Mechanical Behavior of Mixed-Conductivity Membrane Systems
In the context of characterizing mixed-conductivity ceramic materials, oxygen conductors and proton-conductive ceramic materials are studied. The research objective range from the fundamental chemical and physical investigation to the determination of limit conditions for the large-scale integration. In addition to oxygen conductivity and chemical stability, the long-term mechanical integrity of the membranes themselves and of the joints in membrane modules must be ensured for the most efficient gas separation, even under complex thermomechanical stress in aggressive ambient atmospheres.
Material parameters are determined to estimate the loading situation and the expected consequences on the material stability. A detailed observation of the fracture behavior and the high-temperature stability is carried out. For this purpose, mechanical fundamental study of elasticity and brittle fracture-failure behavior as well as high-temperature deformation tests are performed. To ensure the mechanical integrity, additional investigations on the effect slow crack growth and on creep failure are carried out, which form the basis for the determination of the material life span. The mechanical investigations are supported by microstructure and phase characterizations. In particular, the porous and defect structure quantitative characterized and evaluated during the material development by using light and scan electron microscopy (SEM).
Material and Material Composites for High-Temperature Fuel and Electrolysis Cells
In the last decades, both high-temperature fuel cells and high-temperature electrolysis cells have reached the potential for commercial application. Nevertheless, the continuous material development and the follow-up analysis after long-term operation have also raised issues concerning the optimization, with regard to the stability of the cell and seal. The objective of the characterization of these materials is to support basic chemical and physical research for the further development and improvement of the materials and to provide material parameters for the analysis and simulation of occurring stress states.
Special consideration is given to the strength of the substrate and its increase through the use of newly developed materials and the stability of the joining materials. Here, glass-ceramic materials have shown limitations due to their low brittle fracture strength and residual viscosity. Slow crack growth and creep failure serve for the determination of the life-span. Microstructure and phase determinations using light and scanning electron microscopy support the mechanical investigations.
Mechanical Characterization of Electrochemical Storage Materials
Interest in electrochemical storage has increased significantly in recent years, especially with regard to battery materials. Like other electrochemical systems, batteries consist of an electrolyte that allows the exchange of ions between positive and negative electrodes during charging and discharging. Currently, most batteries are based on liquid electrolytes due to their better ionic conductivity, but these have disadvantageous behavior in terms of flammability and leakage. Solid state electrolytes are expected to have better thermal and chemical stability and to avoid leakage problems. Important aspects in this scientific field of work are the reliability and mechanical limits under application-relevant conditions for these advanced, novel ceramic materials. Therefore, the goal of the work is to improve the understanding of the structural reliability and microscopic fracture behavior of such materials. The mechanical evaluation is based on a characterization by means of biaxial flexural test and hardness indentation. Fracture toughness is included in the considerations, in addition to fracture strength, hardness and Young's modulus. The particular advantage of the hardness indentation method is that only a very small volume of material is required and the method is macroscopically non-destructive. Micropillar tests allow the local properties of the grain and the grain boundary to be observed, and the analyses are supported by optical and electron microscopy.