Gel-processing Of Pseudo-boehmite And Tin Dioxide Nano-/micro- Composites

The gel-processing technology for multiscale system including nano-sized pseudo-boehmite particulates and micro-sized SnO2 particles composite material was investigated to solve the critical problems in preparation of the anode materials used for aluminum industry and the electrode materials of batteries with satisfactory properties. In gel processing, micro-sized SnO2 particles are dispersed into the pseudo-boehmite sol, the effect of nano-sized pseudo-boehmite colloidal particulates' high specific surface aera, cleanness and driving force in mass transfer, make the micro-sized SnO2 particles dispersing, sintering and moulding easily. In the experiments, the stable pseudo-boehmite colloid was obtained, the micro-sized SnO2 particles were dispersed in the sol uniformly, the thick coatings on the carbon were prepared, and the sintered bulks were received after molding and sintering. In this thesis, the interactions among the nano-sized pseudo-boehmite colloidal particulates and the micro-sized SnO2 particles, and the physical chemistry mechanisms have been analyzed in emphases.The stable pseudo-boehmite sol was prepared with the peptizing agent, HNO3. The measurement of acid dispersion index indicates that the pseudo-boehmite powder canbe peptized completely. When n_H+/n_AlOOH·nH2O =0.097, the corresponding pH value is 3.4. The calculated potential barrier is 6.31 ×10^-18J according to DLVO model. The size of the colloidal particulates is around 5-10nm, being cross-link as is shown by TEM. The peptizing mechanism of pseudo-boehmite powder was analyzed, and the multi- nucleus model of colloidal nucleus and electrical double layers model of micelle were proposed, based on the measurement of conductivity, the analysis of IR infrared spectra and the equilibrium profiles of dissolution-precipitation for Al3+ at different pH values.The viscosity change of the pseudo-boehmite colloid is related to the size, the morphology and the existing state of the nano-sized colloidal particulates. The viscosity change of pseudo-boehmite colloid in gelation is determined. The colloidal particulates is growing and cross-linking with the increase of viscosity according to the observation by TEM. The kinetics model of colloidal particulates aggregation in gelation was built up based on the viscosity change. Corresponding to the viscosity change, the evolution of the colloidal particulates is divided into three stages: ( I ) slow growth;(II )gradual transition;(III)sudden gelation.The dispersion behaviors of SnO2 in pseudo-boehmite sol and the stability of the slurry were investigated. The SnO2 particles which was observed with size of about 0.1-0.8 μrn by TEM, can be dispersed in the sol with 15(Wt)% solid load of pseudo-boehmite and pH values of 3.4 by mechanical stirring, and the highest content of SnO₂ in the slurry can reach to 60(Wt)%. The gravitational sedimentation of the micro-sized particles in the sol of pseudo-boehmite is the main factor related to the kinetic stability, according to the experiment on the sedimentation of SnO2 particles in the dispersing system. The measurement of the slurries' viscosity and the observation of the dispersing state of micro-sized SnO2 particles among nana-sized pseudo-boehmite colloidal particulates by TEM, reveals the two effects of micro-sized particles' addition. In one hand, the micro-sized particles promoted the gelation faster and the viscosity of slurry increase sharply, which increase the friction between the particle and the media, and prevent the sedimentation of the micro-sized particles. In the other hand, the polymerization of nano-sized pseudo-boehmite colloidal particulates on the surface of SnO2 micro-sized particles increases the steric effect and prevents the aggregation of the micro-sized particles, which strengthens the stability ofthe slurry. The model of gelating process promoted by micro-SnO2 particles is presented based on the experiment and the stability mechanisms in the slurry.The stable slurry obtained by the micro-SnO2 particles dispersing in the pseudo-boehmite sol can be used for preparing the thick coating and bulk materials directly. Firstly, the membrane with the thickness of 0.2-0.5 u m was obtained by dip coating technology in the experiment. There was no visible crack on the surface, while a lot of micro-cracks were observed by SEM. The function of micro-sized particles' filling to get rid of cracks was analyzed from the viewpoints of cracking mechanism. Secondly, the xerogel coming from the slurry drying, was pressed in dry to form the 16X3 green pellets. During the cause of sintering at 1200°C, the shrinkage is up to 23.15%, and the relative density is up to 68.34%. The nano-sized particulates uniform dispersing between the micro-sized particles were observed by SEM. The obvious sintering neck had come into being among the different particles. According to the thermodynamic analyses, there are two types of pores existing in composite bulk materials, thus the mechanisms of quality transfer are different. The driving force of nano-sized particulates in sintering is greater, so they can be sintered easily. The high specific surface area and the great specific surface energy of nano-sized pseudo-boehmite contribute to the mass transfer in the causes of sintering; resulting in the increase of the densification of the sintered bulk.The effects by introducing rare earth metal element Ce to the pseudo-boehmite/SnO2 composite material were studied experimentally. The crystal phases of Al-Sn-0 and Al-Sn-O-Ce composite material were analyzed, and the lattice constants of SnC>2 in composite materials ware calculated. As shown by X-ray diffraction, there are no diffraction peaks of C1-AI2O3 phase in Al-Sn-0 composite materials when the heat treatment temperature is below 1200°C, and the volume of crystal lattice of SnC>2 in Al-Sn-0 composite material is bigger than that of pure SnO2 powder. After addition of rare earth metal oxide, there is no the diffraction peaks of (X-AI2O3 phases also in Al-Sn-O-Ce composite materials when the heat treatment temperature is up 1200°C. This indicates that the addition of nano-CeO2 powder increases the phase transformation temperature to 01-AI2O3.Hence, it is concluded that the active pseudo-boehmite colloidal particulates with high specific surface area enable the micro-sized particles' dispersing and molding inthe gel-processing technology of pseudo-boehmite. The interactions among the nano-sized particulates and micro-sized particles improve the processing properties, and makes process, structure and property controllable in the preparation and syntheses of materials.