Computational design of metal oxides to enhance the wetting and adhesion of silver-based brazes on yttria-stabilized-zirconia

Abstract

Ag–CuO is a broadly used reactive air brazing (RAB) system for effectively bonding ceramics and metal interfaces, especially for sealing yttria-stabilized zirconia (YSZ) to metals in solid-oxide fuel cells (SOFCs). To understand the superior performance of this braze, density functional theory (DFT) calculations were employed to investigate two mechanisms that can potentially increase the work of adhesion (Wadh) and hence reduce the wetting angle of Ag on YSZ. It was found while the formation of Ag–dissolved O clusters at the Ag–YSZ interface can promote wetting, a much greater wetting angle reduction comes from the formation of CuO interlayers between Ag and YSZ. Further, the Wadh of an Ag/CuO and CuO/YSZ interface was found to be significantly higher than that of an Ag/YSZ interface. Based on simulation-obtained insights into metal to oxide bond formation, a simple descriptor was developed to predict the Ag/oxide interface energies, the Ag/oxide Wadh, and to search for potential oxide interlayers capable of promoting the wetting and adhesion of Ag on YSZ. Many simple metal oxides (single cation) were examined, however their Wadh with Ag were less than that of an Ag/CuO interface. Expanding the search to multi-cation oxides led to several promising candidates, such as CuAlO2, CuGaO2, and Cu3TiO4; all of which are also stable in the reducing SOFC conditions. Depending upon their solubility in molten Ag, these newly-identified oxides could either be pre-applied as wetting promoting interlayers or directly incorporated into Ag to form new reactive air brazes.