Defects and porosity in zirconia-based nanomaterials: a study by slow-positron beam technique

A variety of advantageous thermal, electrical and mechanical properties of zirconium di-oxide (ZrO2, zirconia) make zirconia-based materials widely used in many industrial areas, in particular, in ceramic industry and other high-temperature applications. Doping of the ZrO2 host lattice by proper metal cations is a prerequisite of stabilisation of the high- temperature cubic and tetragonal phases down to room temperature as well as improvement of other functional properties. The use of nanopowders as initial substances in manufacturing ZrO2-based nanoceramics by sintering leads to well-homogenised materials of a low porosity. Due to an appreciable volume fraction of grain boundaries (GBs), pores and nanometer-sized open-volume defects associated to GBs become significant in nanopowders. Obviously, positron as well as positronium (Ps) atom becomes efficient
probes of microstructure evolution during production of ZrO2-based functional nanomaterials by sintering.
In the present contribution, investigation of several zirconia-based nanopowders as well as ceramics, obtained by sintering these nanopowders, will be reported. Nanopowders under study were doped with metal cations of various valency (Mg2+, Y3+, Cr3+, Ce4+) and differed also in thermal treatment. Doppler broadening (DB) measurements using slow-positron beam were conducted in the positron energy E ranging from 0.03 eV to 35 keV and the ordinary S and W shape parameters as well as the relative 3γ fractions were evaluated as functions of E. In Figure, an example of measured S(E) curves is given illustrating the sintering induced disappearance of open volume defects and para-Ps formation as well as grain growth could be observed. The VEPFIT models were fitted to the measured S(E), W(E) curves. The DB experiments were supplemented with the conventional positron lifetime, X-ray diffraction (XRD) and mass-density (MD) measurements. Nature and depth distributions of open-volume defects will be discussed on the basis of the slowpositron beam results correlated with the data on positron lifetimes, XRD and MD.