THERMOCHEMICAL REDUCTION OF YTTRIA-STABILIZED-ZIRCONIA CRYSTALS - OPTICAL AND ELECTRON-MICROSCOPY

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, lum inescence, and analytical electron microscopy. After TCR, undoped crys tals show an extinction band centered at about 1.8 eV which is attribu ted to Mie scattering from precipitates with an average size of 10 nm, as observed by analytical transmission electron microscopy. In crysta ls 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 Go-doped crys tals a broad band at 0.9 eV, emerging at 1750 K, was attributed to met allic colloids observed to have an average dimension of 200 nm. In all cases, the precipitate distribution is very inhomogeneous. These resu lts agree qualitatively with the predictions of the Mie theory for the extinction bands associated with metal colloids embedded in the matri x. Electron microscope observations indicate that the degradation of t he optical properties experienced by previously reduced YSZ crystals d uring oxidation at elevated temperatures is a bulk effect probably rel ated to cavities and dislocations.