Abstract
Reactive air brazing has been widely used in fabricating solid oxide fuel/electrolysis cell (SOFC/SOEC) stacks. However, the conventional Ag–CuO braze can lead to (I) over oxidation at the steel interconnect interface caused by its adverse reactions with the CuO and (II) many voids caused by the hydrogen-induced decomposition of CuO. The present work demonstrates that the Ag particle interlayer can be used to join yttria-stabilized zirconia (YSZ) electrolytes to AISI 441 interconnect in air instead of Ag–CuO braze. Reliable joining between YSZ and AISI 441 can be realized at 920 °C. A dense and thin oxide layer (~2 μm) is formed at the AISI 441 interface. Additionally, an interatomic joining at the YSZ/Ag interface was observed by TEM. Obtained joints displayed a shear strength of ~86.1 MPa, 161% higher than that of the joints brazed by Ag–CuO braze (~33 MPa). After aging in reducing and oxidizing atmospheres (800 °C/300 h), joints remained tight and dense, indicating a better aging performance. This technique eliminates the CuO-induced issues, which may extend lifetimes for SOFC/SOEC stacks and other ceramic/metal joining applications.
Highlights
As electrochemical energy conversion devices, solid oxide fuel/electrolysis cells (SOFCs/SOECs) are increasingly widely applied in practice due to high efficiency, fuel diversity, and ultra-low pollution [1,2,3,4,5]
It is apparent that a tight joint was formed under this condition, and the Ag particle interlayer was completely densified without any voids after sintering
The yttria-stabilized zirconia (YSZ) electrolyte was successfully joined to the AISI 441 interconnect using the Ag particle interlayer in air under an assembly pressure of ~2 MPa
Summary
As electrochemical energy conversion devices, solid oxide fuel/electrolysis cells (SOFCs/SOECs) are increasingly widely applied in practice due to high efficiency, fuel diversity, and ultra-low pollution [1,2,3,4,5]. The joining between solid oxide cells and ferritic stainless steel should be stable during the subsequent long-term high-temperature service [20,21]. For the glass/glass–ceramic materials, their composition can be tailored to match the thermomechanical requirements of the SOFC/SOEC stack components [25,26,27] Problems such as the crystallization during sintering/operation and inherent brittleness of the glass-based sealants may be detrimental to the long-term stability of joints [28,29]. The rapid growth of this oxide layer has been proven to be a key factor for the RAB joint failure during aging [22] Another issue for the Ag–CuO braze is that CuO is thermodynamically unstable when exposed to the reducing atmosphere in fuel (i.e., anode) chambers [7,36]. As for the aging test in the oxidizing atmosphere, the compressed air flowed into the above horizontal tube furnace with a constant gas flow of 6 L/h
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