Abstract
Many performances of nanostructured ceramic coatings are better than that of conventional ceramic coatings, such as ductility, microhardness and adhesive strength.One of precondition and key technologies on preparing nanostructured coatings is how to get a high-quality nano-feedstock powders. The nanostructured powers synthesized by the spray drying process possess great developed prospects. However, during the spray drying process, the quality of nanostructured powders will be infected and the nano-grain of powders will grow up. The aim of this thesis is to synthesize the spherical nanostructured ZrO2 reunion powers via the emulsion method assisted with homogenous precipitation for solving the problem which cannot be solved by spray drying process. All research details of this thesis are described as follows.Using urea as precipitant, zirconium nitrate as salt solution and the nano ZrO2 powder was prepared with homogeneous precipitation in closed reaction system. The effects of reaction temperature, reaction time, urea/Zr(NO3)4 ratios(molar concentration),concentration of Zr(NO3)4 solution and calcining temperature on the average particle size, particle size distribution and crystal structure of the nanostructured ZrO2 powder were investigated. The nanostructured ZrO2 powder and its precursor powders were characterized by means of TG-DTA, XRD, BET and laser particle size analyzer. The results indicate that the ultimate reaction pH value is close related with the lattice parameters of nanostructured ZrO2 powders, the higher the final pH value is, the largerthe lattice parameter will be.The stable W/O emulsion system was successfully confected in this paper, and the relationship of emulsion viscosity, conductivity and the content of water phase and surfactant were analyzed. A ternary phase diagram which is made up of A1area and A2 area about emulsion system was plotted. The well spherical, excellent distribution zirconia powders were prepared with emulsion located at both A1 area and A2 area respectively, and the morphology of powders was characterized by SEM and TEM. The result shows that the particle size of zirconia powders prepared with emulsion located at A1 area is about of 50 nm, in contrast, the other zirconia powders prepared with emulsion located at A2 area possess 10um grains.The nanostructured ZrO2 reunion powders were prepared via reverse emulsion method in closed system and were characterized by means of SEM and laser particle size analyzer. The experimental results indicated that the particle size distribution and morphology of the prepared nanostructured ZrO2 reunion powders has little relationship with reaction temperature, reaction time, urea/Zr(NO3)4 (molar concentration ratio) and concentration of Zr(NO3)4 solution in a stable emulsion system. However, the water/oil ratios, water/surfactant ratios and rotate speed will impact the particle size distribution and morphology of reunion powders.
According to preliminary exploratory experiment, the BET value of sample was selected as the observation index of orthogonal experiment which was adopted to prepare nanostructured ZrO2 reunion powders, and the best optimum technology parameters for preparation of powders were obtained. In addition, the nanostructured ZrO2 reunion powders prepared via optimum technology parameters were characterized and analyzed by BET, XRD, SEM, TEM, Zeta potential instrument and laser particle size analyzer. At the same time, the effects of calcination temperature and heating rate during thermal treatment process on the properties of reunion powders were also investigated. The results indicate that nanostructured ZrO2 reunion powders with excellent dispersion and sphericity can be obtained by adopting the optimize technical parameters, and the mean diameter of reunion powders is about 25 um, the BET value is 56.13m2/g. Specially, the effect of heating rate during thermal treatment process on the morphology as well as the forming mechanism of reunion nanoprticles were studied.A contrastive study on size distribution between the as-prepared powders and the spray drying powders was adopted. Moreover, nanostructured ZrO2 coatings CA and CB were respectively prepared by plasma spraying technology through using the above mentioned two kinds of powders as feedstock powders, and adhesion strength and resist crack spread force of as-spraying coatings were investigated. The experimental results show that the adhesion strength and resist crack spread force of CA coating is higher than that of CB coating.
Keywords: Nano ZrO2powder, Homogeneous precipitation, Spherical reunion powder,Reverse emulsion
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