PHYSICOCHEMICAL AND MECHANICAL PROPERTIES OF CROFER 22 APU FERRITIC STEEL APPLIED IN SOFC INTERCONNECTS

The paper presents the results of investigations of the physicochemical and mechanical properties of the Crofer 22 APU steel designed for application in metallic interconnects forming the key components of solid oxide fuel cells (SOFCs). Microstructural and hardness studies of non-metallic inclusions and the matrix were carried out. Based on compression tests of raw Crofer 22 APU and the steel after 600 hrs of cyclic oxidation in air at 800°C, the composition of non-metallic inclusions and their influence on the strength properties of the steel were determined

Experimental review of the performances of protective coatings for interconnects in solid oxide fuel cells

Ferritic stainless steel interconnects are used in solid oxide fuel cells; however, coatings are required to improve their performance. Although several types of coatings have been proposed, they have been scarcely investigated under similar conditions. This study compares the characteristics of uncoated Crofer 22 APU and eight different coatings on Crofer 22 APU for up to 3000\ua0h at 800\ua0\ub0C. The coatings were deposited at various research laboratories around the world, and the experiments were performed at Chalmers University of Technology, Sweden. Cross-sections of the samples were analysed using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The (Co,Mn)-based coated steels showed more than 50-fold lower chromium evaporation and at least 3 times thinner Cr2O3 scale thickness compared to uncoated steel. The coated steel samples showed lower area-specific resistance (ASR) values than the uncoated steel after 3000\ua0h of exposure, irrespective of the coating thickness, composition and deposition method

Interface Reactions between Conductive Ceramic Layers and Fe-Cr Steel Substrates in SOFC Operating Conditions

To improve contact resistance and protect the cathode from chromium poisoning, perovskite (La,Sr)CrO3, (La,Sr)CoO3 and spinel MnCo2O4 coatings were applied onto the surfaces of Fe-25Cr (DIN 50049) steel by means of the screen-printing method. The oxidation process of the coated steels under cyclic-oxidation conditions showed high compactness of the protective layer, good adhesion to the metal substrate, as well as acceptable area specific resistance level for SOFC metallic interconnect materials, which was the result of the structural modification of the coating/steel interface reaction, i.e. the formation of an intermediate, chromia-rich multilayer between the conductive coating and the metal substrate. The cross-sectional morphology and nanostructure of interface products formed during long-term thermal oxidation in air and the H2/H2O gas mixture at 1073K were characterized using SEM-EDS and conventional TEM-SAD. Cr-vaporization tests showed that the perovskite and spinel coatings may play the role of barriers that effectively decrease the volatilization rate of chromia species. [doi:10.2320/matertrans.MB201013

Synthesis and physicochemical properties of yttrium oxide doped with neodymium and lanthanum

Przedmiotem badań są proszki o składzie polikrystalicznego tlenku itru, które preparowano przy użyciu dwóch metod: EDTA gel processes i współstrącania. Otrzymane proszki posiadają silnie rozwinięte aglomeraty, złożone z drobnych owalnych ziaren. Badania rentgenograficzne proszków otrzymanych metodą EDTA gel processes potwierdziły jednofazowy ich skład, natomiast w proszkach otrzymanych metodą współstrącania oprócz tlenku itru zidentyfikowano tlenek lantanu. Na podstawie pomiarów spektrofotometrycznych w zakresie 500-1000 nm, stwierdzono, że najwyższą przeźroczystość, około 7% wykazują próbki preparowane metodą EDTA gel processes po spiekaniu techniką HIP.The presented research involves yttrium oxide powders prepared using two methods: EDTA gel processes and co-precipitation procedure. The obtained powders feature strongly developed agglomerates of fine, oval particles. X-ray diffraction studies of powders obtained using the EDTA gel processes confirmed their single-phase composition, while in the powders obtained by means of co-precipitation the lanthanum oxide was found to be present along the yttria phase. Basing on the spectrophotometric measurements in the range of 500-1000 nm it was determined that the highest transparency of approximately 7% was exhibited by samples prepared via the EDTA gel processes after sintering involving HIP

Physicochemical Study of the Self-Disintegration of Calcium Orthosilicate (β→γ) in the Presence of the C12A7 Aluminate Phase

The β-γ polymorphic transition of calcium orthosilicate (C2S) is a key phenomenon in cement chemistry. During this transition, the compound expands due to structural changes and a significant reduction in its density is observed, leading to its disintegration into a powder with a very high specific surface area. Owing to this tendency of the C2S material to “self-disintegrate”, its production is energy-efficient and thus environmentally friendly. A physicochemical study of the self-disintegration process was conducted with the aim of determining how the amount of dodecacalcium hepta-aluminate (C12A7) in calcium orthosilicate (C2S) affects the temperature at which the polymorphic transi-tions from α’L-C2S to β-C2S and from β-C2S to γ-C2S undergo stabilization. The applied techniques included differential thermal analysis (DTA), calorimetry and X-ray diffraction (XRD), and they made it possible to determine what C2S/C12A7 phase ratio in the samples and what cooling rate constitute the optimal conditions of the self-disintegration process. The optimal cooling rate for C2S materials with a C12A7 content of up to 60 wt% was determined to be 5 K·min−1. The optimal mass ratio of C2S/C12A7 was found to be 70/30, which ensures both efficient self-disintegration and desirable grain size distribution

Changes in the Phase Composition of Calcium Aluminoferrites Based on the Synthesis Condition and Al₂O₃/Fe₂O₃ Molar Ratio

The presented work concerns the study of the changes in the phase composition of calcium aluminoferrites which depend on the synthesis conditions and the selection of the Al2O3/Fe2O3 molar ratio (A/F). The A/F molar ratio extends beyond the limiting composition of C6A2F (6CaO·2Al2O3·Fe2O) towards phases richer in Al2O3. An increase in the A/F ratio above unity favours the formation of other crystalline phases such as C12A7 and C3A, in addition to calcium aluminoferrite. Slow cooling of melts characterised by an A/F ratio below 0.58, results in the formation of a single calcium aluminoferrite phase. Above this ratio, the presence of varying contents of C12A7 and C3A phases was found. The process of rapid cooling of the melts with an A/F molar ratio approaching the value of four favours the formation of a single phase with variable chemical composition. Generally, an increase in the A/F ratio above the value of four generates the formation of a calcium aluminoferrite amorphous phase. The rapidly cooled samples with compositions of C22.19A10.94F and C14.61A6.29F were fully amorphous. Additionally, this study shows that as the A/F molar ratio of the melts decreases, the elemental cell volume of the calcium aluminoferrites decreases

Oxidation and Electrical Property Studies on Ferritic Steels as Potential Interconnects in Electrochemical Devices for Energy Conversion

This work presents the results of oxidation studies on commercially available Nirosta 4016/1.4016 ferritic steel, which contains 16.3 wt.% chromium, as well as the electrical properties of steel/scale layer systems in order to determine the usefulness of this steel for constructing metallic interconnects in solid oxide fuel cell (SOFC) and solid oxide electrolyzer cell (SOEC) stacks. The E-Brite ferritic steel, consisting of up to 26 wt.% chromium, was chosen as a reference material. High-temperature isothermal oxidation kinetics studies were carried out on both steels at 1073 K for 255, 505, 760 and 1010 h in air atmosphere. These conditions are representative of those present in the cathode compartment of a SOFC and the anode compartment of a SOEC. Area specific resistance (ASR) measurements were performed on steel/scale layer systems, obtained after the previous oxidation of both steels in the above-mentioned conditions, in the air in the temperature range of 573–1073 K using the pseudo-DC four-probe method. On the basis of these studies, complemented by morphology observations, as well as chemical and phase composition analysis of the oxidation products, the usefulness of Nirosta 4016/1.4016 ferritic steel for manufacturing interconnects in energy conversion electrochemical devices operating at 1073 K was confirmed