The electronic structure of α-Al2O2 grain boundaries containing reactive element segregants

Abstract

It has long been known that the addition of small quantities (“doping”) of so-called reactive elements (RE) such as Y, Zr, and Hf to high-temperature Al2O3 scale-forming alloys improves oxidation resistance. The presence of reactive elements at grain boundaries lowers the growth rate of the α-Al2O3 scales, but the cause of the reduced scale growth kinetics is not fully understood. Explanations based on steric effects and explanations based on reducing the grain boundary electronic conductivity have been proposed. We have used density functional theory to study the structural and electronic properties of two Σ7 bicrystal grain boundaries containing Y, Hf, and Zr substitutional defects on Al sites. The presence of RE substitutional defects plays a minimal direct role in reducing the density of electronic states near the valence-band maximum. However, Hf4+ or Zr4+ substitutions at the grain boundary repel the positively charged oxygen vacancy VO2+. As VO2+ contributes a defect state above the valence-band maximum but below the Fermi energy, this indirectly lowers the density of current carrying holes and thus the electronic conductivity of the grain boundary. Replacing Al3+ ions with Hf4+ or Zr4+ ions also makes the grain boundary positively charged, further reducing the hole density.