Research On The Process And Mechanism Of BSCF Ceramics’ Flash Sintering And Flash Joining

BSCF ceramics are mainly used for SOFC fuel cell cathode materials and high temperature gas separation,but the current sintering technology for BSCF ceramics is usually pressureless sintering and hot press sintering,which takes longer time and needs higher temperature;as for joining,methods such as solid phase diffusion bonding or brazing are used,which also requires harsh process conditions such as high temperature and long time,and there are also problems that may cause large residual stress in the joint.Therefore,this subject chooses to realize the rapid low-temperature sintering of BSCF ceramics and the rapid low-temperature joining of BSCF ceramics under the assisted electric field,that is,to achieve the flash sintering and flash joining of BSCF ceramics.BSCF ceramics were sintered at a low temperature and rapidly densified by the flash sintering under an electric field,and a relatively dense sintered body was obtained.Then,the temperature field of the sample during the flash sintering was simulated and analyzed using the ABAQUS 6.14.The temperature distribution in the constant current stage of flash sintering was explored.The temperature in the central part of the sample was higher,and the temperature near the surface was lower.There is a large temperature difference between the surface and the central part of the sample;The influence of different process parameters such as temperature,current density and sintering time on the temperature field of flash sintering was explored,it is found that the increase of the furnace temperature will increase the temperature of the sample as a whole;the extension of the sintering time will make the sample’s temperature rises,and the temperature field tends to be stable when the sintering time is longer;the increase of current density will increase the temperature of the entire sample,and the maximum temperature of the sample is proportional to the square of the current density.Then the BSCF ceramic is joined by flash joining,and the connection interface was intact,the base material remained intact,the interface was reliably connected,and there were no obvious welding defects.Then the stress field of the connected components during the flash joining was simulated and analyzed.In the stress field during the flash joining,it is found that the stress is distributed throughout the connected component,and the tensile stress is distributed near the surface of the connected component,the compressive stress is distributed in the central part of the connected component;and the effects of different process parameters such as furnace temperature,current density,and connection time on the stress field of the connection component are explored.It is found that the increase of the current density will increase the stress value of the whole connected component,which is proportional to the quadratic square of the current density;the extension of the connection time will increase the stress value of the entire connected component proportionally;and the furnace temperature does not have a significant impact on the stress field of the connected component.Finally,the mechanical properties of the BSCF ceramic flash joining joint and the RAB joint are compared by finite element simulation.First,the residual stress of the RAB joint formed by BSCF ceramic and Ag-based solder is simulated and calculated,and the thermal expansion coefficient of the solder layer is adjusted by adding a reinforcing phase to the solder to optimize the residual stress of the joint.Through simulation analysis,the distribution of residual stress formed during the cooling stage of RAB is obtained,and the effect of adding reinforcement phase on the residual stress distribution of the joint is analyzed.It is found that the residual stress of the joint is mainly concentrated near the brazing area,and the addition of the reinforcing phase can effectively reduce the residual stress value in the joint;the shear process of the brazed joint and flash joint are simulated,and the stress distribution of the joint during the shear process is analyzed,then we find that the joint stress is concentrated on the the edge of the lower surface near the connection area of the left ceramic block,that is where the weak area are;and the curve of the maximum Max Principal stress of the this area of the joint with the applied shear load is given.The shear strength of different joints is compared.It is known that the shear strength from small to large is as follows: non-reinforced phase RAB joint,2.5wt.% reinforced phase RAB joint,5wt.% reinforced phase RAB joint,joint obtained by flash joining.