Anti-tumor activity of self-charged (Eu,Ca):WO3 and Eu:CaWO4 nanoparticles

Non-stoichiometric (Eu,Ca):WO₃ and Eu:CaWO₄ nanoparticles with anti-tumor activity are synthesized in a sol?Cgel method by adding excessive Eu^3?+? and Ca^2?+? ions to tungsten oxide crystal structure. Colorimetric assay shows that 10?nm (Eu,Ca):WO₃ and Eu:CaWO₄ nanoparticles can effectively inhibit growth of mammary cancer cells without any harm to normal cells. Nanoparticles are characterized by X-ray diffraction, high resolution transmission electron microscopy and fluorescence optical spectrometry. Nanomaterials, insoluble in synthesized water, have complicated self-charging surfaces that trap mammary cancer cells. Surface self-charging effect is suggested as the inhibition mechanism.     

Processing and sintering of yttrium-doped tungsten oxide nanopowders to tungsten-based composites

Innovative chemical methods are capable of fabricating nanoscale tungsten oxide compounds doped with various rare-earth elements with high purity and homogeneity, which can be processed under hydrogen into nanostructured oxide-dispersed tungsten composite powders having several potential applications. However, hydrogen reduction of doped tungsten oxide compounds is rather complex, affecting the morphology and composition of the final powder. In this study, we have investigated the reduction of tungstic acid in the presence of Y and we provide the experimental evidence that Y₂O₃ can be separated from Y-doped tungstic acid via hydrogen reduction to produce Y₂O₃-W powders. The processed powders were further consolidated by spark plasma sintering at different temperatures and holding times at 75 MPa pressure and characterized. The optimized SPS conditions suggest sintering at 1400 °C for 3 min holding time to achieve higher density composites with an optimum finer grain size (3 µm) and a hardness value up to 420 H _V. Major grain growth takes place at temperatures above 1300 °C during sintering. From the density values obtained, it is recommend to apply higher pressure before 900 °C to obtain maximum density. Oxides inclusions present in the matrix were identified as Y₂O₃·3WO₃ and Y₂O₃·WO₃ during high resolution microscopic investigations.     

Growth of single-crystal tungsten whiskers during CO reduction of NiWO<Subscript>4</Subscript>

Experimental evidence is presented that the growth of tungsten whiskers during CO reduction of NiWO₄ follows the vapor-liquid-solid mechanism. A liquid phase appears at 940°C owing to the melting of NiWO₄-Na₂WO₄ eutectic regions which seem to result from the segregation of sodium impurities present in the reagents used. The presence of the liquid phase ensures comparable rates of Ni and W liberation from the crystal lattice of nickel tungstate, leading to the formation of α-W + NiW two-phase regions. The directional growth of tungsten whiskers during further CO reduction of the tungstate melt is associated with W deposition on the growth surfaces of α-W single crystals in the two-phase regions.     

Influence of the composition of precursors and reduction conditions on the properties of magnesiothermic molybdenum powders

We have investigated the preparation of molybdenum powders by reducing the oxide compounds MoO₃, MgMoO₄, and CaMoO₄ with magnesium vapor at residual argon pressures in the range 5–20 kPa and temperatures in the range 700–800°C. Using the MgMoO₄ and CaMoO₄ compounds as precursors, we have obtained molybdenum powders with specific surface areas of up to 20 m²/g. The powders have a mesoporous structure. The reduction of the molybdenum compounds under such conditions was accompanied by separation of the reaction products due to the removal of magnesium oxide from the reaction zone.     

Combustion synthesis of tungsten powder from sodium tungstate

Tungsten powders were prepared by Self-propagating High-temperature Synthesis (SHS) method directly from hitherto unreported system: sodium tungstate (Na₂WO₄) and magnesium (Mg). The adiabatic temperatures of self-propagating combustion reactions with different amount of Mg in Na₂WO₄ + Mg system were calculated. The influences of different starting conditions (molar ratios of Mg/Na₂WO₄ and relative densities of samples) on the compositions and microstructure of reaction products were investigated. It shown that, the complete reduction of WO₃ required a 60% excess of magnesium over the stoichiometric molar ratio Mg/Na₂WO₄ of 3. The amount of the impurities is minimal at appropriate relative density. At last, tungsten powders can be obtained after the acid enrichment and distilled water washing.     

Synthesis of tungsten bronze powder and determination of its composition

Sodium tungsten bronze powders were synthesized by thermal reduction of a gas/melt system at high temperature. Samples having a cubic structure with different compositions were prepared. The initial melt included Na₂WO₄, WO₃ and 10–40% mol. NaCl while the reducing gas was hydrogen at 750 °C. An original mechanism of controlling the powders size and distribution was suggested and discussed. A quantitative novel and simple method to determine the bronze composition based on TGA data was developed. An increase in the NaCl content led to a decrease of the crystals size and improved the powder uniformity. Fine powders, in the 2–5 μm size range, were synthesized from melt with 40% mol of NaCl. The stoichiometry parameter x of the obtained bronzes ranged from 0.8 to 0.92. An excellent agreement between x values determined by the classical XRD route and the proposed TGA method was demonstrated.     

Effects of process parameters on tungsten boride production from WO3 by self propagating high temperature synthesis

In the present study, the production parameters of tungsten boride compounds by self-propagating high-temperature synthesis (SHS) method and following leaching process were investigated. In the SHS stage, the products consisting of tungsten borides, magnesium oxide, magnesium borate, and also minor compounds were obtained by using different initial molar ratios of WO₃, Mg and B₂O₃ as starting materials. In the leaching step, Mg containing byproducts, i.e. MgO and Mg₃B₂O₆, existed in the selected SHS product synthesized at 1:8:2.5 initial molar ratio of WO₃:Mg:B₂O₃ were leached out by using aqueous HCl solution to obtain clean tungsten boride compounds at different experimental parameters which are time, acid concentration and temperature. The acid leaching experiments of the SHS product showed that optimum leaching conditions could be achieved by using 5.8 M HCl at 1/10 S/L ratio and the temperature of 80 °C for 60 min.     

Preparation of high-purity and ultrafine WC-Co composite powder by a simple two-step process

In this study, high-purity and ultrafine WC-6%Co composite crystallites was synthesized by a simple two-step process consisting of the precursor-formation of the mixture of blue tungsten oxide (WO_2.9) and cobaltic oxide (Co₂O₃) and the following deep reduction and carburization with CH₄-H₂ mixed gases. The experimental results revealed that after the first carbothermic reduction stage at 1050 or 1150 °C, a mixture of W, WO₂ and Co₇W₆ was obtained, which was further carburized to the WC and Co phases by CH₄-H₂ mixed gases at 900 °C. With the increase of C/WO_2.9 molar ratio, the particle sizes of first-stage precursor and carbonized product were both decreased. The particle sizes of final products are mainly determined by C/WO_2.9 ratio and reaction temperature at the first stage. When the C/WO_2.9 ratio was in the range of 2.3–2.7, the high-purity WC-6%Co composite powder with the average particle sizes of 160–410 nm could be obtained.     

Fabrication of tungsten powder with sodium tungstate as raw material by SHS method

Tungsten powders were fabricated by Self-propagating High-temperature Synthesis (SHS) method with sodium tungstate (Na₂WO₄) and magnesium (Mg) as raw reactants. The effect of the molar ratios of Mg/Na₂WO₄ on the compositions of combustion products was investigated. The results show that, it is advantageous for more Mg in the raw reactants than the stoichiometric quantity to decrease the impurities in the combustion products and increase the completeness of the synthesis reaction of tungsten. At last, tungsten powders can be obtained after the acid enrichment and distilled water washing.     

Effect of Processing Variables on WC Nanoparticles Synthesized by Solvothermal Route

In this work, the effect of processing variables on the synthesis of WC nanoparticles by solvothermal route has been reported. Nanoparticles of tungsten carbide (WC) have been prepared by taking metallic magnesium (Mg), acetone (C₃H₆O), and tungsten trioxide (WO₃) in an autoclave at 600°C. The XRD results showed that the optimization of the reaction time facilitates the reduction as well as carburization of the tungsten source. The Williamson-–Hall analysis was used to study the effect of synthesis time on the crystallite size and lattice strain of the sample. Differential thermal analysis/thermogravimetric analysis shows the stability of the synthesized product. It has been observed that the synthesis of WC nanoparticles is a multistep process and the complete carburization reaction occurs at particular carburization time.     

The controlled reduction of copper tungstate in H2O/H2 mixtures

The controlled reductions of copper tungstate, copper tungstate plus blue tungsten oxide and CuO plus WO₃ have been studied at temperatures of 500, 600 and 700C using H₂/H₂O mixtures in a glass thermobalance. The copper-tungsten oxide composite powder products have been characterized by X-ray diffraction analysis and scanning electron microscopy. The morphologies of the copper-tungsten oxide products have been related to the morphologies of copper-tungsten powders obtained by dry hydrogen reduction. It is shown that widely varying copper-tungsten oxide morphologies may be obtained as precursors for metal powder production by control of the H₂/H₂O ratio in the reducing gas mixture.     

Processing of low grade tungsten ore concentrates by hydrometallurgical route with particular reference to India

Tungsten, because of its high strength and high melting point occupies a prime position amongst metals. With depletion of high grade resources considerable R and D work is still being carried out in tungsten producing countries around the world for the processing of low grade and secondary resources. The paper gives a brief review of the hydrometallurgical processes developed to recover tungsten from low grade concentrates. The R and D work carried out on purification and recovery of tungsten as tungstic oxide/ammonium paratungstate (APT) from a number of off-grade products such as table concentrate (WO₃=66%, SiO₂=2·2%, S=1·8%), middlings (18–20% WO₃, and 28–30% S) and jig concentrate (4·6% WO₃) are discussed in this paper. It has been found that more than 75% of silica and 90% of sulphur could be removed from the table concentrate by curing with hydrofluoric acid and subsequent roasting of the desilicated product at 650°C. In the case of middlings, it was possible to recover over 90% of tungsten as tungstic oxide by an oxidative roast followed by pressure leaching with soda. A detailed study on the low grade jig concentrate to recover tungsten as APT, showed that over 90% extraction was possible by adopting the pressure leaching-solvent extraction route. Effect of parameters such as soda concentration, time, temperature and pressure during leaching; as well as extraction and stripping behaviour of tungsten from leach solution at different pH and aqueous to organic ratio during solvent extraction with Alamine-336, were studied and a flow-sheet was developed for processing of jig concentrate analysing 4·6% WO₃.     

Ca<Subscript>3</Subscript>WO<Subscript>6</Subscript>: a novel microwave dielectric ceramic with complex perovskite structure

Present work introduces a novel Ca₃WO₆ microwave dielectric ceramic with a complex perovskite structure. The Ca₃WO₆ ceramic was prepared by solid state reaction method and can be well densified at above 1,260 °C for 2 h in air. All the XRD patterns can be fully indexed as a single-phase monoclinic structure (space group P2₁/n). The sharp Raman vibration mode at 810 cm⁻¹ suggests the long range order in the Ca₃WO₆ structure. The best microwave dielectric properties can be obtained in ceramic sample sintered at 1,275 °C for 2 h with a permittivity ~15.3, a Qf value ~29,200 GHz and a TCF value about −30 ppm/°C. Applying the oxide additivity rule, the calculated permittivity agrees well with the measured value. This kind of ceramic might have some potential value for microwave application for its good microwave dielectric behavior. The (Ca_1/2W_1/2) complex cations holding the site of Ti⁴⁺ in perovskite structure would introduce many new systems in complex perovskite compounds in the future.     

Effect of hydrothermal temperature on structure and photochromic properties of WO3 powder

Tungsten trioxide (WO₃) powders were prepared via a simple hydrothermal method. The morphology, structure and photochromic activity of the synthesized WO₃ powders were studied by X-ray diffraction, scanning electron microscopy and UV–vis spectrophotometer combined with color difference meter. The results showed the synthesized WO₃ powders with hexagonal phase got much better photochromic properties than the WO₃ powders with cubic phase, the ones not appear until about 160 °C. Besides, the WO₃ powder synthesized at 120 °C exhibited the best photochromic properties of the samples prepared below 160 °C, the particles of which formed a shape of clusters of cactus with uniform size and good dispersion.     

Mechanochemical synthesis of sodium tungsten bronze nanocrystalline powders

Nanocrystalline powders of sodium tungsten bronze Na_xWO₃ (x∼0.88) have been prepared by mechanochemical process using starting materials of Na pieces and WO₃ powders in a planetary ball mill. The synthesis reactions proceed with increasing milling time and are almost completed after 44 h. Phase-pure nanoparticles of Na_xWO₃ with average size of 17 nm were directly obtained after a simple washing process to remove the by-product of Na₂WO₄. The resistivity was measured in the temperature range from 77 to 300 K. The sample displays semiconducting behavior and can be characterized by three-dimensional variable-range hopping. The mechanochemical process seems to be an attractive route to fabricate tungsten bronzes because of several advantages such as easy preparation, less cost, operating at low temperature and suitability for a large-scale production.     

Effects of co-doped Li+ ions on luminescence of CaWO4:Sm3+ nanoparticles

CaWO₄, CaWO₄:Sm³⁺ and CaWO₄:(Sm³⁺, Li⁺) nanoparticles were synthesized by the precipitation method with addition of PEG 200. The X-ray diffraction patterns show that the obtained samples have a pure tetragonal phase. The CaWO₄ sample shows an emission peak at 418 nm originating from the charge-transfer transitions within the WO₄ ^2? complex. CaWO₄:Sm³⁺ and CaWO₄:(Sm³⁺, Li⁺) samples show emission peaks originating from the f–f forbidden transitions of the 4f electrons of Sm³⁺ ions. The charge compensator of Li⁺ can enhance the emission intensity effectively. It is found that the emission intensity of CaWO₄:(3 mol% Sm³⁺, 4 mol% Li⁺) phosphor is about double that of CaWO₄:3 mol% Sm³⁺ phosphor.     

Phase developments in Pb(Mg<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> and Pb(Zn<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> via B-site precursor route

Phase formation stages of MgWO₄ and ZnWO₄ (precursor compositions for following steps) were investigated by monitoring the reactions of oxide chemicals at various temperatures. Developed phases were examined by using X-ray diffraction (XRD). Successive attempts were also conducted for Pb(Mg_1/2W_1/2)O₃ (PMW) and Pb(Zn_1/2W_1/2)O₃ (PZW) by reacting PbO with the precursor compounds. Stages of phase development in the two compositions were also analyzed. The results are compared with those of another tungsten-containing perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) and its B-site precursor Fe₂WO₆. After PbO addition to the precursor powders, a perovskite phase formed directly (i.e., without any intermediate phases) in the case of PMW. For PbO + ½ZnWO₄, in contrast, the decomposition of ZnWO₄ and preferential reaction with PbO resulted in Pb₂WO₅ and ZnO, instead of the perovskite PZW.     

Thermal Analysis of the Ternary System Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>-WO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

Thermal analysis results indicate that the liquidus surface of the Li₂WO₄-WO₃-Li₂B₄O₇ system consists of the primary phase fields of Li₂WO₄, Li₂B₄O₇, WO₃, Li₂WO₄ · WO₃ (congruent melting), 3Li₂WO₄ · 2Li₂B₄O₇ (congruent melting), and Li₂WO₄ · 3WO₃ (incongruent melting). Low-melting-point compositions are selected that are potentially attractive for the low-temperature synthesis of lithium tungsten bronze powders.     

Hot-wire chemical vapor deposition of WO3−x thin films of various oxygen contents

We present the synthesis of tungsten oxide (WO_3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO_3−x, WO₃/W and WO₃/W/WO_3−x are obtained in a single continuous hot-wire chemical vapor deposition process using only ambient air and hydrogen. The air oxidizes resistively heated tungsten filaments and produces the tungsten oxide species, which deposit on a substrate and are subsequently reduced by the hydrogen. The reduction of tungsten oxides to oxides of lower oxygen content (suboxides) depends on the local water vapor pressure and temperature. In this work, the substrate temperature is either below 250 °C or is kept at 750 °C. A number of films are synthesized using a combined air/hydrogen flow at various total process pressures. Rutherford backscattering spectrometry is employed to measure the number of tungsten and oxygen atoms deposited, revealing the average atomic compositions and the oxygen profiles of the films. High-resolution scanning electron microscopy is performed to measure the physical thicknesses and display the internal morphologies of the films. The chemical structure and crystallinity are investigated with Raman spectroscopy and X-ray diffraction, respectively.