【作者】 田野；

【导师】 黄海明；

【作者基本信息】 北京交通大学 ， 力学， 2019， 博士

【摘要】 与近地轨道速度再入的航天飞行器相比,深空探测与载人登月返回舱以第二宇宙速度再入大气层时会经受更严重的气动加热,对热防护材料的性能提出了更高的要求。轻质炭化材料是未来深空探测及载人航天返回舱热防护结构的关键材料,新型PICA复合材料(Phenolic impregnated carbon ablator)是其中的典型代表。虽然PICA满足了高超声速飞行器热防护轻质与隔热的需求,但其抗烧蚀性能及热防护效率亟待提高。利用碳纤维针刺毡和硼改性酚醛树脂,采用真空浸渍法制备了 PICA,并通过性能表征对其制备工艺进行了优化,确定了 PICA制备的技术途径。通过非等温DSC法及正交试验优化了 PICA的固化工艺;以层间剪切强度为评估指标确定了碳纤维针刺毡的表面处理工艺;研究了碳纤维针刺毡密度及浸渍剂浓度对材料最终密度、压缩强度及导热系数的影响。结果表明:固化升温速率对材料压缩强度影响较大,单纯依靠非等温DSC法和外推法不能得到最优的固化温度,PICA最优固化工艺为120℃ 1h+ 170℃ 1h,固化升温速率为1℃/min,此时PICA的压缩强度最高;在常温下,采用浓硝酸对碳纤维针刺毡进行表面处理时,随着处理时间的增加,PICA的层间剪切强度呈现先增后减的趋势,其中处理时间为2h时,PICA的层间剪切强度最高;随着碳纤维针刺毡密度的增加,PICA材料密度和常温导热系数线性增加,压缩强度大幅提高;随着浸渍剂浓度的提高,PICA材料密度线性增加,压缩强度明显提高,但常温导热系数降低。为了提高PICA的抗烧蚀性能及其热防护效率,提出了陶瓷化自适应环境技术,探索了多种陶瓷填料对PICA性能的影响规律。利用ZrC、ZrB2、SiC陶瓷填料分别对PICA进行改性,研究了填料含量对PICA密度、热稳定性、力学性能、导热系数及抗烧蚀性能的影响。结果表明:(1)ZrC、ZrB2、SiC三种不同陶瓷填料对PICA密度、残炭率、压缩强度以及常温导热系数的影响规律基本一致。随着陶瓷填料含量的增加,PICA密度增加,残炭率提高,压缩强度下降,常温导热系数线性增加。(2)将适量的ZrC、ZrB2、SiC三种陶瓷填料分别引入PICA均能大幅降低线烧蚀率,这是因为在气动热环境下陶瓷填料改性的PICA表层陶瓷化,这个陶瓷层不仅能够增加PICA炭化层的强度,抑制表层的机械剥蚀,而且能够改善烧蚀表面的抗氧化性能,防止碳纤维的氧化,减少烧蚀表面的热化学烧蚀,从而抑制烧蚀过程中PICA的表面后退,进而达到降低线烧蚀率、改善抗烧蚀性能、提高热防护效率的目的。但是,三种陶瓷填料对PICA抗烧蚀性能的影响表现出了不同的规律:(ⅰ)PICA线烧蚀率随ZrC含量的增加逐渐降低,而随ZrB2、SiC含量的增加均呈现先减后增的趋势,其中ZrB2含量为11%时PICA线烧蚀率与无任何添加PICA相比仍大幅降低,但当SiC含量为11%时PICA线烧蚀率则与无任何添加PICA的基本相当。(ⅱ)随着ZrC含量的增加,最大烧蚀表面温度逐渐升高;随ZrB2含量的增加,最大烧蚀表面温度也逐渐升高,但当ZrB2含量达到7%后,最大烧蚀表面基本保持不变;随着SiC含量的增加,最大烧蚀表面则逐渐降低,与ZrC、ZrB2的趋势正好相反,且当SiC含量从5%增加到11%时,其最大表面温度基本保持不变。(ⅲ)随ZrC、ZrB2含量的增加,PICA的烧蚀背面温度均呈现逐渐降低的趋势,然而SiC含量对PICA烧蚀背面温度则影响较小。为了预估PICA在气动热作用下的热响应,开发了一套适用于PICA的热解层导热系数测量系统,获得了 PICA的热解层导热系数,完善了 PICA的性能评估体系。该系统包括测量装置及热解层导热系数识别程序两部分,其测试原理是:通过测量试件在恒定温度加热条件下的瞬态热响应,利用反向参数识别方法对其导热系数进行反演。测量装置主要包括气密外壳、中心测量装置、伺服电机起重装置、气压控制系统、进料口、承载结构、承力杆、试件托盘、压力传感器以及控制面板系统等组成部分。热解层导热系数识别程序是基于炭化材料的一维热解层模型和反向参数识别方法,利用Matlab编写的参数识别计算程序。利用该测量系统对不同气压下PICA热解层导热系数进行测试,验证了热解层导热系数识别程序的有效性,研究了不同气压对PICA热解层导热系数的影响。参数识别结果表明:开发的热解层导热系数测量系统能够有效测量炭化材料的热解层导热系数;随着气压的减小,PICA热解层导热系数逐渐减小;气压越低,温度与PICA热解层导热系数之间的非线性越明显。更多还原

【Abstract】 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.更多还原