Atomic structural environment of grain boundary segregated Y and Zr in creep resistant alumina from EXAFS

Abstract

Dopants Y and Zr at 100 p.p.m. level (Y or Zr to Al atomic ratio) in ultra-high purity polycrystalline alumina have been found to be mainly segregated to the alumina grain boundaries. The atomic structural environments around the Y and Zr segregants have been investigated by Extended X-ray Absorption Fine Structure (EXAFS). On average, Y ions in α-Al 2O 3 grain boundaries are coordinated by four oxygens, at a distance of 2.30 Å, which corresponds nearly to the Y–O bond length in cubic Y 2O 3, and Zr ions are coordinated by five oxygens at a distance of 2.14 Å, which is approximately the same as the average Zr–O bond length in monoclinic ZrO 2. However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at 100 p.p.m. concentrations in α-Al 2O 3 occupy grain boundary sites with well defined nearest neighbor cation–oxygen bond lengths similar to those in their parent oxides but with the next nearest neighbor cation–cation distances varying considerably from site to site. From EXAFS, the Y grain boundary saturation concentration is estimated to be 6.0 atoms/nm 2, which is consistent with the estimate from STEM of 4.4 atoms/nm 2. The differences in the Zr–O and Y–O nearest neighbor distances and coordination numbers in Al 2O 3 grain boundaries are related to the Y–Al and Zr–Al size mismatches.