) developed a phase composition comprising of 3CaO·Al₂O₃, 12CaO·7Al₂O₃ and CaO.     

Influence of MgO on the formation of Ca3SiO5 and 3CaO·3Al2O3·CaSO4 minerals in alite-sulphoaluminate cement

The influence of MgO on the formation of Ca₃SiO₅ and 3CaO·3Al₂O₃·CaSO₄ minerals in alite-sulphoaluminate cement is reported in this paper. The results show that adding a suitable amount of MgO can lower the clinkering temperature, promote the formation of Ca₃SiO₅ and 3CaO·3Al₂O₃·CaSO₄ minerals, and help in the coexistence of the two minerals in the clinker. MgO may obviously decrease the formation of Ca₃Al₂O₆, and increase the SiO₂ content incorporated into the interstitial phase.     

Mechanochemical synthesis of kaolin-KH2PO4 and kaolin-NH4H2PO4 complexes for application as slow release fertilizer

A milling process to reduce kaolin to amorphous phase in the presence of KH₂PO₄ or NH₄H₂PO₄ and allow mechanochemical (MC) reaction for incorporation of KH₂PO₄ and NH₄H₂PO₄ into the kaolin structure was investigated in this work. Mixtures of kaolin and KH₂PO₄ and NH₄H₂PO₄ in separate systems were prepared by milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotational speeds from 200 to 700 rpm were performed to evaluate incorporation of KH₂PO₄ and NH₄H₂PO₄ and release of K⁺, NH₄⁺ and PO₄³⁻ ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC process was successfully applied to incorporate both KH₂PO₄ and NH₄H₂PO₄ into the amorphous kaolin structure. Release of K⁺ and PO₄³⁻ ions from the system (kaolin-KH₂PO₄) when dispersed in water for 24 h reached only up to 10%. Under similar conditions for the system (kaolin-NH₄H₂PO₄), release of NH₄⁺ and PO₄³⁻ ions reached between 25 and 40%. These results indicated that the MC process can be developed to allow amorphous kaolin to act as a carrier of K⁺, NH₄⁺ and PO₄³⁻ nutrients to be released slowly for use as fertilizer.     

Preparation of Sr2SiO4:Eu phosphors by microwave-assisted sintering and their luminescent properties

An Eu³⁺ activated strontium silicate phosphor was synthesized using a microwave-assisted sintering with a flux NH₄Cl. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials as Sr_1.9SiO₄:Eu³⁺_0.1 powders sintered at various temperatures in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave-assisted sintering. In the PL studies, the excitation spectrum of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors exhibited a broad band in the UV region centered at about 270 nm which was consistent with the absorption spectra. Both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 617 nm under excitation of 395 nm, with similar luminescent intensity. The results showed that microwave processing has the potential to decrease the sintering time and required energy input for the production of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors without degrading photoluminescence.     

CdWO4 nanorods: Ultrafast synthesis via a PEG-1000 polymer-assisted enhanced microwave synthesis route and their photoluminescence property

Cadmium tungstate (CdWO₄) nanorods have been successfully ultrafast synthesized in several minutes under enhanced microwave irradiation conditions and characterized by XRD, SEM, TEM, and photoluminescence. The products show a very strong photoluminescence peak at 475 nm with the excitation wavelength of 350 nm and a short decay time.     

Synthesis and luminescence properties of nitrided lanthanum magnesium hexaluminate LaMgAl11O19 phosphors

Nitrided LaMgAl₁₁O₁₉ phosphors were prepared by a two-step method involving synthesis at 1550 °C for 4 h, trituration, and firing at 1650 °C for 5 h under a nitrogen atmosphere. Nitrogen was doped into LaMgAl₁₁O₁₉ and bonded with aluminium atoms. The nitrided LaMgAl₁₁O₁₉ phosphors showed plate-like morphology with a rough surface and exhibited strong blue emission at 442 nm and 450 nm, which may be attributed to the energy transition between defect levels. A weak emission band at 590 nm was ascribed to the transition between the V_Al acceptor and the valence band, which was excited at 254 nm.     

Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, has been successfully synthesized using LiF as lithium source. The one-step reaction with stoichiometric composition and relative lower sintering temperature (700 °C) has been used in our experimental processes. The solid-state reaction mechanism using LiF as lithium precursor has been studied by X-ray diffraction and Fourier transform infrared spectra. The Rietveld refinement results show that in our product sintered at 700 °C no impurity phases of VPO₄, Li₅V(PO₄)₂F₂, or LiVPO₄F can be detected. The solid-state reaction using Li₂CO₃ as Li-precursor has also been carried out for comparison. X-ray diffraction patterns indicate that impurities as Li₃PO₄ can be found in the product using Li₂CO₃ as Li-precursor unless the sintering temperatures are higher than 850 °C. An abrupt particle growth (about 2 μm) has also been observed by scanning electron microscope for the samples sintered at higher temperatures, which can result in a poor cycle performance. The product obtained using LiF as Li-precursor with the uniform flake-like particles and smaller particle size (about 300 nm) exhibits the better performance. At the 50th cycle, the reversible specific capacities for Li₃V₂(PO₄)₃ measured between 3 and 4.8 V at 1C rate are found to approach 147.1 mAh/g (93.8% of initial capacity). The specific capacity of 123.6 mAh/g can even be hold between 3 and 4.8 V at 5C rate.     

Synthesis of high efficiency Zn2SiO4:Mn green phosphors using nano-particles

In this study, Zn₂SiO₄:Mn²⁺ luminescent phosphors were prepared by mixing nano-scale ZnO, SiO₂, and MnO₂ particles at the compositions corresponding to 2ZnO + SiO₂ + X mol% MnO₂ (Zn₂SiO₄–X-MnO₂, 0.02 ≤ X ≤ 0.05). The mixing powders were calcined from 900 °C to 1300 °C in air and in N₂ atmosphere. No matter calcined in air or in N₂ atmosphere, Zn₂SiO₄ was the mainly crystalline phase in particles calcined at 900 °C and was the only phase in particles calcined at 1000 °C and higher. The influences of MnO₂ concentration and calcining atmosphere and temperature on wavelength of luminescence peak and the emission intensity were further intensively investigated. We would show that the calcining atmosphere had no apparent influences on the physical and photoluminescence (PL) characteristics of Zn₂SiO₄:Mn²⁺ phosphors. The MnO₂ content and the calcining temperature were the main reasons to influence the physical and PL characteristics of Zn₂SiO₄:Mn²⁺ phosphors.     

Single-phased emission-tunable Ca3Si2O7:Ce,Eu phosphors for white light-emitting diodes

A series of single-phased emission-tunable Ca₃Si₂O₇:Ce³⁺, Eu²⁺ phosphors has been prepared by the solid-state reaction method. The phosphors show two intense emission bands at about 450 nm and 610 nm, which are attributed to the 5d→4f transitions of Ce³⁺ and Eu²⁺ ions, respectively. The emission colors of Ca₃Si₂O₇:Ce³⁺, Eu²⁺ phosphors vary from blue (0.148, 0.147) to white (0.309, 0.260), and eventually to orange (0.407, 0.319) by tuning the Eu²⁺/Ce³⁺ ratio. Energy transfer from Ce³⁺ to Eu²⁺ is studied by luminescence spectra and energy transfer efficiency. The results show an electric quadrupole–quadrupole interaction plays an important role in the process of energy transfer. The phosphors with tunable emission are suitable for application in white light-emitting diodes.     

Coating Gd2O3:Eu phosphors with silica by solid-state reaction at room temperature

Silica coating on Gd₂O₃:Eu particles was obtained by a simple method, e.g. solid-state reaction at room temperature. The urea homogeneous precipitation method was used to synthesize the Gd₂O₃:Eu cores. Transmission electron microscopy (TEM) shows that the core particles are spherical with submicrometer size which is the soft agglomerates with nanometer crystallites. The TEM morphology of coated particles shows that a thin film is coated on the surface of Gd₂O₃:Eu cores. Scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analysis indicate that the coating of silica can be used to avoid agglomeration of Gd₂O₃:Eu particles to obtain smaller particles. X-ray photoelectron spectra (XPS) show that silica is coated on the surface of core particles by forming the chemical bond. Photoluminescence (PL) spectra conform that Gd₂O₃:Eu phosphors remain well-luminescent properties by the silica coating.     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.