基于声子调控策略协同优化稀土硅酸盐的热膨胀系数与热导率

High-temperature capable T/EBCs for SiC-based CMCs are invariably multilayered and critically request the better matching on mechanical, thermal and chemical properties between TBC and EBC layers. Rare earth silicates, generally including RE2SiO5 orthosilicates and RE2Si2O7 pyrosilicates, are important EBC materials due to their excellent reliability in the extreme operating combustion environments. Unfortunately, RE-silicate EBC materials typically demonstrated obviously smaller thermal expansion coefficients than TBC materials. This mismatch critically challenged long-term thermal stability and durability of SiC-based CMC component in aerospace propulsion systems. Based on our previous investigations, RE silicates showed attractive merits of both environmental barrier coating material and thermal barrier coating material, including relatively smaller thermal expansion coefficient and moderately low thermal conductivity. It is expected that after the effective optimization and modification of the two key thermal properties, rare earth silicates would possess promising advantages as multi-functional T/EBC protective coating for SiC-based CMCs. This project was inspired by our recent discovery on the striking complex characters of phonon anharmonicity in RE silicates. We found two types of phonon modes that have totally different anharmonicity characters, for instances, phonons with negative and positive mode Grüneisen parameters. These two class of phonons have negative and positive contributions to thermal expansion and they can be tailored to achieve modified thermal expansion. Low frequency phonons mostly have negative mode Grüneisen parameters and dominate low lattice thermal conductivity for their giant anharmonicity. Base on the comprehensive understanding of these phonon characters, it provides the opportunity to coordinatively optimize thermal expansion and lattice thermal conductivity through strategic phonon engineering. The novel concept is rooted from the same mechanism that governs modifications of the two key thermal properties. This project will adopt integrative explorations of high-throughput theoretical modeling and experimental evaluation of representative RE silicates with good high temperature phase stability and relative low thermal expansion coefficient and thermal conductivity. We intend to construct the relationships among chemical composition, crystal structure, phonon anharmonicity, and thermal properties. Thereafter, we will conduct pioneer works on tailoring phonon behaviors through selective doping and strain modification. The underlying scientific idea is to modify the bonding strength and vibrational anharmonicity, and therefore to control the trends of thermal conductivity and thermal expansion. The outputs of this project may support the innovative design of T/EBC system for SiC-based CMCs.

面向极端服役环境的SiC基复合材料采用热障/环境屏障复合涂层抵抗环境侵蚀。稀土硅酸盐是环境屏障涂层的优选材料，但其与热障涂层的热膨胀系数失配是亟待解决的瓶颈难题。研究显示稀土硅酸盐兼具热障与环境屏障涂层材料的基本特性，实现协同优化热导率和热膨胀这两个关键热学性能，有望发挥热障/环境屏障功能一体化的优势，为复合涂层的设计和选材提供创新思路。申请人发现稀土硅酸盐中存在非简谐性特征相反（负/正格林艾森系数）的两类声子，它们对热膨胀的贡献部分抵消并且比例可以调节，据此提出通过声子调控策略设计晶格振动非简谐性和热学性质的科学思路，采用高通量理论模拟与实验评价相结合的研究手段，建立稀土硅酸盐（RE2SiO5和RE2Si2O7）的成分、晶体结构、声子非简谐性与热学性质的关联，揭示操控声子行为对热导率和热膨胀系数的影响规律，获得成果对理解和调控稀土硅酸盐复杂的热膨胀行为和极低的热导率有普适的科学意义。

面向极端服役环境的SiC基复合材料采用热障/环境屏障复合涂层抵抗环境侵蚀。稀土硅酸盐是环境屏障涂层的优选材料，但其与热障涂层的热膨胀系数失配是亟待解决的瓶颈难题。稀土硅酸盐兼具热障与环境屏障涂层材料的基本特性，实现协同优化热导率和热膨胀这两个关键热学性能，有望发挥热障/环境屏障功能一体化的优势，为复合涂层的设计和选材提供创新思路。本项目首先采用第一性原理方法准确计算几种典型稀土硅酸盐陶瓷（稀土单硅酸盐及稀土双硅酸盐）的晶体结构和化学键强度、声子行为、热导率及热膨胀性能，建立了材料化学成分-晶体结构-声子非简谐性-关键热学性能之间的本征关联，阐明了稀土硅酸盐材料具有非简谐性特征相反（正/负格林艾森系数）的两类声子，可以通过声子调控策略设计晶格振动非简谐性，改变两类声子的相对比例进而协调控制材料热学性能。在此基础上，证明了通过离子掺杂、空位调制等方法调控材料的晶体结构和化学键、原子间结合强度、声子非简谐性等关键参量进而调控材料热导率和热膨胀系数。进一步实验制备了纯相致密的、化学成分经设计的掺杂型稀土硅酸盐实验样品，研究了材料的相组成、化学键特征、晶型稳定性、热导率、热膨胀系数等性能。研究结果揭示了操控声子行为对热导率和热膨胀系数的影响规律，对理解和调控稀土硅酸盐复杂的热膨胀行为和极低的热导率有普适的科学意义，并且综合建立了稀土硅酸盐材料体系理论预测与实验快速制备、评价、筛选相结合的研发流程。