Molybdenum-tungsten (Mo-W) alloy sputtering targets are widely utilized in fields like electronics, nanotechnology, sensors, and as gate electrodes for TFT-LCDs, owing to their superior properties such as hightemperature stability, low thermal expansion coefficient, electrical conductivity, and corrosion resistance. To achieve optimal performance in application, these targets’ purity, relative density, and grain size of these targets must be carefully controlled. We utilized nanopowders, prepared via the Pechini method, to obtain uniform and fine powders, then carried out spark plasma sintering (SPS) to densify these powders. Our studies revealed that the sintered compacts made from these nanopowders exhibited outstanding features, such as a high relative density of more than 99%, consistent grain size of 3.43 μm, and shape, absence of preferred orientation.

3Y-TZP ceramics obtained by doping 3 mol.% of Y₂O₃ to ZrO₂ to stabilize the phase transition are widely used in the engineering ceramic industry due to their excellent mechanical properties such as high strength, fracture toughness, and wear resistance. An additional increase in mechanical properties is possible by manufacturing a composite in which a high-hardness material such as oxide or carbide is added to the 3Y-TZP matrix. In this study, composite powder was prepared by dispersing a designated percentage of WC in the 3Y-TZP matrix, and the results were compared after manufacturing the composite using the different processes of spark plasma sintering and HP. The difference between the densification behavior and porosity with the process mechanism was investigated. The correlation between the process conditions and phase formation was examined based on the crystalline phase formation behavior. Changes to the microstructure according to the process conditions were compared using field-emission scanning electron microscopy. The toughness-strengthening mechanism of the composite with densification and phase formation was also investigated.

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• Phase Formation and Mechanical Property of YSZ‒30 vol.% WC Composite Ceramics Fabricated by Hot Pressing
  Jin-Kwon Kim, Jae-Hyeong Choi, Nahm Sahn, Sung-Soo Ryu, Seongwon Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(5): 409.     CrossRef

Tungsten disulfide (WS₂) nanosheets have attracted considerable attention because of their unique optical and electrical properties. Several methods for fabrication of WS₂ nanosheets have been developed. However, methods for mass production of high-quality WS₂ nanosheets remain challenging. In this study, WS₂ nanosheets were fabricated using mechano-chemical ball milling based on the synergetic effects of chemical intercalation and mechanical exfoliation. The ball-milling time was set as a variable for the optimized fabricating process of WS₂ nanosheets. Under the optimized conditions, the WS₂ nanosheets had lateral sizes of 500–600 nm with either a monolayer or bilayer. They also exhibited high crystallinity in the 2H semiconducting phase. Thus, the proposed method can be applied to the exfoliation of other transition metal dichalcogenides using suitable chemical intercalants. It can also be used with highperformance WS₂-based photodiodes and transistors used in practical semiconductor applications.

This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

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• Preparation of W-Ni-Cu Alloy Powder by Hydrogen Reduction of Metal Oxides
  Youn Ji Heo, Eui Seon Lee, Ji Won Choi, Jongmin Byun, Sung-Tag Oh
  Korean Journal of Metals and Materials.2024; 62(5): 334.     CrossRef
• Influence of the initial powder characteristic on the densified tungsten microstructure by spark plasma sintering and hot isostatic pressing
  Ji Young Kim, Eui Seon Lee, Youn Ji Heo, Young-In Lee, Jongmin Byun, Sung-Tag Oh
  Powder Metallurgy.2023; 66(5): 644.     CrossRef

Tungsten carbide is widely used in carbide tools. However, its production process generates a significant number of end-of-life products and by-products. Therefore, it is necessary to develop efficient recycling methods and investigate the remanufacturing of tungsten carbide using recycled materials. Herein, we have recovered 99.9% of the tungsten in cemented carbide hard scrap as tungsten oxide via an alkali leaching process. Subsequently, using the recovered tungsten oxide as a starting material, tungsten carbide has been produced by employing a self-propagating high-temperature synthesis (SHS) method. SHS is advantageous as it reduces the reaction time and is energy-efficient. Tungsten carbide with a carbon content of 6.18 wt % and a particle size of 116 nm has been successfully synthesized by optimizing the SHS process parameters, pulverization, and mixing. In this study, a series of processes for the highefficiency recycling and quality improvement of tungsten-based materials have been developed.

Because of its unique properties, tungsten is a strategic and rare metal used in various industrial applications. However, the world's annual production of tungsten is only 84000 t. Ammonium paratungstate (APT), which is used as the main intermediate in industrial tungsten production, is usually obtained from tungsten concentrates of wolframite and scheelite by hydrometallurgical treatment. Intermediates such as tungsten trioxide, tungsten blue oxide, tungstic acid, and ammonium metatungstate can be derived from APT by thermal decomposition or chemical attack. Tungsten metal powder is produced through the hydrogen reduction of high-purity tungsten oxides, and tungsten carbide powder is produced by the reaction of tungsten powder and carbon black powder at 1300–1700°C in a hydrogen atmosphere. Tungsten scrap can be divided into hard and soft scrap based on shape (bulk or powder). It can also be divided into new scrap generated during the production of tungsten-bearing goods and old scrap collected at the end of life. Recycling technologies for tungsten can be divided into four main groups: direct, chemical, and semi-direct recycling, and melting metallurgy. In this review, the current status of tungsten smelting and recycling technologies is discussed.

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• The Current Status and Securing Strategies of Core Mineral Tungsten Resources
  Dohyun Jeong, Seongmin Kim, Hoseok Jeon
  Journal of the Korean Society of Mineral and Energy Resources Engineers.2023; 60(5): 341.     CrossRef
• Tungsten distribution and vertical migration in soils near a typical abandoned tungsten smelter
  Huihui Du, Yang Li, Dan Wan, Chuanqiang Sun, Jing Sun
  Journal of Hazardous Materials.2022; 429: 128292.     CrossRef

The purpose of this study is to investigate the densification behavior and the corresponding microstructural evolution of tantalum and tantalum-tungsten alloy powders for explosively formed liners. The inherent inhomogeneous microstructures of tantalum manufactured by an ingot metallurgy might degrade the capability of the warhead. Therefore, to overcome such drawbacks, powder metallurgy was incorporated into the near-net shape process in this study. Spark plasma-sintered tantalum and its alloys with finer particle sizes exhibited higher densities and lower grain sizes. However, they were contaminated from the graphite mold during sintering. Higher compaction pressures in die and isostatic compaction techniques also enhanced the sinterability of the tantalum powders; however, a full densification could not be achieved. On the other hand, the powders exhibited full densification after being subjected to hot isostatic pressing over two times. Consequently, it was found that the hot isostatic-pressed tantalum might exhibit a lower grain size and a higher density as compared to those obtained in previous studies.

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• Thermal Stability and Weight Reduction of Al0.75V2.82CrZr Refractory High Entropy Alloy Prepared Via Mechanical Alloying
  Minsu Kim, Hansung Lee, Byungmin Ahn
  journal of Korean Powder Metallurgy Institute.2023; 30(6): 478.     CrossRef

W₂C is synthesized through a reaction-sintering process from an ultrafine-W and WC powder mixture using spark plasma sintering (SPS). The effect of various parameters, such as W:WC molar ratio, sintering temperature, and sintering time, on the synthesis behavior of W₂C is investigated through X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) analysis of the microstructure, and final sintered density. Further, the etching properties of a W₂C specimen are analyzed. A W₂C sintered specimen with a particle size of 2.0 μm and a relative density over 98% could be obtained from a W-WC powder mixture with 55 mol%, after SPS at 1700°C for 20 min under a pressure of 50 MPa. The sample etching rate is similar to that of SiC. Based on X-ray photoelectron spectroscopy (XPS) analysis, it is confirmed that fluorocarbon-based layers such as C-F and C-F2 with lower etch rates are also formed.

Tungsten heavy alloys (W–Ni–Fe) play an important role in various industries because of their excellent mechanical properties, such as the excellent hardness of tungsten, low thermal expansion, corrosion resistance of nickel, and ductility of iron. In tungsten heavy alloys, tungsten nanoparticles allow the relatively low-temperature molding of high-melting-point tungsten and can improve densification. In this study, to improve the densification of tungsten heavy alloy, nanoparticles are manufactured by ultrasonic milling of metal oxide. The physical properties of the metal oxide and the solvent viscosity are selected as the main parameters. When the density is low and the Mohs hardness is high, the particle size distribution is relatively high. When the density is high and the Mohs hardness is low, the particle size distribution is relatively low. Additionally, the average particle size tends to decrease with increasing viscosity. Metal oxides prepared by ultrasonic milling in high-viscosity solvent show an average particle size of less than 300 nm based on the dynamic light scattering and scanning electron microscopy analysis. The effects of the physical properties of the metal oxide and the solvent viscosity on the pulverization are analyzed experimentally.

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• Manufacture of high sensitive Ag-Fe3O4-PDMS nanocomposite pressure sensor through morphology control of conductive filler
  Keunhyuk Ryu, Namhun Kwon, Kun-Jae Lee
  Advanced Powder Technology.2021; 32(7): 2441.     CrossRef
• Grinding behavior of WO3, NiO, Fe2O3 by ultrasonic milling parameters control and preparation of nanocomposite powder
  Keunhyuk Ryu, Kun-Jae Lee
  Advanced Powder Technology.2020; 31(9): 3867.     CrossRef

The gas sensor is essential to monitoring dangerous gases in our environment. Metal oxide (MO) gas sensors are primarily utilized for flammable, toxic and organic gases and O₃ because of their high sensitivity, high response and high stability. Tungsten oxides (WO₃) have versatile applications, particularly for gas sensor applications because of the wide bandgap and stability of WO₃. Nanosize WO₃ are synthesized using the hydrothermal method. Asprepared WO₃ nanopowders are in the form of nanorods and nanorulers. The crystal structure is hexagonal tungsten bronze (MxWO₃, x =< 0.33), characterized as a tunnel structure that accommodates alkali ions and the phase stabilizer. A gas detection test reveals that WO₃ can detect acetone, butanol, ethanol, and gasoline. This is the first study to report this capability of WO₃.

Composites of P25 TiO₂ and hexagonal WO₃ nanorods are synthesized through ball-milling in order to study photocatalytic properties. Various composites of TiO₂/WO₃ are prepared by controlling the weight percentages (wt%) of WO₃, in the range of 1–30 wt%, and milling time to investigate the effects of the composition ratio on the photocatalytic properties. Scanning electron microscopy, x-ray diffraction, and transmission electron microscopy are performed to characterize the structure, shape and size of the synthesized composites of TiO₂/WO₃. Methylene blue is used as a test dye to analyze the photocatalytic properties of the synthesized composite material. The photocatalytic activity shows that the decomposition efficiency of the dye due to the photocatalytic effect is the highest in the TiO₂/WO₃ (3 wt%) composite, and the catalytic efficiency decreases sharply when the amount of WO₃ is further increased. As the amount of WO₃ added increases, dye-removal by adsorption occurs during centrifugation, instead of the decomposition of dyes by photocatalysts. Finally, TiO₂/WO₃ (3 wt%) composites are synthesized with various milling times. Experimental results show that the milling time has the best catalytic efficiency at 30 min, after which it gradually decreases. There is no significant change after 1 hour.

This study focuses on the fabrication of a WC/Co composite powder from the oxide of WC/Co hardmetal scrap using solid carbon in a hydrogen gas atmosphere for the recycling of WC/Co hardmetal. Mixed powders are manufactured by mechanically milling the oxide powder of WC-13 wt% Co hardmetal scrap and carbon black with varying powder/ball weight ratios. The oxide powder of WC-13 wt% Co hardmetal scrap consists of WO₃ and CoWO₄. The mixed powder mechanically milled at a lower powder/ball weight ratio (high mechanical milling energy) has a more rapid carbothermal reduction reaction in the formation of WC and Co phases compared with that mechanically milled at a higher powder/ball weight ratio (lower mechanical milling energy). The WC/Co composite powder is fabricated at 900°C for 6 h from the oxide of WC/Co hardmetal scrap using solid carbon in a hydrogen gas atmosphere. The fabricated WC/Co composite powder has a particle size of approximately 0.25-0.5 μm.

Transition-metal oxide semiconductors have various band gaps. Therefore, many studies have been conducted in various application fields. Among these, methods for the adsorption of organic dyes and utilization of photocatalytic properties have been developed using various metal oxides. In this study, the adsorption and photocatalytic effects of WO₃ nanomaterials prepared by hydrothermal synthesis are investigated, with citric acid added in the hydrothermal process as a structure-directing agent. The nanostructures of WO₃ are studied using transmission electron microscopy and scanning electron microscopy images. The crystal structure is investigated using X-ray diffraction patterns, and the changes in the dye concentrations adsorbed on WO₃ nanorods are measured with a UV-visible absorption spectrophotometer based on Beer-Lambert’s law. The methylene blue (MB) dye solution is subjected to acid or base conditions to monitor the change in the maximum adsorption amount in relation to the pH. The maximum adsorption capacity is observed at pH 3. In addition to the dye adsorption, UV irradiation is carried out to investigate the decomposition of the MB dye as a result of photocatalytic effects. Significant photocatalytic properties are observed and compared with the adsorption effects for dye removal.

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• Photocatalytic Properties of WO₃ Thin Films Prepared by Electrodeposition Method
  Kwang-Mo Kang, Ji-Hye Jeong, Ga-In Lee, Jae-Min Im, Hyun-Jeong Cheon, Deok-Hyeon Kim, Yoon-Chae Nah
  Journal of Korean Powder Metallurgy Institute.2019; 26(1): 40.     CrossRef
• Photocatalysis of TiO₂/WO₃ Composites Synthesized by Ball Milling
  Su-Yeol Yu, Chunghee Nam
  Journal of Korean Powder Metallurgy Institute.2018; 25(4): 316.     CrossRef

This study is carried out to obtain basic data regarding oxidation and reduction reactions, originated on the recycling of waste tungsten hard scraps by oxidation and reduction processes. First, it is estimated that the theoretical Gibbs free energy for the formation reaction of WO₂ and WO₃ are calculated as ΔG_1,000K= -407.335 kJ/mol and ΔG_1,000K = -585.679 kJ/mol, from the thermodynamics data reported by Ihsan Barin. In the experiments, the oxidation of pure tungsten rod by oxygen is carried out over a temperature range of 700-1,000°C for 1 h, and it is possible to conclude that the oxidation reaction can be represented by a relatively linear relationship. Second, the reduction of WO₂ and WO₃ powder by hydrogen is also calculated from the same thermodynamics data, and it can be found that it was difficult for the reduction reaction to occur at 1,027°C, in the case of WO₂, but it can happen for temperatures higher than 1127°C. On the other hand, WO₃ reduction reaction occurs at the relatively low temperature of 827oC. Based on these results, the reduction experiments are carried out at a temperature range of 500-1,000°C for 15 min to 4 h, in the case of WO₃ powder, and it is possible to conclude that the reduction at 900°C for 2h is needed for a perfect reduction reaction.

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• A Basic Study on the Recycling of Wasted Cemented Carbide by the Zn Bath Process(Ⅰ)
  Kyung-Sik Kim, In-Ho Kim, Chan-Gi Lee, Chang-Bin Song
  Journal of the Korean Institute of Resources Recycling.2020; 29(6): 35.     CrossRef