Thermo-stimulated evolution of crystalline structure and dopant distribution in Cu-doped Y-stabilized ZrO2 nanopowders

Abstract

This work deals with the effect of Cu doping on thermal stability of the structural properties of Y-stabilized ZrO2 nanopowders and dopants’ spatial distribution. The powders were synthesized by a co-precipitation technique, calcinated at Tc = 500–1100 °C during 2 h and studied by x-ray diffraction (XRD) and transmission electron microscopy. Calcination at Tc = 500 °C results in the formation of ZrO2 nanocrystals with tetragonal phase predominantly. The shifts of XRD peak positions of Cu-doped powders to larger angles in comparison with those of Cu-free ones testify to the Cu presence inside nanocrystals. The Tc increase results in two main processes: (i) the non-monotonic shift of XRD peak positions and (ii) the phase transformation (tetragonal to cubic and both of them to monoclinic). This observation was explained by, at first, Cu atoms incorporation into the nanocrystal volume from the surface complexes (Tc = 500–700 °C) and then their outward diffusion followed by the formation of crystalline CuO (Tc > 700 °C). Phase transformation sets in at Tc = 700 °C, when monoclinic phase appears. Its contribution rises till Tc = 1000 °C. The mechanism of monoclinic phase formation is supposed to be consisted of the out-diffusion of interstitial Cu ions due to their shift from lattice sites. This promotes an appearance of the channels for Y out-diffusion via cation vacancies and results in phase transformation. The sintering process stimulated by CuO formation is proposed to be responsible for appearance of cubic phase at 1000–1100 °C.