Preparation And Application Of Carbon Aerogel And Its Composites

Carbon aerogel is a kind of amorphous glass state carbon nanomaterials with low density,high specific surface area and high porosity.Due to its excellent adsorption,electrochemical inertness and high temperature insulation properties,it has potential in the field of hydrogen storage,high temperature insulation,adsorption,electrochemistry and other fields as a new generation of functional materials.However,the main factors limiting the scale production of aerogels are four aspects:long production cycle,high cost,environmental pollution and poor mechanical properties.First of all,the main way to prepare carbon aerogels is supercritical drying.Although this method can theoretically eliminate the surface tension in drying process,the equipment is expensive and occupies a large area,and it is dangerous to operate under high temperature and pressure.The cycle of solvent replacement is long.It is necessary to replace the water in wet gel with supercritical drying medium(such as carbon dioxide,petroleum ether or ethanol,etc.),and then discharge the drying medium.This process needs to be slow to avoid damaging the supercritical state of the solvent.In recent years,atmospheric drying method that immersing wet gel into solvent with low surface tension coefficient and repeating it.The water in wet gel was replaced by a low surface tension solvent,during this period,it is necessary to use dozens of times or even hundreds of times of gel volume solvent.These solvents,such as ethanol and acetone,are mostly organic solutions are volatile,toxic and difficult to recycle.These have great pollution to the environment.Atmospheric drying can only reduce the drying shrinkage force in the gel process by solvent replacement,so it is difficult to prepare materials with large size or small pores.Besides the imperfect preparation methods of carbon aerogels,the properties of their intrinsic materials are also defective.Carbon aerogels are amorphous porous glass carbon materials,therefore,it has poor toughness and high brittleness.When carbon aerogels need to bear certain loads,brittle fracture occurs easily.In order to ensure the stability of materials,the comprehensive mechanical properties of carbon aerogel composites should be improved by adding reinforcement phase.Therefore,in order to realize the industrialization of carbon aerogels,a large size carbon aerogel composite material with stable overall performance is required by an efficient and environmentally friendly process.In this paper,on the basis of conventional atmospheric drying method,by studying the effects of different gelation parameters,such as catalyst content,gel temperature and water content on the microstructure and properties of carbon aerogels,the mechanical properties of the materials were improved,so that the porous structure and macroscopical shape could be maintained under the condition of atmospheric drying without a long solvent displacement step.The experimental results show that under high catalyst concentration,carbon aerogels have higher density,smaller particle size,lower porosity and superior mechanical properties,and are more suitable for high load-bearing functional materials.When the ratio of resorcinol to catalyst R/C is 300,the density of carbon aerogel can reach 1.13g/cm3.and the compressive strength is as high as 126.8MPa.At low catalyst concentration,carbon aerogels have smaller density,larger particle size and higher porosity.They can be used as light adsorption functional materials.When R/C is 2000,the density of carbon aerogels is only 0.30g/cm3,but the compression performance is still 6.5MPa,which basically meets the mechanical properties of functional materials.When the initial gel temperature changes,the density and compressive strength of carbon aerogels begin to increase with the increase of gel temperature,but this increase is not linear.When the initial gelation temperature is 30 degrees,the density of carbon aerogels is 0.30g/cm3,and the compressive strength is 6.5MPa.When the gel temperature rises to 45 degrees,the density of carbon aerogels is 0.40g/cm3,and the compression strength is 11.5MPa When the gel temperature finally increased to 90 degrees,the density of carbon aerogels was 0.46g/cm3,and the compressive strength was as high as 38.2MPa.It can be found that increasing the gelation temperature will only slightly increase the carbon aerogel density,but significantly improve the mechanical properties of the carbon aerogel,which may be related to the catalytic mechanism of the catalyst in the sol-gel process.In addition,the effects of aging time and water content on the performance of carbon aerogels were also studied.However,the experimental results showed that the effect of increasing aging time and regulating water content on carbon aerogels was limited.In view of the large brittleness of carbon aerogels,this paper adopts the method of adding organic fiber felt into the sol process.The formula chosen is R/C=1500,W/R=17.The carbon aerogel prepared by the formula is more balanced and has a larger application range.The properties of three kinds of organic fibers(phenolic,polyacrylonitrile and viscose)and carbon fibers were compared.The density of phenolic,polyacrylonitrile and viscose based composites was 0.40 g/cm3,0.38 g/cm3 and 0.43 g/cm3,respectively.Compared with the compressive strength of the carbon aerogel 10.8MPa,different carbon fibers reinforced aerogels have different degrees of enhancement.The compressive strength of phenolic,polyacrylonitrile and viscose bases is 11MPa,22MPa and 32MPa respectively.From the stress-strain curves,the three of them are transformed from brittle fracture to ductile fracture,and the failure strains are 6.9%,9.2%and 11.7%respectively.The improvement of mechanical properties makes it possible to prepare large sized carbon aerogel composites.The size of aerogels prepared in this experiment can reach 200 X 200 X 40mm,which can meet the size requirements of most application environments.