Characterization of ceramic/glass composite seals for solid oxide fuel cells.

摘要

Solid oxide fuel cells (SOFCs) require seals that can function in harsh, elevated temperature environments. Comprehensive characterization and understanding of seals is needed for commercially viable SOFCs. The present research focuses on a novel ceramic/glass composite seal that is produced by roller compaction or tape casting of glass and ceramic powders and a proprietary organic binder. Upon heat treatment, micro-voids and surface anomalies are formed. Increased heating and cooling rates during the heat treatment resulted in more and larger voids. The first goal of the current research is to suggest an appropriate heating and cooling rate to minimize the formation of microstructural defects. After identifying an appropriate cure cycle, seals were thermally cycled and then characterized with laser dilatometry, X-Ray diffraction, and sonic resonance. From these experiments the crystalline phases, thermal expansion, and elastic properties were determined. Subsequently compression testing with an acoustic emission (AE) sensor and post-test microstructural analysis were used to identify the formation of damage.;The research also focuses on the study of Weibull statistics and thermal responses for cured seals. The green seal was initially cured for 1 thermal cycle based on the aforementioned appropriate thermal cycle. The cycled seal was then characterized with a laser dilatometer to identify the glass transition, softening temperature and thermal expansion properties. High temperature ring-on-ring tests were also performed to study the effect of glass transition and softening temperatures on mechanical responses. In addition, Weibull statistics were conducted to determine the cumulative probability of failure/damage in seals.;The third part of the research focuses on the construction and use of a controlled leak testing facility for investigating different interfaces involved in sealing electrolyte-supported cells. Simultaneous leak testing with an acoustic emission (AE) sensor was used to identify damage in seals. Two analytical models have been developed and are compared to the experimental leak rate results. The first model is a hydromechanics model based on Navier-Stokes equations. The approximate interfacial distances between stack components were measured using a long-distance microscope lenses. The interfacial distances thus measured served as an input to the model. For the second model, an average-based Reynolds equation was used to approximate the leak rate across the rough interfaces. Post-test optical profilometry was used to study surface textures of different stack components and to calculate the average surface roughness parameters for the model.;Finally, the research focuses on the comparative study between the characteristics of two sealing compositions for SOFCs. The leak rates of each composition were compared with the controlled facility capable of incorporating different mechanical loading, different surface roughness, stack configurations and thermal cycles. The leak test set up was again equipped with an AE sensor to detect any micro-damage in the composite seals. In addition, a two level factorial design was applied on the first sealing composition to identify the main and the interactive factors for leak rates. MINITABRTM was also used to determine a regression based leak rate model. Based on the statistical model, new factors were further introduced to investigate the leak rates for the second composition. It was observed that leak rates depended on the sealing composition. Numbers of interfaces for a specific stack configuration have the most significant effect on leak rates. Thus, the present study could be useful for understanding critical factors that affect leak rates. By fully understanding the characteristics of these ceramic/glass composite seals, next generation seals can be fabricated for better efficiencies.