Nanosized Tungsten Carbide Research Articles

Microstructural evolution of nanosized tungsten carbide during heatup stage of sintering of electroless nickel-coated nanostructured WC–Co powder

In this research, microstructural evolution of nanosized tungsten carbide during heatup stage of sintering of a novel electroless nickel-coated nanostructured WC–Co powder was investigated. Toward this purpose, a mechanical milling process was executed on commercial microcrystalline WC–Co (mc-WC) to achieve nanostructured WC–Co (nc-WC) powder. Electroless nickel plating was performed on the as-milled powder to obtain nickel-coated nanostructured WC–Co (Ni/nc–WC). The nc-WC and Ni/nc–WC powders were subjected to cold-pressing in a uniaxial die followed by heatup stage of sintering from 25°C to a temperature range of 1000–1300°C under argon atmosphere. The microstructural characterizations were carried out by X-ray diffractometry (XRD), high resolution field emission scanning electron microscopy (HR FESEM) and high resolution transmission electron microscopy (HRTEM). The ball milling process resulted in the formation of nc-WC powder containing nanosized WC with average grain size of ~15nm. A uniform nickel layer with a thickness of <100nm was formed around Ni/nc–WC particles through nickel plating. A two-step grain growth trend was observed during heatup of nc-WC: a slow grain growth step at temperatures ≤1000°C, which led to WC grain size of ~76nm, and a rapid step by heating to the temperature range of 1100–1300°C which caused a substantial increase in WC grain size to ~925nm. In contrast, WC grain size in Ni/nc–WC sample varied in the range of ~15–250nm with temperature rising from 25 to 1300°C representing about 72% reduction in WC grain size for Ni/nc–WC compared to nc-WC. In case of nc-WC, surface faceting of nanosized WC occurred upon heating to ≤1000°C; meanwhile, on heating to 1100–1300°C, the coalescence mechanism was operative accounting for the rapid grain growth. As for Ni/nc–WC, surface faceting of WC grains was greatly suppressed. This led to the retardation of the coalescence mechanism, making it possible to form ultrafine-grained Ni/nc–WC material.

Electrochemical preparation of tungsten, tungsten carbide and cemented tungsten carbide

Tungsten is conventionally made by the reduction of tungsten oxide with hydrogen, tungsten carbide by the reaction of tungsten with carbon and cemented tungsten carbide by the sintering together of tungsten carbide with a metal, usually cobalt. This paper reports the electrochemical preparation of tungsten, tungsten carbide and cemented tungsten carbide starting with calcium tungstate, carbon and cobalt oxide, depending upon the desired end product. The starting materials were mixed and pelletised, and the pellets then made the cathode in a calcium chloride based melt. On the application of a modest potential (∼3 V), the calcium tungstate was reduced to tungsten, the mixture of calcium tungstate and carbon to nanosized tungsten carbide and the mixture of calcium tungstate, cobalt oxide and carbon to cemented tungsten carbide, all in ∼1 h. The products were characterised and the reaction path determined.

Mechanically Activated Synthesis of Tungsten Carbide Nanoparticles from Tungsten Oxide

Solid state carbothermic reduction of tungsten oxide (WO3) to nanosized tungsten carbide (WC) particles was achieved by heating mechanically activated mixture of tungsten oxide and graphite at different temperatures under vacuum condition. KCl and Ni were added to the mixture for some samples. The morphology and chemical composition of products, as well as particles size and their distribution were compared by X-ray diffraction and field emission scanning electron microscopy. Mechanical activation of WO3-C powder mixture did not yield WC phase whereas it was possible to produce WC nanoparticles by heating at 1250 °C for 2 h. KCl additive caused fine and homogeneous particle and Ni additive assisted the growth of WC particles.

Nanosized tungsten carbide synthesized by a novel route at low temperature for high performance electrocatalysis

Tungsten carbide (WC) is a widely used engineering material which is usually prepared at high temperature. A new mechanism for synthesizing nanoscaled WC at ultralow temperature has been discovered. This discovery opens a novel route to synthesize valuable WC and other carbides at a cost-efficient way. The novel formation mechanism is based on an ion-exchange resin as carbon source to locally anchor the W and Fe species. As an intermediate, FeWO4 can be formed at lower temperature, which can be directly converted into WC along with the carbonization of resin. The size of WC can be less than 2 nm. The catalyst made with Pt nanoparticles supported on nanosized WC-GC (WC-graphitized carbon) shows enhanced electrocatalytic activity for oxygen reduction reaction. The result also indicates that the Pt nanoparticles deposited on WC-GC are dominated by Pt (111) plane and shows a mass activity of 257.7 mA mg−1Pt@0.9 V.

Synthesis of Nano-WC by High-Energy Ball Milling

Nanosized tungsten carbide (nanoWC) has been widely studied and applied in many industries as hard materials since it has good combination of high hardness and strength. Thermal mechanical alloying method consisting of high-energy ball milling and subsequent carbonization is a common synthetic approach to prepare nanoWC. In this paper, a special milling process has been reviewed, in which the dielectric barrier discharge plasma (DBDP) is introduced, for the preparation of nanoWC. The DBDP milling renders the W+C powders with specific flake-like structure possible owing to the cooperative effect of DBDP and mechanical milling. The obtained W+C powders are activated significantly within shortened milling period. Furthermore, pure nanoWC powders can be synthesized after subsequent carbonization at a significantly lowered temperature. DBDP milling is demonstrated to be an efficient way to synthesize nanoWC.

Nano-tungsten carbide decorated graphene as co-catalysts for enhanced hydrogen evolution on molybdenum disulfide

A novel electrocatalyst of layered MoS2 supported on reduced graphene oxide (RGO) decorated with nano-sized tungsten carbide (WC) shows an enhanced catalytic performance in the hydrogen evolution reaction, which could be attributed to the presence of a conductive and electrocatalytically-active nano-WC dispersant and the positive synergistic effect between nano-WC/RGO and layered MoS2.

Strip-Like Nanosized Tungsten Carbide as Catalyst for Oxygen Reduction Reaction

not Available.

Fabrication of Nanosized Tungsten Carbide Ceramics by Reactive Spark Plasma Sintering

Binderless tungsten carbide ceramics were produced by reactive spark plasma sintering, using tungsten nitride and carbon black as starting materials. The phase and microstructure evolution during the sintering process was investigated. It was found that the tungsten nitride decomposed into nano tungsten with a particle size of about 25 nm firstly, and then reacted with carbon black to form carbides. Due to the high activity of the in situ formed carbides, samples could be densified at a temperature as low as 1350°C. The bulk carbides possessed an average grain size of 270 nm and Vickers’ hardness of 25.4 GPa.

Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite

Solid-state carbothermic reduction of tungsten oxide (WO 3) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO 3 and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO 2.72, WO 2 and metallic tungsten (W), were observed at 741 °C, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.

Plasma synthesis of nanosized W–Co composite powder followed by carburization with a methane–hydrogen mixture

Nanosized tungsten carbide and cobalt composite powder was synthesized by a two-step reaction via a thermal plasma process. Ammonium paratungstate (APT) and cobalt oxide were used as the precursors. The injected precursors were vaporized in the plasma flame and the subsequent reduction of the vaporized precursors produced uniformly mixed nanosized tungsten and cobalt composite powder. The subsequent carburization of this powder by CH4–H2 mixture resulted in the formation of nanosized WC–Co composite powder with a particle size <110 nm. This new method represents an alternative route that avoids the post-treatment required for the WC1−x produced when methane was used for direct carburization in the powder production step.

Fabrication of High Purity and Nano-sized Tungsten Carbide Particles using Chemical Complex Solution

High purity and nano-sized tungsten carbide (WC) particles have been successfully prepared using chemical complex solution which is composed of tungstic acid, citric acid and ammonia solution. The precursor, which was obtained by drying the chemical complex solution, was heated to 1423 K under Ar atmosphere. Total carbon (C) amount of the heat-treated precursor was controlled by molar ratio of citric acid and tungstic acid. In case of this experimental system, W-C bonds have been already formed at 723 K. The carbonization temperature was lower than that of the conventional methods that used C and WO3 or W, therefore particle growth has been suppressed. The sizes of the obtained WC particles were under 100 nm and the amounts of iron and cobalt as impurities were under 10 ppm.

Combustion synthesis of nano-sized tungsten carbide powder and effects of sodium halides

The synthesis of nano-size tungsten carbide powder has been investigated with a WO3 + Mg + C + carbonate system using alkali halides. The effects of different types of alkali halides on combustion temperature and tungsten carbide formation were discussed. Sodium fluoride had a notable effect on the particle size of the product and the degree of transformation from the initial mixture. A small amount of ammonium carbonate activated the carburization of tungsten carbide by the gas phase carbon transportation. X-ray diffraction data and particle analysis showed that the final product synthesized from a WO3–Mg–C–(NH4)2CO3–NaF system contains pure-phase tungsten carbide with a particle size of 50–100 nm.

Agglomeration of tungsten carbide nanoparticles in exposure medium does not prevent uptake and toxicity toward a rainbow trout gill cell line

Due to their increased production and use, engineered nanoparticles are expected to be released into the aquatic environment where particles may agglomerate. The aim of this study was to explore the role of agglomeration of nanoparticles in the uptake and expression of toxicity in the rainbow trout ( Oncorhynchus mykiss) gill cell line, RTgill-W1. This cell line was chosen as model because it is known to be amenable to culture in complete as well as greatly simplified exposure media. Nano-sized tungsten carbide (WC) with or without cobalt doping (WC-Co), two materials relevant in the heavy metal industry, were applied as model particles. These particles were suspended in culture media with decreasing complexity from L15 with 10% fetal bovine serum (FBS) to L15 to L15/ex, containing only salts, galactose and pyruvate of the complete medium L15. Whereas the serum supplement in L15 retained primary nanoparticle suspensions, agglomerates were formed quickly in L15 and L15/ex. Nevertheless, scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) elemental analysis revealed an uptake of both WC and WC-Co nanoparticles into RTgill-W1 cells irrespective of the state of agglomeration of nanoparticles. The localisation seemed to be restricted to the cytoplasm, as no particles were observed in the nucleus of cells. Moreover, reduction in cell viability between 10 and 50% compared to controls were observed upon particle exposure in all media although the pattern of impact varied depending on the medium and exposure time. Short-term exposure of cells led to significant cytotoxicity at the highest nominal particle concentrations, irrespective of the particle type or exposure medium. In contrast, long-term exposures led to preferential toxicity in the simplest medium, L15/ex, and an enhanced toxicity by the cobalt-containing WC nanoparticles in all exposure media. The composition of the exposure media also influenced the toxicity of the cobalt ions, which may dissolve from the WC-Co nanoparticles, with cells reacting much more sensitively toward cobalt ions in the absence of FBS. However, the toxicity observed by ionic cobalt alone did not explain the toxicity of the WC-Co nanoparticles, suggesting that the combination of metallic Co and WC is the cause of the increased particle toxicity of WC-Co. Taken together, our findings indicate that minimal exposure media can lead to rapid agglomeration of nanoparticles but that agglomeration does not prevent uptake into cells and the expression of toxicity.

Plasma synthesis of tungsten carbide and cobalt nanocomposite powder

Nanosized tungsten carbide–cobalt composite powder was synthesized by a thermal plasma process, in which ammonium paratungstate (APT) and cobalt oxide were reacted with a gas mixture containing CH 4, H 2, and Ar. The reduction and carburization of vaporized precursors produced nanosized tungsten carbide (WC 1− x )–cobalt composite powder, which contained small amounts of W 2C and/or W phase. The particle size of synthesized tungsten carbide powder was less than 20 nm. The synthesized composite powders were also subjected to a hydrogen heat treatment to fully carburize WC 1− x , W 2C, and W phases to the WC phase as well as to remove excess carbon in the product. Finally nanosized WC–Co composite powder of particle size less than 100 nm was obtained.

Plasma Synthesis of Tungsten Carbide Nanopowder from Ammonium Paratungstate

A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC1−x) powder, which sometimes contained a small amount of W2C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H2) on the product composition and particle size were investigated. The produced tungsten carbide (WC1−x) powder was &lt;20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC1−x and W2C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size &lt;100 nm was obtained.

Sintering of nanosized tungsten carbide produced by gas phase synthesis

The preparation of ultrafine tungsten carbide bulk solids is of common interest in respect of the increased mechanical properties of such materials. Nanosized tungsten carbide powders are synthesized by ultra-rapid condensation from radio frequency plasma. The samples are compacted by hot pressing (HP) and spark-plasma sintering (SPS) at temperatures between 1500 and 1800°C. The raw powders and compacted samples are investigated with X-ray diffraction (XRD) to study the phase composition and crystallite size. The materials are also further characterized with electron microscopy methods (TEM and SEM). The objective of this study is to investigate the sintering behavior of the material, grain-growth development, and phase development in the WC-W 2 C system due to its technological importance.

Tungsten carbide nanopowder by plasma-assisted chemical vapor synthesis from WCl6–CH4–H2 mixtures

Nanosized tungsten carbide powder was prepared by a thermal plasma process using tungsten hexachloride (WCl6) as the precursor. The reduction and carburization of the vaporized precursor by methane–hydrogen mixtures produced nanosized WC1−x powder, which sometimes contained WC and/or W2C phase. The effects of the molar ratio of reactant gases, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H2) on the product composition and grain size were investigated. The tungsten carbide powder produced by the plasma process showed particle sizes less than 20 nm. The produced powder was heated in hydrogen to fully carburize the WC1−x, and W2C phases to the WC phase as well as to remove excess carbon in the product. Finally, WC powder with particle size less than 100 nm was obtained.

Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide – A review

Abstract Nanocrystalline WC–Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC–Co (average WC grain size

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.%.

Synthesis of nanosized tungsten carbide powder by the chemical vapor condensation process

The chemical vapor condensation (CVC) process was applied to produce nanosized tungsten carbide powders. Characteristics of the synthesized powders with variations in the CVC parameters were studied. Loosely agglomerated WC 1− x powders could be obtained. The size of the CVCed powders decreased with increasing CVC reaction temperature.