富GeTe 的Sb2Te3(GeTe)n 基热电材料的制备与性能研究

PREPARATION AND PROPERTIES OF GETE RICH SB2TE3(GETE)N BASED THERMOELECTRIC MATERIALS

徐啸

11649013

博士

物理学

何佳清

2020-06-05

2020-07-29

哈尔滨工业大学

深圳

在新能源领域的众多应用场景里，热电材料因能将电能与热能进行直接相互转换的特性，具有独一无二的优势。尤其是中温区热电材料，因其在低品位废热发电或自驱动电子器件供能方面有广泛的应用前景而备受关注。热电材料研究的核心是提高材料的热电优值ZT以增加其在使用温区范围内的转化效率。本论文以p型中温区热电材料，富GeTe的Sb2Te3(GeTe)n为研究对象，重点关注载流子浓度对其输运性能的影响问题，通过空位工程、施主掺杂或等价掺杂等手段提升其性能表现，探索Sb2Te3(GeTe)n的特征结构与性能之间的关联性并研究其热电性能优化的物理机制。此外，本文还将Sb2Te3(GeTe)n的特征结构及其表征方法分别拓展到Sb2Te3(SnTe)n及Mn4Si7样品中，并系统的优化了其热电性能。本论文获得的主要研究成果如下：通过熔融法及放电等离子烧结法系统的制备了一系列不同组份的Sb2Te3(GeTe)n样品，证实了n=17时的组份在773 K时, ZT值最具有优化潜力，达到1.9左右。通过对比不同热处理后的Sb2Te3(GeTe)17样品，首次发现Sb2Te3(GeTe)n样品中的空位缺陷会在长时间退火过程中迁移、重构、形成长程的gap缺陷，从而有效优化样品的热电性能。最终，经过7天退火的Sb2Te3(GeTe)17样品在773 K时ZT值可以达到2.4，且在 323 K到 773 K的温度范围内ZTave高达1.5。通过研究Bi掺杂的Sb2Te3(GeTe)17样品以及BiI3双位置掺杂的Sb2Te3(GeTe)17样品，确认施主掺杂的方式能够在掺杂含量极少的情况下，将样品的载流子浓度降低至2.5×1020 cm-3。此外，BiI3掺杂还会向Sb2Te3(GeTe)17样品中引入Te空位簇，这种新的散射源能有效散射声子，使得室温晶格热导率从1.0 Wm-1K-1降低至0.8 Wm-1K-1。最终在BiI3掺杂1.5%样品中，热电优值ZT在723 K达到2.4，且在 323 K到 773 K的温度范围内ZTave值达到1.4。通过在Sb2Te3(GeTe)17基体中掺入Yb降低Ge空位形成能，从而引入大量空位缺陷来实现对高频声子的散射，使得样品晶格热导率由掺杂前的0.75 Wm-1K-1降低至0.42 Wm-1K-1（x=0.005时）。最终，样品ZT值在773 K时达到最高2.4，ZTave在 323 K到 773 K的温度范围内高达1.5。系统地研究表明，其重复性和热稳定性极佳，非常适合用来做热电器件的研究。由此组份及常规n型PbTe材料制备而成的器件在温差为423 K时，实测转换效率约为8%，输出功率约为3W。此外，在Sb2Te3(GeTe)17基体中掺入S会增加Ge空位形成能，从而降低样品的载流子浓度，最终在掺杂含量x=0.1时，在773 K时可将基体的最大ZT值提升至2.2。此外，为了扩展Sb2Te3(GeTe)n的特征结构带来的性能优势，我们还合成了一系列富SnTe的Sb2Te3(SnTe)n样品，并观察到稠密的Sn空位缺陷以及类gap结构。与Sb2Te3(GeTe)n样品一样，此种gap结构是大量的阳离子空位迁移重构形成的二维缺陷，它的存在能优化样品的晶格热导率，使得室温晶格热导率由SnTe的3 Wm-1K-1降至n=8时的0.8 Wm-1K-1。最终，Sb2Te3(SnTe)8样品在723 K时可取到最大ZT值1.12。同时，在同样具有特殊微观结构的高锰硅型热电材料的性能研究中，我们还通过使用STEM-HAADF技术对Mn4Si7组份进行研究，直接获得了样品里Mn原子层与Si原子层的精确比例，克服高锰硅体系研究中实际组份难以确认的问题。最终，在B掺杂x=0.04的Mn4Si7样品中，ZT峰值在773 K时可以达到约0.55，在300 K到823 K时ZTave值约为0.4。

Thermoelectric (TE) materials, which can convert heat into electricity (or vice versa) directly, contain unique advantages when applying in the new energy field. In particular, the medium-temperature TE materials have drawn the attentions of worldwide researchers for their promising application prospects in recycling waste heat power or supplying for self-driven electronic devices. The core task for TE materials research is to improve the ZT value, leading to a marvelous conversion efficiency at the application temperature range. In this thesis, the p type medium-temperature TE materials, GeTe-rich Sb2Te3(GeTe)n, were found to have promising TE performance, however, limited by their intrinsically high carrier concentration. Thus, we enhanced the electrical and thermal properties by vacancy engineering, donor doping or equivalent doping method and investigated the mechanisms about structure characteristics and performance optimization. Additionally, the unique structure of Sb2Te3(GeTe)n was further fabricated in Sb2Te3(SnTe)n sample and the TEM analysis techniques were successfully applied to the Mn4Si7 sample in this work. The main achievements can be summarized as follow:A series of Sb2Te3(GeTe)n samples with different components were prepared by melting followed spark plasma sintering (SPS) method, confirming that the ZT value of n=17 could reach about 1.9 at 773 K, which depicted the best performance in these compounds. With suitable thermal treatments, it was reported for the first time that the Ge vacancy defects in Sb2Te3(GeTe)17 samples preferred to migrate, reconstruct and form long-term gap defects during this long annealing process, leading to a better thermoelectric performance. Finally, the ZT value of Sb2Te3(GeTe)17 sample with 7 days’ annealing was up to 2.4 at 773 K, and a record ZTave value of 1.5 was achieved from 323 K to 773 K.Owing to the systematic studying of Bi doped Sb2Te3(GeTe)17 samples and BiI3 doped Sb2Te3(GeTe)17 samples, the carrier concentration could be suppressed effectively to 2.5×1020 cm-3 by the donor doping method. Additionally, vast anion vacancy clusters were also indicated by Cs-corrected transmission electron microscopy (TEM) in the BiI3 doped samples. This kind of defects could effectively scatter phonons, reducing the lattice thermal conductivity from 1.0 Wm-1K-1 to 0.8 Wm-1K-1. Eventually, a high ZT value of ∼2.2 at 723 K and a large average ZT value of ∼1.4 between 323 and 773 K were achieved in the 1.5 at% BiI3-doped Sb2Te3(GeTe)17 sample.Doping Yb into Sb2Te3(GeTe)17 matrix could reduce the Ge vacancy formation energy, resulting in large amounts of Ge vacancy defects, so that the lattice thermal conductivity was reduced from 0.75 Wm-1K-1 at x=0 to about 0.42 Wm-1K-1 at x=0.005. Eventually, a maximum ZT value of about 2.4 at 773 K, and a high ZTave value of 1.5 from 323 K to 773 K were obtained. Moreover, a further investigation showed that the Yb doped sample contained excellent repeatability and thermal stability, which was suitable for thermoelectric devices. Finally, when the temperature gradient was 423 K, the measured conversion efficiency is about 8% and the output power is about 3W. Additionally, Doping S into Sb2Te3(GeTe)17 matrix could increase the Ge vacancy formation energy, thereby reducing the carrier concentration. Finally, when the doping content x=0.1, the peak ZT value reached 2.2 at 773 K.To expand the performance advantages of Sb2Te3(GeTe)n brought by the microstrucutre, the SnTe-rich Sb2Te3(SnTe)n samples were firstly synthesized in this work, and dense Sn vacancy defects and gap-like structures were directly observed in our Sb2Te3(SnTe)8 sample. As the same as the Sb2Te3(GeTe)n sample, this gap structure was a kind of two-dimensional defect formed by the migration and reconstruction of cation vacancies. Attributing to this extra defect, the lattice thermal conductivity decreased from 3 Wm-1K-1 of SnTe to 0.8 Wm-1K-1 of Sb2Te3(SnTe)8 sample. Finally, the maximum ZT value could reach 1.12 at 723 K. Moreover, for the optimization of high manganese silisides which also contained a kind of speicial microstructure, here, the aberration-corrected scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) technique was used to identify the crystal structure of Mn4Si7, and the accurate ratio between Mn atom layers and Si atom layers could be directly obtained. Finally, in the B-doped Mn4Si7 sample, the peak ZT value of 0.55 at 773 K, and the ZTave value of 0.4 from 300 K to 823 K could be achieved at x=0.04.

热电材料 Sb2Te3(GeTe)n Ge 空位 功率因子 热导率 ZT

Thermoelectric material Sb2Te3(GeTe)n Ge vacancy power factor thermal conductivity ZT

中文

联合培养

南方科技大学

徐啸. 富GeTe 的Sb2Te3(GeTe)n 基热电材料的制备与性能研究[D]. 深圳. 哈尔滨工业大学,2020.