Les conditions nécessaires pour obtenir des céramiques transparentes (absence de porosité, absence de seconde phase) requièrent une totale maitrise de chacune des étapes (synthèse, mise en forme, frittage) intervenant dans le processus, ce qui rend encore difficile l'industrialisation d'un procédé d'élaboration reproductible. Dans ce contexte, ce travail de thèse s’est focalisé à développer et évaluer la capacité du procédé de frittage micro-ondes à améliorer la robustesse du processus d'élaboration de spinelle transparent. La mise en place d'un dispositif de dilatométrie optique et une nouvelle technique de calibration originale, basée sur la fusion d'oxyde, ont permis de caractériser le frittage micro-ondes du spinelle pur avec une plus grande confiance. Même si aucun effet lié au procédé micro-onde n’a pu être mis en évidence pour le frittage du spinelle pur (trajectoire de frittage, mécanisme de densification et évolution de la porosité identiques), ces travaux ont démontré que l’impact de dopants pouvait être amplifié en présence du rayonnement micro-ondes. Ainsi, un décalage des courbes de retrait vers les basses températures a été observé lors du frittage micro-ondes du spinelle dopé avec TiO2 et MgO. L'existence d'un couplage particulier entre les défauts ponctuels chargés (lacunes, cations interstitiels) et le champ électrique pourrait être à l'origine de ce phénomène. Ces travaux de thèse ont également démontré la faisabilité de fritter des pièces de spinelle carrées de grandes dimensions (< 65 mm) compatibles avec un post-traitement HIP, en vue d'obtenir des pièces transparentes. Pour cela, un four micro-ondes monomode 915 MHz a été automatisé et une cellule de frittage adaptée a été développée. A l'issue du traitement HIP, les pièces ont présenté une transparence et des propriétés mécaniques (dureté; ténacité) comparables à celles des pièces pré frittées par voie conventionnelle. / The elaboration of transparent ceramics (no porosity, no second phase) requires a strict control of all steps of the process (synthesis, shaping, sintering). As a result, the industrialization of a reproducible process is still difficult to achieve. In this context, this thesis has focused on developing and evaluating the ability of the microwave sintering process to improve the robustness of the transparent spinel elaboration process.The set-up of an optical dilatometer and a new original calibration method, based of melting of different oxides, allowed to characterize the microwave sintering with a greater confidence. Even if no microwave effect has been observed on the sintering of pure spinel (identical sintering trajectory, densification mechanism and porosity evolution), this work has shown an amplification of the impact of dopants during microwave sintering. A shift of the shrinkage curves towards the low temperatures was observed during the microwave sintering of the spinel doped with TiO2 and MgO. The existence of a particular interaction between charged point defects (vacancies, interstitial cations) and the electric field could explain this phenomenon.This work thesis has shown also the feasibility of sintering of large square spinel pieces (< 65 mm) compatible with post treatment HIP in order to produce transparent ceramics. For this purpose, the microwave system working at 915 MHz was automatized and a new sintering configuration was developed. After a post-treatment, the transparency and mechanical properties (hardness; tenacity) of microwave pre-sintered samples were comparable with those of samples sintered by conventional heating.

The CoFe2O4 nanocrystals are highly attractive due to their magnetic properties - large coercivity (up to 2 T at low temperatures) with moderate saturation magnetization (80 A.m2.kg-1), remarkable chemical stability and mechanical hardness. Upon RE doping, a significant change of the properties is expected. This thesis is focused on investigation of Co1-LaxFe2O4, x = 0.05 - 0.5 and CoLaxFe2-x O4 x = 0.05 - 0.2 nanoparticles prepared by sol-gel and microemulsion method, respectively, with varying particle size according to the final heat treatment. In the former case, the particles were embedded in amorphous SiO2 matrix, while in the latter case, the samples were matrix-free. The samples were characterized using X-ray diffraction and Mössbauer spectroscopy. Measurements of the temperature dependence of the ZFC-FC magnetization revealed that the blocking temperature is above the room temperature. The values of coercivity and saturation magnetization strongly depend on particles size determined by method of preparation. The higher coercivity values of about 2 T at 10 K show the samples prepared by the sol-gel method. The obtained results are discussed in the context of preparation method, particle size and level of La doping.

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2020. / This dissertation presents the results of the ab-initio based computational studies of spinel lithium manganese oxide (LiMn2O4) bulk, surfaces, and the adsorption of an organic electrolyte, ethylene carbonate. The spinel LiMn2O4 is one of the most promising cathode materials for Lithium-ion batteries because of its affordability, nontoxicity, and improved safety compared to commercially used LiCoO2. However, it also suffers from the irreversible capacity due to the electrolyte-cathode interactions which lead to manganese (Mn) dissolution. Using the spin-polarized density functional theory calculations with on site Coulomb interactions and long-range dispersion corrections [DFT+U−D3−(BJ)], we investigated the bulk properties, surface stability and surface reactivity towards the ethylene carbonate (EC) during charge/discharge processes. Firstly, we explored the structural, electronic, and vibrational bulk properties of the spinel LiMn2O4. It was found that the bulk structure is a stable face-centred cubic structure with a bandgap of 0.041 eV and pseudo-gap at the Fermi level indicating electronic stability. Calculated elastic constants show that the structure is mechanically stable since they obey the mechanical stability criteria. The plotted phonon curves show no imaginary vibrations, indicating vibrational stability. To study the charge/discharge surfaces, we modelled the fully lithiated and the partially delithiated slabs and studied their stability. For the fully lithiated slabs, Li-terminated (001) surface was found to be the most stable facet, which agrees with the reported experimental and theoretical data. However, upon surface delithiation, the surface energies increase, and eventually (111) surface becomes the most stable slab as shown by the reduction of the plane in the particle morphologies. Finally, we explored the surface reactivity towards the ethylene carbonate during charge/discharge processes. The ethylene carbonate adsorption on the fully lithiated and partly delithiated facets turn to enhance the stability of (111) surface. Besides the strong interaction with the (111) surfaces, a negligible charge transfer was calculated, and it was attributed by a large charge rearrangement that takes place within the surfactant upon adsorption. The wavenumbers of the C=O stretching showed a red shifting concerning the isolated EC molecule

Thesis (Ph.D. (Physics)) -- University of Limpopo, 2019 / LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion batteries which, despite its high average voltage of lithium intercalation, suffers crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the energy density of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of Ni incorporation and its effect on the electrochemical properties of this spinel are not fully understood. Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and LiNixMn2-xO4 have been calculated out using density functional theory employing the pseudo-potential plane-wave approach within the generalised gradient approximation, together with Virtual Cluster Approximation. The structural properties included equilibrium lattice parameters; electronic properties cover both total and partial density of states and mechanical properties investigated elastic properties of all systems. Secondly, the pressure variation of several properties was investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are consistent with the available experimental and literature results. The average Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties predicted the nature of bonding for both pure and doped systems with partial density of states showing the contribution of each metal in our systems. All systems are shown to be metallic as it has been previously observed for pure spinel LiMn2O4, and mechanical properties, as deduced from elastic properties, depicted their stabilities. Furthermore, the cluster expansion formalism was used to investigate the nickel doped LiMn2O4 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while iv maintaining the predictive power and accuracy of first-principles density functional methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31 generated various structures with different concentrations and symmetries. The findings predict that all nickel doped LMO structures on the ground state line are most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16, Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups they were further investigated by calculating the mechanical and vibrational properties, where the elastic constants and phonon vibrations indicated that the structures are stable in accordance with stability conditions of mechanical properties and phonon dispersions. Lastly, a computer program that identifies different site occupancy configurations for any structure with arbitrary supercell size, space group or composition was employed to investigate voltage profiles for LiNixMn2-xO4. The density functional theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000K, which is in excellent agreement with experiments. It was also found that the configurational entropy is much lower than the maximum entropy at 1000K, indicating that higher temperatures are required to reach a fully disordered solid solution. The maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the experimental value. The temperature has a negligible effect on the Li intercalation voltage of the most stable composition. The approach presented here shows that moderate Ni doping of the LiMn2O4 leads to a substantial change in the average voltage of lithium intercalation, suggesting an attractive route for tuning the cathode properties of this spinel. / National Research Foundation (NRF)

No description available.

<p>At solid oxide fuel cell (SOFC) operating temperatures (650^oC--800^oC), the chromia scale growth on the interconnect surface and chromium poisoning of cathode can lead to performance degradation of the whole cell. A spinel coating can be effective for blocking chromium outward diffusion to overcome this issue. In this thesis, two spinel-forming systems, Zn-Mn-O and Co-Cu-Mn-O were studied to identify a suitable coating.</p> <p>In-situ high temperature XRD was used to identify the phases in the Zn-Mn-O system between 600^oC and 1300^oC. The results showed that cubic spinel phase was stable only at high temperatures (above 1200^oC) and when the temperature decreases, the cubic phase tends to deform to tetragonal structure. In addition, the conductivity results showed low conductivities (below 3 S/cm) at SOFC operating temperature. Thus, the Zn-Mn-O system is not suitable for SOFC interconnect coatings.</p> <p>Another potential coating material analyzed was the Co-Cu-Mn-O system. This system exhibited promising conductivity values. Electrodeposition was used to apply Co-Cu-Mn-O coatings on both ferritic stainless steel and chromium-based alloy (Cr-5Fe) followed by oxidation in air at 800^oC. The spinel coating formed nicely on the stainless steel substrate. However, on the chromium plate, nitride formation, blistering and metal isolation were the common problems that occurred during the oxidation process. In order to improve the quality of coating on the chromium alloy, different heat treatments were explored, such as annealing in reducing atmosphere, oxidation in pure oxygen / mixed gas and decreasing the oxidation temperature. The objective of modifying the heat treatment was to produce adherent, dense coatings.</p> / Master of Applied Science (MASc)

No description available.

<p>U ovom radu prikazani su rezultati ispitivanja strukturnih i magnetnih osobina čistih cink- ferita, ZnFe₂O₄ i cink-ferita sa dodatkom indijuma Zn_1-xIn_xFe₂O₄ i itrijuma, ZnY_xFe_2-xO₄, gde je 0 &le; x &le; 0,6. Prahovi na bazi cink-ferita su sintetisani koristeći nisko temperaturnu metodu sinteze iz tečne faze &ndash; metodu koprecipitacije. Osnovni cilj ove doktorske disertacije je bio da se utvrdi veza između uslova sinteze, uticaja različitih katjona, strukture i osobina čistih cink-feritnih nanočestica, kao i cink-ferita sa dodatkom indijuma i itrijuma. Radi praćenja uticaja veličine čestica dobijenog praha na strukturu i osobine ovih materijala, sintetisani čist cink-ferit je kalcinisan na različitim temperaturama. Posebno je razmatran i uticaj dodatka različitih jona na distribuciju i preraspodelu katjona u spinelnoj strukturi. Pored toga ispitivan je uticaj tako pripremljenih prahova na njihove magnetne osobine. Rentgenostrukturna i TEM analize potvrdili su da ispitivani uzorci spadaju u klasu nanomaterijala spinelne strukture. Analiza Raman i M&ouml;ssbauer spektara je ukazala na moguću raspodelu katjona između tetraedarskih i oktaedarskih mesta, te formiranje delimično inverznog spinela. Ispitivanja magnetnih osobina su pokazala da histerezisne petlje ne pokazuju saturaciju u prisustvu jakog magnetnog polja, &scaron;to je potvrdilo superparamagnetnu i jednodomensku prirodu čestica. Pokazano je da pored uticaja veličine čestica, dodatak različitih katjona (u ovom slučaju itrijum i indijum) ima veliki uticaj na uređenje strukture, a posledično i na magnetno pona&scaron;anje ispitivanih nanočestičnih sistema.</p> / <p> This thesis presents the results of the investigation of the structural and magnetic properties of nanostructured zinc ferrites, ZnFe2O4 and zinc ferrites supstituted with different amount of indium and yttrium, Zn1-xInxFe2O4 and ZnYxFe2-xO4 (0 &le; x &le; 0,6). Powders based on zinc ferrites were synthesised by a low temperature wet-chemical method &ndash; coprecipitation. The main purpose of this thesis was to establish the relationship between the synthesis, dopants, structure and properties of zinc ferrite based materials. Nanoparticles of ZnFe2O4 were calcined at different temperatures in order to elucidate the influence of the particle size on the magnetic properties of the obtained nanoparticles. In addition, we have investigated the effect of dopant addition on cation distribution in spinel structure, in order to modify the magnetic properties and to obtain the magnetic ceramics with improved properties compared to the bulk-counterparts. The results of X-ray and TEM analyses confirmed the nanosized nature and spinel type structure of the investigated samples. Raman and M&ouml;ssbauer spectroscopy studies implied on the possible cation distribution between the tetrahedral and octahedral sites and formation of the partially inversed spinel. The study of the magnetic properties showed that hysteresis loops do not saturate even in the presence of high magnetic fields, which confirmed the superparamagnetic and single domain nature of the samples. These observations imply that, besides the particle size, doping (e.g. yttrium and indium) causes significant structural rearrangements which in turn induce changes in magnetic behavior of the investigated nanoparticulate systems.</p>

Size and Shape Controlled Synthesis and Superparamagnetic Properties of Spinel Ferrites Nanocrystals Qing Song 216 pages Directed by Dr. Z. John Zhang The correlationship between magnetic properties and magnetic couplings is established through the investigations of various cubic spinel ferrite nanocrystals. The results of this thesis contribute to the knowledge of size and shape controlled synthesis of various spinel ferrites and core shell architectured nanocrystals as well as the nanomagnetism in spinel ferrites by systematically investigating the effects of spin orbital coupling, magnetocrystalline anisotropy, exchange coupling, shape and surface anisotropy upon superparamagnetic properties of spinel ferrite nanocrystals. A general synthetic method is developed for size and shape control of metal oxide nanocrystals. The size and shape dependent superparamagnetic properties are discussed. The relationship between spin orbital coupling and magnetocrystalline anisotropy is studied comparatively on variable sizes of spherical CoFe2O4 and Fe3O4 nanocrystals. It also addresses the effect of exchange coupling between magnetic hard phase and soft phase upon magnetic properties in core shell structured spinel ferrite nanocrystals. The role of anisotropic shapes of nanocrystals upon self assembled orientation ordered superstructures are investigated. The effect of thermal stability of molecular precursors upon size controlled synthesis of MnFe2O4 nanocrystals and the size dependent superparamagnetic properties are described.

Biomass gasification offers the chance to produce carbon neutral, renewable fuels. One of the main problems facing the commercialization of biomass gasification technology is the presence of large quantities of methane and carbon dioxide in the biogas. Catalytic reforming of these wastes allows for effective utilization of biomass derived syngas. In most reforming studies, impregnation methods are the primary synthesis technique. Impregnation methods often lead to poor dispersion and are un-reproducible from batch to batch. In the development of a novel catalyst for reforming applications, another preparation method is implemented, controlled adsorption (CA). Ni/Al2O3 and Co/Al2O3 prepared by CA are compared against catalysts that were prepared by a more traditional method, dry impregnation (DI). It is found that controlling the metal deposition provides catalysts with higher dispersion and consequently higher activity for methane dry reforming. NiAl2O4 catalysts prepared by Pechini synthesis were also studied for catalytic conditioning of biomass derived syngas. Physicochemical characterization revealed unique structural properties, indicated a high degree of mobility of nickel in the aluminate structure, and demonstrated the regeneration properties of nickel aluminates under harsh reaction conditions, which will be important at extended reaction times when catalyst regeneration becomes necessary. Fourfold coordinated nickel species are believed to be responsible for high, stable methane dry reforming activity and metallic nickel is believed to be the active site that allows for high, stable conversion during methane dry reforming.