Showing papers on "Molybdenum published in 1999"

Molybdenum Disulphide Lubrication

Abstract: History - early beginnings, range of applications occurrence and extraction - occurrence molybdenum and its compounds - oxides of molybdenum, chemical uses of molybdenum properties of molybdenum disulphide - intercalation compounds, oxidation mechanism of lubrication - fundamentals of friction, load-carrying capacity formation of molybdenum disulphide films - burnishing of soft films, effects of moisture and other vapours on film formation properties of molybdenum disulphide films - friction, effects of moisture and other vapours transfer in lubrication -transfer of molybdenum disulphide, nature and location of the transfer source lubrication by molybdenum disulphide alone -different techniques of use sputtering and other physical deposition processes - effect of substrate, pulsed laser deposition bonded films - types of bonded film, friction and wear properties of bonded films composites - ceramic and inorganic composites, electrical brushes and sliprings use in oils and greases - interaction between molybdenum disulphide and liquids, pastes and dispersions other lamellar solid lubricants -occurrence and properties, transition metal dichalcogenides corrosion and fretting - the chemical environment selection and use - selecting the type of solid lubricant.

Boron-doped molybdenum silicides for structural applications

Abstract: The addition of as little as 1 wt.% (=3 at.%) boron improved the oxidation resistance of Mo5Si3 by as much as five orders of magnitude over a temperature range of 800–1500°C. The mechanism of oxidation protection is the formation of a borosilicate glass scale that flows to form a passivating layer over the base intermetallic. The compositional homogeneity range for T1 (Mo5Si3Bx) phase was determined to be much smaller than that reported previously by Nowotny. Compressive creep measurements show that materials based on the phase assemblage of T1-T2 (Mo5SiB2)–Mo3Si have high creep strengths similar to single phase Mo5Si3. Electrical resistivity of selected compositions was also measured and varied from ≈0.06 mΩ-cm at room temperature to 0.14 mΩ-cm at 1500°C. Temperature coefficient of resistivity (TCR) was estimated to be on the order of 1×10−4 C−1 for most compositions.

Molybdenum silicide based materials and their properties

Abstract: Molybdenum disilicide (MoSi2) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm3). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi2 composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi2-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.

Elasticity, shear strength, and equation of state of molybdenum and gold from x-ray diffraction under nonhydrostatic compression to 24 GPa

Abstract: Lattice strains were measured as a function of the angle ψ between the diffracting plane normal and the stress axis of a diamond anvil cell in a layered sample of molybdenum and gold. The sample was compressed over the range 5–24 GPa and the lattice strains were measured using energy-dispersive x-ray diffraction. As ψ is varied from 0° to 90°, the mean lattice parameter of molybdenum increases by up to 1.2% and that of gold increases by up to 0.7%. A linear relationship between Q(hkl), which is related to the slope of the measured d spacing versus 1−3 cos2 ψ relation, and 3Γ(hkl), a function of the Miller indices of the diffracting plane, is observed for both materials as predicted by theory. The pressure dependence of the uniaxial stress t for gold from this and other recent studies is given by t=0.06+0.015P, where P is the pressure in GPa. The uniaxial stress in molybdenum can be described by t=0.46+0.13P. Using gold as an internal pressure standard, the equation of state of molybdenum depends strongly ...

Passivation of Stainless Steels in Hydrochloric Acid

Abstract: High alloyed stainless steels (Fe20Cr20Ni, Fe20Cr20Ni6Mo, and Fe20Cr20Ni6Mo0.2N) were polarized in 0.1 M HCl + 0.4 M NaCl solution for 10 min and 2 h between {minus}75 and 800 mV vs. saturated calomel electrode (SCE). The composition of the passive films was analyzed by angle-resolved X-ray photoelectron spectroscopy, and showed that oxide particles are formed during the initial stage of the passivation process. It is suggested that they are formed by deprotonation of hydroxide. For prolonged exposure, a uniform oxide film is developed under the hydroxide layer. The composition of the film is strongly dependent on the potential. Molybdenum decreases while Cr, Fe, and Ni increase with the potential. The content of Cr in the hydroxide layer decreases with the potential. {beta}-MoCl{sub 2} is identified in the film on the Mo-containing steels. Chloride is present in both the oxide and hydroxide layers. The film forced at {minus}75 mV(SCE) on the Mo-free alloy contains over 50% more Cl{sup {minus}} compared to the film formed on Mo-containing steels. It is suggested that one of the beneficial effects of Mo in stainless steels is that it forms soluble oxochloro complexes, which thereby lower the amount of Cl{sup {minus}} in the film and makesmore » the steel more resistant to pitting.« less

Structure of the Molybdenum Site of Dimethyl Sulfoxide Reductase

Abstract: Molybdenum K-edge X-ray absorption and Mo(V) electron paramagentic resonance (EPR) spectroscopies have been used to probe the metal coordination in oxidized and reduced forms of both wild-type and a site-directed mutant of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) reductase. We confirm our earlier findings (George, G. N.; Hilton, J.; Rajagopalan, K. V. J. Am. Chem. Soc. 1996, 118, 1113−1117) that the molybdenum site of the oxidized Mo(VI) enzyme possesses one terminal oxygen ligand (MoO) at 1.68 A, four thiolate ligands at 2.44 A, and one oxygen at 1.92 A and that the dithionite-reduced Mo(IV) enzyme possesses a desoxo species with three or four Mo−S at 2.33 A and two different Mo−O ligands at 2.16 and 1.92 A. Mo(V) EPR indicates the presence of one exchangeable oxygen ligand, most likely an Mo−OH, in the signal-giving species, probably originating from the MoO of the oxidized enzyme (Em8.5(IV/V) = +37 mV, Em8.5(V/VI) = +83 mV). The addition of dimethyl sulfide, in the reverse of the physiological...

New routes to transition metal nitrides: and characterization of new phases

Abstract: Transition metal nitrides form a class of materials with unique physical properties which give them varied applications, as high temperature ceramics, magnetic materials, superconductors or catalysts. They are commonly prepared by high temperature conventional processes, but alternative synthetic approaches have also been explored, more recently, which utilize moderate-temperature conditions. For example, high surface area γ-Mo 2 N nitride powders (fcc phase) are prepared from commercial oxide MoO 3 through a topotactic transformation process. Of prime importance is the nature of the precursor, because it may yield new nitride phases unattainable by other synthetic routes. A novel promising method to nitride synthesis has been developed using sulfides as starting materials. The ammonolysis reaction has been applied first to the preparation of two binary molybdenum nitrides: Mo 5 N 6 (filled 2H-MoS 2 structure) and δ-MoN (NiAs-type structure) from MoS 2 , and then extended to other metals such as W, Cr or Ti, as well as molybdenum- and tantalum-based ternary systems. Fine reactive molybdenum sulfide precursor powders (S g ≥200 m 2 g –1 ) have been synthesized in thiocyanate melt. On the other hand, alkali metal ternary oxides offer potential as nitridation precursors. For example, a binary nitride Nb 4 N 5 (defect NaCl-type structure) results from ammonolysis of sodium or potassium niobates whereas LiNb 3 O 8 is transformed into a mixed valent ternary nitride LiNb 3 N 4 (filled 2H-MoS 2 structure). Another illustration of the Li + inductive effect is given in the direct synthesis of LiMN 2 from Li 2 MO 4 (M=Mo, W). The nitrides Mo 5 N 6 , δ-MoN and Nb 4 N 5 show superconducting behavior at T<12 K.

Substrate temperature effects on the splat formation, microstructure development and properties of plasma sprayed coatings: Part II: case study for molybdenum

Abstract: The effects of substrate temperature on plasma sprayed molybdenum splat morphology, deposit microstructure development, and properties have been investigated. The molybdenum splat morphology changes from a fragmented (splashed) to a more contiguous disk shape with increasing substrate or deposit surface temperature. Furthermore, the flattening ratio of the splats prepared on glass is significantly higher than those prepared on steel or molybdenum substrates. With an increase in substrate temperature the deposit exhibits improved lamellar structure with less interlayer pores and fine particle debris (arising from splashing). The oxygen content in the deposit increases with substrate temperature associated with increased surface oxidation subsequent to deposition. The thermal conductivity, hardness and intesplat contact area are enhanced with increased substrate temperature. The fracture characteristics change gradually from interlamellar to translamellar, indicating enhanced interlamellar bonding among splats. The residual stress changes gradually from tensile to compressive with increasing temperature, its magnitude depending on the substrate materials and associated thermal mismatch stresses. A higher substrate temperature dramatically improves the adhesion and bonding of the splats and, therefore, enhances the physical and mechanical properties of the coatings.

A manganese molybdenum phosphate with a tunnel: hydrothermal synthesis, structure and catalytic properties of (NH3CH2CH2NH3)10H3O)3(H5O2)Na2[MnMo12O24(OH)6(PO4)4(PO3OH)4][MnMo12O24(OH)6(PO4)6(PO3OH)2] · 9H2O

Abstract: A manganese molybdenum phosphate, (NH3CH2CH2NH3)10(H3O)3(H5O2)Na2[MnMo12O24(OH)6 (PO4)4(PO3OH)4][MnMo12O24 (OH)6(PO4)6(PO3OH)2]·9H2O, has been hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction. The structure of this compound may be considered to be two [Mo6O12(OH)3(PO4)2(HPO4)2]7- units bonded together by a manganese atom, although several P–O groups are not protonated on account of coordination to a Na+ cation. One-dimensional tunnels were formed in the solid. A probe reaction of the oxidation of acetaldehyde with H2O2 using this compound as catalyst was carried out in a liquid–solid system, showing that the manganese molybdenum phosphate has high catalytic activity in the reaction.

Nitride compound semiconductor light emitting element and its manufacturing method

Abstract: A nitride compound semiconductor light emitting element is made by stacking a metal layer made of one of elements: palladium (Pd), scandium (Sc), vanadium (V), zirconium (Zr), hafnium (Hf), tantalum (Ta), rhodium (Rh), iridium (Ir), cobalt (Co) and copper (Cu), and another metal layer made of one of elements: titanium (Ti), nickel (Ni), molybdenum (Mo), tungsten (W) and magnesium (Mg), to increase the adhesive strength of its electrodes with a semiconductor layer, reduce the contact resistance of the electrodes to improve the ohmic characteristics, and improve the external quantum efficiency by combination of thin-film metals with a transparent electrode.

Preparation and characterization of sno2 and moox-sno2 nanosized powders for thick film gas sensors

Abstract: This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

Molybdenum Metabolism in Plants

Abstract: Among the micronutrients essential for plant growth and for microsymbionts, Mo is required in minute amounts. However, since Mo is often sequestered by Fe- or Al-oxihydrox-ides, especially in acidic soils, the concentration of the water-soluble molybdate anion available for uptake by plants may be limiting for the plant, even when the total Mo content of the soil is sufficient. In contrast to bacteria, no specific molybdenum uptake system is known for plants, but since molybdate and sulfate behave similarly and have similar structure, uptake of molybdate could be mediated unspecifically by one of the sulfate transporters. Transport into the different plant organs proceeds via xylem and phloem. A pterin-bound molybdenum is the cofactor of important plant enzymes involved in redox processes: nitrate reductase, xanthine dehydrogenase, aIdehyde oxidase, and probably sulfite oxidase. Biosynthesis of the molybdenum cofactor (Moco) starts with a guanosine-X-phos-phate. Subsequently, a sulfur-free pterin is synthesized, sulfur is added, and finally molybdenum is incorporated. In addition to the molybdopterin enzymes, small molybdopterin binding proteins without catalytic function are known and are probably involved in the storage of Moco. In symbiotic systems the nitrogen supply of the host plant is strongly influenced by the availability of Mo in soil, since both bacterial nitrogenase and NADPH-dependent nitrate reductase of mycorrhizal fungi are Mo enzymes.

Catalyst and process for preparing 1,3-propanediol

Abstract: A solid, particulate catalyst composition comprising an active nickel component in which the nickel constitutes from 25 to 60 wt% of the catalyst composition; a molybdenum component in which the molybdenum constitutes from 5 to 20 wt% of the catalyst composition; and from 10 to 50 wt% of a binder material comprising at least one of oxides of silicon, and silicates and oxides of zinc, aluminium, zirconium, magnesium and calcium, each of the aluminium, calcium and zinc present in an amount no greater than 2 wt%; and a process for the selective hydrogenation of 3-hydroxypropanal to 1,3-propanediol in aqueous solution, using said catalyst composition.

Pseudoelastic beta titanium alloy and uses therefor

Abstract: A nickel-free beta-titanium alloy containing, in weight percent, between 10.0 and 12.0 percent molybdenum, between 2.8 and 4.0 percent aluminum, between 0.0 and 2.0 percent chromium and vanadium and between 0.0 and 4.0 percent niobium. This alloy possesses pseudo-elastic properties associated with stress-induced martensitic transformation and is suitable for medical uses.

The structural role of metal-organonitrogen subunits in the molecular manipulation of molybdenum oxides

Abstract: Although solid state metal oxides are of both fundamental and practical interest, the designed synthesis of such materials remains an elusive goal. However, valuable synthetic guidelines may be derived from a consideration of the plethora of Nature's remarkable materials which contain composites of molecules or microstructures in which inorganic components coexist with organic components. The presence of organic subunits can profoundly influence the crystallization of the inorganic microstructure, offering a powerful tool for the design of novel materials. In the specific case of molybdenum oxide phases, metal-organoamine molecules, fragments, an even polymeric coordination complex cations may be exploited in the self-assembly of complex hierarchical materials.

Anodically deposited manganese-molybdenum oxide anodes with high selectivity for evolving oxygen in electrolysis of seawater

Abstract: Manganese-molybdenum oxide electrodes were prepared by anodic deposition on an IrO2-coated titanium substrate at a constant current density of 600Am−2 from baths containing 0.2M MnSO4 and 0–0.1M Na2MoO4 at 90∘C and pH 0.5. These electrodes were characterised for oxygen evolution in the electrolysis at 1,000Am−2 in 0.5M NaCl solution at 30∘C and pH 8 or 12. The most active and stable oxygen evolving anode exhibited 100% efficiency for oxygen evolution, and an efficiency of 98.5% for over 1,500 h at pH 12 and of 96.5% for over 2,800 h at pH 8 of continuous electrolysis. X-ray diffraction measurement and XPS analysis indicated that the deposits consist of a nanocrystalline single γ-MnO2 type phase, and manganese and molybdenum in the deposits are in the Mn4+ and Mo6+ states. The electrochemical studies showed that the manganese-molybdenum oxide electrodes drastically reduced the electrocatalytic activity for chlorine evolution to the undetectable level, resulting in 100% efficiency for oxygen evolution, although the addition of molybdenum slightly increased the oxygen overpotential.

High surface area silicon carbide doped with zirconium for use as catalyst support. Preparation, characterization and catalytic application

Abstract: Zirconium doped SiC with a surface area from 88 to 200 m 2 g −1 was synthesized using the shape memory concept method followed by calcination in air at a temperature of ≤480°C. The material obtained was composed of β-SiC and small ZrO 2 particles dispersed throughout the material matrix and a significant amount of an amorphous phase containing Si, Zr and O. Molybdenum oxycarbide, the active isomerization phase, supported on such a material displayed a similar behavior to that obtained on pure SiC for the n -heptane isomerization reaction. A comparison made with the molybdenum oxycarbide catalyst supported on pure ZrO 2 showed that the Zr doped SiC was not simply made of silicon carbide coated with a layer of ZrO 2 on the surface but probably an amorphous phase containing Si, Zr and O which displays a similar behavior as pure SiC.

Molybdenum substitutional doping and its effects on phase transition properties in single crystalline vanadium dioxide thin film

Abstract: Molybdenum (Mo) doped vanadium dioxide thin films were synthesized using a Mo striped vanadium (V) target during pulsed laser ablation process. The film structure was characterized by high resolution x-ray diffraction, x-ray rocking curve and Rutherford backscattering/channeling measurements. The results show that the full width at half magnitude of the x-ray rocking curve is as narrow as 0.0074°, comparable to that of the (0001) sapphire substrate, 0.0042°, in this study. The ratio of the aligned-to-random backscattered yield reaches 5%, implying that the growth is that of the single crystalline epitaxy. The result of angular scans for both V and Mo atomic channelings reveals that Mo atoms successfully take sites of the V sublattice as a substitutional dopant. It has been noted that the degradation of the phase transition properties of the film upon doping is closely related to the conductivity in the semiconductor phase.

Liquid-phase catalytic oxidation of unsaturated fatty acids

Abstract: Liquid-phase catalytic oxidation of oleic acid with hydrogen peroxide in the presence of various transition metal/metal oxide catalysts was studied in a batch autoclave reactor. Azelaic and pelargonic acids are the major reaction products. Tungsten and tantalum and their oxides in supported and unsupported forms were used as catalysts. Alumina pellets and Kieselguhr powder were used as supports for the catalysts. Tungsten, tantalum, molybdenum, zirconium, and niobium were also examined as catalysts. Tertiary butanol was used as solvent. Experimental results concluded that tungsten and tungstic oxide are more suitable catalysts in terms of their activity and selectivity. The rate of reaction observed in the case of supported catalysts appears to be comparable or superior to that of unsupported catalysts. In pure form, tungsten, tantalum, and molybdenum showed strong catalytic activity in the oxidation reaction; however, except for tantalum the other two were determined to be economically unfeasible. Zirconium and niobium showed very little catalytic activity. Based on the experimental observations, tungstic oxide supported on silica is the most suitable catalyst for the oxidation of oleic acid with 85% of the starting oleic acid converted to the oxidation products in 60 min of reaction with high selectivity for azelaic acid.

The role of molybdenum in Mo-doped V-Mg-O catalysts during the oxidative dehydrogenation of n-butane

Abstract: A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium–magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V–Mg–O mixed oxides (23.8 wt% of V2O5) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. SBET, XRD, FTIR, FT-Raman, XPS, H2-TPR. MgMoO4, in addition to Mg3V2O8 and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V5+-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V–Mg–O catalysts. The incorporation of MoO3 favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO3 and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V–Mg–O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.