Abstract
The present work focuses on a family of complex cobalt 114 oxides RBaCo4O7+δ (R = Y, Ca, Gd, Tb, Dy, Ho, Yb), which are believed to be promising candidates for low-temperature oxygen storage applications. Polycrystalline and pelletized samples of YBaCo4O7+δ were prepared both by a standard solid-state reaction and wet chemical reaction routes, using three different complexing agents. It was found that the solid-state reaction is the most optimal synthesis method of YBaCo4O7+δ, because it allows to obtain high-purity samples quickly, easily, and cost-effectively. Therefore, all the other samples synthesized in this thesis were prepared by solid-state synthesis, including the samples utilized for electrical and thermogravimetric measurements. The modeling of the defect structure of YBaCo4O7+δ was based on the structural investigations and the analysis of data reported in the literature. Nominally stoichiometric YBaCo4O7 phase was chosen as a reference state and considered to have neutral formal charge. Since di- and trivalent cobalt ions occupy the same site in the crystal lattice of YBaCo4O7+δ, Kröger-Vink compatible notation was chosen to describe cobalt defects. In order to test the proposed defect model oxygen partial pressure dependence of oxygen non-stoichiometry was studied by thermogravimetric measurements in the temperature range of 450–1050 °C. The curves of oxygen non-stoichiometry versus oxygen partial pressure were fitted according to the defect model. The fitting of this model to the experimental data allowed for estimating important thermodynamic parameters of YBaCo4O7+δ, including standard enthalpies and entropies for oxidation and Anion-Frenkel disorder. The resulting standard oxidation enthalpy was equal to -46.8 ± 3.2 kJ/mol. This result was consistent with the results reported in literature (-45 kJ/mol) and with results obtained in this thesis directly from the oxidation process by means of TG-DSC measurements (-50 ± 5 kJ/mol). Then, in order to correlate the oxygen non-stoichiometry and electrical conductivity, the latter was measured by a standard four-probe method as a function of oxygen partial pressure in a wide range of temperature. The mobility of electron holes was estimated by combining these data with calculated concentration of electron holes. The temperature dependence of mobility was described in terms of a small polaronic hopping model, and the enthalpy of migration of the electron hole was calculated to be 19.9 ± 2.1 kJ/mol. Temperature dependencies of electrical conductivity and the Seebeck coefficient were investigated under equilibrium conditions in the temperature range 25–1000 °C. The obtained data revealed that YBaCo4O7+δ exhibits typical p-type semiconductor behavior in the whole investigated temperature region. The transport mechanism of YBaCo4O7+δ was verified by analyzing the experimental conductivity data by means of different transport models, including Arrhenius, Motts 3D variable range hopping and small polaronic hopping models. It was found that the latter model provides a better fit to the experimental data than other models. Successful fitting of small polaronic hopping model to the electrical conductivity versus temperature data allowed to determine activation energy of conductivity in YBaCo4O7+δ, which was found to be 18.2 ± 0.6 kJ/mol. From the Seebeck coefficient versus temperature data measured in argon, the thermopower activation energy of the YBaCo4O7+δ was estimated to be 1.3 ± 0.3 kJ/mol, which is much smaller than activation energy for electrical conductivity. This confirmed the hypothesis that the conduction mechanism in YBaCo4O7+δ is due to thermally activated hopping in the whole temperature range studied. Moreover, by comparing this value with activation energy of mobility, it was concluded that only the mobility of electron holes is activated. It was also found that Heikes’ formula could only be used to describe the thermopower of YBaCo4O7+δ at temperatures above 650 °C. By means of this formula, the concentration of electron holes as a function of temperature was calculated from Seebeck coefficient data. Thermogravimetric measurements were also used to investigate the properties of pure and substituted 114 oxides which are important for practical applications, including reversibility of the low-temperature process, oxygen uptake/release rates, and dynamic oxygen storage capacity. Experiments have shown that all the investigated compounds, with the exception of Ca0.5Y0.5BaCo4O7+δ, can absorb/desorb large amounts of oxygen with relatively fast oxygen uptake/release speeds and in a highly reversible manner. It was suggested that the reason for the difference associated with Ca0.5Y0.5BaCo4O7+δ is most likely related to the defect chemistry of this oxide rather than its structure or unit cell volume as it was reported in the literature. The thermoelectric performance of YBaCo4O7+δ oxide was evaluated by calculating the power factor and comparing its value with that of the best-known thermoelectric oxides, such as NaCo2O4, Ca3Co4O9+δ, and others. It was suggested that in order for this material to be used in applications in the field of thermoelectric power generation, its electrical conductivity, and Seebeck coefficient values must be increased by means of doping/substitution.