具有多型体特征的Ge2Sb2Te5材料的结构特性与热电性能研究

As a typical phase-change material, pseudo-binary Ge2Sb2Te5 system exhibits a polymorphic character with multiple phases of the same composition but different structures, providing additional strategies for decoupling transport properties. Ge2Sb2Te5 in its stable, trigonal phase exhibits a layered crystal structure with mixed chemical bonds, possessing a low thermal conductivity and high electrical conductivity, which suggests its potential as a promising thermoelectric material. However, there have been few reports utilizing the polymorphic character for thermoelectric performance enhancement. Also, the current understanding on the transport properties and the structural origins is still inadequate for single-phase Ge2Sb2Te5. Therefore, this project focuses on polymorphic Ge2Sb2Te5 thermoelectric materials. We will synthesize polycrystalline Ge2Sb2Te5 bulk materials with controlled phase structures. Combining first-principle calculation, microstructure characterization and transport measurement, we will reveal the effect of crystal structure and defects on transport properties of the stable trigonal phase, and rationally optimize the thermoelectric performance by structural design. Furthermore, by virtue of the polymorphic character of Ge2Sb2Te5, we will then develop two-phase composites consisting of trigonal and cubic phases, and study the effect of secondary phase and interfaces on thermoelectric properties. Through an elaborate design of the phase structures, we expect to further lower the thermal conductivity and maintain the electrical performance based on the selective scattering of charges and phonons. Finally, by integrating the above strategies involving the design of crystal, defect and phase structures, it is expected that high thermoelectric performance will be obtained in Ge2Sb2Te5-based polymorphic materials.

作为典型的相变存储器材料，赝二元体系Ge2Sb2Te5具有成分相同、但结构不同的多物相共存这种多型体结构特征，为电热输运调控提供了新的空间。其三方稳定相表现出强弱键共存的层状晶体结构，具有低热导率和高电导率，而多相的空间涨落有望进一步降低热导率，使其成为有潜力的新型热电材料。然而，该体系多型体结构对优化热电性能的作用尚未引起关注，对单相材料输运性质及其微观结构机理的认识还不清晰。因此，本项目以Ge2Sb2Te5材料为研究对象，在制备相结构可控多晶块体的基础上，深入研究三方相中的晶体结构和缺陷结构对输运性质的影响，通过针对性的结构调控提高热电性能；进一步利用多型体特征，设计同质异构三方/立方两相涨落材料，研究两相界面对电热输运的影响机制；通过控制两相的含量、尺寸和分布，利用界面的选择性散射降低热导率并保持或提高电性能。最终通过单相和多相结构设计，开发高性能Ge2Sb2Te5基多型体热电材料。

赝二元体系Ge2Sb2Te5（GST225）化合物具有成分相同、但结构不同的多型体结构特征，为电热输运调控提供了新的空间。然而，该体系多型体结构对优化热电性能的作用尚未引起关注，对单相材料输运性质及其微观结构机理的认识还不清晰。为此，本项目重点开展了GST系列化合物的热电性能、GST225三方相的电热输运性质与热电性能优化、多相结构GST材料的设计与热电性能等相关研究。项目进展顺利，成功制备了组成可控、元素分布均匀的系列成分GST稳定相块体材料；阐明了GST225稳定相化合物的能带特征；通过In固溶引入共振能级，提高了塞贝克系数；通过Se/S掺杂降低了热导率；zT值由约0.5提升到0.78。通过磁控溅射方法制备了GST225非晶薄膜，通过控制退火工艺实现了对立方和三方相结构的控制，揭示了热电性能与相结构的依赖关系；探索了通过相结构调制来协同优化材料力学性能和热电性能的新途径。项目资助发表论文6篇，申请专利1项，培养博士生1人。