Preparation And Characterization Of Ceramic Filler Modified PICA Composites

Compared with the capsules that return to the atmosphere at low earth orbit velocity,the deep-space exploration and manned lunar landing capsules are subjected to severer aerodynamic heating during reentry at the second cosmic velocity,which means that the higher requirements are put forward for the properties of thermal protection materials.Lightweight charring ablative materials are critical materials of thermal protection system(TPS)used for the future deep-space exploration and manned lunar landing capsules.Phenolic impregnated carbon ablator(PICA),a novel lightweight charring ablative material is the typical example.PICA can satisfy the lightweight and thermal insulation requirement of TPS of hypersonic vehicle,but its ablation resistance performance and thermal protection efficiency urgently need to be improved.Applying needle-punched carbon fiber felt and boron modified phenolic resin,PICA is prepared by vacuum impregnation method using.The curing process of PICA is optimized by non-isothermal DSC method and orthogonal test.The surface treatment process of needle-punched carbon fiber felt is determined,based on the interlaminar shear strength.The effects of the density of needle-punched carbon fiber felt and impregnant concentration on the density,compressive strength and thermal conductivity of PICA are investigated.The test results can be conclude as follow:(1)the heating rate during the curing process has an obvious effect on the compressive strength and the suitable curing temperature obtained only based on the non-isothermal DSC method and extrapolation method is difficult.The optimum curing process of PICA is 120℃ 1h +170℃ 1h and the curing heating rate is 1℃/min,based on considering the better compressive property as the standard.(2)When the needle-punched carbon fiber felts are treated with concentrated nitric acid,the interlaminar shear strength of PICA firstly increases and then decreases with treatment time increasing.The interlaminar shear strength of PICA is the highest when the treatment time is 2h.(3)With the density of carbon fiber needle felt increasing,the density of PICA material and the thermal conductivity at room temperature increase linearly,and the compressive strength greatly improves.(4)With impregnant concentration increasing,the density of PICA material increases linearly and the compressive strength increases remarkably,but the thermal conductivity at room temperature decreases.In order to improve ablation resistance performance and thermal protection efficiency of PICA,a self-adaptive thermal environment technology based on ceramic is proposed and the influence rules of different ceramic filler on the properties of PICA are explored.The PICA is modified by introducing ZrC,ZrB2 and SiC ceramic fillers repectively and the effects of different ceramic fillers on density,thermal stability,mechanical properties,thermal conductivity and ablation resistance of PICA are investigated.The results show that(1)the influence rules of ZrC,ZrB2 and SiC ceramic fillers on density,char yield,mechanical properties and thermal conductivity are consistent.With the content of ceramic fillers increasing,the density of PICA increases,the char yield increases,the compressive strength decreases,and the thermal conductivity at room temperature increases linearly.(2)Introducing moderate ZrC,ZrB2 and SiC ceramic fillers respectively can all significantly reduce the linear ablation rate of PICA.This can be attributed to ceramic of surface layer of PICA.The ceramic layer not only can increase the strength of the char layer and inhibit the mechanical erosion of surface layer,but also can improve antioxidant properties of ablation surface of PICA,prevent the oxidation of carbon fiber,inhibit thermal chemical ablation of ablation surface and thereby reduce the surface recede of PICA during ablation process,and thus achieve the goals of decreasing the linear ablation rate and improving ablation resistance performance and thermal protection efficiency.However,the effects of the three ceramic fillers on the ablation resistance performance of PICA present different rules:(ⅰ)the linear ablation rate of PICA decreases with ZrC content increasing,but the linear ablation rates both firstly decrease and then increase with ZrB2 content and SiC content increasing.Hereinto,the linear ablation rate of PICA with the 11%ZrB2 content is far lower than that of PICA without ceramic filler and the linear ablation rate of PICA with the 11%SiC content is almost equal with that of PICA without ceramic filler,(ⅱ)The maximum ablation surface temperature increases with ZrC content increasing.When ZrB2 content increases from 0%to 7%,the maximum ablation surface temperature of PICA increases gradually.And then the maximum ablation surface temperature of PICA remains about the same,when ZrB2 content continues to increase.The changing trend of maximum ablation surface temperature with the SiC content is contrary to those with ZrC content and ZrB2 content.The maximum ablation surface temperature gradually decreases when the SiC content increases from 0%to 5%and the maximum surface temperature is basically unchanged when the SiC content increases from 5%to 11%.(ⅲ)The back-face temperatures both decrease with ZrC content and SiC content increasing,but the effect of SiC content on the back-face temperature of PICA is not obvious.In order to estimate the thermal response of PICA in aerodynamic heating environment,we develope a measuring system of the thermal conductivity of pyrolysis layer,obtain the thermal conductivity-of pyrolysis layer of PICA and perfect the performance evaluation system of PICA.The system includes the measuring equipment and the parameter identification software of thermal conductivity of pyrolysis layer.The testing principle of this measuring system is that the transient thermal response of test sample is recorded when the test sample is heating at constant temperature,and then the thermal conductivity of pyrolysis layer is identified by the inverse method.The measuring equipment comprises airproof body,central measuring unit,servo motor lifting device,air-pressure control system,sample inlet port,bearing structure,sample tray,air-pressure sensor,control panel and so on.The identification software of thermal conductivity of pyrolysis layer is the written computer codes on MATLAB,based on the pyrolysis layer model for the charring ablative materials and the inverse parameter identification method.And then the thermal conductivity of pyrolysis layer of PICA is measured by the measuring system under different air pressure,in order to verify the validity of identification method of thermal conductivity of pyrolysis layer and investigate the effect of air pressure on the thermal conductivity of pyrolysis layer of PICA.The results of parameter identification show that the developed measuring system can effectively measure the thermal conductivity of pyrolysis layer of charring ablative materials.With the air pressure decreasing,the thermal conductivity of pyrolysis layer of PICA is gradually decreased.With the air pressure decreasing,the nonlinear relationship between temperature and thermal conductivity of pyrolysis layer of PICA becomes more obvious.