基于杂化气凝胶构筑超轻质防热材料及其微结构调控

Facing to more complicated and rigorous thermal environment including oxidizing, high heat flux and strong aerodynamic shearing, traditional low density ablative materials bring out many defects such as great ablative recession, easy oxidization and poor structural strength of charred layers. Thus, future ablative materials must firstly deal with aforementioned problems on the premise of meeting the requirement of light-weighting. In this project, ultra-lightweight ablative composites will be designed and achieved based on hybrid aerogel. By controlling chemical reaction and tuning the gel time of hybrid reactive system，hybrid aerogel will be synthesized through co-gelating reaction and achieved the well dispersion of SiO2 aerogel in organic aerogel’s skeleton at molecular level. Furthermore, chemical reaction mechanism will be studied for multi-components of hybrid system. Consequently, combined with the merits of oxidation resistance for inorganic silica and high efficient ablation for phenolic skeletons, ultra-lightweight ablative composite is to be constructed and fabricated based on the microstructural tuning and optimization of hybrid aerogel. It is expected that ablative efficiency, insulation, structural strength of charred layer, anti-oxidization and other properties will be optimized and modified by microstructural tuning of hybrid aerogel and ablative composite. Specifically speaking, the microstructural parameters of hybrid aerogel and its composite cover pore structure, phasic morphology, interfacial phase, and so on. Relying on simulated ablating test and many other characterizations, we will manage to investigate and reveal the evolution mechanism and oxidation erosion process during ablating of composite and to obtain the ultra-lightweight ablative composite integrating heat resistance, insulation and anti-oxidation. The fundamental research about new ultra-lightweight ablative composite in this project will provide essential basic theories reserve for thermal protection systems and pave a way for ablative material’s application of future manned lunar-landing, deep space exploration and hypersonic near space vehicles.

在有氧、高热流、高剪切等复杂严酷热环境下，传统低密度烧蚀防热材料存在烧蚀后退量大、易氧化、炭层结构脆弱等不足，未来轻质烧蚀防热材料的发展亟待解决上述问题。本项目通过控制杂化体系溶胶-凝胶化学反应进程、调节凝胶时间，共凝胶反应合成酚醛/SiO2杂化气凝胶，实现SiO2气凝胶在酚醛气凝胶骨架中的分子级分散；进一步研究多组分间化学反应机理。结合SiO2组元抗氧化和酚醛组分高效烧蚀的优点，以高孔隙率碳粘接炭纤维骨架为增强体，基于杂化气凝胶微结构调控与优化构筑超轻质防热复合材料。通过对孔结构、相形态、界面相等微结构调控来实现材料烧蚀性能、隔热性能、炭层强度、抗氧化性等性能的综合优化与控制；利用模拟烧蚀试验等手段研究并揭示超轻质防热材料烧蚀状态下多孔炭化层演化机制和氧化侵蚀过程。获得集防热、隔热、抗氧化等功能于一体的超轻质防热材料，为载人登月、深空探测以及临近空间高超声飞行器防热需求提供基础理论。

针对深空探测、载人登月、临近空间高超声速飞行器等对先进轻质防热材料的需求，本项目力求解决传统烧蚀材料密度大、热导率高、碳层易氧蚀等缺点，开展了以酚醛（PR）气凝胶为骨架的耐烧蚀抗氧化杂化气凝胶材料设计、合成与工程制备研究，并基于杂化气凝胶构筑了系列超轻质烧蚀防热复合材料。重点完成了耐烧蚀抗氧化杂化酚醛气凝胶的合成与结构优化研究、超轻质纤维增强体骨架的微细观结构设计与优化、超轻质防隔热一体化材料的制备与评价等研究。突破了耐烧蚀抗氧化杂化酚醛气凝胶材料的大规模制备、高孔隙率纤维增强体与多元杂化气凝胶材料的界面匹配与优化、超轻质防隔热一体化材料的可控制备等多项关键技术。基本探明了杂化气凝胶多组元间化学反应机理，实现对杂化气凝胶微结构调控与优化，掌握了无机组元与有机组分在高温条件下的化学相互作用、原位陶瓷化机制以及复合材料多孔炭化层形成过程，揭示了微观结构与宏观性能间的内在联系。获得多层次多孔微观结构的杂化气凝胶基体调控方法和耐烧蚀抗氧化超轻质防隔热一体化材料制备方法，成功制备出大尺寸（≥300×300mm）的典型样件，材料密度最低可至0.3g/cm3，室温热导率≤0.10W/(m·K)。制备的杂化型超轻质防隔热一体化材料具有优异的防隔热性能，通过多个典型热流状态的考核。在6 MW/m2的月球再入返回轨道热环境中，碳基纤维增强超轻质烧蚀材料（SLCA）的表面温度达到2800 ℃，而30mm深度背面最高温升仅有100℃左右，具有十分优异的防热和隔热性能；在800 kW/m2/1700 kW/m2的典型弹道热环境中，经过400s烧蚀后硅基纤维增强超轻质烧蚀材料（SLSA）最高表面温度达到1700℃，而20mm厚度背面温升仅有90℃，线烧蚀后退率为9.75×10-4mm/s。烧蚀后，材料表面平整，接近于“零体积”烧蚀，碳化层表面未发生明显氧化，具有突出的防隔热性能和烧蚀维形能力。本项目开发的超轻质烧蚀防热材料在深空探测航天器、载人登月返回舱、临近空间高超声速飞行器等热防护部位具有重大的应用前景。