调控能带结构优化几种三元Zintl合金热电性能

The direct energy conversion between heat and electricity based on thermoelectric effects is attracting great attentions and plays an important role in searching new energy materials. The key issue is how to increase the relatively low thermoelectric efficiency of current thermoelectric materials. The values of the thermoelectric figure of merit for Zintl alloys are small though they have low thermal conductivity. Thus, it is necessary to obtain high Seebeck coefficient and high electrical conductivity to improve their thermoelectric efficiency. The rich solid-state chemistry of Zintl alloys allows us to improve their thermoelectric properties by precise doping. In this proposal, we mainly focus on study of the key factors and the related principles for determining the Seebeck coefficient and the electrical conductivity for several ternary Zintl compounds based on the density-functional theory and the Boltzmann theory. We will explore the effective method to tuning the band structure through studying the effect of doping and defect on the band structure near the Fermi level. Moreover, the method to modify the concentration, effective mass, and mobility of carriers will be studied. Consequently, it will be expected that the high convergence of band valleys, reasonable band gap, light and heavy bands will appear near the Fermi level. Such behavior will be helpful to obtain the high weighted mobility and high electrical conductivity. Also, high thermoelectric powerfactor can be achieved. We will also explore the factors for determining thermal conductivity by employing the finite-displacement method and the perturbation theory. Defect and doping effect on the thermodynamic properties will be studied to search the method for decrease the thermal conductivity. The present proposal will be helpful to reveal the principles for determining the thermoelectric performance. Consequently, the way for improving their thermoelectric efficiency will be found.

热电材料可以实现热电能量的直接转换，正成为新能源领域有前景的研究方向。提高其热电转换效率是急需解决的关键问题。良好的热电性能需要其具有高塞贝克系数和电导率以及低热导率。Zintl合金热导率比较低，但热电优值不高，提高其塞贝克系数和电导率可以改善其热电性能；其丰富的固态化学结构性质有利于实现精确掺杂来调控其热电性能。本课题拟以几种三元Zintl合金为对象基于密度泛函理论和玻尔兹曼输运理论研究影响其塞贝克系数和电导率的关键因素。研究掺杂和缺陷对费米能附近能带结构的调控机制，探索控制载流子浓度、有效质量、迁移率的方法，在费米能附近实现多能谷汇聚、合适带隙、轻重带共存从而增大其载流子权重迁移率和提高电导率，有效改善其热电功率因子。基于有限位移法和微扰理论模型研究掺杂和缺陷对其热导率的影响方式。最终目的是揭示影响这些Zintl材料热电性能的内在物理机制，发现提高其热电转换效率的方法。

热电材料能够实现热能和电能之间的直接相互转换，有利于提高能源利用效率并实现能量转换系统的微型化和全固态化，是新能源领域一个重要的研究方向。Zintl合金复杂的晶体结构和较大的原子质量起伏使其具有非常低的晶格热导率，此类材料具有电子晶体-声子玻璃的特征，同时其丰富的固态化学结构性质有利于实现精确掺杂来调控其热电性能。如何提高其热电转换效率是急需解决的关键问题，而理解其热电行为的关联因素和内在规律是提升其热电性能的关键。本项目围绕几种类型Zintl相化合物，采用第一性原理与半经典玻尔兹曼理论相结合的方法，系统研究了影响其热电性能的关键因素。1）我们揭示了Zintl化合物（A5M2Pn6、A3MPn3、A2MPn2）中阴离子基团排布方式差异的成因及其对电子结构和热电性质影响方式；2）提出了利用孤对电子效应形成半满中间带的方法提高电导率的同时保持大的塞贝克系数；3）不同散射机制（中性杂质散射、畸变势散射、离子杂质散射、压电散射）下，热电性质对载流子有效质量、费米能附近能谷数量和简并度、晶体各向异性大小的依赖性，发现无论在何种散射机制下，提高费米能附近能谷简并度均有利于热电性能的提升；4）提出了增加价带和导带边缘能谷数量和汇聚程度的方法；5）揭示了费米能附近电子态密度大小和有效质量的影响因素，对带隙的关联因素如电负性进行了讨论。6）探索了此类材料达到塞贝克系数和电导率平衡时的最优载流子浓度，发现A5M2Pn6中Ca5Al2Sb6需要的载流子浓度最低，Sr5Al2Sb6需要的载流子浓度最高。7）发现Zintl化合物中溶质原子的溶度上限对通过掺杂提高载流子浓度的有效性具有重要影响，而高溶度上限一般发生在掺杂原子与被取代原子的电子组态比较接近的情况。