Robust copper braze for hermetic sealing of solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) are becoming of increasing interest as a primary power source in today's industrial market. The voltage of a single cell under load is approximately 0.7 volts necessitating the use of many cells in series to generate useful electrical potentials, which gives rise to the SOFC stack. One of the key technical challenges in improving the long term performance and reliability of stacks is in the effective sealing of stack interfaces, particularly in planar stacks for which a hot seal (700-900°C) is required. SOFC stack seals must be: resistant to oxidation/volatilization in oxidizing and reducing atmospheres, must wet and bond to the joining members (both ceramic and metal), form a hermetic seal to prevent hydrogen leakage, and have a coefficient of thermal expansion (CTE) close to that of the adjoining components to limit thermally induced stresses. Active metal copper-based brazes present a novel approach to sealing SOFCs by means of robust mechanical/thermal properties providing strong, hermetic braze-interconnect and braze-YSZ interfaces. A commercially available active braze alloy utilizing no precious metal additives was tested and compared to custom synthesized braze compositions fabricated and tested at MSU. Two testing configurations were evaluated for this sealing study, utilizing dense YSZ substrates joined to 25.24mm, 430SS coupons as well as 25mm 440SS pressure test fixtures. Active braze alloys require a protective atmosphere to facilitate chemical bonding with YSZ and results show excellent performance in moderate vacuum (10-4 to 10-5 mbar) and argon atmosphere. Sample characterization was performed by electron microscopy, energy dispersive x-ray spectroscopy, pressurized rupture and leak tests, differential thermal analysis, thermal gravimetric analysis and thermodynamic evaluation. Robust copper-based brazes show potential for the use of sealing in SOFC applications. The brazes display desirable characteristics for sealing applications including the formation of chemically bonded braze joints, formation of a protective oxide barrier and high strength properties. Evidence of silicon diffusion into the YSZ may be problematic for long-term SOFC operation, however, development of a siliconfree braze has yielded excellent performance near that of the commercially available brazing powder.