Single-phase WC Research Articles

Preparation ultrafine WC based on the new tungsten metallurgy system of “no ammonia transformation - no hydrogen reduction”

The preparation of ultra-fine tungsten carbide (WC) is a difficult and hot topic in tungsten industry. The industrial production process requires the need to transform tungsten concentrates to produce ammonium paratungstate (APT) intermediate products followed by hydrogen reduction to produce W, and finally carburization to produce WC. Due to the formation of H2O in the hydrogen reduction process, it is difficult to obtain <100 nm tungsten powder materials. Therefore, in this study, a novel method was introduced that synthesizes ultrafine WC via hydrothermal synthesis of ZnWO4 followed by a carbothermic reduction–carburization process. The findings of this study showed that ZnWO4 with a particle size of 29 nm was synthesized from Na2WO4 solution. This can solve the problem of ammonia‑nitrogen wastewater discharge in the process of APT preparation, and prepare nano-scale raw material. In addition, zinc is volatile at high temperature, which has been widely used in the field of recycling waste cemented carbide. Based on this, the WC mixed powder was obtained by carbothermic reduction of ZnWO4 under the condition of excess C addition, which can inhibit the agglomeration of particles and avoid the production of H2O. Then the single-phase WC with a particle size of 60 nm was obtained by a carburization reaction process at 1300 °C for 3 h. The research intersects tungsten metallurgy with tungsten material preparation, shortens the production process and reduces waste water discharge.

Synthesis of tungsten carbide nanopowders by direct carbonization of tungsten oxide and carbon: Effects of tungsten oxide source on phase structure and morphology evolution

In the paper, WC nanopowders are successfully prepared by carbothermal reduction method, and the effect of tungsten oxide source on the phase structure evolution and products properties of the as-synthesized WC nanopowders has been investigated. Four tungsten oxide powders are chosen as tungsten oxide sources, e.g., rods-like WO3 , WO3 nanopartiles, WO3 micro-particles and WO2.9 micro-particles. Compared with other three tungsten oxide sources, the WO3 micro-particles possesses small particle size, less agglomerates and good dispersity and the uniform tungsten oxide-carbon mixture after ball milling can be easily obtained. The appropriate tungsten oxide source can result in lower processing temperature (≤1200 °C) and shorter holding time (≤3 h). Single-phase WC powders with average particle size of 100 nm and uniform particle distribution can be achieved by micro-particle-like WO3 at 1100 °C for 3 h. The as-prepared WC products by other three tungsten oxide sources exhibit problems of more aggregates, non-uniform particle size and large particle size (250 nm), respectively. In addition, the method can provide a facile, low-cost, efficient, and industrially feasible pathway for large scale preparation of WC nanopowders.

Tungsten Carbide Encapsulated in Grape-Like N-Doped Carbon Nanospheres: One-Step Facile Synthesis for Low-Cost and Highly Active Electrocatalysts in Proton Exchange Membrane Water Electrolyzers

Tungsten carbide (WC) is an alternative to the costly and resource-constrained Pt-based catalysts. Herein, a one-step facile and easily scalable approach is reported to synthesize ultrafine WC nanocrystals encapsulated in porous N-doped carbon nanospheres (NC) by simple self-polymerization, drying, and annealing. It is worth mentioning that this developed method has four novel features: (1) the synthesis process, without any hard template or hydrocarbon gas feeding, is, notably, very facile and efficient with low cost; (2) the carbon coating on WC nanocrystals not only restrains coarsening of particles but also creates strong coupling interactions between the nanocrystallines and the conductive carbonaceous matrix; (3) uniform grape-like WC@NC nanospheres with high specific surface area can be obtained in a large scale; and (4) single-phase WC can be achieved. As a result, WC@NC demonstrates remarkable hydrogen evolution reaction (HER) electrocatalytic performance with overpotentials of 127 and 141 mV at a current density of 10 mA cm-2 and Tafel slopes of 56.3 and 78.7 mV dec-1 in acid and alkaline media, respectively. Our density functional theory calculations manifest that the strong synergistic electronic effect between WC and its intimately bonded carbon shell vastly boosts the HER electrocatalytic activity. WC@NC catalysts as a cathode are further tested in a home-made electrolyzer with 0.78 A cm-2 achieved at a cell voltage of 2 V at 80 °C and operated stably at 200 mA cm-2 for more than 20 h.

Size-controlled synthesis of high-purity tungsten carbide powders via a carbothermic reduction–carburization process

In this paper, a novel method is proposed to synthesize high-purity tungsten carbide (WC) powders with different sizes via carbothermic reduction of yellow tungsten trioxide (WO3) followed by the further carbonization process. The effects of the reaction temperature, reaction time and C/WO3 molar ratio on the phase transition and morphology evolution of the products are investigated in detail. The results reveal that the morphology of the final products is mainly determined at the carbothermic reduction stage, and the particle size of WC is significantly affected by the C/WO3 molar ratio and reaction temperature. It can be concluded from experimental results that particle size of WC increased with the increased of temperature, but decreased with increased of C/WO3 molar ratio. When the C/WO3 molar ratio is 2.7–3.5, the single phase WC with a size of 178–825 nm can be obtained after further carbonization at 1200 °C.

Fabrication of ultrafine and high-purity tungsten carbide powders via a carbothermic reduction–carburization process

In the current study, ultrafine and high-purity tungsten carbide (WC) powders are successfully prepared by a two-step process: carbothermic reduction of WO3 followed by carbonization reaction. The effects of the C/WO3 molar ratio, reaction temperature and reaction time on the phase transition and morphology evolution of the products are investigated. During the carbothermic reduction process, all the oxygen in yellow tungsten trioxide (WO3) is removed by carbon to generate a mixture of W, W2C and WC at 1100 °C; and then the as-prepared powder is mixed with an appropriate content of carbon black and carbonized at 1200 °C. The carbon content in the finally obtained WC powders is almost equal to the theoretical value. Furthermore, it is found that a high C/WO3 molar ratio at the first stage is beneficial for decreasing the particle size of the WC powders. When the C/WO3 molar ratio is 3.5, the single phase WC with a particle size of about 200 nm can be obtained. Therefore, this carbothermic reduction–carburization process may provide a simple, low-cost, and high efficiency route to prepare the WC powders in a large-scale.

Pressureless sintering of binderless tungsten carbide

The aim of this study was to produce dense, single phase polycrystals. The research was carried out on the submicron tungsten carbide powder without additives, with either a carbon or tungsten additive and on the powder with both additives. The primary task of carbon was to reduce surface oxide impurities which passivate WC grains; tungsten in turn bounds free carbon in the WC. The authors manufactured fine-grained, dense (96–98% T.D.) and single-phase WC polycrystals, using the technique of pressureless sintering at the temperature not exceeding 2000°C. A positive effect of carbon addition on tungsten carbide sinterability was observed, whereby a dense, fine-grained polycrystals can be obtained at 1900°C. It was also observed that a significant excess of temperature of sintering process resulted in a strong abnormal grain growth of WC grains.

Effect of Carbon Content on Morphology, Size and Phase of Submicron Tungsten Carbide Powders by Salt-assisted Combustion Synthesis

Abstract The diluent NaCl was introduced into the WO 3 -Mg-C-Na 2 CO 3 system to prepare submicron tungsten carbide (WC) powders via salt-assisted combustion synthesis. The products were analyzed by SEM, EDS and XRD, and the effects of C content on morphology, average particle size and phase composition of the products were studied. Results show that on basis of the m = 0.125 (the number of moles of Na 2 CO 3 ), when the number of moles of carbon in raw material increases from l =2 to 2.25 and 2.5, before leaching, the product is made up of a small number of large size particles and a large number of small size particles; after leaching, samples are composed of aggregates of submicron particles, and the sintering phenomenon between particles is very weak, indicating low degree of aggregation. Particle size distribution of leached product almost falls into the normal distribution, and the particle sizes range from 200 nm to 350 nm. Under the condition of l =2.25, a main target product is WC, and the content of by-products W 2 C is extremely few. That is, k = 2.0 (the number of moles of NaCl), m = 0.125, and l =2.25 are the process conditions for single-phase WC synthesis.

XPS study of surface chemistry of tungsten carbides nanopowders produced through DC thermal plasma/hydrogen annealing process

Abstract X-ray photoelectron spectroscopy (XPS) has been employed to characterize the surface composition and bonding of the tungsten carbides nanopowders produced through a DC thermal plasma/hydrogen annealing process. The XPS results were complemented with those from Raman spectroscopy, high-resolution transmission electron microscopy, and evolved gas analysis. The products of the DC plasma synthesis are the high-surface-area multicarbide mixtures composed mainly of crystalline WC1−x and W2C. The materials are contaminated with a pyrolitic carbonaceous deposit which forms ∼1 nm thick graphitic overlayers on the nanoparticles’ surface. The underlying carbides are not oxidized in ambient air, and show no interfacial compounds underneath the graphitic overlayers. When annealed in hydrogen, the multicarbide mixtures undergo transformation into the single-phase WC nanopowders with an average particle size of 50–60 nm. The surface of the passivated and air-exposed WC nanopowders is stabilized by an ultrathin, no more than 0.5 nm in thickness, chemically heterogeneous overlayer, involving graphitic, carbon-to-oxygen, and WO3 bonding. Oxygen presents at coverages above a monolayer preferentially in the bonding configurations with carbon. The surface segregations of carbon are normally observed, even when the bulk content of carbon is below the stoichiometric level.

The synthesis of single phase WC nanoparticles/C composite by solid state reaction involving nitrogen-rich carbonized polyaniline

Abstract Single phase tungsten carbide nanoparticles (WC-NPs), (mean particle diameter 5.4 nm), distributed over carbonized polyaniline (C-PANI) nanotubes/nanosheets were synthesized by a solid state reaction between WO 3 and nitrogen-rich carbonized polyaniline at 1000 °C in a reducing atmosphere. The resulting composite was characterized by X-ray diffractometry, electron microscopy, thermogravimetry in oxidizing and reduction atmospheres and elemental analysis. We suggested that the synthesis of WC as a single phase was facilitated by reactive C atoms with dangling bonds, formed upon nitrogen removal.

Tungsten carbide (WC) nanopowders synthesized via novel core–shell structured precursors

WC powders with a size in the range of 20–60nm were prepared through carbothermal reduction of a novel core–shell structured precursor in vacuum. The samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The thermolysis process of the precursor has been investigated by themogravimetric analysis and differential thermal analysis (TG–DSC). The results revealed that the single phase WC nanopowders were synthesized at 980°C for 1h. Spectra of XPS indicate that the surface of the specimen mainly consists of W, C and O three species elements only. The effects of experimental parameters and reaction mechanism have been explored. Mainly due to the homogeneous chemical composition of the precursor, the synthesizing temperature was greatly lower than the conventional method.

Electrochemical Stability of Tungsten and Tungsten Monocarbide (WC) Over Wide pH and Potential Ranges

Tungsten monocarbide (WC) is a low cost electrode material that has shown much promise in electrocatalytic applications, yet, a comprehensive study of its electrochemical stability has been absent from literature. In this work, the electrochemical stability of single phase WC and its parent metal, tungsten (W), are determined over a wide range of pH and potential values using a variety of electroanalytical techniques. Central to this study are chronopotentiometric titration measurements, which are a quick and effective way to determine the potential–pH regions of stability for these materials. Cyclic voltammetry, chronoamperometry, and linear sweep voltammetry (LSV) measurements are also presented. X-ray photoelectron spectroscopy and scanning electron microscopy verified the results of these electroanalytical measurements. Our findings show that WC exhibits trends in electrochemical stability that are similar to those of W, which have been the focus of many electrochemical studies. Notable differences in the potential limits of stability and extent of surface oxide formation on WC and W exist and may have important implications for electrochemical applications. The results of this paper should prove useful for the selection of electrolytes, electrode conditioning parameters, and device operating conditions for a wide variety of electrochemical applications using WC.

Synthesis of nanosized tungsten carbide from phenol formaldehyde resin coated precursors

Nanosized tungsten carbide was synthesized from phenol formaldehyde resin (PF) coated tungsten precursors. The process has three steps in which nanosized tungsten particles were first coated with PF, then the precursors were carburized at 950°C, and finally the carburized powders were treated in flowing wet hydrogen atmosphere at 940°C to remove the uncombined carbon. The obtained powders were characterized using X-ray diffraction analysis (XRD), field-emission scanning electron microscopy (FESEM), small angle X-ray scattering (SAXS), and combustion-gas-volume method. The results indicated that single-phase WC could be synthesized using excessive PF as carburizer at a much lower temperature compared with using mixed carbon black. After wet hydrogen treating, the mean size of the obtained WC particles was 94.5 nm and the total carbon content was 6.18 wt.%.

Preparation of single phase tungsten carbide by annealing of sputtered tungsten-carbon layers

Tungsten carbide layers were prepared by sputtering from a stoichiometric WC target and subsequent annealing. Carbide formation was found at temperatures above 800°C. Annealing in pure hydrogen ambient results in a carbon depletion in the layers and the formation of a dominant W 2C phase. We demonstrate that propane added to the annealing ambient stimulates a transformation of the tungsten-carbon layers to a stoichiometric WC phase. The variation of the propane concentration allows a continuously alteration of the layer structure between single phase WC and a mixed layer with dominant W 2C and the adjustment of different values of the electrical resistance and the optical constants.

Preparation of Conductive Tungsten Carbide Layers for SiC High Temperature Applications

ABSTRACTThin tungsten carbide films of different compositions were prepared by DC magnetron sputtering of tungsten and carbon and subsequent annealing in different environments. The onset of carbide formation was around 800°C. Annealing in a pure hydrogen ambient generally results in carbon depletion in the layers with the formation of a dominant W2C phase. Adding propane enhances the carbon content in the layers and stimulates the formation of the WC phase. On silicon nitride substrates, variation of the propane concentration in an annealing environment allows a continuous alteration of the layer structure between polycrystalline single phase WC and a mixed layer with dominant W2C and with it, the adjustment of different values of the electrical resistance. In contrast, on thin (100)SiC layers a textured W2C phase was grown after annealing in propane/hydrogen at 900°C whereas at higher temperatures the formation of silicides was observed. In addition, the chemical composition and the temperature dependence of the electrical specific resistance were investigated and are also discussed.

Chemical vapour deposition of tungsten carbides on tantalum and nickel substrates

Tungsten carbide films have been deposited by low-pressure chemical vapour deposition from a WF 6/C 3H 8/H 2 mixture on Ta and Ni substrates. Single-phase WC films could be deposited on Ta in a broad vapour composition range at 900 °C. A mixture of WC and W 2C was deposited in the temperature range 700–850 °C, while an amorphous film was obtained at 650 °C. The temperature behaviour suggests that deposition of carbon is a limiting factor in the growth process. The deposition process on Ta could be separated into two parts: a fast substrate reduction step of WF 6 leading to the formation of metallic W followed by a slower formation and deposition of WC. The growth behaviour on Ta was also affected by tantalum carbides at the film-substrate interface. A different growth behaviour was observed on Ni. It was found that several η-carbides (i.e. Ni 2W 4C and Ni 6W 6C) were formed during a fast initial growth stage. Later on, the η-carbides reacted with carbon under the formation of WC and free Ni particles. It was also found that the carbon deposition rate on Ni substrates was higher than on Ta. This was explained by a catalytic process where Ni particles on the film surface favoured carbon deposition.

Low Pressure CVD of Tungsten Carbides

Tungsten carbide films have been deposited by CVD from a WF 6 /C 3 H 8 /H 2 gas mixture on several different substrate materials (Ta, Si, SiC and C). Single-phase WC films could easily be obtained on Ta substrates at 900 °C using a total pressure of 100 mTorr and a high linear gas flow velocity (7 m/s). It was found that the low pressures favoured the growth of carbon-rich films and made the deposition zone for WC longer. The phase composition of the films and deposition rates were also strongly affected by the substrate material. The substrate dependence was attributed to the chemical reactivity of WF 6 .