Aging Behavior of Reactive Air Brazed Seals for SOFC

Abstract

In this study the gas-tightness of reactive air brazed YSZ-steel joints and their long-term stability at high temperatures in air and dual atmospheres are investigated. Silver-based braze compositions like Ag-4CuO are used for reactive air brazing of gas-tightness samples. During aging in air at 850°C for 800 h all tested samples remained hermetic. However, microstructural analysis reveals the formation of voids within the brazing zones and severe changes of the interfacial oxide layers. It is shown, that due to the diffusion of chromium and manganese from the steel substrate the thickness of the reaction layers increases and the phase composition changes. Furthermore, in previous work a decrease of the bending strength of reactive air brazed YSZ-steel joints due to the microstructural changes is observed. In contrast, the aging in dual atmospheres at 850°C causes strong degradation and the total loss of gas-tightness. Reducing atmospheric conditions lead to different reactions profiles of the silver based brazes in contact with the YSZ electrolyte. Complex reaction products are identified and reported.

Reactive air brazing of 3YSZ ceramic to aluminized Crofer22H stainless steel using Ag–CuO fillers

“…However, the overreaction between CuO and Crofer22H occurred to form a brittle composite layer of Cu/Cr/Mn/Fe oxides at the interface 27,28 . Furthermore, volatile phases (e.g., CrO 3 and Cr(OH) 2 O 2 ) would be formed in a high‐temperature oxidizing atmosphere.…”

“…However, the overreaction between CuO and Crofer22H occurred to form a brittle composite layer of Cu/Cr/Mn/Fe oxides at the interface. 27,28 Furthermore, volatile phases (e.g., CrO 3 and Cr(OH) 2 O 2 ) would be formed in a hightemperature oxidizing atmosphere. Subsequently, these phases are reduced in the cathode and deposited on its surface, eventually leading to a sharp deterioration in the properties of cells, known as the Cr poisoning of SOFC.…”

Microstructure and mechanical properties of Al2O3 ceramic joints achieved by Ag‐SiO2 braze in air

“…Meanwhile, several challenges of the RAB method when using Ag‐CuO braze remain to be solved 27 . For instance, CuO could be reduced into Cu by H 2 when joints are exposed to a reducing atmosphere in gas sensors or other electrochemical devices, and micropores could emerge and develop into air pockets in the joints, severely deteriorating gas tightness and mechanical properties 28 . It has been confirmed that large‐scale pores are observed when Al 2 O 3 joints obtained by Ag‐8 mol% CuO are exposed under dual (oxidizing and reducing) atmospheres at 800°C for 100 h 29 …”

Joining 3YSZ Electrolyte to AISI 441 Interconnect Using the Ag Particle Interlayer: Enhanced Mechanical and Aging Properties

“…The decomposition of CuO through the reaction CuO + H 2 →Cu + H 2 O and the subsequent solid solution of Cu into the Ag matrix can lead to many voids [3,39]. Besides, these voids can offer rapid paths for hydrogen permeation into the joint central region (where the hydrogen reacts with diffused oxygen to generate gaseous water pockets), consequently weakening the gas-tightness and mechanical properties of joints [36,[40][41][42].…”