热电转换过程中基于缺陷工程的电声结构与电热输运行为调控

发布时间：2018-01-03 20:16

本文关键词：热电转换过程中基于缺陷工程的电声结构与电热输运行为调控　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：在无机功能固体中,电子和声子同时存在且彼此之间存在固有的耦合关系,共同决定其本征的物化特性,并使其表现出诸多新奇的物理现象和多样化的应用属性。因此,对无机固体中的电子、声子结构与输运行为进行的研究既能实现其本征物化性能的灵活调节,又为新奇物化现象的探索和新型功能材料(例如热电效应和热电材料)的设计指引了方向。热电材料因能够直接实现热能和电能的相互转换,是近年来能源转换材料领域的研究热点。然而热电三参数之间的反比耦合使得热电材料的ZT值及热电器件的能量转换效率一直徘徊在较低的水平,严重制约了热电科技的发展。理论分析表明热电性能本质上由材料的电子、声子结构及其输运行为共同决定,且二者常常以晶体中无处不在的晶格缺陷为媒介发生耦合作用;另一方面,晶格缺陷所对应的晶格周期性的破坏也必然会对材料的电子、声子结构及电、热输运行为产生影响。上述事实表明在热电转换过程中有望利用晶格缺陷对电子、声子结构和电、热输运行为进行调控,进而实现热电性能的优化。本论文工作基于缺陷工程思想,通过向目标热电材料体系中引入特定的晶格缺陷(置换原子、空位等),探究缺陷对体系电子、声子结构及电、热输运行为的影响,并在此基础调控和优化体系的热电性能。同时,作者结合缺陷表征和理论计算分析了缺陷调控的内在机理,发展了超晶格层间电荷转移等调控策略,建立了基于缺陷工程的"缺陷结构—热电性能"构效关系。文末,作者基于自己对缺陷工程策略的理解,重新审视了缺陷调控自由度的问题,提出了一种基于缺陷工程的"多自由度"协同调控的研究思想。论文的主要内容包括以下几个方面:1、本章中,作者选取BiCuSeO天然超晶格热电材料为研究对象,通过等价离子掺杂的方式在不损害电学输运性能(即热电功率因子,PF)的前提下显著降低了材料的晶格热导率及总热导率,从而优化其热电性能。借助于BiCuSeO复合化合物独特的层状结构([Bi_2O_2]~(2+)绝缘层和[Cu_2Se_2]~(2-)导电层交替堆叠的超晶格结构),通过La、Ag单独掺杂和共掺杂两种掺杂方式置换相应层中的Bi、Cu离子,得到等价离子掺杂样品。电热输运性能测试表明:由于潜在的能带收敛效应,La-Ag共掺杂样品在提高载流子浓度的同时保持较高的载流子迁移率和Seebeck系数,因而获得了优化的PF,同时借助于掺杂原子的声子散射效应,体系的热导率进一步降低,最终使ZT值得到了优化。在三种掺杂样品中,La-Ag共掺杂的效果优于单独掺杂的样品,其在755 K时ZT值约为0.46,相对于纯样提高了近70%。该项工作表明等价离子双掺杂策略能够同步改善BiCuSeO超晶格材料的热电性能,同时这种亚层交替堆叠的特殊结构也为在同一物质中实施异层双(多)缺陷协同调控研究提供了理想的实验平台。2、在上一章工作中,由于使用等价离子掺杂,电导率提升效果受限,功率因子整体偏低。针对这一问题,本章中,作者提出使用空位缺陷来优化其电热输运性能,同时针对传统的单空位缺陷中可能损害电学性能的弊端,作者设计制备了新型的双空位类型缺陷,通过在合成中同时引入Bi、Cu双空位首次在BiCuSeO基热电材料中实现电、热输运性质的协同优化。结果表明基于空位点缺陷、超晶格界面以及晶界等分级结构的"全尺度"声子散射可以最大幅度降低系统的热导率,使双空位样品在在750 K得到0.37 Wm-1K-1的极低热导率。与此同时,正电子湮没谱测试表明在Bi-Cu双空位样品中,空穴电荷有从Bi空位([Bi_2O_2]~(2+)绝缘层)向Cu空位([Cu_2Se_2]~(2-)导电层)转移的趋势,可以显著提高Bi_(0.975)Cu_(0.975)SeO样品中空穴载流子的浓度,增强其电学输运性能,并最终在750 K时得到0.84的高ZT值,相比于纯样和单空位样品均有显著的改善。该项工作表明由Bi-Cu双空位引发的层间电荷转移以及基于超晶格结构的全尺度声子散射能够协同调控BiCuSeO基热电材料的电学和热学输运性质,大幅度优化其热电性能。同时这种异层双空位协同调控的实验策略也为设计制备高性能的热电材料提供了新的思路。3、通过简单固相反应合成了 Cu_2ZnSnSe_4纯相和空位掺杂样品。分析结果表明样品中含有本征的Se空位和体缺陷,导致Cu、Zn和Sn空位引入后主要以缺陷簇的形式存在,且空位的引入可以造成无序晶畴的产生,引起样品的部分多晶化。电热输运性能测试结果表明空位的引入提高了材料的载流子浓度,进而提高了电导率;同时基于空位缺陷簇和晶体孔洞的强烈声子散射作用以及无序晶畴引起的多晶化,使材料的晶格热导率和总热导率均有所降低,从而实现了对材料电导率和热导率的同步优化,并克服了 Seebeck系数下降的影响,使得空位样品的ZT值与原始样相比均有了明显提升。例如Sn空位样品Cu_2ZnSn_(0.98)Se_4的ZT值在750 K时达到了 0.44,是同温度下原始样性能的3倍。由于此类物质组成元素丰度高且不含污染、高毒的重金属元素,本章的研究为寻求经济、低毒的高效热电材料开辟了新的探索方向。
[Abstract]:In inorganic solids, and the coupling relationship between inherent and between electrons and phonons, determined their intrinsic physicochemical properties, which show many novel physical phenomena and diverse applications of electronic properties. Therefore, inorganic solid in the study of phonon structure and transport behavior which can not only realize the intrinsic physicochemical properties of the flexible adjustment, but also to explore novel physicochemical phenomena and new functional materials (such as thermoelectric effect and thermoelectric materials) direction. The design of thermoelectric materials due to direct conversion of thermal energy and electric energy, energy conversion material is a hot research field in recent years. However the inverse coupling between three parameters makes the thermoelectric energy conversion efficiency of thermoelectric materials ZT and thermoelectric devices has been hovering at a low level, which seriously restrict the development of thermoelectric technology. Theoretical analysis table The thermoelectric properties of materials consisting essentially of electronic structure and phonon transport behavior, joint decisions, and the two often to lattice defects in crystals everywhere as medium coupling; on the other hand, the corresponding lattice defect lattice periodic damage is bound to electronic materials, the phonon structure and electrical. The heat transport effect. The fact that the thermoelectric conversion process is expected to be used in lattice defects on electronic structure and phonon, electricity, heat transport regulation, so as to realize the optimization of the thermoelectric properties. This thesis based on defect engineering ideas, through the introduction of lattice defects specific to the target system (atomic replacement of thermoelectric materials the vacancy, etc.), explore the defects of the system of electronic and electrical, phonon structure, thermal transport behavior, and thermoelectric properties based control and optimization system. At the same time, as a combination of defect characterization And theoretical analysis of inherent mechanism of defect control, the development of super lattice interlayer charge transfer control strategy, based on the project "the defect structure defects of thermoelectric properties" structure-activity relationship. At the end of this paper, author's own understanding of defect engineering based on the strategy, review the defects of regulation of degrees of freedom. This paper proposes a collaborative defect control project "multi degree of freedom based on" research ideas. The main contents of this paper include the following aspects: 1. In this chapter, the author chooses BiCuSeO natural superlattice thermoelectric materials as the research object, through the equivalent ion doping way without damaging the electrical transport properties (thermoelectric power factor PF), under the premise of significantly reduced material total lattice thermal conductivity and thermal conductivity, thereby optimizing the thermoelectric properties. Based on the layered structure of BiCuSeO composite unique compounds ([Bi_2O_2]~ (2 +) insulation layer and [Cu_2Se_2] ~ (2-) superlattice structure alternately stacked conductive layer), through La, Ag single doped and Co doped replacement two doping layer in the corresponding Bi, Cu ion, equivalent ion doped samples. The electric transport performance tests show that the convergence effect due to band potential, La-Ag Co doped samples in improving the carrier concentration while maintaining high carrier mobility and Seebeck coefficient, and the optimization of the PF, and by the phonon scattering effect of dopant atoms, the thermal conductivity of the system is further reduced, making the ZT is optimized. In three kinds of doped samples, effect is better than that of La-Ag doped Co doped samples ZT, its value is about 0.46 at 755 K, compared with the pure sample increased by nearly 70%. of the work showed that the equivalent ion doped strategy can improve BiCuSeO synchronous thermoelectric properties of superlattice materials, and the sub layer are alternately stacked special The structure also is in the same material in the implementation of ISO layer double (multiple) defect cooperative regulation research provides an ideal experimental platform.2, in the last section, due to the use of the equivalent ion doping, the conductivity enhancement effect is limited, the power factor is low. To solve this problem, in this chapter, the author puts forward the vacancy defects to optimize the electric transport properties at the same time, aiming at the drawbacks of single vacancy defects may damage the electrical properties of traditional double vacancy type defects produce a new design for the author, through the synthesis and introduced Bi, Cu for the first time in the electrical double vacancy of BiCuSeO based thermoelectric materials, the collaborative optimization of thermal transport properties. The results show that the vacancy defect based on Superlattice interface and grain boundary classification structure of the "full scale" can greatly reduce the phonon scattering system of thermal conductivity, the double vacancy in 750 samples in K to 0.37 Wm-1K-1 very low Thermal conductivity. At the same time, the positron annihilation spectra show that in Bi-Cu double vacancy samples, hole charge from Bi vacancies ([Bi_2O_2]~ (2+) insulation layer) to the Cu vacancy ([Cu_2Se_2]~ (2-) conductive layer) transfer, can significantly improve the Bi_ (0.975) Cu_ (0.975) SeO samples of the hole carrier concentration and enhance the electrical transport properties, and finally get the high ZT value of 0.84 at 750 K, significant improvements compared to the pure samples and samples have a single vacancy. This work showed that the charge transfer caused by Bi-Cu double vacancy layer and between full scale phonon scattering superlattice structure based on synergistic regulation of BiCuSeO based thermoelectric the material of the electrical and thermal transport properties, greatly optimize the thermoelectric properties. At the same time, the experimental strategy of different layers of divacancies coordinated regulation also provides new ideas for the design and preparation of.3 thermoelectric materials with high performance, through a simple solid state reaction The pure and Cu_2ZnSnSe_4 vacancy doping samples. Analysis results show that samples containing Se vacancies and defects, the intrinsic cause Cu, Zn and Sn after the introduction of the main vacancy defect clusters to form, and the introduction of vacancies can cause disordered crystalline has caused some more crystallization samples. The thermoelectric transport properties test the results show that the vacancy was improved by the introduction of the carrier concentration in the material, thus improving the conductivity and crystallization; strong phonon scattering effect of vacancy defect clusters and crystal holes and disordered crystal domain caused by the material based on the lattice thermal conductivity and thermal conductivity decreased, so as to achieve simultaneous optimization of material conductivity and thermal conductivity, and to overcome the effect of Seebeck coefficient decreased, the vacancy of ZT value of the sample and original sample has improved significantly. For example, Sn Cu_2ZnSn_ (0.98) Se_4 vacancy samples at ZT value of 7 At 50 K, it reached 0.44, which is 3 times higher than that of the original sample at the same temperature. Due to the high abundance and no pollution of such substances, the research of this chapter opens up a new direction for searching for economical, low toxic and high efficiency thermoelectric materials.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB303

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