ISBN 978-3-8439-2174-9

978-3-8439-2174-9, Reihe Energietechnik

Steffen Wolf
Spannungs- und Teilchengrößenanalyse in den Funktionsschichten der Festelektrolytbrennstoffzelle (SOFC)

180 Seiten, Dissertation Universität Stuttgart (2014), Softcover, A5

Zusammenfassung / Abstract

The main topic of this thesis is to study and correlate mechanical, electrical and thermal properties of plasma sprayed layer microstructures in metal supported Solid Oxide Fuel Cells (SOFC). The investigations focus on the measurement, evaluation and characterization of functional materials and coating systems. This work aims to improve the mechanical properties of a SOFC and increase its performance by analyzing the elastic stress states in the processing step of reduction. Additionally, the main degradation mechanisms and strategies to avoid these are discussed further. The analysis of the precursor powders has shown, that these consist of mainly three phases: cubic and tetragonal zirconia and Y2O3. These measurements also show, that although the precursor powders are multiphase in nature, only the cubic high temperature phase dominates (96-98 vol.%) the plasma sprayed layers in the resulting coating with the powders of the vendors Medicoat and Sulzer-Metco. Furthermore, it could be shown, that in the plasma spraying process of the nano scaled precursor powders the element Y is selectively evaporating from the solid solution. This is the case because of the relatively high surface area of the powder. By measuring the lattice parameter of the cubic phase, the residual concentration of Y in zirconia could be deduced. These results supported the assumption, that using nano scaled powders in the suspension based plasma spraying process are not beneficial. Main reasons for that can be found in the limited conveying ability of the precursor powders, the high porosity of the sintered end product, the multiphase end layer, the loss of Y by evaporation and the large particle size in the resulting coating. The ex-situ mechanical stress analysis of the electrolyte material has shown, that the as-sprayed state inherits elastic residual stresses. This is caused by the multilayer spraying process. In this process a high temperature material is sprayed on a colder material surface and causes a temperature gradient based elastic tensile stress of about 150 MPa in the direction of the cathode. The tensile stress causes the formation of a small crack network in the electrolyte. These stresses can only be reduced in the reduction processing step. In this step, however, tensile stresses of about 300 MPa and introduced in the electrolyte by the reduction of NiO to Ni. This reduction causes a volume contraction of about 40 %.