Kazimierz Przybylski

Application of Fe–16Cr ferritic alloy to interconnector for a solid oxide fuel cell

Iron-base scaling-resistant alloys (Fe–Cr) as materials for interconnectors of planar-type solid oxide fuel cells (SOFC) are proposed for application in automobile industry because of their advantages in comparison with other Ni- or Co-based alloys and ceramic materials (e.g. (La,Sr)CrO3). The oxidation kinetics of Fe–16Cr alloy (SUS 430) has been studied in H2–H2O gas mixtures (pH2/pH2O=94/6 and 97/3), and in air in the temperature range of 1023–1173 K for 3.6 up to 1080 ks, in the conditions simulating the anode and cathode environments in SOFC. It has been found that the oxide scale, composed mainly of Cr2O3, grows in accordance with the parabolic rate law. The dependence of the parabolic rate constant, kp, on temperature can be described as kp=6.8×10−4 exp(−202.3 kJ mol−1/RT) for the H2–H2O gas mixture with pH2/pH2O=94/6. The determined parabolic rate constant is independent of the oxygen partial pressure in the experimental range of 5.2×10−22 to 0.21 atm at 1073 K, which means that the growth rates of scale on Fe–16Cr alloy in the above-mentioned atmospheres are comparable. The increase in electrical resistance of the chromia scale growing on Fe–16Cr alloy vs. time, calculated from kp and the specific resistance of Cr2O3 scale, in comparison with the constant electrical resistance of a ceramic interconnector, made of (La,Sr)CrO3, indicates the necessity to modify the studied alloy surface. At 1073 K, the resistance of the Fe–16Cr alloy coated with La0.6Sr0.4CoO3 by a spray-pyrolysis method is low, the average of 45 mΩ cm2 in the H2–H2O gas mixture (pH2/pH2O=94/6) and the average of 20 mΩ cm2 in air, in comparison with the ceramic interconnector, La0.85Sr0.15CrO3, 0.5 cm thick. This indicates the applicability of SUS 430 alloy as interconnector for SOFC.

The role of microstructure on pesting during oxidation of MoSi2 and Mo(Si,Al)2 at 773 K

The pesting behavior of MoSi2 and Mo(Si,Al)2 has been examined in air at 773 K to clarify the origin and mechanism of pesting phenomena and the effect of aluminum on pesting phenomena. The initial cracks play a much more important role than the grain boundaries and the initial oxide layer in pesting. Mo and Si oxidize to amorphous Mo-Si-O simultaneously with about a 200% volume expansion. Therefore, large stress appears at the cracktips and induce many new cracks. MoO3 vaporizes from the Mo-Si-O layer on the external surface and crack surfaces causing the oxides in the initial cracks to become porous. Oxygen has a short-circuit path to enter the sample in the cracks. Therefore, the partial pressure of oxygen is sufficiently high to allow oxidation of Mo in the materials. The platelet-like MoO3 grows on the external surface and also in the cracks. Finally, the sample distintegrates into powder. Pesting of Mo(Si,Al)2 occurs in the same way, however, its rate is much lower than that of MoSi2. The role of Al is to decrease the initial crack density of the samples from the melt. Other effects of Al might be to decrease the oxygen flux toward the oxide-intermetallic interface and to increase the plasticity of the amorphous oxide being formed in the cracks.

Microstructure of Fe–25Cr/(La, Ca)CrO3 composite interconnector in solid oxide fuel cell operating conditions

A 20 μm thick coating of La 0.8 Ca 0.2 CrO 3 perovskite on Fe-25Cr steel substrates was obtained by the screen-printing method. The interaction of the (La, Ca)CrO 3 coating with the steel substrate occurring during oxidation in air at 1073 K for 200h was investigated. The microstructure, phase and chemical analyses of the coating after oxidation were examined. It has been revealed that the metal substrate-coating interfacial zone has a two-layer structure built of mainly FeCr 2 O 4 and LaCrO 3 , which provide the electric contacts between the interconnector material and the film, thus creating conditions for prolonged exploitation of solid oxide fuel cells.

Synthesis and properties of MgB2 obtained by SHS method

Synthesis and properties of bulk magnesium diboride (MgB2) samples obtained by self-propagating high-temperature synthesis (SHS) are presented. The magnetic field and the temperature dependencies of the dispersive and absorption ac magnetic susceptibility as well as resistivity of the MgB2 specimen were measured and analysed. The superconducting transition is very sharp and very sensitive to the ac and dc magnetic fields. The critical current density was extracted from the absorption susceptibility via a superconducting critical state model. We concluded that our MgB2 is a hard, type II superconductor and that the SHS method is suitable to fabricate material with strong pinning centers of MgO. 2003 Elsevier Science B.V. All rights reserved.

Measuring Adhesion of Cr2O3 and Al2O3 Scales on Fe-Based Alloys

Adhesion of oxide scales on metals and alloys is one of the key parameters to better understand spallation phenomena. Three adhesion tests are described, from which interface fracture energy may be derived. In the inverted blister test, a circular crack is propagated due to water pressurizing at the scale-metal interface. In the three-point bending test, the interface is mechanically loaded using a rigid counterplate glued to the external surface of the scale. In the tensile test, the oxide scale is forced to spall, releasing the elastic energy stored. Examples of measurements are given, where it is shown that adhesion values depend on the test used, particularly on the mixed mode angle at the crack tip. Introduction The life time of metallic parts submitted to high temperature corrosion is often limited by scale spallation leading to rapid oxide loss or to initiation of breakaway corrosion by iron oxide formation on chromiaor alumina-formers. Measuring spallation can be done after cooling by surface image analysis, or during service by continuous mass measurements. Temperature drop to initiate spallation is, for example, a simple experiment to quantify this type of degradation. Scale spallation is the result of a complex process where two important parameters play a role: stresses and adhesion. Oxide scales grow in compression, and more compressive stress is added by cooling. Such stresses are currently determined by bending of monofacially oxidized specimens, X-ray diffraction or Raman spectroscopy. On the contrary, adhesion is a parameter much more difficult to evaluate. In this paper, three mechanical tests used for determining adhesion of oxide scales on their substrate are described and several results in chromia-or alumina-forming systems are reported. General description of methods A good adhesion test must be designed to first create an interfacial crack between oxide and substrate and then measure its propagation in order to evaluate the interfacial fracture energy Gi (called “mechanical adhesion” or simply “adhesion”) in the following. It must be noted that Gi values differ from the thermodynamic adhesion energy Wad derived from the metal, oxide and interface surface energies: Wad = γm + γox – γi (1) It is generally considered that Gi is several orders of magnitude higher than Wad, due to dissipative contributions: Gi = C( Wad –We + Wp – Wseg ) (2) Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 631-638 doi:10.4028/www.scientific.net/MSF.461-464.631

Segregation of Neodymium in Chromia Grain-Boundaries during High-Temperature Oxidation of Neodymium Oxide-Coated Chromia-Forming Alloys

The influence of MOCVD reactive-element-oxide (REO) coatings (Nd2O3) onhigh-temperature, chromia-forming alloy oxidation was investigated. REOcoatings decreased steel oxidation rates and greatly enhanced oxide scaleadherence. Uncoated and coated F17Ti samples were oxidized over thetemperature range 700–1050°C in air at atmospheric pressure. SIMSexperiments were performed on oxidized-coated samples in order to determinethe RE distribution through the oxide scale. Nd was distributed across theoxide layers with a higher concentration in the outer part of thescale. Transmission-electron microscopy (TEM) investigations were performedto more precisely locate the RE through the scale. Transverse crosssections, prepared on oxidized Nd2O3-coatedFe–30Cr (model system), showed that Nd, associated with Cr and O,segregated at chromia grain boundaries. It is thought that this is the maincause of the beneficial effects usually ascribed to the RE inchromia-forming alloys. The effect of chromia grain-boundary segregation onchromia growth mechanism and its influence on the reactive-element effect(REE) are discussed.

IDEAL-Cell, Innovative Dual mEmbrAne fueL-Cell: Fabrication and Electrochemical Testing of First Prototypes

The IDEAL-Cell concept is based on the junction between a PCFC anode/electrolyte section and a SOFC cathode/electrolyte section, through a mixed proton and oxide ion conducting porous ceramic membrane, operating in the temperature range 600-700°C. Recombination of ions takes place within the junction, or central membrane (CM), and water vapor is evacuated through open porosity. The first results on the fabrication of multilayered samples reproducing the IDEAL-Cell structure are reported here, together with electrochemical tests carried out on selected samples in order to evaluate the performances of the chosen materials and to demonstrate the feasibility of this innovative concept of fuel cell.

Tg measurements of the oxidation kinetics of Fe-Cr alloy with regard to its application as a separator in SOFC

The high-temperature oxidation behavior of a ferritic alloy (SUS 430) in a SOFC environment, corresponding to the anode (H2/H2O gas mixture) and cathode (air) operating conditions, was determined with regard to application of the alloy as a metallic separator material in SOFC. The oxidation kinetics of Fe-16Cr alloy (SUS 430), was studied by thermogravimetry in H2/H2O gas mixtures with pH/pHO=94/6 and 97/3 and in air, in the temperature range 1023-1223 K, for 3.6 up to 1080 ks. It was found that the protective oxide scale, composed mainly of Cr2O3 with uniform thickness and excellent adhesion to the metal substrate, grows in accordance with the parabolic rate law. The dependence of the parabolic rate constant, kp, of the scale on temperature obeys the Arrhenius equation: kp=6.8×10-4 exp (-202.3 kJ mol-1R-1T-1) for H2/H2O gas mixtures with pH/pHO=94/6. The determined kp was independent of the oxygen partial pressure in the range from 5.2×10-22 to 0.21 atm at 1073 K, which means that the rates of growth of the scale on Fe-16Cr alloy in the above-mentioned atmospheres are comparable. The oxidation test results on Fe-16Cr alloy in H2/H2O gas mixtures and air demonstrate the applicability of SUS 430 alloys as a separator for SOFC.

Role of Minor Element Addition in the Formation of Thermally Grown Alumina Scales

The effect of Y+Zr+Hf and La+Ce on the oxidation behavior of FeCrAl alloys was tested at 1373 K in laboratory air under atmospheric pressure. Oxidation tests performed under different oxygen isotope atmospheres indicated that after 8 h of oxidation, the mixed diffusion of oxygen and aluminum controlled the alumina scale growth. After 96 h of oxidation, the oxide scale mainly grew via oxygen diffusion. The chemical analyses of the alumina scales allowed location of the reactive elements at the alumina grain boundaries and at the metal-oxide interface after the longer oxidation tests, but never after the shorter tests. These results are discussed in relation with the transient oxidation stage necessary for incorporation of the reactive elements into the growing oxide scale.

Interfacial Interactions between some La-Based Perovskite Thick Films and Ferritic Steel Substrate with Regard to the Operating Conditions of SOFC

An overview is presented on the oxidation kinetics, electrical properties and microstructure investigations of the oxide products formed on Fe-25 wt.-%Cr steel uncoated and coated with electrical conducting films of (La,Ca)CrO3 or (La,Sr)CoO3 in air and H2/H2O gas mixture at 1023−1173 K for up to 480 hrs with regard to their application as the SOFC metallic interconnect. The application of the Fe-25Cr steel in SOFC operating at 1073 K requires its surface modification to improve the electrical conductivity of chromia scale forming on the uncoated steel surface. The thick films of (La,Ca)CrO3 and (La,Sr)CoO3 with the thickness range of 20−100 μm, coated on the Fe-25Cr steel by screen-printing method helped solve this problem. TEM-SAD, SEM-EDS and impedance spectroscopy investigations have shown significant influence of the multilayer products formation at the substrate steel/coating films interfacial zone on the electrical properties of the metallic interconnect.