The electrical conductivity of dense and nanoporous zirconia‐based thin films is compared to results obtained on bulk yttria stabilized zirconia (YSZ) ceramics. Different thin film preparation methods are used in order to vary grain size, grain shape, and porosity of the thin films. In porous films, a rather high conductivity is found at room temperature which decreases with increasing temperature to 120 °C. This conductivity is attributed to proton conduction along physisorbed water (Grotthuss mechanism) at the inner surfaces. It is highly dependent on the humidity of the surrounding atmosphere. At temperatures above 120 °C, the conductivity is thermally activated with activation energies between 0.4 and 1.1 eV. In this temperature regime the conduction is due to oxygen ions as well as protons. Proton conduction is caused by hydroxyl groups at the inner surface of the porous films. The effect vanishes above 400 °C, and pure oxygen ion conductivity with an activation energy of 0.9 to 1.3 eV prevails. The same behavior can also be observed in nanoporous bulk ceramic YSZ. In contrast to the nanoporous YSZ, fully dense nanocrystalline thin films only show oxygen ion conductivity, even down to 70 °C with an expected activation energy of 1.0 ± 0.1 eV. No proton conductivity through grain boundaries could be detected in these nanocrystalline, but dense thin films.

The mixed ionic-electronic conductor (MIEC) (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3-δ (BSCF) is a renowned material with applications in membrane reactors and as cathodes in solid-oxide fuel cells. Despite BSCF's large oxygen permeabilities, long-time phase instability at intermediate temperatures has been reported. However, the mechanism of this decomposition is still unclear. Here, we present a study of the synthesis of BSCF and compare our results with those obtained from long-time decomposition. Rietveld and Le Bail analysis as well as transmission electron microscopy studies were applied to investigate the reaction sequence in BSCF formation. We are now able to draw the following conclusion about the reaction mechanism: the formation as well as decomposition is due to a reversible reordering of the hexagonal AO 3 -layer stacking sequence in the cubic perovskite, which can occur if the cubic BSCF is kept at temperatures below T ) 1173 K for long time periods, thereby leading to the decomposition of BSCF into a three-phase mixture. The driving force for this reaction was identified to occur at the cobalt site because cobalt prefers a low-spin configuration in the 3+ oxidation state. This reaction occurs only at temperatures below T ) 1173 K because of the oxidation of cobalt at low temperatures.

Pulsed Laser Deposition (PLD) was used to prepare thin fi lms with the nominal composition La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ (LSCF). The thin fi lm microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target-to-substrate distance, laser fl uence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin fi lm micro-and nanostructure. A map of the LSCF fi lm nanostructures is presented as a function of substrate temperature (25-700 ° C) and oxygen background pressure (0.013-0.4 mbar), with fi lm structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack-free LSCF fi lms with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 ° C. The obtained knowledge on the structure allows for tailoring of perovskite thin fi lm nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin fi lms. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense fl at fi lms.

No abstract

No abstract