Research On The Composite Electrodeposition Preparing Of ?Co,Mn?₃O₄ Spinel Coating And Its Oxidation Property And Electric Conductivity

Interconnect is one of the key components of solid oxide fuel cell?SOFC?.The performance and cost of the interconnect has a direct influence on the service performance,life and economic benefits of the SOFC.With the progress that the operation temperature of SOFC reduced to medium temperature,metal has become possible to be selected as interconnect materials instead of ceramics.Because ferritic stainless steel?FSS?has the characteristics such as better matching thermal expansion coefficient?CTE?with other components of the batteries,lower cost and excellent machining performance,it has become the most potential SOFC interconnecting material However,in the work environment of SOFC,Fe-Cr alloy exhibited insufficient antioxidant capacity,and adjacent cathode poisoning resulted by the outside volatilization of Cr,which will cause the serious reduction of the electrical performance of SOFC,and the commercial development,popularization and application of SOFC.Aimed at the bottlenecks of FSS interconnect,a kind of composite electrodeposition process with short process,low cost,convenient for control and easily to be applied in actual industrial production has been used to prepare a Co-Mn₃O₄guiding layer on the surface of FSS substrate E-brite.?Co,Mn?₃O₄spinel coating was obtained through thermal conversion process.SEM,EDS,XRD phase analysis,high temperature antioxidant property and ASR testing method have been adopted to character the Co-Mn compound spinel protective coatings.Research results show that the coating has a good binding ability with the substrate,good antioxidant capacity in high temperature environment,excellent Cr blocking ability and high temperature electric conductivity.Through the research,a kind of effective method for FSS interconnect modification and SOFC interconnect material development.Main research contents and results of the dissertation include:?1?A new kind of pre-treatment method combining anode electrolytic corrosion and strike plating process has been proposed.The oxidation film on the surface of the E-brite can be removed completely,and the formed Co deposition nucleation can be used as the basis of the growing membrane during the subsequent electro co-deposition process of Co-Mn₃O₄.The binding strength between the deposition coating and the substrate has been enhanced.?2?The compound deposition models of Co-Mn₃O₄has been analyzed,and the dynamic model of the composite electro-deposition process has been set up.The key parameters affect the content of Mn₃O₄particles have been found out.Under optimal process conditions?an electrolyte p H5.0,cathodic current density20 m A/cm²and stirring speed 500 r/min?,the Co-Mn₃O₄composite guiding layer was prepared with the composite electro-deposition process.The Mn content in the coating was 18.3%,the thickness was about 8-10?m,the Mn₃O₄particles dispersed uniformly and was well bounded with the substrate.?3?The Co-Mn₃O₄composite coating on the E-brite sample surface was completely transformed into?Co,Mn?₃O₄composite spinel coating with thermal conversion method.The thermal conversion parameters were optimized through experiments.The heat transformation mechanism and the elements diffusion behavior at the interface were studied.At the first stage,Co coating matrix formed double oxide structure with Co₃O₄at the external and Co O at the internal.Mn₃O₄particles changed into Mn₂O₃partly at the same time.?Co,Mn?₃O₄phase was formed at the interface between manganese oxide and Co₃O₄firstly.With the constant penetration of oxygen,the deposition coating eventually transformed into?Co,Mn?₃O₄spinel phase.And Cr migrated from the matrix to the deposit/matrix interface,then formed a dense oxide layer with 2?m thickness.?4?The oxidation performance of?Co,Mn?₃O₄spinel coated samples under800?was studied.The isothermal and cyclic oxidation experiment results show that the heat conversion process of?Co,Mn?₃O₄spinel coated E-brite was classified into two stages as rapid oxidation and slow oxidation.The parabolic oxidation rate constants within 100h was 4.38×10^-14g²cm^-4s^-1and1.31×10^-14g²cm^-4s^-1,respectively.Element inter-diffusion between coating and E-brite matrix was imbalance,which caused kirkendall holes in the coating.The thickness of Cr₂O₃layer at the coating/substrate interface has not increased.And there is almost no Cr exists in coating outside the oxide layer.It shows that the?Co,Mn?₃O₄spinel coating can significantly reduce the oxidation rate of the substrate,and the volatilization of Cr toward outside can be blocked.Also,the Cr poisoning of the adjacent SOFC cathode,which will cause the reduction of the battery performance can be effectively avoid.?5?Four probe method was taken to measure the ASR under the simulated working environment of SOFC.The electrical conductivity of the?Co,Mn?₃O₄spinel coating was studied.Experimental results show that the ASR value of?Co,Mn?₃O₄spinel coated samples after 500h oxidation was 7.9m?·cm²,which was only 19.3%of the naked E-brite ASR value in the same conditions.Compared with Co₃O₄,the mixing of Mn has reduced the resistivity of the spinel coating and improved the electrical conductivity.The relationship between ASR value and oxidation time was derived by mathematical method.The ASR value of the?Co,Mn?₃O₄spinel coated E-brite after 40000h of oxidation at 800?was calculate to 70.8m?·cm²,which was below the limit of SOFC interconnection ASR value of 100m?·cm²,and the service demands of SOFC can be satisfied.