Recent advances in supercritical hydrothermal preparation and modification of nano-zirconia

With the development of aviation gas turbines in the aviation engine industry towards high flow ratio, high thrust-to-weight ratio, high inlet temperature, and the commercialization of 5G technology in the electronics industry, strict requirements are placed on the performance of industrial materials. Due to excellent mechanical properties (strong fracture toughness and strength), thermal stability (low thermal conductivity, matching thermal expansion coefficient), and chemical catalytic potential (tunable surface acidity and strong metal-support interaction), nano-zirconia has been widely used in high-tech fields such as structural ceramics, thermal barrier coatings, denture materials, catalysts, and solid fuel cell electrolytes. Traditional nanomaterial technology has a slow reaction rate, which is time-consuming, and mostly requires calcination to remove some impurities. Therefore, a preparation method with a simple process, fast reaction rate, and low cost is urgently needed. Supercritical water (SCW) with low density and low dielectric results in a significant decrease in the number and strength of hydrogen bonds, exhibiting similar non-polar character as organic substances, thereby enabling near-complete miscibility with a wide range of organic compounds and gases. Due to high nucleation rate and fast reaction rate, supercritical hydrothermal synthesis (SCHS) technology can provide a new choice for the controllable preparation of nano-zirconia. In this paper, the relationship between the structural characteristics (particle size and crystal form) and properties (mechanical, thermal, catalytic, electrical, and optical properties) of nano-zirconia is introduced. The failure mechanisms of zirconia materials, including sintering densification, high-temperature phase transformation, low-temperature degradation, and thermo-mechanical stress mismatch, were revealed. Further, the flow heat transfer and crystal growth laws in the supercritical hydrothermal synthesis of zirconia were summarized. It can be found that the mixing state determines the quality of the synthesized nanoparticles (particle size and size distribution), and the nucleation process consumes more energy than the chemical reaction and growth stages. Furthermore, the crucial functionalization strategies, including surface modification and ion doping, used for the synthesis of nano-zirconia materials were thoroughly analyzed. On the one hand, different surface modifications can change the physical and chemical properties of the powder surface, such as surface atomic layer structure and functional groups, surface hydrophobicity, wetting and adhesion properties, etc. On the other hand, ion doping can break through the intrinsic performance limitations of pure zirconia. Moreover, the industrialization of supercritical hydrothermal synthesis technology was evaluated, and the design concept of the first domestic supercritical hydrothermal synthesis of nano-zirconia pilot plant with the largest Reynolds number built by Xi′an Jiaotong University was emphasized. Finally, suggestions and prospects from three dimensions were provided, including core equipment breakthroughs, synthesis process optimization, and industrialization. In terms of the core reactor, subsequent research is needed to experimentally validate computational models for other mixers and improve mixers to prevent nanoparticle clogging. For modification, the failure of surface modifiers at high temperatures and the hydrolysis matching of precursor ion doping deserve attention. With regard to scale-up, further work should focus on the application and improvement of large-scale supercritical hydrothermal zirconia synthesis plants with integrated energy systems. In addition, the resource utilization of materials in the reaction water is another focus.