Insights into the PLD process via ICP-MS analysis of LSCF model-type thin films

Abstract

Alternative forms of energy storage and conversion are gaining more and more importance in the 21st century, especially with regard to environmental and efficiency problems of combustion engines used today. Solid oxide fuel cells (SOFC) are electrochemical devices providing the conversion of chemical energy in fuels (e.g. H2, methane or other liquid energy sources) into electricity with a very high conversion efficiency. To meet the requirements, the development of new electrode materials with high catalytic activity for oxygen reduction and other tailored characteristics are necessary. To further improve the efficiency of today’s SOFCs and thus making them also economically more competitive, a reduction of their operation temperature is aimed at. Among others, lanthanum strontium cobalt ferrite (LSCF) offers high activity for oxygen reduction and is already successfully used as cathode material in SOFCs. The performance as well as the degradation behaviour of these materials, however, is strongly dependent on their stoichiometry. Therefore, accurate measurement of the elemental composition is absolutely necessary for a knowledgebased improvement of LSCF and related cathode materials. In this work, the production and analysis of LSCF thin films is investigated. The thin films were produced with a technique called pulsed laser deposition (PLD). Thereby the material is ablated from a target using an UV-laser and deposited as a dense thin film on the substrate. The obtained thin films were used as model system to investigate the elemental composition and learn about the Sr segregation at SOFC operation conditions. As an analytical method for Sr quantification, inductively coupled plasma-mass spectrometry (ICP-MS) was used. From previous work it is known that at temperatures above 500 °C Sr segregates to the surface of the thin films and forms water soluble Sr species. The aqueous solutions needed for ICP-MS measurements were obtained via two different steps. The first one is a H2O etching and the second one a HCl etching method. In the first step, only water soluble species are dissolved. The amount of water soluble Sr species can be determined in this kind of experiments. In the second step, the remaining LSCF film was completely dissolved with HCl and the stoichiometry of the deposited film could be calculated from the corresponding ICP-MS results. These liquid ICP-MS measurements gave hints that the lateral distribution of cations in the deposited thin films on 4 samples per PLD run are not homogeneous. This finding was the starting point to a more detailed investigation of the PLD process. ii Therefore, PLD parameters were varied and a significant improvement in the homogeneity of the thin films could be achieved. Increasing the substrate to target distance turned out to be a crucial factor for film stoichiometry. The lateral distribution of cations within the film was characterised with a laser ablation (LA) system connected to the ICP-MS equipment. By this technique it was possible to analyse a solid sample without the need of dissolving the sample and thereby losing spatial information. Lateral elemental variations of a sample of 4.5 cm in diameter could be mapped in a single measurement run. By using an appropriate software, images of the elemental distribution of films deposited with PLD could be obtained. Such elemental distribution images are a unique feature of the used techniques and are a large step forward in understanding the dynamics and elemental distribution within the plasma plume in the PLD process. Having improved the homogeneity of the samples of one PLD run, several annealing experiments were executed to study the segregation behaviour of Sr under conditions similar to those in an operating SOFC. The samples were annealed in dry and humidified synthetic air at 800 °C for different periods of time. From this experimental setup, time depending information about the segregation could be obtained. Furthermore it could be seen that the amount of humidity in the gas stream is of great importance in the formation process of water soluble Sr species. In a dry atmosphere about 3.5 monolayers of SrO are formed at the surface of the sample. Only 2.5 monolayers of SrO are formed in a dry atmosphere, but the formation process under this conditions is four times faster than in under humid conditions. This information is important for our co-workers in the Christian Doppler Laboratory in which framework this work was conducted.