No abstract

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition metal ions. Such relations are of high interest as they offer the possibility to influence the catalytic activity of perovskites for the application in solid-oxide fuel-and electrolyser cells. Combining theoretical and experimental approaches, we explore the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing non-stoichiometric LaFeO3 (LFO). In the theoretical part, formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the DFT+U results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe2O3 and La2O3) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases in detail by X-ray diffraction and dedicated microscopy methods, namely scanning electron microscopy (SEM) and (scanning) transmission electron Microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). Instead of a variation of the La/Fe ratio, a mixture of two phases, Fe2O3/LaFeO3, was observed resulting in an invariant charge state of Fe, which is in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previously published studies on this material system.

Aiming to achieve the highest combustion efficiency and less pollutant emission, a catalytic coating for cylinder walls in internal combustion engines was developed and tested under several conditions. The coating consists of a La0.8Sr0.2CoO3 (LSCO) catalyst on an aluminum-based ceramic support. Atomic force microscopy was applied to investigate the surface roughness of the LSCO coating, while in situ diffuse infrared Fourier transform spectroscopy was used to obtain the molecular understanding of adsorption and conversion. In addition, the influence of LSCO-coated substrates on the flame quenching distance was studied in a constant-volume combustion chamber. Investigations conclude that an LSCO coating leads to a reduction of flame quenching at low wall temperatures but a negligible effect at high temperatures. Finally, the influence of LSCO coatings on the in-cylinder wall-near gas composition was investigated using a fast gas sampling methodology with sample durations below 1 ms. Ion molecule reaction mass spectrometry and Fourier transform infrared spectroscopy revealed a significant reduction of hydrocarbons and carbon monoxide when LSCO coating was applied.

The enduring demand for ever-increasing storage capacities inspires the development of new few nanometer-sized, high-performance memory devices. In this work, tri-n-octylphosphine oxide (TOPO)-stabilized sub-10 nm monoclinic HfO2 nanocrystals (NC) with a rod-like and spherical shape are synthesized and used to build up microscale and nanoscale test devices. The electrical characterization of these devices studied by cyclic current–voltage measurements reveals a redox-like behavior in ambient atmosphere and volatile threshold switching in vacuum. By employing a thorough spectroscopic and surface analysis (FT-IR and NMR spectroscopy and XPS), the origin of this behavior was elucidated. While the redox behavior is enabled by residual moisture present during clean-up of the NC and thin film preparation, which leads to a partial desorption of TOPO from the NC surface, threshold switching is obtained for dry TOPO-stabilized HfO2 NC in microchannel as well as in nanoelectrode devices addressing only a few sub-10 nm TOPO-stabilized HfO2 NC. The results show that integration of sub-10 nm HfO2 NC in nanoscale devices is feasible to build up switching elements.