Perturbed-angular-correlation study of phase transformations in nanoscaledAl2O3-coated and noncoatedZrO2particles synthesized in a microwave plasma

Abstract

The phase transformations of nanoscaled (n-) ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-coated and noncoated ${\mathrm{ZrO}}_{2}$ particles synthesized in a microwave plasma have been investigated by perturbed-angular-correlation (PAC) measurements of the electric quadrupole interaction (QI) of ${}^{181}\mathrm{Ta}$ on Zr sites between 290 and 1600 K. For the phase identification and structural characterization the QI parameters of the nanoscaled particles are compared to those of coarse-grained ${\mathrm{ZrO}}_{2}$ which were measured between 290 and 2160 K. The PAC spectra of the nanoscaled particles in the as-prepared state are characterized by a broad distribution of strong, axially asymmetric QI's, which reflects a highly disordered oxygen environment of the Zr sites. Upon annealing, the tetragonal phase is the first well-crystallized structure to emerge at about 500 K, both in coated and noncoated $n\ensuremath{-}{\mathrm{ZrO}}_{2},$ in contrast to the previously reported annealing reaction of $n\ensuremath{-}{\mathrm{ZrO}}_{2}$ synthesized by gas-phase condensation in which the tetragonal phase has not been observed. This disorder-order transformation is partially reversible upon cooling. In $n\ensuremath{-}{\mathrm{ZrO}}_{2}{/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}$ synthesized in a microwave plasma the monoclinic phase can be completely suppressed up to 1600 K. In the noncoated particles monoclinic ${\mathrm{ZrO}}_{2}$ starts to appear at 600 K. At 1400 K the transformation from the monoclinic to the tetragonal phase of noncoated $n\ensuremath{-}{\mathrm{ZrO}}_{2}$ was observed to occur with a transformation rate of $\ensuremath{\lambda}=6.9(1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ In nanoscaled coated and noncoated tetragonal ${\mathrm{ZrO}}_{2}$ the ${T}^{3/2}$-temperature dependence of the quadrupole frequency ${v}_{q}$ is weaker than in the same phase of the coarse-grained compound, suggesting a decrease of the mean-square vibrational amplitudes with decreasing particle size.