| The interconnect is a critical stack component of solid oxide fuel cell (SOFC). As the reduction of SOFC operation temperature to an intermediate temperature of about 600-800℃, Cr2O3-forming ferritic stainless steels are the most favorable interconnect material candidates for SOFC. Nevertheless, commercial Fe-Cr alloys are still facing serious challenges of insufficient oxidation resistance and poisoning of the cathode by Cr vaporization, which impedes their long-term applications in the SOFC. So far, development of metallic interconnect is still a vital problem to be overcome in SOFC.This dessertation focuses on the research of SOFC metallic interconnect. Firstly, the effect of Mn content on Fe-Cr alloy in terms of nucleation and growth of oxides, oxidation kinetics and electrical conductivity is investigated. Then, the effect of the conductive perovskite (LaCoO3) and spinel (MnCo2O4, NiCO2O4) ceramic coatings, coated by the sol-gel process, on the oxidation kinetics behavior and electrical property of SUS 430 alloy in SOFC cathode atmosphere is also studied. Finally, novel alloys of Ni-based Ni-Mo-Cr alloy and Fe-based Fe-Cr-Mn alloy are designed and developed for metallic interconnect application. The performances of these novel alloys in SOFC cathode and/or anode atmosphere are systemically characterized and evaluated. The following conclusions are achieved:(1) During the initial oxidation stage (within 1 min) at 750℃in air, Cr is selective preferential oxidized to form Cr2O3 protective layer, which is independence of the Mn content. The oxidation rate, oxides phases and oxide layer morphology are all similar for Fe-Cr-xMn (x=0.0,0.5,1.0 and 3.0 wt.%) alloys. However, during the subsequent oxidation process, multi-stage oxidation kinetics is observed for Fe-Cr-Mn alloys containing 0.5 wt.% or 1.0 wt.% Mn. The first slow oxidation stage corresponds to slow growth of a Cr2O3 layer, and the faster second stage is the result of the rapid diffusion of Mn ions passing through the established Cr2O3 layer to form (Mn, Cr)3O4 spinel on the top of Cr2O3 layer. The oxidation of no Mn or higher Mn content Fe-Cr-Mn alloy is always controlled by the outward diffusion of Cr or Mn ions, respectively, and no multi-stage phenomenon is observed.(2) During the whole oxidation process, the oxidation rate and the thickness of the oxide layer of the Fe-Cr-Mn alloy gradually increase with the increasing of Mn content, while the oxidation resistance and the electrical conductivity of the alloys decrease. Cr2O3 is the only product on the surface of Fe-Cr-Mn alloy when Mn is lower than 0.1 wt.% in the alloy. Double oxide layers, (Mn, Cr)3O4 spinel layer formed on the top of Cr2O3 sublayer on the alloy surface, are formed when Mn is between 0.5 wt.%-1.0 wt.%. Densely Mn2O3 oxide layer forms on the outermost surface as the Mn content reaches 3.0 wt.%. Generally considering the oxidation resistance and electrical conductivity, the Mn content of Fe-Cr-Mn alloy must be controlled in the range of 0.5 wt.%-1.0 wt.%.(3) The perovskite (LaCoO3) and spinel (MnCo2O4, NiCo2O4) ceramic coatings can lower the oxidation rate, enhance the oxidation resistance, and improve the electrical conductivity of oxide layer of SUS 430 alloy. Spinel coatings can restrain the growth of Cr2O3 and the outward diffusion of Cr. However, the LaCoO3 coating cannot completely restrain the outward diffusion of Cr and Mn ions.(4) The thermal expansion coefficient (TEC) of the novel Ni-Mo-Cr alloy is smaller than 14×10-6/℃for temperature below 800℃. After oxidation at 750℃in cathode or anode atmosphere for 1000 h, the oxide layer of the Ni-Mo-Cr alloy presents a double layer structure with densely spinel (Cr-free NiMn2O4 in cathode atmosphere, MnCr2O4 in anode atmosphere) formed on top of the Cr2O3 sublayer, together with an intermetallic compound MoNi3 layer underneath the oxide layer. The novel Ni-Mo-Cr alloy has excellent high temperature oxidation resistance and electrical conductivity and the Cr deposition and poisoning effects of the Ni-Mo-Cr interconnect on the LSM cathode is also slight. Below 800℃, the oxidation resistance, electrical conductivity and Cr evaporation of the novel Ni-Mo-Cr alloy are all superior to the traditional commercial metallic interconnect alloys.(5) Slightly adjusting the Mo and Cr content of Ni-Mo-Cr alloy has almost no effect on the TEC and oxide layer composition, phase and morphology. However, the oxidation rate, oxide layer thickness and area specific resistance (ASR) all increase with the decreasing of Mo and Cr content.(6) The TEC of the novel Fe-Cr-Mn alloy is 12.23×10-6/℃for the temperature below 800℃. After oxidized at 750℃for 1000 h in cathode or anode atmosphere, the oxide layer of the Fe-Cr-Mn alloy mainly consists of Cr2O3, Mn-Cr spinel and Mn-rich surface oxide (Mn2O3 in cathode atmosphere, MnO in anode atmosphere). TiO2 internal oxide is also formed in the substrate adjacent to the alloy/oxide layer interface. The oxidation rate, thickness of oxide layer and ASR gradually increase with the oxidation temperature increasing. At 750℃, the novel Fe-Cr-Mn alloy is promising for SOFC metallic interconnect application.(7) In an anode atmosphere, the existing H2O and/or H2 in oxides affects their defect structure and hence changes the diffusion behavior of the cations (such as Cr and Mn) and oxygen ions, resulting in a faster oxidation rate, thicker oxide layer and higher ASR than that in air. However, the H2/H2O reducing atmosphere apparently enhances the adherence of the layer to the substrate.
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