Ni-Ta-Si Brazes for Planar Solid Oxide Fuel Cell Applications

Abstract

Introduction Solid oxide fuel cells (SOFC) are high efficiency devices for converting the chemical energy from a wide range of fuels and energy-carriers directly into electricity [1]. One of the major challenges for the viability of commercial SOFC devices is the development of suitable sealing technologies to prevent air and fuel crossover at SOFC operating temperatures of ~750⁰C. Today, reactive air brazing (RAB) of Ag-based brazes is commonly used for this purpose. Unfortunately, due to the high diffusivity of both H and O in silver, diffusing H and O react to form micro-pores in silver-based brazes which eventually develop into a porous structure that degrades the seal, reduces the mechanical robustness of the joint, and limits the lifetime of commercial SOFC devices to ~10,000 hours [2]. The present work reports on a new, computationally identified family of Ni-Ta-Si brazes developed to replace conventional Ag-based brazes in SOFC applications. Experimental Methods Here, Thermo-Calc© software was used to identify candidate alloy systems with a melting range suitable for SOFC brazing applications. To limit the number of candidate alloy systems, radioactive, toxic, prohibitively expensive, and chemically unstable elements were eliminated from consideration. Ni-based alloy systems with all possible combinations of the remaining 25 elements from the periodic table were then studied to find alloys with a melting range between 900 and 1000°C. Of the 276 possible ternary alloy combinations, Thermo-Calc© analyses were performed on 172 systems, 104 systems remained unanalyzed because of a lack of Thermo-Calc© data, and 19 Ni-based candidate systems were proposed for experimental investigation. These candidate alloys were physically fabricated using arc-melting in suitable atmospheres and characterized using differential scanning calorimetry (DSC) to determine liquidus and solidus temperatures, thermogravimetric analysis (TGA) to determine the high temperature oxidation resistance, and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) to determine the microstructural and compositional stability. Results and Discussion Of the 19 systems, Ni-Ta-Si alloys with and without boron melting point suppressant additions displayed promising ductility, melting, and high temperature oxidation resistance behavior. Specifically, the liquidus and solidus temperatures for Ni20Ta7Si and Ni20Ta7Si1B (the boron percentage is a nominal composition which may be an overestimate of the actual boron composition due to boron vaporization during alloy fabrication) are 1163.1°C/1122.9°C and 1017.0°C/1065.8°C, respectively, showing that minor boron additions lowers the Ni20Ta7Si melting range by ~100°C. This melting range is comparable to that of the commercial braze BNi2 (Ni82.4Cr7Si4.5B3.1Fe3), which is 1000°C/970°C. Further, the low melting range of Ni20Ta7Si1B occurs in an alloy that does not contain Cr (Cr has been shown to poison SOFC electrodes) [3]. Figure 1 shows TGA results for different Ni-Ta-Si(B) samples as well as a commercial BNi2 braze at 750°C in air. The Ni20Ta7Si1B sample shows good oxidation resistance similar in magnitude to that of BNi2. Figure 1 also shows that, in addition to lowering the Ni-Ta-Si melting point, 1 wt.% of boron significantly modifies the Ni-Ta-Si microstructure (and presumably the oxidation mechanism) through the formation of a Ta enriched surface reaction zone, and a compositionally homogenized surface reaction zone. Conclusions Here a systematic computational-experimental approach was developed to search for, fabricate, and characterize new braze candidates for future SOFC application.Surprisingly, alloys systems with 20 wt.% of Ta (Ta has a melting point of 3020°C) still melt at temperatures as low as 1065⁰C. Boron additions into the Ni-Ta-Si system further reduce the alloy melting point, alter the oxidation mechanism, and improve the oxidation resistance.Although additional wetting, joint strength, and thermal cycling experiments are needed, the Ni-Ta-Si(B) system may be a promising new family of brazes for SOFC or other applications. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.