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, grain size distribution) of powders have been observed and can be attributed to different synthesis conditions (mainly temperature gradients) within the UHV chamber. The existence of necks between single particles 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 process are documented by high-resolution micrographs. Furthermore, some particles exhibit a shell-like structure with monoclinic ZrO2 in the outer region and tetragonal ZrO2 in the inner region. This is an indication of (i) the martensitic phase transition character of zirconia in general, and (ii) the existence of a critical particle size in n-ZrO2 responsible for the metastability of a tetragonal polymorph in nanosized zirconia powder. Lattice distortions in some directions can be concluded from image reconstruction of the digitized micrograph of a single nanosized ZrO2 particle.