Preparation And Characterizations Of T-ZrO₂ And M-ZrO₂ Nanoparticles

ZrO₂ nanoparticles are widely used in the fields of catalysis, glaze materials, coatings and ZrO₂ ceramic materials, due to their excellent physical and chemical properties. However, phases, morphologies, dispersity, particle sizes and size uniformity of ZrO₂ nanoparticles are important factors for the performances of the products. ZrO₂ exists three phases: monoclinic（m）, tetragonal（t）, or cubic（c）. ZrO₂ is also a typical material with a phase-dependent potential application in a number of technologies. For example, due to high oxygen ionic conductivity and phase transformation properties, cubic and tetragonal ZrO₂ are promising candidates for fuel cell electrolytes, oxygen sensors, phase-transformation-toughened structural materials and zirconia-containing nanomaterials, respectively, while monoclinic ZrO₂ is important for catalysis, gate dielectrics, and bioactive coatings on bone implants because of relatively high chemical stability, dielectric constant and good biocompatibility. Morphologies and dispersity of nanoparticles significantly affect the fluidity and filling property of the nanopowders, disperse spherical nanoparticles always have good fluidity and high filling property. Fine nanoparticles have large specific surface areas always result in low temperature sintering ability due to high surface energy. Additionly, uniform size is another important factor for sintering dense nanocrystalline ceramics with uniform microstructures. A narrow particle size distribution is desired because large particles will grow at the expense of small particles during sintering, resulting in coarse-grained ceramics. Therefore, in order to obtain high quality ZrO₂ products or devices, specific pure phase ZrO₂ nanoparticles with fine average size, good dispersity, equiaxial morphology and narrow size distribution should be prepared first. However, nanoparticles always tend to agglomerate together because of huge surface areas and surface energy. The existence of agglomerates will reduce fluidity, catalytic activity and sintering activity of nanopowders, which seriously limits its application. Hence, it is vital to resolve agglomeration of the nanoparticles. Usually, there are two kinds of methods to improve the dispersity of the nanoparticles. One is bottom-up method, taking some measures in the synthesis process to reduce the agglomeration, like using surfactants, the other is top-down method, breaking the bulk materials into nanoparticles by using physical or chemical actions, like ball milling method. Although by a few more elaborate methods such as solvothermal method or sol-gel using zirconium alkoxides and surfactants as precursors, disperse ZrO₂ nanoparticles could be obtained, these methods have their own disadvantages such as low purity of ZrO₂, relatively high cost and difficulty in control of reaction parameters. Therefore, a facile method with low cost for synthesis of high-purity, ultrafine, and agglomerate-free t-ZrO₂ and m-ZrO₂ nanoparticles is necessary.In this work, the starting t-ZrO₂ powders were synthesized by simple precipitation using inorganic zirconium salts as precursors without any surfactants. But the obtained t-ZrO₂ nanoparticles are with severe agglomeration. Then our study offers a corrosion approach to improve the dispersity of t-ZrO₂ nanoparticles. By using corrosion method, disperse pure t-ZrO₂ nanoparticles with average sizes of 4.5 and 6 nm and narrow size distributions of 2-11 and 3-12 nm were produced by calcining the precipitates at 400°C for 2 h and 500°C for 0.5 h and then HCl-corroded at 120°C for 75 h, respectively. Finally, dense m-ZrO₂ nanocrystalline ceramic with an average grain size of 110 nm and a relative density of 99.9% was sintered by two-stage sintering（heating to 1150°C without hold and decreasing to 1000°C with a 10 h hold） using the obtained disperse zirconia nanoparticles calcined at 500°C for 0.5 h and HCl-corroded at 120°C for 75 h with size ranging from 3-12 nm as raw materials.Compared to t-ZrO₂ nanoparticles, synthesis of pure m-ZrO₂ nanoparticles smaller than 10 nm is a more challenging task, few reports can be found in the literature, where disperse fine m-ZrO₂ nanoparticles have been produced. Since the tetragonal phase is stable at room temperature as the consequence of the dominance of the surface energy contribution to the Gibbs free energy of formation in this size range. In this work, we have adopted two methods to prepare m-ZrO₂ nanoparticles. One method is that ZrO₂ powders coexisting monoclinic and tetragonal phase were prepared by precipitation method using zirconium oxychloride and NH4 OH as raw materials and then m-ZrO₂ nanoparticles were obtained by using H2SO4 corrosion in an autoclave at 120°C for 75 h. Although the obtained m-ZrO₂ nanoparticles are dispersed, the morphologies are irregular. The other is hydrothermal method. By adjusting the kinds of mineralizers and the concentrations of the zirconium oxychloride solutions, finally disperse pure m-ZrO₂ nanoparticles with an average size of 4.6 nm and size distribution of 2-11 nm were obtained by using urea as mineralizer and reacting in an autoclave at 180°C for 6 h. Using this nanoparticles as raw material, pure m-ZrO₂ nanocrystalline ceramic with a relative density of 95% and an average grain size of 95 nm was prepared by applying a two-step pressureless heating schedule（heating to 950°C with a 10 h hold and increasing to 1150°C with a 1 h hold）.