有序双过渡金属二维材料MXenes稳定性与热电转换性能的研究

The recently discovered two-dimensional, ordered, double transition metal carbides or also known as MXenes could exhibit the excellent electrical, lattice transport properties and thermoelectric materials, some of them might be used as the advanced electronic or electrical materials . At the moment, characterizing the intrinsic transport and thermoelectric properties of those MXenes by means of a single layered form experimentally is quite challenging and even not possible. The relevant theoretical study is also scarce. Therefore, in this project, we first employ the first principles calculations based on density functional theory to investigate the structural parameters, stability, electronic band structure and phonon spectrum of MXenes. Combining the obtained results with classical Boltzmann transportation theory, and together by developing more reliable and highly efficient physical models for electron or phonon scattering processes, the electrical conductivity, lattice thermal conductivity and thermoelectric figure of merit of all theoretically suggested ordered, double transition metal MXenes systematically and thoroughly in this project. From the computed results, those MXenes structures with prominent electrical conductivity, lattice thermal conductance and promising thermoelectric properties, which can be used as the novel functional electrical materials, will be elucidated. By comparing the computed stabilities and other properties for intrinsic and chemically functionalized MXenes, we would like to reveal the influences of chemical compositions and functionalized groups on the structural stability, transport and thermoelectric properties of them, providing the fundamental guidelines for the future experimental synthesis of novel transition metal MXenes.

有序双过渡金属二维材料MXenes是近期发现的一类极具优良电、热传导性质和热电转换性能潜力的二维材料，可作为先进电工材料应用于电子和电工行业。鉴于现有实验技术手段尚无法对单层MXenes材料热电性能进行表征，而相关理论研究也极为缺乏。本项目拟在基于密度泛函理论的第一性原理结构稳定性优选、电子能带结构和晶格声子谱计算基础上与经典玻尔兹曼运输原理相结合，同时发展和完善现有的电子弛豫时间及半经验声子散射物理模型，进而可靠和高效的对潜在有序双过渡金属单层MXenes二维材料电子和晶格传导性质、热电转换性能进行定量计算。理论预测出具有超高电或者热传导性质、以及优异热电性能的基于MXenes二维结构的新型电工材料，揭示可用于调控有序双过渡金属MXenes材料结构稳定性、传导性质和热电性能的物理机制和作用规律，指明未来试验合成此类材料的研究方向。

热电材料是一类可以将热能直接置换为电能的特殊能量转换材料，对于高效率的回收和再利用人类生产和生活中产生的大量废热起到关键作用。二维材料借助其独特的层状晶体结构带来的空间尺度压缩效应对电子能带和声子热运输产生的调控作用，在热电转换领域相较于传统的热电材料在热电性能方面表现出独特的优势。本项目针对一大类由双过渡金属碳化物构成的二维材料MXenes数百种晶体结构的热力学稳定性、表面修饰基团类型、电子能带结构、磁性、声子谱、电导率、热导率、Seebeck系数以及热电性能品质因子开展了系统性的第一性原理高通量理论研究。研究发现双过渡金属MXenes材料及其对应的前驱体四元MAX陶瓷大部分热力学都是稳定存在的；MXenes二维材料表面修饰基团稳定性与修饰基团价态和过渡金属价电子个数相关性很高，且表面修饰对电子能带结构、声子色散谱调控作用十分显著；大部分MXenes二维材料都是金属类型结构，半导体类型多出现在表面O修饰结构中；MXenes电导率普遍很大，表面修饰结构中电子热导率对总的热导率起到主导作用；金属类型MXenes热电性能由于热导率较高，Seebeck系数小，其总体ZT数值很小，热电性能不突出。表面修饰半导体类型MXenes电导率适中，Seebeck系数大，热电性能突出。基于高通量计算，本项目发现了Cr2TiC2T2(T=-OH, -F和-O)这一类ZT>2的高性能热电材料体系，应用前景巨大。本项目系统和完整的采用高通量第一性原理电子和声子运输计算方法，实现了对MXenes这一类材料电子、声子运输性质以及热电性能的综合预测和评估，揭示了表面修饰基团和过渡金属搭配组合对MXenes电子能带、声子色散等微观物理性质的精细调控机制，对未来高性能热电材料的研究起到启迪作用。