NaEu0.96Sm0.04(MoO4)2 as a promising red-emitting phosphor for LED solid-state lighting prepared by the Pechini process

NaEu_0.96Sm_0.04(MoO₄)₂ was prepared by the Pechini method (P phosphor) and as a comparison, also by solid-state reaction technique (S phosphor). The photo-luminescent properties, the morphology and the grain size were investigated. The phosphors show broadened excitation band around 400 nm, high intensity of Eu³⁺⁵D₀→⁷F₂ emission upon excitation around 400 nm, and appropriate CIE chromaticity coordinates. Intensive red light-emitting diodes (LEDs) were fabricated by combining the phosphor and a 400 nm InGaN chip for the first time, which confirm that the phosphor is a good candidate for near UV LED. The luminescent intensity of P phosphor prepared at 700 °C is near that of S phosphor prepared at 800 °C. In addition, P phosphor shows advantages of lower calcining temperature, shorter heating time, and smaller grain size. Considering all these factors, the suitable method for preparing the promising near UV LED phosphor NaEu_0.96Sm_0.04(MoO₄)₂ is recommended to be the Pechini process at 700 °C.     

Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis

Spherical-shaped Li₄Ti₅O₁₂ anode powders with a mean size of 1.5 μm were prepared by spray pyrolysis. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li₄Ti₅O₁₂. The powders post-treated at temperatures of 800 and 900 °C had the single phase of spinel Li₄Ti₅O₁₂. The powders post-treated at a temperature of 1000 °C had main peaks of the Li₄Ti₅O₁₂ phase and small impurity peaks of Li₂Ti₃O₇. The spherical shape of the precursor powders was maintained after post-treatment at temperatures below 800 °C. The Brunauer-Emmett-Teller (BET) surface areas of the Li₄Ti₅O₁₂ anode powders post-treated at temperatures of 700, 800 and 900 °C were 4.9, 1.6 and 1.5 m²/g, respectively. The initial discharge capacities of Li₄Ti₅O₁₂ powders were changed from 108 to 175 mAh/g when the post-treatment temperatures were changed from 700 to 1000 °C. The maximum initial discharge capacity of the Li₄Ti₅O₁₂ powders was obtained at a post-treatment temperature of 800 °C, which had good cycle properties below current densities of 0.7 C.     

An analysis of Y2O3:Eu thin films for thermographic phosphor applications

A comparative study of the luminescent properties of Y₂O₃:Eu³⁺ phosphor powders and thin films sputtered from targets prepared from combustion synthesized powders is reported. Thin films of (Y_0.96Eu_0.04)₂O₃ were deposited on silicon substrates. Films deposited at 600 °C had both monoclinic and cubic phases of Y₂O₃, which developed to an oriented cubic phase after annealing. Films and powders showed a linear dependence of the intensity of the ⁵D₇→⁷F₂ (611 nm) transition with temperature in the range 26-660 °C with an average rate of change of 1.8×10⁻⁴ °C⁻¹. The rate of change appears to be dependent on the Eu³⁺ concentration. This work shows that these thin films can be used as thermographic phosphors for remote temperature measurements.     

Solvothermal synthesis and luminescent properties of YAG:Tb nano-sized phosphors

Nano-sized Tb-doped YAG phosphor particles were synthesized by a mixed solvo-thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. The formation of YAG:Tb was investigated by means of XRD and IR spectra. The pure crystalline-phase YAG was prepared under moderate synthesis conditions (300 °C and 10 MPa), indicating that ethanol partly replaces water as the solvent, thus favoring the formation of YAG. TEM images showed that YAG:Tb phosphor particles sintered at 300 °C were basically of spherical shape, with good dispersion about a particle size of around 80 nm. The crystalline YAG:Tb showed green emission with ⁵D₄−⁷F₆ (544 nm) as the most prominent group. The PL intensity and crystallinity of YAG:Tb phosphors increases with increasing synthesis temperature, and reaches maximum brightness at 300 °C, which is lower than that exhibited by a commercial product.     

Photoluminescence analysis of α-Al2O3 powders doped with Eu and Eu ions

Alumina (Al₂O₃) powders doped with europium trivalent (Eu³⁺) were prepared by a low-temperature (∼280 °C) combustion synthesis technique. When the powder was heat treated at 1200 °C for 2 h in the presence of flowing ammonia (NH₃), α-Al₂O₃ crystalline ceramic powders was obtained. The analysis of the luminescence showed that Eu³⁺ was reduced to europium divalent (Eu²⁺) after the heat-treatment process. Under ultraviolet (UV) lamp excitation (λ=254 nm) these powders containing sub-microcrystalline structures present bright red (Al₂O₃:Eu³⁺) and green (Al₂O₃:Eu²⁺) luminescence indicating that this material is a potential candidate for applications in phosphor technology.     

Characterization and luminescent properties of SiO2:ZnS:Mn and ZnS:Mn nanophosphors synthesized by a sol-gel method

ZnS and SiO₂-ZnS nanophosphors, with or without different concentration of Mn²⁺ activator ions, were synthesized by using a sol-gel method. Dried gels were annealed at 600 °C for 2 h. Structure, morphology and particle sizes of the samples were determined by using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The diffraction peaks associated with the zincblende and the wurtzite structures of ZnS were detected from as prepared ZnS powders and additional diffraction peaks associated with ZnO were detected from the annealed powders. The particle sizes of the ZnS powders were shown to increase from 3 to 50 nm when the powders were annealed at 600 °C. An UV-Vis spectrophotometer and a 325 nm He-Cd laser were used to investigate luminescent properties of the samples in air at room temperature. The bandgap of ZnS nanoparticles estimated from the UV-Vis data was 4.1 eV. Enhanced orange photoluminescence (PL) associated with ⁴T₁→⁶A₁ transitions of Mn²⁺ was observed from as prepared ZnS:Mn²⁺and SiO₂-ZnS:Mn²⁺ powders at 600 nm when the concentration of Mn²⁺ was varied from 2-20 mol%. This emission was suppressed when the powders were annealed at 600 °C resulting in two emission peaks at 450 and 560 nm, which can be ascribed to defects emission in SiO₂ and ZnO respectively. The mechanism of light emission from Mn²⁺, the effect of varying the concentration on the PL intensity, and the effect of annealing are discussed.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

Synthesis and VUV spectral properties of nanoscaled Zn2SiO4:Mn green phosphor

Nanoscaled Zn₂SiO₄:Mn²⁺ green phosphor with regular and uniform morphology was synthesized by hydrothermal method at a low temperature of 140 °C. The structure and morphology of the phosphor was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of the hydrothermal temperature and the time on the crystallite structure and the vacuum ultraviolet (VUV) photoluminescence (PL) properties were evaluated. The as-synthesized nanoscaled Zn₂SiO₄:Mn²⁺ phosphor exhibited intensive broad emission around 523 nm, which was attributed to the ⁴T₁→⁶A₁ transition of Mn²⁺. The PL intensity increased along with the increasing hydrothermal temperature and time. The heat-treated phosphors exhibited higher PL intensity than the corresponding samples prepared using the conventional solid-state reaction.     

Phase evolution in gel-precipitated LaAlO3 ceramics

Lanthanum aluminate ceramic powders could be prepared by a combined gel precipitation process from metal chlorides using ammonia. A slight modification in the conventional gel precipitation technique was carried out by introducing a step of ultrasonication followed by centrifugal washing of the gel. The dried gels produced pure phase lanthanum aluminate powders on calcination at 1100 °C for the combined gel-precipitated powders, and at 600 °C for the washed gel. The phase evolution was studied and it was found that the delay in obtaining monophasic LaAlO₃ in the combined gel-precipitated powder owed to the crystallization of an impure phase LaOCl. This phase was not detected in the washed gel (WG) powders. TEM micrographs showed a uniform morphology for the calcined WG powders, which were in contrast to the irregular particles in the gel-precipitated (GP) powders. The uniform morphology was assigned to the ultrasonic effects during washing of the gel.     

Annealing temperature effect on microstructure,magnetic and microwave properties of Fe-based amorphous alloy powders

Fe₇₄Ni₃Si₁₃Cr₆W₄ amorphous alloy powders were annealed at different temperature (T) for 1.5 h to fabricate the corresponding amorphous and nanocrystalline powders. The influences of T on the crystalline structure, morphology, magnetic and microwave electromagnetic properties of the resultant samples were investigated via X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer. The results show that the powder samples obtained at T of 650 °C or more are composed of lots of ultra-fine α-Fe(Si) grains embedded in an amorphous matrix. When T increases from 350 to 750 °C, the saturated magnetization and coercivity of the as-annealed powder samples both increase monotonously whereas the relative real permittivity shows a minimal value and the relative real permeability shows a maximal value at T of 650 °C. Thus the powder samples annealed at 650 °C show optimal reflection loss under −10 dB in the whole C-band. These results here suggest that the annealing heat treatment of Fe-based amorphous alloy is an effective approach to fabricate high performance microwave absorber with reasonable permittivity and large permeability simultaneously via adjusting T.     

A novel blue magnesium strontium aluminate-based phosphor for PDP application

A novel blue light-emitting phosphor, Eu²⁺-doped magnesium strontium aluminate (MgSrAl₁₀O₁₇:Eu²⁺), for plasma display panel (PDP) application was developed. X-ray diffraction (XRD) patterns disclosed that the phosphor annealed at 1500 °C for 5 h was a pure MgSrAl₁₀O₁₇ phase. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm. The phosphor shows strong and broad blue emission under vacuum ultraviolet (VUV) light excitation. After baking at 400-600 °C and irradiation with VUV light for 300 h, the phosphor still keep excellent VUV luminescence properties exhibiting good stability against high temperature baking and VUV irradiation. The decay time was short as 1.09 μs and the quantum yield was high to 0.77±0.02. All the characteristics indicated that MgSrAl₁₀O₁₇:Eu²⁺ would be a promising blue phosphor for PDP application.     

Synthesis of bismuth titanate with urea as fuel by solution combustion route and its dielectric and ferroelectric properties

Preparation of ferroelectric bismuth titanate (Bi₄Ti₃O₁₂) is carried out by solution combustion route with urea as fuel at much lower calcinations temperatures. The single phase bismuth titanate was obtained after calcinations at 800 °C. SEM micrographs of the calcined powders show agglomerated, flaky and foamy morphology, which is typical of combustion synthesis and that of sintered ceramics shows the grain formation. Behavior of dielectric constant and dielectric loss as a function of temperature of as-prepared sample are reported in this communication. Ferroelectric to paraelectric phase transition occurs at the temperature T_c ∼ 660 °C. Its remnant polarization (2P_r) is very less of the order of 0.012 μC/cm².     

Microstructures and electrical properties of La0.8Sr0.2MnO3 films synthesized by sol-gel method

La_0.8Sr_0.2MnO₃ (LSMO) thin films were fabricated on alumina substrates by an improved sol-gel dip-coating process. It was found that multiple dip-coating process could not be performed until the pre-firing temperature reached 600 °C. Different amounts of LSMO powders were added to precursor solution with an aim to avoid cracks in LSMO thin films during calcining caused by the shrinkage mismatch between the film and the substrate. The structure and surface morphology of the films prepared from precursors with and without LSMO powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the addition of 56.4 wt.% LSMO powders into the sol-gel precursor solution significantly modified the microstructure of films. A single LSMO perovskite phase was obtained on alumina substrate after calcining at 800 °C for 4 h by the improved sol-gel method. The sheet resistance of the films prepared with different processing parameters was measured by four-point dc method. Results indicated that the sheet resistance of films decreased with increasing the number of coating applications and the amount of LSMO powders.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Characterization and photoluminescence properties of some MLn2(1−x)O4:2xEu or 2xTb systems (M=Ba or Sr, Ln=Gd or La)

BaGd₂O₄, BaLa₂O₄ and SrLa₂O₄ powders doped with different concentrations of Eu³⁺ or Tb³⁺ are prepared by combustion synthesis method and the samples were further heated to 500, 700 and 900 °C to improve the crystallinity of the materials. The structure and morphology of materials have been examined by X-ray diffraction and scanning electron microscopy. It is remarkable that all the samples of BaGd₂O₄, BaLa₂O₄ and SrLa₂O₄ have similar morphology. The SEM images show homogeneous aggregates of varying shapes and sizes, which are composed of a large number of small elliptical shaped crystallites with an average diameter of about 0.5-3.0 μm. Photoluminescence for all materials increases with increase of temperature and shows a maximum for the samples heated to 900 °C with 4 mole% doping of Eu³⁺ or Tb³⁺ ions. The luminescence is almost same for all powders when doped with same concentration of Eu³⁺.     

Synthesis and characterization of lithium ferrite by oxalate precursor route

Nanocrystalline lithium ferrite (LiFe₅O₈) powders have been synthesized by oxalate precursor route. The effects of Fe³⁺/Li⁺ mole ratio, and annealing temperature on the formation, crystalline size, morphology and magnetic properties were systematically studied. The Fe³⁺/Li⁺ mole ratio was controlled from 5 to 3.33 while the annealing temperature was controlled from 600 to 1100 °C. The resultant powders were investigated by differential thermal analyzer (DTA), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). DTA results showed that LiFe₅O₈ phase started to form at around 520 °C. XRD indicated that LiFe₅O₈ phase always contained α-Fe₂O₃ impurity and the hematite phase formation increased by increasing the annealing temperature ?850 °C for different Fe³⁺/Li⁺ mole ratios 5, 4.55 and 3.85. Moreover, lithium ferrite phase was formed with high conversion percentage at critical annealing temperature 750–800 °C. Single well crystalline LiFe₅O₈ phase was obtained at Fe³⁺/Li⁺ mole ratio 3.33 and annealing temperatures from 800 to 1000 °C. Maximum saturation magnetization (68.7 emu/g) was achieved for the formed lithium ferrite phase at Fe³⁺/Li⁺ mole ratio 3.33 and annealing temperature 1000 °C.     

Dosimetric properties of rare earth doped LiCaBO3 thermoluminescence phosphors

Lithium Calcium borate (LiCaBO₃) polycrystalline thermoluminescence (TL) phosphor doped with rare earth (RE³⁺) elements has been synthesized by high temperature solid state diffusion reaction. The reaction has produced a very stable crystalline LiCaBO₃:RE³⁺ phosphors. Among these RE³⁺ doped phosphors thulium doped material showed maximum TL sensitivity with favorable glow curve shape. TL glow curve of gamma irradiated LiCaBO₃:Tm³⁺ samples had shown two major well-separated glow peaks at 230 and 430 °C. The glow peak at 430 °C is almost thrice the intensity of the glow peak at 230 °C. The TL sensitivity of the phosphor to gamma radiation was about eight times that of TLD-100 (LiF). Photoluminescence and TL emission spectra showed the characteristic Tm³⁺ peaks. TL response to gamma radiation dose was linear up to 10³ Gy. Post-irradiation TL fading on storage in room temperature and elevated temperatures was studied in LiCaBO₃:Tm³⁺ phosphor.     

Fuel additives and heat treatment effects on nanocrystalline zinc ferrite phase composition

Nanocrystalline ZnFe₂O₄ powder was prepared by the auto-combustion method using citric acid, acetic acid, carbamide and acrylic acid as fuel additives. Pure spinel zinc ferrite with the crystallite size of about 15 nm can be obtained by using acrylic acid as fuel additive. Samples prepared using other fuel additives contain ZnO impurities. In order to eliminate ZnO impurities, the sample prepared with citric acid as fuel additive was annealed at different temperatures up to 1000 °C in air and in argon. Annealed powders have pure ZnFe₂O₄ phase when annealing temperature is higher than 650 °C in air. Sample annealed at 650 °C in air is paramagnetic. However, annealed powders become a mixture of Fe₃O₄ and FeO after annealing at 1000 °C in argon atmosphere due to Zn volatility and the reduction reaction.     

Hydrothermal preparation and photoluminescent properties of MgAl2O4: Eu spinel nanocrystals

Nanoplates of the MgAl₂O₄ spinel doped with Eu³⁺ ions were prepared by a microwave assisted hydrothermal method. Structural properties of the precursor calcined at 700 and 1000 ^oC powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to the obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The average spinel particle size was determined to be 11 nm after calcination at 700 °C and it increased up to 14 nm after calcination at 1000 °C. The photoluminescent properties of prepared powders with different Eu³⁺ ion concentrations (0-5% mol) were investigated using excitation and emission spectroscopy at room and low temperatures (77 K).