Phase Transformation as a Function of Particle Size in Nanocrystalline Zirconia

Abstract

ABSTRACTBulk zirconia undergoes a pressure-induced transformation from a (low pressure) monoclinic phase to a high pressure tetragonal phase, at around 3GPa (above 900K). 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 the small tetragonal particles transform to the monoclinic structure at about the critical size. Coarsening under these conditions produces irregular particle size distributions. We estimate that the surface-stress induced internal pressure in the tetragonal nanoparticles can be as high as 5 GPa and the corresponding surface stress is about 6N/m.