The influence of pH on the formation of microstructure and electrochemical characteristics of solid electrolytes 5 YSZ

This paper examines how precipitation pH affects the phase stability, microstructure, and electrochemical characteristics of solid electrolytes based on zirconia stabilized with 5 mol.% yttria (5 YSZ), which we synthesized by chemical coprecipitation. We obtained precursors under three pH conditions (acidic pH < 2, near-neutral pH = 3–7, and alkaline pH - 10) using nitrate salts and NH4OH as a precipitant. We characterized the products by SEM, X-ray diffraction, Raman, and IR spectroscopy, as well as DC conductivity measurements. The results demonstrate that precipitation pH critically determines the homogeneity of Y3+ ion distribution in the precursor. Acidic conditions hinder yttrium coprecipitation, resulting in pronounced segregation of the elements and a high content of the monoclinic phase after calcination and milling. Alkaline conditions ensure quantitative precipitation but cause partial compositional fluctuations and severe agglomeration, so they necessitate intensive milling, which induces lattice deformations and phase transformations. Near-neutral conditions (pH 5–6) promote homogeneous coprecipitation of Zr4+ and Y3+ ions, forming precursors with a uniform element distribution and weakly agglomerated particles. This homogeneity ensures the complete removal of coordinated hydroxyl groups during heat treatment, maintaining a high concentration of oxygen vacancies and the stability of the tetragonal phase. Ceramics that we sintered from powder obtained under neutral conditions achieve relative density (98.7%) and ionic conductivity (36.4 mS/cm at 850 °C). Solid oxide electrolytes based on stabilized zirconium dioxide function widely as materials for parameter control sensors (oxygen sensors in propulsion systems) and also serve as the basis for the creation of solid oxide fuel cells.