Preparation And Properties Of Ni-Co Spinel Oxide Doped By Oxide Particles

With the continuous development of intermediate temperature solid oxide fuel cells（IT-SOFCs）,metal materials have gradually replaced ceramic materials for application research.Currently,type 430 ferritic stainless steel is widely used in IT-SOFCs interconnection materials due to its matching thermal expansion coefficient（CTE）with anode and cathode,good overall performance,low cost,good electrical conductivity and excellent oxidation resistance.However,there are still two problems in the commercial application of FSS at high temperatures.First of all,the Cr element in stainless steel will form high resistivity chromium oxide with the thickening of the oxide layer,resulting in serious degradation of the performance of SOFC battery stacks.Secondly,the volatilization of Cr is prone to cathode poisoning,which further affects the service life of SOFC.The best way to solve these problems is to prepare a coating with good oxidation resistance and good electrical conductivity on the surface of stainless steel.Therefore,in order to overcome the difficulty of ferritic stainless steel as connection material,the application of spinel coating is proposed.In order to investigate the effect of La₂O₃and Al₂O₃on the oxidation resistance and electrical properties of Ni-Co alloy,the（Ni,Co）₃O₄coatings doped with La₂O₃and Al₂O₃were synthesized on the surface of SUS430 stainless steel by codeposition method.The surface and cross section structure of the sample were analyzed by SEM and X-ray diffraction.After oxidizing at 800℃for 1000 h,a four-layer structure including Cr₂O₃/（Mn,Cr）₃O₄,（Ni,Co,Fe）₃O₄,（Ni,Co）₃O₄/（Ni,Co）O and（Co,Fe）₃O₄was formed on the surface of Ni-Co coated steel.There were some holes in Ni-Co coated steel due to the diffusion of metal elements inside and outside,and the continuous growth of Cr oxide layer made the surface specific resistance（ASR）of alloy coated sample larger.The overall oxidation gain of Ni-Co-La₂O₃composite coated steel was lower than that of Ni-Co and Ni-Co-Al₂O₃composite coated steel during the whole long oxidation time.The oxidation rate constant（K_p）of Ni-Co-La₂O₃coated steel was8.4×10^-14g²·cm^-4·s^-1,which was 73%and 68%lower than that of Ni-Co coated steel and Ni-Co-Al₂O₃composite coated steel,respectively,during the oxidation time of100-1000 h.The results showed that the addition of La₂O₃in Ni-Co alloy could significantly enhance the oxidation resistance of the matrix compared with Al₂O₃as the composite spinel coating of SOFC metal connectors.After adding 3 g/L,9 g/L and 15 g/L La₂O₃to Ni-Co alloy plating solution,the experimental results were as follows:after long-term oxidation for 1000 h,3 g/L La₂O₃doped Ni-Co composite coated steel had the worst oxidation resistance,the thickest Cr₂O₃thickness and the largest ASR.The 15 g/L La₂O₃doped Ni-Co composite coating steel had the best oxidation resistance,the thinest Cr₂O₃thickness and the smallest ASR.After oxidizing at 800℃for 1000 h,the ASR of Ni-Co-La₂O₃coating samples increased slowly from 8.2 mΩ·cm²to 13.1 mΩ·cm².In addition,according to the fitting formula,the ASR of Ni-Co-La₂O₃composite coating samples after oxidation for 40,000 h was estimated to be only 64.59 mΩ·cm²,which was 51%and 29%lower than that of Ni-Co and Ni-Co-Al₂O₃composite coating samples.This indicated that the addition of La₂O₃in Ni-Co alloy could significantly improve the electrical properties of the interconnnection than that of Al₂O₃.