<p> This thesis reports a systematic study of geometrically frustrated f.c.c. double perovskites with both monoclinic (P2₁/n) La₂LiMO₆ and cubic (Fm3m) Ba₂YMO₆ symmetries, where M=Mo (S=1/2), Re (S=1) and Ru (S=3/2). The roles of both the spin quantum number, i.e. quantum spin fluctuations, and the local site symmetry, i.e. orbital ordering, on the determination of the ground magnetic state were studied. All the compounds were prepared by solid state reaction and the structural information and magnetic properties of the compounds were collected using diffraction techniques (X-ray and neutron), de susceptibility, heat capacity, muon spin relaxation (μSR) and solid state NMR. </p> <p> The S=3/2 materials, La₂LiRuO₆ and Ba₂YRuO₆, while highly frustrated with frustration indices f ~ 16 and 8, respectively, both show antiferromagnetic (AF) long range ordering at 24K and 37K, respectively. The Neel temperature of the latter compound was determined for the first time by the heat capacity and neutron diffraction. This compound shows an unusual AF transition as two broader peaks were observed in the susceptibility while La₂LiRuO₆ shows a typical AF behavior. There is about 1 % of Y/Ru site mixing observed by 89Y MAS NMR in Ba₂YRuO₆. </p> <p> For the S=1 materials, monoclinic La₂LiReO₆ shows collective singlet like behavior as zero magnetization was observed in the ZFC susceptibility and a static and diluted spin system was indicated by μSR. On the other hand, the cubic phase, Ba₂YReO₆ surprisingly shows a spin glass behavior confirmed by μSR while no Y/Re site mixing was observed by MAS NMR. It is also a surprising observation that this compound retains cubic symmetry down to 3K where it would have a structure transition subject to the Jahn-Teller theorem. </p> <p> Finally, the S=1/2 compounds, La₂LiMoO₆ and Ba₂YMoO₆, show quite different magnetic behavior. Monoclinic La₂LiMoO₆ shows the presence of at least short range order achieved at 18K according to the heat capacity and μSR measurements. Ba₂YMoO₆ retains cubic symmetry down to 3K and no Jahn-Teller distortion was observed at the limit of the resolution of neutron diffraction. This compound surprisingly appears to remain paramagnetic down to 2K, yet evidence for a collective singlet state was observed by a paramagnetic Knight shift measurement in NMR. This is consistent with an existing theoretical prediction. </p> <p> An extended study on other S=1/2 Mo analogues, Ba₂LuMoO₆ and Ba₂ScMoO₆ is also presented. Both compounds show cubic structure confirmed by X-ray diffraction and paramagnetic behavior down to 2K in the susceptibility. </p> / Thesis / Doctor of Philosophy (PhD)

L'objectif principal de la présente dissertation est d'étudier le rôle des fissures pointues sur le comportement mécanique des cristaux sous charge à l'échelle atomique. La question d'intérêt est la façon dont un cristal pur, qui contient une seule fissure en équilibre mécanique, se déforme. Deux métaux ont été considérés: l'aluminium, qui est ductile à toute température, et le fer, transformé de ductile en fragile à une température décroissante inférieure à T=77K. Les forces de cohésion dans les deux métaux ont été modélisées via les potentiels phénoménologiques "n-body". A (010)[001] mode I nano-crack a été introduit dans le réseau cristallin parfait de chacun des métaux étudiés en utilisant des déplacements appropriés attribués par l'élasticité anisotrope. A T=0K, des configurations de fissures à l'équilibre ont été obtenues par minimisation d'énergie avec un type mixte de conditions aux limites. Les deux modèles ont révélé que les configurations de fissures restaient stables sous une gamme finie de contraintes appliquées en raison de l'effet de piégeage en treillis. La présente thèse propose une nouvelle approche pour interpréter le comportement mécanique intrinsèque des deux systèmes métalliques sous le chargement. En particulier, la réponse ductile ou fragile d'un système cristallin peut être déterminée en examinant si la barrière de piégeage en treillis d'une fissure préexistante est suffisante pour provoquer le glissement de dislocations statiques préexistantes. Les résultats des simulations ainsi que les données expérimentales démontrent que, selon le modèle proposé, l'aluminium et le fer sont ductiles et fragiles à T=0K, respectivement. / The principal aim of the present dissertation is to investigate the role of sharp cracks on the mechanical behaviour of crystals under load at the atomic scale. The question of interest is how a pure crystal, which contains a single crack in mechanical equilibrium, deforms. Two metals were considered: aluminium, ductile at any temperature below its melting point, and iron, being transformed from ductile to brittle upon decreasing temperature below T=77K. Cohesive forces in both metals were modeled via phenomenological n-body potentials. A (010)[001] mode I nano-crack was introduced in the perfect crystalline lattice of each of the studied metals by using appropriate displacements ascribed by anisotropic elasticity. At T=0K, equilibrium crack configurations were obtained via energy minimization with a mixed type of boundary conditions. Both models revealed that the crack configurations remained stable under a finite range of applied stresses due to the lattice trapping effect. The present thesis proposes a novel approach to interpret the intrinsic mechanical behaviour of the two metallic systems under loading. In particular, the ductile or brittle response of a crystalline system can be determined by examining whether the lattice trapping barrier of a pre-existing crack is sufficient to cause the glide of pre-existing static dislocations on the available slip systems. Simulation results along with experimental data demonstrate that, according to the model proposed, aluminium and iron are ductile and brittle at T=0K, respectively.

La synthèse Fischer-Tropsch (FT) permet de convertir un mélange d'hydrogène et de monoxyde de carbone (gaz de synthèse) sélectivement en hydrocarbures avec une distribution large de longueur de chaine. Le gaz de synthèse peut être produit à partir de différentes ressources comme le gaz naturel, le charbon et la biomasse. A la lumière de la volonté de diversifier les sources d'énergies, la synthèse FT peut apporter une contribution cruciale pour la production de carburants liquides. Les catalyseurs à base de Cobalt supportés sur alumine sont utilisés pour produire des cires lourdes. L'activité et la sélectivité dépendent des propriétés structurales et texturales du catalyseur. Cette étude a pour but d'établir un lien entre les propriétés structurales des catalyseurs à base de Cobalt supportés sur alumine et des paramètres cinétiques spécifiques. Pour atteindre cet objectif, il a été mis en oeuvre une étude cinétique en régime permanent couplé à la technique « SSITKA » sur différents échantillons de Cobalt. En utilisant cette méthodologie, il a été trouvé que la conversion en CO sur 5 catalyseurs à base de Cobalt dépend uniquement du nombre de site initial sur la surface atomique de Cobalt réduit. Aucune influence de la taille de particule, de l'orientation de la phase cristalline ou du promoteur n'a pu être mis en évid ence. Les expériences SSTIKA réalisées sur une longue durée ont permis d'estimer le nombre de sites actifs dans les conditions de travail. Enfin, la modélisation cinétique a démontré que l'espèce la plus abondante sur la surface est le monoxyde de carbone adsorbé et que deux intermédiaires distincts de surface conduisent à la production de méthane et des hydrocarbures plus lourds / The Fischer-Tropsch synthesis (FTS) converts a mixture of hydrogen and carbon monoxide (syngas) selectively into hydrocarbons with a large chain length distribution. Syngas can be produce from different resources such as natural gas, coal and biomass. In the light of energy resource diversi fication, FTS can make a crucial contribution to the production of liquid fuels. Alumina supported cobalt catalysts are used to produce heavy waxes. The activity and selectivity depend on the structural and textural properties of the catalyst. This study aims at establishing a link between the structural properties of alumina supported cobalt catalysts and specific kinetic parameters. To this purpose, the steady-state and SSITKA kinetics over different cobalt samples have been carried out. By using this met hodology, it was found that the CO conversion over 5 cobalt catalysts only depends on the initial number of reduced cobalt surface atoms. No influence of the cobalt particle size, phase orientation or promotor could be identified. SSITKA experiments during long-term catalyst testing allowed estimating the number of active sites under working conditions. Further modelling showed that the most abundant surface species is adsorbed carbon monoxide and that two distinct surface intermediates lead to the production of methane and higher hydrocarbons

Les substrats métalliques ont été élaborés par des traitements thermomécaniques (laminages et recuits)sur des alliages Fe48%Ni et Ni5%W dans le but d'obtenir une hypertexture Cube indispensable à l'épitaxie de l'YBaCuO et du silicium dans la fabrication des câbles supraconducteurs et des cellules photovoltaïques à couches minces. Le rôle des éléments d'alliages tels que le soufre et le niobium sur la recristallisation et la formation de l'hypertexture Cube a été étudié dans le Fe48%Ni. Il a été montré que l'ajout du soufre favorise le développement de la texture Cube alors que l'ajout du niobium empêche la formation de la texture Cube. Le soufre se combine avec le Mn pour former les précipités MnS qui contribuent à l'augmentation de la différence d'énergie stockée entre l'orientation Cube et les orientations de laminage à froid (ECube/autres) quand le soufre augmente. Ce gap d'énergie explique explique l'acuité de la texture Cube avec l'ajout du soufre. Contrairement au soufre, l'ajout du niobium empêche la formation de la texture Cube, ce résultat s'explique par le fait la différence d'énergie stockée entre l'orientation Cube et les orientations de laminage diminue avec l'ajout du niobium. Pour expliques tous ces résultats, les analyses de microstructures et textures ont été faites par la technique EBSD et l'énergie stockée a été estimée à partir de la diffraction des neutrons sur les états déformés. / Substrate tapes were prepared by cold rolling and annealing of a Fe48%Ni and Ni5%W alloys in order to obtain Sharp Cube {100}<001> oriented substrate for photovoltaic thin films and superconductor cables in particular.The effect of microalloying elements sulfur and niobium on recrystallization and sharp Cube formation was studied in Fe48%Ni. It was shown that the addition of sulfur promotes the formation of Cube grains while the addition of niobium prevents the Cube grains formation. Regarding sulfur, it combines with manganese to form the MnS precipitates wich increases the stored energy difference between Cube component and others cold rolled components ECube/other when sulfur is added. This stored energy difference explains the sharpness of the Cube texture when sulfur is added. On the contrary the niobium microalloying element addition prevents the formation of Cube grains. This could be explained by the fact that stored energy of cold-rolled components decreases with the addition of niobium and thus decreases Cube grains fraction when niobium is added. In order to explain these results, the development of Cube texture during recrystallization has been investigated in detail by EBSD, furthermore, the effect of stored energy has been studied by carrying out neutron diffraction measurements on the deformed states.

No description available.

This thesis is an attempt to understand ways to improve thermo-mechanical and structural properties of nano-structured materials. A detailed study on computational design and analysis of metallic nanowires is carried out. Molecular dynamic simulation method is applied. In particular, FCC metallic nanowires, NiAl, and CuZr nanowires are studied. Various bottom-up approaches are suggested with improved structural and thermo¬mechanical properties. In the first part of the thesis, Cu nanowires are considered. Existence of a novel and stable pentagonal multi-shell nanobridge structure of Cu under high strain rate tensile loading is reported. Such a structure shows enhanced mechanical properties. A three-fold pseudo-elastic-plastic shape recovery mechanism in such nanowires is established. This study also shows that the length of the pentagonal nanobridge structures can be characterized by its inelastic strain. It is also reported that an initial FCC structure is transformed into a new HCP structure. The evidence of HCP structure is confirmed with the help of experimental data published in the literature. Subsequent to the above study, a novel mechanism involving coupled temperature-stress dependent reorientation in FCC nanowires is investigated. A detailed map is generated for size dependent stress-temperature induced solid-solid reorientation in Cu nanowires. In the second part of the thesis, deformation mechanisms in NiAl based intermetallic nanowires are studied. A novel mechanism of temperature and cross-section dependent pseudo-elastic/pseudo-plastic shape and strain recovery by an initial B2 phase of NiAl nanowire is reported. Such a recoverable strain, which is as high as ~ 30%, can potentially be utilized to realize various types of shape memory and strain sensing phenomena in nano-scale devices. An asymmetry in tensile and compressive yield strength behavior is also observed, which is due to the softening and hardening of the nanowires under tensile and compressive loadings, respectively. Two different deformation mechanisms dominated by twinning under tension and slip under compression are found. Most interestingly, a superplastic behavior with a failure strain of up to 700% in the intermetallic NiAl nanowires is found to exist at a temperature of 0.36Tm. Such superplastic behavior is attributed to the transformation of the nanowire from a crystalline phase to an amorphous phase after yielding of the nanowire. In the last part the work, another type of nanowires having Cu-Zr system is considered. A novel stress induced martensitic phase transformation from an initial B2 phase to BCT phase in a CuZr nanowire under tensile loading is reported. It is further shown that such a stress induced martenistic phase transformation can be achieved under both tensile as well as compressive loadings. Tensile-compressive asymmetry in the stress-strain behavior is observed due to two different phase transformation mechanisms having maximum transformation strains of ~ 5% under compressive loading and ~ 20% under tensile loading. A size and temperature dependent tensile phase transformation in the nanowire is also observed. Small nanowires show a single step tensile phase transformation whereas the nanowires with larger size show a two step deformation mechanism via an intermediate R-phase hardening followed by R-phase yielding. A study of energetic behavior of these nanowires reveals uniform distribution of stress over the nanowire cross-section and such stress distribution can lead to a significant improvement in its thermo-mechanical properties. Similar improvement is demonstrated by designing the nanowires via manipulating the surface configuration of B2-CuZr system. It is found that the CuZr nanowires with Zr atoms at the surface sites are energetically more stable and also give a uniform distribution of stresses across the cross-section. This leads to the improvement in yield strength as well as failure strain. An approach to design energetically stable nano-structured materials via manipulating the surface configurations with improved thermo-mechanical properties is demonstrated which can help in fundamental understanding and development of similar structures with more stability and enhanced structural properties. Further ab-initio and experimental studies on the confirmation of the stability of the nanowires via manipulating the surface site is an open area of research and related future scopes are highlighted in the closure.

Plastic deformation of metals and alloys are invariably accompanied by the development of texture. The origin of texture is attributed to the deformation micro-mechanisms associated with processing. The face-centered cubic (FCC) metals and alloys are known to exhibit two distinct types of textures when subjected to large strain rolling deformation, namely, (i) Cu-type texture, commonly seen in high/medium stacking fault energy (SFE) materials, (ii) Bs-type texture in low SFE materials. The circumstances that could result in the formation of Bs-type texture in low SFE materials still remains an open question and no definite mechanism has been uniquely agreed upon. Apart from the SFE, grain size could also influence the deformation mechanism and hence the deformation texture. It is well known that in materials with grain sizes less than 100 nm (referred to as nano-crystalline materials), the microstructures contain large fraction of grain boundaries. This subsequently introduces a variety of deformation mechanisms in the microstructure involving grain boundary-mediated processes such as grain boundary sliding and grain rotation, in addition to slip and twinning. A clear understanding of texture evolution in nano-crystalline materials, particularly at large strains, is a topic that remains largely unexplored. The present work is an attempt to address the aforementioned issues pertaining to the evolution of deformation texture, namely, (i) the effect of SFE and (ii) the effect of grain size, in FCC metals and alloys. Nickel-cobalt alloys are chosen as the model system for the present investigation. The addition of cobalt to nickel leads to a systematic reduction of SFE as a function of cobalt content. In this thesis, three alloys of Ni-Co system have been considered, namely, nickel – 20 wt.% cobalt, nickel – 40 wt.% cobalt and nickel – 60 wt.% cobalt. For a comparison, pure nickel has also been subjected to similar study. Chapter 1 of the thesis presents a detailed survey of literature pertaining to the evolution of rolling textures in FCC metals and alloys, and chapter 2 includes the details of the experimental techniques and characterization procedures, which are commonly employed for the entire work. Chapter 3 addresses the effect of stacking fault energy on the evolution of rolling texture. The materials subjected to study in this chapter are microcrystalline Ni-Co alloys. The texture evolution in Ni-20Co is very similar to pure Ni, and a characteristic Cu-type rolling texture is observed. The evolution of texture in these materials is primarily attributed to the intense dislocation activity throughout the deformation stages. In Ni-40Co, a medium SFE material, the rolling texture was predominantly Cu-type up to a strain of ε = 3 (95% thickness reduction). However, beyond this strain level, namely at ε = 4 (98%), the texture gets transformed to Bs-type with orientations maxima predominantly close to Goss ({110} <001>) position. Simultaneously, the Cu component which was dominant until 95% reduction has completely disappeared. The analysis of microstructures indicate that deformation is mostly accommodated by dislocation slip up to 95%, however, at ε > 3, Cu-type shear bands get initiated, preferably in the Cu-oriented ({112} <111>) grains. The sub-grains within the shear bands show preferred orientation towards Goss, which indicates that the Cu component should have undergone transformation and resulted in high fraction of Goss component. In Ni-60Co alloy, Bs-type texture forms in the early stages of deformation (ε ~ 0.5) itself and further deformation results in strengthening of the texture with an important difference that the maximum in orientation distribution has been observed at a location close to Goss component, rather than at exact Bs-location. The development of Bs-type texture is accompanied by the complete absence of Cu and S components. Extensive EBSD analyses show that the deformation twinning gets initiated beyond 10% reduction and was found extensively in most of the grains up to 50% reduction. At higher strains, tendency for twinning ceases and extensive shear banding is observed. A non-random distribution of orientations close to Goss orientation was found within the shear bands. The near-Goss component in the Ni-60Co alloy can be explained on the basis of deformation twinning and shear banding. Thus, a reasonable understanding of the deformation texture transition in the extreme SFE range has been developed. In chapter 4, the effect of fine grain size on the evolution of rolling texture has been addressed. Nanocrystalline (nc) nickel-cobalt alloys with a mean grain size of ~20 nm have been prepared by pulse electro-deposition method. For a comparison, nc Nickel (without cobalt) with similar grain size has also been deposited. For all the materials, a weakening of the initial fiber texture is observed in the early stage of room temperature rolling (ε ~ 0.22). A combination of equiaxed grain microstructure and texture weakening suggests grain boundary sliding as an operative mechanism in the early stage of rolling. At large strain (ε = 1.2), Ni-20Co develops a Cu-type texture with high fractions of S and Cu components, similar to pure Ni. The texture evolution in Ni-40Co and Ni-60Co alloys is more towards Bs-type. However, the texture maximum occurs at a location 10° away from the Goss. The evolution of Cu and S components in nc Ni-60Co alloy takes place simultaneously along with the α-fiber components during rolling. Microstructural investigation by TEM indicates deformation twinning to be more active in all the materials up to 40% reduction. However, no correlation could be drawn between the texture evolution and the density of twins. The deformation of nc Ni-20Co alloy, is also accompanied by significant grain growth at all the stages of rolling. The increase in grain size, subsequently, renders the texture to be of Cu-type. However, Ni-40Co and Ni-60Co alloys show high grain stability. The absence of strain heterogeneities such as shear bands, and the lack of significant fraction of deformation twins indicate that the observed Bs-type texture could be due to planar slip. The increase in deformation beyond 70% reduction caused a modest reduction in the intensity of deformation texture. The microstructural observation indicates the occurrence of restoration mechanisms such as recovery/ recrystallization at large strains. The overall findings of the investigation have been summarized in chapter 5. The deformation mechanism maps relating stacking fault energy with amount of strain and with grain size are proposed for micro- and nano- crystalline materials respectively.