Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Air Braze Filler Metals

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...

Joining of engineering ceramics

“…In a subsequent study by the same research group, the addition of TiO 2 was found to be beneficial in reducing the contact angle on LSCF-6428, just as with alumina as a substrate. 243 However, the addition of TiO 2 enables SrTiO 3 to be formed as an interfacial reaction product, the extent of which needs to be carefully controlled to produce mechanically robust joints with suitable levels of electrical resistivity. Another potential application for RAB is in the joining and sealing of planar SOFCs.…”

Diffusion Barriers Minimizing the Strength Degradation of Reactive Air Brazed Ba0.5Sr0.5Co0.8Fe0.2O3-δ Membranes during Aging

“…Reactive brazing in air has been shown to be suitable for joining BSCF to high-temperature-resistant metals [ 25 , 26 , 27 , 28 ]. In addition, other OTM materials such as LSCF [ 29 , 30 ], BCFN [ 31 ], or dual-phase membranes [ 32 , 33 , 34 ] were joined using reactive air brazing. Suitable metallic joining partners show a similar thermal expansion to the ceramic OTM.…”

Dual Atmosphere Isothermal Aging and Rapid Thermal Cycling of Ag-Ni and Ag-CuO Stainless Steel to Zirconia Braze Joints

“…Historically, reactive elements that oxidize during brazing in air have been added to promote silver wetting on various ceramic substrates including zirconia, 11,12 lanthanum strontium cobalt ferrite (LSCF), 13,14 and gadolinium-doped ceria (GDC). 15 Critically, the oxides in these Reactive Air Brazes (RABs) (most commonly CuO, 8 V 2 O 5, 16 etc.…”