Hydrothermal Preparation Of Low-dimensional Zirconia Crystal And Its Growth Mechanism

Low-dimensional zirconia crystal has been widely used in many fields such as optical-electronics component, catalyzer, sensors, cells, functional composites. Therefore, the preparation of one-dimension and zero-dimension zirconia crystal becomes a hot area of research. At present, although certain work has been carried on the preparation by hydrothermal method, studies on the influence factors are not comprehensive and the growth mechanisms of zirconia crystal with different dimensions still need to be investigated further. To solve these above problems, it is necessary to study the factors and mechanism thoroughly and provide a reasonable explanation to its growth mechanisms. The main research contents are as follows:First, the ideal short rod-like zirconia crystal is prepared and the processing of hydrothermal method is optimized as: 250℃/48 hours/10M NaOH. The influence of reaction temperature time mineral concentration, adopted material precursor solvent are analyzed.Under the condition of 48 hours and 10M NaOH, at the temperature of 150℃, the product is spherical particles. Increasing the temperature above 200℃, the length of crystal varies little, while the length/width ratio increases.Under the condition of 250℃, 1M NaOH and less than 48 hours, the length of rod-like crystal increases with the increase of reaction time. While the time continues to increase, the rod-like crystal grows gradually to laminar one.Under the condition of 250℃ and 48 hours, the length of rod-like crystal increases and the ratio of length/width decreases with rising mineral concentration.Under the condition of 250℃,48 hours and 10M NaOH, adopting the PEG makes the ratio of length/width increase in comparison with the unadopted conditions. While adopting the TiO2, the rod-like zirconia crystal grows to thin and longer rod with the ratio of length/width of 37:1. This should be attributed to the absorption of TiO2 unit to the surface of zirconia changing the superposition rate and preferredorientation.The structure of zirconia gel varies with different pH values in the co-precipitation process, which in turn would have effects on the crystal growth in hydrothermal environment. The precursor obtained under pH=9.5 possesses a higher tetragonal content and a smaller particle size, which is in favor of the growth of one-dimensional zirconia. When the reactant between Zr(NCh)4 and NaOH is used as precursor, the crystal growth is influenced by the absorption of NO3" onto the crystal surface. When the solvent is the mixture of water and alcohol, the ratio of length/width is improved, while the length .decreases. This is because the absorption of hydroxy groups changes the habit of crystal growth and the winding of CH3—CH2 — chain constitutes a micro reacting room.Under different conditions, little amount of zirconia nanotube^ nanowire^ nanobelt is found, which should be attributed to the network of zirconia gel serving as a similar function of template.Second, the growth mechanism of tetragonah cubic ^ monoclinic zirconia is studied according to the theoretic model of anion coordination polyhedra. It is found that monoclinic is the primary phase in hydrothermal environment. And the experimental results confirm that the actual growth habit accords with the theory well.The reasons for anisotropic growth are analyzed according to the Ostwald ripening mechanism and the Unit Combination Theory. In different hydrothermal environments, there existed different type of growth units, the superimposition of which could be accelerated by the dehydration of hydroxy groups under alkaline condition.Third, nano-sized spherical zirconia crystal is prepared by co-precipitation method > hydrothermal method and solvothermal method. The zirconia powder prepared by hydrothermal method is lOnm in diameter, presenting integrate shape, well dispersion and narrow distribution of particle size. The growth of zirconia crystal is affected by both temperature and mineral concentration: the hydroxy group restrainthe growth of crystal by absorption, on the other hand, the dispersing effect decreases due to the dehydration of hydroxy group and the chelating of the molecular chains.According to the nucleation rate equation, it is pointed out that varying the temperature can control the grain size. The nucleation critical number/ = (2r/y /3A/u) is calculated, and the critical number can be changed by adjusting the super saturation A/u. The growth of spherical crystal is found to be in accordance with the Ostwald ripening mechanism.Combining the three types of crystal .growth mechanisms under hydrothermal condition with the experimental results, the tetragonal phase is believed to be the main phase at the beginning of the reaction, so the in situ crystallization mechanism should be dominant. With the reaction going on, however, the grains grow into monoclinic phase and the dissolution-crystallization mechanism is dominant.