鉍-銻-碲單晶奈米線之製備與熱電性質研究

Abstract

在許多熱電材料相關研究顯示，鉍-銻-碲材料在接近室溫有良好的熱電性質，並且在低維度下，其物理行為將會隨著尺寸的不同而改變，此時量子效應則明顯的影響了其物理的特性和行為，因此聲子在物體裡面的物理行為也有所改變，於是我們合成單晶BixSb2-xTe3-y奈米線量測熱電性質，期望可以看到熱電性質的提升。首先使用脈衝雷射沉積系統將Bi0.5Sb1.5Te3鍍於二氧化矽基板，將附有一層薄膜的基板放置石英管並封真空，薄膜以330 ℃至350 ℃熱處理5天，經過熱處理後奈米線會生長於薄膜表面，直徑由幾十至幾百奈米，而長度則為幾微米至幾十微米，將奈米線懸放至量測晶片，透過選區繞射分析奈米線的結晶性並知其生長方向為[110]，使用X射線能量色散儀分析成分，利用掃描式電子顯微鏡影像測量出奈米線的截面積，使用四點量測奈米線的電阻率ρ。使用晶片上的加熱器及溫度計可量測席貝克係數(Seebeck coefficient) S，利用三倍頻方法(3ω method)量測熱傳導率κ，由量測出的三項熱電係數可得熱電優質係數(figure of merit) ZT=S2T/ρκ，此論文量測三根單晶奈米線，等效直徑分別為、350奈米、280奈米和240 奈米，當奈米線的尺寸小於聲子的平均自由徑，會影響聲子的傳遞，因而降低其熱傳導係數，使ZT值增加。以240奈米的奈米線為例，量測的最高ZT值為0.51Bismuth−antimony−telluride alloy, is one of the famous thermoelectric materials in room temperature. Single-crystalline BixSb2-xTe3-y nanowires were synthesized by thermal annealing method[1], with diameter in range 100 nm to 1 μm and the length up to 40 μm. The thin films for nanowires growing were deposited by pulsed laser deposition (PLD) on SiO2/Si substrates. The thermal annealing condition of nanowire growth is annealing at 330 ℃ to 350 ℃ for five days. Transmission electron microscopy (TEM) result shows that BixSb2-xTe3-y nanowires grown by thermal annealing method were single-crystalline and selected area diffraction (SAD) shows the growth direction oriented along [110]. The energy dispersive X-ray (EDX) data show the atomic ratio of nanowires are Bi:Sb:Te = 0.75:1.25:3. Furthermore, the BixSb2-xTe3-y nanowires were suspended on Si3N4/Si-platform. Scanning electron microscope (SEM) image used to characterize the cross-sectional areas of the nanowire. The thermoelectric properties measurements of nanowire including electric resistivity, Seebeck coefficient, thermal conductivity and ZT were investigated in this work. Here we report thermoelectric performance Bi0.75Sb1.25Te3 nanowires with diameter of 300 nm, 280 nm and 240 nm. By selecting the different diameter nanowires, the ZT values represent the dependence of the cross-section area. Indicate that the improved efficiency originates from phonon effects. The free flow of heat by phonon and electrons is resisted by various scattering processes resulting in a finite value of thermal conductivity. Such as phonon – phonon scattering already limit the mean-free path to a smaller sample size, which is related to the crystal size for a specimen with cross-section area [2] [3] [4]. Therefore ZT increased as the cross-sectional areas decreased, and the highest ZT is 0.51.for the smallest cross-section area nanowire.