Effect of Cu2+ ion incorporation on the phase development of ZrO2-type solid solutions during the thermal treatments

Abstract

The amorphous precursors of the ZrO2–CuO system at the ZrO2-rich side of the concentration range were prepared by co-precipitation from aqueous solutions of the corresponding salts. Thermal behavior of the amorphous precursors was monitored using X-ray powder diffraction, Raman spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray spectrometry, differential scanning calorimetry and thermogravimetric analysis. The crystallization temperature of the amorphous precursors rose with an increase in the CuO content for ∼180 °C. Maximum solubility of Cu2+ ions in the ZrO2 lattice (∼23 mol%) occurs in the metastable products obtained upon crystallization of the amorphous precursors. The results of Raman spectroscopy indicate that the incorporation of Cu2+ ions stabilize the tetragonal ZrO2 polymorph. A precise determination of lattice parameters, using both Rietveld and Le Bail refinements of the powder diffraction patterns, showed that the axial ratio c/a in the ZrO2-type solid solutions with a Cu2+ content ≥20 mol% approach 1 (formation of t″-form of tetragonal phase). The terminal solid solubility limit of Cu2+ ions in the ZrO2 lattice rapidly drops with an increase in treatment temperature (up to 1000 °C) that is followed by the formation of and increase in phases structurally closely related to tenorite and monoclinic ZrO2. Low thermal stability of the t-ZrO2-type phase was attributed to the reduction of the sintering temperature in the presence of CuO and a significant difference in size and shape of zirconia and tenorite particles, which prevent surface interactions.