Makoto Nanko

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.

Solid-state reaction kinetics of LaCrO3 from the oxides and determination of La3+ diffusion coefficient

LaCrO{sub 3} is a p-type semiconductor, and it is stable in both oxidizing and reducing atmospheres at elevated temperatures. LaCrO{sub 3} doped with Ca or Sr exhibits high electrical conductivity. Due to these properties, alkaline earth-doped LaCrO{sub 3} is used as an interconnector for solid oxide fuel cells (SOFC), which are attractive because of their high energy conversion efficiency and clean exhaust gas. Since the interconnector is used at elevated temperatures with a large difference of oxygen pressure between anode and cathode, it is extremely important to clarify its transport properties to understand demixing and creep. In several oxides with the perovskite structure, oxide ion diffusivity has been reported. On the other hand, cation diffusivity, which probably governs the demixing and creep processes, has been scarcely reported. The parabolic rate constant for the solid-state reaction of 1/2La{sub 2}O{sub 3} + 1/2Cr{sub 2}O{sub 3} = LaCrO{sub 3} was measured at temperatures between 1,483 and 1,688 /K in air and at 1695 K in oxygen pressures between 2.1 {times} 10{sup 4} and 1.5 {times} 10{sup {minus}5} Pa.

Formation and disappearance of an internal oxidation zone in the initial stage of the steam oxidation of Fe–9Cr–0.26Si ferritic steel

Abstract High temperature steam oxidation of the commercial Fe–9mass%Cr–0.26mass%Si steel (ASME T91) was carried out at 973 K, and the formation and disappearance of an IOZ (Internal Oxidation Zone) is discussed based on the microstructure observation by OM, and TEM with EDS. The IOZ formed at the initial stage and disappeared within about 100 ks. Sample oxidized for 61.2 ks showed a continuous IOZ, and TEM observation at the alloy/IOZ interface clarified that a sheet-like amorphous SiO2 layer formed intermittently. On the sample oxidized for 626.4 ks, the IOZ had disappeared and a sheet of amorphous SiO2 was located at the alloy/inner scale interface. The inner scale progressively grew next to the SiO2 layer due to an increase of oxygen potential at the interface because of the extremely low oxygen permeability in the amorphous SiO2.

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.

Internal and External Oxidation of Pt(Al) Solid Solution at Elevated Temperatures

High temperature oxidation of Pt(Al) solid solution and Pt(Al) with Pt 3 Al was investigated in order to prepare a Pt wire with an external Al 2 O 3 film for a hot-wire technique to measure thermal conductivities of fused metals. At temperatures from 1373 to 1673 K in air, Pt-4,6 mol % At (solid solution) indicated internal oxidation. Continuous Al 2 O 3 scales developed on Pt-9.4 mol % At (solid solution) and Pt-13.1 mol % (the two-phase alloy). Extemal scales formed on those alloys after high temperature oxidation under low oxygen pressure of Ar-10%H 2 -0.6%H 2 O gas mixture. The oxidation rate increased with increasing At content and oxygen partial pressure. The Al concentration at the transition from internal to extemal oxidation was determined by using oxygen permeability through Pt-Al alloys calculated from internal oxidation kinetics.

Formation of Mo(Si, Al)2 layer on Mo dipped in Al melt saturated with Si and the effects of transition metals added in the melt

Dipping Mo into Al liquid with saturated Si, a dense Mo(Si, Al)2 layer was formed on the Mo surface. Microstructure observation and chemical composition analysis of the Mo(Si, Al)2 layer reveal that the intermetallic layer consists of needle-like grains with diameter of submicrons and Al–Si liquid at their grain boundaries. Mo(Si, Al)2 grains are grown by the dissolution-precipitation process in the Al(Si) liquid located between Mo(Si, Al)2 and Mo. The product layers at the corners of Mo had radial cracks because the Mo(Si, Al)2 layer generated at the interface between the Mo(Si, Al)2 layer and Mo. Addition of Cr into the Al–Si liquid bath can prevent the formation of radial cracks.

Formation of Intermetallic Compounds on Refractory Metals in Aluminum-Silicon Liquid

Publisher Summary This chapter explores that for energy serving and ecological issue, an increase in operation temperature of gas turbine systems has been required to increase energy conversion efficiency. New high temperature materials have been required with higher applicable temperatures than that of Ni based superalloys. Disilicides such as MoSi 2 has a promising material because of excellent resistance for high temperature oxidation. Since MoSi 2 has low creep strength and high temperature strength, the intermetallic compound is reasonable for application of coating to prohibit high temperature Oxidation. Dipping the refracto U metals, Mo and Nb, into molten A1 liquid saturating Si, aluminodisilicides were formed on the surface of the metals. The existence of A1 and Si at the grain boundary of the intermetallic layer and the interface between the refractory metals and the intermetallic layer was revealed from the results of detail observation and electron probe microanalysis (EPMA). The supply of AI and Si is carried out by passing the A1-Si liquid at the grain boundary. The reaction for the formation of alumino-disilicide occurs at the A1-Si liquid at the interface between the refractory metals and the intermetallics.