Reactive Air Brazing Process And Mechanism Of 3YSZ Ceramics And Crofer22H Stainless Steel

Solid Oxide Fuel Cell(SOFC)is a clean and efficient electrochemical energy conversion equipment,with low cost,high energy conversion efficiency,fuel flexibility and a series of advantages,has received wide attention,the most promising is the flat-panel SOFC,in order to obtain sufficient power output needs to build multiple cell units in series battery stack,of which 3YSZ ceramic electrolyte and stainless steel support frame between the good seal is the key technology to ensure long-term stable service of SOFC.SOFC needs to be used in high temperature bipolar atmosphere(oxidative and reductive),so it puts forward high requirements for air tightness,mechanical properties,chemical stability and electrical insulation of sealing structures.At present,the reactive air brazing method represented by Ag-CuO brazing material can achieve a good connection between 3YSZ ceramic and stainless steel,but in the use of Ag-CuO brazing to connect ferrite stainless steel Crofer22H,there is an over-reaction between CuO and stainless steel substrate,the volatile Cr element in stainless steel leads to deterioration of battery performance and Ag-based brazing material at high temperatures will soften and other problems,in order to solve these problems,this paper through the stainless steel surface prefabricated protective layer,Exploring the best brazing process parameters and designing composite brazing methods improves the performance of the joints.The quenching morphology and interfacial microstructure of Ag-CuO solder on3YSZ surface at different holding time and temperature were studied.At 1050°C for more than 15 min,the wetting and spreading of Ag-xCuO(x=2,4 mol.%)filler on the surface of 3YSZ ceramics were stable,and the contact angles were about 65°and 45°.The existence form of CuO in liquid filler did not change with the extension of holding time.There was only Ag-rich L2 liquid phase in Ag-2 mol.%CuO filler,and there was both Ag-rich L2 liquid phase and CuO-rich L1 liquid phase in Ag-4 mol.%CuO filler.When the temperature was above 970°C for 30 min,the spreading of Ag-xCuO(x=2,4 mol.%)filler on the ceramic surface was also stable,and the contact angles were about 65°and 45°.The increase of temperature had no significant effect on the existence of CuO in Ag-2 mol.%CuO filler,which was a single L2 liquid phase.For Ag-4 mol.%CuO filler,the increase of temperature would lead to the decrease of the content of L1 liquid phase.The continuous and dense Al₂O₃ protective layer with a thickness of about 1.3?m was prepared on the surface of Crofer22H stainless steel by air reaction aluminizing.There is also a certain amount of diffusion Al element near the surface of stainless steel.3YSZ ceramics and Crofer22H stainless steel were brazed with Ag-CuO filler metal.The effects of CuO content,brazing temperature and holding time on the microstructure and mechanical properties of the interface were studied.The typical interface microstructure of the joint is 3YSZ/Ag(s,s)+CuO/Cu Al₂O₄/Al₂O₃/Crofer22H stainless steel.When using Ag-2mol.%CuO filler,the composition of the filler is only L2 liquid phase,the interface reaction degree is low,the bonding force between the filler and the base metal is poor,and the joint strength is close to 35MPa at different temperatures and holding time.When using Ag-4mol.%CuO filler at different holding time,the content of L1 liquid phase in the filler is basically unchanged,and the liquid phase is more inclined to react with the aluminized stainless steel to form a thicker interface reaction product.The joint strength is improved and stabilized at about45MPa.The increase of temperature will lead to the gradual decrease of the content of L1 liquid phase in Ag-4 mol.%CuO filler metal.When brazing is carried out below1000°C,the content of L1 liquid phase is too high,which will completely consume the Al₂O₃ thin layer on the surface of stainless steel.There is a composite oxide layer in the joint,which is accompanied by defects such as holes.Maximum joint strength of?45MPa can be obtained by brazing at 1050°C.Further increase of temperature will reduce the content of L1 liquid phase in the filler metal,reduce the degree of interfacial reaction,and reduce the joint strength.The composite filler was prepared by adding metal Pd powder on the basis of Ag-4mol.%CuO filler.The addition of Pd does not have a significant effect on the microstructure of the joint interface.All the added Pd elements are dissolved in the Ag matrix,which can improve the high temperature strength of Ag-CuO filler at 600°C,and make it increase from?12.2 MPa without Pd powder to?35 MPa with 12 mol.%Pd powder.