Optimization of powder and ceramic processing, electrical characterization and defect chemistry in the system YbxCa1-xMnO3

Mixed valence alkaline earth manganites RE$_{x}$A$_{1-x}$MnO$_{3}$ with very rich and complex crystal physics and chemistry have attracted a great deal of attention recently. Two innovative potential applications for rare earth manganites are non-volatile memories based on resistive switching and waste heat recovery techniques by thermoelectric generators. The resistive switching mechanism and thermoelectric properties strongly depend on the concentration and nature of the charge carriers. Defect chemistry mechanism involved in these process have not been understood clearly yet. The lack of information in the literature is certainly also due to complexity of this material system, since manganese cations possess several valence states in the quaternary oxide. The other reason possibly is due to the formation of cracks or deformation of ceramic compounds in to the powder during electrical measurements in the reduction regions. Therefore, in the present thesis comprehensive studies were performed for the first time in order to find the way for reducing or preventing crack formation that occurs during sintering. For this purpose the effect of different calcination and sintering conditions on the physical properties of stoichiometric, non-stoichiometric (Ca-excess and Mn-excess) and 0.1 to 10 at. % of Yb-doped CaMnO$_{3}$ ceramics were investigated in dependence of temperature $T$ and partial pressure of oxygen $p(O_{2})$. After finding the required parameters to produce desired crack free-ceramics such as calcination temperature, sintering gas composition types and heating/cooling rates, etc., electrical characterizations were performed including DC-conductivity and impedance spectroscopic measurements. Bulk conductivity measurements are performed by impedance spectroscopy accompanied by modeling the compounds in terms of an electrical equivalent circuit in a wide range of temperatures and frequencies. These experiments reveal the main role of grain boundaries and electronic and ionic contributions in conductivity. A novel schematic illustration based on the double-Schottky-barrier model for polycrystalline ceramics is proposed which clearly describes the contribution of different resistance components in electric transport properties. Migration of oxygen vacancies and their participation in conductivity are discussed in the present study and the results are confirmed by observing oxygen released using a ZrO$_{2}$ oxygen sensor during dilatometry measurements in a wide range of temperatures. The defect chemistry model is proposed to clarify the details of the chemistry of point defects. This theoretical model accompanied by experimental DC-conductivity measurements, dilatometry analysis, SEM, EDX, XRD-measurements, Raman spectroscopy, iodometric titration and thermogravimetric (TGA) experiments reveal ionic as well as electronic charge transport conductivity contribution in dependence of the oxygen partial pressure $p(O_{2})$ for donor-doped Yb$_{x}$Ca$_{1-x}$MnO$_{3}$ systems.. A comparison of the experimental observations and the theoretical defect chemical models clearly show the way for controlling charge carriers in dependence of $p(O_{2})$ and dopant concentrations. A negative slope of electrical conductivity versus $p(O_{2})$ indicates that n-type conductivity occurs. A $p(O_{2})$ independent conductivity (plateau region) is observed for both undoped and donor-doped CaMnO$_{3}$. Electrical measurements accompanied by thermal analysis and phase purity assessment reveal that the origin of the plateau region in conductivity is due to intrinsic or extrinsic electronic compensation. In this region probably due to formation of secondary phases oxygen ions do not considerably influence the electron and hole concentrations. In addition, the origin of the drastic decrease in conductivity at reduction region is elucidated. Overall, the present experiments clearly demonstrate the effect of donor dopant concentration, formation of oxygen vacancies and corresponding change in density of Mn$^{3+}$-Mn$^{4+}$ pair sites on phase transition, charge migration and conductivity mechanism of the complex systems Yb$_{x}$Ca$_{1-x}$MnO$_{3}$. The obtained information probably will be useful for better understanding the conduction mechanism in future technological applications