90,425 research outputs found
Studies on Bis(imido) molybdenum complexes containing unsaturated hydrocarbon ligands
Get PDFThis thesis describes the synthesis and characterisation of molybdenum bis(imido) complexes containing unsaturated hydrocarbon ligands. A principal objective of the work was to examine the effect of various imido substituents on the coordination number of the complex and the orientations adopted by olefin and acetylene ligands. Chapter One highlights areas of transition metal chemistry relevant to the thesis, with particular emphasis on the psuedo-isolobal analogy between cyclopentadienyl and imido ligands, A convenient one-pot synthesis of molybdenum bis(imido) complexes of the type Mo(NR)(NR')Cl(_2).DME (R=R'=l-adamantyl, 2-t- BUC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu) is described in Chapter Two. Mo(N-l- adamantyl)(O)Cl(_2).DME has been synthesised, and its structure determined by single crystal X-ray diffraction.- The preparation of olefin complexes Mo(NR)(NR')(C(_2)H(_4))(PMe(_3))n (R=R'=l-adamantyl, n=l; R=R'=2-t-BuC(_6)H(_4), n=2; R=2,6-i-Pr(_2)C(_6)H(_3), R'=t- Bu, n=l) is outlined in Chapter Three. Structural information derived from NMR data has allowed comparison with metallocene-like olefin adducts. Chapter Four describes the synthesis of complexes containing σ-bound phenyl ligands (Mo(NR)(NR')(σ-C(_6)H(_5))(PMe(_3)) (R=R'=l-adamantyl, 2-t-BuC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu)) as potential precursors to benzyne complexes. Chapter Five describes the preparation of diphenylacetylene complexes Mo(NR)(NR')(PhC=CPh)(PMe(_3)), structural information derived from NMR data allows comparison with previously known metallocene-like acetylene complexes. Full experimental details for Chapters Two to Five are given in Chapter Six
XPS spectra of G-Mo<sub>2</sub>C (A)survey; (B) Mo 3d; (C) C 1s and (D) O 1s.
No full text<p>XPS spectra of G-Mo<sub>2</sub>C (A)survey; (B) Mo 3d; (C) C 1s and (D) O 1s.</p
Enhanced flux pinning and formation of Ba4Y2CuMoOy in top-seeded melt growth processed YBa2Cu3O7-d superconductors with Mo additions
No full textThe effect of Mo addition (0-10 wt%) on the superconductivity of top-seeded melt growth (TSMG) processed YBa2Cu3O7-y (Y123) superconductors was studied. The low level Mo addition (<= 1 wt%) led to a small decrease of the superconducting transition temperature (T-c) and increase of the critical current density (J(c)). The J(c) improvement induced by the low level Mo additions appeared as a peak effect at the intermediated magnetic fields and peak position shift to the lower magnetic fields with increasing Mo content. The enhanced flux pinning caused by Mo additions seems to be attributed to the partial Cu substitution by Mo, YBa2(Cu1-xMox)(3)O7-d. The high level Mo additions (2-10 wt%), however, led to a large J(c) decrease and broad superconducting transition due to the formation of low-T-c phases and the increased volume of the non-superconducting Mo-containing phase. The second particle phase formed by the high level Mo additions was identified as Ba4Y2CuMoOy (Mo4211) by x-ray diffraction (XRD) and scanning electron microscopy energy dispersive x-ray (SEM EDX) analysis
Comportamento eletroquimico e caracterização microestrutural dos aços sinterizados e nitretados por plasma contendo MO e Ni
Get PDFDissertação (Mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas.A fabricação de produtos via metalurgia do pó (M/P) é amplamente empregada na indústria, porém em aços sinterizados a presença de poros residuais decorrentes do processso de fabricação afeta o seu desempenho face à corrosão eletroquímica. Aplicou-se um tratamento superficial nas ligas, nitretação por plasma, visando melhorar a resistência à corrosão. Neste trabalho foram produzidas ligas na forma binária por M/P, Fe-5% Mo e Fe-10% Ni as quais foram sinterizadas a uma temperatura de 1250°C durante 2h e posteriormente nitretadas por plasma a uma temperatura de 540°C em uma mistura de 75%N2/25%H2 por 2h. O objetivo deste trabalho é avaliar o comportamento destas ligas sinterizadas, sinterizadas/nitretadas, frente à corrosão. Os ensaios eletroquímicos empregados para avaliar o comportamento das mesmas foram: Ecorr vs tempo, potenciodinâmico, potenciostático, realizados em uma solução de KNO3 1,25M, de onde derivaram os valores para os cálculos da taxa de corrosão. A Microestrutura das ligas sinterizadas e camada nitretada, foram analisadas por Microscopia Eletrônica de Varredura (MEV) e análise de energia dispersíva (EDAX). Entre as amostras de Fe-10%Ni sinterizadas e sinterizadas/nitretadas, não ocorreu diferença significativa nos ensaios potenciodinâmicos. O ensaio potenciostático revelou uma densa camada de óxido formada na liga sinterizada Fe-5%Mo, indicando a sua passivação. Em ambos os casos as amostras nitretadas não exibem uma melhora na resistência à corrosão
A Review of Creep Deformation and Rupture Mechanisms of Cr-Mo Alloy for the Development of Creep Damage Constitutive Equations under Lower Stress
No full textThis paper presents a review of creep deformation and rupture mechanisms of low Cr-Mo alloy for the development of its creep damage constitutive equations under lower stress. The most popular KRH (Kachanov-Robatnov-Hayhurst) formulation was not necessarily developed and calibrated for low stress and can not depict the creep strain accurately under multi-axial state of stress due to the three-dimensional generalization method used. This paper summarizes a critical analysis on the cavity nucleation and growth, and the deformation mechanisms and creep damage evolution characteristics at the temperature ranging for 723 K to 923 K (450 °C to 650 °C), particularly under low stress level (0.2-0.4ογ) in order to form the physical base for the development of creep damage constitutive equation. Moreover, it covers the influence of the stress level, states of stress, and the failure criterion
Electrical, morphological and structural properties of RF magnetron sputtered Mo thin films for application in thin film photovoltaic solar cells
Get PDFMolybdenum (Mo) thin films were deposited using radio frequency magnetron sputtering, for application as a metal back contact material in ‘‘substrate configuration’’ thin film solar cells. The variations of the electrical, morphological, and structural properties of the deposited films with sputtering pressure, sputtering power and post-deposition annealing were determined. The electrical conductivity of the Mo films was found to increase with decreasing sputtering pressure and increasing sputtering power. X-ray diffraction data showed that all the films had a (110) preferred orientation that became less pronounced at higher sputtering power while being relatively insensitive to process pressure. The lattice stress within the films changed from tensile to compressive with increasing sputtering power and the tensile stress increased with increasing sputtering pressure. The surface morphology of the films changed from pyramids to cigar-shaped grains for a sputtering power between 100 and 200 W, remaining largely unchanged at higher power. These grains were also observed to decrease in size with increasing sputtering pressure. Annealing the films was found to affect the resistivity and stress of the films. The resistivity increased due to the presence of residual oxygen and the stress changed from tensile to compressive. The annealing step was not found to affect the crystallisation and grain growth of the Mo films
Characterization and Reactivity of Mo₂C
No full textTwo types of Mo₂C have been investigated: polycrystalline β-Mo₂C and single crystal α-Mo₂C. The β-Mo₂C material was synthesized via a temperature-programmed method, and then characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), CO chemisorption, and N₂ physisorption. The catalytic activity of the β-Mo₂C was tested for cumene hydrogenation under high pressure conditions, and the effect of sulfur and oxygen poisons on cumene hydrogenation was also investigated.
As a complement to the work done on polycrystalline β-Mo₂C, UHV studies of single crystal α-Mo₂C were undertaken to provide fundamental information about a well-characterized Mo₂C surface. The (0001) surface of α-Mo₂C was investigated using XPS and low energy electron diffraction (LEED). It was found that an ion-bombarded surface could be prepared as either Mo-terminated or C-terminated by choosing either low annealing temperatures (~1000 K) or high annealing temperatures (~1500 K), respectively. CO and O₂ adsorption was also studied on α-Mo₂C (0001) using thermal desorption spectroscopy (TDS), XPS, Auger electron spectroscopy (AES), and LEED. Finally, thiophene adsorption was investigated on α-Mo₂C (0001).Ph. D
Melting properties of a simple tight-binding model of transition metals. I. The region of half-filled d-band
No full textWe present calculations of the free energy, and hence the melting properties, of a simple tight-binding model for transition metals in the region of d-band filling near the middle of a d-series, the parameters of the model being designed to mimic molybdenum. The melting properties are calculated for pressures ranging from ambient to several megabars. The model is intended to be the simplest possible tight-binding representation of the two basic parts of the energy: first, the pairwise repulsion due to Fermi exclusion; and second, the d-band bonding energy described in terms of an electronic density of states that depends on structure. In addition to the number of d-electrons, the model contains four parameters, which are adjusted to fit the pressure dependent d-band-width and the zero-temperature pressure-volume relation of Mo. We show that the resulting model reproduces well the phonon dispersion relations of Mo in the body-centered-cubic structure, as well as the radial distribution function of the high-temperature solid and liquid given by earlier first-principles simulations. Our free energy calculations start from the free energy of the liquid and solid phases of the purely repulsive pair potential model, without d-band bonding. The free energy of the full tight-binding model is obtained from this by thermodynamic integration. The resulting melting properties of the model are quite close to those given by earlier first-principles work on Mo. An interpretation of these melting properties is provided by showing how they are related to those of the purely repulsive model
Studies on molecular Oxo and Imido complexes of the group 6 metals and supported chromium oxide polymerisation catalysts
Get PDFThis thesis describes studies directed towards the preparation of bis (imido) complexes of the Group 6 metals containing olefin, phosphine, acetylene, and alkyl ligands with particular emphasis on their relationship with Group 4 bent metallocenes. The polymerisation mechanism of the Phillips catalyst (CrO(_3)/SiO(_2)) is examined using XPS (X-Ray Photoelectron Spectroscopy) and in situ mass spectroscopy. Chapter 1 highlights properties of some of the important ligand classes that are used throughout the remainder of this thesis. Chapter 2 describes a high yield one-pot synthesis of molybdenum bis (imido) complexes of the type Mo(NR')(NR")Cl(_2).DME. In addition a number of attempts to extend this strategy to other metals are described, including a novel synthesis of the chromium complex [Cr(_2)Cl(_9)] [NHEt(_3)](_3). Chapter 3 describes the synthesis, characterisation, and reactivity of the bis (imido) bis (phosphine) complexes Mo(NAr)(_2)(PMe(_3))(_2) and [Mo(N(^t)Bu)(µ-N(^t)Bu)(PMe(_3))](_2). Further studies on bis (imido) olefin complexes of the type Mo(NAr)(_2)(PMe(_3))2(η(^2)-C(_2)H (_2)) and Mo(N(^t)Bu)(_2)(PMe(_3))(η(^2)-C(_2)H(_4)) was undertaken and concentrated on their structural relationship to Group 4 metallocene species. Chapter 4 describes the preparation, structure, and reactivity of some bis (imido) acetylene complexes Mo(NR)(_2)(PMe(_3)) (PhC=CR’) (R= Ar, (^t)Bu; R'= Ph, H). Preparation of bis (imido) alkyls was undertaken in attempts to generate benzyne and alkylidene derivatives. Chapter 5 studies the Phillips polymerisation catalyst using XPS and mass spectroscopy. Model systems were used to probe reactive surface species their use reveals a number of features that are difficult to observe with the actual catalyst. Aspects of molecular chemistry have been examined which are believed to relate directly to the heterogeneous system allowing a possible polymerisation mechanism to be postulated. Chapter 6 gives experimental details for Chapters 2-5. Philip William Dyer (November 1993
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
Get PDFBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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