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標題: 以真空電弧熔煉法製備p型FeSi2-XAlX之相變化及熱電性質之研究
Phase Transformation and Thermoelectric Performance of p-type FeSi2-XAlX Prepared by VAR
作者: 張凱棋
Chi, Chang Kai
關鍵字: Vacuum Arc Remelting (VAR);真空電弧熔煉;FeSi2;thermoelectric;二矽化鐵;熱電
出版社: 材料工程學研究所
本研究以真空電弧熔煉法製備p型FeSi2-XAlX之熱電塊材,再藉由不同持溫時間之800 ℃的熱處理,使材料相變化為具有半導體性質之β相結構。分別利用感應耦合電漿質譜分析儀、電子微探分析儀、X光能量散佈光譜儀、熱差分析儀、X光繞射儀、場發射電子顯微鏡、熱電量測系統等分析儀器,觀察FeSi2-XAlX之相變化和熱電性質,探討不同持溫時間之熱處理對FeSi2-XAlX的相變化趨勢和熱電性質的影響。
由ICP-MS結果顯示,熔煉後之Si元素的莫耳比之數值較小,因此本實驗統一多添加4 wt% Si。在DTA、XRD和FE-SEM的相變化分析結果中,發現摻雜Al的含量增高時,出現β相的熱處理時間愈久。原因為Al降低包析與共析溫度,造成不同反應速率之包析反應和共析反應變慢。而且摻雜Al還會造成α相在2θ=69.76°的位置產生(2 0 0)之優選方向。熱電力量測方面,熱電優值之最佳值出現在FeSi1.985Al0.015經過熱處理16小時後,其值為Z=2.6×10-5(K-1),其中熱電力S=87.03(μV/K)、電阻率ρ=9.02×103(μΩ㎝)和熱導度κ=3.24(W/mK)。由於未摻雜Al元素之FeSi2為本質半導體的性質,因此較低的載子濃度使能隙較大,而導致電阻率過高。而其它摻雜Al含量較高之成分,則因為β相變化速率太慢,造成熱電力太低,導致其熱電優值都低於FeSi1.985Al0.015。

In this study, we have prepared the bulk materials of p-type FeSi2-XAlX by means of Vacuum Arc Remelting (VAR). The materials subsequently were heat treated at 800℃ for various times to make them transfor into semiconducting β phase. The Composition, Phase transformation and thermoelectric properties were characterized by means of Inductively Coupled Plasma-Mass Spectrometer (ICP-MS), Electron Probe X-Ray Micro-Analyzer (EPMA), Differential Thermal Analysis (DTA), X-Ray Diffraction (XRD), Field-Emission SEM (FE-SEM) and thermoelectric measurement system, respectively.
Shown by ICP-MS result, the mole ratio value of Si is less than the composition planned originally, so we added excess 4 wt% Si to all of the samples prepared. The results in DTA,XRD and FE-SEM, show that the heat treatment time for the appearance of β phase increases with increasing the content of Al-doped. The reason is that the reaction rates of both pertectoid and eutectoid reactions are slowed down due to the effect of lowering pertectoid and eutectoid temperature with Al dopant. It is evident that preferred orientation arises in the direction (2 0 0) at 2θ=69.76° with Al-dopant. About the thermoelectric properties, the optimum figure of merit Z=2.6*10-5(K-1) is composed of thermoelectric power S=87.03(μV/K), electrical resistivity ρ=9.02*103(μΩ㎝) and thermal conductivity κ=3.24(W/mK), which was obtained in FeSi1.985Al0.015 after 16 hours 800℃ heat treatment. Because pure FeSi¬2 is an intrinsic semiconductor, the higher electrical resistivity was due to the lower carrier concentration and the higher energy gap. And the other sample which were doped with higher Al content. Possess lower values of the figure of merit than FeSi1.985Al0.015, because the transformation rate of β phase is too slow to obtain a better thermoelectric property within 16 hours.
Appears in Collections:材料科學與工程學系

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