Preparation Of Carbon Aerogel And Thermal Insulation Composite Materials By Molten Salt Method

The thermal protection system of the new high-speed aircraft is in urgent need of high performance thermal insulation material which can withstand ultra-high temperature,light weight and low thermal conductivity.Carbon aerogel has inherent characteristics of ultra-high temperature resistance of carbon materials（over 2000℃under inert or vacuum atmosphere）,and its high temperature thermal conductivity is low.In this paper,the composite material was prepared by using the original prefabricated fiber impregnated with sodium resorcinol-furfural sol,thus,carbon aerogels with low density and small pore size can be prepared without supercritical drying,solvent replacement or high ambient pressure,which lays a foundation for the preparation of carbon aerogel composites.Firstly,suitable carbon precursor and molten salt were selected to prepare the bulk carbon aerogel with non-expansion volume and micron-size macropores.To reduce the intensity of the gel reaction and the product density,organic solvents were added to the mixture of carbon precursor and molten salt.The effects of isopropyl alcohol and methanol as solvents on the structure and properties of the prepared carbon aerogel were studied,and the pore formation mechanism of salt in sol-gel process was studied.Finally,the structure and properties of the carbon aerogel composite prepared by molten salt method were studied.The main results are as follows:（1）In this study,resorcinol（R）and furfural（F）were selected as the reactive monomers and zinc chloride（Z）as the molten salt,the effects of zinc chloride molten salt on gel time and carbon aerogel density were studied.Molar ratio Z/R=0 the prepared sol was left standing at room temperature for 11h before it was fully gel,and the prepared carbon aerogel density was very high,up to 0.98g/cm³.Molar ratio Z/R=0.32 the prepared sol became gel after 10min,and the prepared carbon aerogel had a higher density,up to 0.82g/cm³.Molar ratio Z/R=0.5 prepared sol,after being placed at room temperature for 7min,began to smoke and heat,and the volume expanded to some extent.There were micron-sized pores on the surface and inside,and the gel reaction was intense.The prepared carbon aerogel had uneven surface,and the density was 0.62g/cm³.Therefore,the preparation of carbon aerogel by molten salt method requires adding an appropriate amount of solvent,diluting the concentration of the reactant,moderating the violent degree of the gel reaction in the presence of zinc chloride salt,appropriately increasing the gel time,reducing the viscosity of the sol,which is conducive to impregnating the prefabricated fiber and reducing the density of carbon aerogel and its composite materials.（2）The effect of isopropanol as solvent on the structure and properties of carbon aerogel was studied.Isopropyl alcohol was studied with resorcinol（I/R）of molar ratio on the multistage aperture carbon aerogel,as the design of the molar ratio of I/R value rose from 1.8 to 11.0,the density of multistage pore carbon aerogels from 0.69 g/cm³reduce to 0.14 g/cm³,the BET specific surface area from 797.0 m²/g gradually decreased to 648.4 m²/g,when the mole ratio of I/R is set to 11.0,zinc chloride and resorcinol（Z/R）molar ratio is set to 3.2,the carbonization temperature is set to 900℃,Multistage pore carbon aerogel with density as low as 0.14g/cm³ and specific surface area as high as 648.4m²/g can be prepared.The results of nitrogen adsorption test,SEM and mercury injection test showed that the pore structure of multistage pore carbon aerogel prepared with isopropanol as solvent was mainly composed of a large number of micropores,a small number of mesoporous and large pores distributed in the range of0.03～30μm.（3）In the system of resorcino-furfural sol with methanol as solvent,the effects of zinc chloride amount on the shrinkage characteristics,pore structure and microstructure of carbon aerogel were studied.The specific surface area of carbon aerogel sa mples prepared without zinc chloride was small,with a value of 15.69 m²/g and a pore volμme of 0.006cm³/g.The specific surface area of carbon aerogel prepared by adding zinc chloride salt gradually increased with the increase of the amount of zinc chlori de salt.When the molar ratio of zinc chloride and resorcinol reached 2.5,the specific surface area reached 1081.77 m²/g.As can be seen from the SEM images,with the increase of the amount of zinc chloride,the surface of carbon aerogel samples became more uneven and contained more nano pores.However,the particle size and pore size of carbon aerogel did not change significantly.（4）Pore formation mechanism of salt in sol-gel process was studied.The pore mechanism of zinc chloride template in sol-gel process should be studied,first of all,it is necessary to effectively retain the nano pores formed by zinc chloride template in RF gel.These nano pores lose the support of zinc chloride template and are usually destroyed to a large extent in the process of atmospheric pressure drying.By utilizing the principle of the disappearance of solid-liquid interface,supercritical drying can reduce the destruction of atmospheric dry interfacial tension on the nanopores in RF gel.At the same time,in order to better conduct supercritical drying of RF gel without support of zinc chloride template,the washed and desalted RF gel was immersed in ethanol to replace the water in the pores of the RF gel with ethanol with low surface tension.Then CO₂ supercritical drying was carried out to successfully synthesize RF aerogel with rich micropores and its derivative carbon aerogel.RF aerogel synthesized without zinc chloride template has relatively large average pore diameter and particle size.RF aerogel synthesized without zinc chloride template has relatively small pore surface area（6.34 m²/g）.With the increase of the amount of zinc chloride,the micropores specific surface area of synthesized RF aerogel gradually increased,and the mesoporous specific surface area changes very little.Moreover,the average pore diameter and particle size of the synthesized RF aerogel did not change significantly with the increase of the amount of zinc chloride,which indicated that zinc chloride salt played a crucial role in the formation of RF aerogel micropores.In the sol-gel process,a part of the zinc chloride salt combines with the water molecules generated by the reaction of resorcinol and furfural to form Zn Cl₂·4H₂O.Some of the remaining zinc chloride is embedded in the gel skeleton（which is the gel skeleton,not the hole,which forms a microhole after being washed with salt water）,another part of zinc chloride salt and Zn Cl₂·4H₂O were dispersed in the pores between the skeletons.（5）Carbon aerogel composites（CF/CA）prepared by molten salt method were studied and characterized.The microstructure of CF/CA was studied by N ₂ adsorption desorption and SEM.The N₂ adsorption-desorption isotherm curve of CF/CA had typical characteristics of type I isotherm,indicating that CF/CA had a large number of micropores and a small number of mesoporous.As can be seen from the SEM figure,the carbon fiber reinforced material of CF/CA prepared is closely combined with the carbon aerogel matrix.The carbon aerogel fills the gap between the carbon fibers,and there are no micron cracks in the carbon aerogel matrix.The compression strength of CF/CA was studied by universal mechanical testing machine.When the strain was 0.1,the compression stress was 6.10 MPa,which was obviously better than the compression strength of carbon aerogel composites prepared by sol-gel method（1.72 MPa）.The high temperature thermal conductivity of CF/CA was studied by laser flash method.As the temperature measured in argon atmosphere increased from 600℃to 1800℃,the thermal conductivity of CF/CA gradually increased from 0.2381 W/m·K to 0.6904W/m·K,which was significantly lower than the high temperature thermal conductivity of micron pore carbon foam（1.745 W/m·K）.