Crystal defects and phase transitions of nanocrystalline yttria-stabilised zirconia induced by high-energy ball milling

Abstract

Yttria-stabilised zirconia (YSZ) is a promising electrolyte for SOFCs and gas sensors. In this study, the particle size of a co-precipitated 5 mol% yttria-stabilised zirconia (5 YSZ) powder was refined from 10.47 μm to 130 nm via high-energy ball milling to improve its sinterability and ionic conductivity. The ball milling process increased the specific surface area of the 5 YSZ powder from approximately 11 to 22 m2 g−1. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) results indicated that the 5 YSZ crystallites decomposed into irregular shapes with the evolution of point, linear, and planar defects. An increase in the milling duration increased the number of oxygen defects in the 5 YSZ powder, as revealed by the X-ray photoelectron spectroscopy results. The tetragonal-to-monoclinic phase transformation occurring in the powder was investigated by X-ray diffraction, Raman spectroscopy, HRTEM, and selected-area electron diffraction pattern analyses. The ball-milled powders could be easily densified, but the presence of too many crystal defects and the large fraction of the m-ZrO2 phase were detrimental to the further densification of the 5 YSZ powders. In spite of the high sintering temperature (1500 °C) used in this study, the maximum relative density of 99.67% could be achieved for the powder ball-milled for 60 min at the rotor speed of 1500 rpm. Moreover, the ionic conductivity of 5 YSZ was improved significantly from 20.6 to 36.2 mS cm−1 (850 °C) after the high-energy ball milling process.