Extrapure WO<Subscript>3</Subscript> for the preparation of CdWO<Subscript>4</Subscript> single crystals

A process has been developed for the preparation of extrapure WO suitable for crystal growth. The method involves thermal decomposition of ammonium paratungstate (APT) powder at 230–250° C, resulting in the formation of soluble ammonium metatungstate (AMT). The AMT solution is then purified and hydrolyzed. The resultant purified APT is converted to WO₃. The reaction yield is at a level of 95%. The contents of metallic impurities, especially those of polyvalent impurities (Fe, Mn, Cu, Pb, V, Cr, and others), capable of influencing the optical properties of crystals, was comparable to their detection limit: 10^?6 to 10^?5 wt %. The purification process was tested using a pilot-scale system. The purity was confirmed by the synthesis of high-quality CdWO₄ crystals.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

A thermal dehydration study of WO3·2H2O

Step-wise thermal dehydration of WO₃·2H₂O has been investigated and the various hydrated tungsten trioxide phases characterized. A cubic phase of tungsten trioxide with 0·36 moles of water is obtained by heating WO₃·2H₂O to 498 K.     

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.     

Microstructural and electrical properties of sintered tungsten trioxide

Tungsten trioxide sintered wafers were prepared from WO₃ powder obtained when ammonium paratungstate is decomposed in air at moderate temperature. Two wafer series of five samples were sintered under the same conditions in the temperature range 600–1000 °C. One of these wafers series was submitted to a subsequent annealing at 700 °C under a hydrogen atmosphere. All samples were characterized at room temperature by X-ray diffraction and electrical measurements. X-ray spectra show that WO₃ ceramic presents a mixture of the triclinic and monoclinic phases before the reduction process. After the reduction process, WO₂ and four hydrogen tungsten bronze phases are present in wafers. Capacitance measurements showed that the samples submitted only to the sintering process changed the dielectric constant with the frequency according to the Debye model. The reduced WO₃ shows a semiconductor behavior, as determined by electrical resistivity measurements.     

Investigation on electrical transport properties of nanocrystalline WO<Subscript>3</Subscript> under high pressure

The electrical transport properties of nanocrystalline tungsten trioxides (WO₃) under high pressures have been investigated by various electrical measurements up to 36.5 GPa. The discontinuous changes in direct-current resistivity under high pressures result from two electronic phase transitions at 4.3 and 10.5 GPa and two structural phase transitions at 24.8 and 31.6 GPa. Hall-effect measurement shows that the nanocrystalline WO₃ is n-type semiconductor within the whole investigated pressure range. The carrier concentration decreases monotonously with increasing pressure, but mobility increases first and then decreases at 10.4 GPa. Through alternate-current impedance measurement, it can be found that the variation of the ratio of grain boundary resistance to grain resistance synchronizes with that of the mobility under high pressures, indicating that the grain boundary plays more important role in the carrier transport process of nanocrystalline WO₃. The discontinuous changes of resistance and relaxation frequency of grain and grain boundary also provide the evidence for electronic phase transitions.     

Electrochromic modulation of near-infrared light by WO<Subscript>3</Subscript> films deposited on silver nanowire substrates

Silver nanowire (Ag NW) transparent conductive electrodes with high conductivity and optical transmittance are fabricated. Then, WO₃ films are deposited on Ag NW electrodes by an electrochemical deposition method. The WO₃/Ag NW films act as obvious optical modulators in the visible region. More importantly, the WO₃/Ag NW films have distinct advantage on NIR modulation over conventional WO₃/ITO electrode. Meanwhile, the WO₃/Ag NW films own high electrochromic efficiency of 86.9 cm² C^?1 at NIR region of 1100 nm. Furthermore, electrochromic devices (ECDs) based on Ag NW substrates are fabricated in this study, which exhibit excellent cycling stability and distinct modulation of near-infrared light compared with ITO-based ECDs. This work is the first study that reports the application of Ag NW-based electrochromic films and electrochromic devices in modulation of NIR light. It exhibits bright prospects that the electrochromic materials deposited on Ag NW electrodes may find potential application in thermal control and emission detectors for spacecraft.     

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.     

Effect of annealing temperature on the characteristics and sensing properties of WO<Subscript>3</Subscript> nanowires

A uniform WO₃ nanowire structure was prepared by two-step thermal oxidation method on Si substrate. WO₃ nanowires show different morphology and crystal structures after annealing at different temperatures. The influence of annealing temperature on WO₃ nanowires was investigated by SEM, TEM and XRD. Higher crystallization property and lower surface state was obtained with higher annealing temperature. The gas sensing properties of the WO₃ nanowires with various annealing temperatures to NO₂ with the concentration ranging from 1 to 4 ppm were examined at different temperatures ranging from room temperature to 200 °C. The results indicate that WO₃ nanowires can greatly lower the working temperature of sensors and sensors based on WO₃ nanowires show p-type or n-type sensing behaviors depending on annealing temperatures. Possible sensing mechanism of p-type WO₃ nanowires and the influence of annealing temperature on sensing types was explained. This work might supply new ideas about gas sensing mechanisms and open a new way to develop p-type WO₃ sensing materials.     

The formation of WO3 nanorods using the surfactant-assisted hydrothermal reaction

Monoclinic tungsten oxide (WO₃) nanorods were grown using the hydrothermal method on a seeded W foil. The seed layer was formed by thermal oxidation of W foil at 400°C for 30 min. Cetyltrimethylammonium bromide (CTAB) or hexamethylamine (HMT) was used in the reactive hydrothermal bath, along with sodium tungstate dihydrate (Na₂WO₄.2H₂O) and hydrochloric acid (HCl). The concentration of CTAB was varied from 0.01 M to 0.07 M and the concentration of HMT was varied from 0.01 M and 0.05 M. The result showed that CTAB-assisted hydrothermal reaction produced WO₃ nanorods with 4–7 nm diameter, and provided that CTAB concentration was less than 0.07 M. WO₃ nanorods could not be obtained when CTAB concentration was 0.07 M. Columnar structured WO₃ was produced with the presence of HMT in the hydrothermal bath. This was due to decomposition of HMT to form hydroxyl ions (OH^?) that inhibited the growth of nanorods. Cyclic voltammetry (CV) analysis showed better electrochromic property of WO₃ nanorods compared to columnar structured WO₃.     

A simple route to large-scale production of tungsten trioxide nanoparticles

Tungsten trioxide particles in high yield were prepared via a simple solid evaporation route with ammonium paratungstate hydrate as precursor and Ar gas as carrier gas. Detailed characterization by scanning electron microscopy has shown that increasing carrier gas flow rate promotes morphological evolution from large and irregular semi-spherical particles to non-agglomerated quasi-spherical particles, and finally to single-crystalline nanoparticles with an average diameter of 60 nm. The adsorption activity of the tungsten trioxide particles is size-dependent and increased with decreasing particle size. The present method could readily produce large-scale tungsten trioxide nanoparticles with ideal adsorption performance, and can be utilized to fabrication of various semiconductor oxides with advanced properties.     

Improvement of thermoelectric properties of WO3 ceramics by NiO addition

The thermoelectric properties of tungsten trioxide (WO₃) ceramics doped with nickel oxide (NiO) were investigated from 323 up to 1,023 K. The results revealed that doping WO₃ with NiO could promote the grain growth and the density. There was a second phase (NiWO₄) segregation at the grain boundaries in the samples containing more than 1.0 mol% NiO, which inhibited the further grain growth. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the NiO content. As for the power factor (σS ² ), the 1.0 mol% sample has the maximum value of the power factor which is 0.55 μW m^?1 K^?2 at 1,023 K.     

Hydrothermal synthesis of NaLa(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> octahedrons and tunable luminescence by changing Eu<Superscript>3+</Superscript> concentration and excitation wavelength

NaLa(WO₄)₂:Eu³⁺ phosphors with different Eu³⁺ concentrations have been synthesized by a hydrothermal method. The phase is confirmed by XRD analysis, which shows a pure-phase NaLa(WO₄)₂ XRD pattern for all of NaLa(WO₄)₂:Eu³⁺ phosphors. The SEM and TEM images indicate that all of NaLa(WO₄)₂:Eu³⁺ phosphors have a octahedral morphology. These suggest that the Eu³⁺ doping has no influence on the structure and growth of NaLa(WO₄)₄ particles. By monitoring the emission of Eu³⁺ at 615 nm, NaLa(WO₄)₂:Eu³⁺ phosphors show excitation bands originating from both host and Eu³⁺ ions. Under the excitation at 271 nm corresponding to WO₄ ^2? groups, emission bands coming from the ¹A₁ → ³T₁ transition with the WO₄ ^2? groups and the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions of Eu³⁺ are observed. The emission intensity relating to WO₄ ^2? groups decreases with increasing Eu³⁺ concentration. But emission intensities of Eu³⁺ increase firstly and then decreases because of concentration quenching effect. Under the excitation at 395 nm corresponding to ⁷F₀ → ⁵L₆ transition of Eu³⁺, only characteristic Eu³⁺ emission bands can be observed. The results of this work suggest that tunable luminescence can be obtained for Eu³⁺ doped NaLa(WO₄)₂ phosphors by changing Eu³⁺ concentration and excitation wavelength.     

Growth and structure of PbWO<Subscript>4</Subscript> films

Lead tungstate films have been grown on silicon by magnetron sputtering followed by heat treatment at various temperatures. The thermal oxidation of metal-oxide (Pb/WO₃/Si and W/PbO₂/Si) and metal-metal (Pb/W/Si) bilayer systems at temperatures above 870 K yields films that are dominated by monoclinic PbWO₄ and contain WO₃, also monoclinic. The optimal configuration for PbWO₄ synthesis is Pb/WO₃/Si because, even during lead deposition onto tungsten oxide, we observe the formation of lead tungstate, PbWO₄, and subsequent heat treatment increases the percentage of this phase in the film.     

Preparation of silane-WO3 film through sol-gel method and characterization of photochromism

The sol of silane and WO₃ was prepared from tetraethyl orthosilicate (TEOS) and methacryloxy-propyl-trimethoxy silane (KH570) with a novel route as described in previous work, and the aqueous WO₃ solution was prepared from ammonium tungstate. These two sols were mixed by stirring for about 1 hour with a certain ratio. Through sol-gel method, the transparent hybrids coating of organic silane and tungsten oxide was prepared by spraying or dipping on the glass substrates, and then were heat-treated at a certain temperature. The photochromic properties were investigated. AFM was used to investigate the surface structure of the prepared coatings. The crystalline phase was studied through X-ray diffraction. UV lights with different wavelengths were used to get the coloration of the film. The results show that silane-WO₃ film exhibits better photo-chromic properties under UV light irradiation.     

Characterization of various commercial forms of ammonium paratungstate powder

The structures and morphologies of various commercial forms of ammonium paratungstate have been studied and related to the processes used for the production of this material. High temperature crystallization processes are shown to produce the monoclinic pentahydrate 5(NH₄)₂O.12WO₃.5H₂O which yields a cuboid or equiaxed monoclinic powder morphology. Crystallization at room temperature produces the orthorhombic undecahydrate 5(NH₄)₂0.12WO₃.11H₂O which has a lath-like particle morphology. Freeze-dried ammonium paratungstate is shown to be amorphous in nature, to have a chemical composition approaching that of the undecahydrate, and to have a porous, multiparticulate agglomerate particle morphology. The apparent densities of the samples of ammonium paratungstate are explained in terms of the particle morphologies.     

Phase transitions of low-temperature sintering tungsten-doped ZnO-TiO<Subscript>2</Subscript> ceramics

WO₃-doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO₃ addition on the low-temperature sintering behavior, phase transition and dielectric properties of zinc titanate ceramics were investigated. The results show that the densification temperature of ZnTiO₃ ceramics can be reduced from 1150 to 900 °C with WO₃ addition and chemical processing. Small amount of WO₃ (<1.00wt %) accelerated the decomposition of hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ phase, while excessive addition (for example, 3.00wt %) restrained the decomposition. At the same time, the phase transition temperature from hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ is lowered by adding WO₃. WO₃ addition affects the dielectric properties of ceramics. The dielectric properties of WO₃-doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed the best dielectric properties for 1.0% WO₃-doped ceramics.     

Preparation and photoelectrochemical characterization of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanotube array electrode

WO₃/TiO₂ nanotube array electrode was fabricated by incorporating WO₃ with TiO₂ nanotube array via a wet impregnation method using ammonium tungstate as the precursor. TiO₂ and WO₃/TiO₂ nanotube arrays were characterized by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. In order to characterize the photoelectrochemical properties of WO₃/TiO₂ electrode, electrochemical impedance spectroscopy, and steady-state photocurrent (i _ss) measurement at a controlled potential were performed in the supporting electrolyte containing different concentrations of glucose. The photoelectrochemical characterization results reveal that WO₃/TiO₂ nanotube array electrode possesses a much higher separation efficiency of the photogenerated electron–hole pairs and could generate more photoholes on the electrode surface compared with the pure TiO₂ nanotube array electrode. The i _ss for glucose oxidation at WO₃/TiO₂ nanotube array electrode is much higher than that at the pure TiO₂ nanotube array electrode.     

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.     

Polyol mediated synthesis of tungsten trioxide and Ti doped tungsten trioxide: Part 1: Synthesis and characterisation of the precursor material

Polyol mediated synthesis for the preparation of tungsten trioxide and titanium doped tungsten trioxide has been reported. The reaction was carried out using chlorides of tungsten and titanium in diethylene glycol medium and water as the reagent for hydrolysis at 190 °C. Formation of a blue coloured dimensionally stable suspension of the precursor materials was observed during the course of the reaction. The particle sizes of the precursor materials were observed to be around 100 nm. The precursor materials were annealed to give tungsten trioxide and titanium doped tungsten trioxide. The precursor materials were characterised using TGA/DTA, FT-IR, optical spectra, SEM, TEM and powder XRD methods. It was observed that the doping of titanium could be effected at least up to 10% of Ti in WO₃. The TGA/DTA studies indicated that WO_3−x·H₂O is the dominant material that formed during the polyol mediated synthesis. The XRD data of the annealed samples revealed that the crystalline phase could be manipulated by varying the extent of titanium doping in the tungsten trioxide matrix.