Abstract
A study was conducted to explore the effects of Sr doping on the electrical properties of perovskite LaFeO3 thin-film protective conversion coatings grown onto a K41 ferritic stainless steel, a typical interconnect material for intermediate temperature solid oxide cell (SOC) applications. The Sr-doped coatings were prepared in La2O3- and SrO-containing molten carbonate baths with minor added amounts of nitrate salt for accelerated coating formation. For comparison purposes, undoped coatings were obtained using the same carbonate bath, with the only difference being that SrO was replaced by inert MgO. SEM/EDX and XRD analyses were used for coating characterization and confirmed the effective incorporation of Sr but not of Mg into the LaFeO3 layer. Although both the Sr-doped and undoped coatings consisted of a LaFeO3 layer grown above an inner Fe-Cr spinel, the coating thickness of the Sr-doped coating was distinctly higher, approximately 2 µm, which is twice that of the undoped coating. Electrical measurements in terms of Area-Specific Resistance (ASR) were conducted at 700 °C in air and showed that Sr-doping significantly improved the electrical conductivity of the coated K41 steel. Due to the Sr-doping, the ASR values of the coated steel dropped from 60 to 37 mΩ cm2 after 300 h of exposure, in spite of the higher Sr-doped coating thickness. The study concludes that Sr-doped thin-film perovskite coatings appear to be a promising solution for improved SOCs steel interconnect stability at intermediate temperatures.
Highlights
IntroductionThe poor electrical conductivity of the oxidized steel surface and chromia evaporation represents two insidious degradation issues on the oxidizing atmosphere side, causing a progressive and continuous interconnect and cell performance loss over time [5,6]
Among the various types of ceramic and metallic materials used for solid oxide cell (SOC) stack and balance-of-plant components, ferritic stainless steel interconnects play a key role in ensuring the stable and efficient performance of stacks operating in the intermediate temperature range of 600–750 ◦ C over prolonged periods of time [3,4]
The results clearly indicate that rapid coating formation takes place on the K41 steel surface in both the salt systems with minimal differences in open circuit potential (OCP) behavior
Summary
The poor electrical conductivity of the oxidized steel surface and chromia evaporation represents two insidious degradation issues on the oxidizing atmosphere side, causing a progressive and continuous interconnect and cell performance loss over time [5,6]. For this reason, the functionalization of the steel surface using a coating deposition of conductive mixed metal oxides prevalently based on spinel Co, Mn-Co or perovskite La-Co phases is usually needed for stabilizing the long-term interconnect performance on the oxidizing side [7,8,9]. Proceeding from these observations, the effect of spinel sublayers on perovskite coating protection has been the object of subsequent and more detailed investigations by a number of authors
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