Research On Fabrication And Properties Of Novel Lightweight Carbon/Phenolic Ablative Composite

Low-density ablative thermal protection materials were usually composed of highly porous and fibrous substrate as reinforcement and porous resin with low thermal conductivity as matrix.These materials were developed to fulfill the requirement of lightweight,low ablation rate,low thermal conductivity and long re-entry time of thermal protection for heatshield and backshell of planetary entry probes and re-entry capsules for deep space exploration.This dissertation presents the material design,preparation,microstructure controlling of lightweight carbon/phenolic ablative composite with phenolic resin(PR)aerogels as matrix and three-dimensional(3-D)canbon fibrous substrate as reinforcement,and its anti-ablation and thermal insulation performance in the hype r aero-thermodynamic environment.Firstly,carbon-bonded carbon fiber(CBCF)composites were prepared with chopped carbon fibers and dilute PR solution by pressure filtration,which achieve a homogeneous microstructure without agglomeration and layering,and relatively high mechanical and good thermo-physical properties.Secondly,PR aerogels and PR/silicone(PR-Si)hybrid aerogels were synthesized through sol-gel polymerization.Thirdly,lightweight needled carbon fiber felt/phenolic resin(NCF/PR),CBCF/PR,and CBCF/PR-Si aerogel composite were fabricated through vacuum impregnation using CBCF or NCF as substrate,and PR aerogel or PR-Si hybrid aerogel as filler,respectivel.Microtructure,mechanical,and thermo-physical properties were systematically investigated.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,char,pyrolysis zone and vir gin material,accompanied by the characteristic microstructure and textual properties,chemical state,thermal stability and pyrolysis behavior of PR aerogel,ablation/insulation mechanism of the novel lightweight carbon/phenolic ablative composites was discussed.CBCF composites were prepared by a modified pressure filtration technique.A homogeneous microstructure without agglomeration and layering was achieved.The chopped carbon fibers are bonded together at the intersections of adjacent fibers to form a fibrillar network bonded by discrete carbon matrix derived from carbonization of PR.As a consequence of filtration,the carbon fibers in CBCF are oriented preferentially in one plane,the fibers in the plane(which is defined as the XY direction)are distributed with randomly and uniformly,while through the thickness(the z direction)are uniformly distributed over the height of the composite,making CBCF a transverse isotropic material with different material properties in these two directions.The composites with density of 0.162–0.381 g/cm~3 possessed compressive strengths ranged from 0.93–6.63 MPa and 0.30–2.01 MPa,compressive modulus of 19.28–144.48 MPa and 4.51–55.23 MPa,and thermal conductivity at room temperature(RT)of 0.314–0.505 W/(m·K)and 0.139–0.368 W/(m·K)in the XY and Z direction,respectively.The thermal conductivity of CBCF of 0.251 g/cm~3 increase from 0.314 W/(m·K)to 1.888 W/(m·K)and 0.204 W/(m·K)to 0.894 W/(m·K)at RT~1275? in the XY and Z directions,respectively.PR aerogels with density of 0.117–0.227 g/cm~3(porosity of 77–88%)were synthesized through sol-gel polymerization,accompanied by solvent exchange and ambient pressure drying,using commercial available PR pre-polymers as the precursor,hexamethylenetetramine(HMTA)as the catalyst,and ethylene glycol(EG)as the solvent and porogen.The aerogel frameworks were composed of sub-micron aggregates that interconnect in different directions,which were stacked by primary nanoparticles,interconnected and open pores were formed among the particles and aggregates.The PR aerogels contain a large amount of meso-and macropores and mesopores represent the dominant proportion,the aromatic nuclei of the PR molecules are linked by the methylene,dibenzyl ether and N-methylene bridges and formed the backbone of PR aerogels,thus the PR aerogels exhibit good thermal stability and the classic thermal degradation process,which can be mainly divided into three stages,as seen in common PR.A novel phenolic resin/silicone(PR-Si)hybrid aerogel was synthesized through a facile sol-gel polymerization from a co-precursor solution of methyltrimethoxysilane(MTMS),PR,EG and HMTA.The hybrid aerogels possess hierarchically micro-meso-macroporous structure,and higher thermal stability and residual weigh at high temperature than that of the pristine PR aerogel,which is attributed to the MTMS chemically grafted the PR aerogels by the –Si–O–C– bridges,furthermore part of MTMS self-condensed to generate high cross-linked siloxane(–Si–O–Si–)network.CBCF/PR aerogel composite was fabricated through vacuum impregnation using CBCF as 3-D reinforcement and PR aerogels as matrix.The micrographs show that a homogeneous microstructure was achieved in the composite,the aerogels are fully and uniformly occupied the voids between carbon fibers,and are coated with a thin layer on the fiber surface.Moreover,a homogeneous PR aerogel matrix and a superior fiber/resin interface without cracking and fracture.CBCF/PR aerogel composite with density of 0.247,0.288,0.314,and 0.346 g/cm~3 was produced with CBCF of 0.118,0.163,0.192,and 0.227 g/cm~3,respectively.The lightweight composites possessed compressive strengths of 1.14 –2.94 MPa and 0.58–1.59 MPa,compressive modulus of 42.21–80.15 MPa and 15.81–18.73 MPa,and RT thermal conductivity of 0.125–0.154 W/(m·K)and 0.188–0.289 W/(m·K)in the XY and Z direction,respectively.The CBCF/PR aerogel composite has good ablative and thermal insulative properties under a simulated atmospheric re-entry condition(cold wall heat flux of 3.7 MW/m~2,enthalpy of 35 MJ/m~2,surface pressure of 2.5 k Pa,test time of 60 s)simulated by an arc jet wind tunnel: recession rates as low as 0.057 mm/s,mass loss rate approximately 0.136 5 g/s and internal temperature peaks below 100 ? at 40 mm and only 60 ? at 50 mm in-depth position as the surface temperature exceeded 2000 ?.NCF/PR aerogel composite was prepared through vacuum impreg nation of NCF with PR aerogels.Composites with density ranged from 0.270 to 0.370 g/cm~3 were produced with impregnated NCF of 0.164 g/cm~3(porosity of 91%)with PR aerogels of 0.117–0.227 g/cm~3(porosity of 77–88%).The lightweight composites possessed compressive strengths of 1.48–11.02 MPa and 0.83–4.90 MPa,compressive modulus of 49.68–633.39 MPa and 15.23–154.69 MPa,and RT thermal conductivity of 0.180–0.230 W/(m·K)and 0.093–0.131 W/(m·K)in the XY and Z direction,respectively.The NCF/PR aerogel composite has good ablative and thermal insulative properties under a simulated atmospheric re-entry condition(cold wall heat flux of 1.5 MW/m~2,enthalpy of 20 MJ/m~2,surface pressure of 2.5 k Pa,test time of 33 s)simulated by an arc jet wind tunnel: recession rates as low as 0.029 mm/s and internal temperature peaks below 90 ? at 38 mm in-depth position as the surface temperature exceeded 2000 ?.CBCF/PR-Si hybrid aerogel composites with density of 0.312–0.356 g/cm~3 were fabricated through vacuum impregnation of CBCF with PR-Si hybrid aerogels.The lightweight composites possessed compressive strengths of 2.50–4.08 MPa and 0.76–1.77 MPa,compressive modulus of 65.44–211.14 MPa and 15.03–33.93 MPa,and RT thermal conductivity of 0.145–0.240 W/(m·K)and 0.098–0.165 W/(m·K)in the XY and Z direction,respectively.The CBCF/PR-Si hybrid aerogel composite has good thermal ablative and insulative properties in oxyacetylene flame simulated high temperature oxidizing environment(heat flux of 1.1 MW/m~2,test time of 120 s): linear ablation rates as low as 0.073 mm/s,mass ablation rate only 0.016 g/s,and internal temperature peaks at 1260 ? at 20 mm,580 ? at 30 mm and approximately 150 ? at 38 mm in-depth position as the surface temperature approximately 1800 ?.By analyzing the evolution of microstructure of ablated surface,char,pyrolysis zone and virgin material,accompanied by the characteristic microstructure and textual properties,chemical state,thermal stability and pyrolysis behavior of PR-Si aerogel,ablation/insulation mechanism was discussed.