Thermoelectric materials are solid state devices having the capability to convert heat to electrical energy and vice versa. These materials are simple, have no moving parts and use no greenhouse gases. But the major drawback of these materials is their low conversion efficiency. Hence enhancement of thermoelectric efficiency is required to make the use of these devices widespread. Thermoelectric efficiency is related to a parameter termed figure of merit, ZT which is associated with the inter-related transport properties such as Seebeck coefficient, electrical and thermal conductivity. Efficient thermoelectric material should possess high Seebeck coefficient (S), high electrical conductivity () and low thermal conductivity (). The present investigation revolves around improvement of ZT of CoSb3 either by chemical doping or through microstructural modifications. These materials possess structural voids, which can be filled with foreign atoms. The rattling motion of these filler atoms reduces the thermal conductivity of these materials, thereby increasing the thermoelectric efficiency. The rattler atoms chosen for the present study are Mn and Yb. Both coarse and fine-grained MnxCo4Sb12 (x = 0. 0.2, 0.4, 0.8, 1.2 and 1.6) and Yb0.19Co4Sb12 have been synthesized and subjected to various structural and functional property characterizations. The structural study based on Rietveld Analysis and the corresponding difference Fourier maps confirms the void occupancy by Mn and Yb in MnxCo4Sb12 (x 0.2, 0.4 and 0.8) and Yb0.19Co4Sb12. In higher Mn content, x=1.2 and 1.6, Mn was found to partially substitute Co site and partially fill the voids and the remaining precipitated out as free particles. A comparative study of coarse and fine-grained CoSb3 has thrown light in to the grain size effect on the thermoelectric properties. Lowering of grain size helped in enhancement of ZT in CoSb3. Seebeck coefficient (thermoelectric power), electrical and thermal conductivity have been measured for different concentrations of the filler Mn atoms between 300K and 673K. A change in sign of the Seebeck coefficient from negative to positive occurs, when Mn concentration exceeds x=0.8. Electrical resistivity values was found to decrease initially with Mn filling with the minimum value at Mn content, x=0.4 and then gradually increase as Mn content increases. The thermal conductivity value decreases with Mn content in the CoSb3 indicating their rattling property which helps in the enhancement of the overall thermoelectric efficiency. There is a reduction in the value of ktotal in Mn filled CoSb3 than that of the unfilled counterpart. This decrease in the ktotal is a clear indication of the rattling motion of the filler Mn atom in the structural void of CoSb3. Highest ZT of 0.36 is achieved by Mn0.4Co4Sb12 at 373K. Higher concentration of Mn (with x= 1.2 and 1.6) proved to be detrimental in terms of improvement of the value of ZT. Grain size reduction helped in improvement of ZT in Mn0.2Co4Sb12. Maximal ZT of 0.06 at 523K is achieved in hot pressed Mn0.2Co4Sb12. The corresponding coarse-grained material is found to possess ZT of 0.01 at the said temperature. The enhancement can be attributed to high / ratio and high density. Similarly, fine grained Yb0.19Co4Sb12 shows higher ZT compared to the coarse-grained sample because high / and high S.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico / Metals environment aquatic contamination has been a growing problem with serious consequences to life of different species over time, even after interrupted their emission into the environment. Among relevance metals, manganese (Mn) has shown importance to be related to several activities such as oil exploration, coal deposits extraction, fertilizer use in agriculture, among others. In living organisms, Mn is an essential trace element for a number of vital functions, and involves energy regulation by blood clotting. On the other hand, high concentrations of Mn can cause irreversible damage to living organisms primarily affecting central nervous system (CNS). Thus, waterborne Mn toxicity can switch from aquatic species, while metal bioaccumulation in marine fish or freshwater has been found around 0.2 to 19.0 mg/kg dry weight. In this study, silver catfish (Rhamdia quelen) were exposed to different Mn concentrations (4.2; 8.4 e 16.2 mg/L), under two different dissolved oxygen levels, normoxia (7.48 ± 0.28) and hypoxia (3.88 ± 0.41) for 15 days. At the end of protocol, Mn bioaccumulation as well parameters of oxidative stress and mitochondrial viability were evaluated in different tissues. In hypoxic conditions, the highest Mn concentration (16.2 mg/L) showed the lowest silver catfish kidney and brain lipoperoxidation (LPO) levels, while brain reduced glutathione (GSH) levels were increased in lower Mn concentration (4.2 mg/L) and kidney catalase activity was reduced in the same metal concentration, in relation to normoxia. Lowest Mn concentration (4.2 mg/L) in hypoxia showed higher gills mitochondrial viability, compared to normoxia. Hematocrit of silver catfish exposed to two highest Mn concentration (8.4 and 16.2 mg/L) was reduced in normoxia conditions while under hypoxia, these values were similar to control. Plasma except, Mn bioaccumulation in liver, kidney and gills were higher in normoxia than hypoxia. From these results is possible to suggest that hypoxia stimulates the development of adaptive mechanisms and/or hormesis in silver catfish exposed to Mn, mainly because the lower metal bioaccumulation occurred in this oxygen level. Contributing with this, under hypoxia, oxidative damage indicators were lower than those observed in normoxia, which were accompanied by changes in antioxidant system represented by GSH and catalase. In conclusion, our results show in the first time that silver catfish exposed to Mn contamination is able to show a better survival under hypoxia. These findings indicate need for continuing studies in search of molecular mechanisms involved in the adaptation and or hormesis processes, which were suggested here. / A contaminação do meio aquático por metais tem sido um problema crescente para à vida de diferentes espécies, mesmo depois de interrompida sua emissão no ambiente. Entre os metais de relevância, o manganês (Mn) apresenta importância por estar relacionado a uma série de atividades tais como exploração de petróleo, extrativismo em jazidas de carvão mineral, utilização de fertilizantes na agricultura, entre outros. Nos organismos vivos, o Mn constitui um oligoelemento essencial para uma série de funções vitais, e envolve desde a regulação da energia até coagulação sangüínea. Por outro lado, quando em concentrações elevadas, o Mn pode causar danos irreversíveis aos organismos afetando principalmente o sistema nervoso central (SNC). A toxicidade do Mn dissolvido na água pode variar entre as espécies aquáticas, a bioacumulação do metal em peixes marinhos ou de água doce tem sido encontrada em torno de 0,2 até 19,0 mg/kg de peso seco. No presente estudo, jundiás (Rhamdia quelen) foram expostos a diferentes concentrações de Mn (4,2; 8,4 e 16,2 mg/L), sob dois diferentes níveis de oxigênio dissolvido: normóxia (7,48±0,28 mg/L) e hipóxia (3,88±0,41 mg/L), durante 15 dias. Ao final do protocolo, a bioacumulação de Mn bem como, parâmetros de estresse oxidativo e a viabilidade mitocondrial foram avaliados em diferentes tecidos vitais. Em condições de hipóxia, a maior concentração de Mn (16,2 mg/L) mostrou os menores níveis de lipoperoxidação (LPO) em rim e encéfalo dos jundiás, enquanto os níveis de glutationa reduzida (GSH) encefálico foram aumentados na menor concentração de Mn (4,2 mg/L) e a atividade da catalase renal foi reduzida nesta mesma concentração de metal, em relação à normoxia. A menor concentração de Mn (4,2 m/L) em hipóxia proporcionou maior viabilidade mitocondrial das brânquias, em relação à normóxia. O hematócrito dos jundiás expostos às duas maiores concentrações de Mn (8,4 e 16,2 mg/L) foi reduzido em condições de normóxia, enquanto sob hipóxia, estes valores foram semelhantes ao grupo sem a presença do metal. Sob normóxia, a bioacumulação de Mn em fígado, rim e brânquias foi maior que sob hipóxia, excetuando o plasma. A partir desses resultados é possível sugerir que a hipóxia pode ter estimulado o desenvolvimento de mecanismos adaptativos ou de hormesis nos jundiás, em presença de Mn na água, principalmente porque neste nível de oxigênio, a bioacumulação do metal ocorreu em menor grau. Contribuindo com este resultado, sob hipóxia, os indicadores de danos oxidativos foram mais baixos que os observados sob normóxia, os quais foram acompanhados de alterações do sistema antioxidante representado pela GSH e catalase. Neste sentido, nossos resultados apontam para uma melhor sobrevivência da espécie em estudo, sob hipóxia, quando exposta ao Mn. Estes achados indicam a necessidade da continuidade dos estudos, em busca dos mecanismos moleculares envolvidos no processo de adaptação aqui sugeridos.

Durant ce travail de thèse, d’importants paramètres concernant la synthèse de matériaux catalytiques nanostructurés à base de manganèse et d’oxydes de cobalt ont été établis. La corrélation entre les propriétés structurales du catalyseur et l’activité catalytique, ainsi que le mécanisme d’oxydation du CO ont été analysé au moyen d’une grande variété de méthodes expérimentales physico-chimiques.<p>De l’oxyde de manganèse non-stœchiométrique (MnOx) a été préparé par décomposition spinodale d’oxalate de manganèse trihydraté en ayant recours à la technique d’oxydation programmée en température (TPO). Tant l’analyse quantitative relatives à ces données TPO que les résultats obtenus par spectroscopie de structure au front d’absorption des rayons X (XANES), ainsi que par spectroscopie des photoélectrons X (XPS) ont permis d’estimer la stœchiométrie de l’oxyde avec un x situé entre 1.61 et 1.67. En accord avec à la fois la surface spécifique élevée et la combinaison d’isothermes d’adsorption/désorption de type I et IV, la microscopie électronique à transmission à haute résolution (HRTEM) démontre la présence de micro-bâtonnets caractéristiques et « imbriqués » les uns dans les autres, accompagné de particules nanocristalline à l’extrémité de ces bâtonnets.<p>Les découvertes faites par spectroscopie infra-rouge de réflexion diffuse par transformée de Fourier (DRIFTS), par études isotopiques et cinétiques suggère que l’adsorption des deux molécules, CO et O2, est suivie par leur réaction en surface via des intermédiaires de type carbonate/formate, pour finalement produire du CO2. Nous supposons un mécanisme de type Mars-van Krevelen où l’oxygène appartenant à la structure de type MnOx prend part dans l’oxydation catalytique du CO à basse température. Cependant, ces espèces mobiles d’oxygènes ne faisaient pas partie du cœur de phase du réseau d’oxyde, et de ce fait, ont été capables de « sauter » sur la surface et approvisionner les espèces oxygénées nécessaires à l’oxydation du CO déjà adsorbé.<p>Une structure spinelle d’oxyde de cobalt Co3O4 dans lequel le cobalt présente deux états de valence (+2 et +3) a été choisie pour élucider l’effet de la taille des particules sur l’activité lors de la réaction d’oxydation du CO. Tout d’abord, des nanoparticules monodispersées de cobalt métallique présentant une déviation standard en taille inférieure à 8% ont été synthétisées à partir de carbonyle de cobalt (Co2(CO)8) par une méthode optimisée «d’injection chaude». Un contrôle de la taille des nanoparticules dans la gamme 3 à 11 nm a pu être obtenu en variant la température d’injection du carbonyle de cobalt dans une solution de dichlorobenzène et d’acide oléique. La microscopie électronique à transmission (TEM) nous montre que ces particules de cobalt sont quasiment hémisphériques. Ensuite, de la silice poreuse (de type MCF-17) a été imprégnée par des nanoparticules de cobalt, et ensuite activée par TPO menant à des nanoparticules d’oxyde de cobalt. Des études par diffraction des rayons X (XRD) et spectroscopie des photoélectrons X (XPS) ont démontré la structure spinelle Co3O4. Finalement, l’activité des catalyseurs obtenus vis-à-vis de l’oxydation du monoxyde de carbone fut mesurée à 423 K et ce en fonction de la taille des particules. Les particules de Co3O4 présentant une taille allant de 5 à 8 nm se sont révélées les plus actives. Ceci peut s’expliquer par une plus grande mobilité des atomes d’oxygène en surface des nanoparticules d’oxyde de cobalt.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

While group VII 4d Tc and 5d Re have hexagonally close-packed (hcp) ground states, 3d Mn adopts the complex chi-phase which exhibits non-collinear magnetism. Density functional theory (DFT) calculations have shown that without magnetism the chi-phase remains the ground state of Mn implying that magnetism is not the critical factor, as is commonly believed, in driving the anomalous stability of the chi-phase over hcp. Using a tight-binding (TB) model it is found that while harder potentials stabilise close-packed hcp, a softer potential stabilises the more open chi-phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the chi-phase in Mn is shown to be due to 3d valent Mn lacking d states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4d Tc and 5d Re. Subsequently an analytic Bond-Order Potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal d-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the five experimentally observed phases of Mn, alpha, beta, gamma, delta, and epsilon-Mn. Not only does the BOP reproduce qualitatively DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in alpha-Mn, the ferrimagnetic (FiM) ordering in beta-Mn and the AFM ordering in the other phases that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants which require further moments. Magnetic analytic BOPs are also developed for Fe and Fe-Mn. The Fe model correctly reproduces trends in the structural stabilities of the common metallic structures except that AFM hcp is overstabilised. Reproduction of the elastic constants with a 9-moment BOP is reasonable although as is found for the Mn BOP the elastic shear constants require more moments to converge. Vacancy formation energies are close to those determined by experiment and DFT and the relative stabilities of self-interstitial atom (SIA) defects in ferromagnetic bcc Fe are correctly reproduced. The SIA formation energies are found to be better than those calculated with existing BOP models. The Fe-Mn TB and BOP models were challenging to fit and nonmagnetic face-centred cubic (fcc) structures are overstabilised. Furthermore within BOP an incorrect magnetic solution is predicted for one fcc structure resulting in poor reproduction of the DFT stacking fault energies. Refitting the bond integrals might help to better reproduce the nonmagnetic hcp-fcc energy differences while an environment-dependent Stoner parameter could help provide the flexibility needed to correctly capture the magnetic energy differences.

In the Middelplaats mine area of the Kalahari manganese field, two drill holes (MP53 and MP54) intersected anomalously high-grade manganese ore sitting stratigraphically just above an igneous body (likely a dike or sill). Manganese ore located within approximate 5 meters of the contact with the underlying igneous rocks has been substantially metasomatically upgraded from 25 percent manganese, to over 40 percent whilst the dominant manganese species within the ore has been altered to hausmannite. This report demonstrates the metasomatic alteration is related to devolatilization (removal and/or remobilization of H₂O, CO₂ and CaO) due to contact metamorphism caused by the underlying igneous rocks. The Middelplaats mine is situated in the southwest corner of the Kalahari manganese field where the paleo basin shallows out and ends. Within the mine area, several stratigraphic units pinch out or are truncated by the side of the basin. This pinching out of lithological formations has led to the underlying Ongeluk Formation being in contact with the much younger units of the Hotazel Formation. Therefore, geochemical investigation into the nature and source of the igneous rocks was also undertaken to see if the rocks from the two drill holes were related to one another and/or the underlying Ongeluk Formation. Results of these geochemical studies have demonstrated that the Middelplaats igneous rocks (dolerites) from the two drill holes (MP53 and MP54) share a co-genetic source region. There is also reasonable geochemical evidence that the source region of the Middelplaats igneous rocks was substantially similar to the source region of the Ongeluk Formation. This may indicate that the source region of the Ongeluk Formation was reactivated at some later stage resulting in the emplacement of doleritic dikes or sills in the Middelplaats mine area. The Middelplaats igneous rocks were also found to have undergone a slight but pervasive potassic alteration; with most of the original plagioclase feldspar showing some level of replacement by a potassium enriched feldspar. Although no source for this potassic fluid was found, the devolatilization reaction within the manganese ore appears to have released some potassium into the surrounding rocks. This additional potassium may be responsible for some localized potassic alteration.

The Postmasburg Manganese Field (PMF), located in the Northern Cape Province of South Africa, once represented one of the largest sources of manganese ore worldwide. However, the discovery of the giant manganese deposits of the Kalahari Manganese Field (KMF) led to the gradual decline in manganese mining activity in the PMF. Two belts of manganese ore deposits have been distinguished in the PMF, namely the Western Belt of ferruginous manganese ores and the Eastern Belt of siliceous manganese ores. Prevailing models of ore formation in these two belts invoke karstification of manganese-rich dolomites and residual accumulation of manganese wad which later underwent diagenetic and low-grade metamorphic processes. For the most part, the role of hydrothermal processes in ore formation and metasomatic alteration is not addressed. The identification of an abundance of common and some rare Al-, Na-, K- and Ba-bearing minerals, particularly aegirine, albite, microcline, banalsite, sérandite-pectolite, paragonite and natrolite in the PMF ores studied in this thesis, is indicative of the influence of hydrothermal activity. Enrichments in Na, K and/or Ba in the ores are generally on a percentage level for the majority of samples analysed through bulk-rock techniques. The discovery of a Ba-Mn arsenate/vanadate similar to gamagarite may also indicate that the hydrothermal fluid affecting the ores was not only alkali-rich but also probably contained some As and V. The fluid was likely to be oxidized and alkaline in nature and is thought to have been a mature basinal brine. Various replacement textures, particularly of Na- and Krich minerals by Ba-bearing phases, suggest sequential deposition of gangue as well as oreminerals from the hydrothermal fluid, with Ba phases being deposited at a later stage. The stratigraphic variability of the studied ores and the deviation of their character from the pigeon-hole-type classification of ferruginous and siliceous ores in the literature, suggests that a re-evaluation of genetic models is warranted. The discovery of hydrothermallydeposited alkali-rich assemblages in the PMF and KMF provides grounding for further investigation into a possible regional-scale hydrothermal event at least re-constituting the ores. Some shortcomings in previous works include disregard for the highly variable nature of the PMF deposits, the effects of hydrothermal activity of the ores and the existence of stratigraphic discrepancies. This study provides a single, broad model for the development of all manganese deposits of the PMF. The source of metals is attributed to all formations that stratigraphically overly the Reivilo Formation of the Campbellrand Subgroup (including the Reivilo Formation itself). The main process by which metals are accumulated is attributed to karstification of the dolomites. The interaction of oxidized, alkaline brines with the ores is considered and the overlying Asbestos Hills Subgroup BIF is suggested as a potential source of alkali metals.

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Thesis (M.Sc. (Physics)) -- University of Limpopo, 2021 / The stead-fast demand for sustainable lithium-ion batteries (LIB) with competitive electrochemical properties, safety, reduced costs, and long-life cycle, calls for intensive efforts towards the development of new battery cathode materials. The layered transition metal oxides formulated LiMO2 (M: Mn, Ni and Co) have attracted considerable attention due to their capability to optimize the discharge capacity, cycling rate, electrochemical stability and lifetime. The transition metals Mn, Ni and Co (NMC) have been reported to contribute towards enhancement of the performance of NMC based lithium-ion batteries. In this work, the electronic properties of transition metal oxides LiMO2 (M: Mn, Ni and Co) as individual crystal structures are studied using density functional theory (DFT+U) in the local density and generalized gradient approximation (LDA and GGA). The Hubbard U values together with the low spin transition metal in 3+ charge state (Mn3+, Ni3+ and Co3+) predicts the electrical conductivity of the materials. The conductivity is associated predominantly with 3d states of the transition metals (Mn, Ni and Co) and 2d character in oxygen. The LiNiO2 material is high in conductivity, while both LiMnO2 and LiCoO2 are low in electrical conductivity. All independent elastic constants satisfy the mechanical stability criterion of orthorhombic materials implying stability of the materials. However, the phonon dispersion curves display imaginary vibration along high symmetry direction for LiCoO2. The heats of formations predict that the LiNiO2 is the most thermodynamically stable material while the LiMnO2 is the least thermodynamically stable material. The derived interatomic potentials produced NiO and CoO structures with a difference of less than 1% and 9% respectively, from the experimental structures. The structures were melted at temperatures close to their experimental values from molecular dynamics. The radial distribution curves and Nano architectures presented the melting point of NiO and CoO at 2250K and 2000K respectively. All independent elastic constants satisfy the mechanical stability criterion of cubic materials implying stability of the materials. The high electrical conductivity and thermodynamic favourability LiNiO2 suggests that the material can be the most recommendable material as a cathode material and further improved through doping. This will add the overall enhancement of the electrochemical performance while stabilizing structural stability of the cathode material in high energy density Li-ion batteries. / National Research Foundation (NRF)

A catalytic route to hydrogen production via thermochemical water splitting is highly desirable because it directly converts thermal energy into stored chemical energy in the form of hydrogen and oxygen. Recently, the Davis group at Caltech reported an innovative low-temperature (max 850°C) catalytic cycle for thermochemical water splitting based on sodium and manganese oxides (Xu, Bhawe and Davis, PNAS, 2012). The key steps are thought to be hydrogen evolution from a Na₂CO₃/MnO mixture, and oxygen evolution by thermal reduction of solids formed by Na⁺ extraction from NaMnO₂. Our work is aimed at understanding the fundamental chemical processes involved in the catalytic cycle, especially the hydrogen evolution from water. In this project, efforts are made to understand the interactions between the key components (Na, O₂, CO₂, and water) in the hydrogen evolution steps on a well-defined MnO(100) single crystal surface, utilizing x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). While some of the behavior of the catalytic system is observed with the model system developed in this work, hydrogen is only produced from water in the presence of metallic sodium, in contrast to the proposal of Xu et al. that water splitting occurs from the reaction of water with a mixture of Na₂CO₃ and MnO. These differences are discussed in light of the different operating conditions for the catalytic system and the surface science model developed in this work. / Ph. D.