Joining of porous alloys by glass ceramics for energy applications

Solid oxide cells (SOCs) devices are of great interest for effective power generation and for highly efficient conversion of electricity to hydrogen, fuels and chemicals using high temperature electrolysis. Durable SOCs can be achieved only by a reliable integration and joining technology. In this context, glass-based materials, with multiple functional compositions and properties play a key role in the final performance and durability. In SOC stack designs the metallic interconnect must be sealed both to the interconnect frame, thus presenting significant challenges, even considering the high operating temperatures (800-1000°C). Strong efforts are being directed towards the development of lower temperature SOC, operating between 600 and 700°C. More recently, porous metallic alloys became of greater interest as engineering materials for use at medium-high temperatures. Due to open porosity, the porous alloys allow for gas transport, which is an important engineering feature. Porous ferritic stainless steels have been proposed as support structures for fuel cells and gas separation membranes. In this context, metal supported solid oxide cells, (MSOC), utilize a porous metal alloy to provide higher toughness, lower operating temperature and faster start-ups. The greater flexibility, resistance and potentially lower costs are particularly interesting for mobile systems or emergency power unit on board of planes for example. One of the major challenges in the design of solid oxide cell stacks is the selection of materials to join and seal the metallic interconnects. The sealant’s role is fundamental to assure high stack durability and long-term performance. Glass and glass ceramic sealants play a key role in SOC performance and durability as well as in the sealing and joining of metallic interconnects. Joining of porous substrates to bulk alloys present significant challenges. This works aims to expand the knowledge on joining by focusing on bonding of a commercial Höganäs AB porous alloy to an interconnect alloy (Crofer 22 APU) by two different glass-ceramics, for a potential use in MSOC supports or gas separation membranes. Two types of systems were used in this study, labelled as V10 (a Na-containing glass) and HJ11 (a Ba-containing glass) respectively. The microstructure observation and HSM studies indicated that pre-oxidation for the bulk alloy slightly improved the wetting of the substrate by the glass. Reliable bonding with both substrates was achieved for a very specific set-up: pre-oxidized Crofer on top and porous alloy on the bottom and under 15g/cm2 load. Better results in terms of glass-ceramic densification were obtained with the Ba-containing glass (HJ11) and this system was chosen with the previously mentioned joint configuration to assess the tensile strength. The computer tomography (CT scan) of the reference joined sample was also carried out. Tensile tests (a modification of ASTM C633-01 test) were performed on joined samples; the average strength value was found to be 7,91±1,06 MPa. Barium chromate was also identified on the fracture surface and scattered at the interface of the glass and the foam alloy. This MSc thesis has been one of the first attempts to thoroughly examine the joining between a porous alloy and a bulk metallic interconnect, highlighting challenges in terms of compatibility, corrosion resistance and tensile strength.