Demands for zero greenhouse-gas emission vehicles have sharpened with today’s increased focus on global warming. Hydrogen storage is a key technology for the implementation of hydrogen powered vehicles. Metal hydrides can claim higher energy densities than alternative hydrogen storage materials, but a remaining challenge is to find a metal hydride which satisfies all current demands on practical usability. Several metals store large amounts of hydrogen by forming a metal hydride, e.g., Mg, Ti and Al. The main problems are the weight of the material and the reaction energy between the metal and hydrogen. Magnesium has a high storage capacity (7.6 wt.% hydrogen) in forming MgH2; this is a slow reaction, but can be accelerated either by minimizing the diffusion length within the hydride or by changing the diffusion properties. Light-metal hydrides have been studied in this thesis with the goal of finding new hydrogen storage compounds and of gaining a better understanding of the parameters which determine their storage properties. Various magnesium-containing compounds have been investigated. These systems represent different ways to address the problems which arise in exploiting magnesium based materials. The compounds were synthesized in sealed tantalum tubes, and investigated by in situ synchrotron radiation X-ray powder diffraction, neutron powder diffraction, isothermal measurements, thermal desorption spectroscopy and electron microscopy. It is demonstrated that hydrogen storage properties can be improved by alloying magnesium with yttrium or scandium. Mg-Y-compounds decompose in hydrogen to form MgH2 nano-structures. Hydrogen desorption kinetics are improved compared to pure MgH2. The influence of adding a third element, gallium or zinc has also been studied; it is shown that gallium improves hydrogen desorption from YH2. ScAl1-xMgx is presented here for the first time as a hydrogen storage material. It absorbs hydrogen by forming ScH2 and Al(Mg) in a fully reversible reaction. It is shown that the hydrogen desorption temperature of ScH2 is reduced by more than 400 °C by alloying with aluminium and magnesium.

1979 wurde mit CeCu2Si2 erstmalig ein Schwere-Fermionen-Supraleiter entdeckt. Diese Verbindung, entdeckt von Steglich und Mitarbeitern, befindet sich nahe an einem quantenkritischen Punkt, an dem die magnetische Ordnung gerade unterdrückt wird. Der Abstand zu diesem Punkt kann sowohl durch Druck als auch durch Germaniumsubstitution auf dem Siliziumplatz variiert werden. Dabei treten neben der Supraleitung in CeCu2Si2 auch verschiedene magnetische Phasen bei höherem Germaniumgehalt auf. CeCu2Si2 ordnet magnetisch unterhalb von TN = 0.8 K in einer Spindichtewelle, während das Schwere-Fermionen-System CeCu2Ge2 unterhalb von TN = 4.1 K antiferromagnetisch ordnet. In dieser Arbeit wurde die Substitutionsreihe CeCu2(Si1-xGex)2 mittels Neutronendiffraktion untersucht. Ausgehend von Proben mit hohem Germaniumgehalt von x = 0.45, deren magnetische Struktur detailliert untersucht wurde, wurden schrittweise die Eigenschaften von Proben mit kleinerem x erschlossen, um schließlich die (bis dato unbekannte) magnetische Struktur in CeCu2Si2 aufzuklären. Weiterhin wurden Untersuchungen zumWechselspiel zwischenMagnetismus und Supraleitung durchgeführt. Hierzu wurde mit einem selbstentwickelten Aufbau dieWechselfeldsuszeptibilität simultan zu den Diffraktionsexperimenten aufgezeichnet. Durch die direkte Korrelation konnte nachgewiesen werden, dass in CeCu2Si2 keine mikroskopische Koexistenz von Supraleitung und magnetischer Ordnung vorliegt, sondern mikroskopische Phasenseparation. - Die Arbeit ist auch über den Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen mit der ISBN 978-3-86727-587-3 erhältlich. / In 1979 the first heavy-fermion superconductor CeCu2Si2 was discovered by Steglich et al. The system is near a quantum critical point (QCP), where the magnetic order is just suppressed. The distance to the QCP can be variied with hydrostatic pressure as well as by germanium substitution on the silicon site. Next to the superconductivity in CeCu2Si2 one finds distinct magnetic phases while increasing the germanium content. CeCu2Si2 shows a magnetic order of a spin-density-type below TN = 0.8 K, whereas the heavy fermion system CeCu2Ge2 orders below TN = 4.1 K as an antiferromagnet. The focus of this thesis is on neutron-diffraction in the system CeCu2(Si1-xGex)2. Starting with a sample with a high germanium content of x = 0.45, the magnetic structures are investigated in detail. Following a step-by-step approach, samples with reduced x are investigated subsequently to figure out the properties of pure CeCu2Si2, which were not accessible before. Furthermore, the complex interaction between magnetism and superconductivity is investigated in detail. Using a specially designed setup, the ac-susceptibility could be recorded simultaneously during the neutron diffraction experiments. Due to the direct correlation between antiferromagnetic signals and diamagnetic features, the microscopic coexistence of superconductivity and magnetic order can be ruled out. Instead, a phase separation on the microscopic scale is found. - The thesis is also available from the publisher Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen under the ISBN 978-3-86727-587-3.

<p>It is generally believed that one of the key parameterscontrolling the normal state and superconducting properties ofhigh temperature superconductors is the charge carrierconcentration<i>p</i>in the CuO₂planes.By changing the non-isovalent dopingconcentration on the RE site as well as the oxygen content in(RE)Ba₂Cu₃O_7−δ, an excellent tool is obtained tovary the hole concentration over a wide range from theunderdoped up to the overdoped regime.In the present thesis thefocus is on the doping effects on the structural and normalstate electrical properties in Nd-123 doped with Ca, La, Pr,Ca-Pr, and Ca-Th.T he effects of doping have been investigatedby X-ray and neutron powder diffraction, and by measurements ofthe resistivity, thermoelectric power<i>S</i>, and Hall coefficient R_H.T he thermoelectric power is a powerful tool forstudies of high temperature superconductivity and is highlysensitive to details of the electronic band structure.<i>S</i>as a function of temperature has been analyzed in twodifferent two band models.The parameters of these models arerelated to charactristic features of the electron bands and asemiempirical physical description of the doping dependence of<i>S</i>is obtained.So me important results are following:</p><p>(i)<i>The valence of Pr in the RE-123 family.</i>Results from thestructural investigations, the critical temperature Tc, and thethermoelectric power indicated a valence +4 at low dopingconcentration, which is in agreement with results of chargeneutral doping in the RE-123 family.(ii)<i>Hole localization</i>. The results of bond valence sum (BVS)calculations from neutron diffraction data showed that holelocalization on the Pr⁺⁴site was the main reason for the decrease of thehole concentration p.Differ ent types of localization wereinferred by S measurements for Ca-Th and Ca-Pr dopings.(iii)<i>Competition between added charge and disorder</i>. Theresults of RH measurements indicated that Ca doping introduceddisorder in the CuO₂planes in addition to added charge.This could bethe main reason for the observed small decrease of thebandwidth of the density of states in the description of aphenomenological narrow band model.(iv) Empirical parabolic relation between γ and p.S data were analyzed and well described by a two-band modelwith an additional linear T term, γT.An empiricalparabolic relation for γ as a function of holeconcentration has been found.</p><p><b>Key words:</b>high temperature superconductors, criticaltemperature, resistivity, thermoelectric power, Hallcoefficient, X-ray diffraction, Neutron diffraction, NdBa₂Cu₃O_7−δ, hole concentration,substitution.</p>

This dissertation reports on the synthesis, structural and thermal characterization and electrical and thermal transport properties of a variety of strongly correlated spinels. General structure property relationships for electrical and thermal transport are discussed. However, the relationship between thermopower and features of the crystal structure such as spin, crystal field, anti-site disorder, and structural distortions are explored in depth. The experimental findings are reported in the context of improving existing oxide thermoelectric materials, screening for new materials or using thermopower as a unique characterization tool to determine the cation distribution in spinels. The need for improved n-type oxide thermoelectric materials has led researchers to consider mixed valence \((+3/+4)\) manganese oxides. Contrary to previous findings we report herein that the \(LiMn_2O_4\) compound reaches the relatively large n-type thermopower of \(-73 \mu V/K\) which is three times larger than the value observed in other manganese oxides, \(-25 \mu V/K\). The cause of this increase in thermopower is shown to be the absence of a Jahn-Teller distortion on the \(Mn^{3+}\) ions in \(LiMn_2O_4\). By avoiding this structural distortion the orbital degeneracy is doubled and the Koshibae et al.’s modified Heikes formula predicts a thermopower of \(-79 \mu V/K\) in good agreement with the experiment. Altering the \(Mn^{3+/4+}\) ratio via aliovalent doping did not affect the thermopower and is a second evidence of universal charge transport first reported by Kobayashi et al. The role of anti-site disorder was further examined in \(Fe_xMn_{1-x}NiCrO_4\) x=0, ½, ¾, 1 spinels but the effect on thermopower was inconclusive due to the presence of impurity phases. Next, the thermopower as a function of temperature in \(Co_3O_4\) was investigated as a means whereby the Wu and Mason’s 30 year old model for using thermopower to calculate cation distribution in spinels could be revisited. We report evidence that Wu and Mason’s original model using the standard Heikes formula and considering octahedral sites alone leads to a stoichiometrically inconsistent result at high temperatures. Alternate models are evaluated considering Koshibae et al.’s modified Heikes formula and accounting for tetrahedral site contributions. Furthermore, the effect of a possible spin state transition is considered. / Engineering and Applied Sciences

Zr-2.5Nb is currently used for pressure tubes in the CANDU (CANada Deuterium Uranium) reactor. A complete understanding of the deformation mechanism of Zr-2.5Nb is important if we are to accurately predict the in-reactor performance of pressure tubes and guarantee normal operation of the reactors. This thesis is a first step in gaining such an understanding; the deformation mechanism of ZrNb alloys at room temperature has been evaluated through studying the effect of texture and microstructure on deformation. In-situ neutron diffraction was used to monitor the evolution of the lattice strain of individual grain families along both the loading and Poisson’s directions and to track the development of interphase and intergranular strains during deformation. The following experiments were carried out with data interpreted using elasto-plastic modeling techniques: 1) Compression tests of a 100%Zr material at room temperature. 2) Tension and compression tests of hot rolled Zr-2.5Nb plate material. 3) Compression of annealed Zr-2.5Nb. 4) Cyclic loading of the hot rolled Zr-2.5Nb. 5) Compression tests of ZrNb alloys with different Nb and oxygen contents. The experimental results were interpreted using a combination of finite element (FE) and elasto-plastic self-consistent (EPSC) models. The phase properties and phase interactions well represented by the FE model, the EPSC model successfully captured the evolution of intergranular constraint during deformation and provided reasonable estimates of the critical resolved shear stress and hardening parameters of different slip systems under different conditions. The consistency of the material parameters obtained by the EPSC model allows the deformation mechanism at room temperature and the effect of textures and microstructures of ZrNb alloys to be understood. This work provides useful information towards manufacturing of Zr-2.5Nb components and helps in producing ideal microstructures and material properties for pressure tubes. Also it is helpful in guiding the development of new materials for the next generation of nuclear reactors. Furthermore, the large data set obtained from this study can be used in evaluation and improving current and future polycrystalline deformation models. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-09-05 13:51:30.42

The interaction between magnetism and the nuclear lattice is investigated experimentally, using thermal expansion, magnetostriction, specific heat, magnetisation and neutron scattering measurements. Both localised moment systems, as represented by the rare earth compounds Tb2Agln, Pd2Gdln and Cu2Gdln, as well as transition metal compounds, Ni2MnGa and V20 3 have been characterised at low temperatures. Measurements of the lattice properties are important due to the intrinsic coupling of magnetic degrees of freedom to them. The response of the lattice to magnetic order, and also to applied magnetic fields have been probed by the use of the aforementioned techniques. Such techniques allow the direct determination of the coefficient of linear thermal expansion, over a wide temperature range and the forced magnetostriclion in applied fields of 0 to 7T. Indirect determination of the spontaneous magnetostriction and the total magnetic entropy contribution via measurements of isostructural compounds further enhance the range of experimental data available. The dynamic properties are characterised by spin polarised neutron scattering measurements. The experimental results are presented and discussed. Various methods of coupling lattice and electronic degrees of freedom have been investigated. It is argued that in order to fully understand and appreciate the low temperature properties of the materials investigated such a coupling must be taken into account.

Inhomogeneous deformation fields arising from the grain-grain interactions in polycrystalline materials have been evaluated using a crystal plasticity finite element method and extensively compared to neutron diffraction measurements under fatigue crack growth conditions. The roles of intergranular deformation anisotropy, grain boundary damage, and non-common deformation mechanisms (such as twinning for hexagonal close packed crystals) are systematically evaluated. The lattice misorientation field can be used to determine the intragranular deformation behavior in polycrystals or to describe the deformation inhomogeneity due to dislocation plasticity in single crystals. The study of indentation-induced lattice misorientation fields in single crystals sheds lights on the understanding of the scale-dependent plasticity mechanisms. A two-scale micromechanical analysis is performed to study the lattice strain distributions near a fatigue crack tip. The experimental finding of vanishing residual intergranular strain in polycrystals as the increase of the fully reversed loading cycles suggests the intergranular damage be the dominant failure mechanism. Our model predictions are compared to in situ neutron diffraction measurements of Ni-based superalloys under fatigue crack growth conditions. Predicted and measured lattice strains in the vicinity of fatigue crack tips illustrate the important roles played by the intergranular damage and the surrounding plasticity in fatigue growth. Motivated by the synchrotron x-ray measurements of lattice rotation fields in single crystals under indentation, the effect of the orientation of slip systems on the 2D wedge indentation of a model single crystal is investigated. Furthermore, the crystallographic orientations of the indented solids are gradually rotated, resulting changes of lattice misorientation patterns under the indenter. These 2D simulations, as well as a 3D Berkovich indentation simulation, suggest a kinematic relationship between the lattice misorientation and crystalline slip fields. Advanced structural materials such as light-weighted materials, nanocrystalline metals/alloys, and hierarchically structured alloys often encounter unconventional deformation mechanisms. The convolution of crystalline slip and deformation twin are considered in the hexagonal close packed polycrystals. Specifically, we have determined the lattice strain distributions near fatigue crack tips in Zircaloy-4, and the role of tensile-twins on intergranular strain evolution in a wrought Mg alloy, which compare favorable to available neutron diffraction measurements.

Les propriétés d'autodiffusion de l’uranium sont essentielles pour la compréhension d’interaction pastille-gaine dans le réacteur. L'objectif de cette thèse est de déterminer les coefficients d'autodiffusion de l'uranium dans l’$UO_2$ hyper-stœchiométrique qui sont contrôlés, dans certaines conditions thermodynamiques, par les défauts ponctuels. Pour cet objectif, trois études différentes ont été réalisées. La première porte sur la compréhension des défauts d'oxygène et les différents réarrangements du réseau après oxydation. Pour cela, des échantillons d’$UO_2$ et d’$UO_{2+x}$ ont été caractérisés par une diffraction neutronique au sein du laboratoire ILL à Grenoble. Les résultats obtenus de l’analyse par la « Pair Distribution Function » montrent que les ions interstitiels ont tendance à être isolés aux faibles valeurs de x mais ils sont groupés aux valeurs plus élevées de x. La deuxième partie vise à étudier les défauts lacunaires d'uranium, prédominants dans les échantillons d’$UO_{2+x}$ recuits à haute température, qui influent directement sur l'autodiffusion de l'uranium. La méthode non destructive de « Spectroscopie d'annihilation de Positron », implémentée au laboratoire CEMHTI à Orléans, a été appliquée. Les résultats ont montré l'existence des lacunes d'uranium dans le matériau et leurs quantités peuvent être estimées en fonction de la mesure de durée de vie des positrons à l'aide d'un modèle de piégeage. La connaissance de la nature des défauts cationiques et anioniques et des équilibres de défauts aide à comprendre la corrélation entre les propriétés importantes du combustible (e.g, la diffusion, le fluage) et les conditions thermodynamiques (T, pO2). / Uranium self-diffusion properties are essential for the understanding of in-reactor pellet-cladding interaction. The aim of this thesis is to determined uranium self-diffusion coefficients in hyper-stoichiometric uranium dioxide under certain thermodynamic conditions, which indeed are governed by the induced point defects. For that purpose, three separate studies were carried out on virgin material. Firstly, to improve the knowledge of oxygen defects and the rearrangements occurring in the oxygen sub-lattice after oxidation, $UO_2$ and $UO_{2+x}$ samples were characterized by neutron diffraction in ILL Grenoble. The results obtained by a Pair Distribution Function analysis show that interstitial ions tend to be isolated at lower x but cluster at higher x. Secondly, to study the predominant uranium vacancy defects in high-temperature annealed $UO_{2+x}$, which directly influence the uranium self-diffusivity, a non-destructive method – Positron Annihilation Spectroscopy, available in CEMHTI, Orleans has been carried out. The results of Doppler broadening spectroscopy of annihilation of electron-positron pairs has proved the existence of uranium vacancies in the materials, and their concentration can be estimated based on the positron lifetime measurements using a trapping model. The knowledge of the nature relating to both cation and anion defects and defect equilibria are used to understand the correlation between important fuel properties (e.g. diffusion, creep) and thermodynamic conditions (i.e. temperature and oxygen partial pressure).

Ce travail a pour objectif de contribuer à l'étude du magnétisme dans les oxydes isolants de type AB_2O_6 (A= Fe, Co, Ni; B= Ta, Sb). Ces composés cristallisent dans une structure trirutile et présentent une variété très riche de phases magnétiques. Tous les ordres observés peuvent être classés comme antiferromagnétiques mais diffèrent d'un ordre simple de type Néel. En outre, les substitutions comme Fe_xCo_{1-x}Ta_2O_6, Fe_xNi_{1-x}Ta_2O_6 et Co_xNi_{1-x}Ta_2O_6 conduisent à des domaines de coexistence de phases magnétiques et à des points bicritiques dans les diagrammes de phases T(x). Une caractéristique particulièrement intéressante de ces composés est le caractère quasi-bidimensionnel de leur magnétisme, puisque les ions de métaux de transitions A apparaissent dans des réseaux plans séparés par deux plans d'ions non magnétiques B. Une forte anisotropie de champ cristallin est observée sur les sites magnétiques, résultant de la distorsion axiale des octaèdres d'oxygènes qui entourent ces ions A. Nous avons abordé deux aspects complémentaires: théorique et expérimental. Du point de vue théorique, une reformulation du modèle bidimensionnel jusqu'à present utilisée pour décrire la susceptibilité paramagnétique nous a permi d'obtenir des constants d'échange compatibles avec les types d'ordre magnétique planaire observés. Considérant que le caractère tridimensionnel des structures magnétiques révélées par diffraction neutronique indique l'importance du couplage entre plans, quoique faible, à basse température, nous proposons un modèle tridimensionnel coherent avec les ordonnements observés aussi dans le plan ab qu'au long de l'axe c. Du point de vue expérimental, nous faisons des substitutions sur le site non magnétique pour modifier de façon systematique l'espacement entre les plans magnétiques et, en consequence, le couplage entre eux. En particulier, nous avons synthétisé des composés du type ANb_xTa_{2-x}O_6 avec A = Fe, Ni et Co, restant dans le domaine de stabilité de la phase quadratique, puisque la présence de Nb favorise une phase orthorhombique. Une caractérisation structurale et magnétique de ces systèmes est faite par diffraction de rayons X et de neutrons, complétées par des mesures de susceptibilité magnétique, chaleur spécifique et aimantation en fonction du champ appliqué. Nous étudions également des séries avec Sb à la place de Nb où nous avons également effectué des remplacements sur le site magnétique. / This work aims to contribute to the study of magnetism in insulating oxides of the type AB_2O _6 (A = Fe, Co, Ni, Ta = B, Sb). These compounds crystallize in a trirutile structure and present a rich variety of magnetic phases. All the observed orderings can be classified as antiferromagnetic, but they differ from a simple Néel type order. Moreover, substitutions such as Fe_xCo_{1-x}Ta_2O_6, Fe_xNi_{1-x}Ta_2O_6 and Co_xNi_{1-x}Ta_2O_6 produce several regions of phase coexistence and bicritical points in the phase diagram T vs. x. A particularly interesting characteristic of these compounds is the quasi-two-dimensional nature of their magnetism, since the transition-metal ions A appear in layers separated by two planes of the non-magnetic ions B. They also show strong crystal-field anisotropy on the magnetic sites due to axial distortion of the oxygen octahedra surrounding the A ions. Our approach involves two complementary aspects: theoretical and experimental. From the theoretical point of view, a reformulation of the two-dimensional model used until now to describe the paramagnetic susceptibility allows us to obtain exchange parameters compatible with the observed planar magnetic orderings. Considering that the three-dimensional character the magnetic structures revealed by neutron diffraction indicates that the coupling between planes, although weak, plays an important role to low temperatures, we propose a three-dimensional model consistent with the observed orderings both in the ab plane and along the c axis. From the experimental point of view, we make substitutions on the non-magnetic sites, in order to systematically modify the spacing between planes and, consequently, the coupling between them. In particular, we have synthesized compounds of type ANb_xTa_{2-x}O_6 with A = Fe, Ni and Co, remaining in the stability region of tetragonal phase, since the presence of Nb favors an orthorhombic phase. The structural and magnetic characterization of the systems is done via the X-ray and neutron diffraction as well as measurements of magnetic susceptibility, specific heat, and magnetization as a function of the applied field. We also study series with Sb in place of Nb where we additionally include substitutions in the magnetic site.

La fabrication de prothèses dentaires à base de titane a connu un réel essor ces dernières années. Cependant, la production de ces produits reste assez coûteuse, et le choix de producteurs est encore restreint. Les partenaires du projet de recherche Européen Den-ticast® ont identifié un fort potentiel pour la création d'une machine innovante, capable de produire ces pièces à un coût réduit et à un niveau de qualité élevé. Le projet est axé sur le domaine de la production de pièces de métal pour l'usage dentaire, avec comme objectif, le développement d'un nouveau système polyvalent de coulée des alliages, qui doit permettre de couler l'ensemble des alliages dentaires actuellement commercialisés (NiCr, CoCr), mais aussi le titane et ses alliages. Le prototype Denticast® associe, pour la première fois, la fusion par induction et l'injection par centrifugation à haute vitesse, sous vide secondaire. Entièrement programmable, cette machine doit pouvoir évoluer en fonction du développement de nouveaux alliages sur le marché dentaire.Ce projet réunit des partenaires européens de 3 pays différents (France, Allemagne, Italie) et 3 secteurs d'activité : industriel, académique et odontologique, de sorte que chacun ap-porte un aspect complémentaire au projet. Il comporte 3 phases principales : une phase d'étude, une phase de conception et une phase d'évaluation du prototype.Nos résultats montrent que Denticast® est capable de produire des pièces à usage dentaire de qualité, à grains fins et homogènes, avec un niveau de contrainte résiduelle relativement bas, et une fraction volumique de porosité faible. Les caractéristiques mécaniques sont globalement supérieures ou équivalentes aux systèmes commerciaux du marché. / Although the manufacture of titanium dental prostheses has experienced a real rise in the last years, the production of those products remains quite expensive, and the choice of pro-ducers is still limited. The partners of the European research project Denticast® have identi-fied a high potential for the creation of an innovative machine that can produce those dental parts at low cost and high quality level. The aim of this research is the development of a new, innovative and versatile system of casting alloys, which can cast all dental alloys (NiCr, CoCr) as well as titanium and its alloys. The prototype Denticast® associates for the first time the melting by induction and the injection by centrifugation with high speed under vacuum. Totally preset, this machine should be able to evolve according to the devel-opment of new dental materials.The project combines European partners from 3 different countries (France, Germany and Italy) and three sectors: industrial, academic and dental, so that each of them contributes to the project. It presents three main phases: a study phase, a phase of design and the evalua-tion phase of the prototype.Our results show that Denticast® is able to produce quality parts for dental use with a ho-mogeneous and fine grain, with a low level of residual stress and a relatively low volume of fraction porosity. In most cases, the mechanical characteristics are equal or even greater than other systems on the market at present.