Summary 109 8. Summary In the joining technology, composite materials based on glass, glass ceramics (GC) and glass ceramic composites (GCC) become more and more important. Due to the increasingly required multi-functionality of the sealant material (for e.g. coefficient of expansion, flow ability, etc…), the developed sealant material used today is based on glass ceramic composites. With these, a good adhesion of the joining components, a good flow ability of the sealant material during the joining process and a good electrical insulation and mechanical strength have to be achieved. The joining process has to take place at temperatures < 900 °C. The operation conditions of the SOFC are: 600 – 800 °C under oxidizing as well as reducing atmospheric conditions. The present Thesis shows that, based on the use of database and thermochemical methods, sealing materials can be swiftly adjusted to all process and operation requirements. In order to do this especially, with regards of the thermo-mechanical properties of the developed sealant material (coefficient of expansion and flow ability) during the joining process, a new concept has been designed. Joining tests have been achieved on sandwich samples. The starting point of this new concept is a metal/glass matrix composite where the coefficient of expansion was controlled by dispersing a metal phase in the glass matrix. The newly developed concept is based on glass as a matrix and a crystalline phase as a filler material. It is essential that the crystalline phase and the glass matrix composition consist of the same thermodynamic co-existence area of the selected oxide base system. Here, the glass matrix has a lower liquidus temperature compared to the crystalline filler material, while the crystalline phase which consist of a higher coefficient of expansion is used alone to tailor the coefficient of expansion of the joining material. By incorporating a single crystalline phase into the selected glass matrix, the coefficient of expansion of the sealant material matched the value of the joined material component. In this work and on this approach, it has been shown that by incorporating the crystalline filler material barium silicatphase (BS) based on the system BaO-MgO- SiO2 into the selected glass matrix consisting of the same co-existence area from the selected oxide system, the coefficient of expansion would be tailored in the range of 9 to 15·10 -6 K -1 depending on the variation of the mass ratio of the BS-crystalline phase.
110 Summary The previous constitutional phases of the glass ceramic composite from the glass compositions by means of knowledge-based tools have been identified by the XRD analysis. In order to achieve a homogeneous distribution of multi-component phases according to the application conditions, a new approach to tailor the flow properties of the sealant material was proposed. First, multi-component oxide systems were used of low possessing liquidus temperature. This strategy is, however, limited to very narrow compositional ranges. In the second approach, a significant improvement of the flow ability of the developed sealant material was achieved. Hereby, phases with low liquidus temperature were added into the previously developed glass ceramic composite. This above described low melting oxide phase reacted as a "lubricant". Thus, a massive decrease of the joining temperature for the previously developed GCC was achieved. Here, the reduction of the joining temperatures of up to 300 K has been obtained by using a low-melting phase based on the oxide system Bi2O3-B2O3. In addition, experimental measurements which were closely associated to the concept of dynamic viscosity such as rotation and beambendingviscosimetry were achieved. In the long-term tests at operating temperature, the crystallization behavior of the developed sealant materials becomes importance. To estimate the relative crystallization behavior of glass, two approaches (by Weinberg and by Hrubÿ), which describing the glass stability against crystallization and the glass formationability, respectively were used. The developed glass ceramic composites (GCC) used as sealant material in the SOFC 800 as well as SOFC 600 reached a good tightness values (10 -4 - 10 -9 mbar·l·s -1 ).