Jin Yong Y. Kim

Reactive Air Brazing: A Novel Method of Sealing SOFCs and Other Solid-State Electrochemical Devices

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 ~pSOFC! components.

Effects of chrome contamination on the performance of La0.6Sr0.4Co0.2Fe0.8O3 cathode used in solid oxide fuel cells

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 (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-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.

Joining Mixed Conducting Oxides Using an Air-Fired Electrically Conductive Braze

Pacific Northwest National Laboratory, Richland, Washington 99352, USAThe lanthanum strontium cobalt ferrites ~LSCF! are a well-known family of mixed oxygen-ionic/electronic conductors that are ofinterest for use in high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen separation membranes. Oneof the challenges however in developing these types of devices is joining the constituent metallic and ceramic components,particularly with a technique that is compatible with the electrochemically active membrane materials. A new joining process hasbeen developed which employs a silver-copper oxide braze material. In the present study, we found that braze compositionscontaining between 1.4 and 16 mol % CuO offer the best combination of wettability, joint strength, and electrical conductivity. Ourresults indicate that the wettability of the Ag-CuO brazes on LSCF substrates increases with CuO content and that electricalconductivity through the joint remains high as long as the silver content is greater than 66 mol %. Results from long-term,high-temperature resistance experiments confirmed the latter finding, as minimal degradation in conductivity was observed in thevarious test joints over 500 h of testing at 750°C. Data obtained from three-point bend testing showed that joints formed with abraze composition of 1.4 and 16 mol % CuO displayed maximal average flexural strength.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1764782# All rights reserved.Manuscript submitted September 26, 2003; revised manuscript received January 22, 2004. Available electronically June 17, 2004.

Silver-copper oxide based reactive air braze for joining yttria-stabilized zirconia

We are investigating a new method of ceramic-to-metal joining, referred to as reactive air brazing (RAB), as a potential method of sealing ceramic components in high-temperature electrochemical devices. Sessile drop wetting experiments and joint strength testing were conducted using yttria stabilized zirconia (YSZ) substrates and CuO-Ag based air brazes. Results from our studies indicate that the wettability of the braze improves substantially with increasing CuO content, over a compositional range of 1 - 8 mol% CuO, which is accompanied by an increase in the bend strength of the corresponding brazed YSZ joint. The addition of a small amount of TiO2 (0.5 mol%) to the CuO-Ag braze further improves wettability due to the formation of a titanium zirconate reaction product along the braze/substrate interface. However, with one notable exception, the bend strength of these ternary braze joints remained nearly identical to those measured in comparable binary braze joints. SEM analysis conducted on the corresponding fracture surfaces indicated that in the binary braze joints the failure occurs primarily at the braze/YSZ interface. Similarly in the case of the the ternary, TiO2-doped brazes joint failure occurs predominantly along the interface between the braze filler metal and the underlying titanium zirconate reaction layer.

Use of aluminum in air-brazing aluminum oxide

A commercial aluminum foil was used to braze alumina plates in air. Although the outer surface of the aluminum oxidizes in air, the majority of the aluminum underneath remains unoxidized during brazing, allowing the ceramic pieces to be joined together with adequate strength. In fact, the joint exhibits a modest increase in bend strength when exposed to air at 850oC for a prolonged period of time. Joint strength testing and subsequent examination of the fracture surfaces of the joints indicate that the joints are inherently ductile, even after long-term, high-temperature air exposure.

The Effect of Composition on the Wetting Behavior and Joint Strength of the Ag-CuO Reactive Air Braze

One of the challenges in manufacturing solid-state electrochemical devices is in joining the ceramic and metallic components such that the resulting joint is rugged, hermetic, and stable under continuous high temperature operation in an oxidizing atmosphere. A well proven method of joining dissimilar materials is by brazing. Unfortunately many of the commercially available ceramic-to-metal braze alloys exhibit oxidation behavior which is unacceptable for potential use in a high temperature electrochemical device. An alternative braze alloy composition designed for oxidation resistance has been developed to join ferritic stainless steel to a variety of electrochemically active ceramic membranes including YSZ, nickel oxide, and mixed conducting perovskite oxides. The results of this study to date will be discussed.