Phase development of the ZrO2–ZnO system during the thermal treatments of amorphous precursors

Thermal behavior of the amorphous precursors of the ZrO2–ZnO system on the ZrO2-rich side of the concentration range, co-precipitated from aqueous solutions of the corresponding salts, was monitored using X-ray powder diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray spectrometry, differential scanning calorimetry and thermogravimetric analysis. The crystallization temperature of the amorphous precursors increased with an increase in the ZnO content, from 457 °C (0 mol% ZnO) to 548 °C (25 mol% ZnO). Maximum solubility of Zn2+ ions in the ZrO2 lattice (∼25 mol%) occurred in the metastable products obtained upon crystallization of the amorphous precursors. Raman spectroscopy indicates that the incorporation of Zn2+ ions can partially stabilize only the tetragonal ZrO2. A precise determination of unit-cell parameters of the t-ZrO2-type solid solutions, using both Rietveld and Le Bail refinements of the powder diffraction patterns, shows that the increase in the Zn2+ content causes a decrease in c-ax, which in a solid solution with a Zn2+ content above 20 mol% approaches very closely a-ax. The thermal treatment of the crystallization products (up to 1000 °C) leads to a rapid decrease in the terminal solid solubility limit of Zn2+ ions in the ZrO2 lattice that is followed by the partial evaporation of zinc, the formation of and increase in phases structurally closely related to zincite and monoclinic ZrO2. The results of micro-structural analysis indicate that the presence of ZnO promotes the sintering of the ZrO2 crystallization products.