Reactive Air Brazing Mechanism between Stainless Steel and YSZ Ceramics for Solid Fuel Cell

Author:Si Xiao Qing

Supervisor:cao jian


Degree Year:2019





Solid oxide fue l cell(SOFC)is a clean and efficient high-temperature solid electrochemical energy conversion system with low cost,low pollution,high energy conversion rate,fuel diversity and low noise.In order to obtain a sufficient power output,multiple single cells need to be joined together to build a SOFC stack.In particular,achieving a high quality joining of the YSZ ceramic of cell to the stainless steel is the most key technology.The stack needs a long-term operation in the oxidation and reduction atmospheres under the high temperature(800°C).The joint will face great thermochemical and thermomechanical performance challenges.Stable joint microstructure,good air tightness and high joint strength are important indicators to meet the service requirments of the SOFC stack.At present,Ag-based brazes are the best choice for the long-term service of the SOFC stack,especially for the mobile power application.However,the most widely used Ag-CuO braze still has problems such as the excessive reaction with the stainless steel,the large thermal mismatch with stack assembles,and the high joining temperature.In this study,a protective layer was prefabricated on the stainless steel.The formation and protection mechanisam of the protective layer was investigated in detail.The braze composition was optimized,and the strengthening mechanism was analyzed.A novel method for joining SOFC components at low temperature using Ag nanoparticles was also reported.The Al2O3 protective layer was successfully fabricated on the surface of ferritic stainless steel(Crofer22)by reactive air aluminizing.The formation and protection mechanis m of the Al2O3 protective layer were analyzed by optimizing the process.And the melting-solidification-diffusion process of aluminium powders was clarified.The reactive air aluminization process mainly included the following three steps:the formation and solidification of Al liquid phase(660-700°C),the solid interdiffusion between the Al-rich layer and the stainless steel matrix(700-1000°C),and the high temperature epitaxial growth of the Al2O3 protective layer(1000-1100°C).The protective effect of aluminized Crofer22 was studied by high temperature(800°C)oxidation test.It was shown that the best protective effect was obtained by aluminizing Crofer22 at 1100°C.After 2000 h high temperature oxidation,the mass only increased by 0.03 mg/cm2.And the dense Al2O3 protective layer(2μm)was firmly bonded to the stainless steel substrate.A series of Ag-CuO brazes were developed to reactive air braze(RAB)aluminized Crofer22 and SOFC cells(joining position:YSZ ceramic).The protective effect of the Al2O3 protective layer during the joining and service processes was studied.The interfacial joining mechanism between the braze and the base matrix was analyzed.The Al2O3 protective layer effectively protected the Crofer22 matrix during joining and subsequent high temperature(800°C)oxidation(air)and reduction(4%H2-50%H2O-N2)tests.The stability of the joint microstructure was ensured.Two kinds of mechanical interlocks was formed on both sides of the joint,which improved its structure stability.The transmission electron microscopy microscopy(TEM)analys is showed that the Ag element could diffuse into the oxide matrix after brazing.The diffusion depth in YSZ and Al2O3 matrix were 10 nm and 20 nm,respective ly.Both Ag and CuO achieved atomic bonding with YSZ and Al2O3.The Ag-CuO-Al2O3 composite braze was developed by adding Al2O3nanoparticles(10 nm),which was used for RAB joining the aluminized Crofer22and the YSZ ceramic.The RAB joining characteristics of the nano-scale reinforced composite braze were investigated.The interfaicial microstructure evolution and the sintering behavior of the reinforcements were studied by adjusting the content of Al2O3 nanoparticles and optimizing the joining process.The sintering growth of Al2O3 nanoparticles during RAB joining was unavoidable.Further TEM observations confirmed that a micro-nano-Al2O3 composite strengthening effect wasd formed in the as-brazed joints.When the Al2O3 content was 8 wt.%,the joint possessed a maximum interfacial fracture energy of 768 J/m2 at 1050°C for 30 min,which could be attributed to the best micro-nano composite strengthening effect.The good high temperature stability of Al2O3 nanoparticle was determined by aging in air and 4%H2-50%H2O-N2 atmospheres at 800°C for 300 h.The joint maintained a very low gas leakage rate of 2.1×10-3-2.7×10-3 sccm/cm after brazing and aging tests.The interfacial fracture energy also did not change significantly.In order to minimize the residual stress in the joint,the Ag-CuO-LiAlSiO4composite braze was developed by adding a negative expansion coefficientβ-eucryptite(LiAlSiO4:-6.2×10-6/K).A good thermal expansion coeffic ient(CTE)match between the aluminized Crofer22 and the YSZ electrolyte of cell was achieved.By studying the joining process,it was determined that a large assembly pressure of 16 N/cm2 was good for achieving a defect-free joint at 970°C.At the same time,two kinds of mechanical interlocking structures were formed at both sides of the joint interface.When the content of LiAlSiO4 was 6 wt.%,an interfacial fracture energy of up to 930 J/m2 was obtained.The high temperature stability of the composite braze was evaluated by aging in oxidation(air)and reduction(4%H2-50%H2O-N2)atmospheres at 800°C for 300 h.The joint had good microstructure stability,and maintained a very low gas leakage rate of1.2×10-3-1.4×10-3 sccm/cm after brazing and aging tests.The interfacial fracture energy also did not change significantly.The faying surfaces of base matrixs were nanostructured,a 3D Ni/Au nanosheet array surface was obtained.A novel method using a combination of Ag nanoparticle paste and a silver-foam interlayer was developed for joining the Crofer22 and the SOFC cell at 300°C.The interfacial microstructure evolution was systematically studied.It was shown that the 3D nanosheet arry film improved the sintering efficiency both between Ag nanoparticles and substrates,and the Ag-foam interlyer increased the density of the sintered joint.The joint strengthening mechanis m was clarified.A mechanical interlocking was fromed between the 3D nanosheet array and the sintered Ag matrix.At the same time,another mechanical interlocking was formed between the Ag-foam interlayer and the sintered Ag matrix.The composite strengthening effect increased the interfacial fracture energy from124 J/m2 to 352 J/m2.Joints displayed excellent stability in high temperature aging tests.After high temeprature oxidation and reduction at 800°C for 400 h,the joint microstructure and strength still maintained good stability.