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Abstract We have investigated the water solubility, oxygen isotope diffusivity, and oxygen surface exchange coefficient in a humid atmosphere of the system {(CeO2)x(ZrO2)1-x}0.8(YO1.5)0.2 (x = 0–1) by using isotope exchange and secondary ion mass spectrometry (SIMS). The deuterium ion (2D-) was detected from the {(CeO2)x(ZrO2)1-x}0.8(YO1.5)0.2 polycrystals which were annealed in D2O containing atmosphere. The solubility of deuterium in the polycrystals increased with the cerium content (x). The oxygen exchange rate constant (a) in air at T = 973 K shows a maximum at x = 0.2–0.3, which can be correlated to the compositional dependence of electronic conductivity of {(CeO2)x(ZrO2)1-x}0.8(YO1.5)0.2. The effect of water on the surface exchange rate constant was more significantly observed for the samples with higher content of cerium x > 0.6.

Chemical stability of ferritic (Fe–Cr) alloy interconnect material was examined under anode gas atmospheres for use in solid oxide fuel cells (SOFCs). In H2–H2O and CH4–H2O atmospheres, oxide scale layers were formed on the alloy surface. The surface morphology of the alloy was varied with forming oxide scales at 1073 K for an extended annealing time. Secondary ion mass spectrometry (SIMS) was applied to analyze the distribution of major and minor elements as a function of depth. From surface to inner alloy, the following distribution was identified: Mn, Fe rich → Cr rich → Si rich → alloy bulk and Al2O3inner oxides. The parabolic growth rate constants of oxide scales (kp) were calculated for each atmosphere as follows: 3.76 × 10–6μm2s–1for H2–H2O and 5.23  × 10–6μm2s–1for CH4–H2O. The growth rate constants were similar between these two atmospheres.

Chemical stability of ferritic (Fe–Cr) alloy interconnect material was examined under anode gas atmospheres for use in solid oxide fuel cells (SOFCs). In H2–H2O and CH4–H2O atmospheres, oxide scale layers were formed on the alloy surface. The surface morphology of the alloy was varied with forming oxide scales at 1073 K for an extended annealing time. Secondary ion mass spectrometry (SIMS) was applied to analyze the distribution of major and minor elements as a function of depth. From surface to inner alloy, the following distribution was identified: Mn, Fe rich → Cr rich → Si rich → alloy bulk and Al2O3 inner oxides. The parabolic growth rate constants of oxide scales (kp) were calculated for each atmosphere as follows: 3.76 × 10^–6 μm²s^–1 for H2–H2O and 5.23  × 10^–6 μm²s^–1 for CH4–H2O. The growth rate constants were similar between these two atmospheres.

Thermal stresses are a critical factor in the development of reliable, fuel efficient, planar solid oxide fuel cells (SOFCs). In the literature, broad recommendations have been made on the importance of minimising differences in thermal expansion coefficients (TECs) of materials used to fabricate SOFCs. The stresses due to temperature gradients, material property changes, material reliability, and differences in TECs in simple component configurations are reviewed. The purpose of the present paper is to examine the effect of size and to establish the minimum difference necessary in thermal expansion coefficients for a reliable SOFC. Two planar SOFC models with closed form solutions are used to investigate thermal tensile and shear stresses resulting from the fabrication process. For an acceptable probability of failure of less than 10-6in the zirconia electrolyte, the differences in the component TECs must be less than 1% of the zirconia electrolyte TEC.

The effect of the electrode material on the high voltage stoichiometry-polarization (“electrocoloration”) of Fe-doped SrTiO3single crystals has been studied. For different electrode materials (150-nm Ag/15-nm Cr and 150-nm Au/15-nm Cr) secondary ion mass spectrometry (SIMS) was used to measure the depth profile of the 18O-isotope after high-field stress. The results were compared with spatially resolved impedance measurements on electrocolored Fe-doped SrTiO3single crystals. For Au/Cr as well as Ag/Cr electrodes a large dc voltage leads to moving color fronts which are usually correlated with a pronounced stoichiometry polarization of the samples due to electrodes blocking the ionic current. However, the local impedance measurements demonstrate that the conductivity profiles near the cathode depend on the electrode material. This finding is in accordance with the SIMS measurements which indicate that the Ag/Cr-electrodes are, in contrast to Au/Cr-electrodes not completely inactive for the oxygen incorporation into the Fe-doped SrTiO3. The results are discussed in terms of defect chemical models.

Abstract: The characteristic features of the generalized chemical potential diagram are discussed and compared with those of chemical equilibrium calculations and of phase diagram calculations. An emphasis is placed on their applications to interfaces between materials having different chemical bonds such as alloys, ceramics, and salts. To provide a reasonable basis for analyzing complicated concentration profiles resulting from interface reactions between high-temperature materials, a simple thermodynamic model based on a local equilibrium approximation is proposed to represent reactive diffusion paths in generalized chemical potential diagrams. Experimentally observed reaction profiles are examined by taking into account kinetic factors such as diffusion, surface reaction rate, and nucleation at a heterogeneous interface.

The effects of doping Co for the Ga site on the oxide ion conductivity of La0.8Sr0.2Ga0.8Mg0.2O3 have been investigated in detail. It was found that doping Co is effective for enhancing the oxide ion conductivity. In particular, a significant increase in conductivity in the low-temperature range was observed. The electrical conductivity was monotonically increased; however, the transport number for the oxide ion decreased with an increasing amount of Co. Considering the transport number and ion transport number, an optimized amount for the Co doping seems to exist at 8.5 mol % for Ga site. The theoretical electromotive forces were exhibited on H2−O2 gas cell utilizing the optimized composition of La0.8Sr0.2Ga0.8Mg0.115Co0.085O3. The diffusion characteristics of the oxide ion in La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 were also investigated by using the ¹⁸O tracer method. Since the diffusion coefficient measured by the ¹⁸O tracer method was similar to that estimated by the electrical conductivity, the conduction of La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 is concluded to be almost ionic. On the other hand, an oxygen permeation measurement suggests that the oxide ion conductivity increased linearly with an increasing amount of Co. Therefore, specimens with Co content higher than 10 mol % can be considered as a superior mixed oxide ion and hole conductor. The UV−vis spectra suggests that the valence number of doped Co was changed from +3 to +2 with decreasing oxygen partial pressure; the origin of hole conduction can thus be assigned to the formation of Co³⁺. Since the amount of dopant in the Ga site was compensated with Mg²⁺, the amount of oxygen deficiency was decreased by doping Co. Therefore, it is likely that the improved oxide ion conductivity observed by doping with Co is brought about by the enhanced mobility of oxide ion.

Abstract: The interfacial reactions of FeO/Cr, Cu2O/Ni, Cu2O/Fe, and Ti/Al2O3 at elevated temperature and the corrosion behavior of low Cr steels in an aqueous system in a CO2 environment were analyzed by applying the chemical potential diagrams for the Fe-Cr-O, Cu-Ni-O, Cu-Fe-O, Ti-Al-O, Fe-C-O-H-e-, and Fe-Cr-C-O-H-e- systems. These diagrams were constructed using the Gibbs energy changes for formation of the compounds involved. The reaction paths and the diffusion behavior of these interfacial reactions were reasonably explained by a simple model based on a steady state approximation and local equilibrium at the interface, which predicts that the reaction path should be represented by simple lines in the chemical potential diagrams.

The interfacial reactions of FeO/Cr, Cu2O/Ni, Cu2O/Fe, and Ti/Al2O3at elevated temperature and the corrosion behavior of low Cr steels in an aqueous system in a CO2environment were analyzed by applying the chemical potential diagrams for the Fe-Cr-O, Cu-Ni-O, Cu-Fe-O, Ti-Al-O, Fe-C-O-H-e-, and Fe-Cr-C-O-H-e- systems. These diagrams were constructed using the Gibbs energy changes for formation of the compounds involved. The reaction paths and the diffusion behavior of these interfacial reactions were reasonably explained by a simple model based on a steady state approximation and local equilibrium at the interface, which predicts that the reaction path should be represented by simple lines in the chemical potential diagrams.

Abstract: For the GaSb single crystals doped with copper (grown using the Czochralski method without encapsulant in flowing atmosphere of hydrogen) the distribution coefficient of copper in GaSb,k eff=0.0021±0.0006 was found and the copper solubility in GaSb was discussed. The region of copper solubility in GaSb was analyzed on the thermodynamic basis using chemical phase diagram in the Sb−Ga−Cu system. Due to a rather low solubility of copper, its excessive amount in GaSb caused probably an increase of the dislocation density at the end of the GaSb single crystals.

Calcium-doped lanthanum chromites, (La1-xCax) (Cr1-y Cay O3, have been synthesized to investigate effects of calcium doping on sinterability and electrical conductivity. X-ray diffractometric results have revealed that in addition to normal perovskites (La1-xCaxCrO3), chromium-deficient perovskites can exist as a single phase in the composition region 0.1

Yttria stabilized zirconia (8 mol%) composites were fabricated by tape casting with either alumina powder or alumina whiskers, and pressureless sintered. Sintering behaviour, ionic conductivity and mechanical strength were analysed. For all compositions analysed, increasing alumina content reduced the sintered density. For whisker-reinforced zirconia, the rigid whiskers prevented matrix densification along their axis. The ionic conductivity was measured by the complex impedance method from 500–1000 °C and the activation energy for ionic conduction calculated over that range. The ionic conductivity of the alumina-zirconia composites decreased with increasing alumina content as expected by the rule of mixtures. However, the ionic conductivity of the whisker-zirconia composites decreased more than expected possibly due to contamination from the whiskers. The strength of the whisker-zirconia composites was also found to be affected by the porosity. At 5 vol%, the average strength was measured at 39.9 kgf mm-2, which decreased to 24 kgf mm-2 at 20 vol%.