Auswahl der wissenschaftlichen Literatur zum Thema „CALCIUM COBALTITE“

Abstract Thermoelectric (TE) materials are able to generate power from waste heat and thereby provide an alternative source of sustainable energy. Calcium cobaltite is a promising p-type TE oxide because of its intrinsically low thermal conductivity arising from the misfit-layered structure. Its structural framework contains two sub-layers with different incommensurate periodicities, offering different sites for substituting elements; the plate-like grain structure contributes to texture development, thereby providing opportunities to modulate the TE response. In this topical review, we briefly introduce the misfit crystal structure of calcium cobaltite and summarize three efficient strategies to enhance the TE performance, namely (a) elemental doping, (b) optimization of fabrication route, and (c) composite design. For each strategy, examples are presented and enhancing mechanisms are discussed. The roles of dopants, processing routes and phase composition are identified to provide insights into processing-microstructure-property relationships for calcium cobaltite based materials. We outline some of the challenges that still need to be addressed and hope that the proposed strategies can be exploited in other TE systems.

The impact of the non-stoichiometric addition of potassium (K) on the nanostructure and thermoelectric performance of misfit layered calcium cobaltite (Ca₃Co₄O₉) ceramics is reported.

Dans le contexte de transition énergétique vers la neutralité carbone à l’horizon 2050, les piles à combustible à oxyde solide (Solid Oxide Fuel Cells, SOFC) et l’Electrolyse à Haute Température (EHT) présentent un réel potentiel d’utilisation via l’hydrogène comme vecteur d’énergie. L’objectif de cette thèse est la compréhension des processus électrochimiques dans ces systèmes dans l'objectif d'améliorer leurs performances et leur durabilité. La technique retenue est la spectroscopie d’impédance pour l'étude de la réaction de réduction de l'oxygène. Cette réaction est complexe et fait intervenir plusieurs étapes : la diffusion de l'oxygène moléculaire, la dissociation de l'oxygène moléculaire à la surface de l'électrode, la diffusion des atomes d'oxygène ou partiellement ionisés à la surface du solide ou leur incorporation dans le solide, le transfert de charges, la diffusion des ions dans le solide... Alors que la diffusion gazeuse est un processus lent, la diffusion ionique dans le solide est rapide. L'étude fine des spectres d'impédance mesurés sur des cellules symétriques permet de définir les étapes qui limitent la réaction et d’identifier les orientations à prendre pour optimiser les systèmes. Cela suppose la mesure de données fiables. Le test de Kramers-Krönig permet de vérifier la qualité des données. A partir de ces données, il est possible de calculer la fonction de distribution des temps de relaxation caractéristiques des phénomènes impliqués au sein de la cellule mais le nombre de données étant fini, la résolution de l'équation associée à cette fonction n'est pas simple. L'objectif de cette thèse a tout d'abord été de définir une méthodologie pour le traitement rigoureux des données des spectres d'impédance mesurés sur des cellules symétriques constituées d'un électrolyte de cérine dopé au gadolinium sur lequel a été déposé une électrode modèle à base de Ca3Co4O9+δ, un matériau d'électrode innovant, étudié depuis plusieurs années à l'UCCS. Contrairement aux matériaux de l'état de l'art, les cobaltites de calcium ont l'avantage de ne pas contenir de terres rares et surtout de présenter un coefficient de dilatation du même ordre de grandeur que celui des électrolytes utilisés pour ces applications, laissant espérer des durabilités accrues. D'abord utilisé comme électrode modèle, la substitution du calcium par du strontium dans ce composé et son utilisation en composite avec la cérine ont permis d'atteindre les spécificités requises pour l'application : une résistance surfacique spécifique inférieure ou égale à 0,15 Ω.cm² à 700°C. L'étude a ensuite été étendue à la caractérisation de cellules complètes. Cette thèse a bénéficié d'un financement Région Hauts de France, Centrale Lille. Une partie des travaux a été menée dans le cadre du projet MODTESTER, un projet Eurostars Eureka financé par la BPI, et porté par la société Fiaxell, une PME Suisse, et dans le cadre du projet européen NOUVEAU qui porte sur la recherche de nouveaux matériaux d'électrodes et d’interconnecteurs durables et réutilisables pour l'électrolyse de l'eau à haute température
In the context of energy transition towards carbon neutrality by 2050, Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs) offer real potential for use via hydrogen as an energy carrier. The aim of this thesis is to understand the electrochemical processes in these systems, with a view to improving their performance and durability. The technique chosen is impedance spectroscopy to study the oxygen reduction reaction. This is a complex reaction involving several stages: diffusion of molecular oxygen, dissociation of molecular oxygen at the electrode surface, diffusion of oxygen or partially ionized atoms at the solid surface or their incorporation into the solid, charge transfer, diffusion of ions into the solid, etc. Whereas gaseous diffusion is a slow process, ionic diffusion in solids is rapid. The detailed study of impedance spectra measured on symmetrical cells enables us to define the steps that limit the reaction and identify the directions to take to optimize the systems. This requires the measurement of reliable data. The Kramers-Krönig test is used to check the quality of the data. From these data, it is possible to calculate the distribution function of the relaxation times characteristic of the phenomena involved within the cell, but as the number of data is finite, solving the equation associated with this function is not straightforward. The aim of this thesis was first to define a methodology for the rigorous processing of impedance spectra measured on symmetrical cells consisting of a gadolinium-doped ceria electrolyte on which a model electrode based on Ca3Co4O9+δ, an innovative electrode material studied for several years at UCCS, has been deposited. Unlike state-of-the-art materials, calcium cobaltites have the advantage of not containing rare earths and, above all, of presenting an expansion coefficient of the same order of magnitude as that of the electrolytes used for these applications, giving rise to the hope of increased durability. Initially used as a model electrode, the substitution of strontium for calcium in this compound and its use as a composite with ceria enabled the specific features required for the application to be achieved: a specific surface resistance of less than or equal to 0.15 Ω.cm² at 700°C. The study was then extended to the characterization of complete cells. This thesis was funded by the Hauts de France Region and Centrale Lille. Part of the work was carried out as part of the MODTESTER project, a BPI-funded Eurostars Eureka project led by Fiaxell, a Swiss SME, and as part of the European NOUVEAU project, which focuses on the search for new, sustainable and reusable electrode and interconnector materials for high-temperature water electrolysis

Объектами исследования настоящей работы являются катодные материалы на основе сложного оксида Сa3Co4O9+δ. Цель работы – апробация материалов на основе Сa3Co4O9+δ, которые могут быть использованы в качестве катодов для среднетемпературных твердооксидных топливных элементов с протон-проводящими электролитами BaCe0.5Zr0.3Y0.1Yb0.1O3- и BaCe0.7Zr0.1Y0.1Yb0.1O3-. Методом пиролиза цитрат-солевых композиций проведен синтез сложных оксидов Сa3Co4O9+δ, Ca3Co4-xCuxO9 (х = 0.05; 0.1; 0.15; 0.2), BaCe0.5Zr0.3Y0.1Yb0.1O3-δ и BaCe0.7Zr0.1Y0.1Yb0.1O3-. При помощи комплекса современных методов исследования выполнена фазовая, структурная и микроструктурная аттестация оксидов Сa3Co4O9+δ, Ca3Co4 xCuxO9 (х = 0.05; 0.1; 0.15; 0.2), BaCe0.5Zr0.3Y0.1Yb0.1O3-δ и BaCe0.7Zr0.1Y0.1Yb0.1O3-. Термогравиметрическим методом исследована термическая устойчивость Сa3Co4O9+δ на воздухе и в атмосфере аргона. Термическое расширение оксидов Сa3Co4O9+δ и BaCe0.5Zr0.3Y0.1Yb0.1O3-δ изучено методом дилатометрии, доказана их термическая совместимость. Изучена химическая совместимость оксида Сa3Co4O9+δ с электролитными материалами Ba2In1.8W0.2O5.15, 0.7Ba2In2O5·0.3Ba2InNbO6, Ba3Ca1.18Nb1.82O9 δ, BaCe0.5Zr0.3Y0.1Yb0.1O3 δ, а также материалами коллекторных слоев La0.6Sr0.4MnO3-δ и LaNi0.6Fe0.4О3 δ, установлена оптимальная температура припекания катодного материала Сa3Co4O9+δ к электролиту BaCe0.5Zr0.3Y0.1Yb0.1O3-δ. Исследованы температурные зависимости электропроводности Сa3Co4O9+δ и BaCe0.5Zr0.3Y0.1Yb0.1O3-δ на воздухе. Сформированы электроды на основе композитов с различным массовым содержанием Сa3Co4O9+δ и BaCe0.5Zr0.3Y0.1Yb0.1O3-δ на подложках из BaCe0.5Zr0.3Y0.1Yb0.1O3-δ, а также электроды на основе Ca3Co4-xCuxO9 (х = 0; 0.05; 0.1; 0.15) на подложках из BaCe0.7Zr0.1Y0.1Yb0.1O3-δ. Методом импедансной спектроскопии на симметричных ячейках измерены поляризационные характеристики полученных электродов, а также электродов с оксидным коллектором состава La0.6Sr0.4MnO3-δ+2 масс.% CuO.
The object of study in this work is a cathode material based on the Сa3Co4O9+δ. The aim of the work is to study the electrochemical behavior of electrodes based on the Сa3Co4O9+δ with the electrolyte materials BaCe0.5Zr0.3Y0.1Yb0.1O3- and BaCe0.7Zr0.1Y0.1Yb0.1O3-. The synthesis of the Сa3Co4O9+δ, Ca3Co4-xCuxO9 (х = 0.05; 0.1; 0.15; 0.2), BaCe0.5Zr0.3Y0.1Yb0.1O3-δ and BaCe0.7Zr0.1Y0.1Yb0.1O3- complex oxides was carried out by pyrolysis of citrate-salt compositions. Using a complex of modern research methods, phase, structural and microstructural attestation of the Сa3Co4O9+δ, Ca3Co4-xCuxO9 (х = 0.05; 0.1; 0.15; 0.2), BaCe0.5Zr0.3Y0.1Yb0.1O3-δ and BaCe0.7Zr0.1Y0.1Yb0.1O3- oxides were carried out. The thermal stability of the Сa3Co4O9+δ in air and in the argon atmosphere was studied by the thermo gravimetrical method. The thermal expansion of the Сa3Co4O9+δ and BaCe0.5Zr0.3Y0.1Yb0.1O3-δ oxides was studied by dilatometry, and their thermal compatibility was proved. The chemical compatibility of the Сa3Co4O9+δ oxide with the electrolyte materials Ba2In1.8W0.2O5.15, 0.7Ba2In2O5·0.3Ba2InNbO6, Ba3Ca1.18Nb1.82O9 δ, BaCe0.5Zr0.3Y0.1Yb0.1O3-δ, Lа0.6Sr0.4MnO3-δ and LaNi0.6Fe0.4О3-δ collector materials was studied, the optimal temperature of the cathode material Сa3Co4O9+δ annealing to the BaCe0.5Zr0.3Y0.1Yb0.1O3-δ electrolyte was established. The temperature dependences of the electrical conductivity of the Сa3Co4O9+δ and BaCe0.5Zr0.3Y0.1Yb0.1O3-δ in air were investigated. Electrodes based on composites with different mass contents of Сa3Co4O9+δ and BaCe0.5Zr0.3Y0.1Yb0.1O3-δ on substrates of BaCe0.5Zr0.3Y0.1Yb0.1O3-δ, as well as electrodes based on Ca3Co4-xCuxO9 (х = 0; 0.05; 0.1; 0.15) on substrates of BaCe0.7Zr0.1Y0.1Yb0.1O3  were formed. The polarization characteristics of the obtained electrodes, including those with an La0.6Sr0.4MnO3-δ+2 wt.% CuO oxide collector, were studied by the method of impedance spectroscopy on the symmetric cells.

Cette thèse concerne la réalisation des films minces d’oxydes et l’étude de leurs propriétés physiques pour les cellules photovoltaïques (PV) et les modules thermoélectriques. Dans une première partie, les propriétés de l’oxyde de titane TiOx (1,45
This thesis concerns the realization of oxide thin films and the study of their properties for photovoltaic or thermoelectric devices. In the first part, the TiOx properties are studied for use as an optically active transparent conductive oxide to put in front of the PV cells or, as optical coupling layer to interpose between the metal reflector and the absorbent layer of a PV cell. The layers are deposited by pulsed laser deposition (PLD). This method allows to get stoichiometric or oxygen deficient layers by controlling the oxygen partial pressure during the growth. The layers are doped with Nb to enhance electrical conductivity and/or with Nd for the conversion of Ultra-Violet photons to Near Infra-Red photons. Insulating and transparent layers, luminescent layers or conducting and absorbent layers are obtained. The TiO₁,₄₅₋₁,₆₀ films show polaronic or bipolaronic conductivity and exhibited the jump of electrical conductivity with jump height and temperature depending on the nature of the dopants. A second part of the manuscript concerns thermoelectricity in which the properties of cobalt calcium oxide are modulated for an efficient conversion of low temperature gradients centered at 300-365K. The control of the oxygen concentration of films allows to obtain the polymorphic phases CaxCoO₂,Ca₃Co₄O₉ and Ca₃Co₄O₆,₄₋₆,₈ having metallic or semiconducting behavior depending on the deposition temperature. The Ca₃Co₄O₆,₄₋₆,₈ films show high Seebeck coefficients (S) ≥ 1 000 μV/K and low electrical resistivity (3.8 to 6 mΩ.cm). Such interesting values have to be confirmed by additional experiments in order to be used as thermoelectric films

As demand for energy increases, there will be greater need for more efficient energy generating technologies. Much of the energy lost in current generation systems is through heat. Thermoelectricity provides one way to recover waste heat and convert it into useful energy, but the efficiency of commercially-available thermoelectric devices is relatively low, especially at temperatures approaching 800 C. Therefore, advanced materials for use in high-temperature thermoelectric devices are necessary. This thesis examines the thermoelectric properties of electrospun layered calcium cobaltites. Layered cobaltites have high Seebeck coefficients, good electrical conductivity, and poor thermal conductivity, leading to a good figure of merit, ZT (the key measure of thermoelectric efficiency). Cobalt oxides are safe and thermally robust compared to other thermoelectric materials, but ZT values for polycrystalline cobalt oxides are somewhat lower than the accepted benchmark of ZT {u0303} 1. Nanostructuring has the potential to improve the efficiency of thermoelectric materials, so calcium cobaltites were made using the electrospinning technique. Sol-gels of calcium and cobalt acetates dissolved in solutions of polyvinyl alcohol and deionized water were used for electrospinning. After electrospinning, the as-spun material was converted into a fibrous metal-oxide through a three-stage calcination process that heated the material up to 650 C. These samples were then pressed into pellets for thermoelectric characterization. Thermogravimetric analysis clarified that three-stage calcination with {u0303}10-20 C/min heating rates might help produce nanostructures. However, the three-stage calcination procedure was no more effective than single-stage calcination for producing fibrous calcium cobaltites at slower heating rates (e.g. 3 C/min). Calcination at 650 C produced CaxCoO2 with a layered structure similar to NaxCoO2. When calcined up to 800 C, misfit-layered Ca3Co4O9 was produced. The thermoelectric properties of calcium cobaltites made by bulk sol-gel processing and electrospun samples treated with H2O2 were compared to the standard electrospun CaxCoO2 and Ca3Co4O9. The effects of the helium testing environment were also investigated. H2O2 treatment and high-temperature helium appeared to reduce carrier concentration, which was not expected from H2O2 treatment. The Ca3Co4O9 samples exhibited superior ZT values between 0.08 (untreated electrospun) and 0.11 (H2O2-treated) with the sol-gel materials exhibiting ZT {u0303} 0.09, rivalling current polycrystalline Ca3Co4O9. The CaxCoO2 samples had consistent ZT values between 0.02 (sol-gel) and 0.03 (electrospun). Ca-substituted CaxCoO2 was investigated using lithium, sodium, strontium, barium, neodymium, and erbium as dopants. Each dopant improved the thermoelectric properties compared to undoped CaxCoO2. 10% neodymium and erbium substitution accelerated the calcination process, leading to possible Ca3Co4O9 formation at 650 C and improved ZT values of 0.064 and 0.072 at 690 K, respectively. Strontium at 10% concentration improved the ZT to 0.063 at 573 K, likely through lattice strain effects. These results suggest CaxCoO2 could be very efficient given the right composition and processing parameters. The thermal conductivity of every sample was relatively low (between 0.1 and 0.6 W/m/K), with rare-earth doped and peroxide-treated cobaltites exhibiting the lowest thermal conductivity. Electrospun calcium cobaltites are promising thermoelectric materials with potential for high efficiency, and will be crucial for producing safe, efficient energy generation technologies in the future.

Perovskite oxides show wide range of applications in the area of magnetism, ferroelectricity, piezoelectricity, thermoelectricity, gas sensing, catalyst development, solid oxide fuel cell, etc. This is due to flexibility in the structure and compositions that can be tuned by specific element doping. In the perovskite oxide (ABO3), large cation (A) is 12 -coordinated and smaller B-cation is 6 coordinated with oxide ions. Oxide materials are considered as better candidates for thermoelectric applications (interconversion of thermal into electrical energy) due to its non-toxicity and thermal stability at elevated temperature. These are insulating in nature and the conductivity can be increased by doping A and / or B –sites. Perovskite oxides are also used for oxygen monitoring in different applications including control and optimization of combustion of fossil fuels in industries and automobiles, biological and defines places, etc. In the present study, we focused on thermoelectric properties in single perovskite oxides of lanthanum cobaltite and calcium manganite and a double perovskite oxide of dysprosium barium cobaltite. Also, the oxygen sensing behaviour of dysprosium barium cobaltite at elevated temperatures is studied. The thesis contains seven chapters and a summary of respective chapters are given below. The first chapter outlines the basics of thermoelectric and gas sensing applications of both perovskite and double perovskite oxides. In the initial part, thermoelectric phenomena are explained. Thermoelectric effect is the conversion of thermal energy to electrical energy and vice-versa. Higher thermoelectric efficiency (η) can be achieved by maintaining a large temperature difference across the material. The efficiency depends on the thermoelectric figure of merit (zT) of material, which depends on thermopower (S), electrical resistivity (ρ) and thermal conductivity (κ) of the material and hence needs to be optimized. The latter part discusses the oxygen sensing property of distorted double perovskite 112 structure type as it shows advantages over other materials due to oxygen nonstoichiometric. Further, an overview of the relevant literature, objective and scope of the thesis are mentioned. The second chapter elucidates the materials and methods used for the present work. The materials viz. LaCoO3, CaMnO3-δ and DyBaCo2O5+δ, were selected for thermoelectric and oxygen sensing studies. Both the conventional solid state and soft chemistry methods were adopted for the synthesis of these materials. Powders were densified into pellets by hot uniaxial pressing / cold isostatic pressing and various heat treatments were carried out. Samples thus prepared were phase pure as confirmed using powder x-ray diffraction and Rietveld refinement performed for structural analysis. Morphological studies were carried out using scanning electron microscopy and transmission electron microscopy. Further Raman and x-ray photoelectron spectroscopic characterization of these materials were discussed. The transport properties viz. electrical resistivity, thermopower and thermal conductivity of compact pellets were measured at elevated temperatures. Further, the home-built apparatus for room temperature See beck measurements and chemo resistive oxygen sensing were explained in detail as a part of this work. The third chapter describes the effect of monovalent ion doping (Na+ and K+) at A-site of lanthanum cobaltite on thermoelectric properties. Lanthanum cobaltite system exhibit exotic behaviour due to commensuration phenomena of spin, lattice, charge and metal insulator transition. The synthesis, followed by structural refinements by Rietveld method using Fullprof suit program are explained. The results of the transport properties indicate that there is no appreciable change in the See beck Coefficient of K-doped samples throughout the studied temperature range. The Na-doped samples exhibit a decrease in the Seebeck value with increasing Na content at room temperature. At higher temperatures Seebeck value matches with that of the parent sample. This may be due to a change in the ratio of the concentration of Co4+/Co3+ ions which increases the configurational entropy of the system. In conclusion, the highest figure of merit (0.01) found for the Na / K- doped lanthanum cobaltite is for 15 atomic wt. % of doping amongst the studied samples. The fourth chapter explains about Tb/Nb co-doped calcium manganite for thermoelectric applications. The CaMnO3-δ shows enhanced thermoelectric properties, exhibits n-type behavior and the absolute thermopower is found to be 129 µV/K. Here, we investigated the Terbium and Niobium codoped at Ca and Mn-sites respectively. The presence of oxygen non-stoichiometry was confirmed using Raman spectroscopy (Mn3+ peak at 614 cm-1) and δ value was evaluated by iodometric titration. The thermoelectric properties of cold isostatic pressed (CIP) pellets prepared by the solid state and soft chemistry routes are compared. The non-monotonous behavior of absolute thermopower may be due to the increase of Mn3+ in the Mn4+ matrix and also the presence of oxygen defects in compounds. The thermoelectric figure of merit of solid state sample CaMnO3-δ estimated of 0.036 at 825K. The fifth chapter describes the thermoelectric properties of double Perovskite AA’B2O6 (112 type): (RE)BaCo2O5+δ. It is a disordered double perovskite with non-stoichiometry in oxygen and exhibits mixed valences of Cobalt. Resistivity of DyBaCo2O5+δ was found to be 0.09 Ω cm and Seebeck coefficient is found to be 42 µV/K. In order to improve the thermopower value, the Fe is substituted at Co-site. This varies the valences of Cobalt that in turn leads to a higher thermopower. Also, the morphology of thermally etched CIP pellets recorded and correlated with the transport properties. It shows the highest thermoelectric figure of merit of 0.25 at 773 K for 20 at wt % of Fe substituted sample. The sixth chapter explains about oxygen sensing studies of DyBaCo2O5+δ (112 type). The detailed structural and morphological characterization studies were carried out. Thermogravimetric analysis at isothermal temperature 873 K shows fast intake/release of oxygen of this disordered double perovskite structure. The higher chemo resistive oxygen sensitivity at the elevated temperature was measured. Further, the systematic study on the effect of oxygen sensing on the substitution of Fe and Cu at Co-site in DyBaCo2-xM xO5+δ was investigated. The possible bulk diffusion mechanism at higher temperature due to movement of oxygen defects were explained. The highest sensitivity was obtained for x = 0.4 at % of Fe and 0.2 at % of Cu at 973 K and 823 K respectively. The key findings and future aspects are summarized in the chapter-7.

Perovskite oxides show wide range of applications in the area of magnetism, ferroelectricity, piezoelectricity, thermoelectricity, gas sensing, catalyst development, solid oxide fuel cell, etc. This is due to flexibility in the structure and compositions that can be tuned by specific element doping. In the perovskite oxide (ABO3), large cation (A) is 12 -coordinated and smaller B-cation is 6 coordinated with oxide ions. Oxide materials are considered as better candidates for thermoelectric applications (interconversion of thermal into electrical energy) due to its non-toxicity and thermal stability at elevated temperature. These are insulating in nature and the conductivity can be increased by doping A and / or B –sites. Perovskite oxides are also used for oxygen monitoring in different applications including control and optimization of combustion of fossil fuels in industries and automobiles, biological and defines places, etc. In the present study, we focused on thermoelectric properties in single perovskite oxides of lanthanum cobaltite and calcium manganite and a double perovskite oxide of dysprosium barium cobaltite. Also, the oxygen sensing behaviour of dysprosium barium cobaltite at elevated temperatures is studied. The thesis contains seven chapters and a summary of respective chapters are given below. The first chapter outlines the basics of thermoelectric and gas sensing applications of both perovskite and double perovskite oxides. In the initial part, thermoelectric phenomena are explained. Thermoelectric effect is the conversion of thermal energy to electrical energy and vice-versa. Higher thermoelectric efficiency (η) can be achieved by maintaining a large temperature difference across the material. The efficiency depends on the thermoelectric figure of merit (zT) of material, which depends on thermopower (S), electrical resistivity (ρ) and thermal conductivity (κ) of the material and hence needs to be optimized. The latter part discusses the oxygen sensing property of distorted double perovskite 112 structure type as it shows advantages over other materials due to oxygen nonstoichiometric. Further, an overview of the relevant literature, objective and scope of the thesis are mentioned. The second chapter elucidates the materials and methods used for the present work. The materials viz. LaCoO3, CaMnO3-δ and DyBaCo2O5+δ, were selected for thermoelectric and oxygen sensing studies. Both the conventional solid state and soft chemistry methods were adopted for the synthesis of these materials. Powders were densified into pellets by hot uniaxial pressing / cold isostatic pressing and various heat treatments were carried out. Samples thus prepared were phase pure as confirmed using powder x-ray diffraction and Rietveld refinement performed for structural analysis. Morphological studies were carried out using scanning electron microscopy and transmission electron microscopy. Further Raman and x-ray photoelectron spectroscopic characterization of these materials were discussed. The transport properties viz. electrical resistivity, thermopower and thermal conductivity of compact pellets were measured at elevated temperatures. Further, the home-built apparatus for room temperature See beck measurements and chemo resistive oxygen sensing were explained in detail as a part of this work. The third chapter describes the effect of monovalent ion doping (Na+ and K+) at A-site of lanthanum cobaltite on thermoelectric properties. Lanthanum cobaltite system exhibit exotic behaviour due to commensuration phenomena of spin, lattice, charge and metal insulator transition. The synthesis, followed by structural refinements by Rietveld method using Fullprof suit program are explained. The results of the transport properties indicate that there is no appreciable change in the See beck Coefficient of K-doped samples throughout the studied temperature range. The Na-doped samples exhibit a decrease in the Seebeck value with increasing Na content at room temperature. At higher temperatures Seebeck value matches with that of the parent sample. This may be due to a change in the ratio of the concentration of Co4+/Co3+ ions which increases the configurational entropy of the system. In conclusion, the highest figure of merit (0.01) found for the Na / K- doped lanthanum cobaltite is for 15 atomic wt. % of doping amongst the studied samples. The fourth chapter explains about Tb/Nb co-doped calcium manganite for thermoelectric applications. The CaMnO3-δ shows enhanced thermoelectric properties, exhibits n-type behavior and the absolute thermopower is found to be 129 µV/K. Here, we investigated the Terbium and Niobium codoped at Ca and Mn-sites respectively. The presence of oxygen non-stoichiometry was confirmed using Raman spectroscopy (Mn3+ peak at 614 cm-1) and δ value was evaluated by iodometric titration. The thermoelectric properties of cold isostatic pressed (CIP) pellets prepared by the solid state and soft chemistry routes are compared. The non-monotonous behavior of absolute thermopower may be due to the increase of Mn3+ in the Mn4+ matrix and also the presence of oxygen defects in compounds. The thermoelectric figure of merit of solid state sample CaMnO3-δ estimated of 0.036 at 825K. The fifth chapter describes the thermoelectric properties of double Perovskite AA’B2O6 (112 type): (RE)BaCo2O5+δ. It is a disordered double perovskite with non-stoichiometry in oxygen and exhibits mixed valences of Cobalt. Resistivity of DyBaCo2O5+δ was found to be 0.09 Ω cm and Seebeck coefficient is found to be 42 µV/K. In order to improve the thermopower value, the Fe is substituted at Co-site. This varies the valences of Cobalt that in turn leads to a higher thermopower. Also, the morphology of thermally etched CIP pellets recorded and correlated with the transport properties. It shows the highest thermoelectric figure of merit of 0.25 at 773 K for 20 at wt % of Fe substituted sample. The sixth chapter explains about oxygen sensing studies of DyBaCo2O5+δ (112 type). The detailed structural and morphological characterization studies were carried out. Thermogravimetric analysis at isothermal temperature 873 K shows fast intake/release of oxygen of this disordered double perovskite structure. The higher chemo resistive oxygen sensitivity at the elevated temperature was measured. Further, the systematic study on the effect of oxygen sensing on the substitution of Fe and Cu at Co-site in DyBaCo2-xM xO5+δ was investigated. The possible bulk diffusion mechanism at higher temperature due to movement of oxygen defects were explained. The highest sensitivity was obtained for x = 0.4 at % of Fe and 0.2 at % of Cu at 973 K and 823 K respectively. The key findings and future aspects are summarized in the chapter-7.