ADVANCED APPROACHES FOR PRODUCING NANOCRYSTALLINE  AND FINE-GRAINED ZrO₂-BASED POWDERS (REVIEW) 
II. WET CHEMISTRY METHODS: COPRECIPITATION, SOL–GEL PROCESS, AND PECHINI METHOD

Abstract

This part of the review focuses on wet chemistry methods that do not involve pressure effects on the starting components. The features of synthesizing nanocrystalline powders of both unstabilized ZrO₂ and ZrO₂-based systems are described. Under precipitation, the synthesis completeness and physicochemical properties of the powders depend on the concentration of reagents, pH of the environment, precipitate washing quality, and drying methods. For deagglomeration of powders, high-energy grinding in ball mills, azeotropic distillation in an aliphatic alcohol environment with more than three carbon atoms, ultrahigh-frequency radiation, pulsed magnetic field, and their combinations with ultrasonic treatment in the powder synthesis process are employed. Coprecipitation using microemulsions was developed to produce powders with spherical particle morphology. The sol–gel process relies on polycondensation and hydrolysis processes and is believed to be a simple method to synthesize homogeneous powders of complex composition. The use of microwave heating in the powder sol-gel synthesis process and the influence of synthesis conditions on powder sinterability are shown. The preparation of mesoporous ZrO₂-based materials, which are promising for the development of catalysts for their special structural characteristics, is considered. The Pechini method (citrate method, polymer complex method) is used to synthesize highly homogeneous and superfine oxide materials, employing complexation and intermediate preparation of a polymer gel. The properties of powders produced through various modifications of the Pechini method are compared. The potential for producing nanosheets is shown. Zirconia-based powders synthesized with the discussed methods were used in the development of sorbents and photocatalysts, solid oxide fuel cells, various optical materials, advanced thermal barrier coatings, nanofiltration membranes, nanostructured amorphous composites with high strength, and functional materials for medical applications, particularly for diagnosis and therapy.