| With the mach numbers of hypersonic speed vehicles increasing,the aerodynamic heating environment is becoming increasingly serious.The efficient and reliable thermal protection material is one of the key systems for high-performance aircraft with safe flight.It not only requires high temperature and ablation resistance,but also requires low thermal conductivity and low bulk density.The common thermal protection materials are composite multi-layer hybrid structures with C/C or C/SiC on the outside with aerogel on the inside,which have the disadvantages of large thickness and large surface density.It is an urgent to develop a new type of adiabatic and ablative structure-function material.Based on this,this paper presents a high-temperature vacuum insulation panel(HTVIP)material that can withstand high temperatures with low bulk density,low thermal conductivity,ablation resistance,oxidation resistance,and heat insulation.The material consists of a SiC foam core,a C/C shell and a dense CVD SiC encapsulation film.The thermal conductivity of HTVIP is 0.2~0.8 W/m·K with vacuum inside,its density is only 0.6 g/cm~3~1.0 g/cm~3,which is 1/2~1/3 of the traditional materials.According to the characteristics of fiber weaving structure,2D and 2.5D carbon fiber preforms were selected and coated on the surface of SiC foam.The C/C shell was prepared by CVI and PIP methods.The SiC coating was prepared by low-pressure chemical vapor deposition to achieve vacuum encapsulation,then the HTVIP was formed.The thermal conductivity of the C/C shell was measured with a laser thermal conductivity meter.The thermal conductivity of the HTVIP was measured with a flat plate thermal conductivity meter.The microstructure of the shell was characterized with scanning electron microscope and the surface temperature of the HTVIP was observed with infrared thermal radiation imager.A model of the microstructure of the carbon fiber preform was established.The calculation model of the thermal stress of the SiC coating was established.Based on ANSYS software,the heat transfer characteristics of HTVIP was studied,which provided new ideas for high temperature heat insulation fields.The main innovations in this article are as follows:(1)Based on the hypothesis of yarn cross section in carbon fiber fabrics,the prediction model of the fabrics was established,and the influences of different warp/weft density and yarn fineness on the maximum pore volume in fabric were analyzed.(1)When the warp density P_j is 3,the weft density P_w increases from 2 to 9,and the volume fraction increases from 15%to 35%,the maximum pore volume in the 2D fabric decreases from 2.69mm~3 to 0.195 mm~3,and the 2.5D fabric decreases from 2.67 mm~3.To 0.125 mm~3,and at the same volume fraction,the lower the yarn fineness,the smaller the maximum pore volume in 2D and 2.5D fabrics;(2)When P_j+P_w is a certain value,with the increase of P_w,the maximum pore volume in 2D and2.5D fabric decreases,while with the increase of P_j,the maximum pore volume increases.(2)The HTVIP C/C composite shell was densified by CVI and PIP merhods.The densification process and its influence on gas barrier properties were systematically investigated.(1)After 200 hours of CVI,the bulk densities of 2D and 2.5D C/C composite shells reached 1.626g/cm~3 and 1.641 g/cm~3,respectively,but large pores of about 100μm remained inside;(2)After 6 times PIP,the bulk densities of the 2D and 2.5DC/C composite shells reached 1.709g/cm~3 and 1.716 g/cm~3,respectively,and the maximum pore size decreased to 5μm,Porosity between fiber bundles is effectively filled;(3)After CVI and PIP processes,the gas transmission rates of 2D and 2.5D C/C composites were62.9 cm~3/m~2·d·Pa and 59.4 cm~3/m~2·d·Pa,respectively.(3)A dense glass carbon composite coating with low gas transmission was prepared.The microstructure and gas permeability of the glassy carbon coating were studied.(1)The pure glassy carbon coating has large pyrolysis shrinkage and exits many cracks.The gas permeability of 2D C/C coated with glassy carbon is 36.8 cm~3/m~2·d·Pa a,a decrease of 41.5%;(2)A certain amount of 1-5μm carbon powder is added to the phenolic resin to produce a glassy carbon composite coating.The gas permeability of the coated 2D C/C composite shell is reduced to 7.6cm~3/m~2·d·Pa,a drop of 87.9%.(4)Based on the Timoshenko theory,the residual thermal stress model of monolayer-multilayer SiC coating was constructed,and the influence of different layers on gas permeability was studied.(1)The calculations show that the shear stress in single-layer SiC coating is 25 MPa at the edge and the tensile stress is 1.5 GPa in the middle.When the coating is increased to 11 layers,the maximum tensile stress decreases to 0.62 GPa,lower than the tensile strength of SiC coating;(2)Experiments show that 5 layers of SiC on the surface of the 2D C/C composite shell has a gas transmission rate of only 0.001 cm~3/m~2·d·Pa,showing excellent gas barrier effect.After 10 times thermal shock cycles,the gas transmission rate increased to 0.027 cm~3/m~2·d·Pa,the initial rate of rise of HTVIP internal pressure after five CVD depositions was calculated to be 0.8 Pa/min.(5)The test characterizes the HTVIP thermal insulation performance and establishes the HTVIP thermal conductivity prediction model.Based on the ANSYS software,the HTVIP surface heat flow impact is dynamically simulated.(1)The test shows that the maximum thermal conductivity of the 2D and 2.5D C/C composites in the warp direction is 16.21 W/m·K and 10.69 W/m·K,respectively.It indicates the 2.5D braided structure is more conducive to relieve the bridge effect of HTVIP;(2)From 25℃ to 900℃,the 2D HTVIP thermal conductivity increases from 0.368 W/m·K to1.027 W/m·K,an increase of 179%,and the 2.5D HTVIP thermal conductivity increases from 0.326W/m·K to 0.748 W/m·K,an increase of 129%;(3)Simulations show that the front temperature of 2D and 2.5D HTVIP rapidly rises to 1800℃ within 40s,the back temperature rises to 471℃ and 386℃ within 300s,,respectively and the highest values of heat flux of 2D and 2.5D HTVIP shells on the side were 163 kW/m~2·K and 105 kW/m~2·K,respectively,indicating that the 2.5D HTVIP has better insulation than the 2D structure. |
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