Research On Preparation Process Of Silica Aerogel And Its Composite Materials

SiO₂ aerogel has been taken widely attention from all walks of life as a new type of ultra-light porous solid materials.SiO₂ aerogel possesses excellent properties,such as heat resistance,sound resistance,adsorption and transparent,which make it has a broad development prospects in the heat preservation and heat insulation,aerospace,catalyst carrier,and other fields.The purpose of this paper was to reduce the production cost.Low-cost waste powder and acidic silica sol were respectively as raw materials.Silica aerogel was prepared under the condition of normal pressure following by the technology of aging,replacement and modification of the solvent.Different process parameters on the influence of structure and performance of the aerogel were studied.The related mechanisms were discussed.Eventually,aerogels were used to prepare aerogel-fiber composite materials with quartz fiber felt and rock wool board respectively.The important parameters,such as thermal conductivity and the line shrinkage rate of the aerogel-fiber composite materials were studied.The industrial waste silicon powders were used as raw material to prepare aerogels.The increases of the reaction rate and silicon powder conversion rate were caused by the increase of sodium hydroxide amount.But for the sake of excepting Na⁺ conveniently,the mass ratio was determined as silicon powders:NaOH:H₂O for 5:1:40.The best reaction temperature was 70°C.Cation exchange resin was used to remove Na⁺,and the content of Na⁺ had a great reduction between the switching time of 8h to 12h.Gel time was changed by the increase of HNO₃ concentration,the gel time was the shortest when the concentration reached 1.4 mol/L.The best water bath temperature was 80°C.When the aging time was 4 d and the temperature was 55°C,the microstructure of the aerogel was more uniform and parameters were relatively good.N-hexane was selected as replaced solvent,trimethylchlorosilane?TMCS?was selected as modified solvent.When the volume ratio of Et OH:n-hexane:TMCS was10:10:1,the gel porous structure was more obvious and particle size was more uniform.The best replacing and modifying time was 4 d.Aerogel kept good hydrophobic under 400°C.Hydrophobic failure temperature was from 400°C to 500°C.Weightlessness was most obvious between 250°C and 300°C,about 1.8%.Aerogel had a wider exothermic peak between 350°C and 550°C,this was mainly due to the oxidation of-CH₃ groups.The parameters,such as specific surface area could be improved when aerogel being heated at 300°C.When more than 700°C,the density rised in an almost straight line.The specific surface area dropped below 200m²/g with heat treatment temperature of 900°C,particles gathered,basic porous structure was lost.Acidic silica sol was used as raw materials to prepare aerogels.When theconcentration of ammonia increased from 0.2 mol/L to 0.6 mol/L gradually,the gel time decreased firstly and then increased.The gel time was the shortest when the ammonia concentration was 0.4 mol/L.Gel time and the skeleton structure of the aerogel were affected by the pH and water bath temperature.Gel time was the shortest with pH of 6.5,the best water bath temperature was 80°C,at this point,the aerogel skeleton structure was more uniform.Aerogel prepared by silicon sol with concentration of 20%had relatively good chunks,three dimensional frame structure was relatively obvious,the particle size and aperture size was more uniform.The structure and properties of aerogel were influenced by heating mechanism,gradient heating mechanism was more appropriate than constant temperature drying.To a certain extent,the parameters of the aerogel could be improved when heated at 300°C,structure was destroyed at 900°C.The thermal conductivity at room temperature of aerogel-quartz fiber composite material was not more than 0.02 W/m·K,which was more than half of the thermal conductivity of quartz fiber felt at room temperature.When the temperature was 600°C,the thermal conductivity of aerogel-quartz fiber composite material was about 0.045 W/m·K,and microstructure changed obviously.The maximum line shrinkage rate of aerogel-quartz fiber composite material was-1.55%,minimum density was 0.53g/cm³.When the temperature was below 450°C,its compressive strength was more than 2.91 MPa,the flexural strength was above 0.56 MPa.The thermal conductivity of aerogel-rock wool composite material at room temperature was about0.015 W/m·K,the aerogel-rock wool composite material still had a good thermal conductivity of 0.04 W/m·K when the temperature was 600°C.The maximum line shrinkage rate of aerogel-quartz fiber composite material was-1.51%,minimum density was 0.55 g/cm³.When the temperature was below 400°C,the intensity had almost no difference,its compressive strength was more than 1.83 MPa,the flexural strength was above 0.36 MPa.