Elements Diffusion And Optimization Design Of Protective Coatings For Sofc Metallic Interconnects

(Mn,Co)₃O₄spinel is one of the most promising coatings for solid oxide fuelcell （SOFC） stainless steel interconnects. The properties of stoichiometricMn_1.5Co_1.5O₄are preferable to other Mn/Co ratios because of the CTE match to thetypical substrate materials and higher conductivity. However, the effect of elementsdiffusion on coating composition in high temperature can not to be neglected.Previous works showed that Mn/Co ratio changes during operation due to Mndiffusion from substrates. Therefore, in this work, coating compositions, coatingthicknesses and annealing time have been varied to investigate the law of elementdiffusion and optimize protective coating. It will not only provides theory forsubsequent similar research, but also reduces cost for SOFC commercialization.Firstly, to optimize magnetron sputtering parameters and technology to gain thecompact coatings with varying thickness （800nm,1500nm,3000nm） and differentcompositions (pure Co, Mn₂₀Co₈₀、Mn₄₀Co₆₀). Then, isothermal annealing isperformed at800°C in a batch-type furnace with stagnant air for2h,10h,50h,250hand1000h to obtained （Mn,Co）₃O₄spinel phase development. The morphology,chemical compositions （including surface and cross sections）, and structures ofoxide formed are analyzed by SEM, EDS, XRD. Area specific resistance （ASR） wastested as well. Element diffusion （Mn, Co, Cr, Fe） and growth of oxide layer fromthe analysis results and optimizing coating recipe for SOFC interconnect.Compact MnCo spinel layers were obtained by magnetron sputtering withchanging bias and gradient elements followed by annealing successfully. EDSanalysis indicates that Mn/Co ratio approximatedly remain unchanged during thelong oxidation period in Mn₄₀Co₆₀coating. When oxidized1000h, it is close to1:1,and XRD results displays the main phase is Mn_1.5Co_1.5O₄, while elements diffusionhas not reach stable and the main phases are Co₃O₄/Co₂CrO₄and MnCo₂O in purecoating and Mn₂₀Co₈₀coating respectively.Cross section SEM micrographs and EDS line scan data demonstrate thatdiscontinuous silica and alumina distributed in interface between coating andsubstrate; Little cobalt was found to diffusion inward into substrate, a few Fe existsin （Mn,Co）₃O₄layer, while no Cr to the coating layer. Elements outward diffusionfrom substrates follows the order of D_Mn>D_Fe>D_Cr. Oxidation mechanism of oxidelayer followsparabolic law. Considering the stable Mn/Co and total thickness,1,000h is long enough to study elements diffusion in whole lifetime of SOFC.Long term ASR tests indicate that the value of ASR is0.051Ωcm²,0.021Ωcm² and0.036Ωcm²in Mn₄₀Co₆₀coating with800nm,1500nm and3000nm afterannealed1100h, respectively. Besides, the change trend keep stable, it is reasonableto forecast that the ASR value is far less than0.1Ωcm²after40,000h oxidized,especially in1500nm. At the same time, it also suggests Mn₄₀Co₆₀coating with1500nm is the best candidate, which provides a reference standard for SOFCinterconnect research.