HRTEM STUDY OF NANOCRYSTALLINE ZIRCONIA POWDERS

Zirconia powders with grain sizes between 4 and 35 nm were synthesized by the gas condensation technique. Different qualities (grain size, g rain size distribution) of powders have been observed and can be attri buted to different synthesis conditions (mainly temperature gradients) within the UHV chamber. The existence of necks between single particl es has been observed. These necks are due to a first sintering process during the oxidation of the metal/suboxide clusters condensed on the walls in the synthesis chamber. Several stages of this sintering proce ss are documented by high-resolution micrographs. Furthermore, some pa rticles exhibit a shell-like structure with monoclinic ZrO2 in the out er region and tetragonal ZrO2 in the inner region. This is an indicati on of (i) the martensitic phase transition character of zirconia in ge neral, and (ii) the existence of a critical particle size in n-ZrO2 re sponsible for the metastability of a tetragonal polymorph in nanosized zirconia powder. Lattice distortions in some directions can be conclu ded from image reconstruction of the digitized micrograph of a single nanosized ZrO2 particle.