Tungsten Phase Research Articles

Biphasic Catalytic Hydrogen Peroxide Oxidation of Alcohols in Flow: Scale-up and Extraction

We report continuous solvent-free biphasic alcohol oxidation with hydrogen peroxide and in-line separation of the tungsten polyoxometalate catalyst and phase transfer catalyst from the product. Zinc-substituted polyoxotungstate in combination with the selected phase transfer catalyst drives the oxidation reaction to completion within a short residence time (5–10 min) in a silicon Pyrex microreactor. This continuous and small-scale reactor allows for fast optimization of reaction conditions for each substrate and selection of the phase transfer catalyst. Scaling of the production rate (up to 650 times) is achieved with a Corning low flow reactor (LFR) and an advanced flow reactor (AFR). New scaled-up, in-line membrane-based liquid–liquid extraction units at the reactor outlet first separate the tungsten polyoxometalate catalyst with the aqueous waste stream from the organic product stream. A three-stage countercurrent liquid–liquid extraction then removes more than 90% of the phase transfer catalyst from t...

Up-conversion thermometric properties of nanorod-like K2Bi0.98−xEr0.02YbxNb5O15

A series of K2Bi0.98−xEr0.02YbxNb5O15(KBN-x, 0≤x≤0.15) up-conversion nanoparticles (UCNPs) with different particle sizes have been prepared which exhibit a single phase tungsten bronze (TB) structure. It is found that synthesis temperature (Ts) acts an important factor to determine the particle size and nanorod-like particles are obtained for all samples as Ts≤1100°C. Bright up-conversion (UC) green (2H11/2/4S3/2→4I15/2) and weak red (4F9/2→4I15/2) emissions are realized. Based on the fluorescence intensity ratio (FIR) of green emissions, temperature-dependent UC luminescence and thermometric properties of KBN-x UCNPs were investigated. We show that Yb3+-concentration and particle size both have important influence on the sensitivity of KBN-x system.

Microstructure evolution and recrystallization after annealing of tungsten heavy alloy subjected to severe plastic deformation

The microstructure evolution, recovery and recrystallization of the work hardening W phase for 93W-4.9Ni-2.1Fe alloy subjected to severe plastic deformation during annealing treatment are investigated. Microstructure stability is studied by heating at temperature ranging from 900 °C to 1400 °C. Results indicate that tungsten phase of a hot-hydrostatically extruded 93W-4.9Ni-2.1Fe alloy can hold good stability below 900 °C and static recovery of W phase occurs firstly. The recovery process may be attributed to the rearrangement and annihilation of dislocations. Secondly, when the annealing temperature goes beyond 900 °C, the micro-hardness drops sharply which indicates the onset of recrystallization. The micro-hardness starts to stay at a stable level at 1300 °C which indicates full recrystallization has been obtained. In this research, the plastic deformation amount is large (area reduction 75%), dislocation slip start within the elongated tungsten grain, resulting in the formation of high density dislocation walls consisting of dislocation lines and dislocation tangle to be proved by TEM. Then the high density dislocation walls are transformed into fine subgrain boundaries due to continuous and coordination deformation, which further evolve into refined boundaries and fine subgrains. The driving pressure for recrystallization arises from the crystal defects stored in the deformed material. These include high angle grain boundaries, dislocations and dislocation boundaries such as cell or subgrain walls. Thus, the static recovery process and recrystallization of tungsten phase during annealing occur successively.

The formation of tungsten doped Al2O3/ZnO coatings on aluminum by plasma electrolytic oxidation and their application in photocatalysis

Tungsten doped Al2O3/ZnO coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte (0.1M boric acid+0.05M borax+2g/L ZnO) with addition of different concentrations of Na2WO4·2H2O. The morphology, crystal structure, chemical composition, and light absorption characteristics of formed surface coatings are investigated. The X-ray diffraction and X-ray photoelectron spectroscopy results indicate that formed surface coatings consist of alpha and gamma phase of Al2O3, ZnO, metallic tungsten and WO3. Obtained results showed that incorporated tungsten does not have any influence on the absorption spectra of Al2O3/ZnO coatings, which showed invariable band edge at about 385nm. The photocatalytic activity of undoped and tungsten doped Al2O3/ZnO coatings is estimated by the photodegradation of methyl orange. The photocatalytic activity of tungsten doped Al2O3/ZnO coatings is higher thanof undoped Al2O3/ZnO coatings; the best photocatalytic activity is ascribed to coatings formed in supporting electrolyte with addition of 0.3g/L Na2WO4·2H2O. Tungsten in Al2O3/ZnO coatings acts as a charge trap, thus reducing the recombination rate of photogenerated electron-hole pairs. The results of PL measurements are in agreement with photocatalytic activity. Declining PL intensity corresponds to increasing photocatalytic activity of the coatings, indicating slower recombination of electron-hole pairs.

Temperature dependence of micro-deformation behavior of the porous tungsten/Zr-based metallic glass composite

The effect of temperature on the micro-deformation behavior of the porous tungsten/Zr-based metallic glass composite was investigated during cyclic compression by synchrotron based in-situ high-energy X-ray diffraction (HEXRD) and finite element modeling (FEM). Both the metallic glass phase and the tungsten phase remained elastic in the first loading at different temperatures. The metallic glass phase also exhibits “work hardening” behavior, which is attributed to the crystallization of the metallic glass phase during deformation at high temperature. The yield strength of the tungsten phase during the second loading decreased with the increase of temperature while remained almost equal in the third loading. Both the temperature and the pre-deformation from the cyclic loading have great influence on the work hardening behavior of the tungsten phase. The influence of the pre-deformation after the second cyclic loading is greater than the influence of the temperature during the third loading.

Growth and characterization of α and β-phase tungsten films on various substrates

The growth conditions of tungsten thin films were investigated using various substrates including Si, Si/SiO2, GaAs, MgO, and Al2O3, and recipes were discovered for the optimal growth conditions of thick metastable β-phase tungsten films on Si, GaAs, and Al2O3 substrates, which is an important material in spin orbit torque studies. For the Si/SiO2 substrate, the crystal phase of the tungsten films was different depending upon the tungsten film thickness, and the transport properties were found to dramatically change with the thickness owing to a change in phase from the α + β phase to the α-phase. It is shown that the crystal phase changes are associated with residual stress in the tungsten films and that the resistivity is closely related to the grain sizes.

Structural, thermal and magnetic characterization of nanocrystalline Co65Ti25W5B5 powders prepared by mechanical alloying

In this work, nanocrystalline Co65Ti25W5B5 (at.%) powders were prepared by mechanical alloying (MA) of the elemental powder mixture under argon gas atmosphere. The powders were milled during different periods (2.5, 5, 10, 20 and 30h) using a planetary ball-mill (Retsch PM100 CM) at 400rpm. The structural, morphological, thermal and magnetic properties of the nanocrystalline Co65Ti25W5B5 powders were studied by means of X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), differential thermal analysis (DTA) and vibrating sample magnetometer (VSM) techniques. Because of its high melting point, hardness and low solubility in the alloy components, a small amount of tungsten remained in the amorphous matrix during MA. By using the Williamson-Hall method, the crystallite size and lattice strain of the tungsten phase were calculated as about 25nm and 0.48% respectively, for 30h of milling. The DTA curves of the milled powders demonstrated an exothermic peak at about 600 °C, indicating the crystallization of the amorphous phase. The apparent mean activation energy, 303.5±7kJ/mol for 20h milled powders was determined by Kissenger and Ozawa methods. The saturation magnetization (Ms), the coercivity (Hc) and the remanence-to-saturation ratio (Mr/Ms), values were of about 66emu/g, 11Oe and 0.012 respectively, after 30h of milling.

W–steel and W–WC–steel composites and FGMs produced by hot pressing

Tungsten–steel composites and functionally graded materials (FGMs) are being developed for potential application in plasma facing components of fusion devices. Their role is to reduce thermal stresses at the interface between plasma facing tungsten armor and structural material. In this study, uniform composites and graded layers produced from tungsten and steel powders by hot pressing were investigated. Fully dense composites with uniform distribution and good bonding of the phases were formed. A thin layer of Fe2W intermetallic phase was found at the interfaces. To avoid its formation, a third phase (tungsten carbide) was introduced between tungsten and steel. Structure, thermal and mechanical properties of both types of composites in the as-produced and annealed state were characterized.

Micro-mechanical behavior of porous tungsten/Zr-based metallic glass composite under cyclic compression

The micro-mechanical behavior of porous tungsten/Zr-based metallic glass composites with different tungsten volume fraction was investigated under cyclic compression by synchrotron-based in-situ high-energy X-ray diffraction (HEXRD) and finite element modeling (FEM). During cyclic compression, the dislocation in the tungsten phase tangled near the interfaces, indicating that the elastic metallic glass phase restricted dislocation motion and obstructed the deformation of the tungsten phase because of the heterogeneity in stress. After the metallic glass phase yielded, the dislocation tended to propagate away from the interfaces, showing the decrease of the interphase stress affected the direction of motion in the dislocations. The tungsten phase exhibited increased yield strength with the increase of cyclic loading number. Yield stress of the tungsten phase decreased with increasing the tungsten volume fraction during cyclic compression, which was influenced by the elastic strain mismatch between the two phases. The stress heterogeneity and the stress distribution difference between the two phases resulted in that the yield strength of the metallic glass phase decreased with the increase of tungsten volume fraction, and accelerated the formation of shear bands in the metallic glass phase as well as cracks in the tungsten phase. The heterogeneity in stress also excessed the interface bonding strength, inducing interface fracture near interfaces.

Effect of Cu content and heat treatment on the properties and microstructure of W–Cu composites produced by hot extrusion with steel cup

In this paper, the effect of Cu content on the properties and microstructure of W–Cu composites produced by hot extrusion with steel cup and heat treatment was investigated. W–Cu products were sintered at 1150°C under vacuum conditions for 1h and then the sintered billets were hot extruded with steel cup at 950°C with extrusion ratio 10.56. After heat treatment, W–Cu composites with highly comprehensive properties will be acquired. Metallographic microscopy and scanning electron microscope were used to observe microstructure. Experimental results showed that when the Cu content (10–20mass%), a little deformation was observed in W phase. When the percentage composition of Cu is high (30–40mass%), little deformation appeared in W phase, while main deformation occurred in Cu phase. Tungsten phase contiguity in extruded composites is increasing with the decreasing of Cu content. It is hard to achieve the full densification for W–Cu composite with low Cu content (below 20wt.%Cu) by plasticity deformation technology with small deformation amount.

Cover Picture: Materials and Corrosion. 4/2015

Cover:The composition of the high strength austenitic steel Sanicro 25 is characterized by a high chromium content and additions of tungsten, niobium, copper as well as significant levels of nitrogen. The steel microstructures in the bulk of the sample and the sub surface zones after exposure times of 1000, 3000 and 10000 hours in steam at temperatures in the range 600–750 °C were analysed by scanning and transmission electron microscopy as well as X‐ray diffraction. Additionally, the surface oxide scale formation as well as the oxidation induced sub surface microstructural changes were studied using depth profiling by secondary neutrals mass spectrometry. In the as‐received condition the only detectable precipitate was (Nb,Cr)N‐base Z‐phase. The oxidation rates were found to be only slightly higher than those of a typical nickel base wrought material such as alloy 617. In the subsurface zones depletion of scale forming elements was accompanied by enrichment of tungsten and copper rich phases. In spite of the fact that precipitate sizes were, especially after exposure at lower temperatures, quite small unequivocal identification of the phases present in the alloy microstructure was possible by combining EDX and WDX data with results from EBSD analyses. The cover picture illustrates how information about elements present in the various precipitates as analysed by EDX and WDX can be compared with compounds prevailing in the structure database ICSD. Subsequently, an EBSD mapping of the respective best fitting compounds was made. The EBSD mapping reveals precipitates of (Cr,Nb)N‐base Z‐phase, Cr2N, tungsten rich µ‐phase and M23C6 carbides of various composition.More information can be found in: J. Zurek, S.‐M. Yang, D.‐Y. Lin, T. Hüttel, L. Singheiser, W. J. Quadakkers, Microstructural stability and oxidation behaviour of Sanicro 25 during long term steam exposure in the temperature range 600–750 °C, Materials and Corrosion, 2015, 66, 315.

Carbothermic reactions of mechanically milled coal and wolframite mixtures under Ar and N2 atmospheres

The thermal reactions of mechanically activated coal and natural wolframite mixtures have been studied using DTA–TG. The reaction mixture, prepared on the basis of a thermodynamic simulation, was milled in a planetary ball mill for either 2 or 4 h. Unmilled and milled mixtures were subjected to differential thermal analysis under nitrogen and argon atmospheres with the crystalline phase composition determined with XRD. The main differences between nitrogen and argon atmospheres were observed in the raw samples. Milled samples produced primarily an iron tungsten phase, Fe7W6. Under the nitrogen atmosphere, both samples showed a greater extent of reduction than under argon.

Giant Spin Hall Effect and Switching Induced by Spin-Transfer Torque in aW/Co40Fe40B20/MgOStructure with Perpendicular Magnetic Anisotropy

Tomorrow's spintronic MRAM and logic processors could exploit the physics of the spin Hall effect for switching bits, but the materials engineering is challenging. The authors have realized robust perpendicular magnetic anisotropy (PMA) in a layered structure combining the elusive, metastable $\ensuremath{\beta}$ phase of tungsten and a ferromagnetic thin film. The large spin-orbit coupling in $\ensuremath{\beta}$-W yields, after suitable annealing, a very low critical current density for magnetization switching. The authors' structures furthermore are easily fabricated, making them even more technologically promising.

Formation and Stability of Metastable Tungsten Carbide Nanoparticles

The low temperature transformation pathways of tungsten carbide formation in the nanoscale regime were investigated using a reactive carbon molecule and tungsten. WC core/W shell nanoparticles produced by the decomposition of metastable W2C were discovered using TEM. XRD studies revealed both the elemental and carbide phases of tungsten. It was observed that the metastable W2C phase can be stabilized at RT by carbon encapsulation. These findings open new avenues to access core-shell morphologies of refractory carbides and to stabilize W2C nanoparticles at RT.

Alpha- vs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials' properties

Tungsten thin films have exceptional thermal and mechanical properties, compared to other pure metal coatings, making them indispensable in many contemporary high-tech applications. Their superior properties, however, are strongly dependent on film microstructure, specifically including comprising phases, and hence, the employed deposition parameters, making a thorough process control essential. This study therefore investigates the different material properties of the bcc α and the A15 β phase of tungsten with respect to process parameters. The formation of β-W is mainly dependent on process gas pressure. While all films deposited at elevated pressure consist of β-W, the onset of β formation can already be observed for low pressure deposition, if magnetron power is reduced sufficiently. However, β-W formation for all deposition pressures can be suppressed by applying a substrate bias during deposition, leading to an α-W configuration. This indicates that the alpha-to-beta transition during film growth predominantly depends on adatom energy, rather than solely on the inclusion of residual impurities. Simultaneously, mechanical and electrical performance could be improved, while maintaining a nanocrystalline microstructure.

Effect of heat treatment on microstructure and mechanical properties of hot-hydrostatically extruded 93W–4.9Ni–2.1Fe alloy

Microstructure, mechanical properties and strengthening mechanisms of tungsten heavy alloys consolidated by liquid-phase sintered, hot-hydrostatic extrusion and heat treatment were investigated. The results showed that ultimate tensile strength σb of 93W–4.9Ni–2.1Fe alloy with area reduction 75% extruded at 1200°C could reach as high as about 1540MPa and elongation could also reach 9.2%. Meanwhile, ultimate tensile strength and elongation by heat treatment remain 1487MPa and 13.4%, respectively. Transmission electron microscopy observation revealed that as-extruded alloys were characterized by fine subgrains in elongated tungsten grains and dynamically re-crystallized grains in Ni–Fe–W matrix phase. Tungsten subgrains subsequently grew up a bit and nanocrystalline W with most of them being less than 100nm in length showing a needle precipitated in Ni–Fe–W matrix phase after heat treatment at 1000°C for 1h. Results indicate that one of the strengthening mechanisms is interface strengthening between tungsten and matrix phase due to severe plastic deformation, which can be explained via aspect ratio correlated with deformed tungsten particles. The other strengthening mechanism can be attributed to precipitated nanocrystalline W during heat treatment in Ni–Fe–W matrix phase of the as-extruded alloy with area reduction 75%.

Structure of niobium-tungsten alloy films produced by metal sputtering

The solid-solution alloy coatings are obtained via ion-plasma sputtering and the codeposition of ultradispersed Nb and W particles in the entire concentration range of a double system. Samples with 15.8–90.6 at % W are prepared. The sputtered metal particles have sizes ranging between 0.1 and 0.9 nm. The solid-solution alloy structures are based on niobium and tungsten if the tungsten concentration of the coating is, respectively, less and more than 40 at %. At certain ratios between the sizes of ultradispersed W particles and a certain sputterer power, a photoinduced increase in the mobility of particles is observed. As a result, a pure tungsten phase segregates from the tungsten solid solution in niobium and its particles coagulate in the nanocluster superlattice. It is established that superstructure formation depends mainly on the size factor. On account of growth in the thickness of the niobium and tungsten sublayers, metallic phases and solid solutions appear in the coatings and the nanocluster superlattices of one metal are formed in the matrix of another.

Facile Fabrication of WO3 Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting

Single crystalline orthorhombic phase tungsten trioxide monohydrate (O-WO3·H2O, space group: Pmnb) nanoplates with a clear morphology and uniform size distribution have been synthesized by the hydrothermal method and fabricated on the surface of fluorine doped tin oxide (FTO) coated glass substrates with selective exposure of the crystal facet by the finger rubbing method. The rubbing method can easily arrange the O-WO3·H2O nanoplates along the (020) facet on the FTO substrate. The O-WO3·H2O nanoplate can be converted to monoclinic phase WO3 (γ-WO3, space group: P21/n) with dominant crystal facet of (002) without destroying the plate structure. Crystal morphologies, structures, and components of the powders and films have been determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, etc. The band gap energies of the O-WO3·H2O and γ-WO3 nanoplates were determined as ca. 2.26 and 2.49 eV, respectively. Photoelectrochemical prop...

Strategy for silicon based hot-wire chemical vapor deposition without wire silicide formation

Silicide formation of wires during hot-wire chemical vapor deposition (HWCVD) of silicon based coatings is a key challenge which has to be overcome before HWCVD can be transferred successfully into industry. Silicide formation of tungsten wires is not occurring at temperatures of approximately 1900°C and above when maintaining a silane partial pressure below approximately 1Pa. Proceeding silicide formation at the cold ends where the wires are electrically contacted was completely prevented by continuously moving the cold ends of the wires into the hot deposition zone, resulting in a retransformation of the tungsten phase. Thus the maintenance period of a HWCVD manufacturing tool can be freed from wire lifetime.

Influence of Nanopowders on Corrosion Resistance of Welded Joints

The paper presents experimental results on influence of tungsten, molybdenum nanopowders and aluminum oxyhydroxide phase introduced into the molten pool on the corrosion resistance of welded joints. It was shown that the nature of nanopowders significantly impacts intercrystalline corrosion of the welding joint.