Electric Field-assisted Fast Joining Of 3YSZ To Stainless Steel And Rare-earth Zirconate High-entropy Ceramic And Mechanisms

Yttria stabilized zirconia（YSZ）possesses promising characteristics including high temperature resistance,wear resistance,high strength,and high-temperature ion conductivity.The joining between YSZ and other materials can achieve performance complementarity,thereby expanding their application ranges.The design of joining method should consider both the physical and chemical properties of the materials used and develop a feasible method to ensure the reliability of joints and to keep energy consumption at a relatively low level.However,due to the low diffusion coefficient of ceramics,the joining techniques commonly used generally require high temperature and long processing time,resulting in potential thermal damage of the materials and significant energy consumption.Thus,for the technological advancement,it is in great demand to develop a clean energy-driven high-efficiency ceramic joining technique.Flash sintering is a ceramic sintering technique performed under external electric field,which enhances densification by triggering Joule heating and defect avalanche.The comprehensive effect leads to mass transfer and densification of ceramic bodies within few seconds.In addition,this process is accompanied by charge carrier transfer,which can significantly promote wetting of liquid metal on YSZ.Inspired by this,this thesis is dedicated to introduce flash sintering technique into the field of ceramic joining,with a focus on addressing challenges related to high joining temperature and long processing time of conventional methods and developing novel high-efficient sintering-joining techniques to meet diverse application requirements.Cases including 3YSZ/stainless steel and3YSZ/5RE₂Zr₂O₇（RE=La,Nd,Sm,Gd,Yb）high-entropy ceramic were selected based on applications in solid oxide cells and thermal barrier coating,and their feasibility of flash joining and flash sintering-joining was investigated.Further,correlations between parameters,structure and mechanical properties were established and the joining mechanisms were revealed.The findings are as follows:（1）Flash joining of 3YSZ and stainless steel was achieved using Ag-Cu O filler（the current flows from 3YSZ to steel）.The required shortest joining time is 1 s and the lowest temperature is 875 ^oC.The joint strength was optimized to 158±9 MPa.The effect of current density,joining time and furnace temperature were systematically investigated.It was revealed that rapid mass transfer at the 3YSZ/Ag-Cu O interface and the formation of（Ag,Cu）Zr₂intermetallic（IMC）at the Ag-Cu O/stainless steel interface were the key factors for the rapid and robust joining.Finite element analysis was used to determine the effect of Joule heating,and it was revealed that the formation of liquid phase played a key role for the rapid diffusion of Zr,the formation of（Ag,Cu）Zr₂ IMC and thereby fast joining.（2）Based on a home-made wetting test device that allows the current to flow through the solid/liquid/solid phases,we investigated the wetting behavior of Ag-Cu O situated between3YSZ and Mn_1.5Co_1.5O₄（MCO）under the application of electric field.The current flowing from 3YSZ to MCO（positive direction）improve the wetting of 3YSZ/Ag-Cu O interface,and the reversed current（negative direction）improve wetting of both interfaces.The formation of sub-stoichiometric Zr O_2-δand dissolution of 3YSZ are main factors promoting wetting for the positive current.The formation,diffusion and adhesion of Ag–O cluster on both surfaces are the mechanisms promoting wetting under negative current.Based on these effects,we achieved fast joining of 3YSZ to MCO-coated stainless steel within 30 s,and the shear strength was optimized to 62±11 MPa.（3）An‘alternate current（AC）before direct current（DC）’flash sintering-joining technique was developed.By using this technique,rapid sintering of 3YSZ green bodies and its in-situ joining to stainless steel were achieved at 950°C within only 60 s by using Cu as an interlayer,yielding a maximum shear strength of 34±2 MPa.It was revealed that the distribution of sub-stoichiometric Zr O_2-δwithin the 3YSZ significantly influence densification and joining process by altering local conductivity,which influences the distribution of Joule heating.Furthermore,the AC/DC was demonstrated as the most feasible route for achieving sintering-joining by comparing with the experiments using single AC or DC.The mechanism is as follow:first,applying AC field to avoid accumulation of Zr O_2-δ,which results in homogeneous Joule heating and densification,and meanwhile,taking advantage of Joule heating to melt Cu interlayer;then,applying DC to produce Zr O_2-δat the 3YSZ/Cu interface,which promotes wetting and interfacial joining by forming Cu-Zr IMC.（4）Direct joining of 3YSZ to 5RE₂Zr₂O₇ high-entropy ceramic was achieved by performing flash joining under a uniaxial pressure.The required joining temperature and time were 1200°C and 30 s,respectively,and the joint strength reached 26±3 MPa.Correlations between joining parameters（including current density,holding time and uniaxial pressure）,microstructure and joint strength and were established.We demonstrated that the contact resistance significantly enhances local Joule heating,resulting in phase segregation,grain coarsening and intergranular pore formation in the 5RE₂Zr₂O₇near the seam（<100μm）.Increasing uniaxial pressure has three positive effects:1.promoting particle migration,rearrangement and coalescence of grain boundaries;2.reducing contact resistance-induced local over-heating and grain coarsening;3.accelerating plastic flow,which provides larger contact area for diffusion.Furthermore,a flash sintering-joining technique that enables rapid joining of ceramics by triggering flash sintering of a green body interlayer was developed.By using this technique,a multilayer joint system 5RE₂Zr₂O₇/5RE_2-xZr_2+xO_7+x/2/3YSZ was successfully fabricated within only 30 s.In summary,this thesis is dedicated to the development of electric field-assisted joining technique for the ceramic materials.The correlations between‘parameter-structure-property’in the 3YSZ-involved liquid and solid joining systems were clarified.The joining mechanisms were revealed based on the current-induced Joule heating and ionic charge transfer effects,and a novel flash sintering-joining technique,which is of great efficiency and flexibility,was developed.This work is anticipated to offer valuable insights for the advancement of joining techniques for electrolyte ceramic materials.