Ultrafine Tungsten Powders Research Articles

Effect of jet milled ultrafine powder on porous tungsten emitter components by micro injection molding

Micro injection molding (μPIM) is currently the optimal solution for fabricating porous tungsten emitters with complex shapes. Considering the serious agglomeration of ultrafine powder, using this particle size for μPIM is detrimental to the molding of the components and the regulation of the pore structure. In this work, jet milling technology was applied as pretreatment to obtain uniformly dispersed ultrafine tungsten powder, and a needle-like porous tungsten emitter with uniform and adjustable pores was successfully prepared by μPIM. The effects of jet milling on the characteristics of ultrafine tungsten powder and μPIM process were investigated. After the jet milling treatment at 0.75 MPa and 12,000 rpm, the agglomerates in ultrafine powder were completely eliminated and the powder dispersion was optimized. The solid loading of feedstock was increased from 45% to 51%, and the corresponding rheological properties and debinding efficiency were also improved. Jet milled powder exhibited higher sintering activity and gained the same porosity (25%) at 1425 °C, lower than the 1500 °C of the as-received one. The porous tungsten obtained by jet milled powder had pores and skeletons with uniform size and distribution, which contributes to the adjustment of the pores. The pore size decreased from 338 nm to 263 nm, and the closed porosity decreased from 1.5% to 0.6%. This work is beneficial for further understanding the influence of jet milling on powder characteristics, μPIM process and pore characteristics, and highlights its potential application in the field of micro-pores complex parts.

Preparation of ultrafine tungsten powder by fluidized bed hydrogen reduction of WO3-Co3O4

This article investigated the preparation of ultrafine W powder by fluidized bed hydrogen reduction, and the apparent activation energy was determined for various reduction stages by using temperature programmed reduction with hydrogen. The results demonstrate that the introduction of Co3O4 significantly enhances the reduction process of WO3.High Co3O4 content can lead to the formation of CoWO4 in the initial stage and obtain products with high content of Co3W. An increase in the Co3O4 content correlates with a gradual decrease in the particle size of the product. The apparent activation energy for the WO3 → WO2 and WO2 → W processes decreases as the Co3O4 content increases. It can be decreased from 105.59 kJ /mol to 78.72 kJ/mol for the WO3 → WO2 stage, and from 107.38 kJ/mol to 78.38 kJ/mol for the WO2 → W stage, respectively.

Evolution of reduction process from tungsten oxide to ultrafine tungsten powder via hydrogen

Abstract Herein, the evolution of reduction process of ultrafine tungsten powder in industrial conditions was investigated. The transition process of morphology and composition was examined via SEM, XRD, and calcination experiments. The results show that the reduction sequence of WO2.9 was WO2.9 → WO2.72 → WO2 → W on the surface, but WO2.9 → WO2 → W inside the oxide particles. With the aid of chemical vapor transport of WO x (OH) y , surface morphology transformed into rod-like, star-shaped cracking, floret, irregularly fibrous structure, and finally, spherical tungsten particles.

Fabrication of ultrafine-grain and great-performance W–Ni–Fe alloy with medium W content

The tungsten alloy with medium W content (wt.%) is a combination of high strength and high toughness, which has wide potential applications in related fields. In order to fabricate high-performance W–Ni–Fe alloy with medium W content, ultrafine tungsten powder obtained by sol-spray drying-hydrogen reduction method was employed as raw material. The medium W content W–Ni–Fe alloys with different compositions were systematically studied for the first time, i.e., 50W–25Ni–25Fe, 50W–35Ni–15Fe, 70W–15Ni–15Fe and 70W–21Ni–9Fe. The results show that the mechanical properties of medium W content W–Ni–Fe alloys prepared by this method are excellent, the highest tensile strength is 945 MPa, the highest elongation is 31.7%. The tensile strength, density and elongation of tungsten alloy with Ni/Fe ratio of 7:3 are better than those of tungsten alloy with Ni/Fe ratio of 1:1 when W content is fixed. With the same Ni/Fe ratio, the density of tungsten alloy with 50W series is higher than that of tungsten alloy with 70W series, but the tensile strength is lower. The microstructure of tungsten alloys with four components are uniform and fine, the grain size of tungsten is just 3–5 μm. The two main fracture modes of samples are the cleavage fracture of tungsten grains and the dimple fracture between tungsten grains and γ phase.

Hydrothermal synthesis of nanoplates assembled hierarchical h-WO3 microspheres and phase evolution in preparing cubic Zr(Y)O2-doped tungsten powders

Three-dimensional hierarchical h-WO3 and doping tungsten powders have recently attracted considerable attention because of their superior sensing properties and refined grains, respectively. In this article, we report a facile hydrothermal hydrogen reduction process for preparing hierarchical h-WO3 microspheres that self-assemble with nanoplates. Meanwhile, the phase evolution process and evolution mechanisms during the conversion of h-WO3 to W are systemically investigated. Results indicate that the highly homogeneous h-WO3 microspheres are uniformly covered with ultrafine ZrY2(OH)10 micelles, which fully transform into m-WO3 and cubic Zr(Y)O2 after calcination at 600 °C. Microspheres possessing different pore diameters and containing nanosized particles can be obtained by adjusting the hydrogen reduction process. These phase evolution process can provide reasonable guidance for preparing tungsten oxide with high electrochemical properties and ultrafine tungsten powders. The h-WO3 microspheres with an average size of 3 μm consist of nanoplates and the tungsten powders doped with 1.0 wt% Zr(Y)O2 have a mean particle size of approximately 1.4 μm. Comparative test results indicate that the addition of 1.0 wt% Zr(Y)O2 can promote the formation of low-degree particle agglomerates.

Модифицирование сплава марки АК12 частицами ультрадисперсного порошка вольфрама

Модифицирование сплава марки АК12 частицами ультрадисперсного порошка вольфрама

First-principles study of absorption mechanism of hydrogen on W20O58 (010) surface

With the development of modern industrial technology, tungsten products prepared from traditional tungsten powder cannot meet the demands of industry. However, the properties of tungsten products produced from ultra-fine tungsten powder have been greatly improved:they have high strength, high toughness, and low metal plasticity-brittle transition temperature. Hence, it is necessary to carry out theoretical research of the micro-adsorption dynamics during hydrogen reduction of W20O58, which is beneficial to synthetizing ultra-fine tungsten powder. In this article, to comprehend the crystal characteristics of W20O58 (010) surface and provide the theoretical reaction law for hydrogen reduction on W20O58 (010) surface, the absorption mechanism of H2 molecule on W20O58 (010) surface is studied by the first-principles calculation based on density functional theory in a plane wave pseudo-potential framework. The results show that the indirect band gap of W20O58 is 0.8 eV, indicating that it has metallic characteristic. The W20O58 (010) surface has different terminations, i.e., WO-terminated (010) surface and O-terminated (010) surface. After the geometrical optimization of the two surfaces, the W–O bond length and bond angle of W–O–W are both changed. In addition, six absorption configurations of H2 on W20O58 (010) surface, including WO-L-O1c, WO-V-O1c, WO-L-O2c, WO-V-O2c, O-L-O1c and O-V-O1c, are chosen to be investigated. The calculation results show that the WO-L-O1c, WO-V-O1c and WO-L-O2c absorption system are unstable, while the WO-V-O2c, O-L-O1c and O-V-O1c absorption configuration are stable. When H2 molecule is dissociated into two H atoms, the absorption energies of the three stable configurations are-1.164 eV,-1.021 eV and-3.11 eV, respectively. It is obvious that the O-V-O1c absorption configuration is the most stable one. The analysis of density of states reveals that the 1s state of H atom interacts with the 2p and 2s states of O atom. The outermost O1c atom of O-terminated (010) surface contains an unsaturated bond, which results in the formation of bonding between two H atoms and O1c atom. As a result, an H2O molecule is formed and an oxygen vacancy on the surface is generated after absorption reaction. By combining experimental observations with simulation calculations, the mechanism of hydrogen reduction of W20O58 can be revealed from a microscopic view.

First-principles study on adsorption mechanism of hydrogen on tungsten trioxide surface

With the development of modern industrial technology, tungsten products prepared from normal tungsten powder cannot meet the demands of industry. The tungsten product produced from ultra-fine tungsten powder exhibits high strength, high toughness, and low metal plasticity-brittleness transition temperature, which greatly improves the performance of materials. Hence, it is necessary to carry out theoretical research on the micro adsorption dynamics during hydrogen reduction of tungsten trioxide to prepare ultra fine tungsten powder. In order to understand crystal characteristics of WO3 and WO3(001) surface characteristics, and to provide beneficial theoretical support for reaction law of hydrogen reduction on the WO3(001) surface, the mechanisms of H atom adsorption on cubic WO3 and WO3(001) surface are studied by the first-principles calculation based on the density functional theory (DFT) plane wave pseudo-potential method. The results show that theoretically calculated band gap of the cubic crystalline WO3 is 0.587 eV. There are two kinds of WO3(001) surfaces, WO-terminated (001) surface and O-terminated (001) surface. The W-O bond length and the bond angle of W-O-W structure change after the geometric optimization of the surface, and thus the surface relaxation is realized. The WO-terminated (001) surface shows n-type semiconductor characteristics while the O-terminated (001) surface shows p-type semiconductor characteristics. Four adsorption configurations of H atoms on the WO-terminated (001) surface and the O-terminated (001) surface, including H-O2c-H, H-O2 cH-O2c, H-O1c-H, and H-O1cH-O1c, are calculated. Among them, the adsorption energy of the H-O1c-H configuration is the smallest (-3.684 eV) with the shortest bond length of H-O bond (0.0968 nm), and hydrogen atoms lose the most of electrons (0.55e), which indicates that the H-O1c-H adsorption configuration is the most stable one. The band gap of the H-O1c-H configuration increases from 0.624 eV to 1.004 eV after adsorption, while the bandwidth of valence band is almost unchanged. The results about the density of states (DOS) reveal that 1s state of the H atom interacts with 2p and 2s states of the O atom. Strong isolated electron peaks are formed to be at about -8 and -20 eV. The outermost O1c atoms of O-terminated (001) surface contain an unsaturated bond, facilitating the bonding between two H atoms and one O1c atom. Thus, two H atoms and one O1c atom form chemical bonds respectively, and an H2O molecule is generated, leaving an oxygen vacancy on the surface after adsorption reaction. By combining experimental observations with simulation results, the mechanism of hydrogen reducing tungsten trioxide can be elaborated profoundly from a micro view.

The effect of trace nickel additive and ball milling treatment on the near-full densification behavior of ultrafine tungsten powder

Ultrafine W–0.1Ni (wt.%) composite powder was prepared by sol-spray drying and subsequent hydrogen reduction process, followed by mechanical ball milling for 5 or 10h. Sintering densification behaviors of the synthesized composite powders both before and after ball milling treatment were investigated, which were compared with those of ultrafine pure tungsten powder fabricated by the same process for the purpose of clarifying the influence of Ni additive. The results show that the addition of trace nickel has a great effect in accelerating the densification rate of ultrafine tungsten. However, near full dense tungsten bulk material can be produced only if the W–0.1Ni powder is subjected to subsequent ball milling treatment. The W–0.1Ni powder ball milled for 5h can be sintered to 99.4% of theoretical density at 1600°C for 2h, which is 1000°C lower than the traditional microscaled tungsten powder sintered for the same density. At the same time, the addition of trace nickel and ball milling substantially reduce the onset temperature of sintering, recrystallization as well as grain growth. It is also found that the microhardness of the sintered tungsten material is highly dependent on its sintering density and grain size.

Ultrafine Tungsten Powder Obtained with Violet Tungsten Oxide Using Circulatory Oxidization-Reduction Method

Violet tungsten oxide is prepared with pure ammonium paratungstate in the argon gas. Tungsten powder is obtained with violet tungsten oxide through deoxidation in the hydrogen gas (Rate of purity: 99.99 %, dew point: -40 °C), and tungsten powder is oxidized in the air. Tungsten oxide is reduced with tungsten powder in the hydrogen gas. The samples are characterized with the laser particle size distribution measuring instrument and field-emission scanning electron microscope. The results show that ultrafine tungsten powder is obtained through circulatory oxidation twice and reduction three times. The percentage of the particle size distribution of ultrafine tungsten powder is 95.73 % between 0.1 μm and 1.0 μm.

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.

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 m 2/g and 4.65 m 2/g, and the oxygen contents are 0.35% and 0.29%, respectively.

Synthesis and Characterization of Ultrafine Tungsten and Tungsten Oxide Nanoparticles by a Reverse Microemulsion-Mediated Method

Ultrafine tungsten and tungsten oxide powders with controllable particle size and structure have been synthesized by a reverse microemulsion-mediated synthesis method. The nanoparticles were characterized by transmission electron microcopy, X-ray diffraction, and thermal analysis. Synthesis conditions, including water-to-surfactant ratio, annealing temperature, water-to-alkoxide ratio, and drying method, have been shown to be critical in controlling the size and structure of tungsten nanoparticles. Using the same synthesis technique, carbon-supported metallic tungsten and tungsten oxide nanoparticles with controllable size and dispersion can also be prepared. The ultrafine nanoparticles prepared by this synthesis method may find interesting applications in various fields such as catalysis, electronics, illumination, gas sensors, and so forth.

Particle size characterization of ultrafine tungsten powder

The aim of this research was to investigate the particle size measurement of as-supplied and lab-milled ultrafine tungsten powders. The ultrafine tungsten powder derived from X-ray diffraction pure WO 3 by hydrogen reduction process under “dry” reduction conditions was milled for 1, 2 and 4 h, respectively. Then the as-supplied and lab-milled tungsten powders were used to perform particle size measurement by laser diffraction method and Fisher sub-sieve sizer (FSSS) method and BET method through adsorption isotherms. Results show that the mean particle size values of as-supplied and lab-milled powders measured by laser diffraction method and FSSS method were misleading because of the defects of the measurement system or unsuitability of measurement theory. And the calculated particle size of d BET values from the formula 6/(S BET density) were smaller than that of by SEM method, which was resulted from the neglect of the effect of surface roughness and pore area on d BET calculation. The fractal dimension D of surface roughness and external surface area S t of pores were obtained from adsorption isotherms, which were used to modify d BET calculation formula. The modified formula was d BET( M) =k (D/2)×6/(S BET density ), where coefficient k was a constant, it was a function of pores area, D was fractal dimension value of surface roughness. As to the ultrafine tungsten powders in this investigation, k was defined as S BET/ S t, so modified d BET(M) may be expressed as 3 D/( S t density ). By the modified formula, the calculated d BET(M) values of four powders in this study were in good agreement with the SEM sizes.

Short note Dynamic compression behaviour of tungsten powders consolidated by plasma pressure compaction

It has been generally believed that the ductility of tungsten can be increased by decreasing grain size to ultrafine levels (∼100 nm). Pure ultrafine grain tungsten powders of various average particle sizes (2·0, 0·7, and 0·16 μm) were consolidated using the rapid consolidation technique, plasma pressure compaction (P2C), to minimise grain growth. Preliminary results indicate that tungsten powders can be consolidated to high density without abnormal grain growth at 2000°C and 50 MPa with less than 3 min of iso thermal holding time in the P2C process. The densities of consolidated specimens were measured using Archimedes'method, and the microstructure was analysed using SEM. The mechanical behaviour of the consolidated specimens was investigated using the compression Kolsky bar. Dynamic compression properties were examined to determine the effect of grain size, grain structure, and density on yield stress at various strain rates.

The preparation and characterization of ultrafine tungsten powder

The preparation and characterization of ultrafine tungsten powder

Preparing ultrafine tungsten powders by H2 plasma reduction

Preparing ultrafine tungsten powders by H2 plasma reduction

Production and characterization of hydrogen-reduced submicron tungsten powders—Part 1: State of the art in research, production and characterization of raw materials and tungsten powders

This paper ∗ ∗ Part 2 of this paper, which will appear in a subsequent issue of this journal, will be devoted to the controlled decomposition of APT and further reduction of the resulting oxides, to yield high quality ultrafine tungsten powder. deals primarily with the production and characterization of fine tungsten powders prepared by conventional hydrogen reduction techniques. The state-of-the-art in the production and characterization of submicron tungsten powders is presented and discussed herein.

Effect of heating on the structure of ultrafine tungsten powders

The physicochemical properties and structure of ultrafine tungsten powders during heating were examined by electron microscopy and x-ray phase analysis. The results showed that the powder lost up to 10% of its weight during annealing in argon and hydrogen. The powders were single crystals consisting of two phases. Relaxation of the powder surface energy occurred during heating in hydrogen while the particle size grew but the structural characteristics remained unchanged. Approximate calculations were derived which indicate that surface diffusion is the principal mass transport mechanism and is accompanied in the early stages by quasiliquid particle coalescence.

Particle size analysis techniques for metal powders and frits used in thick-film pastes : R. P. Anjard, Sr.Microelectron. J.13 (5), 16 (1982)

The aim of this research was to investigate the particle size measurement of as-supplied and lab-milled ultrafine tungsten powders. The ultrafine tungsten powder derived from X-ray diffraction pure WO3 by hydrogen reduction process under “dry” reduction conditions was milled for 1, 2 and 4 h, respectively. Then the as-supplied and lab-milled tungsten powders were used to perform particle size measurement by laser diffraction method and Fisher sub-sieve sizer (FSSS) method and BET method through adsorption isotherms. Results show that the mean particle size values of as-supplied and lab-milled powders measured by laser diffraction method and FSSS method were misleading because of the defects of the measurement system or unsuitability of measurement theory. And the calculated particle size of dBET values from the formula 6/(SBETdensity) were smaller than that of by SEM method, which was resulted from the neglect of the effect of surface roughness and pore area on dBET calculation. The fractal dimension D of surface roughness and external surface area St of pores were obtained from adsorption isotherms, which were used to modify dBET calculation formula. The modified formula was dBET(M)=k(D/2)×6/(SBETdensity), where coefficient k was a constant, it was a function of pores area, D was fractal dimension value of surface roughness. As to the ultrafine tungsten powders in this investigation, k was defined as SBET/St, so modified dBET(M) may be expressed as 3D/(Stdensity). By the modified formula, the calculated dBET(M) values of four powders in this study were in good agreement with the SEM sizes.