【作者】 周祥发；

【导师】 肖汉宁；

【作者基本信息】 湖南大学 ， 材料学， 2011， 博士

【摘要】 新型航天飞行器在大气层中长航时且以大于6Ma的速度飞行时,遭遇到的气动加热环境极其严重,其表面累计气动加热温度超过800℃。因此,开展高效隔热以及可热控的复合材料研究和设计具有重要的意义。本文首次将相变材料应用到飞行器内部防热系统上,通过利用纳米多孔陶瓷的三维骨架支撑作用,使相变材料比较均匀且充分地分布在多孔陶瓷网状结构之中,开展了热控相变材料熔渗纳米多孔陶瓷的研究。研究表明:多孔陶瓷骨架将75⁸5wt%的相变材料分成无数个纳米级蓄热小单元,当温度超过相变材料的熔点时,相变材料熔化而吸收热量,延缓了热量的传递,控制了温度的急剧上升,且因陶瓷骨架纳米孔的毛细管力作用,不会使相变材料熔体流出,保持了复合材料的定型结构。论文主要从以下几个方面开展研究工作:（1）通过相变材料熔体浸渗多孔陶瓷的力学推导以及计算流体力学软件（CFD/Fluent）的传热模拟,研究了熔渗过程传热传质的动力学机理。根据熔渗实验模型与力学基础理论,推导了相变材料熔体熔渗多孔陶瓷过程中的力学方程,导出了复合材料制备工艺中多孔陶瓷基体孔隙半径条件、相变材料熔体渗入多孔陶瓷孔隙的深度条件以及相变材料热物性参数与多孔陶瓷孔隙深度的关系式;根据体积平均理论与热焓法,模拟了熔渗过程中多孔陶瓷内相变材料熔体的压力分布、温度分布以及速度等。理论上确定了复合材料的制备工艺条件:陶瓷基体理论半径宜小于6.8×10^-5mm,浸渗温度要大于相变材料的熔点小于多孔陶瓷的烧结温度,浸渗时间依据具体的实验工艺制定。（2）依据熔渗制备工艺的研究理论,实验研究了纳米多孔陶瓷的溶胶—凝胶制备工艺和陶瓷基相变复合材料的熔融浸渗制备工艺。实验研究表明,多孔陶瓷基体具有相互连通的孔结构,平均孔径约30nm,密度为0.202g/cm³,孔隙率达90.4%,孔体积为4.39cm³/g,比表面积为527.8m²/g。实验过程中发现随乙醇含量增加,样品的孔径增大,即通过乙醇的不同配比可以调节样品的孔尺寸或结构。当乙醇与正硅酸乙酯的摩尔比为10或20时,合成的孔结构特点的陶瓷基体适宜做相变材料的支撑材料,相变材料的浸渗率最大。选用石蜡作为相变材料时,浸渗率达到75wt%,浸渗时间宜为180²10s;选用糖醇作为相变材料时,浸渗率达到85wt%,浸渗时间宜为250³50s。复合材料的浸渗制备温度适宜超过相变材料熔点之上约30℃。相变复合材料的性能测试表明:石蜡或糖醇相变材料与多孔陶瓷基体相容性好,熔渗过程中不会发生化学反应;相变复合材料的相变温度范围为50¹20℃,蓄热密度最高达到289.9 kJ/kg。石蜡（70#）相变复合材料的抗弯强度为10.1±0.10MPa,屈服强度为0.03MPa,抗压模量为18.8MPa;糖醇（120#）相变复合材料的抗弯强度为25.9±0.10MPa,屈服强度为1.11MPa,抗压模量达到740 MPa。（3）测试了多孔氧化物陶瓷隔热复合材料与相变复合材料的热控效果,并进行了相应的传热模拟研究。在相同测试条件下,当热面温度为500℃时,单一多孔氧化物陶瓷隔热复合材料的冷面温度为184.2℃;石蜡相变复合材料的冷面温度最低为139.3℃;糖醇相变复合材料的冷面温度最低为86.9℃。表明相变复合材料的热控效果优于单一的多孔氧化物陶瓷隔热复合材料,糖醇相变复合材料的热控效果好于石蜡相变复合材料。相变复合材料的传热模拟表明,相变复合材料在接近加热面的位置,相变材料熔化非常快,随着时间的推进,液相区域加长,固-液界面渐渐地向着热流方向扩展,固相区域变短。三种相变复合材料的传热模拟发现,在相同的传热时间,90#相变复合材料（蓄热密度为198kJ/kg）的固-液界面发生距离远小于58#和62#相变复合材料,表明了相变复合材料的蓄热密度是控制温度升高最关键的影响因素。（4）采用具有高吸收热值的ZrOCl₂·8H₂O和硼酸为添加剂,首次研究了高热控性能有机-无机复合相变材料,并以陶瓷纤维为基体,制备了高蓄热密度的陶瓷基相变复合材料。含ZrOCl₂·8H₂O的石蜡相变复合材料制备研究表明,ZrOCl₂·8H₂O以晶体的形式分布在陶瓷纤维结构之中,相变复合材料分别在56.3℃和96¹58℃的温度区域有较大的吸热峰,蓄热密度共计达到536kJ/kg。当ZrOCl₂·8H₂O添加量达到24.5wt%,在样品的加热面,以50℃/min快速升温至600℃并保温1800s时,其冷面温度在114.5¹37.6℃范围有个迟缓过程,其延缓时间为840s,热控效果比较显著。含硼酸的糖醇相变复合材料制备研究表明,硼酸以晶体的形式分布在陶瓷纤维结构之中;相变复合材料分别在51.9℃、151.2℃和158.6℃有较大的吸热峰,蓄热密度共计达到671kJ/kg。当热面温度以50℃/min快速升温至600℃并保温2800s时,含ZrOCl₂·8H₂O的石蜡相变复合材料的冷面温度在115¹28℃区间有一个延缓过程,其延缓时间为900s;含硼酸的糖醇相变复合材料的冷面温度在152¹63℃区间有一个延缓过程,其延缓时间为1310s。表明高蓄热密度的相变复合材料有优异的热控效果。更多还原

【Abstract】 Thermal protection systems （TPS） and heat insulation materials are required for a range of hypersonic vehicles ranging from ballistic reentry to hypersonic cruise vehicles, both within Erath’s atmosphere and non-Earth atmospheres. Phase change materials （PCM） are one of the most preferrd methods to thermal control applications that can effctively delay or modify the temperature rise of the surface of the aircrafts subjected to high thermal flux. This work originately put PCMs to use in the internal themal insulated materilas of the hypersonic vehicles. Porous ceramic matrix serves as the supporting material, which provides structural strength and prevents the leakage of melted PCMs; and PCMs acts as thermal absorb material limiting the temperature abruptly rising of the aircrafts. This study demonstrates as below.Firstly, thermaldynamics, kinetics and statics equations of melted PCM infiltrated porous ceramic marix are derivated according to experimental model and CFD modeling, and then analyze the influence factors including infiltration temperature and infiltration time and so on. Results indicated the infiltraton temperature in theory is higher than the melt point of the PCM and lower than the sinter temperature of the porous cermic matrix, the calculated radius is less than 6.8×10^-5 mm. CFD modeling shows the pressure drop of the melted PCM in porous ceramics distinct and centerline velocity becomes lower along the directions. When infiltration time increases, infiltration depth will rise until it reaches equilibrium; and optimum infiltration depth needs to choose comprehesive infiltration conditions.Secondly, porous ceramics derived by sol-gel processing. Studies show the pore structure of porous ceramics was connected; its porosity was calculated as 90.4%, and surface area was measured as 527.8 m²/g and mean radius was measured as 30.3 nm. It indicates that high mesoporosity when the molar ratios of EtOH/TEOS is 2, 3 and 5. The pore size of porous ceramics becomes larger with the increase of EtOH/TEOS molar ratios; the average pore size of the specimen with E=10, 20 silica are 53.1nm and 56.0nm, respectively. Furthermore, PCM-porous ceramic composite were successfully prepared by melt infiltration pressurelessly; PCM and porous ceramics were chosen as thermal absorb material and supporting material, respectively. Compared with 3 types of pore structure porous silica using as supporting materials of the PCM, the mass percentage of the paraffin impregnated E=10, 20 silica matrices reached to 75% and over than those of the E=2 silica matrix, 68%. The E=10, 20 silica matrices are suitable to serve as supporting materials of the PCM for larger thermal absorption. For paraffin, the optimal infiltration time of preparing the composite was range from 180 to 210 seconds while the mass fraction of the paraffin reached to 75 per cent. For erythritol, the optimal infiltration time of preparing the composite was range from 250 to 350 seconds while the mass fraction of the PCM reached to 85 per cent. There was no reaction among the PCM-porous silica ceramic composite according by the structure analysis with SEM and the chemical analysis with FTIR. The measurement of latent heat, melting point of PCM-porous silica ceramic composite were 63²89.9 kJ/kg, 50¹20℃respectively. The flexural and compressive properties of 3 types of specimens were investigated by two techniques: 3-piont bending and uniaxial compression. Results indicated the bending strength of the erythritol/ceramic composite is 25.9±0.10MPa and paraffin -ceramic composite is 10.1±0.10MPa. The compressive strength of the erythritol-ceramic composite is 1.11 MPa corresponding 0.15% strain and paraffin-ceramic composite is 0.03 MPa corresponding 0.16% strain.Thirdly, experimental and numerical studies are proposed to predict and investigate the thermal absorb characteristics of porous silica infiltrated with phase change materials （PCM） for thermal protection applications. Several types of different solid-liquid phase change composites were introduced into a cylindrical enclosure while it experiences its heat from a heat source setting on the left of the enclosure. Studies show that the cold face temperature of the ceramic aerogel-composite is 184.2℃while the cold face temperatures of the paraffin-porous ceramic and erythritol-porous ceramic are 139.3℃, 86.9℃respectively. The numerical simulation of the porous ceramic composite indicates that transient heat transfer simulation shows good agreement with the experimental data. When heat tranfer time is 600,800,1000 seconds, the numerical caculated temperature is 115.0℃,161.4℃,201.2℃. Thus the simulation method can forecast the experimental conclusions and optimize the geometric structure of the heat insulation composites. The numerical simulation of the phase change compsoite was performed using the volume averaging technique and a finite volume technique was used to discretize the heat diffusion equation while the phase change process was modeled using the enthalpy porosity method. The results are portrayed in terms of temperature distribution and liquid fraction, and the numerical and experimental results showed good agreement. The results illustrated that the higher the latent heat storage capacity the more stability of the thermal performance of the phase change composite.Moreover, PCM-ceramic fiber composite were prepared by melt infiltration, particularly, to be embedded within the endotherms, ZrOCl₂·8H₂O or boric acid, to provide for nonreversible heat absorbing applications. Studies indicate endotherms to be in the fiber-PCM as crystal that dispersed in the PCM-ceramic fiber composite and has very large decomposition enthalpy. The thermal capacity of the PCM-ceramic composite embeded with ZrOCl₂·8H₂O is 536 kJ/kg happened at 56.3℃and 96¹58℃and the thermal capacity of the PCM-ceramic composite embeded with boric acid is 671kJ/kg happened at 51.9℃,151.2℃and 158.6℃. The thermal protection properties of the fiber-PCM composites were designed in the laboratory conditions via using a thermal measurement setup under a simulated thermal environment of the aircraft. The fiber-PCM composites produces here can be helpful for the thermal protection application and exhibited fairly efficient thermal regulation under very high temperatures and for long periods of time.更多还原