Molybdenum Nanopowders Research Articles

Controllable Preparation of Spherical Molybdenum Nano-Powders by One-Step Reduction of APM in Radio Frequency Hydrogen Plasma

Spherical molybdenum nano-powders were in-situ ultrafast synthesized from ammonium paramolybdate (APM) raw materials in a one-step reduction method by radio frequency (RF) hydrogen plasma. Due to the extreme conditions of the RF plasma torch such as its high temperature and large temperature gradient, the injected raw APM powder was quickly gasified and then reduced into nano-sized metal molybdenum (Mo) powder. The influences of APM powder delivery rate and H2 concentration on the properties of the obtained powders were investigated. Field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), nanolaser particle analyzer, and specific surface area method were used to characterize the morphology, phase, and particle size distribution of the powders. The results showed that the nano-sized Mo powder obtained by hydrogen plasma treatment had a quasi-spherical morphology and an average particle size of about 30 nm. The particle size could be successfully adjusted by varying H2 concentrations. In addition, spherical nano-sized MoO3 powder could be obtained when no H2 was added into the RF plasma.

Synthesis of Molybdenum Nitrides

We have studied the effect of precursors on the synthesis conditions and characteristics of molybdenum nitrides. Mo, MoO3, and MgMoO4 powders were nitrided in flowing ammonia at temperatures in the range 500–800°C. The use of molybdenum nanopowder as a precursor has made it possible to reduce the synthesis temperature and time. We have demonstrated the possibility of direct ammonolysis of the double oxide MgMoO4. Using this molybdate, we have obtained a material with a specific surface area up to 29 m2/g, which is a factor of 2 to 3 larger than that reached by nitriding MoO3. In all cases, the synthesis products consisted of the γ- and β-phases of Mo2N, with cubic and tetragonal lattices, respectively.

Research of the Effect of Nanopowder Additives on Some Main Properties of Lubricants

The possibility of improving the basic properties of a lubricating composition based on Litol 24 lubricant, by adding nanopowders (molybdenum, tungsten carbide, tungsten) obtained from their carbide wastes, has been considered. The structure of nanopowders has been studied. Their X-ray spectrum analysis was carried out It has been shown that the addition of a molybdenum nanomodifier to the base lubricant mixture results in improved tribotechnical properties. The optimal concentration of molybdenum nanopowder was determined to be 0.1 wt%. Also paper contains results of operational tests of lubricant composition. The resulting lubricant composition may be a competitive product in the lubricant market

Consolidation of Molybdenum nanopowders by spark plasma sintering: Densification mechanism and first mirror application

Mo nanopowders are synthesized by ball milling and the following hydrogen reduction of MoO3 powders. The densification mechanism of Mo nanopowders during spark plasma sintering (SPS) is analyzed by the combination of the regression of the experimental data on regular SPS and on the SPS multistep pressure dilatometry based on the continuum theory of the sintering. The Mo mirror with 400 nm average grain size and 93% relative density is fabricated by the SPS. The performance of the mirrors made from a single crystal Mo and from the hydrogen-reduced sintered Mo nanopowder is discussed. The microstructure and optical properties of the mirrors are characterized before and after plasma exposure, and no substantial degradation of the reflectivity was observed.

Oxidation effects on spark plasma sintering of molybdenum nanopowders

AbstractSurface‐oxidized molybdenum nanopowders are compacted by spark plasma sintering (SPS). The oxide impurity behavior is analyzed under various sintering temperatures. The densification mechanism of the nanopowders with a melted oxide phase is identified in situ by regression analysis of the experimental data on the temperature‐dependent porosity change and on the SPS multistep pressure dilatometry. To increase the density of the compacted pellets, the nanopowders with the oxide phase are consolidated by SPS using the two in situ oxide removal methods: carbothermic reduction and particle surface cleaning by the electric current flow through the powders. The advantages and disadvantages of these methods in terms of the density, grain size, and mechanical properties of the final products are discussed.

Optimal concentration of nanostructured powder in protective gas

In the present work, a method is developed for determining the optimal concentration of nanostructured powder in the protective gas in welding by a floating electrode in argon. The theoretical analysis is confirmed by experiments with molybdenum nanopowder, which is introduced in the welding bath through a special device. The apparatus used for surfacing of the sample includes a GSP-2 welding head combined with the specially developed device and a VS-300B power source. In surfacing 12Kh18N10T steel samples, 12Kh18N19T steel welding wire (diameter 1.2 mm) is employed. To ensure a satisfactory weld joint, the dendrite dimensions must be minimized. Stable welding is ensured by transfer of a droplet of electrode metal from the end of the welding wire to the welding bath. Hence, the droplet volume must also be minimized. Before optimizing the concentration of nanostructured powder in the protective gas, the influence of the welding parameters on the microstructure of the surfaced metal is established. The results show that the grain size is smallest with a current of 240–260 A and an arc voltage of 28–30 V. In those conditions, the optimal concentration of nanostructured powder in the protective gas is determined. It is found that the optimal concentration is 20 mg per 1 m of weld seam. The use of different concentrations of nanostructured powder in the protective gas results in different microstructure of the applied metal. When the concentration of nanostructured powder in the protective gas is 20 mg per 1 m of weld seam, the branching of the dendrites is least and the dendrite size corresponds to equilibrium structure. On adding nanostructured powder to the liquid bath, the mechanical properties of the weld joints are increased by 7.5% at +20°C and by 6.5% at +500°C.

METHOD OF DETERMINING THE OPTIMAL CONCENTRATION OF NANOSTRUCTURED POWDERS IN SHIELDING GAS

The paper presents the theoretical and experimental studies to determine the optimal concentration of nanostructured powders in the shielding gas. The objective of this study is the development of a defi nition technique for optimal concentration of nanostructured powders in the shielding gas during welding by consumable electrode in the argon medium. Molybdenum nanopowder (NP Mo) was used to confi rm the calculations used in the experimental studies. The injection of the powder into the weld bath was carried out through the special device. The surfacing of samples was carried out in a pilot plant, which consisted of a welding head GSP-2 with the developed device, the power supply had rated current of 300 A. For surfacing of steel samples (austenitic steel with chemical composition: C – 0.12 %, Cr – 18 %, Ni – 10 %, Ti – 1 %) the welding wire with diameter of 1.2 mm was used (chemical composition: C – 0.12 %, Cr – 18 %, Ni – 9 %, Ti – 1 %,). To ensure the quality of the welded joint during welding, the dimension parameters of dendrites should tend to a minimum. A stable welding process is caused by the transition of electrode metal droplets from the end of the welding wire into the weld bath. Therefore, the volume of the electrode metal droplet should also tend to a minimum. Before the start of the optimization of nanostructured powders concentration in the shielding gas, the eff ect of welding mode parameters by consumable electrode in the argon medium on the microstructure of the weld metal was established. The results of the investigations have shown that the minimum grain size is observed at a current strength of 240 – 260 A and arc voltage of 28 – 30 V. In these modes, the studies were conducted to select the optimum concentration of nanostructured powders in the shielding gas. It was found that the optimum concentration of nanostructured powders-modifi ers in the shielding gas is 20 mg/m of the welded joint. It was established that the use of diff erent concentrations of nanostructured powders in the shielding gas makes it possible to obtain a diff erent microstructure of the weld metal. The most lightly branched dendrites and the equilibrium structure according to the dendrites size are achieved at a concentration of nanostructured powder in the shielding gas of 20 mg/m of the weld. When adding nanostructured powders-modifi ers to a liquid weld bath, the mechanical properties of the welded joints increase as compared to the welding process, without the addition of a powder-modifi er at +20 °C by 7.5 %, at +500 °C by 6.5 %.

Two-Step Synthesis of Tungsten and Molybdenum Disulfides

Efficient two-step technique of tungsten and molybdenum disulfides obtaining from metal nanopowders produced by EEW and elementary sulphur is described. Tungsten and molybdenum nanopowders surface area dependence on wire length is studied. Features of metal and sulphur combustion process are discussed. It is determined sulphur excess in reagents 15 wt.% results in mono-phase metal disulfide formation with small free sulphur concentration in reaction products.

Si/Mo/MWCNT Nanocomposites for Lithium Ion Battery Applications

In this study, silicon/multi-wall carbon nanotube (Si/MWCNT) and silicon/molybdenum/multi-wall carbon nanotube (Si/Mo/MWCNT) composites were produced by high speed planetary ball milling. Produced Si/MWCNT composite containing 50 wt.% Si and 50 wt.% MWCNT and dispersingdi erent amount of molybdenum nanopowders (1 wt.%, 3 wt.% and 5 wt.%) Si/Mo/MWCNT composites were produced by high speed planetary ball milling. Surface morphology of produced composite electrodes was characterized using scanning electron microscopy (SEM) and EDS dot-map analyze was performed to investigate dispersion of MWCNT and molybdenum powders in the composite structure. X-ray di raction (XRD) technique was carried out to investigate structure of produced Si/Mo/MWCNT composites. Electrochemical performance of the electrodes were tested between 50 mV and 1.5 V in CR2016 test cell.

Formation of WS<sub>2</sub>/MoS<sub>2</sub> Particles with Nanoscale Layers for Multipurpose Application

Multipurpose tungsten and molybdenum disulfides with nanoscale layered structure formation are studied. Morphology and particle size distribution of initial tungsten and molybdenum nanopowders produced by electrical explosion of wire and used in metal disulfides synthesis are presented. Mechanism of metal nanopowder and elementary sulphur interaction in SHS condition with WS2/MoS2 nanoscale layered particles formation is considered.

Influence of Nanopowders on Corrosion Resistance of Welded Joints

The paper presents experimental results on influence of tungsten, molybdenum nanopowders and aluminum oxyhydroxide phase introduced into the molten pool on the corrosion resistance of welded joints. It was shown that the nature of nanopowders significantly impacts intercrystalline corrosion of the welding joint.

Study of Tungsten and Molybdenum Nanopowders Interaction with Sulphur in SHS Conditions and Synthesized Product Properties

X-ray and transmission electron microscopy results of initial tungsten and molybdenum nanopowders produced by electrical explosion of wire and used in metal sulfides synthesis is presented. Sulphur excess effect on metal nanopowders combustion with elementary sulphur and phase composition of products is studied. Tungsten and molybdenum disulfides obtained by self-propagating high-temperature synthesis under argon pressure 3 MPa and sulphur excess 15 wt. % in initial mixture are crystallized in layered aggregates with layer thickness 20-30 nm.

Mo and MoO3 powders: Structure and resistance to CO

A molybdenum trioxide micropowder (МоО3 MP) with an average crystallite size of 821nm was obtained by the thermal treatment of an initial molybdenum nanopowder (Mo NP) with an average crystallite size of 150nm.The structure of these powders and their resistance to carbon monoxide action at an increased temperature were studied by X-ray diffraction, X-ray fluorescence, and IR spectroscopy.The Mo NP sample contained 88.25% molybdenum and 11.51% oxygen and, as a whole, showed resistance to CO action at 200°C. However, its crystallites were observed to grow under the specified conditions, and their final average size exceeded 1000nm.The obtained MoO3 MP sample contained 67.50% molybdenum and 32.34% oxygen. Мо2С, Мо and Мо4О11 were identified in addition to МоО3 after a treatment with CO at 200°C. It is concluded that the МоО3 MP is unstable under the action of carbon monoxide at the indicated temperature. It is suggested that a dissociative sorption of CO involving МоО3 occurs.

Coating of Synthesized Molybdenum Nanopowder on Aluminium

Coating of Synthesized Molybdenum Nanopowder on Aluminium

Physicochemical properties and activity of nanopowder catalysts in the hydrodesulfurization of diesel fraction

New hydrofining catalysts based on molybdenum, nickel, and aluminum nitride nanopowders produced by electrical explosion are obtained. Their physicochemical properties are studied. The direct sulfurizing of molybdenum nanopowders with sulfides from straight-run diesel oil fraction is shown to be possible. The high hydrodesulfurization capability of nanopowder catalysts is established.

Mechanical production of tungsten and molybdenum nanoparticles for functional composites

High-purity (∼99.2%) tungsten and molybdenum nanopowders (particle size 30–350 nm) may be obtained by the magnetic-thermal reduction of WO3 and MoO3 in conditions of mechanical activation.

Sintering kinetics analysis of molybdenum nanopowder in a non-isothermal process

Compared with commercial micro-sized molybdenum powder, the sintering of molybdenum nanopowder fabricated by high-energy ball milling and subsequent hydrogen reduction was considerably more activated due to its extended surface area. In this study, the sintering kinetics of a molybdenum nanopowder were evaluated by measuring the linear shrinkage in a non-isothermal process and calculating the activation energy for the initial sintering stage. To reach a linear shrinkage of 2 %, the activation energy of the molybdenum nanopowder sintering was 180 kJ, primarily due to the grain boundary diffusion, while the activation energy of the commercial micro-sized powder sintering was 345 kJ, predominantly due to the volume diffusion.

Thermal stability of aluminum, molybdenum, tungsten, and chromium nanopowders and their mixtures

Our experimental studies have revealed that after the mixing of nanopowders and coarsely dispersed powders, the temperature of the start of oxidation of the mixture was not determined by the corresponding temperature of the thermally less stable component, but took a new constant value. This tendency was explained by the mutual effects of the binary electrical layers formed by redox reactions in the surface and subsurface layers of the particles of the powders being mixed.

Effect of hydrogen annealing on the composition and particle size of molybdenum nanopowders

We have studied the behavior of molybdenum nanopowder consisting of particles with an average size of 23.7 nm. The powder was prepared by hydrogen plasma reduction of molybdenum trioxide and contained considerable amounts of oxygen in the form of amorphous molybdenum oxides and hydroxides. Annealing in hydrogen for 1 h at temperatures from 300 to 1000°C is shown to cause crystallization of oxides, which then vaporize to form the volatile hydroxide MoO2(OH)2. Most of the oxygen is removed by annealing below 700°C. Starting at this temperature, particle growth is observed. The main mechanism behind the coagulation of molybdenum particles is vapor transport. Annealing at 1000°C for 30–50 min in hydrogen with a dew point of −5°C increases the particle size by about one order of magnitude.

Natural gas conversion on ZSM-5 zeolites modified with zirconium and molybdenum nanopowders

The joint promoting action of zirconium and molybdenum nanosized powders on the catalytic properties of ZSM-5 zeolite with different silica ratios in the conversion of natural gas into liquid hydrocarbons was studied. By means of thermal analysis and high-resolution transmission electron microscopy, data on the amount and nature of the coke deposit produced during the reaction on the surface of the Zr-Mo/ZSM-5 catalyst were obtained. It was shown that the addition of zirconium to the molybdenum-containing catalyst leads to enhancement of the catalyst activity and selectivity in the formation of aromatic hydrocarbons from natural gas components. It was found that the maximal amount of aromatic compounds is formed in the presence of zeolite having a silica ratio of 40 and containing 0.5% Zr and 4.0% Mo.