Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

Synthesis and photoluminescence properties of Eu doped Sr9Sc(PO4)7 phosphors for white light-emitting diodes

A series of Eu²⁺-activated Sr₉Sc(PO₄)₇ yellowish-green emitting phosphors were synthesized by conventional solid-state reaction. The photoluminescence (PL) properties and concentration quenching mechanism of the as-prepared phosphors were investigated. The emission spectrum exhibits a broad and asymmetric band peaking at 510 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The excitation spectrum exhibits a broad band extending from 250 to 450 nm, which matches well with the emission of near ultraviolet (n-UV) chips (350–430 nm). Non-radiative transitions between Eu²⁺ ions in the Sr₉Sc(PO₄)₇ host have been demonstrated to be attributable to dipole–dipole interactions, and the critical distance was calculated to be 23.1 Å. These results indicate that Sr₉Sc(PO₄)₇:Eu²⁺ phosphor could serve as a promising candidate for application in n-UV white-light LEDs.     

VUV photoluminescence properties of Y1−xGdxVO4:Eu phosphors prepared by ultrasonic spray pyrolysis

The red-emitting (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors were synthesized by ultrasonic spray pyrolysis. The (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors had the tetragonal xenotime structure with a space group of I4₁/amd (1 4 1). The calculated crystallite sizes of the annealed phosphors ranged from 58 to 68 nm. In this study, we discussed the photoluminescence properties of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors under VUV excitation, depending on Gd content. The emission intensity of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors increased with increasing Gd content up to x = 0.5, and then decreased with a further increase in Gd content. The purest red color was obtained for the (Y_0.5Gd_0.5)_0.94Eu_0.06VO₄ phosphors.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

Solid state reaction preparation and enhanced red luminescence of S-doped La2Mo2O9:Prphosphors

Red-light-emitting phosphors of La₂Mo₂O₉:Pr³⁺ and S-doped La₂Mo₂O₉:Pr³⁺ were prepared by high temperature solid state reaction. Under the excitation of 450 nm blue light, all samples produced a red emission peak at 650 nm corresponding to the characteristic transition of Pr³⁺ (³P₀→³F₂). The dependence of Pr³⁺ doping content (x) on the luminescent intensity was analyzed, and the optimal doping content of Pr³⁺ was x=0.07. After a small quantity of sulfur was introduced into the system, the luminescence intensity of phosphors was obviously enhanced. The reasons for the enhancement of luminescence are due to improved crystallization after S doping and the relatively large electronegativity difference between S and Mo. Additionally, the coincidence of the excitation wavelength with the emission of GaN chips may recommend this phosphor system as a potential candidate for use in white light-emitting diodes.     

Enhancement of Ca3Co4O9 thermoelectric properties by Cr for Co substitution

Ca₃Co_4−xCr_xO₉ polycristalline thermoelectric ceramics with small amounts of Cr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Cr has been incorporated into the Ca₃Co₄O₉ structure and no Cr-containing secondary phases have been produced for Cr contents≤0.05. Apparent density measurements have shown that all samples are very similar, with densities around 75% of the theoretical one. Electrical resistivity decreases and Seebeck coefficient slightly raises when Cr content increases until 0.05 Cr addition. The improvement in both parameters leads to higher power factor values than the usually obtained by conventional solid state routes.     

IBLC effect leading to colossal dielectric constant in layered structured Eu2CuO4 ceramic

Dielectric (ε_r′) studies of phase pure T′-type Eu₂CuO₄ ceramics of two markedly different grain sizes (D), prepared by (i) conventional powder mixing and (ii) citrate complexation-Pechini process, have been carried out in the frequency range 0.1 Hz to 1 MHz, and at temperatures −100 °C to 150 °C. ε_r′ is found to be highly grain size dependent. For the sample with coarse bar-like grains (D²~17×6 μm²) ε_r′ is >10³, and for the finer grain size sample with bimodal distribution (D₁~1 μm, D₂~3 μm) ε_r′ is ~10⁵; for both the samples, high ε_r′ value is nearly frequency independent over 500 Hz≤f<100 kHz and T≥30 °C. The impedance spectroscopy (IS) study has clearly shown that both, the coarse- and the fine-grained samples consist of semiconducting grains and insulating grain boundaries that primarily lead to an internal barrier layer capacitance (IBLC) effect. And thus, manifest colossal dielectric constant (ε_r′>10³) in Eu₂CuO₄ ceramics. The smaller grain size (Pechini) sample, with over an order higher number of grains and grain boundary network, showing over an order higher ε_r′ (~10⁵) compared to the coarse grained one, further endorses the IBLC effect.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

One-step hydrothermal synthesis and microwave electromagnetic properties of RGO/NiFe2O4 composite

The reduced graphene oxide (RGO)/NiFe₂O₄ composite was synthesized by a facile one-pot hydrothermal route, which avoided the usage of chemical reducing agent. The reduction of graphene oxide (GO) and the crystallization of NiFe₂O₄ crystals happened in a one-step hydrothermal process. The morphology, microstructure and magnetic properties of the composite were detected by means of XRD, XPS, TEM, EDX, TG-DSC and VSM. The maximum R_L of the RGO/NiFe₂O₄ composite is −39.7 dB at 9.2 GHz with the thickness of 3.0 mm, and the absorption bandwidth with the R_L below −10 dB is up to 5.0 GHz (from 12.7 to 17.7 GHz) with a thickness of 1.9 mm. The introduction of RGO signally enhanced microwave absorption performance of the NiFe₂O₄ NPs. It is believed that such composite will be applied widely in microwave absorbing area.     

Synthesis and photoluminescence properties of La1−xAlO3:xTb green phosphors for white LEDs

Tb³⁺-doped La_1−xAlO₃ phosphor powders are successfully synthesized by the solution combustion method, using citric acid as the combustion fuel. The crystal structure and photoluminescence properties of La_1−xAlO₃:xTb³⁺ phosphors are studied, depending on Tb³⁺ content. The strongest emission peak is found at 543 nm, which originates from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions, indicating green emission. Among the fabricated phosphors, the La_0.9AlO₃:0.1Tb³⁺ phosphor emits the strongest green light. The excellent luminescent properties make it a possible candidate for white light-emitting diodes and various photonic applications.     

The characteristics of X1 type Y2SiO5:Tb phosphor particles prepared by high temperature spray pyrolysis

The X1 type Y₂SiO₅:Tb phosphor particles with high brightness were prepared by spray pyrolysis from spray solution with NH₄F flux material. The phosphor particles prepared by spray pyrolysis at high preparation temperature had spherical shape, fine size and dense morphology. The mean sizes of the phosphor particles prepared at 900 and 1650 °C were 1.3 and 0.9 μm. The emission spectrum of the phosphor particles prepared by spray pyrolysis at 1650^oC had the characteristics of X1 type Y₂SiO₅:Tb phosphor. The photoluminescence intensity of the phosphor particles directly prepared by spray pyrolysis from spray solution with 20 wt.% NH₄F flux of the product at temperature of 1650 °C was 127 and 184% of the X1 and X2 type Y₂SiO₅:Tb phosphor particles post-treated at 1100 and 1300 °C, respectively. The Y₂SiO₅:Tb phosphor particles prepared by spray pyrolysis at 1650 °C had X1 type crystal structure because of short residence time of particles inside hot wall reactor of 0.4 s.     

Structure and magnetic properties of multi-morphological CoFe2O4/CoFe nanocomposites by one-step hydrothermal synthesis

Multi-morphological CoFe₂O₄/CoFe nanocomposites have been synthesized using a facile hydrothermal process. The effects of hydrazine hydrate amount during hydrothermal reaction on the structure and magnetic property of the specimens were studied. With increasing hydrazine hydrate amount, the CoFe₂O₄ transformed to CoFe and the morphology of the specimen changed from granular particles to faceted particles. The saturation magnetization monotonically increased and the coercivity monotonically decreased with increasing hydrazine hydrate amount. The magnetic interactions, determining the magnetic properties of the composites, result from the dominant dipole coupling and relative weak exchange coupling between CoFe₂O₄ and CoFe nanoparticles. The CoFe₂O₄/CoFe nanocomposite prepared with 2?mL hydrazine hydrate exhibited the optimal magnetic properties, with the saturation magnetization of 81?emu/g and coercivity of 636?Oe.     

Green and red upconversion luminescence of Er-doped K0.5Na0.5NbO3 ceramics

Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) lead-free piezoelectric ceramics were synthesized by the solid-state reaction method. The upconversion emission properties of Er³⁺ doped KNN ceramics were investigated as a function of Er³⁺ concentration and incident pumping power intensity. Bright green (~555 nm) and red (670 nm) upconversion emission bands were obtained under 980 nm excitation at room temperature, which are attributed to (²H_11/2, ⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. The upconversion emission intensity can be adjusted by changing Er³⁺ concentration, and the mechanism of upconversion processes involve not only a two-photon absorption but also a three-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its upconversion and piezoelectric property.     

Effect of the O2/N2 ratio on the morphology of ZnO nanostructures grown by a thermal evaporation technique

ZnO nanostructures were synthesized by thermal evaporation of Zn powder in an oxygen and nitrogen gas environment at 1000 °C. No substrates or catalysts were used for the formation of the ZnO nanostructures. The ZnO nanostructures were synthesized inside crucibles with Zn source powder. ZnO nanoneedles began to be found at O₂/N₂ ratios of less than 60/40. As the O₂/N₂ ratio increased to 80/20, the ZnO nanostructures mainly grew in the longitudinal direction. Upon increasing the O₂/N₂ ratio to 100/0, the ZnO nanostructures grew mainly in the transverse direction. CL spectra showed that ZnO with a high crystalline quality was obtained at a O₂/N₂ ratio of 80/20.     

Synthesis of negative thermal expansion materials ZrW2−xMoxO8 (0 ≤ x ≤ 2) using hydrothermal method

Negative thermal expansion materials ZrW_2−xMo_xO₈ (0 ≤ x ≤ 2) have been successfully synthesized by the reaction of a mixture of ammonium tungstate and ammonium molybdate with zirconium oxynitrate using a hydrothermal method. Effect of substituted ion Mo on the microstructure, α-to-β and cubic to trigonal phase transition in resulting ZrW_2−xMo_xO₈ powders was examined by the XRD experiments. It was found that the structural phase transition temperature decreased slightly with increasing substituted content. The cubic to trigonal phase transition was also influenced by substituted content. The resulting products decomposed to WO₃/MoO₃ and ZrO₂ as temperature increasing when x ≤ 0.5 and while x > 0.5, the cubic phase transited to trigonal phase. The effect of substituted Mo on the morphology of resulting products was also investigated by SEM experiments.     

Luminescent properties of Eu-doped YTaO4 powders

YTaO₄:Eu³⁺ powders were synthesized by a flux method with LiCl. The use of a flux in the synthesis of M′–YTaO₄ facilitated the reaction of raw materials, leading to lowering the heating temperature, but not effective at the high firing temperature. The red emission peaks were observed around 613 nm with an excitation wavelength of 254 nm. Emission peaks were composed of two sets around 613 nm and 590 nm, which originated from ⁵D₀–⁷F₂ and ⁵D₀–⁷F₁, respectively. PL intensity of YTaO₄:Eu³⁺ prepared with a small amount of LiCl (10 wt%) significantly depended on the firing temperatures, while that with a larger amount of LiCl (40 wt%) slightly relied on firing temperatures. The highest PL intensity could be obtained at the firing conditions of 1300 °C and 10 wt% LiCl.     

Structural and luminescence properties of RE (RE = Eu, Tb):(MgCa)2Bi4Ti5O20 ceramics

Rare-earth ions (Eu³⁺, Tb³⁺) activated magnesium calcium bismuth titanate [(MgCa)₂Bi₄Ti₅O₂₀] ceramics were prepared by conventional solid state reaction method for their structural and luminescence properties. By using XRD patterns, the structural properties of ceramic powders have been analyzed. Emission spectrum of Eu³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown strong red emission at 615 nm (⁵D₀ → ⁷F₂) with an excitation wavelength λ_exci = 393 nm and Tb³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown green emission at 542 nm (⁵D₄ → ⁷F₅) with an excitation wavelength λ_exci = 376 nm. In addition, from the measurements of scanning electron microscopy (SEM), Fourier transform-infrared (FTIR) and energy dispersive X-ray analysis (EDAX) results the morphology, structure and elemental analysis of these powder ceramics have been studied.     

Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Impedance spectroscopic studies on NiFe2O4 with different morphologies: Microstructure vs. dielectric properties

NiFe₂O₄ powders were prepared by combustion synthesis, polyol-mediated and sol–gel methods. Morphological characterization of sintered disks was carried out using scanning electron microscopy (SEM). Dielectric properties of NiFe₂O₄ synthesized by different routes were investigated over the frequency range of 100 Hz–5 MHz at room temperature. A difference in dielectric constant (ε_r) and dissipation factor (tan δ) of NiFe₂O₄ samples obtained by different synthesis methods has been observed. The observed dissimilarity in the behavior originates from the variation in the microstructure of the samples that is evident in the Cole–Cole plot results.     

The role of the stabilizing agent on the structural and luminescent properties of hydrothermally synthesized ZrO2:Tb3+phosphors

Zirconium oxide powders doped with terbium, synthesized by hydrothermal route from a highly basic solution, were used to determine the role of the basic agent (NaOH, KOH or LiOH) utilized to carry out the hydrothermal synthesis on their morphology, crystalline structure, photoluminescent or cathodoluminescent properties. The synthesized powders showed an amorphous/polycrystalline nature with tetragonal phase crystallites of ZrO₂. The ratio of material undergoing the transformation from amorphous to crystalline is highly influenced by the kind of stabilizing agent used. As for the photoluminescence (PL) and cathodoluminescence (CL) results, the samples present the characteristic emission lines corresponding to inter electronic energy levels transitions for Tb³⁺ ions (⁵D₄ → ⁷F_J (J?=?3–6) transitions). The photoluminescence excitation spectra present a broad peak centered at 250?nm for all samples, however the sample prepared with LiOH as basic agent presented an additional peak at 290?nm which much likely is related to localized centers introduced by Li⁺ ions into the ZrO₂ crystallites.