The Effect of Composition on the Wetting Behavior and Joint Strength of the Ag-CuO Reactive Air Braze
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The Effect of Composition on the Wetting Behavior and Joint Strength of the Ag-CuO Reactive Air Braze K. Scott Weil, Chris A. Coyle, Jin Yong Kim, and John S. Hardy Department of Materials Science, Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. ABSTRACT As interest in high temperature electrochemical membrane devices for energy and gas generation has intensified, it has become apparent that developing an appropriate method of hermetically joining the ceramic and metallic components in these devices will be critical to their success. A recently developed technique referred to as reactive air brazing (RAB) has shown promise in the joining of components for planar solid oxide fuel cells (pSOFC) and oxygen generators. In the study described below, the relationship between braze composition, substrate wetting, and joint strenth was investigated to gain further understanding of the RAB process. It was found that braze wettability and joint strength are inversely related for the simple binary Ag-CuO braze system.
INTRODUCTION A solid state electrochemical device such as a pSOFC functions because of an oxygen ion gradient that develops across the yttria stabilized zirconia (YSZ) electrolyte membrane under ionic transport. To maintain this gradient, and thereby maximize the performance of the device, the electrolyte and the joint that seals this membrane to the device chassis must be hermetic. That is, the YSZ layer must be dense, must not contain interconnected porosity, and must be connected to the rest of the device structure with a high temperature, gas-tight seal. Recent efforts on the tape casting of thin, anode-supported ceramic bilayers have successfully addressed the first two issues [1], the remaining challenge is in joining the ~10µm thick electrochemically active YSZ electrolyte to the metallic structural component such that the resulting seal is hermetic, rugged, and stable under both thermal cycling and continuous high temperature operation. The RAB technique was recently developed as method of joining complex oxides to heat resistant metals for use in fabricating high temperature electrochemical devices such as pSOFCs and oxygen generators [2,3]. RAB differs from traditional ceramic-to-metal joining techniques such as active metal brazing and the Mo-Mn process in two important ways: 1) RAB utilizes a liquid-phase oxide-metal melt as the basis for joining and 2) the process is conducted in an air muffle furnace. The latter difference appears to be critical in electrochemical device fabrication. Exposure of the device to a reducing atmosphere at a temperature greater than ~800°C, typical processing conditions in active metal brazing, is too demanding for many of the complex oxide materials used in these devices. For example, many of the mixed ionic/electronic conducting perovskites that are employed as electrodes will reduce under these heat treatment conditions and may undergo irreversible deterioration via phase separation, causing severe degradation in device performance. An addit
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