Synthesis And Structure Control Of Submicron Silica Solid Foam

The submicron silica solid foams have more advantages in many areas than traditional macroporous ceramics foams,such as high-temperature thermal insulation,high thermally conductive heat sinks,catalyst supports,low dielectric materials and biological materials,etc. It is obviously important for us to find a simple and effective method to prepare the submicron silica solid foam,which will be much better if it can be put into industry.The negative-charged polystyrene beads with different sizes were parepared by emulsion polymerization and emulsifier-free emulsion polymerization and the positive-charged PMMA beads were parepared by emulsifier-free emulsion polymerization .Then the three kinds of polymer beads was used as templates to prepare the monlithic submicron silica solid foams by sol-gel technique at pH=3.8.we studied the affect of charge and size of the polymer beads on the structure of samples.we also studied the thermal stability of the silica aerogels and solid foams .The samples were characterized by XRD,IR,SEM,BET,DSC,TG and TEM technique.It was found that all the samples were made up of amorphous silica.When the positive-charged polystyrene beads were used as templates.The prepared samples were monlithic solid foam,whose pores were uniform. When we used the negative-charged polystyrene beads parepared by emulsifier-free emulsion polymerization to prepare the silica solid foams,no matter the TEOS was hydrolysised or not before added to the latices,we can get the materials with uniform pore morphology.however,When the negative-charged polystyrene beads parepared by emulsion polymerization were used as templates,only TEOS was not hydrolysised before added to the latices,we can get solid foams with the uniform pores, when the positive-charged polystyrene beads were used as templates,the samples were consisted of submicron hollow silica spheres.We conclued that since the charge of the silicate particles and oligomer was oppsite to the positive-charged polystyrene beads at pH=3.8 ,the silicate particles and oligomer were accumulated on the polymer beads. The condensation proceeded until the silica shell layer formed even under weak acidic condition. When the positive-charged polystyrene beads were used as templates,the polymer beads with smaller size were easy to aggregate during sol-gel process for their huge suface energy..Silica aerogels and solid foams annealed under 600℃displayed the high stability in their porous texture,but when they were annealed over 600℃,the character of their porous structure began to decline and when annealed over 1060℃,the porous character in Samples disappeared.The aggregation of nanometer particles in samples'structure and the phase tration caused by heat treatment resulted in decrease of average pore diameter and the pore volume in the surface area.