Defect studies of nanocrystalline zirconia powders and sintered ceramics

Abstract

The main objective of the present paper is to communicate a study of defects behavior in zirconia-based nanomaterials---pressure-compacted yttria-stabilized zirconia (YSZ) nanopowders with different contents of ${\text{Y}}_{2}{\text{O}}_{3}$ and ceramics obtained by sintering the YZS nanopowders. In addition, YZS single crystals were also investigated. Positron annihilation techniques including positron lifetime and coincidence Doppler broadening with a conventional positron source and Doppler broadening experiments on a monoenergetic positron beam were involved in this study as the principal tools. These techniques were supplemented with transmission electron microscopy and x-ray diffraction observations. In order to get better support of the experimental data interpretation, the state-of-art theoretical calculations of positron parameters were performed for the perfect ${\text{ZrO}}_{2}$ lattice and selected defect configurations in the YSZ. Theoretical calculations have indicated that neither the oxygen vacancies nor their neutral complexes with substitutional yttrium atoms are capable of positron trapping. On the other hand, the zirconium vacancies are deep positron traps and obviously are responsible for the saturated positron trapping observed in the YSZ single crystals. In the compacted YSZ nanopowders, a majority of positrons is trapped either in the vacancylike defects situated in the negative space-charge layers along grain boundaries $({\ensuremath{\tau}}_{1}\ensuremath{\approx}185\text{ }\text{ps})$ or in vacancy clusters at intersections of grain boundaries $({\ensuremath{\tau}}_{2}\ensuremath{\approx}370\text{ }\text{ps})$. The intensity ratio ${I}_{2}/{I}_{1}$ was found to be correlated with the mean grain size $d$ as ${I}_{2}/{I}_{1}\ensuremath{\sim}{d}^{\ensuremath{-}2}$. A small fraction of positrons $(\ensuremath{\approx}10\mathrm{%})$ form positronium in large pores $({\ensuremath{\tau}}_{3}\ensuremath{\approx}2\text{ }\text{ns},\text{ }{\ensuremath{\tau}}_{4}\ensuremath{\approx}30\text{ }\text{ns})$. A significant grain growth during sintering of the YSZ nanopowders above $1000\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ was observed.