Research On Structural Regulation And Thermal Insulation Performance Of Polyimide-based Carbon Aerogel Composites

With the breakthrough of technical barriers in space flight,the harsh service environment of advanced high-speed space vehicles sets higher requirements for the performance of thermal protection systems.The demands of practical applications cannot be met by relying on the structural design and layout of the equipment.It is crucial to develop a new thermal insulator with low thermal conductivity,ultra-high temperature stability,moderate mechanical properties and lightweight for the long-life,long-range flight of high-performance spacecraft.Carbon aerogels are three-dimensional nano porous materials consisted of the cross-linking network of carbon nano particles,which combined the advantages of low thermal expansion coefficient,ultra-high temperature stability,low radiation thermal conductivity and a low density.However,the weak inherent non-covalent interactions between the nanoparticles of the carbon aerogel skeletons obtained by the traditional organic aerogels carbonization result in the defects of low mechanical strength,large brittleness and poor formability,which limit the use of carbon aerogels alone as thermal insulators in aerospace application.Based on research on the structural evolution and shrinkage in the preparation of polyimide-based carbon aerogels(CPI-Y%)with micropores,this work adopts phenolic resin-based pre-carbonized foams(CFX-500℃)with continuous phase and good formability as reinforcements of the carbonization precursors composite(CFX-PI),and the phenolic resin-based carbon foam reinforced polyimide carbon aerogel composite(CFX-CPIY-Z℃)is prepared by dipping polyimide sol,gel integral molding,aging,solvent replacement,supercritical drying and co-carbonization.Due to thermal weight loss during the carbonization process,the pre-carbonized foams shrink with the polyimide aerogels,which significantly reduced the gap between the two-phase interface,thus improving the shrinking compatibility between the foam reinforcements and the aerogel matrix.Using this approach,we obtain the crack-free co-carbonized CFX-CPIY-Z℃composite material.Conclusions are listed as follow:(1)In order to obtain polyimide-based carbon aerogels with low thermal conductivity and shrinkage,the influence of sol concentration and carbonization temperature on the properties and microstructure evolution of carbon aerogels were investigated.The shrinkage,carbon yields and density of the carbon aerogels increased with the increase of sol concentration.Carbon aerogels prepared in low sol concentrations(5%-7.5%)exhibited a fiber-mesh crosslinked structure and transformed into a pore structure dominated by micropores.With the increase of sol concentration,the specific surface area and the average pore size of carbon aerogels decreased,while the average particle size of the skeletons and the proportion of micropores increased.When the sol concentration increased to 10%and 12.5%,the carbon aerogels showed dense microscopic morphology,and the nano porous structures were destroyed.With the increase of carbonization temperature,the polyimide-based aerogels prepared by 5%sol-concentration have lower shrinkages,while the micropore rate increased and remained above 74%.When the carbonization temperature reaches to 1000°C,the specific surface area of the carbon aerogel achieved a maximum value of 764.169 m~2/g,while the average pore size dropped to 3.372 nm.Therefore,the sol concentration of 5%,7.5%and the carbonization temperature of 1000°C is conducive to reduce the shrinkage rate of aerogels in the process of high-temperature carbonization and maintain a relatively complete three-dimensional nano porous structure to obtain a polyimide-based carbon aerogel with lower density,larger specific surface area,smaller pore size,higher micropore rate and smaller nanoparticle skeleton.(2)Using polyurethane foam as the template and boric phenolic resin as the carbon source,phenolic resin-based pre-carbonized foam were prepared by liquid phase impregnation and drying(PF-PU Foam),oxidation stabilization(PF-PU Foam-Ox),and high temperature pyrolysis(CFX-500℃)in liquid phase.Pre-carbonized foam with different shrinkages and densities were adopted as reinforcements,and the effects of the concentration of phenolic resin impregnation solution on the physicochemical properties and microstructure of pre-carbonized carbon foams were analyzed.Thermogravimetric analysis showed that the oxidation stabilization treatment could transform the thermoplastic phenolic resin into thermosetting resin which could stably adhere to the polyurethane skeleton during the pyrolysis and the carbonization stage,avoiding the stress and macroscopic defects in the final carbon foam structure.The pre-carbonization weight loss rate of PF-PU Foam-Ox-X prepared using different concentrations of phenolic resin impregnation solution is a crucial factor in regulating the shrinkage of foams.With the increase in the concentration of the impregnation solution,the linear shrinkage and weight loss rate of PF-PU Foam-Ox-X in the pre-carbonization treatment decreased,while the densities and the average pore size of CFX-500℃increased.(3)In order to further improve the shrinkage compatibility between the aerogel matrix and the foam reinforcements and optimize the thermal insulation performance of the composites,the polyimide aerogel prepared with the sol concentration of 5%and 7.5%were filled into the micro pore structure of pre-carbonized foam,followed by co-carbonization.The effects of types of carbon aerogel matrix on the structure and properties of the composite material were studied.Compared with those prepared using7.5%sol concentration,the composites with 5%sol concentration showed smaller interfacial gaps and higher carbon aerogel filling ratios,which led to lower high-temperature thermal conductivities and higher compression strengths.In addition,at the carbonization temperature of 1000°C,the carbon foam reinforced polyimide carbon aerogel composite showed an ultra-low thermal conductivity similar to that of polyimide carbon aerogel monomer under the same preparation conditions,while the density was reduced by nearly 50%compared with the pure aerogel monomer under the same preparation conditions.Therefore,the carbon foam reinforced carbon aerogel composite prepared at 1000°C with the sol concentration of 5%could achieve the enhancement of both of the mechanical properties and thermal insulation performances,and maintained a low density of to meet the lightweight requirements of structural-functional integrated materials in the field of aerospace thermal protection.(4)The composites prepared using different types of pre-carbonized foam matrix are studied to analyze the influence of pre-carbonized foam density on the structure and properties of composites,aiming to develop an efficient way to further inhibit the generation of interfacial gaps in composite materials.Compared to CF40-CPI5-1000°C and CF50-CPI5-1000°C,composites prepared by lower density pre-carbonized foam,i.e.,CF20-CPI5-1000°C and CF30-CPI5-1000°C,showed smaller interfacial gaps due to higher shrinkages and lower carbon yields during carbonization,and maintained a low thermal conductivity above 900°C.Therefore,minimizing of the two-phase interfacial gap is essential to keep a low thermal conductivity of the composites.At 1900°C,the thermal conductivity of CF20-CPI5-1000℃and CF30-CPI5-1000℃were 0.57 W/(m·K)and 0.56 W/(m·K).In addition,CF30-CPI-1000°C showed the highest compressive strength and elastic modulus(0.532MPa,9.091MPa)due to its high density of pre-carbonized foam precursor and small interfacial gap.(5)The co-carbonization process of polyimide aerogel,phenolic resin-based pre-carbonized foam and composite samples were characterized with various techniques.The main thermal weight loss of polyimide aerogel,pre-carbonized foam and composite occurred in a similar temperature range.During the process of co-carbonization,the functional group and surface crystal distribution of the gaseous and solid products of the composites matched well with the characteristics of the monomer phase.The elemental composition of the final products was close to the same,which indicated that similar pyrolysis polycondensation reactions of deoxidation,denitrification and dehydrogenation occurred,which is beneficial to limit the separation of phase interface in the composites.