Reactive air brazing for sealing mixed ionic electronic conducting hollow fibre membranes

Mixed ionic electronic conducting (MIEC) ceramic membranes and high-temperature alloys are candidate materials for applications in high-temperature gas separation systems and solid oxide fuel cells (SOFCs). Ensuring a gas-tight seal between the components is of paramount importance in the operation of such devices. This paper investigates the wettability and joining of representative ceramic-to-ceramic and ceramic-to-metal components by reactive air brazing (RAB) using Ag-Cu alloys. The correlation of the interfacial reaction (including wettability) to the hermeticity of the joints has been demonstrated by elemental mapping using Electron Probe Micro-Analysis with wavelength dispersive spectrometry (EPMA-WDS). The wettability studies described herein demonstrate that RAB is a reliable method to achieve strong, gas-tight bonding between the dissimilar materials. These are the first reported results of successful air-brazed joints between La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) and BaCo0.4Fe0.4Zr0.2O3-delta (BCFZ) hollow fibre membranes to FeCrAlloy components using a 4 mol.% Cu in Ag filler metal composition which delivered an impressive runtime of up to 2000 h (for LSCF). It has been demonstrated that these RAB joints are hermetic and resistant to thermal ageing, making them suitable for membrane-based gas-separation applications. Post-operation EPMA-WDS analysis of the microstructures and compositional distribution of the brazed seals has revealed that their performance is largely dependent upon a reaction zone and an interfacial oxide layer adherent to the FeCrAlloy surface. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.