**Optimization of Properties and Ablation Behavior of CBCF/RF Aerogel Composites**

Author:Wang Zhong Hai

Supervisor:hong chang qing

Database:Doctor

Degree Year:2018

Download:57

Pages:138Size:7957K

Keyword:ablation properties，carbon-bonded carbon fiber composite，Numerical simulation，RF aerogel，silane

With the development of space technology,deep space exploration and planetary exploration have become the focus of aerospace research.As the key subsystem of planetary entry probes and re-entry capsules for deep space exploration,the thermal protection technology becomes the key technology that restricts the performance of the project.Especially,the thermal protection for heatshield and backshell set higher request on lightweight,low ablation rate,low thermal conductivity and long re-entry time.In this dissertation,material design,preparation,microstructure controlling of lightweight ablative composites were presented,unsing aerogels as matrix and carbon-bonded carbon fiber（CBCF）composite as three-dimensional reinforcement,furthermore the ablation performance and mechanism in the hyper aero-thermodynamic environment.Firstly,resorcinol-furfural（RF）aerogels and silane-resorcinol-furfural（Si-RF）hybrid aerogels were synthesized through sol-gel poly-condensation and ambient pressure drying（APD）.Secondly,through vacuum pressure impregnation of self-fabricated carbon-bonded carbon fiber（CBCF）composites with aerogel precursor solutions,low-density CBCF/RF and CBCVF/Si-RF aerogel composites were fabricated.Microtructure,mechanical,and thermo-physical properties were systematically investigated.Thirdly,Anti-ablation and thermal insulation performance was evaluated under extreme environment simulated by arc-jet wind tunnel and oxy-acetylene flame.Through analyzing the evolution of microstructure of ablated surface,c har,pyrolysis zone and virgin material,accompanied by the characteristic microstructure and textual properties,chemical state,thermal stability and pyrolysis behavior of aerogels,ablation/insulation mechanism of these novel low-density ablators were discussed.Finally,an intrinsic mathematical model describing ablation behavior is established,which gives priority to solid phases and includes influences of complex factors such as pyrolytic reaction and mass injection.Moreover,full-scale multi-dimensional model was mathematical modeling by COMSOL-Multiphysics and was solved transiently in a multi-field strong coupling way.The synthesized RF aerogels,contain a large amount of meso-and macropores and mesopores represent the dominant proportion,also exhibit the chemical composition and thermal stability of common PR.CBCF/RF aerogel composite with density of 0.26-0.34 g/cm3 was fabricated through vacuum impregnation using CBCF as 3-D reinforcement and RF aerogels as matrix.A homogeneous microstructure was achieved in the composite.The variation of mechanical and thermal insulative properties with CBCF and RF aerogels are analyzed,respectively.The low-density composites possessed compressive strengths of 0.62–3.27 MPa and0.44–1.09 MPa,compressive modulus of 18.20–73.74 MPa and 10.20–35.17 MPa,and thermal conductivity of 0.189–0.312 W/（m·K）and 0.105–0.168 W/（m·K）in the XY and Z direction,respectively.The CBCF/RF aerogel composite has good ablative and thermal insulative properties under a simulated atmospheric re-entry condition（heat flux of 1.5 MW/m2,enthalpy of 20 MJ/m2,surface pressure of 2.5kPa,test time of 33 s）simulated by an arc jet wind tunnel:recession rates of0.082–0.089 mm/s,mass loss tates of 0.288–0.329 g/s,and internal temperature peaks at 200℃,150℃ below 80℃ at 18,28,and 38 mm in-depth position,respevtively,while the surface temperature exceeded 2000℃.It is confirmed that reradiation mechanism is the main ablation mechanism through the evolution of the recessed surface,char,pyrolysis zone and virgin material after the ablation.The Si-RF hybrid aerogels possess hierarchically micro-meso-macroporous structure and mesopores represent the dominant proportion,and higher thermal stability and residual weigh at high temperature than the pristine RF aerogel,which is attributed to the silane chemically grafted the RF aerogels by the–Si–O–C–bridges,furthermore part of silane self-condensed to generate high cross-linked siloxane（–Si–O–Si–）network.CBCF/Si-RF hybrid aerogel composites with density of 0.31-0.38 g/cm3 were fabricated through vacuum impregnation through CBCF with density of 0.18 g/cm3 and Si-RF hybrid aerogels.The lightweight composites possessed compressive strengths of 1.34-2.94 MPa and 0.58-0.93 MPa,compressive modulus of 15.33-18.73 MPa and 2.21-80.15 MPa,and RT thermal conductivity of0.109-0.136 W/（m·K）and 0.085-0.108 W/（m·K）in the XY and Z direction,respectively.The CBCF/Si-RF hybrid aerogel composite has good thermal ablative and insulative properties in oxyacetylene flame simulated high temperature oxidizing environment（heat flux of 1.1 MW/m2,test time of 60 s）:linear ablation rates as low as 0.117 mm/s,mass ablation rate only 0.274 g/s,and internal temperature peaks at 333℃ at 18 mm,252℃ at 28 mm and approximately 110℃ at 38 mm in-depth position as the surface temperature exceeded 2000℃.The heat blockage effect of SiO2 was verified by analyzing the evolution of alated sample.Based on thermal protection mechanism analysis results of CBCF/RF,an intrinsic mathematical model describing ablation behavior is established,which gives priority to solid phases and includes influences of complex factors such as pyrolytic reaction and mass injection.Using Gaussian peak differentiating and imitating method as well as peak analysis,multistage pyrolysis kinetic model of three pyrolysis stages based on multiple generalized Arrhenius equations is obtained.And there comes a calculation model describing the variation of the material properties parameters over time and temperature by adopting pyrolysis ratio as an interpolation coefficient.Moreover,a flow model of pyrolysis gases in porous media is established based on the conservation of mass and momentum.Net heat flow transferred to the inside is calculated by surface energy balance equation and the thermal response equation of the control body is solved subsequently by the assumption of thermodynamic equilibrium.Based on the mathematical control equation derived,full-scale multi-dimensional model is mathematical modeling by COMSOL-Multiphysics and is solved transiently in a multi-field strong coupling way.The material,geometry and boundary nonlinear effects are considered,and surface recession is characterized by deformed geometry technique.Numerical simulation results in comparison with the arc tunnel experiments show both regularity consistency of response and reasonable relative error.This,in turn,validates the validity of mathematical model and indicates high fidelity of simulation solution.In the post-data processing,line-diagrams and cloud-graphics of property field variables and physical field variables are presented.The analysis of primary and secondary factors in thermal protection is to further supporting material selection optimization and system architecture design.