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

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 oC. 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 oC) 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.