Development of New Brazing Fillers and Process Variants for Reactive Air Brazing (RAB) of Electrochemical Devices

Solid Oxide Fuel Cells and air-separation membranes are promising components for modern energy generation. For both applications the connection between steel and ceramic parts is a fundamental element. Only with a gastight joint the high efficient electrochemical processes can take place. Due to the operating conditions this joint has to be long-term mechanically and chemically stable at high temperatures and during temperature cycling. It has been shown that these demands can be met very well with brazed seams produced by reactive air brazing (RAB). The basis material for the brazing filler is silver, which provides an adequate ductility. The junction of the brazing filler to the ceramic part of the device is realized by addition of small amounts of metal oxide, with the same type of chemical bonding as ceramics, to the silver. The brazing process takes place in a muffle furnace in air-atmosphere. Experimental studies have shown that the currently most used brazing alloy Ag8Cu causes metallurgical interactions at both interfaces. A brittle reaction layer between brazing filler and steel as well as the infiltration of brazing filler into the ceramic lead to failure of the joint. To overcome these problems, brazing alloys with alternative metal oxides are being developed. Silver with addition of one or several metal oxides is tested as brazing filler. Beside this approach, a 2-step-variant of the RAB-process is developed and variations of the brazing temperature are examined. All experiments are accompanied by metallographical characterization and DSC-measurements, thus metallurgical reactions as a function of the temperature can be analyzed in detail. Depending on the material of the join partners, different alloying elements shaped up as alternatives to copper.  For membranes (Ba_0,5Sr_0,5Co_0,8Fe_0,2O_3-δ/X15CrNiSi25-20) aluminum, cobalt and vanadium are promising additives. For SOFCs (YSZ/Crofer22) aluminum, nickel oxide and titanium oxide delivered good results.