High-pressure techniques have become increasingly important in the synthesis of ceramic and metallic solids allowing the discovery of new materials with interesting properties. In this research dense solid oxides have been synthesised at high pressures, and structural investigations have been conducted using x-ray and neutron diffraction. The perovskite LaPdO3 has been synthesised at pressures of 6{10GPa. Neutron diffraction studies have been carried out from 7{260K to investigate any structural distortions, particularly related to the possibility of charge order at low temperatures. No reduction in symmetry associated with charge ordering has been observed; the material appears to remain metallic with only one unique Pd site down to 7K. LaPdO3 adopts the GdFeO3-type Pbnm structure. The PdO6 octahedra exhibit a tetragonal distortion throughout the temperature range with a shortening of the apical Pd{O bonds of 2:5% relative to the equatorial bonds. Attempts to prepare analogues of the perovskite containing smaller rare earths have resulted in multi-phase samples, and further RPdO3 perovskites remain inaccessible although there is evidence for a small amount of the perovskite phase in the products of synthesis attempts with neodymium. Three new oxypnictide superconductors, RFeAsO1 xFx (R = Tb, Dy and Ho) have been synthesised at 7{12GPa. The materials are isostructural with other recently discovered iron arsenide superconductors and have Tc's of 52:8 K, 48:5K and 36:2K respectively, demonstrating a downturn in Tc in the series for smaller R. Systematic studies on TbFeAsO0.9F0.1 and HoFeAsO0.9F0.1 show negative values of dTc=dV in contrast to those reported for early R containing materials. Low-temperature neutron diffraction measurements on both materials, and synchrotron studies on HoFeAsO0.9F0.1 reveal no tetragonal to orthorhombic transitions as observed in early R-containing materials with lower doping levels. Magnetic reflections are evident but they are shown to be from R2O3 and RAs impurities with TN's of 5:5K for Tb2O3, 6:5K for HoAs and 1:7K < TN < 4K for Ho2O3. The implications of these results for superconductivity in the iron arsenides are discussed.

Ce travail de thèse a pour objectif la caractérisation des propriétés physico-chimiques descéramiques de composition (1-x) BaTi0.8Zr0.2O3-x Ba0.7Ca0.3TiO3 préparées par frittage conventionnelet frittage flash (SPS). Les études structurales réalisées au voisinage du point triple (x≈ 0.32) à l’aidede la diffraction des RX de haute résolution (synchrotron) sur poudre ont introduit des modificationsmajeures sur le diagramme de phase température-composition déjà proposé. La réponseélectromécanique géante mesurée est alors corrélée à la dégénérescence du profil de l’énergie libreinduite par les instabilités structurales. De plus, la flexibilité de la polarisation sous contraintesthermique et électrique est couplée à un assouplissement de la maille cristalline. Ces deuxcaractéristiques contribuent ensemble à une réponse électromécanique colossale via une forteactivité des murs de domaine. La dégradation des propriétés diélectriques, ferroélectriques etpiézoélectriques pour les céramiques BCTZ (x=0.32 et 0.5) élaborées par frittage flash estessentiellement attribuée aux fluctuations importantes de composition et à la stabilisation de laconfiguration des murs de domaines avec la diminution de la taille des grains. / The aim of this work is to make a full characterization of the structural, microstructural, dielectric,ferroelectric and piezoelectric properties of the perovskite-structured oxides (1-x) BaTi0.8Zr0.2O3-xBa0.7Ca0.3TiO3 prepared by a conventional solid-state reaction method (conventional sintering) andSPS fabrication technique. Using high-resolution synchrotron x-ray powder diffraction, the structuralinvestigations carried out close to the triple point (x≈ 0.32) have introduced significant corrections tothe previously published composition-temperature phase diagram. The colossal electromechanicalresponse was then correlated to a strongly degenerate free energy landscape caused by structuralinstabilities. Furthermore, the coupling between the high polarization flexibility under electric andthermal stresses and the ‘lattice softening’ gives rise to a giant electromechanical response due tohigh domain wall activities. The decrease of the dielectric, ferroelectric and piezoelectric propertiesof BCTZ ceramics (x=0.32 and 0.5) processed by SPS was essentially attributed to the largecompositional fluctuations and stable domain wall configurations as the grain size decreased.

The contribution at hand introduces computational modeling and realistic simulation concepts for a comprehensive description of the manufacturing and application of densified wood and wooden structures made from molded densified wood.Wood, as a natural material, is characterized by e.g. a very good mechanical load-bearing capacity related to its density. Nevertheless, the ratio between its mechanical properties and its density can be optimized by densification technology for an expanded use of wood in structural engineering. The wood densification process is not only a mechanical process with large and irreversible deformations, it is also denoted by temperature- and moisture-dependent treatments of the wooden specimens. Thus, the introduced approaches to predict the material and structural characteristics of compressed and molded wood consist of an inelastic and multi-physical constitutive modeling of wood at finite deformations as well as the computation of effective structural properties of wood after the thermo-hygro-mechanical densification process. A successful implementation of the modeling concepts into the finite element method (FEM) is presented, which is verified by numerical investigations. A validation of the numerical results is carried out by use of experimental data at beech wood (Fagus Sylvatica, L.), taken from literature.