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標題: | P 型half-Heusler合金材料之熱電性質之研究 Study of Thermoelectric Properties of P Type Half-Heusler Alloy Compound |
作者: | Chia-Cheng Hsu 許嘉政 |
指導教授: | 馬小康(Hsiao-Kan Ma) |
關鍵字: | Half-Heusler,BM-SPS,Seebeck係數,導電率,ZT 值, Half-Heusler,BM-SPS,Seebeck coefficient,Electrical conductivity,ZT, |
出版年 : | 2014 |
學位: | 博士 |
摘要: | 在過去二十幾年來,half-Heusler熱電材料已經吸引了廣泛的研究興趣,而此half-Heusler合金被所感興趣的原因是因為它在700 K以上的溫度擁有很好的熱電性質,此溫度範圍正是接近大多數工業廢熱的熱源溫度範圍,在過去幾年的研究中,half-Heusler奈米結構熱電材料的合成造成其熱電性質顯著的提升。
在本論文中,我們以half-Heusler中MCoSb之 P 型材料系統使用BM-SPS的合成方法,來提升熱電性質。下列有效提升熱電性質的方法也在內文中被討論,如in-situ金屬奈米相來增加電導性及氧化物奈米複材來降低其熱傳導性質,有效的energy filtering effect來增加其Seebeck係數,利用重原子來取代輕原子或較大的原子取代較小的原子,形成的point defect scattering,mass fluctuation scattering和strain field effect的效果造成聲子散射,降低熱傳導係數,並借由BM-SPS製程來形成奈米晶粒,造成boundary scattering來減少熱傳導係數。 在此論文中,首先研究half-Heusler材料中的ZrFexCo1-xSb (x = 0–0.6)系統,此材料利用電弧熔煉來合成,ZrFexCo1-xSb 之合金可由XRD鑑定出α-Fe、ZrCoSb half-Heusler 相和Fe0.63ZrSb相,而Fe0.63ZrSb相隨著鐵摻雜量之增加而增加。另外利用SEM可觀測出此材料系統,隨著Fe摻雜量之增加,有晶格細小化之現象。在Fe之摻雜後,Seebeck coefficient之極性由負變正,另外電導性也隨Fe之摻雜量之增加而增加。在ZrFexCo1-xSb系統上晶格熱傳導也會減少,這是因為Fe置換Co的位置時會產生點缺陷,因此會散射熱聲子,Fe的摻雜有助於提升ZT值,ZrFe0.2Co0.8Sb的ZT值在溫度為900K時最高值,其值為0.036. 在P-type ZrCoSb1-xSnx (x = 0.1 and 0.2)材料系統研究中,而表現出相對性較高的ZT值。最大的ZT值是在ZrCoSb0.8Sn02材料中所發現,在溫度900 K時其值為0.38。此ZrCoSb1-xSnx (x = 0.1 and 0.2)材料系統中可知,由於維持了大量的載子有效質量,造成導電率的提升。最佳化的載子濃度控制,不會造成Seebeck coefficient減少,是ZrCoSb1-xSnx材料系統擁有高ZT的原因。 另外研究half-Heusler 系統中的Zr0.5Hf0.5CoSb1-xSnx/ HfO2 (x = 0.1 and 0.2)奈米複材,在Zr0.5Hf0.5CoSb1-xSnx材料系統中討論晶粒細化對結構和熱電性質的影響,並且證明了利用球磨和SPS製程,會形成奈米HfO2,此奈米HfO2分佈在主基材之邊界上,而這種Zr0.5Hf0.5CoSb1-xSnx/HfO2 (x = 0.1 and 0.2)之結構可以當成是一種奈米複合材料結構。在此研究中主基材Zr0.5Hf0.5CoSb1-xSnx中Sn對Sb之置換,可以有效的提升電導性,另外Zr0.5Hf0.5CoSb0.8Sn0.2 /HfO2奈米複合材料也造成在900 K時有擁有最低的晶格熱傳導為2.04 Wm-1K-1,最高的熱電優值ZT為0.75。 而另一個half-Heusler系統中Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2複材研究中,也對此複材對於微結構與熱電性質進行分析討論。在此研究中,我們證明了在Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8材料系統在進行電弧熔煉過後,會產生Fe3Sn2相,此Fe3Sn2相均勻的分佈在主基材之邊界上,此Fe3Sn2相的含量也隨Fe摻雜量之增加而增加,我們也可將此Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2之結構當成一種複合材料。在此研究中,Fe可置換Co位置的含量約為3.32 at.%。Fe有效的置換Co的位置可有效的提升其熱電優值,最大的熱電優質可在Zr0.5Hf0.5Fe0.1Co0.9Sb0.8Sn0.2/Fe3Sn2之複合材料中發現,其ZT值在900 K時可達0.62。 Half-Heusler (HH) thermoelectric materials have been attracting extensive research interest over the last two decades. Half-Heusler alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In the doctoral dissertation, we use the BM-SPS synthesis methods for optimizing individual parameters to enhance the thermoelectric performance in MCoSb (p-type) based half-Heuslers. Some effective approaches, such as using metallic phase nanoinclusions as dopants to enhance electrical conductivity and oxide nanocomposite to reduce thermal conductivity, low energy carrier filtering to enhance Seebeck coefficient, the Sn/Sb and Hf/Zr substitution result in the point defect scattering, mass fluctuation scattering and strain field effect and nanosized grains formed by BM-SPS to reduce thermal conductivity, are discussed in the thesis. In the ZrFexCo1-xSb (x = 0–0.6) system, the alloys were prepared by arc melting. XRD results indicated that α-Fe, ZrCoSb half-Heusler phase and Fe0.63ZrSb can be found in ZrFexCo1-xSb alloy. In addition, the peak of Fe0.63ZrSb increases with increasing of Fe content. SEM results show a tendency of grain refinement, where the grain size decreases with increasing of Fe content. The Seebeck coefficient changed from negative to positive by Fe doping, and the electrical conductivity increased with increasing Fe content. The lattice thermal conductivity for ZrFexCo1-xSb is considerably reduced, because the point defects induced by Fe substitution for Co intensively scatter the thermal phonons. Fe doping improved the thermoelectric figure of merit ZT of ZrFexCo1-xSb and the maximum ZT value of 0.036 was obtained for ZrFe0.2Co0.8Sb sample at 900 K. In the p-type ZrCoSb1-xSnx (x = 0.1 and 0.2) system, the half-Heusler alloys showed relatively high ZT values. The maximum ZT was 0.38 at 900 K for ZrCoSb0.8Sn02. Electrical conduction was considerably improved when a large positive thermoelectric power due to a large carrier effective mass was maintained. A high ZT was realized in the p-type ZrCoSb1-xSnx system by optimizing carrier concentration without any degradation of Seebeck coefficient. In the half-Heusler Zr0.5Hf0.5CoSb1-xSnx/ HfO2 (x = 0.1 and 0.2) nanocomposites system, we demonstrate that the substitution of Sn for the Sb site in Zr0.5Hf0.5CoSb1-xSnx under the combined process of ball-milling and spark plasma sintering (SPS) is able to promote the occurrence of grain refinement and nanostructured HfO2. The in-situ formed HfO2 nanoparticles were evenly dispersed on the grain boundaries. Such structure morphology can be treated as Zr0.5Hf0.5CoSb1-xSnx/HfO2 (x = 0.1 and 0.2) nanocomposites. The substitution of the Sb site by Sn in Zr0.5Hf0.5CoSb1-xSnx is effective in increasing electrical conductivity. The minimum lattice thermal conductivity of Zr0.5Hf0.5CoSb0.8Sn0.2 /HfO2 that can be reached is 2.04 Wm-1K-1 and the maximum ZT is 0.75 at 900K. In the half-Heusler Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 (x = 0.05, 0.1 and 0.2) system, we demonstrate that the substitution of Fe for the Co site in Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8 under the arc melting process is able to promote the occurrence of a second Fe3Sn2 phase. The in-situ formed Fe3Sn2 phase was evenly dispersed on the grain boundaries and increased with increasing amounts of Fe. Such structure morphology can be treated as Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8/Fe3Sn2 (x = 0.05, 0.1 and 0.2) composites. The maximum substitution of the Co site by Fe is approximately 3.32 at. %. The substitution of the Co site by Fe in Zr0.5Hf0.5FexCo1-xSn0.2Sb0.8 is effective in increasing the ZT value. The maximum ZT of Zr0.5Hf0.5Fe0.1Co0.9Sb0.8Sn0.2/Fe3Sn2 can reach 0.62 at 900K after BM-SPS processing. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/5099 |
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顯示於系所單位: | 機械工程學系 |
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