Preparation And Microstructure Performance Of Quartz Fiber/Phenolic Aerogel Composites For Thermal Protection

A reliable thermal protection system is one of the key technologies for the safe flight of deep space exploration.The characteristics of lightweight,low ablation rate,low thermal conductivity and long service of thermal protection system put forward stringent requirements on material density and thermal conductivity.Traditional carbon phenolic composites have excellent ablation properties,but have problems such as high density and thermal conductivity due to their dense microstructure.Therefore,it is imperative to develop a new generation of ablative materials with low density and excellent thermal insulation properties.This paper presents the material optimum design,microstructure control,thermal and mechanical properties testing and ablation test validation of lightweight quartz/phenolic ablative composite with phenolic resin aerogel（PR）and phenolic-silica hybrid aerogel（PR-Si O₂）as impregnating phases and three-dimensional（3D）needled quartz fiber felt（NQF）as reinforcement.Using the aerogel structure as the matrix can greatly reduce the density and thermal conductivity of the composite material,and achieve the effect of high temperature resistance and efficient heat insulation integration,but there is still a lack of in-depth research on the ablation performance and high temperature performance of two-component organic-inorganic aerogel-based composite material.In this paper,the ratio of raw materials in the synthesis of aerogel matrix was used as a variable to optimize the pore structure of aerogel matrix,and NQF/PR and NQF/PR-Si O₂ aerogel composites were prepared respectively.The ablation/thermal insulation properties of the NQF/PR and NQF/PR-Si O₂ composites were evaluated by using equipment such as butane flame and oxygen-acetylene flame to simulate the medium-low temperature and high temperature oxidative thermal environment.The ablation/thermal insulation mechanism was discussed based on the morphology evolution law and the microstructure and thermophysical properties of the composites.In view of the disadvantages of high preparation cost and complex process of aerogel materials,this paper uses low molecular weight phenolic resin as raw material to synthesize low-density（0.107-0.218 g/cm³）,high porosity（78.0-89.2%）PR aerogel,the aerogel has a mesopore-dominated hierarchical pore structure,uniform microstructure and good thermal stability.PR aerogel is a three-dimensional network structure formed by stacking nanoparticles with each other and has the characteristics of bicontinuous pores.After FT-IR and XPS analysis and testing,it is shown that the PR aerogel is mainly composed of methylene,methylene ether and N-methylene ether-containing benzene ring bridged structures.In order to improve the oxidation resistance of PR aerogel materials,a PR-Si O₂ hybrid aerogel with uniform microstructure was synthesized by co-sol-gel reaction with tetraethyl orthosilicate as raw material,combined with solvent replacement and atmospheric drying.The PR-Si O₂hybrid aerogel has more abundant micropore and mesoporous structure,and has better thermal stability in air atmosphere,and the PR-Si O₂ hybrid aerogel can still maintain the pore size characteristics of the original aerogel and have lower thermal conductivity after carbonization at high temperature,forming an amorphous C-Si O₂ aerogel.In view of the poor oxidation resistance of carbon phenolic composites in the environment below 1700°C,NQF/PR aerogel composites with a density of0.312-0.369 g/cm³ were fabricated by vacuum impregnation with NQF as the three-dimensional reinforcing phase.The NQF/PR aerogel composite has a uniform and complete microstructure.The PR aerogel fully and uniformly fills the pores between the fibers and wraps on the surface of the quartz fiber.The variation law of the thermodynamic properties of the composite with the content of the curing agent is systematically studied.NQF/PR has excellent mechanical properties and extremely low room temperature thermal conductivity.NQF/PR showed excellent ablation and thermal insulation performance at medium-low temperature and high temperature in oxidizing atmosphere.Under the test of butane flame at 1000°C for more than 10 minutes,the material did not show macroscopic damage.In the oxygen-acetylene high temperature oxidizing atmosphere environment over 1700°C,the linear ablation rate and mass ablation rate of NQF/PR are as low as 0.0163 mm/s and0.0161 g/s,and the temperature of the backside at a distance of 25 mm from the ablation surface is as low as 267°C in the monitoring process of 400 s,and it has good ablation resistance and thermal insulation ability.The Si O₂ layer formed on the ablated surface is attached to the surface of the material through fiber connections,and the heat flow is prevented from penetrating into the interior of the material by surface radiation,which improves the material’s anti-oxidative ablation ability.Based on the higher ablation resistance and thermal insulation requirements of composites,the NQF/PR-Si O₂ aerogel composites with a density of 0.221-0.347 g/cm³ were prepared by vacuum impregnation with PR-Si O₂ hybrid aerogel as matrix phase and NQF as reinforcement phase.The NQF/PR-Si O₂ composite has a uniform and complete microstructure.The NQF/PR-Si O₂ exhibits excellent ablation and thermal insulation properties at medium-low temperature and high temperature in an oxidizing atmosphere.Under the test of a butane flame at 1000°C for more than 10 minutes,and the back temperature is reduced by 40°C compared to NQF/PR.In the oxygen-acetylene high temperature oxidizing atmosphere environment over 1700°C,the linear ablation rate and mass ablation rate of NQF/PR-Si O₂ nanocomposites are as low as 0.0119 mm/s and 0.0121 g/s,and the temperature of the backside of the sample at a distance of 25 mm from the ablated surface was as low as205°C during the 400 s monitoring process,and the back temperature was reduced by 62°C compared with NQF/PR.The Si O₂ layer formed on the ablated surface is attached to the surface of the material through fiber connections.With the increase of Si O₂ in the matrix,the dense Si O₂ layer on the surface is denser and has no holes.The ablation performance of the composite is mainly achieved by the mass ejection effect of the ablation layer and the reverse radiation effect of the surface melting layer to achieve active thermal protection requirements.High-efficiency thermal insulation is achieved through the excellent thermal retardation effect of PR-Si O₂ aerogel,so as to solve the synergistic problem of lightweight and efficient heat insulation and ablation conformal of the composite for ablative thermal protection at higher temperatures.