Synthesis of ZrO_2 Nanopartides by Freeze Drying

摘要

The interest in zirconia nanoparticles for either structural or functional applications depending on the stabilized phases has promoted the search for different synthesis methods. One of these methods is freeze drying. The present work deals with the preparation of zirconia nanoparticles by freeze drying using ZrOCl_2 solutions. The freeze-dried powders consist of non-spherical granules made up of soft agglomerates of ZrO_2 with a d_(BET) down to 40 nm and specific surface area values of 2-27 m~2 /depending on the thermal treatment. Granules have a low remaining porosity formed as a consequence of the sublimation of ice. The thermal treatment determined the crystalline phase formed, and so a study of the phase transformations with temperature has been carried out by X-ray diffraction and transmission electron microscopy. The results indicate that at temperatures ≤400℃ and a short duration of treatment, t-ZrO_2 is obtained with an acicular shape, while higher temperatures lead to the transformation into a monoclinic phase, which is accomplished by grain size growth, larger aspect ratio, and increased agglomeration. After sonication of the diluted suspensions, monomodal particle size distributions are obtained, with average sizes of ≦ 100 nm, thus demonstrating that agglomerates are soft and can be easily broken down.