Novel silver/mica multilayer compressive seals for solid-oxide fuel cells: The effect of thermal cycling and material de
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A novel Ag/mica compressive seal was thermally cycled between 100 °C and 800 °C in air to evaluate its stability. The novel Ag/mica compressive seal was composed of a naturally cleaved Muscovite mica sheet and two thin silver layers, and was reported in a previous study to have very low leak rates at 800 °C. In the present study, we examined the thermal cycle stability of the Ag/mica-based compressive seals pressed between mating couples with large and small mismatch in thermal expansion. For comparison, thermal cycling also was conducted on plain mica as well as plain silver only. In addition, the results were compared with published data of a similar mica seal using glass instead of Ag as the interlayers. For mating materials of large mismatch in thermal expansion coefficient (CTE; Inconel/alumina), the Ag/mica seal showed lower leak rates than the plain mica. For mating materials of small mismatch in CTE (SS430/alumina), the leak rates were similar for both the Ag/mica and the plain mica seal. Scanning electron microscopy was used to characterize the microstructure of the mica after thermal cycling. Microcracks, fragmentation, and wear-particle formation were observed on the mica and were correlated to the leak behavior. Overall, the novel Ag/mica seals present good thermal cycle stability for solid-oxide fuel cells, although the leak rates were greater than the corresponding mica seals with glass interlayers. I. INTRODUCTION
Solid-oxide fuel cells (SOFCs) present a new and unique way of generating electrical power for the next generation because of their high efficiency, modular construction, minimal site restrictions, and much lower air pollution.1–3 In addition to stationary applications, SOFCs are also being considered for transportationrelated applications such as auxiliary power units for automobiles.4 One of the critical technical challenges facing SOFC developers is the need for reliable seal technology to allow for the successful production of reliable SOFC stacks. The seals need to offer: (i) very low leak rates; (ii) electrical insulation; (iii) stability during longterm exposure to elevated temperatures and harsh environments (oxidizing, reducing, and humid); and (iv) stability toward the materials in contact (e.g., electrolytes, interconnects, and electrodes). In addition, the seal has to survive multiple thermal cycles (possibly thousands of thermal cycles for transportation applications) during routine operation. A number of approaches such as glass seals,5–7 glass-ceramic seals,8 cement seals, and brazes have been proposed. A major challenge for these a)
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“rigid” seal concepts is the stringent requirement of very close matching of the thermal expansion coefficients (CTEs) of the components being sealed and the sealing material. Otherwise, residual stresses will develop that may cause structural damage to the stack components during thermal cycling. The required close match of CTE has thus greatly limited the selection
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