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Bulk $$ {\hbox{B}}{{\hbox{a}}_x}{\hbox{S}}{{\hbox{r}}_{{1} - x}}{\hbox{C}}{{\hbox{o}}_y}{\hbox{F}}{{\hbox{e}}_{{1} - y}}{{\hbox{O}}_{{3} - \delta }} $$ compositions (BSCF) were synthesized by the solid-state reaction method. The electrical conductivity of ceramic bars was measured using a dc four-probe method as a function of temperature in air up to 970°C. All compositions showed thermally activated p-type semi-conductivity up to ~450°C and then a transition to metal-like conductivity. The small-polaron hopping p-type semi-conductivity depends on the oxygen nonstoichiometry, which increases with increasing temperature. Metal-like conductivity is attributed to the overlap of the transition metal d-electron orbitals with the oxygen p-orbitals. Strontium-rich compositions show higher conductivity. The Co/Fe ratio does not influence much the p-type semi-conduction. Iron-rich compositions revealed more metal-like conduction behavior. The degree of overlap between transition metal d-orbitals and oxygen p-orbitals depends on the Ba/Sr as well as on the Co/Fe ratios

The concept and the design of a micro-solid oxide fuel cell system is described and discussed. The system in this study is called the ONEBAT system and consists of the fuel cell PEN (positive electrode - electrolyte - negative electrode) element, a gas processing unit, and a thermal system. PEN elements of free-standing multi-layer membranes are fabricated on Foturan® and on Si substrates using thin film deposition and microfabrication techniques. Open circuit voltages of up to 1.06 V and power of 150 mW cm−2 are achieved at 550°C. The membranes are stable up to 600°C. The gas processing unit allows butane conversion of 95% and hydrogen selectivity of 83% at 550°C in the reformer and efficient after-burning of hydrogen, carbon monoxide, and lower hydrocarbons in the post-combustor. Thermal system simulations prove that a large thermal gradient of more than 500 °C between the hot module and its exterior are feasible. The correlation between electrical power output – system size and thermal conductivity – heat-transfer coefficient of the thermal insulation material are shown. The system design studies show that the single sub-systems can be integrated into a complete system and that the requirements for portable electronic devices can be achieved with a base unit of 2.5 W and a modular approach.

Strontium barium niobate (Sr2.5Ba2.5Nb10O30) thin films were deposited on (001) SrTiO3 single-crystalline substrates by pulsed laser deposition. The growth nature was investigated by transmission electron microscopy (TEM). Selected-area electron diffraction and high-resolution transmission electron microscopy revealed the existence of six types of grains. These grains grew on the substrate in a partially epitaxial fashion. Geometrical models were built, which were confirmed by TEM observations. Based on the TEM results and geometrical analysis, a crystallographic model was developed. The strain nature resulting from the growth columns is discussed in this report

The mechanism and kinetics of oxygen reduction at thin dense two-dimensional $$ {\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }} $$ (LSCF) electrodes have been investigated in air between 500 and 700 °C with electrochemical impedance spectroscopy and steady-state voltammetry. Dense and geometrically well-defined LSCF films with various thicknesses ranging between 16 and 766nm have been prepared on cerium gadolinium oxide substrates by pulsed laser deposition and structured with photolithography. The current collection was ensured by a porous LSCF layer. A good agreement was found between the experimental data and the impedance of the reaction model calculated with state-space modelling for various electrode potentials and thicknesses. It was evidenced that oxygen adsorption, incorporation into the LSCF and bulk diffusion are rate-determining while charge transfer at the electrode/electrolyte interface remains at quasi-equilibrium. The 16 and 60nm thin dense LSCF electrodes appear to be more active towards oxygen reduction than thicker layers and porous films at 600 and 700°C

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal tr, Riassunto Il Niobato di Stronzio e Bario è una soluzione solida di formula SrxBa1-xNb2O6, (nel seguito SBN), e struttura cristallina tetragonale. Questo composto ha una fase ferroelettrica con asse polare unico e la temperatura di transizione dipende dal rapporto tra concentrazione di Stronzio e di Bario. In questa tesi viene studiata la crescita di film sottili di SBN; lo scopo è di ottenere un sistema modello per lo studio dell'inversion di polarità in domini a 180°, in prossimità della transizione di fase, per l'appunto in film sottili. SBN è stato scelto non solo per l'unicità dell'asse polare ma anche per la possibilità di variare la temperatura di transizione cambiando il rapporto tra stronzio e bario. Questo permette di effettuare lo studio delle proprietà elettriche in intervalli di temperatura non lontani dalla temperatura ambiente. Uno dei vantaggi, oltre a quelli legati alla praticità sperimentale, è la minimizazione dei fenomeni di conduzione. I film per queso studio sono ottenuti con la tecnica nota come "Pulsed laser deposition", abbreviata PLD e il sistema di deposizione è stato costruito durante il lavoro di tesi. Dal punto di vista della modellizzazione è desiderabile avere un sistema il piu possibile prossimo a un monocristallo. Per questo motivo i parametri di deposizione sono ottimizzati per la deposizione di film con l'asse cristallino [001], coincidente con l'asse polare, perpendicolare al piano del film. I campioni cosi ottenuti sono integrati in condensatori a faccie piane e parallele, per mezzo di tecniche standard di foto-litografia. La stabilità della fase ferroelettrica nei film di SBN, risulta essere sensibile alle impurità; la presenza di contaminanti in concentrazione inferiore all' 1% e sufficiente per indurre un abbassamento della temperatura della transizione fase di circa 100°. Anche le deformazioni hanno un influenza sulla transizone di fase; a causa dell'elevata temperatura di deposizione, quando la costante di dilatazione termic

Bulk $$ {\hbox{B}}{{\hbox{a}}_x}{\hbox{S}}{{\hbox{r}}_{{1} - x}}{\hbox{C}}{{\hbox{o}}_y}{\hbox{F}}{{\hbox{e}}_{{1} - y}}{{\hbox{O}}_{{3} - \delta }} $$ compositions (BSCF) were synthesized by the solid-state reaction method. The electrical conductivity of ceramic bars was measured using a dc four-probe method as a function of temperature in air up to 970°C. All compositions showed thermally activated p-type semi-conductivity up to ~450°C and then a transition to metal-like conductivity. The small-polaron hopping p-type semi-conductivity depends on the oxygen nonstoichiometry, which increases with increasing temperature. Metal-like conductivity is attributed to the overlap of the transition metal d-electron orbitals with the oxygen p-orbitals. Strontium-rich compositions show higher conductivity. The Co/Fe ratio does not influence much the p-type semi-conduction. Iron-rich compositions revealed more metal-like conduction behavior. The degree of overlap between transition metal d-orbitals and oxygen p-orbitals depends on the Ba/Sr as well as on the Co/Fe ratios

Strontium barium niobate (Sr2.5Ba2.5Nb10O30) thin films were deposited on (001) SrTiO3 single-crystalline substrates by pulsed laser deposition. The growth nature was investigated by transmission electron microscopy (TEM). Selected-area electron diffraction and high-resolution transmission electron microscopy revealed the existence of six types of grains. These grains grew on the substrate in a partially epitaxial fashion. Geometrical models were built, which were confirmed by TEM observations. Based on the TEM results and geometrical analysis, a crystallographic model was developed. The strain nature resulting from the growth columns is discussed in this report