Advanced Approaches for Producing Nanocrystalline and Fine-Grained ZrO2-Based Powders (Review) II. Wet Chemistry Methods: Hydrothermal, Solvothermal, and Supercritical Water Synthesis

The third part of the review describes wet chemistry methods that involve the application of pressure to the starting solutions. The synthesis of nanocrystalline powders from both unstabilized ZrO₂ and ZrO₂-based systems is discussed. The influence of acidic and alkaline environments on powder morphology is examined. Crystallization under hydrothermal conditions promotes the formation of hierarchical m-ZrO₂ nanorods. The properties of m-ZrO₂ powders produced by the reflux method and by hydrothermal synthesis in acidic and alkaline environments are compared. Hydrothermal homogeneous precipitation in the presence of urea is considered. Microwave heating is identified as an effective approach to increase the crystallinity of hydrothermally synthesized powders and to shorten the reaction time. The process of depositing ZrO₂ coatings on ZrB₂ powders and the hydrothermal corrosion method are described. In the solvothermal (glycothermal) method, organic compounds are used. The advantages of this method include the production of high-purity monodisperse powders. Spherical or rounded particles, nanorods, interconnected nanorods, and nanosheets are synthesized using this approach. Hydrothermal synthesis in supercritical conditions utilizes the properties of water at its critical point (374°C and 22.1 MPa), which facilitates uniform nucleation of primary nanoparticles. The particle morphology and size distribution are found to be influenced by parameters such as reaction temperature, pressure, time, precursor concentration, and pH. The powders synthesized with the methods reviewed are employed in the design of humidity and gas sensors, photocatalysts, functional materials for optical and medical applications, solid oxide fuel cells, thermal barrier coatings, and materials for the automotive industry.