Preparation and high-temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2-based materials

Spent MoSi₂ and MoB were used as raw materials to prepare multilayer MoSi₂/MoB coating on molybdenum by the two-step method of slurry deposition and spark plasma sintering. The results showed dense MoSi₂/MoB coating after sintering while penetrated cracks appeared in MoSi₂ coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi₂/MoB coatings, MoB and Mo₂B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high-temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm²) was obtained for MoSi₂/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi₂ coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi₂/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti-oxidation properties at high temperature.     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo₃Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi₂ grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO₃ gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.     

Stabilization and superconductivity of AlB2-type nonstoichiometric molybdenum diboride by Sc doping

Molybdenum diboride is unique among transition metal diborides because it exists in both hexagonal (AlB₂-type) and rhombohedral structures. However, it is difficult to stabilize the superconducting AlB₂-type phase, which requires either extreme synthesis condition or suitable chemical doping. Here we report the structural and physical properties of Sc-doped nonstoichiometric molybdenum diborides (Mo_0.95Sc_0.05)_1-xB₂ and (Mo_1-ySc_y)_0.71B₂ prepared by the common arc melting method. The AlB₂-type phase is found to form over wide ranges of 0 ≤ x ≤ 0.29 and 0.025 ≤ y ≤ 0.30 for the first time, and bulk superconductivity with T_c up to 7.9 K is observed. T_c increases with increasing x in the (Mo_0.95Sc_0.05)_1-xB₂ series, but evolves nonmonotonically with varying y in the (Mo_1-ySc_y)_0.71B₂ series. Despite this contrast, T_c of both borides follows nearly the same linear dependence on the electron-phonon coupling constant, suggesting that it is mainly controlled by the electron-phonon interaction. In addition, the stabilization of AlB₂-type structure is attributed to the decrease in the number of d electrons as a consequence of Sc doping, which suggests that a similar effect may be achieved by substituting Mo with other d electron-poorer metal elements.     

Study on the reduction of commercial MoO3 with carbon black to prepare MoO2 and Mo2C nanoparticles

A simple method was developed to synthesize MoO₂ and Mo₂C nanoparticles via controlling nucleation and growth in carbothermic reduction of commercial MoO₃ with carbon black. It was found that the appropriate C/MoO₃ molar ratio for preparation of Mo₂C was 2.8, and the carbothermic reduction process followed the sequence: MoO₃ → transport phase (TP) → MoO₂ → Mo₂C. It was revealed that the most crucial issues for controlling number of produced particles of product were migration of Mo source and aid of nucleating agent, which can be achieved by using MoO₃ and carbon black as starting materials. MoO₂ nanosheets with the thickness of 12 nm and lateral size of 60 nm, as well as Mo₂C nanoparticles with particle size of 30 nm were prepared via reduction of MoO₃ with carbon black. However, MoO₂ and Mo₂C produced via reducing MoO₃ by other kinds of carbon sources (activated carbon, graphite) or gas reductants (10% CH₄/H₂, CO) had much larger particle sizes of a few micrometers, which were tens of times than those using MoO₃ and carbon black, due to the too small amount of formed nuclei. The effects of C/MoO₃ molar ratio (0.5-2.8), molybdenum sources and carbon sources on the reaction mechanisms were investigated in detail.     

Hydrodesulfurization of dibenzothiophene over supported and unsupported molybdenum carbide catalysts

A series of γ-Al₂O₃ supported molybdenum carbides [carbided Mo/γ-Al₂O₃ (MCS), Co-Mo/γ-Al₂O₃ (CMCS), and Ni-Mo/γ-Al₂O₃ (NMCS)] and unsupported molybdenum carbide (MCUS) were prepared by the temperature-programmed carburization of their corresponding molybdenum nitrides with 20 % CH₄/H₂. XRD and SEM studies show that unsupported molybdenum carbide catalyst possesses a typical crystalline Mo₂C (FCC structure), while supported molybdenum carbide catalysts possess highly dispersed surface molybdenum carbide species on an alumina oxide support. The results of dibenzothiophene (DBT) hydrodesulfurization over molybdenum carbide catalysts show that the reactivity is strongly dependent on the type of catalyst. Supported molybdenum carbide catalysts possess a higher reactivity than the unsupported molybdenum carbide catalyst. In addition, Co or Ni promoted, supported molybdenum carbide catalyst possesses a higher reactivity than the unpromoted, supported molybdenum carbide catalyst. The reactivity, which is also dependent on the reaction conditions, increases with increasing reaction temperature and pressure and contact time. The CO uptakes of the molybdenum carbide catalysts correlate well with overall activity (total rate) for DBT hydrodesulfurization. The major reaction product is biphenyl, with cyclohexylbenzene next in abundance regardless of the type of catalysts and reaction conditions. It was also found that the molybdenum carbide catalysts exhibit stable initial reactivity due to the stable and weak acidic characteristics of these catalysts.     

Control of multiphase evolution and Al-deficiency in reactive-sintered MoAlB ceramics with excessive Al

MoAlB is a ternary boride of MAB phases with strong resistance to oxidation and ablation at service temperature, thanks to preferential Al diffusion to form a protective oxide scale. However, excessive Al were often necessary in sintering of MoAlB ceramics, suggesting that a liquid-phase densification might occur thus leading to complex microstructures. By quantitative SEM analysis, ~17 mol.% Al₂O₃ phase was found common in MoAlB ceramics. The liquid-phase of Al-Mo-B-O facilitates the direct conversion from MoB to MoAlB before in situ formation of Al₂O₃. An intermediate Mo₃Al₈ phase competes with layered conversion, which limits the insertion rate of Al into B-B layers of MoB to form abundant Al-deficient stacking-faults. Intermetallic Mo₃Al₈ phase precipitates further in the intergranular regions parallel to the crystallization of Al₂O₃, leaving the reprecipitation of remaining Al. Both layered-conversion and intergranular oxides can improve ablation behavior for MoAlB ceramics, as well as fracture toughness and compressive strength.     

pH Dependent formations of dinuclear molybdenum(V) and incomplete cubane molybdenum(IV) complexes with nitrilotriacetate

Nitrilotriacetato oxomolybdenum(V) dinuclear complex (NH₄)₄[cis-Mo₂O₄(nta)₂] · 7H₂O (1) (H₃nta = nitrilotriacetic acid) and its incomplete cubane-type trinuclear molybdenum(IV) complex [Mo₃O(OH)₃(Hnta)₃] · Cl · 3H₂O (2) are isolated in a weak acidic and an acidic solutions, respectively (pH 4.5 and 1.0). The nitrilotriacetate ligand in the anion 1 or the cation 2 displays tridentate mode through the nitrogen atom and the oxygen atoms of β-carboxy groups. Unusual protonations of bridged oxygen atoms in 2 have not effect on the skeleton of molybdenum(IV) cap Mo₃O unit. There are obvious dissociations of molybdenum(V) complexes based on ¹³C NMR observations in solution.     

Incorporation limit of MoO3 in sodium borosilicate glasses

Optimizing the concentration of molybdenum incorporated in a borosilicate glass matrix is essential in the vitrification of high-level radioactive waste. However, the incorporation limit of MoO₃ in fundamental borosilicate systems has been rarely correlated with the local structure of the molybdenum cations. This study investigates the variations in the incorporation limit of MoO₃ in ternary sodium borosilicate glass upon varying the B₂O₃/(SiO₂ + B₂O₃) ratio (i.e., B)_. The incorporation limit of MoO₃ was less than 3 mol% in the low-B region (B < 0.7), where molybdenum cations mainly existed as [MoO₄]²⁻. However, when B was higher than 0.85, the incorporation limit was higher than 6 mol%, and the Raman spectra indicated the presence of octahedrally coordinated molybdenum cations, essential to stabilize the Mo–O–Mo linkage. The variation in the local structure of molybdenum cations can be explained by the available amount of non-framework cations compensating for the negative charge near [MoO₄]²⁻. These results allow the development of glass compositions with a high incorporation limit of MoO₃ simply by controlling the local structure near the molybdenum cations.     

Microstructure,mechanical properties,and corrosion resistance of Mo2(Fe,Ni)B2-based cermets prepared by reaction boronizing sintering

Mo₂(Fe,Ni)B₂-based cermets were successfully prepared by reaction boronizing sintering strategy, and their phase transformation, microstructure evolution, mechanical properties, and corrosion resistance were investigated. Mo₂(Fe,Ni)B₂ was formed by the reaction between (Fe,Ni)₂B and Mo during solid-phase sintering. In the temperature range of 1010–1270 °C, extremely rapid densification occurred, and nearly full densification was obtained at the sintering temperature of 1270 °C. Mo₂(Fe,Ni)B₂-based cermets demonstrated superior mechanical properties with transverse rupture strength of 2140 ± 35 MPa, Rockwell hardness of 83.9 ± 0.1 HRA, and fracture toughness of 22.5 ± 0.6 MPa m^1/2. Moreover, corrosion current density of Mo₂(Fe,Ni)B₂-based cermets was about four orders of magnitude lower than that of Mo₂FeB₂-based cermets, which indicates excellent corrosion resistance.     

Effect of reactant composition on the production of MoSi2 by self-propagating high-temperature synthesis

The effect of reactant composition, particle size of silicon, density of powdered compacts, and reaction atmosphere on the characteristics of molybdenum disilicide produced from molybdenum and silicon powders by self-propagating high-temperature synthesis, was studied in a pressurized reaction chamber at 1.5 bar. The atomic ratio of silicon to molybdenum (Si/Mo) was changed from 1.0 to 2.6 in order to investigate the effect of reactant composition on the characteristics of self-propagating high-temperature synthesis. Stable combustion was observed for the values of atomic ratios of silicon to molybdenum from 1.8 to 2.2 and SHS-produced material consisted of a uniform and single-phased MoSi₂. In the meantime unstable combustion such as oscillatory, spinning, and surface combustion was detected for the values of atomic ratios of silicon to molybdenum less than 1.8 or larger than 2.2. SHS-produced material under unstable combustion includes the impurities of Mo₅Si₃, Mo₃Si, unreacted Mo and Si resulting from the layered or reacted-on-surface structures, which give lower degree of reaction and possibly poor electrical properties of heating element MoSi₂. The value of criterion α suggested by Shkadinskii et al. to differentiate stable combustion from unstable one, is found to be 0.74 for producing molybdenum disilicide by self-propagating high-temperature synthesis. Stable combustion was detected for the values of α greater than 0.74 (α>0.74) to give the uniform and single-phased product while unstable combustion was observed for the values of α less than 0.74 (α<0.74) to result in a non-uniform and multiphase product. This critical value will help the industry to produce uniform and high-purity molybdenum disilicide by self-propagating high-temperature synthesis processes.     

Magnesothermic synthesis of disperse molybdenum carbide in sodium carbonate melts

The physicochemical aspects of the synthesis of molybdenum carbide powders through the magnesothermic reduction of molybdenum oxide in sodium carbonate melts are considered. The reduction reactions are thermodynamically characterized. The composition of the ionic melt is shown to influence the phase composition of the products. In sodium carbonate melts Mo₂C is formed, while sodium chloride melts under the same conditions yield molybdenum powders. The product yield reaches 97% with an about 30% excess of the reducing agent relative to the stoichiometry. Minor metal concentrations are within ～2%. The particle sizes of the powders are determined. The specific surface area of molybdenum carbide powders is 5.53 × 10⁵ m^?1; that of molybdenum is 20.19 × 10⁵ m^?1.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

Polycrystalline Mo₄Y₂Al₃B₆ ceramic (92.84 wt% Mo₄Y₂Al₃B₆ and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10^?6 K^?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm²/s at 25 °C to 4.33 mm²/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g^?1·K^?1, 11.73 W·m^?1·K^?1, and 0.66 × 10⁶ Ω^?1·m^?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Synthesis and Properties of MoSi2–MoB–SiC Ceramics

MoB and SiC particulate reinforced MoSi₂ matrix composites were synthesized in situ from Mo, Si, and B₄C powder mixtures by self‐propagating high‐temperature synthesis (SHS). The SHS MoSi₂–MoB–SiC products were vacuum hot‐pressed (HPed) at 1400°C for 90 min to fabricate high‐density (> 97.5% relative density) bulk composites. Microstructure refinement and improvements in the Vickers hardness and fracture toughness of the HPed composites were observed with increasing B₄C content in the reaction mixture. The HPed composite of composition MoSi₂–0.4MoB–0.1SiC exhibited grain size of 1–5 μm, Vickers hardness of 12.5 GPa, bending strength of 537 MPa, and fracture toughness of 3.8 MPa.m^1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi₂ composites could be promising candidates for structural applications.     

Contact interaction and hot pressing of ZrB2-MoSi2 in CO/CO2 atmosphere

ZrB₂-15 vol% MoSi₂ ceramics were hot pressed in CO/CO₂ atmosphere in the 1700–1900^oС temperature range. During hot pressing, MoSi₂ decomposes into Mo and Si and the phase composition of the as-sintered ceramic results in ZrB₂, (Zr, Mo)B₂, SiC, SiO₂, and MoB. Contact melting between ZrB₂ and MoSi₂ was observed at 1800^oC, corresponding to the formation of (Zr, Mo)B₂. Ceramics obtained at1800–1850^oС had ∼ 500 МPа and 200 MPa strength at room at 1800^oC in vacuum, respectively. The thickness of the oxidized scales upon exposing the samples at 1600 ^oC for 120 min was 30–80 µm and depended on the amount of residual MoSi₂ and (Zr, Mo)B₂. The highest oxidation resistance was observed for the ceramic sintered at 1850 °C.     

Pressureless sintering of high-entropy boride ceramics

High-entropy boride ceramics were densified by pressureless sintering. Green densities of the ceramics varied by composition with the highest green density of 53.6 % for (Hf, Nb, Ta, Ti, Zr)B₂. After pressureless sintering, relative densities up to ∼100 % were obtained for (Cr, Hf, Ta, Ti, Zr)B₂ and (Hf, Ta, Ti, V, Zr)B₂. Two compositions, (Hf, Ta, Ti, W, Zr)B₂ and (Hf, Mo, Ti, W, Zr)B₂ contained secondary phases and did not reach full density. All compositions had average grain sizes less than 10 µm and less than 2 vol % of residual B₄C. This is the first report of conventional pressureless sintering of high-entropy boride ceramics powder compacts without evidence of liquid phase formation.     

The adsorption characteristics of two Mo sites on a sulfided MoO3/γ-Al2O3 surface

The surface structure of the oxidized and ammonia dissolved MoO₃/-Al₂O₃ samples and the adsorption characteristics of the sulfided samples were investigated by Laser Raman Spectroscopy (LRS) and Low Temperature Infrared Spectroscopy (LTIR) techniques.It was verified that there were two kinds of coordinated unsaturated Mo sites (denoted as Mo_A(CUS) and Mo_B(CUS)) on the surface and the precursors of Mo_A(CUS) and Mo_B(CUS) were Mo(O) and Mo(T), respectively. It was also concluded that Mo_A(CUS) sites could adsorb NO and CO, while Mo_B(CUS) could adsorb CO only. The surface concentration of Mo_A(CUS) might be far smaller than that of Mo_B(CUS).     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

Effect of thermal treatment on states of molybdenum in Mo/H–ZSM-5 catalyst for methane dehydroaromatization: ESR and UV–VIS study

Valence and coordination states of molybdenum ions formed upon thermal treatment of Mo/H–ZSM-5 catalyst for methane dehydroaromatization in Ar and Ar/CH₄ media at 573–973 K have been studied by ESR and UV–VIS spectroscopy. For comparison, the characteristic ESR spectra of thermolyzed bulk ammonium heptamolybdate have been studied and analyzed in detail. The nature of earlier observed Mo⁵⁺ ions has been verified, and new paramagnetic states of molybdenum in Mo/H–ZSM-5 catalysts have been detected: Mo³⁺ ions, and Mo⁵⁺ ions in tetrahedral coordination with delocalization of unpaired electron to Al and H or Al and N atoms.     

Synthesis of molybdenum nitrides nanosheets by nitriding 2H‐MoS2 with ammonia

Metal nitrides nanosheets possess remarkable physical and chemical properties such as high electrical conductivities, catalytic properties, energy storage, and conversion efficiency. In this paper, molybdenum nitride (Mo₅N₆, MoN, and Mo₂N) nanosheets were synthesized by nitriding and exfoliating the bulk 2H‐MoS₂ via dropping N from ammonia at high temperature. Molybdenum nitride nanosheets with the thickness of dozens of nanometers were prepared successfully under different conditions. It was found that the reaction between MoS₂ and NH₃ began from about 696°C, and reduction products and reaction mechanisms were strongly dependent on the temperature. When there was MoS₂, the generated Mo₅N₆, MoN, and Mo₂N can exist stably at even 820, 1020, and 1120°C, respectively. However, they will decompose progressively after MoS₂ was consumed completely: at 820°C, Mo₅N₆ started to decompose to δ‐MoN; at 1020°C, the phase evolution process of MoN can be described as follows: δ‐MoN→ γ‐Mo₂N→ β‐Mo₂N→ Mo, while at 1120°C, the β‐Mo₂N will transform to Mo.