Design And Characterization Of High-toughness Carbon Aerogel Composites And Lattice Structure

Carbon aerogel composites can be applied to the thermal protection system in the advanced high speed aerospace vehicles because of their ultra-high temperature stability,ultra-low density and ultra-low thermal conductivity.However,the intrinsic brittleness of carbon aerogels limits their further application in aerospace insulation field.Based on thorough research on the structural evolution and heat-transfer mechanism of carbon aerogels,for the first time,to achieve the improvement of mechanical bearing capacity and integrated properties,this thesis designs and fabricates the carbon aerogel composites and their lattice structure with ultralight weight and high toughness.The as-prepared carbon aerogel composites and lattice structure are characterized to test their high-temperature performance.This study will provide theoretical methods and technical support for the upgrade and service reliability of ultra-high temperature insulation materials.Conclusions are listed as follows:(1)Carbon aerogels with ultra-low thermal conductivity are prepared via the sol–gel polymerisation of resorcinol with formaldehyde,followed by supercritical drying and carbonization.The effects of organic content and catalyst on the microstructure and properties of aerogels are studied.The fabricated carbon aerogels have low density in the range of 0.028-0.196 g/cm~3,ultra-low thermal conductivity in the range of0.0259-0.0707 W/(m·K).Adding fibres to the aerogels not only improves the compressive strength of the composites but also overcomes the brittleness and improve machinability of the aerogel.This ultra-low high-temperature thermal conductivity and good machinability of carbon fibre reinforced carbon aerogel composites may consequently increase their usage in thermal protection systems.Ambient pressure drying is a simple,fast and cost-effective method for large-sized carbon aerogels(CAs)preparation.In this thesis,two effective methods have been used for the preparation monolithic CAs:GO as shrinkage inhibitor was incorporated into the RF matrix to prepare GO/RF.GO can significantly reinforce the network of gels and improve the resistance to collapse during the drying process;strong monolith CAs was successfully prepared without solvent exchange due to the uniform arrange-meet of carbon particles and pores,fine particle size,abundant network structure and enhanced particle neck.(2)A novel super-elastic carbon fibre composite is prepared by impregnating carbon aerogel into carbon-bonded carbon fibre(CBCF)through vacuum impregnation.The compressive strength and modulus of the CBCF is increased after impregnated with aerogel.The compressive strength of CBCF-CA is higher than CBCF and CBCF with surface modification.CBCF-CA has good high temperature stability.After heat treatment at 1600°C,the microstructure,phase and mechanical properties do not change significantly.The compression strength at high temperature does not decline.Meanwhile,the thermal conductivity of the composites will not increase sharply because of the low thermal conductivity of the impregnated carbon aerogel.The CBCF-CA composite will be a new multifunctional material due to its low density,low thermal conductivity,high specific strength,excellent processability,super-elastic property,and high electromagnetic shielding,which can be used for thermal insulation and protection of aerospace.A phenomenological model for CBCF-CA composite under multiple cyclic loads is investigated to fit Mullins effect.The rubber-like behavior of CBCF-CA composites can be simulated effectively by using the nonlinear model in Abques.Meanwhile,the construction and examination of meso-structural finite element models of a CBCF-CA composite is carried out based on X-ray microtomography digital images(IB-FEM).The accurate meso-structural features of the CBCF-CA composites,which are consisted of carbon fiber and pyrolytic carbon,are reconstructed.The heat transfer analysis is performed using the CBCF model reconstructed by the algorithm.(3)Carbon fiber reinforced phenolic resin(C/PR)composite material lattice structure is fabricated by self-balance connection used continuous carbon fiber.The typical failure modes are observed under out-plane,shear,and in-plane compressive load.The strain and failure modes of lattice structure are calculated by mechanics of materials and energy method based on the geometry of the unit cell.Lattice structures filling with RF aerogel significantly reduce the heating rate at heating test.Lattice structures filling with RF aerogel with outstanding ablative ability can be used in thermal protection systems(TPS).Heat transfer characteristics of C/C structurally and thermally integrated thermal protection system(ITPS)are investigated.The experiment and calculation effective thermal conductivity is test and calculated by thermal resistance modes.The results show that the ITPS filled with carbon aerogel can effectively suppress the thermal radiation and reduce the equivalent thermal conductivity of the structural parts.