Phases and phase transformations in nanocrystalline ZrO2

Starting from results from He-pycnometry, electron diffraction, Extended X-ray Absorption Fine Structure Spectroscopy and Perturbed Angular Correlation Spectroscopy the phase transformations and structures of zirconia are described. From a comparison of these results with those obtained on other oxide nanoparticles it is concluded that the phases and structure of nanoparticles are different compared to those of coarse-grained material. The difference of the transformation temperature of bare and coated nanoparticles was used to estimate enthalpy and entropy of the tetragonal → monoclinic transformation for nanoparticulate zirconia. By comparison with results obtained from other nanocrystalline oxides, the following rules were derived: Provided the particles are sufficiently small, particles made of materials showing phase transitions crystallize in the high temperature structure. However, compared to coarse-grained materials of the same structure, the density of nanoparticles is reduced. A first estimation limits this phenomenon to particle sizes well below 10 nm. Those nanoparticles follow the generalized phase diagram postulated by Tammann