Investigating materials with disordered structures using total neutron scattering

The structures of a variety of disordered materials were determined using the technique of total neutron scattering. The synthesis of various polymorphs of Ga2O3 and related materials was investigated and the structures of the hitherto uncharacterised polymorphs were examined in detail. The structure of y-Ga2O3 was found to be a cubic defect spinel with four partially occupied Ga sites, however, the octahedral Ga coordination environments were found to be distorted from the average cubic structure. The cation distribution in y-Ga2O3 was found to depend on particle size and synthesis method. Examination of the structure of E-Ga2O3 revealed that it is analogous to a disordered, hexagonal form of E-Fe2O3. The poorly crystalline product of the thermal decomposition of Ga(NO3)3.9H2O was found to be a nanocrystalline modification of E-Ga2O3, rather than a distinct phase with the bixbyite structure, as had been previously reported. The structure of a novel gallium oxyhydroxide, Ga5O7(OH), was determined to be analogous to tohdite, Al5O7(OH), and in its thermal decomposition pathway was revealed a new Ga2O3 polymorph: orthorhombic K-Ga2O3. A solvothermal synthetic route to spinel structured ternary gallium oxides, of general formula MxGa3-xO4-y, was developed. The structures of the materials where M = Zn or Ni were found to be consistent with those previously published. The materials where M = Co or Fe possess novel, oxygen-deficient compositions and exhibit interesting magnetic behaviour. A series of cerium bismuth oxides of formula Ce1-xBixO2-1/2x were found to adopt the cubic fluorite structure with significant local distortion due to the preference of Bi3+ for an asymmetric coordination environment. A sodium cerium titanate pyrochlore was also structurally characterised and it was determined that, due to the presence of three different cations on the A site, the local structure required a model with reduced symmetry. In situ neutron scattering experiments were carried out on amorphous zeolite precursor gels in the presence of the reaction liquid. These experiments revealed structural features unique to the gel, and proved that the gel undergoes irreversible structural changes on drying. Preliminary analysis of the gel structure indicated that the Na+ cations play an important role in the development of the ordered zeolitic framework, and revealed no strong evidence for the existence of discrete structural building units in the gel

Enhanced heterogeneous nucleation on oxides in Al alloys by intensive melt shearing

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.This study was funded by the EPSRC and Norton Aluminium Limited, UK

Design and characterization of doped Lithium Rich Layered Oxides for Lithium Ion Battery

Lithium-Rich Layered Oxides (LRLO) are opening new frontiers for high-capacity/high-voltage positive electrodes in Li-ion batteries to meet the challenges of green and safe transportation as well as cheap and sustainable stationary energy storage from renewable sources. LRLO exploit the extra-lithiation provided by the Li1.2TM0.8O2 stoichiometries to disclose specific capacities beyond 200-250 mAhg-1 and working potentials in the 3.4-3.8V vs Li. In my thesis, I demonstrated a novel doping strategy by the substitution of cobalt in the transition metal layer of the lattice with aluminum and lithium, resulting in new optimized layered materials, i.e. Li1.2+xMn0.54Ni0.13Co0.13-x-yAlyO2, with outstanding electrochemical performance in full Li-ion batteries, improved environmental benignity and reduced manufacturing costs compared to the state-of-the-art. Furthermore, the last step deals the application of over-lithiation to demonstrate experimentally a Co-free over-lithiated LRLO material, i.e. Li1.25Mn0.625Ni0.125O2. After that, my research focused on a novel approach to investigate the structural complexity of pristine materials, involving the use of supercells, i.e. unit cells larger than the conventional ones, and FAULTS software, to take into account the staking faults defects. A combination of ex situ techniques has been used as a tool to understand the structural evolution of Li1.28Mn0.54Ni0.13Co0.02Al0.03O2. This part of the research identified that significant changes occurred during electrochemical cycling, showed the irreversible changes in the cell parameters and the presence of a new phase. Finally, innovative non-aqueous electrolytes for Li-ion batteries with superior safety features were investigated. Three ionic liquid, Pyr1,nTFSI with n=4,5,8, have been used as addditive to improve liquid electrolytes. These electrolyte formulations have been analyzed by comparing chemical-physical properties and electrochemical stability

Metal droplet entrainment by solid particles in slags : a combined phase field-experimental approach

This doctoral work investigated metal droplet entrainment by solid particles in slags with a combination of two experimental set-ups and two phase field models. The binary model with limited complexity already clarified our view of the interaction between metal droplets and nonreacting solid particles to a great extent. For example, the fact that the movement of one phase with respect to the others influenced the apparent wetting regime is very interesting for the interpretation of experimentally obtained results. Moreover, the two different types of experiments confirmed that a chemical reaction might lay at the origin of the attachment, but that it requires nucleation sites in the form of metal droplets before it takes place. However, the first phase field model assumed nonreactive solid particles. Thus, a model concerning the growth of the solid phase in a realistic quaternary oxide system was also considered. Future work needs to consider the interaction of reacting metal droplets with reacting solid particles in a realistic liquid slag

Nucleation and growth behavior of multicomponent secondary phases in entropy-stabilized oxides

The rocksalt structured (Co,Cu,Mg,Ni,Zn)O entropy-stabilized oxide (ESO) exhibits a reversible phase transformation that leads to the formation of Cu-rich tenorite and Co-rich spinel secondary phases. Using atom probe tomography, kinetic analysis, and thermodynamic modeling, we uncover the nucleation and growth mechanisms governing the formation of these two secondary phases. We find that these phases do not nucleate directly, but rather they first form Cu-rich and Co-rich precursor phases, which nucleate in regions rich in Cu and cation vacancies, respectively. These precursor phases then grow through cation diffusion and exhibit a rocksalt-like crystal structure. The Cu-rich precursor phase subsequently transforms into the Cu-rich tenorite phase through a structural distortion-based transformation, while the Co-rich precursor phase transforms into the Co-rich spinel phase through a defect-mediated transformation. Further growth of the secondary phases is controlled by cation diffusion within the primary rocksalt phase, whose diffusion behavior resembles other common rocksalt oxides

Reclamation of reactive metal oxides from complex minerals using alkali roasting and leaching- an improved approach to process engineering

In nature, the commonly occurring reactive metal oxides of titanium, chromium, aluminium, and vanadium often chemically combine with the transition metal oxides such as iron oxides and form complex minerals. Physico-chemical separation of transition metal oxides from the remaining reactive metal oxides is therefore an important step in the purification of reactive oxide constituents. Each purification step has quite a high energy requirement at present. Current practice in industry yields sulphate and neutralized chloride waste from titanium dioxide enrichment, red mud from bauxite refining, slag and leach residues from vanadium extraction and chromite ore process residue (COPR) from chromate processes. In this review article, a novel alkali-based oxidative roasting and aqueous leaching for the extraction of mineral oxides is explained in the context of the original work of Le Chatelier in 1850, which was unsuccessful in the industrialization of bauxite processing for alumina extraction. However, much later in the 19th century the alkali-based oxidative mineral roasting was successfully developed for industrial scale manufacturing of chromate chemicals, which yields COPR. The crystal chemistry of mineral oxides, namely alumina, titanium dioxide, and chromium oxide in naturally occurring minerals is briefly reviewed in the context of chemical extraction, which is then developed as a model for developing thermodynamic chemical equilibrium principles for analyzing the physical separation and enrichment of such reactive metal oxides by forming water-soluble and water-insoluble alkali complexes. The involvement of the alkali roasting chemistry of non-magnetic titaniferous mineral waste is also reported in the initial separation of rare-earth oxide mixtures for subsequent separation of individual oxides. The paper concludes with a generic approach to process chemistry which minimizes waste generation and therefore helps in reducing the overall process and energy costs. Examples of recovering alkali from high pH solution using carbon dioxide are also demonstrated

Development of spinel-based electrode supports for solid oxide fuel cells

The high temperature oxidation of ferritic stainless steel interconnects results in chromium poisoning of the solid oxide fuel cell (SOFC) electrodes, which is a limiting factor for their utilisation as SOFC interconnects. Chromium-rich spinel materials were studied as electrode supports that would be situated at the interface between interconnect and electrode, in order to reduce the effect of chromium poisoning of the electrodes. The main goal of this thesis was to find chromium-rich spinel materials with good electrical conductivity (σ ≥ 0.1 S∙cm⁻¹) in air and reducing atmosphere, chemically and mechanically stable in SOFC testing conditions. The structure and properties of newly formulated chromium-rich spinels, such as Mn₁₊ₓCr₂₋ₓO₄ (x = 0, 0.5), MnFeₓCr₂₋ₓO₄ (x = 0.1, 1), MgMnCrO₄, MnLiₓCr₂₋ₓO₄ (x = 0.1) and MgMₓCr₂₋ₓO₄, (M = Li, Mg, Ti, Fe, Cu, Ga) were studied aiming at their application as electrode support material for solid oxide fuel cells. Cation distributions were determined by Rietveld refinement from X-ray diffraction (XRD), within the limits of XRD precision and correlated with electrical properties determined experimentally. The chemical stability in reducing conditions was studied and the reduction effects upon materials were evaluated by XRD phase analysis and microstructure analysis. It was found that MnMₓCr₂₋ₓO₄ materials have a limited stability to reduction, only MnCr₂O₄ proved to have good stability when reduced, with negative influence for its p-type semiconductor conductivity. Even though MnFeCrO₄ had limited stability to reduction, in reducing conditions the conductivity changed from p-type to n-type semiconductor. A similar behaviour to reduction was observed for MgFeCrO₄. Also the mechanical and chemical compatibility of some spinels with YSZ was studied in terms of thermal expansion coefficient (TEC/K⁻¹), sintering step and possible chemical reactions. Lithium titanate spinels, starting with LiCrTiO₄, were investigated in terms of structure, properties and spinel - ramsdellite phase transition temperature also with the purpose of new material development. For these materials positive results were obtained in conductivity and chemical stability in reducing conditions. The performance of MnFeCrO₄ and MgFeCrO₄ as electrode support materials was investigated when used alone or impregnated with (La₀.₇₅Sr₀.₂₅)₀.₉₇Cr₀.₅Mn₀.₅O₃, La₀.₈Sr₀.₂FeO₃, Ce₀.₉Gd₀.₁O₂, CeO₂ or Pd. Composite anodes for SOFC were prepared by aqueous infiltration of nitrate salts into porous MnFeCrO₄ and MgFeCrO₄ scaffolds and studied by electrochemical impedance spectroscopy (EIS) in symmetrical cell configuration. The performance of the composite anodes was evaluated in humidified 5%H₂/Ar in order to understand their stability and performance at 850 °C or lower temperature with respect to the porous substrates. It was found that all the impregnated phases adhere very well to the spinel and considerably enhance performance and stability to a level required for SOFC applications. An interesting next step in this work would be to apply such spinel materials on steel interconnects, integrate them into testing SOFC devices and evaluate their effect upon chromium poisoning of the electrodes

Enriched Basaltic Andesites from Mid-crustal Fractional Crystallization, Recharge, and Assimilation (Pilavo Volcano, Western Cordillera of Ecuador)

The origin of andesite is an important issue in petrology because andesite is the main eruptive product at convergent margins, corresponds to the average crustal composition and is often associated with major Cu-Au mineralization. In this study we present petrographic, mineralogical, geochemical and isotopic data for basaltic andesites of the latest Pleistocene Pilavo volcano, one of the most frontal volcanoes of the Ecuadorian Quaternary arc, situated upon thick (30-50 km) mafic crust composed of accreted Cretaceous oceanic plateau rocks and overlying mafic to intermediate Late Cretaceous-Late Tertiary magmatic arcs. The Pilavo rocks are basaltic andesites (54-57·5 wt % SiO2) with a tholeiitic affinity as opposed to the typical calc-alkaline high-silica andesites and dacites (SiO2 59-66 wt %) of other frontal arc volcanoes of Ecuador (e.g. Pichincha, Pululahua). They have much higher incompatible element contents (e.g. Sr 650-1350 ppm, Ba 650-1800 ppm, Zr 100-225 ppm, Th 5-25 ppm, La 15-65 ppm) and Th/La ratios (0·28-0·36) than Pichincha and Pululahua, and more primitive Sr (87Sr/86Sr ∼0·7038-0·7039) and Nd (εNd ∼ +5·5 to +6·1) isotopic signatures. Pilavo andesites have geochemical affinities with modern and recent high-MgO andesites (e.g. low-silica adakites, Setouchi sanukites) and, especially, with Archean sanukitoids, for both of which incompatible element enrichments are believed to result from interactions of slab melts with peridotitic mantle. Petrographic, mineral chemistry, bulk-rock geochemical and isotopic data indicate that the Pilavo magmatic rocks have evolved through three main stages: (1) generation of a basaltic magma in the mantle wedge region by flux melting induced by slab-derived fluids (aqueous, supercritical or melts); (2) high-pressure differentiation of the basaltic melt (at the mantle-crust boundary or at lower crustal levels) through sustained fractionation of olivine and clinopyroxene, leading to hydrous, high-alumina basaltic andesite melts with a tholeiitic affinity, enriched in incompatible elements and strongly impoverished in Ni and Cr; (3) establishment of one or more mid-crustal magma storage reservoirs in which the magmas evolved through dominant amphibole and clinopyroxene (but no plagioclase) fractionation accompanied by assimilation of the modified plutonic roots of the arc and recharge by incoming batches of more primitive magma from depth. The latter process has resulted in strongly increasing incompatible element concentrations in the Pilavo basaltic andesites, coupled with slightly increasing crustal isotopic signatures and a shift towards a more calc-alkaline affinity. Our data show that, although ultimately originating from the slab, incompatible element abundances in arc andesites with primitive isotopic signatures can be significantly enhanced by intra-crustal processes within a thick juvenile mafic crust, thus providing an additional process for the generation of enriched andesite

Plasma oxidation of liquid precursors for complex metal oxides.

Clean energy production and storage are two of the most significant challenges in the 21st century currently limited by the discovery and development of new and advanced materials. Complex oxides and alloys made using earth-abundant elements will play a crucial role in technology development moving forward, however, current preparation techniques are limited by their inability to produce complex oxides and alloys with precise composition control at fast timescales. A concept was proposed to produce mixed metal oxides with composition control through the oxidation of liquid precursors via plasma oxidation. It was hypothesized that the oxidation process can be completed in fast timescales owing to the rapid heating and cooling of the plasma process. Even though the rapid timescales for oxidation can be understood through fast heating processes during plasma exposure, the mechanisms responsible for composition control are not immediately obvious. So, fundamental experiments were carried out to elucidate the nucleation and growth steps responsible for metastable non-stoichiometric oxide formation. Interrupted oxidation experiments completed within twenty seconds revealed the following steps during plasma exposure of liquid droplets: the nucleation of monometallic oxide phases from an amorphous nutrient, solid-state reaction into intermediate mixed oxide phase, and formation of metastable phase. Evidence also suggests the fast kinetics of the oxidation process depends on the enormous heat released from the recombinative reactions among plasma species present in the plasma. The viability of a select set of plasma-synthesized oxides were tested in energy conversion and storage technologies. The technique was successfully used to synthesize W0.99Ir0.01O3-δ alloy which showed high oxygen evolution reaction (OER) activity and stability in acid with an overvoltage reduction in the excess of 500 mV compared to the same composition prepared via standard thermal oxidation route. The structural dilution of iridium with earth-abundant tungsten will enable the efficient use of scarce iridium resources. In alkaline media OER, charge-transfer type double perovskite (La0.9Ca0.1Co0.5Ni0.5O3-δ) prepared via the rapid plasma method shows excellent activity rivaling best performing complex oxide electrocatalysts. Most importantly, the obtained experimental data, combined with density functional theory calculations allows for relating the high OER activity to the strong hybridization of the transition metal 3d and oxygen 2p bands. Again, this technique has been used to fabricate manganese-enriched nickel-manganese-cobalt (NMC) oxides. The resulting NMC materials were tested as cathodes in lithium ion battery and show competitive results compared with NMCs prepared through other routes. This dissertation presents a concept utilizing plasma oxidation of liquid precursors for composition control of complex oxides and alloys. The presented concept could expedite the accelerated discovery and development of advanced materials for energy conversion and storage. Furthermore, the underlying nucleation and growth mechanistic aspects for forming non-stoichiometric oxide phases will add scientific knowledge to our understanding of the synthesis of materials far from equilibrium