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M.Sc. / An annual production of approximately 46 million metric tons of polyolefins worldwide, emphasizes the industrial importance of this product and the polymerisation process. Olefins are the basic building block of the petrochemical industry and are therefore readily available and cheap. Reactivity of olefins decreases from ethylene to propylene to 1-octene and makes the study of polymerisation catalysts more complex, seeing that the activity of a catalyst differs from monomer to monomer. In this study zirconocene complexes with bridged cyclopentadienyl ligands have been prepared and investigated as , possible catalysts for the polymerisation of higher aolefins. Fulvenes have been reductively coupled and used as ligands for zirconium complexes. Steric bulk of the substituents on the ligand have been increased and changes in the polymeric products have been studied. The tacticty, endgroups and chain lengths of the polyolefins have been investigated. There is currently a considerable interest in the development of 'non-metallocene' catalysts as alternatives for the polymerisation and oligomerisation of a-olefins. Chelating diamide complexes of Group 4 metals have been the focus of much attention and these compounds have shown moderate to high reactivity. However, only a few examples of the corresponding chelating alkoxides are known. In this study, alkoxide complexes of zirconium and titanium have been prepared with Schiff bases as ligands. These complexes have been evaluated as polymerisation catalysts and the products have been studied. The titanium complexes were more active than the zirconium analogues. The narrow molecular weight distribution of the polyolefins gave evidence that these catalysts are single-sited catalysts.

An investigation of creep deformation in pure zirconia, using a creep-in-bending method with programmed temperature increase, showed the occurrence of superplasticity at temperatures near the monoclinic to tetragonal phase transformation. An interesting phenomenon was observed in a temporary halt to the creep process which occurred with continuing temperature increase beyond the phase transformation. Temperature dependence tests at a maximum fibre stress of 3140 psi gave overall activation energies for the creep process in pure zirconia in the monoclinic and tetragonal phases of 44 and 103 kcal/mole respectively. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The corrosion behaviour of zirconium based alloys has been primarily investigated by electrochemical impedance spectroscopy (EIS). In-situ autoclave EIS experiments were performed in simulated primary coolant conditions in order to study the high temperature water corrosion of zirconium alloys in PWRs. In-situ impedance response of the corroding material was recorded throughout first kinetic transition. A physical model of the zirconium oxide was proposed in accordance with the microstructural observation' made by SEM analysis. Electrical properties of the oxide was evaluated with equivalent circuit model (ECM) which was constructed according to the physical oxide model. Evolution of various oxide parameters obtained from ECM was analysed in accordance with the microstructure observation made by SEM. A two layer structure consists of a outer porous oxide and an inner barrier oxide, was found to be the most accurate description for the autoclave formed oxide. Supporting evidence from the SEM cross-section and surface analysis of the oxide had shown cracks and pores that were linked and connected with the environment. This observation is also confirmed by the in-situ EIS measurement which has shown porous electrode behaviour throughout the course of oxidation. The porous oxide behaviour was also confirmed by the ex-situ soaking experiment on samples with incremental exposure time. Evolution of inner barrier layer oxide thickness was found to be correlated with kinetic transition which was determined from weight gain measurement. This indicated that barrier layer maybe the oxidation rate controlling layer and its thickness maybe reduced during transition. Thus, a thinner barrier layer would resulted in a rapid corrosion of zirconium alloys. Furthermore, maintaining the barrier layer thickness maybe the possible route to improve zirconium alloy corrosion resistance.

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This work was produced as part of a multidisciplinary study of the corrosion of zirconium alloys undertaken by a consortium of universities working in the MUZIC program; Oxford, Manchester and The Open University. The objective of the project as a whole was to further the understanding of the mechanisms of the breakaway oxidation process and to characterise these corrosion processes within a number of fuel rod cladding materials. This thesis describes laser 3D atom probe characterisation of the nano-scale chemical redistribution of oxygen and other solutes that occurs at the metal-oxide interface during corrosion, and a large body of technique development that was required to achieve this goal. The development of the metal-oxide interface of ZIRLO, a Zr-Nb-Sn-Fe-O alloy, is followed by generating 3D atomic scale reconstructions at four different stages of corrosion. The formation of a sub-oxide ZrO layer is seen during pre-transition oxide development. The ZrO interfacial layer is consumed by the rapid formation of oxide after the breakaway transition. After transition the chemistry of the interface is similar to the early pre-transition case, although an oxygen-saturated layer of metal adjacent to the interface formed during corrosion remains. The ZrO interfacial layer (Zr-ZrO-ZrO₂) and the region of oxygen-saturated material ahead of the metal-oxide interface alter the distribution of minor alloying additions such as niobium and iron. The ZrO layer increases the acceptance of niobium into the oxide, which is otherwise seen to be rejected at the Zr-ZrO2 interface along with iron. Niobium is seen to precipitate out of solution as nano-scale particles near the interface after around 100 days of corrosion. This is not seen in the bulk metal matrix of the corroded material due to the absence of other factors driving the process: the stress at the interface and a very high oxygen concentration in the metal ahead of the interface. The nano-scale niobium particles are found to be of a meta-stable composition. Iron is seen to redistribute in the corroded material and can be correlated with the local oxygen concentration. Similarities are seen in the behaviour of solutes within pre-transition ZIRLO and Zircaloy-4 (Zr-Sn-Fe-O). In both cases no redistribution of tin is seen at the metal-oxide interface. A Zr-Nb-Ti alloy with very poor corrosion resistance was also analysed in this way, and the similarities and differences with chemically-similar ZIRLO are discussed. The segregation of solutes to grain boundaries and solute clustering within the matrix are also examined before and after corrosion.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed February 25, 2010). PDF text: 164 p. : ill. (some col.) ; 4 Mb. UMI publication number: AAT 3386603. Includes bibliographical references. Also available in microfilm and microfiche formats.

The thesis entitled “Variants of Complex Bismuth and Zirconium Oxides: Structure-Property Correlation Studies” consists of five chapters. A short introductory note outlines the synthetic procedures, characterization techniques and evaluated properties such as photocatalysis, second harmonic generation, ionic conductivity and thermal expansion in these materials. Chapter 1 deals with a new solid solution Pb3-xBi2x/3V2O8 (0.20 ≤ x ≤ 0.50), stabilizing the high temperature γ form of Pb3V2O8 in the system Pb3V2O8−BiVO4. Single-crystals of the composition x = 0.50 were grown and the structure is a new variant in palmierite structural type as determined by both single crystal X-ray and powder neutron diffraction. Several refinement strategies backed up by difference Fourier methods were used to arrive at the final crystal structure. ac impedance studies indicate conductivity of the order of 10-4 Ω-1 cm-1 for Pb2.5Bi1/3V2O8. Chapter 2 has two sections and describes the structure property correlation in bismuth based vanadate and phosphate eulytites. Section 2.1 discusses the crystal structure of Pb3Bi(VO4)3, the first eulytite compound containing [VO4]3- moieties. The compound displays incongruent melting behavior. Single-crystals were grown by melt-cool technique adding excess amount of BiVO4. The crystal structure has been characterized by both X-ray and neutron diffraction studies. Section 2.2 describes the crystal structures of four phosphate eulytites A3Bi(PO4)3 ( A = Ca, Cd, Sr, Pb). The crystals were grown from melt-cool technique with considerable difficulty as the compounds melt incongruently. While Pb3Bi(VO4)3 and Pb3Bi(PO4)3 have an unique position for one of the oxygen atoms, Sr3Bi(PO4)3, Ca3Bi(PO4)3, Cd3Bi(PO4)3 display split oxygen atomic sites. The SHG efficiencies measured on polycrystalline samples were 5.3, 3.8, 2.85, 1.21 and 0.64 times that of KDP (KH2PO4) for Pb3Bi(VO4)3, Cd3Bi(PO4)3, Sr3Bi(PO4)3, Pb3Bi(PO4)3 and Ca3Bi(PO4)3 respectively. Chapter 3 describes the isolation of the compound Sr2Bi2/3V2O8, a variant palmierite, in the phase diagram of SrO-Bi2O3-V2O5. The compound was synthesized by ceramic method and it is of interest to note that the Sr(1) site also accommodates Bi as found by single crystal X-ray studies unlike that found in the case described in chapter 1. Chapter 4 has two sections, dealing with synthesis, characterization and photocatalytic properties of trigonal and monoclinic polymorphs of ZrMo2O8, a negative thermal expansion material in its cubic form. Section 4.1 describes the synthesis of trigonal polymorph of ZrMo2O8 by both ceramic and combustion synthesis methods. SEM images show a particle size of 40-50 nm for combustion synthesized samples and 8-10 μm for solid state synthesized ZrMo2O8. The band gap obtained by UV-visible diffuse reflectance spectra for the combustion synthesized and solid state synthesized samples were 2.70 and 2.74 eV and the BET surface area were 1.0 m2/g and 10.0 m2/g. DFT electronic structure calculations reveal the indirect band gap nature of this polymorph. Photo-degradation studies performed on pollutant water show specific affinity to degrade dyes which do not possess anthraquinonic moieties. Section 4.2 describes the single-crystal structure determination and catalytic properties of monoclinic polymorph of ZrMo2O8. The band gap measured for the monoclinic form by UV-visible diffuse reflectance spectra was 2.57 eV. This polymorph was found to be specific towards the degradation of cationic dyes. Chapter 5 discusses a new solid solution ZrV2-xMo5x/6O7 (0 ≤ x ≤ 0.8) identified in the phase diagram of ZrO2−V2O5−MoO3. These compounds were synthesized via the solution combustion method. The resulting products were characterized by powder X-ray diffraction, solid-state UV-visible diffuse reflectance spectra, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photo-catalytic activity shows specificity towards the degradation of non-azo dyes. Single-crystals were grown by melt-cool technique from the starting materials with twice the MoO3 quantity. Since, these crystals belong to a cubic system, space group Pa 3, they were tested for negative thermal expansion using variable temperature single-crystal XRD and indeed they exhibit this property above 370 K.