Synthesis of high-purity ultrafine tungsten and tungsten carbide powders

High-purity ultrafine W or WC powder was prepared via a two-step process composed of the carbothermic pre-reduction of WO_2.9 and the following deep reduction with H₂ or carbonization with CH₄+H₂ mixed gases. The effects of C/WO_2.9 molar ratio and temperature on phase composition, morphology, particle size, and impurity content of products were investigated. The results revealed that when the C/WO_2.9 ratio was in the range from 2.1:1 to 2.5:1, the carbothermic pre-reduction products consisted of W and a small amount of WO₂. With changing C/WO_2.9 ratio from 2.1:1 to 2.5:1, the particle sizes were gradually decreased. In order to prepare ultrafine W or WC powder, a relatively high C/WO_2.9 ratio and a lower reaction temperature at this stage were preferred. After the second reaction, the final products of ultrafine W and WC powders with a high purity could be obtained, respectively.     

水热合成钨前驱体-碳热还原-碳化制备超细纳米级碳化钨

提出了一种以水热合成的PbWO₄为原料,然后通过碳热还原-碳化获得超细WC的方法。以PbWO₄为钨中间产品,避免了氨氮试剂的引入;采用碳还原的方式可避免水蒸气的产生,抑制了钨粉的长大。结果表明：在初始pH为7.0、反应温度为160 ℃,反应时间为4.5 h的条件下,Na₂WO₄溶液中99.9%（质量分数）以上的W以PbWO₄的形式回收。然后采用低温碳还原PbWO₄,在C:W摩尔比为5、950 ℃的条件下还原3 h,获得了W和C的混合物,该混合物中预加富余的C有助于抑制钨粉的团聚。然后将W和C混合物高温碳化,在1200 ℃下反应6 h,获得了粒径约为60 nm的WC粉末。     

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.     

Effect of vanadium on synthesis of WC nanopowders by thermal processing of V-doped tungsten precursor

The effect of vanadium on the synthesis of WC nanopowders by carbon thermal processing of V-doped tungsten precursor has been discussed. The V-doped tungsten precursor was prepared by a wet chemical method with ammonium tungstate and ammonium vanadate as its starting materials. The precursor was carbonized in the vacuum furnace using phenol formaldehyde resin as a carbon agent. The results of XRD revealed that the tungsten oxide and vanadium oxide obtained from the precursor preparation formed V–O–W bronze with the structure of WO₃ · 0.33H₂O. The carbonization reactions of WO₃ with 1 wt% of vanadium took place in a temperature range from 900 to 1050 °C to obtain V-doped WC nanopowder. The results of particle size measurement and morphological analysis show that the vanadium effectively inhibits the particle growth of tungsten carbide powder during carbonization processes, resulting in the particle size to be within the range from 64 to 184 nm after heat treatment in the temperature range from 900 to 1200 °C. V₂O₃ particles decomposed from V–O–W bronze can act as a nucleation aid for tungsten during reduction, and those on the surface of tungsten powder can hinter the growth of tungsten carbide crystal by the pinning effect.     

Synthesis of Cr-doped APT in the evaporation and crystallization process and its effect on properties of WC-Co cemented carbide alloy

WC grain size has significant effect on WC-Co cemented carbide alloy properties. In order to inhibit WC grain growth during sintering process, grain growth-inhibitor Cr₃C₂ is usually added to tungsten carbide powder in advance through mechanical milling. While, homogeneous distribution of Cr₃C₂ in the tungsten carbide powder is difficult to achieve and result in abnormal growth of WC grains. For this purpose of growth-inhibitor uniform distribution, (CH₃COO)₃Cr is added into ammonium tungstate solution during evaporation and crystallization process to prepare Cr-doped APT powder, which can be used as precursor for ultrafine-grained WC-Co cemented carbide alloy preparation. Compared with conventional APT powder, the Cr-doped APT has smaller particle size and bulk density, moreover, chromium is evenly distributed within it. The Cr-doped APT is then used to produce Cr-doped tungsten powder, which also has smaller particle size than that of conventional tungsten powder. Cr-doped tungsten powder is subsequently prepared into tungsten carbide powder and WC-Co cemented carbide alloy through carbonization and sintering process, respectively. Compared with conventional WC-Co cemented carbide alloy, the obtained WC-Co cemented carbide alloy has smaller mean WC grain size (0.36 μm), and more uniform microstructure. Furthermore, the phenomenon of WC grain abnormal growth during sintering process is not observed, because the grain growth-inhibitor Cr₃C₂ is well dispersed in tungsten carbide and cobalt composite powder. Results show that the obtained WC-Co cemented carbide alloy presents better mechanical properties (HRA, bending strength, coercive force) than those of conventional WC-Co cemented carbide alloy. Accordingly, the novel addition of (CH₃COO)₃Cr during the evaporation and crystallization process is the key factor of ultrafine-grained WC-Co cemented carbide alloy production.     

Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure,composition and properties of WC powders

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO₃ by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO₃ and V₂O₅ by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO₄) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.     

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

Solid-state carbothermic reduction of tungsten oxide (WO₃) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO₃ and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO_2.72, WO₂ 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.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

In-situ synthesis of WC–Co composite powder and densification by sinter-HIP

In the present study, a complete route which integrates in-situ synthesis of WC–Co composite powder and sinter-HIP is proposed to prepare the ultrafine tungsten carbides. Owing to the in-situ reduction and carbonization reactions of WO_2.9, Co₃O₄ and carbon black powders at 1000 °C, the composite powder with pure phase constitution and ultrafine particle size is synthesized with a rapid procedure. The WC–Co bulk material prepared by the sinter-HIP densification of the composite powder exhibits homogeneous and ultrafine microstructure, as well as the excellent mechanical properties. The proposed method shows potential to be developed as a promising industrial route owing to its advantages of low-cost raw materials and short-term in-situ reactions.     

A short and facile process to synthesize WC-Co cemented carbides

WC-Co cemented carbides are widely used in the fields of military, aerospace, mining and cutting industry etc. In this paper, a new two-step method for the preparation of WC-Co cemented carbides was proposed. First, the mixture of yellow tungsten trioxide (WO₃) and cobaltic oxide (Co₂O₃) were reduced by carbon black to remove all the oxygen. Then, the carbothermic reduction products were precisely mixed with an appropriate amount of carbon black to directly prepare WC-Co cemented carbides. The effects of C/WO₃ ratio on the phase composition, morphological evolution, particle size and mechanical properties of products are investigated. The experimental results revealed that when the C/WO₃ molar ratio was above 2.7, all oxygen in the raw material mixture were removed by carbon black and a mixture of W₂C and η-phase were obtained after the first step of carbothermic reduction at 1150 °C for 2 h; then, the mixture of carbothermic reduction product and an appropriate content of carbon black was compacted, and the green compact was first carbonized at 1200 °C for 2 h and then sintered at 1450 °C for 4 h to prepare cemented carbides. With the increase of C/WO₃ ratio at the first stage, the content of η-phase with a low melting point increased, which resulted in the large grain size of WC in the finally prepared cemented carbide. Compared with the traditional method of preparing cemented carbides, the cemented carbides prepared by the current method showed a higher hardness and toughness. Furthermore, the addition of a proper content of the VC in the second stage can significantly inhibit the grain growth of WC and further increase the hardness of cemented carbides.     

Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres

Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO₃ powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH₄, or mechanical activation followed by thermal activation of tungsten and carbon precursors.In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H₂, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO₃ in H₂ containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.     

An investigation of the fabrication of tungsten carbide–alumina composite powder from WO3, Al and C reactants through microwave-assisted SHS process

Possibility of synthesis of tungsten carbide–alumina composite powder from WO₃–Al–C mixture via microwave-assisted SHS process in a domestic microwave oven has been investigated. By comparison of the results of thermodynamic calculations with experimental findings, it was found that during microwave heating of WO₃:2Al:C mixture, synthesis process initiates by vigorous exothermic reaction of WO₃ with Al which results in a great deal of heat. Major portion of tungsten carbide phase in the product is W₂C, whose formation is supposed to be related to the high thermodynamic stability of this compound at high temperatures. W₂C formation could also be related to carbon loss phenomenon in the mixture, as a consequence of some carbon burn. It has been concluded that addition of excess carbon to the initial mixture together with extension of the microwave processing time, increase the amount of WC phase in the product in expense of W₂C. Experimental results showed that only small amounts of W₂C remain in the product with around 80 mol% excess initial carbon and about 10 min of microwave heating time.     

An investigation on the mechanochemical behavior of the Ca–C–Cu2O–WO3 quaternary system to synthesize the Cu–WC nanocomposite powder

Fundamental aspects of the reaction path in the Ca–C–Cu₂O–WO₃ quaternary system to synthesize a copper matrix nanocomposite with reinforcement particles of tungsten carbide have been studied. The mechanism of reactions was specified through the analysis of the relevant sub-reactions. In the presence of carbon as a reducing agent (without Ca), the carbothermal reaction partially occurred even after 40 h of milling. On the other hand, calcium (without C) reduced both Cu₂O and WO₃ after 15 min of milling in a self-sustaining mode. In the simultaneous presence of Ca and C, the products included Cu and W₂C as well as a significant amount of remaining unreacted W. The Cu–WC nanopowder, with no trace of W₂C, was synthesized by the addition of excess carbon to the initial mixture. SEM observations indicated that the composite powders were agglomerated and the range of the particle size was within 100 nm. Elemental mapping spectra showed a relatively uniform distribution of WC in the Cu matrix.     

Reactive spark plasma sintering of binderless WC ceramics at 1500 °C

Binderless WC ceramics were prepared by reactive spark plasma sintering, using tungsten trioxide, tungsten and carbon black as the starting materials. Phase assemblages and microstructure of the as-sintered ceramics were investigated. It was found that graphite existed as an impurity phase due to the volatilization of WO₃, and W could compensate for the WO₃ loss to form WC with a single phase. Benefiting from the enhanced sinterability, WC ceramics with high relative density and good hardness could be obtained at temperature as low as 1500 °C.     

Comparative kinetic and structural analyses of nanocrystalline WC powder synthesis from pre-reduced W under pure and diluted CH4 atmospheres

W nanoparticles derived from WO₃ during heating in H₂ were carburized at 900-1100 K in pure, Ar and H₂-diluted CH₄ atmospheres. It was aimed to elucidate carburization behavior of pre-reduced W powders under various atmospheres and to establish optimal conditions for the synthesis of nanocrystalline WC. Weight measurements, XRD and SEM were used to characterize the products at various stages of the reaction. At 900 K, carburization was limited to the formation of W₂C owing to slow C diffusion. At 1000 K, WC particles were obtained within ~ 75 min using the diluted gas mixtures, while under pure CH₄ atmosphere carburization reaction practically stopped due to pyrolytic carbon skin formed on particle surfaces where C supply was more than consumed. WC powders with particle size 40-65 nm and grain size 15-25 nm were synthesized at 1100 K in a short time under the gas atmospheres studied.     

A low-cost,efficient, and industrially feasible pathway for large scale preparation of tungsten nanopowders

Tungsten (W) is the most commonly used high-temperature refractory metal in many critical fields such as aerospace, military and electronic industries etc. This paper proposes a low-cost, efficient, and industrially feasible pathway for large scale preparation of tungsten nanoparticles via the combination of carbothermic reduction and hydrogen reduction processes. The new strategy involves the preparation process of pre-reduction W powder by reducing commercial WO₃ with insufficient carbon black at 1050 °C or 1150 °C to avoid the residue of carbon, and the deep reduction process of pre-reduction W powder by hydrogen 725 °C. By this process, most of the oxygen in WO₃ was reduced by carbon, and W particles with a much smaller size could be obtained owing to the absence of the volatile tungsten oxide, such as WO₂(OH)₂, which leads to the serious increase of particle size during the hydrogen reduction process. Tungsten nanoparticles with average particle sizes of about 40 nm and 75 nm have been successfully synthesized at 1050 °C and 1150 °C, respectively, with the residual carbon content as low as about 0.01%. This process can be readily extended to a large-scale industrial production of W nanopowders. Additionally, this new strategy has great potential to prepare other pure metals (or nanopowders) from their metal oxides via combining of carbothermic reduction (main process) and further reduction of other reducing agents.     

Preparation of ultrafine tungsten powders by in-situ hydrogen reduction of nano-needle violet tungsten oxide

Ultrafine tungsten powders with a grain size below 0.5 μm are key raw materials for fabricating ultrafine cemented carbides. Conventional hydrogen reduction technique has been utilized to prepare the ultrafine tungsten powders. In the present work, highly pure nano-needles of violet tungsten oxide (WO_2.72) were reduced by dry hydrogen. Nucleation and growth of the metallic tungsten in the early stage of hydrogen reduction have been studied by XRD, FESEM and HRTEM. Mechanism of formation of nano-size tungsten powders is proposed and a concept of in-situ hydrogen of the nano-needle WO_2.72 is presented. Empirical relations between an average diameter of nano-needle WO_2.72 and an average particle size of the resultant tungsten powders in both stage of nucleation and industrial conduction have been established. These empirical relations could be a reasonable guidance for suitably choosing the raw materials of nano-needle WO_2.72 to prepare ultrafine tungsten powders. It has been determined that the BET special surface areas of the in-situ hydrogen-reduced tungsten powders with the average particle size of 0.2 μm and 0.3 μm, which were produced from the raw nano-needle WO_2.72 powders with the average diameter of 60 nm and 80 nm, are 6.03 m²/g and 4.65 m²/g, and the oxygen contents are 0.35% and 0.29%, respectively.     

Conditions for the in-situ formation of carbide coatings on diamond grains during their sintering with CuWC binders

In present work, self-assembled WC/W coatings on diamond grains, sintered with Cu matrix were firstly obtained by gas phase transport mechanism. Conditions for spontaneous coating formation were also determined. The coatings can improve adhesion strength between diamond grains and a binder. Discovered method can be used for production of heat sink and heat spreader devices, as well as cutting tools. As functional additives (precursors) for coating formation, WC and WO₃ were used. Morphology and composition of the coatings were identified by XPS, SEM, and TEM techniques. Coating consisting of mixture of metallic tungsten and tungsten carbide is formed on diamond when the Cu + WC binder is used. No coating is formed when copper is modified with WO₃ nanoparticles; introduction of 20% Fe to the binder leads to formation of a coating containing 100% tungsten in metallic form. Hence, conditions for formation of the tungsten (tungsten carbide) coating onto the diamond surface during sintering with a metal binder include the presence of a nucleating agent WC and a metal that catalyzes graphitization. It was found that the driving force for spontaneous coating formation is a chemically activated gas-phase mass transfer of WO₃ to the diamond surface, its chemisorption and subsequent reduction.     

Synthesis mechanism and sintering behavior of tungsten carbide powder produced by a novel solid state reaction of W2N

Nanosized WC powder was synthesized by solid state reaction between tungsten nitride (W₂N) and carbon black. The reaction path and the phase evolution during the synthesis were studied. It was found that W₂N firstly decomposed to tungsten metal in the middle stage of synthesis, and then it reacted with carbon black to form W₂C and WC in sequence. Using this WC powder as the starting material, binderless WC hardmetals were prepared by hot pressing, and the microstructure and mechanical properties of the material were investigated.     

Synthesis of nanostructured tungsten carbide via metal-organic chemical vapor deposition and carburization process

Nanostructured tungsten carbide particles were successfully synthesized by metal-organic chemical vapor deposition in a spouted bed followed by carburization in CH₄/H₂ atmosphere in the temperature range 700–900 °C. The carburization process was a little bit complex, which involved the coating of carbon on the outer surface of the decomposed W(CO)₆ precursor particles and then followed by carbon diffusion into the particles, leading to the formation of nanostructured WC via an intermediate metastable phase W₂C. The carbon deficient phase W₂C was formed initially at lower carburization temperature and then transformed to stable WC phase by increasing the temperature and holding time.