Microstructure Regulation And Thermal Protection Performance Of Carbon Foam-Reinforced Carbon Aerogel Composites

Hypersonic aircrafts are usually exposed to extraordinarily high temperature,which sets higher demands for thermal protection materials.Novel lightweight thermal protection materials with low thermal conductivity,high strength,and good reliability are urgently required.Carbon aerogels,which can be used as ultra-high temperature thermal protection materials,have high specific extinction coefficient,low thermal expansion coefficient,low thermal conductivity and excellent thermal stabilities.However,the inherently weak non-covalent interactions among the nanoparticles within carbon aerogel frameworks lead to poor mechanical performances,which limits their practical applications.In this thesis,lightweight phenolic resin carbon foams were used to reinforce carbon aerogels.The obtained lightweight carbon foam-reinforced carbon aerogel composites with low thermal conductivity and high compression strength show great potential as ultra-high temperature thermal protection materials.Precarbonized phenolic resin foams were impregnated with resorcinolformaldehyde(RF)sol in vacuum,aged at 50 °C,exchanged with ethanol,dried supercritically,and finally carbonized by copyrolysis.The structure-performance relationship was separately studied from carbon aerogels and carbon foams.Firstly,the structure of carbon aerogels was studied by changing a series of parameters such as catalyst content,formaldehyde content,aging temperature,aging time and so on.The influences of these parameters on the structure of carbon aerogels were studied,and the mechanisms were further analyzed.The results showed that the specific surface area,pore volume,shrinkage and density of carbon aerogels were positively correlated with the contents of catalyst.The particle size of carbon aerogels was negatively correlated with the contents of catalyst.The pore volume,average pore size,and shrinkage of carbon aerogels increased with the contents of formaldehyde,while the average particle size of aerogels decreased.The aging temperature has no obvious effect on the pore structure of carbon aerogels.And the aging temperature was negatively correlated with the density of carbon aerogels.After prolonging aging time,the mesoporous volume and the skeleton density of carbon aerogels increased,while the specific surface area of carbon aerogels remained unchanged.Therefore,the amount of catalyst and formaldehyde have great influence on the structure of carbon aerogel.Secondly,the effects of the structure of carbon aerogel and the types and density of phenol foams on composite structure and properties were studied respectively.The prepared carbon aerogel-reinforced carbon foams composite showed a high mechanical performance,and its thermal conductivity increased slowly with temperature.The composite maintained low thermal conductivity even at high temperatures.We demonstrated that the low temperature and medium temperature thermal protection performance of the composite was improved with catalyst contents reduced.As catalyst content increased,the ultra-high temperature thermal protection performance of the composite improved.Moreover,commercial phenolic resin foams(CH),low-density phenolic resin foams(CL)and medium-density phenolic resin foams(CM)were used as matrices to enhance the mechanical performances of carbon aerogels.The result showed that using CL as matrices,the prepared composites maintain low thermal conductivity and high compressive strength.The thermal conductivity of CCL-900 composites with limited interfacial gap was as low as 0.31 W/m·K and the compressive strength of CCL-900 was 0.51 MPa.Low-density phenolic resin foams exhibited both low thermal conductivity and high after compounding with aerogels.Then,CL were integrated with aerogels in different precursor concentrations to optimize the preparation process parameters.As a result,the compressive strength of the composite was adjustable in the range of 0.95～0.11 MPa.The thermal conductivity of the composite at 800 °C was about 0.1 W/m K.Finally,the copyrolysis mechanism of the composite was studied.Dehydrogenation,deoxygenation and decarbonization reactions occurred during copyrolysis process of the phenolic resin foam-reinforced aerogels composite.