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Les travaux présentés dans ce manuscrit concernent des niobates de formulation Ba2LnNb4O15 (Ln = La ou Nd) et de structure "Tetragonal Tungsten Bronze" (TTB). Ces travaux se situent à la charnière de la chimie et de la physique des matériaux diélectriques et visent à mettre en relation structure cristalline et propriétés diélectriques. L'étude a été menée sur des TTB élaborés sous forme céramique et monocristalline. Après détermination de conditions optimales de croissance, par fusion de zone en four à image, des sections monocristallines ont été obtenues et caractérisées. Les résultats obtenus suggèrent des liens étroits entre composition, modulations structurales apériodiques et comportement diélectrique. Dans le même temps, deux nouvelles solutions solides céramiques ont été explorées, avec des schémas de substitution différents menant notamment à une réflexion très large sur les liens cristallochimie-ferroélectricité dans cette famille de TTB. Ces solutions solides présentent un crossover relaxeur-ferroélectrique, un comportement original et déjà observé dans d'autres solutions solides issues de cette famille de TTB. Des caractérisations avancées (mesures pyro- et piézoélectriques, cycles de polarisation) et des études structurales résolues en composition et en température ont permis d'établir des diagrammes de phases diélectriques montrant l'existence d'un état ferroélectrique métastable. Finalement, la présence d’une modulation structurale bidimensionnelle a pu être confirmée, dans les monocristaux et dans les céramiques, et l'ensemble des résultats obtenus pointent son implication dans les comportements cristallochimiques originaux rencontrés dans ces TTB. / The present work deals with Ba2LnNb4O15 (Ln = La ou Nd) niobates crystallizing with the "Tetragonal Tungsten Bronze" (TTB) structure. These researches, at the interface of chemistry and physics of dieletrics, aim at establishing structure / dielectric properties relationships. They were performed on TTB materials elaborated in ceramic and single crystal forms. Following optimization of growth parameters with an image furnace, single crystals were obtained and characterized. The results obtained suggest that composition, aperiodically modulated structure and dielectric behavior are closely tied in TTBs. Meanwhile, two new ceramic solid solutions with different substitutions schemes were investigated, deepening the insight on crystal-chemistry and ferroelectricity of TTBs. These solid solutions exhibit a relaxor-ferroelectric crossover, an original behaviour previously observed in solid solutions derived from the same family of TTB niobates. Advanced physical characterization (ferro-, pyro- and piezoelectric measurements, polarization loops), and composition/temperature resolved structural studies, allowed for the determination of a dielectric phase diagrams showing the presence of a metastable ferroelectric state. Finally, the existence of a two-dimensional structural modulation in single crystals and ceramics has been confirmed, and the whole set of experimental results points towards its implication in the original dielectric behavior observed in these TTBs.

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In this work, in-depth structural and electrical characterisation is used to study a family of “empty” tetragonal tungsten bronzes (TTBs), A2₄A1₂B1₂B2₈O₃₀. An initial investigation into the effect of the A1-cation size on the properties of empty Ba₄R₀.₆₇◻₁.₃₃Nb₁₀O₃₀ TTBs (where R is the A1-cation and R = La, Nd, Sm, Gd, Dy and Y; ◻ = vacancy) was performed. These were determined to be metrically tetragonal by powder x-ray diffraction, with decreasing R cation size inducing increased crystal anisotropy. This tetragonal structural distortion, driven by contraction in the ab-plane, is shown to stabilise c-axis ferroelectricity; a direct correlation between tetragonality and the ferroelectric Curie temperature, T[sub]C, is demonstrated. Further examination of the relaxor ferroelectric (RFE) to ferroelectric (FE) crossover in Ba₄(La₁₋ₓNdₓ)₀.₆₇◻₁.₃₃Nb₁₀O₃₀ TTBs using detailed structural studies employing variable temperature, high resolution neutron, synchrotron X-ray and electron diffraction revealed a common superstructure with 2√2 × √2 × 2 cell with respect to the basic tetragonal aristotype cell. However, they display different degrees of order/disorder which can disrupt polar order (ferroelectricity). La-rich analogues exhibit a disordered regime between the low and high temperature ferroelectric and non-polar phases. Although polar, this disordered regime is non-ferroelectric, however, large polarisation may be established with an applied electric field, but relaxes back to the disordered phase upon removal of the field. Substitution of Nd for La at the A1-site leads to destabilisation of the disordered phase and reintroduces “normal” ferroelectric behaviour. Finally, isovalent substitution of Sr²⁺∙ for Ba²⁺ is shown to lead to the development of relaxor behaviour at higher dopant concentrations in Ba₄₋ₓSrₓDy₀.₆₇◻₁.₃₃Nb₁₀O₃₀, (x = 0, 0.25, 0.5, 1, 2, 3; ◻ = vacancy). With increasing x the unit cell contracts in both the ab- plane and c-axis coinciding with a decrease in T[sub]C and development of relaxor behaviour for x ≥ 2. This observation is rationalised by differing cation occupancies: for x ≤ 1, Sr²⁺ principally occupies the A2-site while for x ≥ 2 significant Sr²⁺ occupation of the A1-site leads to the observed RFE characteristics. The FE to RFE crossover is discussed in the context of a previously proposed TTB crystal chemical framework with the A1-site tolerance factor identified as the dominant influence on relaxor behaviour.

Thesis on Electron beam welding of ODS materials is divided into theoretical and experimental part. In the first part the theoretical research work focuses on the accurate description of the candidate materials in a research fusion reactor ITER. Following are detailed constructional ODS materials and tungsten alloy as a material of the first wall. The second part of the theoretical research deals with various possibilities of joining high-melting dissimilar materials such as brazing or electron beam welding as well, which will be established in the experimental section. The experimental part of the thesis deals with a joining ODS steel MA 956 with sintered carbide WC-Co and ODS tungsten. These materials are welded by electron beam. Individual samples are distinguished used filler material, preheating or welding parameters. This thesis contains photographic documentations of microstructure the welding samples, results from scanning electron microscopy, and measurements of microhardness across the weld metal.

Vertebroplasty is a new technique in orthopedic surgery for stabilizing fractured vertebra. In this technique acrylic bone cement as a biocompatible material is injected through a cannula inside of vertebra. There are several concerns in this technique that the most serious one is cement leakage out of vertebra. The main reasons are improper viscosity and lack of visibility. Clinicians who practice vertebroplasty use commercial highly concentrated radiopaque acrylic bone cement (more than 25%BaSO[subscript 4] or ZrO[subscript 2]) or a cement with manually added radiopaque agents. High density materials with attenuation under X-ray are good alternatives compared to conventional radiopaque agents (BaSO[subscript 4] or ZrO[subscript 2]) in acrylic bone cement for application in vertebroplasty. In the first part of this study, thermal and rheological properties of modified acrylic bone cement with conventional radiopaque agent (Barium Sulfate, BaSO[subscript 4]) are studied. Additions of barium sulfate are in the form of substitute or excess. In substitute formulation, barium sulfate is replaced with the same weight of powder and liquid to powder ratio kept constant. In the excess formulation, barium sulfate added as excess and liquid to powder ratio decreased. In the second part of this study, high density radiopaque agents are used as alternative radiopacifier. Experimental design technique is used to study the effect of X-ray conditions, concentration, type, and size of radiopaque agents on the visibility of bone cement. The visibility of bone cement was quantified by the measurement of contrast index. In the first project, it was found that the setting time increased with the increase of concentration of radiopacifier in substitute formulation of barium sulfate bone cement. With increase of barium sulfate concentration, excess formulations showed higher residual monomer but for substitute cement, we had a decreasing trend. Acrylic bone cements with excess formulation had higher initial viscosity compared to reference or substitute but the variation of viscosity with time was lower for substitute formulation and cements had higher working time. In the second project, contrast index was the same for barium sulfate, tungsten, and zirconium in the lower voltage but in higher voltage of X-ray lamp, tungsten and zirconium gave higher contrast index. Variation of current in X-ray lamp changed the contrast index of cement slightly compared to the effect of voltage. Bone cement with nano tungsten had higher contrast index compared to the cement with micro size tungsten although micro size zirconium as radiopacifier gave higher contrast index than nano size zirconium.

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Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2008. / In this study the coordination of ligands with several coordination sites, 2-aminoazoles (2- amino-4-methylthiazole), 2-aminobenzothiazole, 2-aminobenzoimidazole and 2- aminothiazoline and a biguanidine (N-(2-methylphenyl)imidodicarbonimidic diamide) to soft metal centres [gold(I) (group 11), chromium(0) (group 6) and tungsten (0) (group 6)] was investigated. The aminoazoles have three coordination sites, an exocyclic amine nitrogen, an endocyclic imine nitrogen and an endocyclic thioether sulphur. The biguanidine ligand has three sites for deprotonation, one central amine and two imine nitrogens, and at least five sites available for nitrogen coordination.

Transition metal fluorides represent an important family of complex solids displaying a variety of different properties and interesting phenomena. Despite their remarkable behaviour, these classes of materials have not received much attention and the rationalization of their behaviour is still lacking a systematic approach. This thesis aims to contribute to the field by examining previously unknown or understudied complex fluorides. The compounds were selected for their intriguing physical properties that range from superconductivity to multiferroism. The discovery of superconductivity in the iron pnictides sparked new interest in materials with layered ZrCuSiAs-type structure. Herein the properties of one of these systems, namely SrFeAsF, will be discussed. We have found that it behaves as a poor metal and undergoes a tetragonal (P4/nmm) to orthorhombic (Cmma) structural transition at T = 180 K, accompanied by a spin density wave in magnetic susceptibility and electrical resistivity. Below T < 150 K, the Fe moments order in antiferromagnetic spin-stripes. Electron doping with La3+ is a successful route to obtain superconducting phases, with maximum Tc = 27 K (x = 0.2). The isostructural AeMnPnF series (Ae = Sr, Ba; Pn = P, As, Sb) was also investigated to elucidate the influence of transition metal d-electrons and size effects of Ae and Pn on the physical properties. The isoelectronic replacement of Ae and Pn leads to a significant distortion in the tetragonal building blocks. All d5 Mn fluorides investigated here are insulating antiferromagnets with TN ~ 350 K. Due to the coexistence of electronic and magnetic ordering, the tetragonal tungsten bronze (TTB) materials KxM2+ xM3+ 1-xF3 (x = 0.4 – 0.6; M = transition metal) are potential multiferroics. The type of structural distortion adopted by these systems is strongly dependant on the M2+/M3+ ratio. For instance, our high-resolution diffraction study on K0.5Mn0.5Cr0.5F3 has revealed a small orthorhombic distortion, which indicates full chemical order of Mn2+ and Cr3+ on all crystallographic sites. K0.5Mn0.5Cr0.5F3 remains orthorhombic Ccc2 on cooling through the ferromagnetic transition at TN = 23 K. On heating, the structure is acentric up to T = 373 K, where a change to tetragonal P42/mbc symmetry marks the transition from ferroelectric (polar) to paraelectric (apolar) states. High-pressure diffraction experiments have shown that the Ccc2 structure is robust upon pressurization with anisotropic axial compressibility up to the maximum pressure applied p = 18 GPa. The crystal structure of related mixed-valence TTB fluoride K0.6Cr2+ 0.6Cr3+ 0.4F3 is influenced by the presence of Jahn-Teller active Cr2+. The structural analysis described here revealed the presence of a small polar monoclinic distortion (P112) providing a clear signature of full charge order (CO). On heating, the gradual loss of CO leads to two consecutive structural phase transitions to orthorhombic (Pba2, T = 423 K) and then tetragonal (P42/mbc, T = 823 K) lattices, the latter is the signature of the ferro- to paraelectric transition. Below T = 150 K, increased X-ray exposure time leads to CO-melting and the stabilization of a new, charge-disordered orthorhombic phase (Cmm2), with a phenomenology similar to the CO manganites. In highpressure diffraction experiments, a further transition to tetragonal P4bm symmetry is found at p = 6 GPa. The magnetic susceptibility points towards a complex spin arrangement, with two transitions at TN = 33 K and 6 K. The results presented herein show the richness of the structural, electronic and magnetic phase diagrams of transition metal fluorides and clearly demonstrate that systematic studies on these systems will greatly enhance our current understanding of the underlying mechanisms of important phenomena such as superconductivity and ferroelectricity.

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