Causes And Control Of The Pore On The Sealing Interface Of Ag-Cu Alloy Brazing Oxygen Permeable Membranes And Metal Supports

Mixed ionic/electronic conducting oxygen transport membranes (MIEC-OTM)have been used for oxygen separation, partial oxidation of methane to syngas,selective oxidation of hydrocarbon, etc. For an industrial application, the gas-tightjoining of the membranes to their metal components is an engineering challenge in theindustrial application. In recent years, due to good ductility and oxidation resistance,Ag-Cu alloys have been studied to braze the oxygen-permeable membranes and theirmetal supports in air. However, many big pores formed at the joints interfaces duringbrazing the membrane modules and their metal supports in air. And the sealingperformance deteriorates seriously. In our study, the sources of pore formation havebeen investigated thoroughly and some regulating methods were proposed andconducted. The results are summarized as follows:For brazing oxygen-permeable membranes to their metal supports by usingAg-Cu alloys in air, many big pores will form at the joints interfaces during brazingthe membrane modules to their metal supports. Because oxygen solubility in liquidsilver is much higher than that in solid one, the dissolved oxygen will precipitate inthe cooling process and form a lot of pores at the joining interface. When brazing isconducted in argon by using the pre-oxidized Ag-Cu alloys or Ag-Cu alloys, theprocess of the pores formation can be described as follows: during thehigh-temperature brazing, the copper oxide at high-temperature decomposes andoxygen is released. The released oxygen dissolves in the molten Ag-based alloy andprecipitates in the cooling process and forms a lot of pores at the joining interface.The oxygen of Ag-Cu alloys in argon atmosphere comes from the oxygen-permeablemembranes.Two regulating methods are proposed to reduce the pores:(1) the oxygenbubbles are thoroughly discharged out of the molten brazing alloy before its completesolidification. Theoretically, this can be achieved by the directional solidification technology. But the experimental results show that it is very difficult to conduct theregulating method under the brazing condition.(2) By adjusting the composition ofAg based alloys to react with the released oxygen to form the oxides instead of thebubbles during the solidification. The Ag-Cu-Ni alloys and Ag-Cu-Mn–based alloyshave been tried to braze the oxygen-permeable membranes and their metal supports inargon. The results show that few pores are found at the sealing interface. Thisindicates that by adjusting the alloy composition can effectively control the poresformation. But the reaction products (nickel oxide and manganese oxide) with highmelting temperature could deteriorate the spreading and wettability of the brazingalloys on the substrates. More optimizations need to be done on the alloycompositions.