Co-doping effects of Y3+ and Sc3+ on the crystal structures, nanoparticle properties and electrical behavior of ZrO2 solid solutions prepared by a mild urea-based hydrothermal method

Abstract

Weakly agglomerated nanocrystalline (ZrO₂)_0.92(Y₂O₃)_{0.08 − x}(Sc₂O₃)_x (x = 0–0.08) powders (8.4–9.9 nm in size) in a cubic structure with high surface area (145–153 m² g⁻¹) were synthesized by a two-step hydrothermal process in the presence of urea: a stock solution of metal nitrates and urea was heated at 80 °C for 24 h and then at 180 °C for 48 h. The co-doping effect of Y³⁺ and Sc³⁺ ions on the crystal structures, nanoparticle properties (crystallite size, surface area and microstrain), and sintering and electrical behaviors of the (ZrO₂)_0.92(Y₂O₃)_{0.08 − x}(Sc₂O₃)_x solid solutions were systematically investigated by means of X-ray diffraction and transmission electron microscopy combined with energy dispersive X-ray analysis, scanning electron microscopy, BET specific area analysis and ac impedance spectroscopy. The lattice parameter and average crystallite size of the as-prepared nanocrystallites tend to decrease with increasing scandia content. In the Arrhenius plots over the measurement temperature range of 400–800 °C, a curvature towards higher activation energy with decreasing temperature appears, which is remarkable for the compacted sample doped with 8 mol% Sc₂O₃ (yttria-free) after sintering at 1400 °C, although not for those doped with 8 mol% Y₂O₃ (scandia-free) and with 1–7 mol% Sc₂O₃. When scandia is substituted by yttria, the lattice conductivity decreases monotonically due to the difference in the sizes of Y³⁺ and Sc³⁺.