Synthesis Of Tungsten Carbide Research Articles

(Digital Presentation) Electrochemical Conversion of CO2 into Tungsten Carbides in Molten Salts

Huge amounts of anthropogenic emissions of the greenhouse gas carbon dioxide into the Earth's atmosphere are one of the key factors causing global warming. To mitigate the consequences of the severe climate changes caused by this phenomenon, over the last two decades great efforts of researchers have been directed towards the development of sustainable, environmentally friendly, carbon neutral and, if possible, not very expensive (in terms of used energy and inexpensive consumables) technologies for capture, conversion and storage (CCS) of CO2.Electrochemical conversion of CO2 using molten salts can rightfully be classified as CCS technology. In this case, carbon dioxide from various sources of its generation (fossil fuel power plants, industrial enterprises with a high carbon footprint) can be captured by molten salt (as a result of its physical dissolution, or chemical absorption by molten salt), and then electrochemically be converted into high value-added carbon-containing compounds: (a) carbon monoxide [1]; (b) carbon allotropes of various structures and modifications [2]; (c) refractory metal carbides [3], and various composites based on them. The reaction path and composition of the cathode products will depend on the electrolysis conditions. Elemental carbon synthesis precursor can be – carbon dioxide, directly dissolved in the molten salt mixture (direct reduction of CO2), as well as the carbonate anion, formed as a result of carbon dioxide interaction with oxide ions which are presented in the electrolyte bath (indirect reduction of CO2).This work presents the result of research concerning the electrochemical synthesis of the powders of tungsten carbides (WC and W2C) in chloride melt NaCl-KCl (1:1) under carbon dioxide pressure at the temperature range 700 – 800 оС. Refractory metal precursors are its oxy-anions (WО3; W2O7 2-; Меn x[WO4]nx-2; WO3F3 3- where Me – Na; K; Li; Mg; Ca; n – valance of metal Me). The formation of the new forms of tungsten electrochemical active particles in electrolyte is realized by the changing (control) of acidity of the melt. Carbon source is CO2, which was introduced into the electrolyzer under the excessive pressure of 0.1 – 1.7 MPa. The creation of excessive gas pressure is necessary condition for the increasing of the rate of electrolytic process (current densities) throw the rise of CO2 solubility in chloride melt. The general scheme of the high-temperature synthesis of tungsten carbides by the method of Molten Salt Carbon Electrochemical Transformation (MS-CCT) is presented in Fig. 1.The electrochemical investigations of partial and joint electroreduction of tungsten carbide precursors were carried out by the method of cyclic voltammetry. The areas of potentials and current densities, where the joint electrochemical discharge of tungsten carbide precursors (a narrow range of deposition potentials) occurs up to refractory metal and carbon takes place were found.Electrolytical synthesis of nano-sized (10 – 30 nm) powders of tungsten carbides (WC, W2C) and composites WC-C (up to 5 wt % of free carbon); W2C-WC; WC-C-Pt was carried out from the melts of different chemical composition; and the characterization of obtained products was fulfilled by the methods of chemical analysis, X-ray diffraction, DTG, BET adsorption, scanning and transmission electron microscopy.Synthesized composite materials based on tungsten carbides of various compositions were investigated as a cathode material in the reaction of electrolytic splitting of water for hydrogen production in a sulfuric acid solution [4]. The obtained results showed that the best activity has a composite of tungsten monocarbide WC with a content of free carbon up to 5 wt.%. The hydrogen onset potential for this electrode is -0.02 V, the overvoltage of hydrogen release at ik = 10 mA/cm2 is -110 mV, the exchange current is 7.0×10-4 A/cm2, the Tafel slope – -85 mV/dec.The presence of free carbon on the surface of tungsten carbides electrode improves its catalytic activity, increasing the area of the active surface. The catalytic activity of electrodes made of tungsten monocarbide increases with the introduction of platinum (up to 10 wt %) into the composite.References Kaplan V, Wachtel E, Gartsman K et al (2010) Conversion of CO2 to CO by electrolysis of molten lithium carbonate. J Electrochem Soc 157:B552–B556.Novoselova I.A., Kushkhov Kh.B., Malyshev V.V., Shapoval V.I. (2001) Theoretical foundations and implementation of high-temperature electrochemical synthesis of tungsten carbides in ionic melts. Theor. Found. Chem. Eng. 35:175–187.Novoselova I.A., Kuleshov S.V., Volkov S.V. et al (2016) Electrochemical synthesis, morphological and structural characteristics of carbon nanomaterials produced in molten salts. Electrochim Acta 211:343–355.Novoselova I, Kuleshov S, Fedoryshena E et al (2018) Electrochemical synthesis of tungsten carbide in molten salts, its properties and applications. ECS Trans 86:81–94. Figure 1

Synthesis of Tungsten Carbides in a Copper Matrix by Spark Plasma Sintering: Microstructure Formation Mechanisms and Properties of the Consolidated Materials.

In this study, the synthesis of tungsten carbides in a copper matrix by spark plasma sintering (SPS) is conducted and the microstructure formation mechanisms of the composite materials are investigated. The reaction mixtures were prepared by the high-energy mechanical milling (MM) of W, C and Cu powders. The influence of the MM time and SPS temperature on the tungsten carbide synthesis in an inert copper matrix was analyzed. It was demonstrated that the milling duration is a critical factor for creating the direct contacts between the W and C reactants and increasing the reactive transformation degree. A WC-W2C-Cu composite was fabricated from the W-C-3Cu powder mixture milled for 10 min and subjected to SPS at a temperature of 980 °C for 5 min. The formation of unconventional microstructures with Cu-rich regions is related to inter-particle melting during SPS. The WC-W2C-Cu composite showed a promising combination of mechanical and functional properties: a hardness of 300 HV, an electrical conductivity of 24% of the International Annealed Copper Standard, a residual porosity of less than 5%, a coefficient of friction in pair with a WC-6Co counterpart of 0.46, and a specific wear rate of the material of 0.52 × 10-5 mm3 N-1 m-1.

Optimization of pulse bi-directional electrolysis in-situ synthesis of tungsten carbide by response surface methodology

The response surface methodology was used to optimize the parameters of pulse bi-directional electrolysis (PBE) in-situ synthesis of tungsten carbide (WC) in NaF-KF molten salt system. The process of PBE synthesis of WC is a short process method of in-situ synthesis of WC powders in molten salt electrolyte. WC were used both as cathode and anode. A symmetrical bi-directional pulse is applied between the two electrodes. Tungsten ions dissolved by forward pulse, and WC were produced in situ with carbon near the electrode during reverse pulse. During the research, it was found that the factors affecting the phase of the experimental product were Current intensity, Pulse period and Electrolysis duration. In this paper, the cathode and anodic dissolved mass and the Proportion of WC in the product were used as the response values. The Box-Behnken center combined test and response surface analysis are used to optimize the factors that affect significantly, and the optimal process parameters were determined. Finally, the model was experimentally verified. Under the optimized experimental conditions, high-performance tungsten carbide nanopowders were prepared with short-process and high-efficiency.

High-pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of cBN particle size and volume fraction on their microstructure and properties

WC-cBN composites without any additives were synthesized by High Pressure-High Temperature (HPHT) method. The effect of sintering temperature, cBN content, and particle size on the densification behavior, microstructure, and mechanical properties of WC-cBN composites were investigated. It was found that the low sinterability of cBN particles resulted in poor bonding between WC grains and difficulty in forming grain boundaries, leading to difficulties in synthesizing fully dense WC-cBN composites at a pressure of 5.5 GPa. Under high pressure conditions, the phase transformation from cBN to the low-hardness hexagonal hBN was inhibited. The low sintering temperature and the inhibition effect of WC grain growth by the large cBN particles can deteriorate the hardness and fracture toughness of the composite. However, the hardness and toughness of the WC-cBN composites increased significantly with increasing cBN content. Excellent mechanical properties can be obtained by sintering composites containing cBN with small particle size and large content at high temperature with the hardness of 32.8 GPa and fracture toughness of 9.95 MP·m1/2.

Activation energy of phase transformations at high-temperature synthesis of tungsten carbide by electrothermal explosion under pressure

The effect of activation energy on phase transformations (transitions) in the W–C system during the synthesis induced by an external heat source was investigated by electrothermal explosion (ETE) under pressure. The ETE technology combines self-propagating high-temperature synthesis (SHS) with additional sample heating by Joule heat – electric current passing through the synthesized mixture, and it makes it possible to determine the chemical reaction rate that is highly susceptible to external impacts such as pressure, concentration, sample shape, any film present on combustion products, etc. The chemical reaction rate, i.e. external source current, may be controlled by changing the activation energy. The study was conducted in the following conditions: temperature Т = 293÷3700 K; carbon concentration of 49.8–50.2 at.%; quasi-static compression at P = 96 MPa; external source voltage and current density V = 10 V, I = 20 МА/m2, respectively; samples 8 mm in diameter weighing 6 g. The Т–τ thermogram of the W–C system was used to determine the following parameters: four stages of the synthesis process, temperatures of special points of phase transformations, temperature boundaries of phases and process activation energy. Thermograms of intermediate states are presented as isothermal plateaus of phase transformations. The analysis of experimental results and the physical representation of the process make it possible to assert that temperature plateau parameters are the effective value of activation energy for synthesis mode maintenance. Each of the 4 W–C mixture synthesis stages is described. Pre-explosion stage I – sample heating in the temperature range of Т = 293÷563К, endothermic reaction, effective activation energy for synthesis mode maintenance Q = 2.96 kJ, and taking into account 1-mole mass Еа = 111.6 kJ/mol. Low-temperature (563–1190 К) stage II – ignition, Q = 5.46 kJ, Еа = 109.2 kJ/mol. High-temperature stage (III) in the range of Т = 1190÷2695К, order–disorder transformation, Q = 14.25 kJ, Еа = 424 kJ/mol. Finally, Stage IV occurs in the range of Т = 2695÷3695К, Q = 14.31 kJ, Еа = 143.2 kJ/mol. It was shown that the limiting stage with the highest activation energy is the melting process.

High pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Tungsten carbide (WC) with different amounts of Cubic boron nitride (cBN) were synthesized by High Pressure-High Temperature (HPHT) method. The mapping correlation between thermodynamic condition, cBN addition, and microstructure, mechanical properties of WC–cBN composites was established and analyzed by response surface methodology. The main factors affecting the properties of composites were identified by ANOVA. The optimum thermodynamic condition was calculated. It was found that a minor phase transformation of cBN into the low-hardness hBN occurred at a temperature of 1300 °C and intensified at 1500 °C. The homogeneously dispersed cBN particles in the WC matrix promoted an improvement of hardness and fracture toughness, but the phase transition of cBN and its truss effect can dramatically reduce the mechanical properties. The Vickers hardness and fracture toughness of the well-sintered WC-cBN bulks reached a high value of 34 GPa and 13.6 MPa·m1/2, which are improved by 17% and 52% respectively compared with the pure WC samples sintered under similar high-pressure level.

New Type of Catalyst for Efficient Aerobic Oxidative Desulfurization Based On Tungsten Carbide Synthesized by the Microwave Method.

Herein, we present a new type of high-performance catalyst for aerobic oxidation of organosulfur compounds based on tungsten carbide. The synthesis of tungsten carbide was performed via microwave irradiation of the precursors, which makes it possible to obtain a catalyst in just 15 min. The synthesized catalyst was investigated by a variety of physicochemical methods: X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy, and N2 adsorption/desorption. It was shown that active centers containing tungsten in the transition oxidation state (+4) play a key role in the activation of oxygen. The main factors influencing the conversion of dibenzothiophene (DBT) were investigated. It should be noted that 100% conversion of DBT can be achieved under relatively mild conditions: 120 °C, 3 h, 6 bar, and 0.5% wt catalyst. The catalyst retained its activity for at least six oxidation/regeneration cycles. The simplicity and speed of synthesis of the proposed catalyst in combination with its high activity and stability open broad prospects for its further use both for oxidative desulfurization and for other reactions of aerobic oxidation of organic substrates.

ELECTROREDUCTION OF DITUNGSTATE AND CARBONATE ANIONS IN CHLORIDE MELT

Electrocatalysis is one of the actively developing fields of application of tungsten carbides. For the synthesis of catalytically active carbides (materials with a large specific surface area, small particle size and structural defects) a large number of different technologies are being developed in the world. The method of high-temperature electrochemical synthesis is promising one. For its successful realization, it is necessary to study in detail the electrochemical behavior of each carbide component (tungsten and carbon) and the features of their partial and joint discharge. The aim of this paper is a voltammetric study of the partial and joint electroreduction of Na2W2O7 and Li2CO3 in molten NaCl–KCl electrolyte under CO2 pressure at a temperature of 750 °C.
 As a result of research, it was found that in the system Na,K|Cl–Na2W2O7–Li2CO3–CO2 joint reduction of tungsten carbide synthesis components occurs from lithium complexes of tungstate (LixWO4)2-x and carbonate- (LixCO3)2-x anions at potentials -1.65 – -1.8 V. Introduction of СО2 into the system (creation of its excess pressure in the cell) is necessary for the binding of oxide anions O2-, released during the discharge of anionic complexes, into a carbonate complex. The released oxide anion in the near-electrode layer inhibits the cathodic process. Also, a necessary condition for the sustainability production of tungsten monocarbide WC is the presence of free carbon, which is formed during the decomposition of CO2.
 Nanosized composites of tungsten carbides with free carbon WC/C (5 wt%) were obtained by potentiostatic electrolysis at a potential of -1.8 V as a cathode product. The properties of the obtained compounds were analyzed by XRD, SEM, BET, and Raman spectroscopy. Tungsten carbide has a particle size of ~ 10 nm and consists of hollow spherical structures. The synthesized composite is mesoporous material with a specific surface area of ~ 140 m2/g.
 The properties of the synthesized compo­site, namely: structural defects, the presence of free carbon, spherical morphology, nanometer size and high specific surface area, make it possible to use it as an effective electrocatalyst, for example, in the reaction of hydrogen evolution in acidic aqueous solutions.

Metal (Mo, W, Ti) Carbide Catalysts: Synthesis and Application as Alternative Catalysts for Dry Reforming of Hydrocarbons-A Review.

Dry reforming of hydrocarbons (DRH) is a pro-environmental method for syngas production. It owes its pro-environmental character to the use of carbon dioxide, which is one of the main greenhouse gases. Currently used nickel catalysts on oxide supports suffer from rapid deactivation due to sintering of active metal particles or the deposition of carbon deposits blocking the flow of gases through the reaction tube. In this view, new alternative catalysts are highly sought after. Transition metal carbides (TMCs) can potentially replace traditional nickel catalysts due to their stability and activity in DR processes. The catalytic activity of carbides results from the synthesis-dependent structural properties of carbides. In this respect, this review presents the most important methods of titanium, molybdenum, and tungsten carbide synthesis and the influence of their properties on activity in catalyzing the reaction of methane with carbon dioxide.

Synthesis of tungsten carbide from bimodal tungsten powder produced by electrical explosion of wire

In the present study, bimodal tungsten carbide WC powders are successfully prepared by a two-stage process: electrical explosion of wire (EEW) synthesis of bimodal tungsten powder followed by a carburization reaction. The conditions of the EEW synthesis have been determined, resulting in the preparation of the bimodal powder. During the carburization process, the tungsten carbide WC is formed through the transition phase W2C. The carbon content in the obtained WC powders is very close to the theoretical value. Also, it was found that a high C/W molar ratio at the carburization stage does not affect the carburization process significantly. Optimal conditions for the tungsten carbide WC synthesis have been determined: calcination at 1200 °C with 8 h dwell and C/W molar ratio to be 1.4. The yield of tungsten carbide is 99 wt% under these conditions. Thus, the carburization process of bimodal tungsten powder may provide a low-cost and high-efficiency route to prepare the WC powders for various applications.

Ultrafine Cemented Carbides with Cobalt and Iron Binders Prepared via Reactive <i>In Situ</i> Sintering

Reactive sintering of cemented carbides involves mechanical and thermal activation of precursor elemental powders, followed by in-situ synthesis of tungsten carbide. This approach promotes formation of ultrafine microstructure favored in many cemented carbide applications. Our study focuses on the effect of mechanical activation (high-energy milling) on the properties of powder and following thermal activation (sintering) on the microstructure characteristics and phase composition. Reactive sintering proved effective – an ultrafine grained microstructure of cemented carbides with Co and Fe binders was achieved. Formation of tungsten carbide grains was complete at low temperature during reactive spark plasma sintering, resulting in textured microstructure with anisotropic grain formation and growth.

Vacuumless synthesis of tungsten carbide in a self-shielding atmospheric plasma of DC arc discharge

The goal of this paper is to simplify and reduce the cost of manufacturing tungsten carbide. The work presents the results of experimental studies that prove the possibility of synthesizing fine and ultrafine crystalline phases of tungsten carbide in the DC arc discharge plasma ignited in open air by a developed vacuumless technique. The synthesis is implemented in the process of arc discharge burning between the graphite rod (anode) placed inside the cavity of the graphite cathode in the presence of graphite and tungsten powders used as initial precursors. The cathode cavity (reaction zone) is shielded from oxygen by a stream of carbon monoxide and carbon dioxide gases, which are generated during the arc discharge burning in an open air. After studying the influence of main process parameters (parameters of the discharge circuit and graphite electrode's dimensions) and optimizing the system, the authors obtained tungsten carbide phases without impurities of oxide phases. This allows simplifying significantly the technology of tungsten carbides synthesis by excluding the stage of vacuum creation.

Mechanochemical synthesis of tungsten carbide powders induced by magnesiothermic reduction of WCl6 and Na2CO3 raw materials

The present study explains the synthesis of tungsten carbide (WC and W2C) powders from WCl6 and Na2CO3 containing blends via magnesiothermic reduction induced by mechanochemical reactions. Mechanochemical synthesis (MCS) was conducted at room temperature in a high-energy ball mill (SPEXTM 8000D Mixer/Mill) utilizing tungsten carbide vial/balls. A subsequent purification treatment using HCl and distilled water was applied on the mechanochemically synthesized (MCS’d) powders in order to leach out unwanted NaCl and MgO by-products. The optimum production conditions of tungsten carbide powders were examined in regard of MCS time and weight amounts of Na2CO3 and Mg reactants. Phase, microstructural and thermal characterizations of the MCS’d and leached powders were performed using x-ray diffractometer (XRD), stereo microscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS), particle size analyzer (PSA), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). Stoichiometric or 50 wt% excess Mg added WCl6-Na2CO3-Mg mixtures resulted in the formation of both WC and W2C powders with an amount of WCl6, WO2Cl2 and W phases after MCS and leaching processes. However, WCl6 and WO2Cl2 were disappeared, and only W2C, WC and W phases were achieved from the 50 wt% excess Na2CO3 added WCl6-Na2CO3-Mg mixtures. The best results were obtained for the WCl6-Na2CO3-Mg mixtures with 70 wt% excess Na2CO3 which pure WC powders owning an 191 nm average particle size yielded after MCS for 5 h and leaching with 7 M HCl.

Facile, low-temperature synthesis of tungsten carbide (WC) flakes for the sensitive and selective electrocatalytic detection of dopamine in biological samples

Transition metal carbides have shown potential for use in electrochemical applications due to their excellent electronic conductivity, stability and electrocatalysis.

Electrochemical Synthesis of Tungsten Carbide in Molten Salts, Its Properties and Applications

Currently, the most used material for electrocatalysis is platinum black. The use of this metal leads to the high costs of electrocatalytic devices. It is necessary to reduce Pt amount in the catalysts or to replace it by less expensive materials. Tungsten carbides (WC and W2C) are very attractive materials for catalysis due to unique properties: high electronic conductivity, corrosion resistance and ability to withstand the action of catalytic poisons. Because of the relative cheapness of the production methods, these materials can be an alternative to expensive platinum. For application as catalyst WC need to be produced in the form of nanodispersed powders with high surface area. In industry, WC is obtained by carbonizing of tungsten metal powders (or its oxide) with carbon black or carbon-containing gases. Commercial WC has micron sizes with a specific surface area of up to 2-5 m2/g. There are a number of non-traditional methods for production of nanosized WC. Among these methods, the high-temperature electrochemical synthesis (HTES) in molten salt is very promising one. Depending on the conditions of HTES, the ultradispersed powders and coatings of WC of high purity can be obtained; doping and modification of the product during the synthesis can be implemented. This method also allows to solve very important problem - electrochemical utilization of carbon dioxide with production a new chemical product. The purpose of this work was to determine the systems and conditions for the electrolytic production of tungsten carbide in molten salts, to establish its chemical and phase composition, to study the morphological and structural features. Also the evaluation of possibility of WC application as an electrocatalyst in the reaction of hydrogen evolution was carried out. The peculiarities of the partial and joint electroreduction of the synthesis components (tungsten and carbon from their oxygen-containing compounds) on platinum and glassy carbon cathodes were studied by cyclic voltammetry. Two compositions of electrolytic baths were used: (1) Na,К|Cl–Na2WO4–NaPO3; (2) Na,К|Cl–Na2W2O7. Carbon dioxide dissolved in the melt under excess pressure up to 17 atm was used as the carbon component of the synthesis. Mechanisms and kinetic features of electrode reactions were determined. The region of potentials and currents was chosen for the realization of electrochemical synthesis of carbides in the studied systems. The synthesis temperature was 700-750 °C. The cathode current density (ik) = 0.05 – 0.2 A/cm2. The highly dispersed powders were produced by electrolysis. The physico-chemical properties of the products were studied by XRD, SEM, TEM, Raman spectroscopy, BET, DTG methods. Electrocatalytic testing was carried out in a standard three-electrode cell. A graphite rod with a layer of catalytic inks was used as a working electrode. The catalytic ink was a suspension of carbide product in a 5 mas. % solution of Nafion. Non-working side part of an electrode was isolated. A platinum counter electrode and a silver chloride reference electrode were used. It has been established by XRD method that depending on conditions and parameters of electrolysis, the composite mixtures of WC and W2C carbides with nano-dimensional carbon structures and platinum can be obtained at the cathode. The parameters of hexagonal α-WC crystal lattice (a = 2.9081 ± 0.005, c = 2.8211 ± 0.01) were calculated and the average particle size (5 – 10 nm) was established. The specific surface area of the powders that was 25-30 m2/g was determined by the BET method. According to the results of TEM, DTG and Raman spectroscopy, the produced WC powders contain up to 4 wt. % of free carbon, which has an amorphous structure and covers the WC nanocrystallites as a "coat". Different morphologies of WC powders were found by SEM, TEM methods: fibers and rods, layered particles, “curdled” conglomerates (Fig.1). Electrocatalytic testing was carried out on different samples of composite mixtures: (1) WC: W2C; (2) WC: xPt; (3) WC: nC. Studies have shown that the overvoltage of hydrogen evolution (η) from 0.1 M solution of H2SO4 is in the range of 0.14 ÷ 0.5 V depending on the composition, morphology and conditions of synthesis. So for WC powders containing platinum (up to 60 mass %), the value of η is the smallest – 0.14 V. The electrodes consisting of a mixture of WC and W2C have the largest value of η. It has been established that it is possible to produce the composite mixtures based on ultradispersed tungsten monocarbide of different composition by the HTES method. These products have the required properties for the application in electrocatalysis. Fig 1. SEM images of electrolytic WC. Figure 1

Electrochemical Synthesis of Tungsten Carbide in Molten Salts, Its Properties and Applications

The peculiarities of the partial and joint electrodeposition of tungsten and carbon on platinum and glassy carbon cathodes in the chloride-oxide melt NaCl–KCl–NaPO3–CO2 have been studied by cyclic voltammetry. The potential and current density ranges and bath component rations for the high-temperature electrochemical synthesis of WC and W2C powders have been determined. Compositions, physicochemical properties, structural and morphological peculiarities of carbide products have been studied by chemical, BET and XRD analyses, SEM, TEM, Raman and Auger spectroscopy. The investigation showed that composite mixtures of WC and W2C with carbon nanomaterials (CNMs) of different composition and single-phase nano-sized powders of hexagonal α-WC of different morphology with a specific surface of up to 25 m2/g can be obtained by the electrolytic method. The electrocatalytic activity of tungsten monocarbide in the hydrogen evolution reaction in acid solution has been assessed.

Preparation of Tungsten Carbide from Scheelite Concentrate using Concentrated Energy Fluxes

The results of the synthesis of tungsten carbide from the oxide phase of scheelite concentrate are given. It is shown that the reduction and carbonization of tungsten occur upon exposure of the concentrate to high energy densities, provided that a protective atmosphere and a reducing agent are present.

3D Printing of Carbides Using Renewable Resources

Here we present preliminary results on fabrication of 3D complex shapes of porous carbide material by additive manufacturing. Porous carbides feature unique properties such as high melting point, high chemical stability, low thermal expansion coefficient, low density, high surface area and high specific strength [1]. The current state of the art for fabrication of carbide parts includes incomplete sintering and templating method. However, these methods use non-renewable carbon precursors, which is a matter of concern for the sustainability of the Earth. Furthermore, the shape of the carbide parts strongly depends on the use of mold and making of a 3D complex shape is challenging in these methods. Here we postulate the fabrication of 3D complex shapes of porous carbide material by 3D printing of a biopolymer composite followed by carbothermal reduction reaction. Recently we reported an environment friendly and sustainable approach for synthesis of tungsten carbide using a renewable biopolymer composite [2], which resembles a gel material. The renewable biopolymers eliminate the use of non-renewable carbon precursors and the Bingham plastic nature of the composite allows for layer-by-layer fabrication. The additive manufacturing will eliminate the dependence of mold and it can be possible to fabricate 3D intricate shapes, which are challenging in current methods. The potential application of such 3D printed carbide shapes includes lightweight structural material, catalysts supports, high temperature filters and insulations. The biopolymer composite we used here was composed of iota-carrageenan (IC) and chitin. The IC powder was mixed to chitin in a 1:4 weight ratio. As our initial target was to fabricate 3D printed shapes of tungsten carbide (WC), we mixed tungsten trioxide (WO3) nanoparticles with the biopolymer powders such that a molar ratio of WO3:C equal to 1:6 was achieved. Ultrapure water was added to the biopolymer-WO3 nanoparticle mixture and stirred with a homogenizer to obtain the gel-like composite. The gel composite was loaded into a syringe and extruded into 3D shapes using a paste extruder (Discov3ry) coupled with a 3D printer (LulzBot TAZ 5). Figure 1a shows such a 3D printed shape of the polymer composite. Once the printing was done, the 3D printed composite was dried overnight to obtain a 3D shape of xerogel (Figure 1b). The 3D shape of the xerogel was heat treated at 1300 °C for 3 hours in nitrogen atmosphere to obtain 3D shapes of WC (Figure 1c). The composition of the heat-treated sample was confirmed by the x-ray diffraction (XRD) spectroscopy, as only peaks of WC were observed in the XRD pattern. Ongoing work is 1) characterizing of the shrinkage of the 3D printed parts during drying and heat treatment; 2) implementing a design of experiments to obtain different dimensions of cellular architectures and understanding the effect of printing on those cellular architectures; 3) characterizing the mechanical properties of the 3D printed cellular architectures of WC.

Effect of precursor mass on product phase composition in plasma dynamic synthesis of tungsten carbide

An interest in WC1-x cubic tungsten carbide results from its catalytic properties similar to those of platinum group metals and the synergistic effect between WC1-x and Pt in reactions of hydrogen evolution and hydrogen oxidation. However, according to the phase diagram of the W–C system, the cubic phase WC1-x only exists in a narrow range of temperature stability (about 2798–3058 K), which makes it difficult for being obtained. To date, there are different methods for synthesizing tungsten carbide powder with a low content of cubic phase that complicates the study of WC1-x properties. A direct plasma dynamic synthesis is known as one of the promising methods to produce WC1-x. The aim of this work is to find the optimal amount of tungsten precursor to obtain cubic tungsten carbide with a high purity by plasma dynamic method. The synthesized products were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns showed that the main phase was cubic tungsten carbide with negligible content of hexagonal tungsten carbide W2C and pure tungsten W. According to a quantitative analysis of synthesized products, which were obtained using masses of initial tungsten equal to 1.0, 0.7, 0.6 and 0.5 gram, the yield of WC1-x phase was 84, 89, 95 and 92 wt%, respectively. The results of TEM displayed that the synthesized powders consist of crystallites, having the size less than 100 nm (WC1-x), and a carbon matrix. This carbon was not detected in XRD due to its presence as an amorphous phase.

Wide band gap and conducting tungsten carbide (WC) thin films prepared by hot wire chemical vapor deposition (HW-CVD) method

In this letter, we report synthesis of tungsten carbide (WC) thin films having wide band gap (3.22–3.3eV) with high electrical conductivity (80–1260S/cm) by HW-CVD using heated W filament and Tetra-fluoro-methane (CF4) gas. Formation of WC was confirmed by low angle XRD, Raman spectroscopy and x-ray photoelectron spectroscopy. UV–Visible spectroscopy analysis revealed that the synthesized films have high transmission at wavelength 500nm. Electrical properties measured using Hall measurement show that these films are semiconductor. The obtained results imply that the growth of WC thin films is mainly from the atomic species (W and C) evaporated from the hot filament. The HW-CVD opens a novel route to synthesize wide band gap and conducting WC at a cost-efficient way for DSSCs and hydrogen evolution reaction (HER).