Controllable Preparation Of Uniformly Dispersed Nano-Scale Metal Oxides

This thesis reports a new method for the preparation of controllable nano-scale ZnO and MgO. The preparation of nano-scale ZnO and MgO from zinc carbonate hydroxide and magnesium carbonate hydroxide precursors by temperature programmed calcination is studied. The effect of a number of factors on the structure, appearance, particle size and its distribution in the products is described. 1 , The preparation method of carbonate hydroxide salts was studied.Zinc carbonate hydroxides with different formula are synthesized by a novel method developed in our laboratories using separate nucleation and aging steps using different precipitating agents. The effect of different starting materials and their concentrations on the properties of the precursors as well as the effect of different precursors on the properties of nano-scale ZnO is discussed. The results showed that the precursors can be prepared by the addition of either NH4HCO3, (NH4)2CO3 or Na2CO3 to Zn(OH)2 at room temperature. ZnO derived from these precursors varied insize and apparence. Small ZnO crystallites can be obtained at either low orhigh concentration of starting materials.Magnesium carbonate hydroxides with the same formula can be prepared using different starting materials. The effect of preparation temperature and aging time of the precursor on the properties of the resulting MgO was studied. The results showed that magnesium carbonate hydroxide with the same formula can be obtained using NH4HCO3, (NH4)2CO3 or Na2CO3 as precipitating agents at 60 C for 2h. 2^ The effect of calcination conditions on nano-scale ZnO and MgO was studied.The results showed that with increasing calcining temperature, the nanoparticles grow larger and the crystals of them become more well-formed. Hexagonal ZnO with well formed crystals was prepared by calcining the zinc carbonate hydroxide precursor at 500C for Ih. Cubic MgO with well formed crystal was prepared by calcining the magnesium carbonate hydroxide precursor at 950C for Ih. The calcination time has little effect on the nature of the nanoparticles. The size of the nanoparticles slowly becomes larger and more uniform with increasing calcination time. 3 , The advantages of the method of preparation of nano-scale metal oxides from carbonate hydroxide salt precursors is analysed.The nanoparticles prepared under different conditions were characterized by XRD, TEM, TG-DTA, EXAFS, IR, BET, UV etc. The results showed that the crystallinity, purity and the dispersibility of thesenanoparticles with a narrow size distribution are good. The UV absorption properties of nano-scale ZnO are better than ordinary ZnO.The reasons for the superiority of zinc carbonate hydroxide and magnesium carbonate hydroxide over other precursors was studied theoretically by calculation of the decomposition kinetics. It was shows that the CO2 emitted by zinc carbonate hydroxide when heated was initially adsorbed on the ZnO, which had the effect of breaking up the precursor crystallites, preventing the agglomeration of the particles and reducing the capillary condensation. Compared with other hydrate or carbonate precursors, the precursors having low decomposition temperatures can produce smaller nanoparticles.