Preparation And Properties Of NiO-NiFe₂O₄ Coatings

Solid oxide fuel cell(SOFC)has the prospect of being widely used because of its all-solid structure and high energy conversion rate.Generally,the single cell can only provide a voltage of 1 V,so SOFC interconnects are required to connect the single cell in series to form a battery stack to the practical application.Ferrite stainless steel has become one of the most promising materials due to its good oxidation resistance,great surface oxide scales conductivity and suitable thermal expansion.However,there are challenges to be resolved for ferritic stainless steel interconnects in 600～800℃.Sustaining chromium formation which-increases electrical resistance over time due to the growth of oxide scale,at the same time,the high volatility of Cr migrate to the cathode and poison them,degrading the electric performance of SOFC.Deposit protective coating is the most effective method to solve those problems at present.NiFe2O4 spinel has the ability to absorb Cr element,also the coefficient of thermal expansion(CTE)is matched with FSS interconnects,NiO has a strong ability to inhibit the outward diffusion of Cr.NiO-NiFe2O4 double-layer coating is expected to be applied to FSS.In order to inhibit the elements diffusion between the coating and the substrate,the stainless steel substrate was pre-oxidized to form Cr2O3 before the coating was deposited.In this work,Ni-NiFe2 coatings were deposited on pre-oxidized SUS430 stainless steel by DC magnetron sputtering technology,followed by thermal oxidation to convert to the NiO-NiFe2O4 oxide layers in 800℃.The oxidation process and oxidation mechanism were investigated and the electrical performance of the oxide scale were characterized.The effect of Ni layer thickness on the high-temperature properties of the coated stainless steel was also studied.The results are as follows:(1)The short-term oxidation showed that,the Fe element in the NiFe2 coatings was preferentially oxidized to Fe2O3 on the surface,Ni was oxidized by O diffusion.The oxidized coatings were divided into a multilayer structure,consisting of an inner Cr2O3 layers,followed by NiFe2O4 and NiO,and a Fe2O3 layer on the top.(2)The long-term oxidation showed that the oxidized coatings were divided into a multilayer structure,consisting of an inner Cr2O3 layers,followed by NiO,and NiFe2O4 spinel layer on the top.NiO-NiFe2O4 layers not only inhibited the growth of Cr2O3 layer,also reduced the outward diffusion of Cr.As the thickness of the NiO layer increased,the coating’s ability to inhibit the outward diffusion of Cr was enhanced,after 10 weeks of oxidation,the spinel layer contained almost no Cr element.(3)The ASR of the 1 h Ni,2 h NiFe2 samples and 2 h Ni,2 h NiFe2 samples after oxidation for 10 weeks were 28.08 mΩ·cm2 and 29.16 mΩ·cm2 respectively,which met the electrical conductivity requrements of the interconnects coatings.