熱電材料為少數能有效回收廢熱的綠能材料。評斷熱電材料效能之熱電優值ZT = S2σ/κ，當中Seebeck係數(S)、電導率(σ)與熱導率(κ)皆與電子濃度有關，但熱導率除了與電子濃度有關外，也與聲子熱傳導有關，本實驗將Bi2Te3-xSex一系列塊材以放電加工法製作為奈米微粒，希望藉由材料奈米化將聲子熱傳導降低。由XRD及TEM可確認經由放電加工法製作不同成分之微粒，與塊材相比無任何雜相的產生，而TEM的觀察結果發現微粒尺寸大多小於10 nm，但當中混有少量大於100 nm之多晶顆粒。本實驗採用瞬態ZT量測法，測得Bi2Te2.1Se0.9微粒以冷壓成型製作之塊材其ZT值約為原塊材1/10，換算之熱電功率因子(S2σ)約為原塊材1/500，估計熱導率約為原塊材之1/40，此結果顯示，製作為奈米材料後，可確實有效的降低其晶格熱傳導，但同時因奈米微粒界面大幅增加導致界面間電子散射機率增加，進而使得電阻率及熱導率有大幅度的改變。另外Raman的結果也顯示奈米化的樣品有疑似界面造成之表面振動模(Surface mode)特徵峰的產生。 The thermoelectric(TE) materials can convert heat directly to electricity as temperature gradient built up, so it is an important method to regain waste heat. There are more performance parameters that play a very important role in a TE module characterization. These parameters are thermal conductance (κ), electrical conductivity (σ) and the Seebeck coefficient(S) which combined as follows: ZT = S2σ/κ.The parameter Z is usually referred to as “figure-of- merit”. The thermal conductance (κ) of TE materials can be reduced by nano-technic. For this study , the Bi2Te3-xSex nanoparticles were fabricated to research the size effect on the properties of TE materials. Bi2Te3-xSex nanoparticles were fabricated by electrical discharge method (EDM). All of Bi2Te3-xSex nanoparticles were the same phase with bulk by X-ray diffraction (XRD) patterns. The transmission electron microscopy (TEM) result shows that most nanoparticle with the average size below 10 nm. A transient method was used to measure ZT values. The ZT value of cool-pressed Bi2Te2.1Se0.9 nanoparticle sample is about 1/10 of bulk in room temperature. Considering that thermo-powerfactor S2σ is about 500 times higher than bulk sample, we can estimate the thermal conductivity of cool-pressed nanoparticles sample was reduced dramatically to only 1/40 of its bulk sample. The reduced κ maybe derive from more scattering coming from the large number of grain boundaries between nanoparticles. But so small κ imply that electronic concentration of sample maybe also affect by nanoized process. The additional peaks appearing on Raman spectra indicate that possibility surface-mode phonon existed in nanoparticle samples.
