碲化鉍銻奈米線及銀摻雜在硒化錫奈米晶體的合成及熱電性質分析

The synthesis and thermoelectric properties of Bi-Sb-Te nanowire and Ag doped SnSe nanocrystals

錢佳華(Chia-Hua Chien)

指導教授 ： 李志浩 ； 陳洋元

國立清華大學/原子科學院/工程與系統科學系/博士(2017年)

摘要

近來，熱電應用因其將廢熱轉化為電無有害排放的能力，引起了越來越多的興趣。與塊材相比，許多研究已經能夠實驗指出低維材料的物理性質不同。在奈米尺度上，藉由阻擋聲子傳遞能降低晶格熱導率（κph）來增強熱電效率，此外，透過摻雜濃度效應，調整費米能階的位置以達到最佳功率因數（σα2），是提高zT值的另一個策略。在本論文中，我們基於上述概念合成了高質量和大規模奈米材料，並分為兩個系統來研究熱電性能。
在第一部分中，藉由聚焦離子束，Bi0.8Sb1.2Te2.9奈米線的熱電性質被原位研究，其直徑從750修剪到490和285 nm。雖然電導率和熱導率都隨著直徑減小而顯著降低，但兩種物理性質清楚地表現出隨著直徑改變而有不同的行為，與Callaway模型的理論預測相比，觀察到更低的熱導率。結果顯示，熱導率的降低除了尺寸效應外，還有歸因於Ga離子照射產生的缺陷造成額外的聲子散射。在750nm處顯示出最大Seebeck係數和zT優值，然後隨著尺寸減小而線性減小。該研究不僅全面地了解了尺寸和缺陷對熱電性質的影響，而且提出了一種通過離子照射來調控奈米線熱導率的可能方法。
在第二部分中，使用兩步驟且無表面活性劑的溶液生長，來合成以克為單位的p型Ag摻雜SnSe奈米晶體。藉由高解析電子顯微鏡，研究SnSe奈米晶體的形成機制，且熱導率的量測下發現800 K附近有明顯相變。利用火花電漿法燒結製備的SnSe樣品，其熱電性質在3 ％Ag摻雜的SnSe中有顯示出zTmax（在850 K下為0.8）。zTmax值比原始SnSe高約40％，其結果主要歸因於Ag摻雜對載子濃度和功率因數的增加。此研究結果表明，這種簡單的化學方法適合製造高品質的SnSe奈米晶體，並且也可以應用於其他異向性結晶材料。

關鍵字

熱電材料 ； 碲化鉍銻 ； 硒化錫 ； 奈米線 ； 奈米晶體

並列摘要

Thermoelectric applications have attracted increasing interest recently due to its capability of converting waste heat into electricity without hazardous emissions. Compare with the bulk materials, many researches have been able to experimentally reveal that physical properties were different in low dimensional materials. In nanoscale, the enhancement of the thermoelectric efficiency by the reduction of lattice thermal conductivity (κph) due to phonon blocking. In addition, tuning the position of Femi level to achieve the optimum power factor (σα2) by doping concentration effect is another strategy to improve the zT value. In this thesis, we have synthesized the high quality and large-scale nanomaterials based on the mentioned concept and divided into two systems to investigate the thermoelectric properties.
In the first part, the thermoelectric properties of a Bi0.8Sb1.2Te2.9 nanowire (NW) were in-situ studied as it was trimmed from 750 down to 490 and 285 nm in diameter by a focused ion beam. While electrical and thermal conductivities both indubitably decrease with the diameter reduction, the two physical properties clearly exhibit different diameter dependent behaviors. The much lower thermal conductivities were observed as compared with the theoretical prediction of Callaway model. The consequence indicates that in addition to the size effect, extra phonon scattering of defects created by Ga ion irradiation was attributed to the reduction of thermal conductivities. The size dependence of Seebeck coefficient and figure of merit (zT) show the maximum at 750 nm, then decrease linearly with size decrease. The study not only provides the thoroughly understanding of the size and defect effects on the thermoelectric properties but also proposes a possible method to manipulate the thermal conductivity of NWs via ion irradiation.
In the second part, A two steps, surfactant-free solution growth process was utilized to synthesize p-type Ag doped SnSe nanocrystals in gram quantities. The formation mechanism of SnSe nanocrystals studied by the high resolution transmission electron microscopy. A clear phase transition near 800 K was discovered in the temperature dependence of thermal conductivity. The thermoelectric properties of SnSe pellets prepared by spark plasma sintering, exhibit a significant increase of zTmax (0.8 at 850 K) in the 3 % Ag doped SnSe. The zTmax value is about 40 % higher than that of the prinstine SnSe. The consequence is mainly attributed to the enhancement of carrier concentration and power factor by Ag doping. Our results demonstrate that this facile chemical method is amenable to fabricate high quality SnSe nanocrystals and might also be applied to other anisotropic crystalline materials.

並列關鍵字

Thermoelectric material ； BiSbTe ； SnSe ； nanowire ； nanocrystal

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