Hydrothermal Synthesis Control Method Of Alumina Micromophology And Analysis On Its Microstructure

Alumina is an important inorganic porous structure material.Porous alumina materials,such as granular porous alumina ceramics,fibrous alumina porous materials and nano-porous three-dimensional network alumina aerogels,have a broad application prospects in the fields of catalyst and thermal protection due to their special mechanical and thermal insulation properties.Alumina possesses on many phase states,and most of them are transient or metastable in different temperature ranges.Under different preparation conditions,alumina presents different micromorphology.Hence,in theory and practice,it is meaningful to understand the influences of different synthetic conditions on the microstructures of alumina.In this paper,the alumina precursors were synthesized by the sol-gel method and hydrothermal method.Then,the alumina powder was prepared after high temperature solid phase sintering.The influences of silicon element contents,sintering temperature and hydrothermal treatment on the micro-morphology of alumina powder were investigated,which provided certain theoretical basis and experimental support for the constructions of new materials based on their unique morphological characteristics.The fractal models of thermal conductivity of alumina powder were established by the fractal theory to predict thermal conductivity of alumina.In the experiment,aluminum tri-sec-butoxide was chosen as the alumina precursor and the aluminum sols were prepared by the sol-gel method.Then,two kinds of precursors,i.e.aluminum sols and ones after hydrothermal treatment(nanosize rod-like boehmite),were mixed with Al F3 and silica sols to prepare alumina powder after high temperature solid-phase sintering,respectively.The influences of silicon element content and sintering temperature on the micro-morphology of alumina powder were discussed.Bohmite was prepared by hydrothermal method to investigate the effect of precursor hydrothermal treatment on alumina growth.The results show that the micromorphology of alumina is changed from hexagonal flake corundum to whisker mullite with the increase of silicon content.Moreover,the addition of a small amount of Si inhibits the growth of flake alumina crystals.With the increase of the amount of Si,the aspect ratio of mullite whisker increases.The sintering temperature has little effect on the morphology and size of the flake crystal when ?-Al2O3 is formed.The aspect ratio of mullite whisker increases with the increase of sintering temperature.The alumina prepared by hydrothermal treatment is slightly different in morphology and phase state compared with that of alumina prepared by aluminum sols.According to the SEM images of alumina powder,the fractal dimensions of alumina powder were calculated by fractal theory based on numerical simulation software.Meanwhile,two-dimensional thermal conductivity fractal models of alumina powder were constructed to predict the thermal conductivity and porosity of alumina.The results show that alumina has fractal characteristics and the thermal conductivity decreases with the increase of porosity.The results of dissipative particle dynamics(DPD)simulation show that all the components of the solute are uniformly dispersed in the solvent and no agglomeration phenomenon appears under the effect of shear.