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Unusual severe corrosion and wearled to early failure of SiC-based mechanicals eals when operated under specific conditions in water. In order to understand the mechanisms, seal tests with different SiC types were conducted. Results show that electrical discharge causes the failure und electrical resistance of seal ring material must be considered as a key property. Selecting the correct seal material combination is essential to gain long life time.

Solid oxide fuel cells are operated at temperatures from 1073 K to 1273 K due to the use of ceramic materials as solid electrolyte. In view of the high temperatures and the required long operating times of at least 40000 hours particularly great demands are made on the stability and chemical compatibility of the materials used. The aim of the present work is to conduct relevant studies of the ceramic LaCrO$_{3}$ - based interconnect in order to provide a contribution to its further development. Moreover, these studies contribute towards a better understanding of the mechanisms which can lead to a deterioration of the function of SOFC modules. Apart from the investigation of the defect thermodynamics, the influence of the formation of defects on the mechanical properties of the partially substituted materials based on lanthanum chromite is studied in more detail. The kinetics of defect formation complements the spectrum of these studies. The chemical interactions between the materials used in the SOFC were studied under the conditions prevailing during cell operation. The formation of phases was determined by X-ray diffraction analysis. Ca-containing lanthanum-chromite specimens of different composition currently considered as candidate materials for the ceramic interconnect were prepared and characterized. For comparison, two material grades produced for technical application were incorporated in the investigation programme. These were the ceramic interconnect materials La$_{0.80}$Sr$_{0.20}$Cr$_{0.97}$V$_{0,03}$O$_{3-\delta}$ and LaCr$_{0.78}$Mg$_{0.05}$Al$_{0.16}$O$_{3-\delta}$. Equilibrium constants for the formation of oxygen vacancies in the above-mentioned lanthanum chromites were determined by thermogravimetry. The parameters for establishing disorder equilibria in the Ca-doped specimens resulted from kinetic measurements which were also performed with the aid of the thermobalance set up. The mechanical studies tonfirm an increase in fracture toughness, strength and Young's modulus with diminishing oxygen partial pressure, i.e. increasing amount of oxygen vacancies.

EPR measurements on yttria-doped tetragonal zirconia polycrystals (Y-TZP) were performed in order to study the modifications of the structure of the material following hydrothermal treatments. By X-ray irradiation of the samples, the structural defects near the oxygen vacancies were magnetically activated by electron trapping. Y2O3 aggregates were identified by EPR signals of defects of F+ type, coordinated with yttrium ions, and defects such as Y2+ (4d1) in four-fold coordination, randomly tetragonally distorted. The analysis of these defect sites, used as local probes, supports the existence of structural modification of the material and the formation of α-Y(OH)3 clusters. © 1993 Chapman & Hall.