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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Some properties of vanadium and niobium pentafluorides

Cavell, Ronald George January 1960 (has links)
The reactions of various amines with vanadium and niobium pentafluoride have been studied. Pyridine and ammonia reacted with vanadium pentafluoride reducing vanadium from the pentavalent to the tetravalent state and forming monosolvated complexes of vanadium tetrafluoride. The products isolated were pyridinetetrafluorovanadium IV, (Py) VF₄, and amminotetrafluorovanadium IV, (NH₃) VF₄. Ethylenediamine also reduced vanadium pentafluoride to the tetravalent state but formed a more highly solvated denvative of vanadium tetrafluoride. The product was identified as tris (ethylenediamine) tetrafluorovanadium IV and has been formulated as the octahedral complex [V(en)₃] F₄. Niobium pentafluoride reacted with ammonia to form a 2:1 adduct of niobium pentafluoride, diammonopentafluoroniobium V, (NH₃) ₂NbF₅. The product formed on reaction of ethylenediamine with niobium pentafluoride had the composition NbF₅ (en)₁.₆. It has been considered as a highly solvated 1:1 adduct ethylenediaminepentafluoroniobium V. No reduction of niobium has been observed in amine reactions. The infrared spectra and magnetic suceptibilities of the complexes have been measured and the dissociation pressure of diamminopentafluoroniobium V has been determined. The solubilities and melting points of the complexes were also investigated. The pentafluorides of vanadium and niobium, in contrast to the highest chlorides of the same metals, are not solvolysed by the amines studied here. This common behaviour of fluorides, coupled with an apparent maximum coordination number of six for vanadium has been used to explain the reduction of vanadium pentafluoride by the amines. The reduction of vanadium occurs because the association of vanadium pentafluoride in the liquid state has saturated the coordination number of vanadium, so that any solvated product formed by the amines has an unstable coordination number which must be lowered. The fact that solvolysis, which provides a method of reducing the coordination of the central atom without reducing its valence state, cannot occur with fluorides, requires that the reduction in coordination number of vanadium must proceed through an alternate mechanism with concomitant reduction of the valence of vanadium. This interpretation is consistent with the behaviour of niobium pentafluoride which forms solvated derivatives in which niobium has increased its coordination number to seven or eight, the probable maximum coordination number of niobium. Structures have been suggested for the products of the amine reactions. The properties of the pyridine and ammonia derivatives of vanadium pentafluoride, (Py)VF₄ and (NH₃) VF₄, plus the improbable coordination of five for vanadium in the monomer suggests that the compounds are polymers. The most probable structure is a long chain of VF₄. Base units linked by fluorine bridge bonds such as have been proposed to explain association of vanadium pentafluoride. The ethylenediamine derivative of vanadium pentafluoride, [V(en)₃] F₄, is probably a monomeric compound, for there is no need to postulate association to increase the coordination number of vanadium to six. The many complex ions formed by ethylenediamine suggest that it is reasonable to postulate the formation of the [V(en)₃]⁺⁴ ion. This is the first hexacoordinate cationic complex of tetravalent vanadium, with the exception of vanadyl complexes, to be reported. The niobium pentafluoride complexes (NH₃) ₂NbF₅, (en)NbF₅ may be septacoordinate monomers, isostructural with iodine heptafluoride, or octacoordinate fluorine bridged polymers. The proposed structures are based only on the few qualitative properties that are available for the compounds. Complete structure determinations are required for verification of the structures proposed. A nuclear magnetic resonance study of vanadium pentafluoride has yielded only the F¹⁹ chemical shift of vanadium pentafluoride of -335 ppm relative to trifluoroacetic acid. The broad nuclear resonance absorption obtained has been explained in terms of rapid fluorine exchange occuring in conjunction with quadrupole broadening due to the electric quadrupole moment of vanadium. / Science, Faculty of / Chemistry, Department of / Graduate
22

Etude de la microstructure et des transitions de phases électroniques et cristallines de couches épitaxiales de VO₂ déposées sur différents substrats / Synthesis, structural and physical characterizations of phase transition thin films for micro and nanoelectronic applications.

Thery, Virginie 10 November 2017 (has links)
Les travaux de recherche présentés dans ce manuscrit concernent l'étude du rôle des déformations (épitaxiale et d'origine thermique) sur les transitions structurales et électriques du dioxyde de vanadium. A cet effet, nous avons synthétisé des films minces de VO₂ par évaporation à faisceau d'électrons et par ablation laser. La géométrie des déformations est contrôlée en modifiant, d'une part, la nature des substrats et, d'autre part, l'épaisseur des dépôts. Dans le cas de la croissance sur des substrats de saphir (Al₂ O₃ ) orientés (001), le fort désaccord de réseau entraîne une croissance par coïncidence de domaine, de sorte que les déformations résiduelles résultent exclusivement du désaccord de coefficient de dilatation entre la couche et le substrat. L'étude de la transition structurale par diffraction des rayons X et l'étude de la résistivité électrique via un dispositif 4 pointes ont montré que la déformation en tension selon l'axe cᵣ conduit à une augmentation de la température de transition (> 68◦ C). L'apparition d'une phase intermédiaire a été mise en évidence au cours de l'étude structurale en température. La croissance sur des substrats de TiO₂ orientés (001) et (111) est caractérisée par un désaccord de réseau de plus faible (∼ 1%) avec une épaisseur critique de 4 nm, à partir de laquelle des dislocations sont créées en vue de relaxer l'énergie élastique. L'étude des transitions électriques et structurales a mis en évidence que l'évolution des transitions résulte d'une compétition entre les déformations épitaxiales, les déformations d'origine thermique et la présence de lacunes d'oxygène à l'interface. / The research presented in this manuscript deals the study of the effect of strain (epitaxial or thermal) on the structural and the electrical transitions of vanadium dioxide. VO₂ thin films have been synthesized by e-beam deposition and Pulsed Laser Deposition methods. The strain geometry is controlled by modifying, on the one hand, the nature of the substrates and, on the other hand, the thickness of thin films. In the case of (001) sapphire substrates (Al₂ O₃ ), the important lattice mismatch leads to a domain matching epitaxial growth mechanism, so that the residual strain solely result from the film/substrate thermal expansion mismatch. The study of the structural phase transition, using X-ray diffraction, and the study of the metal-insulator transition, using a 4-probes device, showed that the tensile strain along the cᵣ axis leads to an increase of the transition temperature (> 68◦ C). The appearance of an intermediate phase was demonstrated during the study of the structural phase transition. Growth on (001)- and (111)-TiO₂ substrates is characterized by a weaker lattice mismatch (∼ 1%), with a critical thickness of 4 nm, from which dislocations are created to relax the elastic energy. The study of electrical and structural transitions has shown that the evolution of transitions results from a competition between epitaxial distorsion, thermal distorsions and the presence of oxygen vacancies at the interface.
23

Surface studies of thin films with a focus on potentially protective films on vanadium

Asunskis, Daniel John January 1900 (has links)
Doctor of Philosophy / Department of Chemistry / Peter M.A. Sherwood / Thin films can be created on the surface of a metal, protecting it from oxidation and corrosion. Phosphate films have historically been a common choice for these corrosion resistant films. In this dissertation, the oxidation of vanadium metal by water and atmosphere is studied. Also, a series of phosphate films on the surface of vanadium metal were created and are studied as potential corrosion resistant films. Lastly, an independent study identifying the oxidation state of copper in a biological sample is carried out. To characterize these thin films, X-ray Photoelectron Spectroscopy (XPS) is employed. The reaction of vanadium metal with the atmosphere and distilled, de-ionized, water is studied. The core level and valence band results are explored and compared to calculated valence band spectra for some vanadium oxides. The etching of vanadium metal and reaction of the etched metal with a phosphoric acid solution are studied. Synthesized vanadium phosphate compounds serve as model compounds for the analysis of a phosphate coating created on the surface of vanadium metal by the reaction of vanadium metal with phosphoric acid by a newly developed bench top method. The core level and valence band regions for the compounds and coating are discussed along with cluster and band structure calculations for interpretation. The variation in the coating on vanadium metal by biasing the metal at different potentials during reaction is also studied. Coatings are also created on vanadium metal using different forms of phosphorus oxy-acid. An analysis of the various coatings is performed by XPS and accompanied by predictive calculations. In an additional study, the oxidation state of copper in a biological compound is identified. The analysis makes use of satellite features commonly seen in XPS to make the determination. A discussion of the origin of these features and the energy of the shifts is given, along with the results for the other core level XPS regions for the compound.
24

A multinuclear magnetic resonance study of vanadium (V) complexes and equilibria

Harrison, A. T. January 1986 (has links)
No description available.
25

Non-aqueous polyvanadate chemistry

Bakri, Ridla January 1998 (has links)
No description available.
26

Supercritical fluids as solvents and antisolvents

Webster, Jeremy M. January 2000 (has links)
No description available.
27

ELECTROMAGNETIC TRANSITIONS IN VANADIUM-51 AND MANGANESE-53

Goodman, Allan Stephen, 1942- January 1971 (has links)
No description available.
28

Electrochemical analysis and kinetic study in the presence of chelating agents

Chow, Lu 05 1900 (has links)
No description available.
29

Oxidative dehydrogenation of n-octane using vanadium-based hydrotalcite-like compounds.

Bux, Mayashree. January 2010 (has links)
The oxidative dehydrogenation of alkanes provides a potential route to higher value products such as olefins and aromatic compounds. Alkanes are of low environmental impact and their increasing availability has prompted extensive research in the field of alkane activation. The use of hydrotalcite-type compounds (HTlc), to achieve such reactions, has received much attention over recent years. Specifically, hydrotalcites and hydrotalcite-like compounds are promising catalysts because they provide a route to mixed-metal oxides with variable composition and significantly high metal-oxide distribution. Vanadium containing hydrotalcite-like catalysts were synthesized via the co-precipitation route and doped with either barium, cesium or boron using the wet impregnation method. These catalysts were characterized using electron microscopy, inductively coupled plasma - optical emission spectroscopy, X-Ray diffraction, differential thermal analysis, BET surface area measurements and infrared spectroscopy. The catalysts were then tested in a fixed bed reactor using n-octane as the feed and air as the oxidant. The effect of fuel-air ratios and contact time was studied on the unpromoted Mg-V-HTlc. The conversion of n-octane and the selectivity and yields to the products were quantified using gas chromatography and used to determine optimum reaction conditions. The effect of promoters on the conversion of n-octane and the selectivity of the catalyst in terms of products was determined under the optimum reaction conditions. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2010.
30

A study of the vanadium oxide bronze 0-VOB, and vanadium oxides V2O5 and VO2, using hyperfine interaction techniques.

Naicker, Vishnu Visvanathan. January 1999 (has links)
One of the main interests in the vanadium oxides V2O5 and VO2 is that, when doped with a metal such as Fe, these oxides display semiconductor-to-metal transitions at certain critical temperatures. These transitions are also accompanied with changes in the crystallographic phases of the oxides. This thesis describes the use of hyperfine interactions at dopant sites in the vanadium oxides V2O5 and VO2 to infer information on the phase transitions that take place in these oxides. The hyperfine interaction techniques of Mossbauer Spectroscopy and Time Differential Perturbed Angular Correlation (TDPAC) are used to study the hyperfine parameters in the Fe - V2O5 system and Cd - V2O5 system, respectively. X-ray powder diffraction spectroscopy were also conducted on the samples to establish the phases created. A large part of this project was spent in the design of apparatus. The apparatus constructed were (i) a furnace to perform a solid state reaction in order to introduce Fe into V2O5, the maximum operating temperature of the furnace being 1473 K, (ii) a Mossbauer sample chamber and sample holder which enabled the sample to be heated up to a temperature of 873 K, and (iii) a device constructed to determine the electrical conductivities of powder samples at temperatures ranging from 773 K to room temperature. For the Mossbauer studies, the Fe-V2O5 system was studied as a function of the Fe concentration. Six symmetric doublets, with intensities changing as the Fe concentration changed, were observed. Correlating the Mossbauer components of the individual spectra with the phases identified using powder x-ray diffraction patterns in terms of the reflection intensities, allowed two of the doublets to be assigned to lattice sites in the vanadium oxide bronze system, θ-YOB, a further two doublets to substitutional and interstitial sites in the Fe doped V2O5 system, respectively, and the fifth doublet to the super-paramagnetic Fe2O3 phase. The sixth doublet observed was attributed to an unresolved crystallographic phase observed in the x-ray diffraction spectra at large Fe concentrations. The magnitude of the quadrupole splittings of the doublets assigned to the vanadium oxide bronze and the Fe-V2O5 systems indicate that the electronic environment of the Fe atoms in the bronze phase displays a greater symmetry than those in the V2O5 phase. In order to gain insight on the semiconducting nature of the Fe doped V2O5 and the θ-VOB phases, temperature dependent Mossbauer measurements ranging from 300 K to 573 K, together with electrical conductivity measurements, were performed on a few samples. The temperature dependent Mossbauer spectra displayed the usual second order Doppler shift of the isomer shifts for the various components as a function of temperature, but no significant change in the magnitude of the quadrupole splittings. From this result, on the basis of the Duncan-Golding correlation diagram, the valence state of the Fe ions was inferred to be 3+. No components were observed (with increasing temperature) that could be correlated with the population of Fe2+ states. This therefore suggests that the semiconducting properties of the Fe doped V2O5 phase and the θ-VOB phase are associated with electron hopping between V4+ - V5+ valence sites rather than Fe3+ - Fe2+ valence sites. 111In-TDPAC measurements were made on V2Os and VO2. For V2O5, the measurements yielded one distinct substitutional cation site for the 1llCd ions, with quadrupole coupling constant vQ =88,1(3) MHz, and asymmetry η =0,619(3) In VO2, temperature dependent TDPAC measurements yielded two well defined quadrupole coupling frequencies for the 1llCd probe nuclei, the first, vQ =43,0(7) MHz, observed at room temperature, corresponding to a monoclinic or triclinic phase of VO2, and the second, vQ =89,1(1) MHz, observed at 423 K and above, corresponding to the rutile phase of VO2. / Thesis (Ph.D.)-University of Durban-Westville, 1999.

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