On the size-dependent phase transformation in nanoparticulate zirconia

We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and undergo a phase transformation when the particle 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 change in the driving force for coarsening. Grain size distributions below the critical size for the transforrnation are log-normal, but as the transformation 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 forces are restored after the phase transformation is complete.