Showing papers on "Molybdenum published in 2005"

Optimization of Mo-Si-B intermetallic alloys

Abstract: Mo-Si-B intermetallics consisting of the phases Mo3Si and Mo5SiB2, and a molybdenum solid solution (“α-Mo”), have melting points on the order of 2000 °C. These alloys have potential as oxidation-resistant ultra-high-temperature structural materials. They can be designed with microstructures containing either individual α-Mo particles or a continuous α-Mo phase. A compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength. If the α-Mo phase could be further ductilized, less α-Mo would be needed to achieve an adequate value of the fracture toughness, and the oxidation resistance would be improved. It is shown that microalloying of Mo-Si-B intermetallics with Zr and the addition of MgAl2O4 spinel particles to Mo both hold promise in this regard.

Electrodeposition of Manganese and Molybdenum Mixed Oxide Thin Films and Their Charge Storage Properties

Abstract: Manganese and molybdenum mixed oxides in a thin film form were deposited anodically on a platinum substrate by cycling the electrode potential between 0 and +1.0 V vs Ag/AgCl in aqueous manganese(II) solutions containing molybdate anion (MoO42-). A possible mechanism for the film formation is as follows. First, electrooxidation of Mn2+ ions with H2O yields Mn oxide and protons. Then, the protons being accumulated near the electrode surface react with MoO42- to form polyoxomolybdate through a dehydrated condensation reaction (by protonation and dehydration). The condensed product coprecipitates with the Mn oxide. Cyclic voltammetry of the Mn/Mo oxide film-coated electrode in aqueous 0.5 M Na2SO4 solution exhibited a pseudocapacitive behavior with higher capacitance and better rate capability than that of the pure Mn oxide prepared similarly, most likely as a result of an increase in electrical conductivity of the film. Electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy clearly...

Epitaxial growth and properties of MoOx(2<x<2.75) films

Abstract: We report the growth of epitaxial molybdenum oxide (MoOx,2

Molybdenum: biological activity and metabolism

Abstract: Molybdenum and tungsten are available to all organisms, with molybdenum having the far greater abundance and availability. Molybdenum occurs in a wide range of metalloenzymes in bacteria, fungi, algae, plants and animals, while tungsten was found to be essential only for a limited range of bacteria. In order to gain biological activity, molybdenum has to be complexed by a pterin compound, thus forming a molybdenum cofactor. In this article I will review the way that molybdenum takes from uptake into the cell, via formation of the molybdenum cofactor and its storage, to the final modification of molybdenum cofactor and its insertion into apo-metalloenzymes.

The role of surface oxidation in friction processes on molybdenum nitride thin films

Abstract: Molybdenum nitride Mo 2 N films were deposited on 440C steel substrates by reactive magnetron sputtering. The film composition was analyzed with EDX/WDX. X-ray diffraction was used to determine their crystallographic structure. Friction tests, with an alumina ball as a counterpart, were carried out in the temperature range of 20–400 °C. Film surface inside and outside wear track was studied with Raman spectroscopy. We discuss the crucial role of surface oxide layer, created due to thermal and tribo-oxidation, in friction and wear processes. Room temperature friction leads to residual tribo-oxidation within the wear track. In normal humidity conditions, it results in low friction values μ ≈0.4 and very low wear. At higher temperatures, brittle oxide layer is easily worn out and friction increases. Above 250 °C, oxide softening leads to pronounced decrease of friction.

Synthesis and Characterization of Superconducting β-Mo2N Crystalline Phase on a Si Substrate: An Application of Pulsed Laser Deposition to Nitride Chemistry

Abstract: Molybdenum nitride β-Mo2N was synthesized on Si substrates by pulsed laser deposition of molybdenum metal under nitrogen radical irradiation. X-ray diffraction studies using a multiaxes diffractometer revealed that the crystal phase had a tetragonal structure uniaxially oriented with (100) planes parallel to the substrate surface. The composition was determined to be Mo2N0.85 by calibrated X-ray photoelectron spectroscopy. These results demonstrated that the well-crystallized tetragonal-phase β-Mo2N was synthesized by deposition, rather than a cubic γ-phase with randomly distributed nitrogen vacancies. The nitride β-Mo2N was superconducting below 5.2 K.

Atomic Layer Deposition of TaN, NbN, and MoN Films for Cu Metallizations

Abstract: Transition metal nitrides, metal silicides, and metal-silicon-nitrides are considered the most promising diffusion barrier materials for next generation ultra large scale integration (ULSI) microelectronics. The semiconductor industry has long used Ti, Ta, and W based materials, and their material properties have been very well studied. Recently, tantalum-based materials have been attracting particular interest. The barrier properties of materials based on other transition metals have been little studied. In this work, tantalum nitride films were deposited, with four new reducing agents used to reduce tantalum and obtain the desired TaN phase. As well, the deposition of niobium and molybdenum nitride films was investigated. All films were deposited by the atomic layer deposition (ALD) method, which ensures excellent conformality and large area uniformity of the films. The problem in depositing TaN films by ALD is that in volatile tantalum precursors the tantalum usually exists in oxidation state +V which is difficult to reduce to the +III state needed in cubic TaN. The new reducing agents examined in this study were trimethylaluminum (TMA), tert-butylamine (BuNH2), allylamine (allylNH2), and tris(dimethylamino)silane (TDMAS). In addition to reducing tantalum, TMA also acted as a carbon and aluminum source, BuNH2 and allylNH2 as nitrogen sources, and TDMAS as a silicon precursor. ALD of niobium nitride and molybdenum nitride films was studied at lower temperatures than reported earlier. Both NbNx and MoNx films were deposited from the corresponding metal chloride precursors (NbCl5 and MoCl5, respectively) using ammonia as nitrogen source. No additional reducing agent was required. The deposition parameters, compositions, crystallinity, and electrical properties were studied for all deposited films. Barrier characteristics were investigated for Ta(Al)N(C), NbNx, and MoNx films. The work function values were measured for Ta(Si)N films deposited at two different temperatures.

Heterogenization of chiral molybdenum(VI) dioxo complexes on mesoporous materials and their application in catalysis

Abstract: Optically active molybdenum(VI) dioxo complexes bearing hydrosalen derivatives as ligands were synthesized, grafted on the surface of MCM-41 and MCM-48 and examined as catalysts for asymmetric epoxidation. In case of cis-β-methylstyrene and trans-β-methylstyrene moderate enantiomeric excesses of up to 31% can be reached when the reaction is carried out at room temperature. The catalysts can also be applied for non-chiral oxidation reactions.

Flexural Strength and Shear Strength of Silicon Carbide to Silicon Carbide Joints Fabricated by a Molybdenum Diffusion Bonding Technique

Abstract: The capability to form robust SiC-to-SiC joints is needed to enable the fabrication of complex SiC-based structures. In this work, molybdenum foils are used to develop a diffusion bond with SiC during a vacuum heat treatment at 1500°C for 10 h. The bond consists of a central region of Mo 2 C with Mo 5 Si 3 +carbon and/or Mo 5 Si 3 C phases at the SiC interface. Flexure strength determined using a standard 4-point flexural test method (ASTM C1161) over the temperature range from room temperature to 1100°C showed that the molybdenum foil joined SiC failed at a somewhat lower value (263-50 MPa) than similarly tested monolithic chemically vapor deposited (CVD) SiC without any bond (443-197 MPa). Differences in elastic properties and coefficient of thermal expansion between SiC and the phases produced in the molybdenum foil joint region likely result in the formation of slightly larger flaws in the SiC near the joint, which are fracture initiation sites that result in lower flexural strength values for molybdenum-joined SiC. Shear strength results obtained using a double-notched specimen are also compared for the joined and monolithic specimens, which provides fracture strength data that can be used for comparisons. The shear strength values for the molybdenum-joined SiC (264-61 MPa) are found to be within the data scatter of the monolithic material (397-31 MPa). These results indicate that a molybdenum foil diffusion bonding technique can be used to produce joints that are as strong as continuous bars of monolithic SiC in shear-type loading, but they have lower flexural strength values than monolithic SiC. However, the flexure strength values for molybdenum foil-joined SiC are within the range of values or higher than values reported in literature for the joining of SiC using reaction forming, reaction-bonding, polymer precursors, and niobium diffusion bonding.

Ni-Cr-Co alloy for advanced gas turbine engines

Abstract: A wrought age-hardenable nickel-chromium-cobalt based alloy suitable for use in high temperature gas turbine transition ducts possessing a combination of three specific key properties, namely resistance to strain age cracking, good thermal stability, and good creep-rupture strength contains in weight percent 17 to 22 chromium, 8 to 15 cobalt, 4.0 to 9.1 molybdenum, up to 7 tungsten, 1.39 to 1.65 aluminum, 1.50 to 2.30 titanium, up to 0.80 niobium, 0.01 to 0.2 carbon, up to 0.01 boron, up to 3 iron, up to 1.5 tantalum and less than 0.02 zirconium, with a balance of nickel and impurities. Certain alloying elements must be present in amounts according to two equations here disclosed.

Quantum chemical modeling of CO oxidation by the active site of molybdenum CO dehydrogenase.

Abstract: The catalytic mechanism of molybdenum containing CO dehydrogenase has been studied using hybrid DFT methods with quite large chemical models. The recent high-resolution X-ray structure, showing the surprising presence of copper linked to molybdenum, was used as a starting point. A pathway was initially found with a low barrier for CO bond formation and CO2 release. However, this pathway did not include the formation of any SCO2 species, which had been suggested by experiments with an n-butylisocyanide inhibitor. When these SCO2 structures were studied they were found to lead to deep minima, making CO2 release much more difficult. A large effort was spent, including investigations of other spin states, varying the number of protons and electrons, adding water, etc., until a plausible pathway for SC bond cleavage was found. In this pathway a water molecule is inserted in between molybdenum and the SCO2 group. Full catalytic cycles, including electron and proton transfers, are constructed both with and without SC bond formation. When these pathways are extended to two full catalytic cycles it can be understood why the formation of the SC bond actually makes catalysis faster, even though the individual step of CO2 release becomes much more difficult. These results agree well with experimental findings. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 888–898, 2005

Enzymatic and catalytic reduction of dinitrogen to ammonia: Density functional theory characterization of alternative molybdenum active sites

Abstract: We used density functional calculations to model dinitrogen reduction by a FeMo cofactor containing a central nitrogen atom and by a Mo-based catalyst. Plausible intermediates, reaction pathways, and relative energetics in the enzymatic and catalytic reduction of N2 to ammonia at a single Mo center are explored. Calculations indicate that the binding of N2 to the Mo atom and the subsequent multiple proton–electron transfer to dinitrogen and its protonated species involved in the conversion of N2 are feasible energetically. In the reduction of N2 the Mo atom experiences a cycled oxidation state from Mo(IV) to Mo(VI) by nitrogenase and from Mo(III) to Mo(VI) by the molybdenum catalyst, respectively, tuning the gradual reduction of N2. Such a wide range of oxidation states exhibited by the Mo center is crucial for the gradual reduction process via successive proton–electron transfer. Present results suggest that the Mo atom in the N-centered FeMo cofactor is a likely alternative active site for dinitrogen binding and reduction under mild conditions once there is an empty site available at the Mo site. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

A spectroscopic study on the 12-heteropolyacids of molybdenum and tungsten (H3PMo12−nWnO40) combined with cetylpyridinium bromide in the epoxidation of cyclopentene

Abstract: The epoxidation of cyclopentene with hydrogen peroxide catalyzed by 12-heteropolyacids of molybdenum and tungsten (H3PMo12−nWnO40, n = 1–11), 12-tungstophosphoric acid and 12-molybdophosphoric acid combined with cetylpyridinium bromide as a phase transfer reagent was carried out in acetonitrile. Among 13 heteropolyacids investigated, catalyst of H3PMo6W6O40 showed the highest activity, giving a conversion of 60% and a selectivity of 95% in the epoxidation of cyclopentene. The fresh catalysts and the catalysts under reaction condition were characterized by UV–vis, FT-IR and 31P NMR spectroscopy, which has revealed that all of the molybdotungstophosphoric acids were degraded in the presence of hydrogen peroxide to form a considerable amount of phosphorus-containing species. The active species resulted from H3PMo6W6O40 are new kinds of phosphorus-containing species, which is different from {PO4[WO(O2)2]4}3−.