Thermochemical reduction of yttria-stabilized-zirconia crystals: Optical and electron microscopy

Abstract

Thermochemical reduction (TCR) at high temperatures has been performed on yttria-stabilized zirconia (YSZ), undoped and doped with Ni:Er, Co, or Pr. Investigations were carried out using optical absorption, luminescence, and analytical electron microscopy. After TCR, undoped crystals show an extinction band centered at about 1.8 eV which is attributed to Mie scattering from precipitates with an average size of 10 nm, as observed by analytical transmission electron microscopy. In crystals codoped with Ni:Er, TCR induced a broad extinction band centered at 1.3 eV beginning at 1600 K. Metallic Ni colloids contributing to Mie scattering have an average size of 100 nm. Similarly, in Co-doped crystals a broad band at 0.9 eV, emerging at 1750 K, was attributed to metallic colloids observed to have an average dimension of 200 nm. In all cases, the precipitate distribution is very inhomogeneous. These results agree qualitatively with the predictions of the Mie theory for the extinction bands associated with metal colloids embedded in the matrix. Electron microscope observations indicate that the degradation of the optical properties experienced by previously reduced YSZ crystals during oxidation at elevated temperatures is a bulk effect probably related to cavities and dislocations.