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No description available.

Azospirillum brasilense reduced nitrate in W-containing Mo-limited medium, but at lower rates than in W-free medium. However nitrate reduction by Paracoccus denitrificans was completely suppressed under the same conditions. Nitrite reductase activity of growing A. brasilense was negatively affected by tungstate. Nitrite accumulation was shown to be the result of an inhibitory effect of nitrate on nitrite reductase activity. Both resting whole cell and cell-free extract preformed nitrite reductase activities were equally affected by increasing levels of tungstate. Preformed nitrate reductase activity of the cell-free extract was shown to be more sensitive to increasing concentrations of tungstate than whole cell activity, suggesting that the cytoplasmic membrane served as a protective barrier against tungsten inactivation of nitrate reductase.

In this study, the possibilities to produce metallic glass as binder phase in hard metal by means of powder metallurgical methods have been investigated. The aim of the study was to do an initial investigation about metallic glass as alternative binder phase to cobalt in hard metal. Production of samples with metallic glass forming alloys and an amorphous powder as binder phase in hard metal by means of quenching and hot pressing have been performed. Moreover, mechanical alloying of metallic glass forming powder to achieve amorphicity has been performed. The samples and powders were analyzed by means of XRD, LOM, STA, SEM and EDS. The results showed that no glass formation of the binder phase was achieved by quenching, hot pressing or mechanical alloying. However, interesting information about glass formation by means of metallurgical methods was obtained. The main conclusion was that production of metallic glass by means of metallurgical methods is complicated due to changes in the binder phase composition throughout the production process as well as requirements of high cooling rates when quenching and high pressures when hot pressing.

Layered metal sulfides MoS2 and WS2 exhibit highly anisotropic surface chemistry. Adsorption of molecules is stronger on the atomic layer edges than on atomic planes. The edges are catalytically active in the petroleum hydrodesulfurization, while the layer planes are inert. Dispersing MoS2 and WS2 on the nanometer scale can also lead to the onset of photocatalytic properties due to the bandgap tuning by quantum confinement. In this work, we aim at determining how the adsorption on surface sites is altered for the nanoparticles compared to the bulk sulfides (micron-sized particles). A comparative study of the MoS2 and WS2 nanoparticles and bulk materials is done by attempting the adsorption of small molecules (N2, CO, acetone, and acetonitrile) to probe the surface sites. MoS2 and WS2 nanoparticles were synthesized by thermal decomposition of the metal hexacarbonyls in presence of sulfur in high-boiling solvents. The size range is 5-30 nm from Transmission Electron Microscopy. Transmission Infrared Spectroscopy was used to monitor the spectra of the probe molecules. A dedicated experimental setup has been constructed that consists of a high-vacuum chamber with a base pressure of 5×10-7 Torr. At the lowest achievable temperature of the sample (-145°C), N2, CO, and acetone were found to not adsorb strongly enough to be retained in vacuum on these materials. Acetonitrile was found to adsorb on these materials at -145°C and to desorb between -90°C and -50°C. The nanomaterial samples adsorbed significantly more acetonitrile than the corresponding bulk sulfides, as judged by the infrared signals intensity. Qualitatively, adsorbed acetonitrile species on nanodispersed and bulk sulfides are the same. It is likely that most of the adsorbed acetonitrile observed is physisorbed as ice or adsorbed on the sulfur-terminated terraces. At the final stages of desorprtion, distinctly different adsorbed species are seen whose CN stretching IR bands are shifted to higher frequencies. It is likely that these minority species are at monolayer or submonolayer coverages. The exact nature of the species requires further studies. / Department of Chemistry

A substantial raise in recuperator effectiveness has been established in the past by improving the fabricating and joining configurations regarding the manufacturing of compact recuperators. Further advancement of state-of-the-art recuperators requires providing for increased temperatures and pressures. 1bis can only be achieved by incorporating high temperature materials into the recuperator design. Although many high temperature materials have been identified in past research, less of these can be utilized in new concepts due to difficulties regarding fabricating and joining. However recently, in an independent study, a tungsten-copper alloy was identified through detailed material selection methods as a suitable material for high temperature applications. The validity of tungsten-copper regarding fabricating and joining, to establish a leak tight structure still needs to be demonstrated. The aim of the study is to carry out a comprehensive review of existing recuperator technologies and design methodologies as well as to investigate the manufacturability of tungsten-copper for use in a recuperator design of limited size. More specifically, the objectives entail the following: (1) The comprehensive review of existing recuperator technologies and recuperator design methodologies, (2) The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material and (3) The determination of the feasibility of fabrication of the design and the applicability of the selected W -eu alloy in the design. The fabrication technique that is presented in the design entailed the use of 2.Irm tungsten carbide drill bits to machine the correct recuperator profile, while the recuperator unit was joined by utilizing a mechanical fastening system. Although diffusion bonding was initially identified as the ideal joining technique for the recuperator of this research, restrictions and limitations relating to the use of diffusion bonding has lead to the identification of a fastening system as the technique used. Evaluation of the fabricated recuperator revealed that several factors were outside the initially specified values, inter alia the flatness tolerance of recuperator plate geometries and machined slots precision. These factors contributed to a leaJdng recuperator structure when tested. The most likely contributing factors for the latter relate to non-conforming tolerances achieved in the fabricated design, residual stresses induced by the machining process as well as design issues relating to the recuperator plate geometries. The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material, requires re-evaluation. Similar work will ensure a design of a high quality when provision is made for advanced surface fmishing of machined parts (notably the recuperator plate geometries), slight modifications to the design as well as stress relieving of machined components for the purpose of eliminating any residual stresses thatJnight be present. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

A substantial raise in recuperator effectiveness has been established in the past by improving the fabricating and joining configurations regarding the manufacturing of compact recuperators. Further advancement of state-of-the-art recuperators requires providing for increased temperatures and pressures. 1bis can only be achieved by incorporating high temperature materials into the recuperator design. Although many high temperature materials have been identified in past research, less of these can be utilized in new concepts due to difficulties regarding fabricating and joining. However recently, in an independent study, a tungsten-copper alloy was identified through detailed material selection methods as a suitable material for high temperature applications. The validity of tungsten-copper regarding fabricating and joining, to establish a leak tight structure still needs to be demonstrated. The aim of the study is to carry out a comprehensive review of existing recuperator technologies and design methodologies as well as to investigate the manufacturability of tungsten-copper for use in a recuperator design of limited size. More specifically, the objectives entail the following: (1) The comprehensive review of existing recuperator technologies and recuperator design methodologies, (2) The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material and (3) The determination of the feasibility of fabrication of the design and the applicability of the selected W -eu alloy in the design. The fabrication technique that is presented in the design entailed the use of 2.Irm tungsten carbide drill bits to machine the correct recuperator profile, while the recuperator unit was joined by utilizing a mechanical fastening system. Although diffusion bonding was initially identified as the ideal joining technique for the recuperator of this research, restrictions and limitations relating to the use of diffusion bonding has lead to the identification of a fastening system as the technique used. Evaluation of the fabricated recuperator revealed that several factors were outside the initially specified values, inter alia the flatness tolerance of recuperator plate geometries and machined slots precision. These factors contributed to a leaJdng recuperator structure when tested. The most likely contributing factors for the latter relate to non-conforming tolerances achieved in the fabricated design, residual stresses induced by the machining process as well as design issues relating to the recuperator plate geometries. The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material, requires re-evaluation. Similar work will ensure a design of a high quality when provision is made for advanced surface fmishing of machined parts (notably the recuperator plate geometries), slight modifications to the design as well as stress relieving of machined components for the purpose of eliminating any residual stresses thatJnight be present. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

The use of low-friction coatings on machine elements is steadily increasing, and they are expected to play an important role in the reduction of fuel consumption of future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribofilms, which then cover the coating surface and its counter surface. It is within these tribofilms that sliding takes place, and their properties largely determine the performance. The role of the coating is then not to provide low friction, but to supply support and constituents for the tribofilm. In this thesis, the formation of such tribofilms has been studied for a number of different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, such as increased surface roughness, varied surrounding atmosphere and reduced availability of the tribofilm constituents has been given special attention. For TaC/aC coatings, the formation of a functioning tribofilm was found to be a multi-step process, where wear fragments are formed, agglomerated, compacted and eventually stabilized into a dense film of fine grains. This formation is delayed by a moderate roughening of the coated surface. Coatings based on tungsten disulphide (WS2) are often able to provide exceptionally low friction, but their use is restricted by their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties can be achieved by addition of for example carbon, but the achievable hardness is still limited. When titanium was added to W-S-C coatings, a carbidic hard phase was formed, causing drastically increased hardness, with retained low friction. Titanium oxides in the tribofilms however caused the friction to be high initially and unstable in the long term. In a study of W-S-N coatings, the effects of humidity and oxygen were studied separately, and it was found that the detrimental role of oxygen is larger than often assumed. Low friction tribofilms may form by rearrangement of coating material, but also by tribochemical reactions between constituents of the coating and its counter surface. This was observed for Ti-C-S coatings, which formed WS2 tribofilms when sliding against tungsten counter surfaces, leading to dramatic friction reductions.

The reaction between pentacarbonylbis(trimethylsilyl)ethynetungsten(0), W(CO)5(&amp / #951 / 2-btmse), and triphenylbismuthine, Bi&Oslash / 3, yields pentacarbonyltriphenylbismuthinetungsten(0), W(CO)5(Bi&Oslash / 3), as the sole product. The kinetics of the substitution of btmse by Bi&Oslash / 3 was studied by means of quantitative FT-IR Spectroscopy. The starting complex was prepared photochemically from hexacarbonyltungsten, W(CO)6, in the presence of excess btmse in n-hexane and identified by FT-IR and NMR Spectroscopies. The substitution reactions were performed in cyclohexane solutions at different concentrations of both leaving and entering ligand to observe the dependence of observed rate constant, kobs, on the concentration of entering and leaving ligands and also, at different temperatures to evaluate the activation enthalpy (&amp / #8710 / H&amp / #8800 / ) and the activation entropy (&amp / #8710 / S&amp / #8800 / ). The IR extinction coefficients for CO stretching were determined for both the starting complex W(CO)5(&amp / #951 / 2-btmse), and the product W(CO)5(Bi&Oslash / 3). Quantitative IR Spectroscopy does not show any significant reduction in the total amount of substance (material balance). Formation of W(CO)6 (in small amount) was attributed to the decomposition of the product, W(CO)5Bi&Oslash / 3. From the evaluation of kinetic data, a mechanism was proposed in which the rate determining step is the cleavage of btmse ligand from the starting complex, W(CO)5(&amp / #951 / 2-btmse) and the formation of solvated complex, W(CO)5(solvent). Thus, the reaction is essentially dissociative. The large positive value of activation entropy (&amp / #8710 / S&amp / #8800 / ) and large value of activation enthalpy (&amp / #8710 / H&amp / #8800 / ) are indicative of a dissociative mechanism.

In this study a bidentate ligand containing two ferrocenyl moieties, N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine, was prepared by condensation reaction of ferrocenecarboxyaldehyde and ethylenediamine on refluxing in benzene. The molecule was identified by IR, Raman, UV-VIS, 1H-, 13C-NMR spectroscopies. Then, this bidentate ligand was reacted with pentacarbonylbis-(trimethyl)silylethyne)tungsten(0). The ligand substitution reaction in dichloromethane yielded the new complex, tetracarbonyl [N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine]tungsten(0) (W(CO)4(BFEDA)). This new complex was isolated from the reaction solution in the form of orange crystals and fully characterized by elemental analysis, IR, UV-VIS, MS, 1H- and 13C-NMR spectroscopies. As a bidentate ligand, [N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine] binds the metal atom in the two cis positions in the pseudooctahedral geometry of the tungsten-complex. Electrochemistry of the tetracarbonyl [N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine]tungsten(0) was studied by cyclic voltammetry, and controlled potential electrolysis combined with the UV-VIS spectroscopy. One irreversible oxidation and three reversible oxidations were observed in the cyclic voltammogram. One of these reversible and the irreversible oxidations are attributed to tungsten and the other two reversible oxidations to iron centers. It is found that the two ferrocene groups started communication with each other after formation of tungsten-complex. N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine was also reacted with photogenerated pentacarbonyl(tetrahydrofuran)tungsten(0) complex and the pentacarbonyl [N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine]tungsten(0) (W(CO)5(BFEDA)) as an intermediate on the route to the tetracarbonyl[N,N&#039 / -bis(ferrocenylmethylene)ethylenediamine]tungsten(0) was isolated from the reaction medium in the form of red crystals and fully characterized by, IR, 1H- and 13C-NMR spectroscopies. The conversion of W(CO)5(BFEDA) to the W(CO)4(BFEDA) in dichloromethane by a ring closure mechanism was observed by IR spectroscopy.