Jin Yong Kim

The silver-copper oxide–based reactive air brazing technique was developed as a method of joining complex-shaped ceramic parts. To investigate the viability of this approach for high-temperature application, a series of air-brazed alumina joints were independently exposed to either oxidizing or reducing atmosphere at 800 °C for 100 h. Those samples that were thermally aged in air maintained good joint strength, similar to that of the original as-brazed samples. Microstructural analysis revealed no significant change in joint microstructure after long-term oxidation at elevated temperature, indicating excellent stability of the Ag–CuO-based filler metal in this environment. On the other hand, exposure of the air-brazed alumina joints to hydrogen under the same aging conditions resulted in a measurable decrease in joint strength. Scanning electron microscope analysis conducted on the fracture surfaces of the broken hydrogen-exposed specimens indicated that the source of joint failure was debonding along the interface between the filler metal and alumina substrate. This was due in large part to internal reduction of CuO precipitates within the filler metal to copper and accompanied by the simultaneous formation of porosity at these sites, both within the bulk of the joint as well as along the filler metal/substrate interfaces. Pore formation was noticeably present in filler metals prepared with a high concentration of copper oxide.

Chrome poisoning effects of various Cr-containing metal sources on the electrochemical performance of the La0.6Sr0.4Co0.2Fe0.8O3La0.6Sr0.4Co0.2Fe0.8O3 (LSCF6428) 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–MnCr–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.

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 (SOFCs). In the present study, yttria stabilized zirconia (YSZ) 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-TiO2CuO-Ag-TiO2 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 TiO2TiO2 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.