Nanocrystalline Tungsten Carbide Research Articles

Synthesis of Nanocrystalline Tungsten Carbide by Intermittent Microwave Heating Method as Eletrocatalyst Support

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Atomic-scale structure and composition of tungsten carbide reinforced diamondlike carbon films

This study presents the true atomic-scale structure and composition of hard/lubricious nanocomposite diamondlike carbon (DLC) films. Nanocrystalline tungsten carbide (∼2–4 nm precipitates) in mixed sp3/sp2 amorphous C–H matrices was characterized using the combination of three-dimensional atom probe (3DAP) tomography and transmission electron microscopy. Excellent correspondence in structural and chemical layering was observed (alternating ∼25 nm thick carbon-rich and tungsten-rich layers) with 3DAP also revealing C32+ and C2H4+ species in the carbon-rich layers and W2+, WC2+, C+, and C2+ species in the tungsten-rich layers. 3D morphology and chemical partitioning of the WC nanoprecipitates in the tungsten-rich layers are also discussed.

Two-dimensional focusing of high-energy X-rays by thermal-gradient crystals for small-angle X-ray scattering

A novel method for focusing X-rays in two dimensions by thermal-gradient crystals in symmetrical Laue geometry is described. A 225 kV stationary tungsten tube delivers an X-ray beam with a source diameter of about 1.0 mm (full width at half-maximum). The focal point at the detector at a distance up to 16 m from the source is of the same size. The beam at the focusing crystals at half the distance between the source and the detector has typical dimensions of 30 × 30 mm. The intensity of the focal point can be increased by more than 200 times by applying a thermal gradient of about 2.2 K mm−1on the focusing crystals. The described method and apparatus are designed for small-angle X-ray scattering at high photon energies up to 60 keV, where the high penetration power allows experiments on strongly absorbing materials in transmission mode. Particle sizes up to 3000 Å can be detected. First measurements on nanocrystalline tungsten carbide and Teflon yield radii of gyration of 540 Å and 815 Å, respectively.

Integrated molding of nanocrystalline tungsten carbide powder with stainless steel

Nanopowder of tungsten carbide ( d < 100 nm) was prepared by the planetary ball milling of the commercial powder with the particle size of the submicron order. The WC powder was densified by the spark plasma sintering. The milled powder was possible to be densified at a temperature which was 500 °C lower than densification temperature for the commercial powder. The tungsten carbide body sintered at 1100 °C for 3 min under 50 MPa mainly consisted of nanograins with the grain size of <100 nm. It exhibited superplastic deformation behavior under compression at 1200 °C to a strain of >2 without brittle fracture. Applying such superplastic property, we successfully covered the inwall of the cylindrical metal mold made of stainless steel with tungsten carbide layer of the 2 mm in thickness by using spark plasma sintering method.

Effect of substrate temperature variation on nanostructured WC films prepared using HFCVD technique

Nanocrystalline tungsten carbide thin films are deposited on quartz substrates using hot-filament chemical vapor deposition technique. The influence of the substrate temperature on the nanostructured WC films is studied. The scanning electron microscopy indicates that the size of nanoparticles increases from 50 to 150 nm with an increase of substrate temperature from 400 °C to 800 °C. The crystalline structures, chemical bonds, and nanocomposition of WC films are characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering, and energy dispersion spectroscopy. The evolution of crystalline structures from α-WC to α-W 2C following variation of substrate temperature is observed.

Low Temperature Synthesis of Tungsten Carbide by Embedment-Direct Reduction Carburization Process

Tungsten carbide (WC) powder has been widely studied because of its catalytic properties in some organic reactions. Nanocrystalline tungsten carbide powders with large surface area can provide a large number of catalytic active centers (situ) to reactions. In this article, a new process of preparing Nanocrystalline tungsten carbide powder by embedment-direct reduction and carburization was studied. The nanocrystalline WC powder with average 40 nm grain size was synthesized by embedment-direct reduction and carburization at 1300°C and 3 hours' heat preservation.

Synthesis of nanocrystalline hexagonal tungsten carbide via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium

Nanocrystalline tungsten carbide (WC) was synthesized via a simple route by the reaction of metallic magnesium with sodium carbonate and tungsten hexachloride in an autoclave at 600 °C. X-ray powder diffraction pattern indicated that the product was hexagonal WC, and the cell constant was a = 2.913 Å, c = 2.838 Å. Transmission electron microscopy image showed that it consisted of particles with an average size of 20 nm. The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 500 °C in air.

Improved Performance of Direct Methanol Fuel Cells with Tungsten Carbide Promoted Pt∕C Composite Cathode Electrocatalyst

Oxygen reduction reaction (ORR) on tungsten carbide promoted Pt/C (Pt-W 2 C/C) composite catalysts prepared by the intermittent microwave irradiation method has been investigated in acidic media. The exchange current density for the ORR on Pt-W 2 C/C is more than 2 orders of magnitude higher than that on a conventional Pt/C catalyst with a significantly improved onset potential. A direct methanol fuel cell (DMFC) using a Pt-W 2 C/C cathode achieved a power output of 190 mW cm -2 , which is 36% higher than that of a conventional Pt/C cathode with a much higher Pt loading. The results show that the incorporation of tungsten carbide greatly enhances the electrocatalytic activities of the Pt/C catalyst and significantly improves the DMFC performance, probably due to the synergetic catalytic effect between nanocrystalline tungsten carbide and Pt catalysts.

Preparation of nano-crystalline tungsten carbide thin film electrode and its electrocatalytic activity for hydrogen evolution

Nano-crystalline tungsten carbide thin films were fabricated on pure flake Ni and foam Ni substrates by plasma enhanced chemical vapor deposition. SEM results showed that the films were composed of uniform grains with a diameter of around 10 nm. The electrocatalytic properties of the film for hydrogen evolution reaction in alkaline solution were investigated. The film showed an attractive catalytic activity in hydrogen evolution. The exchange current density of hydrogen evolution reaction was 4.02–4.22 × 10 −4 A cm −2. When the over-potential was 263 mV, the activation energy of hydrogen evolution was 45.62–45.77 kJ mol −1. Nano-crystalline WC thin film electrode had a high catalytic stability according to the result of cyclic voltammetry.

Synthesis of nanophase tungsten carbide by electrical discharge machining

Tungsten carbide nanopowders were synthesized successfully by electric discharge machining followed by annealing under a nitrogen atmosphere. The tungsten workpieces were initially melted and evaporated on the working surface during the electric discharge machining process, and then the tungsten powders were reacted with the carbon electrode and the working medium of kerosene to form the nanocrystalline WC 1− x powders. The powders produced were characterized by XRD, SEM, and TEM. When annealing the powders under an N 2 atmosphere, the cubic phases of WC 1− x gradually changed to hexagonal W 2C and then were transformed fully to nanocrystalline hexagonal WC at 1200 °C, with the nanocrystalline tungsten carbide encapsulated in a carbon shell. On the other hand, under an H 2 atmosphere, the WC 1− x phase changed via a W 2C phase to reduced powders of pure tungsten at 1000 °C or were reduced directly from WC 1− x to elemental W.

Magnetron sputtered W–C films with C 60 as carbon source

Thin films in the W–C system were prepared by magnetron sputtering of W with coevaporated C 60 as carbon source. Epitaxial deposition of different W–C phases is demonstrated. In addition, nanocrystalline tungsten carbide film growth is also observed. At low C 60/W ratios, epitaxial growth of α-W with a solid solution of carbon was obtained on MgO(001) and Al 2O 3(001) at 400 °C. The carbon content in these films (10–20 at.%) was at least an order of magnitude higher than the maximum equilibrium solubility and gives rise to an extreme hardening effect. Nanoindentation measurements showed that the hardness of these films increased with the carbon content and values as high as 35 GPa were observed. At high C 60/W ratios, films of the cubic β-WC 1− x ( x=0–0.6) phase were deposited with a nanocrystalline microstructure. Films with a grain size <30 Å were obtained and the hardness of these films varied from 14 to 24 GPa. At intermediate C 60/W ratios, epitaxial films of hexagonal W 2C were deposited on MgO(111) at 400 °C. Polycrystalline phase mixtures were obtained on other substrates and hexagonal WC could be deposited as minority phase at 800 °C.

Structure and mechanical properties of tungsten carbide films deposited by off-plane double bend filtered cathodic vacuum arc

High quality nanocrystalline tungsten carbide films were deposited by a filtered cathodic vacuum arc technique at room temperatures. X-ray diffraction and atomic force microscopy were used to characterize the crystalline structure and surface morphology of the films. Substrate bending methods and nanoindenter were used to measure internal stress, hardness, and Young’s modulus of the films. The film structure evolves from a single hexagonal α-W2C phase to a mixture phase, which is composed of a hexagonal α-W2C, a cubic WC1−x, and a cubic W phase, as the substrate bias, which is always negative, is increased to 200 V. As the substrate bias is further increased, the film structure changes gradually to a single α-W2C phase again. The variation trends of internal stress, hardness, and Young’s modulus with the substrate bias are similar. They increase with substrate bias, reaching a maximum of 7.2, 26, and 270 GPa, respectively, at a substrate bias of 200 V, then decrease with further increase of substrate bias.

Nanocrystalline tungsten carbide encapsulated in carbon shells

Nanocrystalline tungsten carbide with an average grain size of 12 nm was synthesized using the ion arc method. The characterization was carried out by means of XRD, TEM, and TGA. The experimental results indicate that the as-synthesized powders are mainly cubic WC 1 − x. Besides, a small amount of α-W 2C is also observed. These tungsten carbide powders are encapsulated within crystalline graphite. Owing to the ultrafine sizes of the powers, oxidation of the graphite occurs at about 350 °C. The nanocrystalline tungsten carbide transforms into WO 3 at about 670 °C.

Mechanochemical Synthesis of Nanocrystalline Tungsten Carbide Powders

Nanocrystalline tungsten carbide powder mixtures of WC, W2 C, and cubic WC1–z were prepared by the mechanochemical method of reactive high energy ball milling WO3–Mg mixtures containing graphite as a source of carbon for the W phase. The synthesis is completed in two steps, namely the explosive reactive synthesis of α-W by the reductive reaction of Mg with WO3 and the diffusion reaction of high activity α-W with C. The chemical regularities of mechanochemical synthesis of the refractory compounds are given. After grinding for 50 h at a milling rate of 250 rev min−1, all WO3 and C diffraction peaks disappeared and only diffraction peaks of tungsten carbides and MgO remained. The final product, with an average grain size of about 4–20 nm was obtained after MgO was removed by HCl solution. PM/0795

Synthesis of full-density nanocrystalline tungsten carbide by reduction of tungstic oxide at room temperature

Among the hard alloys, WC alloys find wide industrial applications as tips for cutting tools and wear-resistant parts. Their intrinsic resistance to oxidation and corrosion at high temperatures also makes them desirable as a protective coating for devices at elevated temperatures. In the industrial scale of production, WC is prepared by a direct union of the elements at a temperature of 3,273 to 3,473 K. Accordingly, the high cost of preparation is a disadvantage of this process. Here, the authors report a novel technique for preparing a large amount of WC powder using a simple method. This process is based on mechanical solid-state reduction (MSSR) followed y solid-state reaction (SSR) during room-temperature ball milling (a high energy ball mill, Fritsch P6, was used at a rotation speed of 4.2 s{sup {minus}1}) of a mixture of WO{sub 3}, Mg, and C powders.