MoSiB alloys show promise as the next-generation turbine blade material due to their high-temperature strength and oxidation resistance afforded by a protective borosilicate surface layer. Powder processing and reactive synthesis of these alloys has proven to be a viable method and offers several advantages over conventional melt processing routes. Microstructures obtained have well-dispersed intermetallics in a continuous matrix of molybdenum solid-solution (Mo-ss). However, bend testing of pure Mo and Mo-ss samples has shown that, while the powder processing route can produce ductile Mo metal, the hardening effect of Si and B in solid-solution renders the matrix brittle. Testing at elevated temperatures (200°C) was performed in order to determine the ductile-to-brittle transition temperature of the metal as an indication of ductility. Methods of ductilizing the Mo-ss matrix such as annealing and alloying additions have been investigated.

The purpose of this study is to develop catalysts for conversion of synthesis gas (H₂ and CO) to higher alcohols, primarily ethanol and propanol. Crude oil is consumed at a rate of more than 20 million barrels a day in the United States, mainly for producing fuels and chemical feedstocks. However, the total amount of crude oil is limited, and alternative ways of producing alcohols as precursors for chemical feedstocks are desirable. In this study, using a known K/MoS₂/metal oxide catalyst as the starting point, two different approaches were explored to improve catalytic properties: 1) Co promotion on K/MoS₂/mixed metal oxide (MMO) catalysts, and 2) Preparation of K/MoS₂/metal oxide catalysts with molybdenum carbide as a precursor, instead of molybdenum oxide. With respect to Co promotion on K/MoS₂/MMO catalysts, the effect of varying the Co content in the K/Mo-Co/MMO catalysts prepared by a co-impregnation method did not produce significant changes in catalytic acitivities or selectivities. It was due to the premature precipitation of cobalt molybdate during synthesis. Cobalt molybdate precipitation can generally be prevented by using water as a solvent, but this approach is not appropriate for this study because of the use of hydrotalcite-derived mixed metal oxide as the support. Co loadings on K/Mo/MMO-Co catalysts did not change selectivities significantly, either. However, they changed catalytic activities, represented by gas hourly space velocity (GHSV) required to obtain 8% conversion while maintaining high selectivities for higher alcohols. As a result, C ₂₊ alcohol productivities reached 0.01g(alcohol)/g(catalyst)/hr with Co loadings higher than 8%. With respect to using Mo2C as the precursor of Mo species instead of MoO3, comparisons between catalysts with different precursors for Mo species and different pretreatments were investigated. In this study, both K/Mo catalysts supported on MgO and α-Al₂O₃ showed similar tendencies of catalytic activities and selectivities. The highest C₂₊ alcohol selectivities and productivities were obtained on presulfided MoO₃ catalysts on both supports. In comparison of K/Mo ₂C catalysts with different pretreatments, higher C₂₊ alcohol selectivities and lower MeOH selectivities were obtained on presulfided catalysts compared to non-pretreated catalysts.

We have carried out structural studies on bovine pancreatic phospholipase A2 (BPLA2) and two proteins involved in molybdenum cofactor (Moco) biosynthesis pathway. In addition, molecular-dynamics simulations and other analyses have been performed to corroborate the findings obtained from the crystal structures. Crystal structures of the three active-site mutants (H48N, D49N and D49K) of BPLA2 were determined to understand the mechanism by which the mutant H48N is able to catalyze the reaction of phospholipid hydrolysis and to see the effect of the loss of Ca 2+ ion in the active site of D49N and D49K mutants. We found that Asp49 could possibly play the role of a general base instead of His48 in the case of the H48N mutant. In the case of D49N and D49K mutants, the active site of the enzyme is perturbed, whereas the overall tertiary structure of these mutants is intact. In addition, a total of 24 invariant water molecules were identified in all of the crystal structures of BPLA2 available in its archive, PDB. Out of these, four water molecules are essential for the catalytic activity, whereas, the remaining water molecules play a role in the stability of the enzyme. In addition, structural studies on two proteins MoaC and MogA involved in Moco biosynthesis pathway have been carried out. For the first time, crystal structure of MoaC bound with GTP molecule has been reported. The gene id TTHA0341, which is mentioned as MoaB in the CMR database, was annotated as MogA based the comparative analysis of sequences and structures (with the present work and the structures available in the literature). The role of N-and C-termini of MoaB and MogA proteins were proposed that these residues might stabilize the substrate and/or product molecule in the active site. In addition, the residues involved in the oligomerization are compared with MD simulations. The molecular docking studies show that MoaB proteins show more preference to GTP than ATP. The comparison of the two active (MPT and AMP-binding) sites revealed that MPT-binding site is preferred over AMP-binding site for nucleotide binding.

We have deposited bulk, monolayer and few-layers as well as large-scale 2D layered MoS2 thin films by pulsed DC magnetron sputtering from an MoS2 target. MoS2 has gained great attention lately, together with other layered Transition Metal Dichalcogenides (TMDCs), for its unique optical and electrical properties with thickness-dependent bandgap. MoS2 also transitions from an indirect to a direct bandgap when thinned down to monolayer. This is intriguing in the fabrication of novel solar cells and photodetectors. Sputter-deposition has the advantage of producing large-scale, high-quality films, which is paramount for layered MoS2 to be applicable on an industrial level. The quality in terms of crystallinity and c⊥-texture of sputtered bulk MoS2 was evaluated as a function of several deposition process parameters: process pressure, substrate temperature and H2S-to-Ar ratio. X-ray Diffraction (XRD) results revealed that the high substrate temperature of 700 °C together with reactive H2S process gas improved the quality regardless of pressure. However, the quality was slightly improved further with increasing pressure up to 50 mTorr. We also found that the quality improved with increasing temperature up to 700 °C using pure Ar as the process gas. Rutherford Backscattering Spectrometry (RBS) analysis showed that with the addition of H2S the stoichiometry of MoSx improved from MoS1.78 using pure Ar to fully stoichiometric MoS2.01 at 40% H2S in the H2S/Ar mixture. Cross-sectional Transmission Electron Microscopy (TEM) imaging revealed the high-quality 2D layered structure of the MoS2 films and a maximum thickness of 5 nm of c⊥-growth MoS2 before the onset of the undesirable c∥-growth. These results provide a solution with respect to the ongoing challenge of obtaining high quality and good stoichiometry of sputtered TMDC films at elevated temperatures. Formation of monolayer and few-layers MoS2 was confirmed by Raman and Photoluminescence (PL) spectroscopy. The peak separation of the E12g and A1g Raman-active modes for MoS2 monolayer was measured to 19.3 cm-1 on SiO2/Si, increases substantially in the transition to bilayer MoS2 and exhibits bulk values from four layers MoS2 and above. This result serves as a good indicator of monolayer as well as few-layers MoS2 formation. The monolayer film exhibits a strong photoluminescence peak at 1.88 eV owing to its direct optical bandgap, as compared to the indirect one of bilayer and thicker films. X-ray Photoelectron Spectroscopy (XPS) spectra of the monolayer MoSx film indicate successful sulfurization of the molybdenum atoms and absence of residual sulfur. XPS also showed ideal stoichiometric MoS2.03 ± 0.03 of the monolayer film. Furthermore, a uniform MoS2 monolayer was successfully grown on a 4" SiO2/Si wafer, demonstrating the large-scale uniformity that can be achieved by sputter-deposition, making it highly applicable on an industrial level.

No description available.

Thesis: M.S., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1978 / Includes bibliographical references. / M.S. / M.S. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Dissertation submitted in partial compliance with the requirements for a Masters Degree in Technology: Homoeopathy, Durban University of Technology, 2008. / Introduction This study investigated the effectiveness of homoeopathic dilutions of abscisic acid (ABA), molybdenum and allopurinol on inhibiting or promoting the germination of barley seeds (Hordeum vulgare cv. Stirling, ex Caledon, Western Cape, South Africa, 1998 harvest). Recent research involving ABA and seed germination has shown mixed results, with Bruni (2001), finding there to be statistically significant biological effects, but Couchman (2001) not. Objective/Aim/Purpose The purpose of this study was to evaluate the effectiveness of homoeopathic dilutions of ABA, molybdenum and allopurinol (two substances which have an effect on ABA metabolism), especially those above the 10-23 level (Avogadro’s dilution limit), on germination, in light of recent findings. Abscisic acid, a plant hormone and molybdenum, a trace element, both play an essential role in inducing dormancy of the seed. Allopurinol, a therapeutic drug, has also been shown to affect ABA metabolism and therefore seed germination. The study used all three substances individually and in combination, in homoeopathic dilutions ranging from 4CH to 200CH potency. Methodology There were 7 treatments with 5 potencies per treatment (4CH, 9CH, 15CH, 30CH and 200CH). Each potency level for each treatment had a control, which meant there were 5 controls per treatment. The seeds (distally cut) were placed in 9cm Petri dishes (20 seeds in each), with 5 repetitions, 100 seeds per dilution level with one control of 20 seeds. There were thus 600 (120 x 5) seeds per treatment and 4200 seeds in total (600 x 7 treatments). Seeds were germinated in the dark at a constant temperature. Counts were done every 24 hours for 3 days and the data recorded. The criterion for germination was radical emergence. Results The data was analysed statistically using Univariate Analysis of Variance (STATISTICA version 6). The results showed statistically significant interaction between treatments and potencies and a One-Way Anova was then used to analyse each treatment to determine the effectiveness of each potency. Statistically significant differences were noted between potencies for each treatment. From the results it was clear that the most effective treatment for stimulating germination was the treatment utilizing homoeopathic dilutions of allopurinol. The most effective treatment for inhibiting germination was the treatment utilizing ABA in homoeopathic dilutions. The 30CH (10-60) showed a statistically significant effect on the stimulation of germination across almost all treatments, whereas the 15CH (10-30) showed a statistically significant effect in inhibiting germination in most treatments. Conclusion It is evident from the results of this study that all the treatments produced distinct biological effects, whether it be stimulating germination or inhibiting germination in homoeopathic dilution.

This study focuses on the visible light response of hetero-structures of TiO2-graphene- MoS2 for solar energy harvestings. The commercial P25 TiO2 nano-particles, and selfprepared layered reduced graphene oxides (RG) and MoS2 were assembled for the targeted hetero-structure materials as visible-light responsible solar harvesting cocatalysts. The hydrothermal method was applied for nano-material synthesis, the reduction of graphene oxides, and bonding formation. Multiple characterization methods (SEM-TEM, XRD, XPS, UV-VIS, PL, FT-IR, TGA) have been applied to understand the electron-hole pair separation and recombination, and performance tuning in their visible-light photo-catalysis rhodamine B (Rh.B) degradations process Compared to TiO2, an obvious red shift of light absorption (from 3.1 eV to 2.6 eV) of the as-prepared RG-TiO2 was observed by UV-vis analysis, and an enhanced photocatalytic degradation of the Rhodamine B (Rh.B) using the as-prepared RG-TiO2 was also observed in a Xe lamp exposure test. The explication of these two approaches to photocatalytic improvements were concluded as the energy gap changing, the formation of Ti-O-C chemical bonds between TiO2 and RG for charge transfer and the reduction of the band gap, as well as a likelihood of up-conversion photoluminescence mechanism (UCPL). The synthesis temperature was found to be critical factor to control binding formation and agglomeration of nano-materials. The lower and higher temperatures induced ineffective formations of preferable bonding structures and the significant agglomeration. The optimal synthesis temperature was found to be within 120 ℃-150 ℃ in the TiO2-RG system. For better understanding of the Ti-O-C bonding, a heterostructure of TiO2 nanotube arrays with GO (TNA-GO) was synthesized using the Langmuir-Blodgett (LB) assembly method. The band gap of this assemble was very close to the previous TiO2-RG synthesized below 120 ℃, which is very close to that of TiO2 nano-particles. This lead to the conclusion on the significance of the Ti-O-C bonding in the visible-light-responsible photo-catalysis solar harvestings. This study revealed the fundamental mechanisms on the bonding formations and the significant visible-light-response of hetero-structcures between commercial-available, inexpensive and non-toxic TiO2 and layered materials, such as the zero-band-gap graphene and the smaller-band-gap MoS2. This mechanisms understanding will greatly sustain applications of economical-effective and environmental-safe TiO2.

Mass dependent isotope fractionation of stable isotopes between meteorites and planetary materials has been used to assess processes that occurred during formation of Earth and its core. However, thus far little is known about the mass dependent isotope fractionation of Mo in the solar system, and at high temperatures in the Earth, in particular during mantle processes. Molybdenum is a refractory and moderately siderophile element. The processes that might have fractionated Mo in the early solar system include condensation and evaporation of dust grains, metal-silicate segregation, core crystallization, silicate and sulphide melting and aqueous alteration. In order to investigate the processes fractionating Mo isotopes, it is first necessary to assess how much fractionation takes place during mantle melting, estimate the isotopic composition of the bulk silicate Earth, and then make comparisons with primitive and differentiated meteorites. I present double spike Mo isotope data for forty-two mafic and seven ultramafic samples from diverse locations, and nineteen extra-terrestrial samples. The delta^98/95Mo values of all the terrestrial samples (normalized to NIST SRM 3134) exhibit a significant range from +0.53±0.21 to -0.56±0.09‰. The compositions of mid-ocean ridge basalts (MORBs) (+0.03±0.07‰, 2s.d.) and ultramafic rocks (+0.38±0.15‰, 2 s.d.) are relatively uniform and well resolved, providing evidence of fractionation associated with partial melting. In contrast intraplate and ocean island basalts (OIBs) display significant variability within a single locality from MORB-like to strongly negative (-0.56‰). The most extreme values measured are for nephelinites from the Cameroon Line and Trinidade, which also have anomalously high Ce/Pb and low Mo/Ce relative to normal oceanic basalts. The observed relationships between delta^98/95Mo and Ce/Pb, U/Pb and Mo/Ce provide evidence that sulphide plays a critical role in retaining Mo in the mantle and fractionating its isotopic composition in basaltic magmas. If residual sulphides are responsible the Mo isotopic composition, Mo budget of the bulk silicate Earth will be misrepresented by values estimated from basalts. On this basis a revised best estimate of the Mo content in the bulk silicate Earth (BSE) ranging between 251 to 268 ppb is derived, approximately 6 times higher than previously assumed, and similar to the levels of depletion in refractory siderophile elements such as W, Ni and Co. This significantly ameliorates the argument for Mo removal via late stage sulphide extraction to the core. The Mo isotopic composition of the BSE (0.35‰) is distinct from the delta^98/95Mo values found in primitive and iron meteorites. Although Mo isotopic fractionation varies between different phases within a single iron meteorite, and occurs during fractional crystallization in asteroidal cores, most iron meteorites have ddelta^98/95MoSRM3134 (-0.14 to -0.06‰) that are similar to ordinary and CI carbonaceous chondrite (-0.12 to -0.09‰). This range of delta^98/95Moo is not only significantly lighter than the BSE, but also enstatite chondrites, which have delta^98/95Mo values of 0.04 to 0.13‰. Several possible explanations are proposed. (A) Core-mantle differentiation fractionates Mo isotopes. The recently proposed Mo effect of sulphide liquid removal is likely to be minor because this should have generated a light Mo isotope composition for the BSE. However, isotopic fractionation associated with metal-silicate partitioning may be responsible for the heavy Mo in the BSE. (B) A distinct isotopic composition for the late material that contributed Mo to the BSE. Enstatite chondrites (or other putative groups of chondrites with a heavy Mo isotope composition) and sulphur-rich components form the cores of impacting bodies are the most likely candidates that could deliver heavy Mo to Earth. (C) The Mo isotopic composition of the Solar System is heterogeneous in a mass dependent fashion such that heavier Mo isotopes are enriched in the section of the disk from which Earth accreted. There are some difficulties behind each of these models and further work is needed to determine which is correct.

The development of molybdenum - silicon - boron (Mo-Si-B) composites having a combination of high temperature strength, creep, and oxidation residence has the potential to substantially increase the efficiency of gas turbines. The refractory nature of the αMo, Mo3Si (A15), and Mo5SiB2 (T2) phases results in good strength and creep resistance up to 1300°C. At this temperature, the formation of a borosilicate surface scale from the two intermetallic phases is able to provide oxidation resistance. However, realization of these advantages has been prevented by both a high brittle to ductile transition temperature and difficulty in forming the initial surface borosilicate to provide bulk oxidation resistance. This dissertation addresses two factors pertaining to this material system: 1) improvements to powder processing techniques, and 2) development of compositions for oxidation resistance at 1300°C. The processing of Mo-Si-B composites is strongly tied to their mechanical properties by establishing the αMo matrix, limiting impurity content, and reducing silicon supersaturation. These microstructural aspects control the brittle to ductile transition temperature which has traditionally been too high for implementation of Mo-Si-B composites. The processing here built upon the previously developed powder processing with silicon and boron nitrides which allowed for a low oxygen content and sintering of fine starting powders. Adjustments were made to the firing cycle based upon dew point measurements made during the hydrogen de-oxidation stage. Under a relatively high gas flow rate, 90% of the total water generated occurred during a ramp of 2°C /min between 450 and 800°C followed by a hold of 30 minutes. The oxidation resistance of Mo-Si-B composites was studied for a wide range of compositions. Silicon to boron atomic ratios were varied from 1 to 5 and iron, nickel, cobalt, yttria, and manganese were included as minor additions. In all these compositions, the αMo volume fraction was kept over 50% to ensure the potential toughness of the composite. For the oxidized surface glass, a silica fraction of 80 to 85% was found to be necessary for the borosilicate to have a sufficiently high viscosity and low oxygen permeability for oxidation resistance at 1300°C. For the Mo-Si-B bulk composition this corresponds to a Si/B atomic ration of 2 to 2.5. Higher viscosity compositions failed due to spallation of poorly attached, high silica scales. Lower viscosity compositions failed from continuous oxidation, either through open channels or repetitive MoO3 bubble growth and popping. Additionally, around 1% manganese was necessary for initial spreading of the borosilicate at 1300°C. In conjunction with flowing air to prevent MoO3 accumulation, oxidation weight loss rates below 0.05 mg/cm2-hr were measured. Finally, a theory is proposed here to describe the mechanisms responsible for the development of oxidation resistance. This theory involves three stages associated with: 1) generation of an initial surface borosilicate, 2) thickening of the borosilicate layer, and 3) slow parabolic oxidation controlled by the high silica surface scale.