Gd2O3:Eu phosphor powders prepared using a size-controllable droplet generator

Gd₂O₃:Eu phosphor powders were prepared by a filter expansion aerosol generator (FEAG) process capable of changing the mean size of droplets. The change in the mean size of the Gd₂O₃:Eu phosphor powders according to the concentrations of polyethylene glycol added to spray solutions was caused by the difference in the mean size of the droplets produced via the FEAG process. The mean sizes of droplets produced by the FEAG process were affected by the surface tension and viscosity of the spray solutions. The mean sizes of the Gd₂O₃:Eu phosphor powders obtained from the spray solutions with the same concentration of metal salts changed from 1.5 to 4.2 μm according to the concentrations of polyethylene glycol and citric acid added to the spray solutions. The maximum photoluminescent intensity of the phosphor powders obtained from the spray solutions with polymeric precursors and boric acid flux was 144% of that of the phosphor powders obtained from the aqueous spray solutions without flux.     

Synthesis of Bi2Fe4O9 via PVA sol-gel route

Bi₂Fe₄O₉ powders were synthesized by a sol-gel process using polyvinyl alcohol (PVA) as a complexing agent. Differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Field emission scanning electron microscope (FSEM) techniques were used to characterize precursor and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of Bi₂Fe₄O₉ was investigated. XRD analysis showed that single-phase Bi₂Fe₄O₉ was obtained from the Mⁿ⁺/-OH = 2:1 and Mⁿ⁺/-OH = 1:1 precursors at the temperature of 700 °C. For the precursor with Mⁿ⁺/-OH = 4:1, pure Bi₂Fe₄O₉ formed at the temperature of 800 °C. Bi₂Fe₄O₉ powders clacined at 700 °C from Mⁿ⁺/-OH = 2:1 precursor shows weak ferromagnetism.     

Solvent and stabilizer free growth of Ag and Pd nanoparticles using metallic salts/cyclotriphosphazenes mixtures

Cyclotriphosphazene is used as a sacrificial solid-state template to synthesize a range of Ag and Pd nanoparticles with diverse geometries by thermal treatment using MLn/N₃P₃(O₂C₁₂H₈)₃ mixtures. The Pd and Ag nanoparticles are synthesized by solid-state pyrolysis of AgPPh₃[CF₃SO₃]/N₃P₃(O₂C₁₂H₈)₃ and PdCl₂/N₃P₃(O₂C₁₂H₈)₃ mixtures with molar relationships of 1:1, 1:5 and 1:10 respectively, in air and at 800 °C. The morphology of the as-prepared nanoparticles is found to depend on the molar ratio of the precursor mixture, the preparation method and of the nature of the metal. Ag and Pd, microcrystals were thermally grown on Si from the respective 1:1 precursors while that metal foams were grown from 1:5 ratios precursors on SiO₂ wafers. High resolution transmission electron microscopy investigations reveal in most cases small crystals of Pd. HRSTEM measurements indicate that the formation of the Pd and Ag nanoparticles occurs through a phase demixing and dewetting mechanism. This approach has potential to be a useful and facile method to prepare metallic nanoparticles without requiring solutions or surfactants for application in electronic, catalytic and sensor materials and devices.     

A study of sintering and magnetic parameters of spinel lithium ferrite: II. Lithium ferrite containing Bi2O3

High-dispersity powders of spinel lithium ferrite, Li_0.5Fe_2.5O₄, containing different amounts of Bi₂O₃ were prepared by thermal treatment of mixtures of /Fe₃(HCOO)₆(OH)₂/HCOO.4H₂O, LiHCOO.H₂O and Bi(HCOO)₃ obtained by spray drying. It was found that sintering of lithium ferrite in the presence of Bi₂O₃ leads to very high densities of the products at 900 – 1000°C. This is due to the formation, by Li_0.5Fe_2.5O₄ and Bi₂O₃, of an easily melting eutectic. The study of the magnetic properties showed that the presence of Bi₂O₃ ensured the formation of lithium ferrites with good characteristics at considerably lower temperatures than those usually observed.     

Generation of metal and metal oxide nanoparticles by liquid flame spray process

A liquid flame spray (LFS) process has been investigated for the generation of single component nanoparticles. In the LFS process, a solution consisting of metal nitrate dissolved in water is sprayed into a turbulent, high temperature H₂-O₂-flame. The primary spray droplets evaporate and subsequent reactions in the flame produce metal or metal oxide vapours which nucleate to final particulate form. In the study, the process characteristics were examined to produce 10–60 nm particles from silver, palladium and iron containing precursors. A systematic study using variable process parameters proved that the size of the generated nanoparticles is set by the mass flow rate of the metal precursor, only. The geometric standard deviation of the size distributions was seen to vary in a limited range of 1.35–1.4. The particle size was verified by aerosol instrumentation, the composition and morphology by X-ray diffraction (XRD) and transmission electron microscopy (TEM), correspondingly. The Ag and Pd particles were seen to consist of pure metals. For iron, the presence of all three of the following compounds were detected: Fe, Fe₂O₃ and Fe₃O₄.     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC₂H₅)₅, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.     

The effect of annealing on the photoluminescence of layered perovskite H1.77[Sr0.8Bi0.21Ta2O7] powders

The microstructural and luminescent properties of H_1.77[Sr_0.8Bi_0.21Ta₂O₇] powders under different dehydrating conditions were investigated. The powders were obtained by protonating Bi₂SrTa₂O₉ powders in 3 M aqueous HCl solution, and were dehydrated by a post-annealing in air from 250 to 700 °C. The photoluminescence was enhanced for all dehydrated powders, especially for the sample annealed at 400 °C. The thermogravimetric analysis showed that there is an obvious dehydration temperature range from 200 to 400 °C, which was corresponding to the changing of the microstructure and photoluminescence in the same temperature range. The mechanism behind the experimental results was discussed.     

Phase Composition of Nanocrystalline Titania Synthesized under Hydrothermal Conditions from Different Titanyl Compounds

Data are presented on the phase composition and physicochemical properties of nanocrystalline TiO₂ powders prepared via hydrothermal treatment of aqueous titanyl sulfate (TiOSO₄), titanyl nitrate (TiO(NO₃)₂), and aqua complex titanyl oxalate acid (H₂TiO(C₂O₄)₂) solutions and amorphous titanyl hydroxide (TiO₂ · nH₂O) gel (synthesized by precipitating H₂TiCl₆ with an excess of aqueous ammonia) at 423 and 523 K for 10 min to 6 h at solution concentrations from 0.07 to 0.5 M. The synthesized samples were characterized by x-ray diffraction, thermogravimetry, transmission electron microscopy, and nitrogen BET surface area measurements. It is shown that, independent of the precursor, short-term (10 min) hydrothermal treatment leads to the formation of nanocrystalline anatase (crystallite size d = 10–30 nm), a metastable form of titania. Upon an increase in hydrothermal-treatment time to 6 h, the systems studied exhibit different behaviors. Nanocrystalline anatase may persist (titanyl sulfate solutions and amorphous titanyl hydroxide gel) or transform into rutile (d = 50–100 nm), the thermodynamically stable form of TiO₂, through recrystallization processes (titanyl nitrate solutions; 0.07 M H₂TiO(C₂O₄)₂ solutions, partial conversion at 423 K and full conversion at 523 K; 0.28 M H₂TiO(C₂O₄)₂ solutions, full conversion at 423 K). Also possible is the formation of mesoporous anatase (0.28 M H₂TiO(C₂O₄)₂ solution, 523 K, 70- to 90-nm aggregates of crystallites, 10- to 20-nm closed pores containing the solution). A model is proposed according to which the formation of mesoporous oxides is possible at comparable rates of anatase nucleation and growth.     

Influence of Primary Milling on Structural and Electrical Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Ceramics Obtained by Sintering Process

In this article, the influence of primary mechanical milling of precursors on the microstructure and dielectric properties of Bi₄Ti₃O₁₂ ceramics was studied. Precursor material (mixture of Bi₂O₃ and TiO₂ powders) was ground by a high-energy attritorial mill for (1, 3, 5, and 10) h. Bi₄Ti₃O₁₂ ceramics were obtained by a solid-state reaction process, synthesized at an intermediate temperature (800 °C) and finally sintered at high temperature (1140 °C). Structure studies were performed by X-ray diffraction (XRD) and scanning electron microscopy techniques. XRD patterns were analyzed by the Rietveld method using the DBWS 9807a program. The thermal properties of the studied materials were measured using differential thermal analysis and thermal gravimetric techniques. These studies indicate that one-, three-, and five-hour primary high-energy ball milling followed by sintering is a promising technique for pure Bi₄Ti₃O₁₂ ferroic ceramics preparation. The investigation of Bi₄Ti₃O₁₂ shows that ceramics obtained from a precursor and milled for 5 h have the best dielectric properties.     

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC₂H₅)₅, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.     

Thermisches Spritzen neuartiger SiC und TiC Pulver mit oxidischer Binderphase

Processing of newly developed SiC and TiC Powders with Oxide‐Ceramic Matrix by means of Thermal Spraying Intermediate results of an actual project are presented. The investigations concern newly developed SiC and TiC powders, their processing by means of thermal spraying and the characterization of produced coatings. Development and optimization of powders, thermal spray process and spray parameter optimization are carried out on permanent feedback. The powder production line is spray‐drying, sintering and conditioning. The binder matrix phase is aluminum oxide / yttrium oxide. The produced powders SiC – Al₂O₃/Y₂O₃ and TiC – Al₂O₃/Y₂O₃ show different specific chemical compositions and morphologies each. Carbide contents of >65 % are aimed at. Applied thermal spray processes are atmospherical plasma spraying and high velocity oxygen fuel spraying. The results demonstrated characterize feedstock powders as well as produced coatings. The investigations are done by means of light and scanning electron microscopy.     

Effect of two-step sintering on rare earth (RE = Y2O3, Pr6O11) doped ZnO–Bi2O3 varistors processed from ‘nano-precursor’ powders

Nano size ZnO–Bi₂O₃ varistor precursor powders containing Y₂O₃ and Pr₆O₁₁ rare earth dopants were prepared by low temperature refluxing at 80 °C. Effect of rare earth dopants, densification by two-step sintering, evolution of microstructures and their influence on varistor properties were investigated. Chemically synthesized nano- precursor varistor powders produced controlled grain size in two-step sintering in which the average sintered ZnO grain size was reduced to at least half compared to the conventionally processed ZnO–Bi₂O₃ varistors. The study revealed that such grain size reduction is highly beneficial to attain enhanced varistor properties.     

Hydrothermal synthesis of bismuth sodium titanate particles with different morphologies

Nanostructured Na_0.5Bi_0.5TiO₃ particles have been synthesized by a hydrothermal synthesis method using a layered titanate H_1.07Ti_1.73O₄·nH₂O as a Ti precursor. The obtained Na_0.5Bi_0.5TiO₃ particles showed different morphologies including plate-like, wire-like, and cubic-like structures in different hydrothermal conditions. The effect of the NaOH concentration on the growth and morphology evolution of hydrothermally derived Na_0.5Bi_0.5TiO₃ powders were investigated. It was found that alkaline concentration had a great effect on the phase and morphology of the resultant powders. The dissolution–recrystallization and in situ topotactic transformation mechanisms were suggested in different alkaline concentrations according to the evolution process.     

Preparation of spherical Pb(Zr,Ti)O3 powders by ultrasonic spray pyrolysis

Spherical Pb(Zr,Ti)O₃ powders were prepared from the aqueous acetate-base solution by ultrasonic spray pyrolysis. The raw materials were Pb(C₂H₃O₂)-3H₂O, ZrO(C₂H₃O₂)₂ and a mixture of Ti(C₃H₇O)₄ and C₅H₇O₂. A single-phase PZT was formed at 900 C in air and at 700 C in O₂ atmosphere. The PbTiO₃ phase, as an intermediate product, was observed in the formation of PZT and no PbZrO₃ phase was detected under our experimental conditions. The intensity of the PbTiO₃ peak decreased with increasing reaction temperature in air, while the reverse effect of reaction temperature was observed in O₂. Spherical and irregular-shaped particles coexisted in the powder containing the minor PbTiO₃ phase, while the particles of a single-phase PZT have spherical morphology with little evidence of irregular-shaped particle formation.     

Improved chemical route for quantitative precipitation of lead zirconyl oxalate (PZO) leading to lead zirconate (PZ) powders

An improved chemical route is described to precipitate quantitatively a molecular precursor lead zirconyl oxalate tetrahydrate, PbZrO(C₂O₄)₂·4H₂O (PZO) at room temperature. In this route, soluble ammonium zirconyl oxalate, (NH₄)₂ZrO(C₂O₄)₂ (AZO), is initially prepared by chemical reaction of 0.1-M solution of zirconyl nitrate with 0.2-M solution of ammonium oxalate. The anionic species ZrO(C₂O₄)₂⁻² thus obtained is exchanged by adding equimolar solution of lead acetate to precipitate single-phase PZO with higher yield. The improvement in the presently described route is to drive exchange reaction to its completion by suitably manoeuvring the choice of precursor, pH and reaction conditions, in such a way that single-phase PZO is precipitated without any destabilization of anionic species ZrO(C₂O₄)₂⁻². The pyrolysis of PZO produced stoichiometric, monophasic, orthorhombic PbZrO₃ (PZ) powders at temperature T=700°C. Various physico-chemical techniques were used to characterize both molecular precursor PZO as well as PZ powders obtained by its pyrolysis. All our experimental results related to the synthesis and characterization of these powders are presented in this paper.     

Molten salt synthesis and luminescence properties of Bi4Si3O12 powders

In this paper, Bismuth silicate (Bi₄Si₃O₁₂, BSO) powders were prepared by reaction in molten salts (NaCl and KCl) using Bi₂O₃ and SiO₂ powders as raw materials. The effects of salt contents and excessive contents of Bi₂O₃ and SiO₂ were characterized by X-ray diffraction and scanning electron microscopy. The properties of Bi₄Si₃O₁₂ powders were analyzed via fluorescence spectroscopy. Forming mechanism of platelets was discussed. The experimental results suggest that Bi₄Si₃O₁₂ powders are produced at 780 °C for 4 h with 40 wt% salts and 5 wt% excessive content of Bi₂O₃. The peaks of excitation spectrum and emission spectrum for BSO powders are separately located at 266 and 457.6 nm. Compared with crystal materials, the excitation spectrum and emission spectrum of Bi₄Si₃O₁₂ powders indicate blue shift. Interface reacting mechanism can be used to explain the coarsening process of bismuth silicate platelets produced in molten salt synthesis. In addition, dissolution–precipitation mechanism also plays an important role in the synthesis of bismuth silicate platelets.     

Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis,characterization and application in recyclable photodegradation of organic dyes under visible light irradiation

Heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by a two-step method. First, Fe₃O₄ nanoparticles with the size of ca. 10 nm were synthesized by chemical method at room temperature and then heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by hydrothermal method at 180 °C for 24 h with the addition of 10 wt% Fe₃O₄ nanoparticles into the precursor suspension of Bi₂O₂CO₃. The pH value of synthesis suspension was adjusted to 4 and 6 with the addition of 2 M NaOH aqueous solution. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe₃O₄/Bi₂O₂CO₃ photocatalysts were obtained, respectively. Both photocatalysts demonstrate superparamagnetic behavior at room temperature. The UV–vis diffuse reflectance spectra of the photocatalysts confirm that all the heterostructured photocatalysts are responsive to visible light. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. The heterostructured photocatalysts prepared in this study exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of MB and MO, and they can be easily recovered by applying an external magnetic field.     

Preparation, sintering, and microstructures of strontium barium bismuth tantalate layered perovskite ceramics

The solid solutions of Ba-doped SrBi₂Ta₂O₉ layered perovskite ceramic powders have been successfully prepared via a two-step process using BiTaO₄ as a precursor. The lattice constants of the solid solutions monotonically increase with increasing barium-ion content. The sinterability of (Sr_1–xBa_x)Bi₂Ta₂O₉ powders is significantly improved by increasing the barium-ion content. When the specimens with high barium-ion contents are sintered at 1100°C, they thermally decompose to form rod-like grains and the matrix expands, leading to a lower density. The addition of barium ions to SrBi₂Ta₂O₉ also results in significant variation in the morphology of the sintered specimens and the occurrence of c-axis preferred orientation which is ascribed to the anisotropic growth of plate-like grains. The precise control of the barium-ion content as well as the sintering conditions is critical for obtaining densified barium-ion doped SrBi₂Ta₂O₉ ceramics with a pure, layered perovskite structure.     

Morphologies and properties of nickel particles prepared by spray pyrolysis

Nickel particles were prepared by spray pyrolysis of aqueous solutions of NiCl₂·6H₂O with reducing gas. Wet pyrolysis lowered the onset and the finishing temperatures of the individual chemical transformations by at least 100°C to contribute high conversion to nickel and its completion of sintering. Densification by increasing the furnace set temperature outweighed the adverse effects of the dry sintering and short residence time of the precursor particles. The initial salt concentration increased the size of the nickel particles with their densification, resulting from the enhanced rate of nickel nucleation.