On the size-dependent phase transformation in nanoparticulate zirconia

We have studied the structures of zirconia nanoparticles formed by plasma-s praying an organo-metallic precursor. Inspection of the particles in the TE M reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, whi le larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the z irconia particles coarsen and undergo a phase transformation when the parti cle size is of the order of 18 nm, for reasons associated with the surface energy, and the occurrence of this phase transformation produces a sudden c hange in the driving force for coarsening. Grain size distributions below t he critical size for the transformation are log-normal, but as the transfor mation occurs, the size distribution changes to a markedly less skewed form . The development of this distribution is followed to establish whether it grows self-similarly, or returns to log-normality once normal driving force s are restored after the phase transformation is complete. (C) 2000 Elsevie r Science S.A. All rights reserved.