Influence of Alumina$S Addition on the Thermoelectric Performance of Zn4sb3 Alloys

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Project title

Code/計畫編號

MOST106-2221-E019-025

Translated Name/計畫中文名

氧化鋁之添加對銻化鋅合金熱電特性影響研究

Project Coordinator/計畫主持人

Pee-Yew Lee

Funding Organization/主管機關

National Science and Technology Council

Department/Unit

Department of Optoelectronics and Materials Technology

Website

https://www.grb.gov.tw/search/planDetail?id=12226447

Year

2017

Start date/計畫起

01-08-2017

Expected Completion/計畫迄

31-07-2018

Bugetid/研究經費

1168千元

ResearchField/研究領域

材料科技

"中溫熱電材料包含Tellurides (PbTe/GeTe/AgSbTe2)、Half-Heusler(MNiSn, M=Zr, H, Ti)、FilledSkutt eruide(CoSb3, CeFe3.5Co0.5Sb12) 、Antimonides(Zn-Sb) 、Silicides(MgSi2, Fe2Si,Mn2Si, Sr2Si) 、 Oxides(NaxCoO2、Ca3Co4O9)等多種材料系統，如考慮材料可考度、高ZT特性、P/N電性匹配等因素，則以碲化鉛(PbTe)系列熱電材料為最佳選擇，但此材料之組成為鉛碲，鉛為嚴重環境污染物，對地球環境與人類健康有潛在的威脅，而碲元素的價格在2011年開始飆漲，價格較往年平均值成長3~5倍，雖然今年起價格已回跌40%，但仍高達每磅100美元，除此外碲在地表的蘊藏量甚至比鉑(Pt)少，故其在未來的價格波動幅度預期是漲多跌少之趨勢，因此碲化鉛(PbTe)熱電材料在未來之使用勢必因價格/ 蘊藏量/環保等因素考量而受到嚴格的檢驗，相對於此，β-Zn4Sb3因為原料價格便宜、地表蘊藏量豐富與無毒的特性，並具備低熱傳導係數及在670K時擁有高達1.3的熱電優值，故考量對環境友善與原料成本之因素，β- Zn4Sb3在未來將會是比PbTe更具應用潛力的中溫熱電材料，但由於Zn4Sb3體系本身的材料脆性和在相轉變過程中由於熱膨脹係數變化而產生的微裂紋，導致材料具有較差的力學性能和可加工性，大大限制了β- Zn4Sb3材料的商業化應用。因此開發出可製造具有優良熱電性能與高機械強度之Zn4Sb3材料的嶄新製程，最近已成為熱電相關研究學者的努力目標。近年來應用奈米複合材料(Nanocomposite)結構來提升材料性能的概念，已被廣泛應用於很多新穎材料的開發，奈米熱電複合材料塊材的研發當然亦不自外於此，應用此設計概念，申請人於在目前進行之「摻雜γ-Al2O3之Bi-Sb-Te熱電複合材料特性研究」研究計畫(105/08/01~106/07/31，計劃編號: MOST-105-2221-E-019-012)中，發現結合機械合金與真空熱壓製程製備之具微米/奈米之複合結構的 γ-Al2O3/Bi0.4Sb1.6Te3熱電複合材料塊材，藉由γ-Al2O3之添加，γ-Al2O3/Bi0.4Sb1.6Te3塊材在298~423 K的 ZT值為1.0~1.22，明顯高於其他研究團隊利用不同方式製備之Bi0.4Sb1.6Te3熱電塊材，此特殊行為是多數的γ-Al2O3粉末都均勻散佈於Bi0.4Sb1.6Te3基材晶界上所致，故據上述結果可知在Zn4Sb3合金中，若添加之γ-Al2O3粉末亦可均勻散佈於Zn4Sb3基材晶界上，則此種具微米/奈米結構之Zn4Sb3熱電複合材料塊材的熱電優值(ZT)應可隨溫度增加而升高，而ZT值也可望因此獲得大幅提升。綜上所述，將γ-Al2O3粉末引入β-Zn4Sb3合金中預期可提升β-Zn4Sb3合金的熱電與機械性能，故本研究之目的在探討利用粉末冶金技藝來製備具微米/奈米複合材料結構之γ-Al2O3/Zn4Sb3熱電塊材的可行性，並進一步瞭解塊材的結構與熱電及機械性質的關聯性。研究工作之進行先利用高能量震動式球磨機製備出Zn4Sb3合金粉末，接著將不同重量百分比的γ-Al2O3粉末混合Zn4Sb3合金粉末後，再以同型之球磨機合成γ-Al2O3/Zn4Sb3複合材料粉末，最後利用真空熱壓技術將此複合材料粉末製備成複合材料塊材，所製備之複合材料粉末與塊材將依序分別以X光繞射儀、SEM 掃瞄式電子顯微鏡、TEM穿透式電子顯微鏡及HRTEM高分辨穿透式電子顯微鏡進行分析及研究其微觀組織結構外，另亦針對熱電複合材料塊材進行席貝克係數、電傳導率、熱傳導率、ZT值的檢測及微硬度試驗，藉此瞭解此等塊材的熱電與機械性質，詳細評估實驗所得之數據資料後，將可獲知具微米/奈米複合材料結構之 γ-Al2O3/Zn4Sb3熱電材料塊材的結構與熱電性能的關聯性，同時也可開發出應用高能量球磨及真空熱壓製程製備具高熱電性能之奈米熱電複合材料塊材的最佳化條件。" "PbTe-based thermoelectric materials with high ZT (>1), including AgPbmSbTem+2, PbTe, and PbxSb1-xTe3 have become the most promising thermoelectric (TE) materials in the intermediate temperature range. Nevertheless, they always face similar challenges in terms of containing high-cost or toxic elements, especially lead (Pb) or tellurium (Te). As is well known, Pb is harmful to human health, and the European Union has regulated its usage in electronic devices. On the other hand, the price of Te is over US $100 per pound and its atomic abundance is even lower than that of platinum (Pt). Therefore, commonly available and inexpensive thermoelectric materials have received increasing attention and it is necessary to develop high-performance but Pb/Te free materials for industrial applications. Recently, P-type β-Zn4Sb3 compound has re-attracted increasing interest because it has exceptional thermoelectric properties in the intermediate temperature range. The maximum ZT value reaches 1.3 at 670K because of its extremely low thermal conductivity originated from its complex crystal structure. Moreover, its constituent elements, zinc (Zn) and antimony (Sb), are earth abundant, environmentally friendly, and nontoxic. However, the fragility and the microcracks result from the phase transition greatly decrease the mechanical property and process ability, which limits its commercial application. Therefore, to fabricate β-Zn4Sb3 bulk material with not only high thermoelectric performance but also high mechanical durability is of vital significance. Recently, numerous attempts have been made to increase the ZT value of TE materials. One effective method is to reduce the lattice thermal conductivity by refining the grain size based on incorporating additional phonon scattering sites inside the grain boundary regions. In our on-going MOST-supported project: “Thermoelectric properties of γ-Al2O3 doped Bi-Sb-Te materials” (MOST 105-2221-E-019-012), the γ-Al2O3/Bi0.4Sb1.6Te3 nanocomposites were prepared by a combination of mechanical alloying (MA) and vacuum hot pressing (VHP) techniques. We found the ZT values as high as 1.22 and 1.21 were achieved at 373 and 398 K for samples containing 1 and 3 wt % Al2O3 nanoparticles, respectively. These ZT values are higher than those of several reported monolithic Bi0.4Sb1.6Te3 samples prepared through BM and hot pressing or spark plasma sintering. The TEM results indicated the achieved high ZT values may be attributable to the unique nano- and microstructures in which Al2O3 nanoparticles are dispersed among the grain boundary or in the Bi0.4Sb1.6Te3 matrix grain. Consequently, the thermoelectric properties and mechanical properties of β-Zn4Sb3 materials are expected to be improved by obtaining nano/microstructured γ-Al2O3/Zn4Sb3 with doping γ-Al2O3 particles into β-Zn4Sb3 matrix. Therefore, the goal of this proposal is to investigate the feasibility of preparing new γ-Al2O3/Zn4Sb3 nanocomposites by a combination of mechanical alloying (MA) and vacuum hot pressing (VHP) techniques. The phase transform and the microstructure formation during the MA processes, and the influences of different MA and VHP parameters on TE properties and mechanical properties will be studied. The structure of as-prepared nanocomposite powders and bulk nanocomposites materials will be examined by XRD, SEM, and TEM. In addition, the measurement of thermal conductivity, Seebeck coefficient and electrical conductivity as well as hardness tests on the bulk nanocomposites materials also will be conducted in order to understand the thermoelectric and mechanical property of the resultant γ-Al2O3/Zn4Sb3 bulk nanocomposite materials. After comprehensive evaluation of all the experimental results obtained in this work, the influence of the microstructure on the thermoelectric property can be elucidated. The optimum conditions for the production of high thermoelectric performance bulk γ-Al2O3/Zn4Sb3 nanocomposite through ball milling and vacuum hot pressing routes also can be established."

Keyword(s)

奈米熱電複合材料塊材
熱電優值
機械合金
真空熱壓
Bulk nanocomposite thermoelectric materials
Figure of merit
Mechanical alloying
Vacuum hot pressing

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Influence of Alumina$S Addition on the Thermoelectric Performance of Zn4sb3 Alloys

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