Thermoelectrics and Oxygen Sensing Studies of Selected Perovskite Oxides
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.
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