Effect of ammonium dimolybdate (ADM) on the reduction of molybdenum trioxide

Ammonium dimolybdate ((NH₄)₂Mo₂O₇) or molybdenum trioxide (MoO₃) is used as starting raw materials for manufacturing Mo powders. In the initial step, usually carried out in rotary calciners, (NH₄)₂Mo₂O₇ or MoO₃ is reduced to MoO₂. Agglomeration of powder due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ and due to melting of MoO₃ occurs during this reduction step resulting in several manufacturing issues. The reduction from (NH₄)₂Mo₂O₇ involves an endothermic reaction however, reduction of MoO₃ occurs only by exothermic reaction. It is hypothesized that addition of (NH₄)₂Mo₂O₇ to MoO₃ will decrease agglomeration of powders due to the endothermic reaction involved in the reduction process. The current paper details experiments carried out to verify the hypothesis. MoO₃ containing varying amounts (NH₄)₂Mo₂O₇ were reduced at 550 °C, 650 °C and 750 °C in hydrogen atmosphere. The results show lower agglomeration of powder with addition of (NH₄)₂Mo₂O₇. The thermal analysis results confirm reduction of MoO₃ at lower temperatures with the addition of (NH₄)₂Mo₂O₇.     

B4C/TiB2 composite powders prepared by carbothermal reduction method

B₄C–(10–20 vol%)TiB₂ composite powders have been synthesized with the temperature of 1650–1800 °C by carbothermal reduction process using boron acid, carbon black and TiO₂ powder as the starting materials. B/C mole ratio of the starting materials is ascertained, thermodynamics temperature of the reactions is calculated and the effect of ball milling on the composite powders is discussed. The experimental results indicate that B/C mole ratio of the starting materials and composite powders are 4.4 and 3.98–4.03, respectively. The purity of the gained powders is more than 99 wt%. Wet ball milling eliminates the size of the B₄C/TiB₂ composite powders from 30–40 to 3–5 μm by decreasing the conglomeration of the composite powders. XRD and EDS results show that the composite powders are composed of B₄C and TiB₂.     

Synthesis and hydrogen reduction of nano-sized copper tungstate powders produced by a hydrothermal method

The pure nano-sized copper tungstate (CuWO₄) powders were prepared by hydrothermal method and consequent annealing at 500 °C for 120 min. The thermogravimetric analysis was used to study dehydration processes, and the scanning electron microscopy (SEM) indicated that CuWO₄ particles were mostly spherical in the size range from 60 to 90 nm. Hydrogen reduction at 800 °C for 60 min converted the CuWO₄ to W–Cu composite powders. The hydrogen reduction results showed that nano-sized CuWO₄ particles calcining at 500 °C for 120 min indicated finer microstructure than the other calcination temperatures of 0 °C, 400 °C, 620 °C, 650 °C and 700 °C. W–Cu particles were observed finest and homogeneous in the size range from 90 to 150 nm by SEM images. Homogeneous distribution of W and Cu particles was clearly demonstrated by elemental mapping. Encapsulation of Cu phase by the W phase was observed by EDS and TEM. From FFT and HRTEM images, the orientation relationship of (01-1)_Cu ∥(01-1)_W and a semicoherent interface between W and Cu phases could be observed. A good correlation between the HRTEM image and the calculated lattice misfit (δ) was obtained.     

Precipitation of Cr-rich phases in rapidly solidified Ni–20Al–8Cr (at.%) powders

The evolution of the microstructure in rapidly solidified Ni–20.9Al–8Cr–0.49B (at.%) powders after different continuous and isothermal heat treatments at temperatures up to 1100 °C has been studied by electron microscopy and microanalysis. Powders in the rapidly solidified condition have a dendritic microstructure consisting of Ni₃Al dendrites and a NiAl phase in the interdendritic regions. Chromium is in solid solution in both phases. This microstructure is stable when heating at 10 K min⁻¹ up to 750 °C. When the powders are heated up to 950 °C, partial dissolution of the NiAl phase and the precipitation of very small chromium-rich particles take place.The microstructure of the powders after annealing at temperatures between 750 and 1100 °C for different times is characterised by the dissolution of the β-NiAl phase and the simultaneous precipitation of various Cr-rich phases. α-Chromium, the metastable X-phase, and dark polygonal Cr₅B₃ precipitates have been identified.The segregation of chromium and boron in the form of borides removes these elements from the intermetallic matrix, so the content of both elements should be optimised to preserve their beneficial influence on the ductility of the γ′-Ni₃Al phase.     

Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Microstructure and mechanical properties of Fe3Al-based alloys with strengthening boride precipitates

Five quaternary Fe–Al–B–M (M = Ti, Hf, Zr, V, W) alloys based on Fe₃Al with strengthening boride precipitates were produced by vacuum induction melting. The alloys were investigated with respect to their microstructure and mechanical behaviour up to 1000 °C. The mechanical properties were determined by tensile tests, 4-point-bending tests, high-temperature compression tests up to 1000 °C as well as creep tests at 650 and 750 °C. Microstructural and phase analysis were carried out by light optical microscopy, scanning electron microscopy, X-ray diffraction analysis and differential thermal analysis. The alloys were tested in the as-cast state, after homogenisation at 1200 °C for 48 h and after annealing at 800 °C for 624 h. Compared to a corresponding binary alloy the examined alloys exhibit significantly improved mechanical high-temperature properties as well as stable microstructures without considerable loss of ductility.     

Preparation of Eu-activated strontium orthosilicate (Sr1.95SiO4:Eu0.05) phosphor by a sol–gel method and its luminescent properties

Eu²⁺-activated Sr₂SiO₄ phosphor was successfully synthesized by a sol–gel method using sodium silicate and SrO as the starting materials. The wavelength of the emission peak and the emission intensity of the phosphor powders were influenced by the pre-treating temperature. The maximum emission intensity of the phosphor was found as pre-treated at 1200 °C in air and then heated at 1300 °C in the reducing atmosphere (10% H₂ + 90% He). As the pre-treating temperature was <1200 °C, the composition of the phosphor powder was not uniform, which leads to decrease of the emission intensity, whereas >1200 °C, the decrease of the emission intensity may be caused from the reversible phase transformation of Sr₃SiO₅ → Sr₂SiO₄ at 1300 °C, which also shows the red-shift behavior.     

Carbothermal synthesis of ZrC powders using a combustion synthesis precursor

ZrC powders were synthesized by carbothermal reduction of a combustion synthesized precursor derived from zirconium nitrate, urea, and glucose mixed solution. The results showed that the obtained precursor was comprised of polyporous blocky particles. The precursor powders were subsequently calcined under argon at 1200–1600 °C for 3 h. The transformation of ZrO₂ to ZrC, by adopting this route, occurred at 1300 °C. The preparation of ZrC experienced an intermediate phase of ZrOxCy. ZrC powders synthesized at 1500 °C are characterized by the spherical shape, small particle size (120–180 nm in diameter), low oxygen content (1.4 wt.%) and non-aggregated particles.     

Study on oxidation mechanism and kinetics of MoO2 to MoO3 in air atmosphere

The oxidation mechanism and kinetics of MoO₂ to MoO₃ in air atmosphere from 750 K to 902 K have been investigated in the present work. These results show that temperature has significant effects on the oxidation process. It is found that the produced MoO₃ has a tendency to form a big platelet-shaped particle and the surface appears to be smooth at the high reaction temperature (902 K); while at the low reaction temperature (750 K), the micrographs of final products MoO₃ become rough and irregular. The intermediate product Mo₄O₁₁ will be formed only when the temperature is above 810 K. It is found that the oxidation reaction was controlled by the interface chemical reaction at the reaction interface (from MoO₂ to Mo₄O₁₁) and diffusion (from Mo₄O₁₁ to MoO₃), respectively, by using the dual-interface reaction model in the temperature range of 810 K to 902 K. While in the temperature range of 750 K to 779 K, the oxidation reaction (one-step reaction, from MoO₂ to MoO₃ directly) was controlled by the diffusion model.     

Architecture,optical absorption,and photocurrent response of oxygen-deficient mixed-phase titania nanostructures

The optical absorption and photoelectric properties of oxygen-deficient titania (TiO₂) nanostructures consisting of anatase nanotubes and rutile film layer were investigated. The nanostructures were prepared by electrochemical anodization followed by long-time annealing at four temperatures – 450, 550, 650 and 750 °C. Various characterization techniques, including X-ray photoelectron spectroscopy depth profiling, revealed that elemental stable zones (structural regions in which the concentrations of O and Ti are stable) formed within the TiO₂ nanostructures at high annealing temperatures (650 and 750 °C) have O/Ti atomic ratios significantly less than 2. A direct relationship between oxygen vacancy concentration and annealing temperature was established on the basis of this finding. Measurement of the optical absorption spectra of the TiO₂ nanostructures revealed a blue-shift in the absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies. This observation is in agreement with the first-principles calculations of the absorption coefficients of anatase Ti_nO_2n?1 and Ti_nO_2n?2 structures, in which the oxygen vacancy concentration can be adjusted by varying the supercell size. The contrary photocurrent responses of the TiO₂ nanostructures under ultraviolet and visible light were measured. A strong photocurrent response under filtered visible light (λ > 500 nm) was found for the TiO₂ nanostructures annealed at 650 and 750 °C, which suggests that the dominant positive effect of oxygen vacancies exceeds the adverse impact of other features associated with thickening of the rutile film layer at high annealing temperatures, such as a reduction in the specific surface area and an increased charge recombination rate.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Effects of R2CO3 (R = Li,Na and K) on the reduction of MoO2 to Mo by hydrogen

In the present work, the influence of different factors (additives and temperatures) on the morphologies and sizes of as-prepared metal Mo powders by the hydrogen reduction of MoO₂ in the temperature range of 1053 K to 1353 K are investigated. When pure MoO₂ powders are reduced, the morphologies and sizes of as-prepared metal Mo powders are nearly kept the same as the raw materials. However, it is found that adding a small amount of additives (Li₂CO₃, Na₂CO₃ and K₂CO₃) into MoO₂ has a great influence on that of prepared metal Mo powders. The particles become uniformed, ultrafine and spherical powders. In addition, the particles size of the as-prepared metal Mo powders will become larger as the increase of reaction temperature due to the grains growth. This work provides a new method for the preparation of uniformed, ultrafine and spherical metal Mo powders.     

Thermal stability of the Pr3Pt4 compound

The temperature stability of the Pr₃Pt₄ compound was investigated by isothermal annealing at different temperatures, X-ray diffraction (XRD) and differential thermal analysis (DTA). It was found that the decomposition reaction Pr₃Pt₄ → PrPt + PrPt₂ took place at temperatures ranging approximately from 360 °C to 830 °C and it was an exothermal reaction. After annealing at 500–750 °C for 7 days, the Pr₃Pt₄ compound decomposes completely into the two neighboring compounds PrPt and PrPt₂.     

Preparation of MoO2 sub-micro scale sheets and their optical properties

MoO₂ sub-micro sheets have been synthesized in a large scale on silicon substrate with MoO₃ and C powders as raw materials using a novel chemical vapour deposition method. The lengths and widths of MoO₂ sheets are in the range of several to dozens micrometers, and the thickness of MoO₂ sub-micro sheets is ～200 nm. Transmission electron microscopy and high-resolution electron microscopy show that the MoO₂ sheets are of single crystal with a monoclinic structure. The sheets exhibit fluorescent emissions at 304.4, 343.5 and 350.6 nm when the 220 nm light excitation is applied at room temperature, and the emissions result from some defects and the electron transition between valence band and conduction band. UV–vis spectrum shows the MoO₂ sub-micro sheets have absorption peaks between 200and 300 nm and the emissions should be attributed to the defect states of MoO₂. Furthermore, the band-gap is estimated to be approximately 4.22 eV. The growth mechanism of the two-dimensional MoO₂ sub-micro scale sheets is also discussed.     

A study on mechanochemical behavior of MoO3–Mg–C to synthesize molybdenum carbide

The influence of Mg value in the MoO₃–Mg–C mixture on the molybdenum carbide formation and the mechanism of reactions during mechanochemical process were investigated. In keeping with this aim, magnesium and carbon contents of the mixture were changed according to the following reaction: 2MoO₃ + (6 − x) Mg + (1 + x) C = (6 − x) MgO + Mo₂C + x CO. The value of x varied from 0 to 6. Differential thermal analysis (DTA) results for sample with stoichiometric ratio (x = 0) revealed that in the early stage, carbon reduced the MoO₃ to MoO₂ and subsequently highly exothermic magnesiothermic MoO₂ reduction occurred after magnesium melting. Also, it was indicated that the exothermic reaction temperature shifted to before magnesium melting in the 11 h-milled sample (x = 0) and all the exothermic reactions happened, simultaneously. According to the experimental findings, molybdenum carbide (Mo₂C) was synthesized in the mixture powder with stoichiometric ratio (x = 0) after 12 h milling process and the type of reactions was mechanically induced self-sustaining reaction (MSR). However, at lower Mg content in the MoO₃–Mg–C mixture (0 < x ≤ 2), the magnesiothermic reduction occurred in MSR mode and activated the carbothermal reaction. Further decrease in Mg value (2 < x ≤ 3) resulted in MSR mode magnesiothermic reaction and gradual carbothermal reduction. In samples with lower magnesium contents, partial molybdenum oxide reduction proceeded through a gradual mode magnesiothermic reaction.     

Mullite/molybdenum ceramic–metal composites

Dense (>98 th%) homogeneous mullite/Mo (32 vol.%) composites with two different Mo average grain sizes (1.4 and 3 μm) have been obtained at 1650°C in vacuum and in reducing condition. Depending on the Mo grain size and processing atmosphere, the K_IC ranges from 4 to 7 MPa m^1/2 and σ_f from 370 to 530 MPa. The MoO₂–2SiO₂·3Al₂O₃–Mo system was found to be compatible in solid state, and a solid solution of ≈4 wt% of MoO₂ in mullite at 1650°C was detected. A solid state dewetting of MoO₂ from the surface of the Mo particle takes place during sintering. It was found that the absence of MoO₂ in the mullite/Mo composites by processing in reducing conditions increases the strength of the metal/ceramic interface and the plasticity of the Mo metal particles, thus strengthening the composite by a crack bridging mechanism. As a result, the K_IC and the σ_f values of the ceramic–metal composite were found to be ≈4 times and ≈2 times higher than the ones corresponding to the mullite matrix.     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.