A silver-based joining technique referred to as reactive air brazing has been recently developed for joining high-temperature structural ceramic components of the type used in gas turbines, combustion engines, heat exchangers, and burners. It was found that additions of copper oxide to silver exhibit a tremendous effect on both the wettability and joint strength characteristics of the subsequent braze relative to polycrystalline alumina substrates. The effect is particularly significant at low copper oxide content, with substantial improvements in wetting observed in the 1-8 mol% range. The corresponding strength of the brazed polycrystalline alumina joints appears to be maximized at a copper oxide content of 8 mol%, with a maximum room temperature flexural strength approaching that of monolithic alumina. While further increases in oxide content lead to improved wetting on polycrystalline alumina, the effect on joint strength is deleterious. It appears that the formation of a continuous brittle copper-based oxide layer along the interface between the braze and alumina faying surface is responsible for the poor mechanical behavior observed in joints fabricated with higher copper oxide content brazes.

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High-temperature electrochemical devices operate via an ion gradient that develops across a solid electrolyte. Consequently, hermeticity across this membrane is paramount. Not only must the electrolyte contain no interconnected porosity, but it must be connected to device chassis with a gas-tight seal. Here we report a new method of brazing developed specifically for solid-state electrochemical applications. We demonstrate that the seal is hermetic and resistant to thermal aging, can be thermally cycled under rapid heating rates with no measurable loss in seal strength, and has shown promise in sealing full-size planar solid oxide fuel cell ͑pSOFC͒ components.

A novel method of ceramic-to-metal joining, referred to as reactive air brazing, was investigated as a potential method of sealing planar solid oxide fuel cells ͑SOFCs͒. In the present study, yttria stabilized zirconia ͑YSZ͒ and Fecralloy were selected as subject materials to simulate the cell-to-frame seal in planar SOFC. YSZ plates were joined with Fecralloy foils, using a range of CuO-Ag and CuO-Ag-TiO 2 braze compositions. Metallographic analysis revealed that a majority of the CuO in the braze preferentially migrates to the braze/Fecralloy interface, indicating a stronger affinity and interaction between the CuO and the alumina scale that had formed on the Fecralloy than with the YSZ substrate. The addition of TiO 2 to the braze had no significant effect on the microstructure or mechanical properties of the YSZ/Fecralloy joints, unlike what had been observed previously in YSZ/YSZ joining. Four-point bend tests indicated that joint strength improves with increasing CuO content up to 8 mol % CuO, the maximum concentration of copper oxide tested, likely due to the concomitant improvement in braze wettability. A maximum bend strength of 101 MPa was achieved using the 8 mol % CuO braze composition, demonstrating the feasibility of this joining technique for sealing planar SOFCs.

Chrome poisoning effects of various Cr-containing metal sources on the electrochemical performance of the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 ͑LSCF6428͒ cathode have been investigated. It was found that chromia-forming metals caused significant fade in power density due to the chrome poisoning, while alumina-forming alloys exhibited no influence on the cell degradation. This degradation caused by the chrome poisoning was accelerated at higher operating temperatures. Microstructural analysis conducted on the cells tested with chromia formers exhibited the formation of a strontium chromate phase in the entire cathode, leading to the homogeneous distribution of Cr in the cathode. Although the Mn-containing chromia former such as Crofer22 formed a continuous layer of the Cr-Mn oxide scale on the mesh surface, it was not effective enough to prevent Cr poisoning of the cathode. In contrast, alumina formers such as Haynes214 and Kanthal formed a continuous layer of the alumina scale, resulting in no Cr contamination in the LSCF6428 cathode.