Semiconductive α-Al2O3/Sr3Al2O6 Oxide Layer Formed on Fe–Cr–Al Alloy

Abstract

The surface oxide layer of Fe–Cr–Al alloy was modified for use as the metal support of SOFCs. The Fe–Cr–Al alloy was firstly coated with LSCF and pre-heat-treated in a vacuum at 1000 °C for 1 h. In comparison with heat treatment at 700 °C, in which the surface oxide was identified as columnar γ-Al2O3/Sr3Al2O6, durability was greatly improved while maintaining electric conductivity at the same level. High-resolution STEM/TEM analysis revealed that the morphology was a similar columnar structure arranged in the same direction. The primary component was α-Al2O3 polycrystal (>80%) and the secondary component was Sr3Al2O6 (∼15%). After the formation of columnar γ-Al2O3/Sr3Al2O6 in the low temperature range (<800 °C), γ-Al2O3 was transformed to a more stable α-Al2O3 by increasing the temperature to 1000 °C. Ionic diffusion paths, which cause growth of the oxide during operation, were reduced in the columnar structure. Electronic conduction similarly occurred for both γ-Al2O3/Sr3Al2O6 and α-Al2O3/Sr3Al2O6, which was enhanced along the interface between Al2O3 and Sr3Al2O6. Thus, we obtained a stable semiconductive Al2O-based surface oxide layer with a columnar α-Al2O3/Sr3Al2O6 structure.