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A series of Ag-CuOx compositions has been developed for potential use as high-temperature brazing filler metal alloys, specifically for joining and sealing the ceramic and metal components employed in solid-state electrochemical devices such as solid oxide fuel cells, gas concentrators, and sensors.[1,2] These alloys differ from most braze filler metals in that (1) they readily wet a wide variety of ceramics, (2) brazing can be conducted directly in air without an inert cover gas or use of surface reactive fluxes, and (3) the resulting joint is inherently resistant to oxidation at high temperature.[3–7] The latter issue is a particularly significant challenge in high-temperature devices. Recent studies on the oxidation behavior of commercial active metal brazes, such as Nioro ABA and Gold ABA (Wesgo Metals), have shown that they are unreliable at temperatures beyond 500 C, due to oxidation along the interface between the filler metal and ceramic substrate.[8,9] As part of an on-going investigation of these alloys, we examined the effects of palladium on the solidus and liquidus temperatures in the Ag-CuOx system, as well as on the subsequent wetting characteristics relative to a model set of alumina substrates.[10–12] As shown in Figure 1, two invariant reactions exist in the binary system, a eutectic at 942 C and xAg = 0.99 and a monotectic at 967 C and xAg = 0.36.[13–15] Extending between these two points is a two-phase liquid miscibility JENS T. DARSELL, formerly Graduate Student, Washington State University, Pullman, WA 99164, is Postdoctoral Researcher, Pacific Northwest National Laboratory. K. SCOTT WEIL, Staff Scientist, is with Pacific Northwest National Laboratory, Richland, WA 99352. Contact e-mail: [email protected] Manuscript submitted September 14, 2007. Article published online June 6, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

gap in which a silver-rich (L1) and a copper-oxide–rich liquid (L2) coexist. Shown in Figure 2 is the effect of palladium addition on the solidus and liquidus temperatures in the Ag-CuOx system. Results from differential thermal analysis indicate that the addition of palladium can increase both the liquidus and solidus temperatures by as much as 350 C relative to comparable binary compositions.[10–12] Findings from an associated series of sessile drop experiments show that the added palladium leads to reduced wetting between the resulting filler metal and alumina substrate.[11] An analogous effect has been observed with YSZ substrates.[12] That is, there is a trade-off in properties for the Pd-Ag-CuOx system. On one hand, the increase in liquidus temperature raises the upper temperature limit at which devices joined with these materials can be deployed and also permits two or more brazed seals to be employed in a given device (one ‘‘high-temperature’’ and the other ‘‘low-temperature’’) without concerns of filler metal remelting. On the other hand, the reduction in filler metal wettability implies a potential reduction in filler metal adhesion and subsequent joint strength. The pres