NanoZirconia (ZrO2 nanoparticles) has been used as a catalyst in various organic reactions, in photocatalytic and piezoelectric applications, as well as dental and optical coatings. It exists in three crystallographic phases; cubic, tetragonal and monoclinic. Within these forms, monoclinic zirconia is the most thermodynamically stable phase at room temperature. The other phases, cubic and tetragonal are stable at high temperatures. However, by reducing the particle size to nano-range or by doping the zirconia, the high temperature phases can be stabilized at room temperature.
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Nanozirconia can be synthesized by various methods such as sol-gel, hydrothermal, solvothermal, and thermal decomposition methods and through the pyrolysis of zirconium oxychloride salt precursor. Using these methods, the size and shape of the zirconia nanoparticles can be controlled. Researchers have reported the synthesis of tetragonal nanozirconia from the precursor zirconyl hydroxide. It was found that the obtained hydrogel was calcined at 600°C which led to the formation of nanocrystalline zirconia in the tetragonal phase. Dehydration and dissolution-reprecipitation were the reason behind the stabilization of small crystals of tetragonal zirconia. To prevent the formation of the stable monoclinic phase, the fabrication was carried out in a glass vessel in basic solutions. Nanozirconia consists of discrete metal oxide clusters enclosed by surface hydroxyl groups. The hydroxyl groups are the reactive parts of the molecule therefore they form an integral part of the matrix and can react with acid or base.
Nanozirconia has many remarkable properties. These include high thermal expansion coefficient, low thermal conductivity, superior mechanical characteristics, increased fracture toughness, strength, hardness, phase stability, good chemical resistance, and superplastic deformation. Because of this, they are being utilized in various industrial and engineering applications like high durability coating, medical prosthetics, cutting tools, synthetic jewellery, high density grinding media, wear components, seals, valves, aviation engines, tiles for space shuttles, missiles and many more. They are used in solid oxide fuel cells and in gas sensors of O2 and NOx. The stabilized nanozirconia is also suited for high temperature energy conversion systems, due to its high oxygen ion transport ability and good long-term stability.
Nanozirconia also has many prospective biomedical applications like in anticancer, antioxidant activities and cytotoxicity via reactive oxygen species. Researchers have reported the synthesis of small spherical shaped nanozirconia by using E. globulus leaf extract improved bioactivity.
Furthermore, Nanozirconia has found application in the ceramic dental material. There are different types of zirconia ceramics material that are available for dental applications such zirconia ceramics are yttrium tetragonal zirconia polycrystals, glass-infiltrated zirconia-toughened alumina, magnesia partially stabilized zirconia and zirconia-containing lithium silicate ceramics. Researchers have reported that tetragonal-zirconia (t-ZrO2) is capable of controlling the adsorption activity and can alter the pH of the external solution and can also be used in a number of radiochemical separation and concentration procedures.
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