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.

No abstract

This paper reports on the wetting behavior, reactivity, and long-term electrical conductance of a series of ternary filler metals being considered for brazing lanthanum strontium cobalt ferrite ͑LSCF͒-based oxygen separation membranes. Mixed ionic/electronic conducting perovskite oxides such as LSCF and various doped barium cerates are currently being considered for use in hightemperature electrochemical devices such as oxygen and hydrogen concentrators and solid oxide fuel cells. However, to take full advantage of the unique properties of these materials, reliable joining techniques need to be developed. Furthermore, if the proposed joining technique were to yield a hermetic ceramic-to-metal junction that was also electrically conductive, it would additionally benefit the device by allowing current to be drawn from or carried to the electrochemically active mixed conducting oxide component without requiring a separate current collector. A newly developed brazing technique known as air brazing is one such method of joining. In its present form, air brazing uses a silver-copper oxide based filler metal that can be melted directly in air to form a compliant joint that is electrically conductive. Recently, it has been shown that the addition of titania can enhance the wetting behavior of Ag-CuO filler metals on alumina. Here the effect of this wetting agent on the surface wettability, long-term electrical resistance at 750°C, and reactivity with La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF-6428 or LSCF͒ substrates is discussed.Because of their unique mixed ionic/electronic conductive nature, a number of complex metal oxide perovskites including LSCF-6428 ͑La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ ͒ exhibit electrochemical properties that are useful in gas sensing and measurement devices. 1 While the current demand for oxide-based sensors has grown to nearly $3 billion dollars per year, the use of these materials as cathodes 2 and fuel reformation catalysis agents in solid oxide fuel cells ͑SOFCs͒ represents an even larger market. Additional applications being investigated include their use in electrically driven ion transport devices for gas separation, partial hydrocarbon oxidation, and waste reduction and recovery. 3 For example, mixed conducting oxide membranes offer the potential to separate oxygen from air with far greater efficiency and at one-third lower cost than the cryogenic processing technology used today. And unlike cryo-separation oxygen transport membranes operate at high temperature, making them ideally suited for direct integration with coal gasification plants. 4 Electrochemical devices designed to exploit the unique properties of these advanced functional ceramics require reliable joining techniques. However, the low tolerance of many perovskite compounds for reducing environments and the high operating temperatures of these devices are practical contraints that tend to shorten the list of candidate joining technologies that can be considered. For many mixed conducting oxides, joining must take place in air to...

No abstract