Evaluation of Silica-based/Nickel and Borate-based/ Silver Glass Composites Seals for Solid Oxide Fuel Cells.

摘要

The increasing demand for energy and the necessity to overcome the depletion of fossil fuel supplies requires that alternative energy sources be developed. Solid Oxide Fuel Cells (SOFCs) are one of the alternative technologies to minimise our dependence on fossil fuel due to their numerous advantages including high efficiency, long-term stability, fuel flexibility and low emissions. However, the development of reliable sealing techniques remains a crucial challenge to overcome to allow usable efficiency and facilitate commercialization.;Sealing technology has been object of research for several years. Nevertheless, the optimal solution is yet to be found. The use of a glass composite approach is attractive as it allows the possibility of engineering the properties of the seal, by independently adjusting the particle size distribution and volume fraction of the additives. In the present work, the interaction between various SiO₂ based glasses with nickel and B₂O₃ based glasses with silver were studied. Results as a function of additive particle size distribution (7-100 microns) and volume fraction (0-18%) will be presented.;Micrographs, X-ray patterns and CTE measurements showed that the proposed systems have adequate characteristics for usage as seal for fuel cells due to the inertness of the additive particles with the respective glass matrix and predictable long-term chemical and thermal stability.;The use of DTMA as a technique to calculate the onset of residual stresses, explores the influence of the additive and its interfacial interactions on the dissipation of energy during deformation. The multi-frequency test lead to an activation energy for stress relaxation between 400 and 600 kJ/mol depending on the different additive content. Furthermore, the temperature difference between de T_g and the onset of residual stresses was calculated showing that increments on the additive content results on a larger temperature range that allows stress relaxation.;The mechanical response under compression test was also investigated to identify the potential deformation of a stack during service. The results showed that the glass composites can experience large deformations during the entire service cycle and not only during the isothermal service hold. Moreover, the microstructure in terms of crystalline phase evolves with the test temperature and the applied force, showing an increase of the crystals volume fraction when either the temperature of the applied load increase. The microstructures showed that the additive is getting aligned during deformation, providing an increased resistance to compression against flow of the viscous glass composite.;Finally the measurement of the residual stresses as function of cooling rate and additive content revealed that the residual stresses development is minimised for a combination of service conditions including cooling rate under 20 °C/min and glass composite containing a minimum of 12 %vol. Such operating conditions should contribute to maximise the service life of a SOFC stack.