Microstructure and Electrical Characterization of Thermoelectric Nanocrystalline Bi2 Te3 Synthesized by Mechanical Alloying

Nanocrystalline bismuth telluride, Bi2Te3 has been synthesized by mechanical alloying (MA) the powder mixture composed of Bi and Te in the inert gas atmosphere within a short duration of 15 min. Microstructure characterization of the prepared powder has been investigated adopting the Rietveld’s method using X-Ray diffraction (XRD) data and analyzing high-resolution transmission electron microscopy (HRTEM) images. The XRD pattern of 15 min milled powder is composed of reflections of major Bi2Te3 phase with other two minor phases. Detailed structural information of these minor phases has been reported here which was not explored in any previous works. XRD analysis reveals that the stoichiometric Bi2Te3 phase has been formed after 1h of milling and there is no phase transition up to 10h of milling. HRTEM study also reveals the formation of single phase nanostructured Bi2Te3 phase. The DC (direct current) conductivities of 15min, 30min and 10h ball milled powders are measured within the temperature range of 359 to 633K. The electrical conductivity (σ) of Bi2Te3 indicates the semiconducting nature of the sample. There is no significant change of electrical conductivity of Bi2Te3 phase since its formation after 15min to 10h of milling, as there is no noticeable change in crystallite size.

本研究通过机械合金化（mechanical alloying, MA）工艺，在惰性气体气氛下，以15分钟的短时时长，由铋（Bi）与碲（Te）的混合粉末合成了纳米晶碲化铋（Bi₂Te₃）。本研究采用基于X射线衍射（X-Ray diffraction, XRD）数据的瑞特韦尔德法（Rietveld’s method），并结合高分辨透射电子显微镜（high-resolution transmission electron microscopy, HRTEM）图像分析，对所制备粉末的微观结构进行了表征与研究。15分钟球磨所得粉末的XRD图谱中，除主相Bi₂Te₃的衍射峰外，还存在两个次要相的衍射峰。本研究报道了此前尚无文献探究的这两个次要相的详细结构信息。XRD分析结果显示，球磨1小时后即可生成化学计量比的Bi₂Te₃相，且球磨至10小时的过程中未发生相变。HRTEM研究同样证实了单相纳米晶Bi₂Te₃相的形成。本研究测试了15分钟、30分钟及10小时球磨粉末在359~633K温度区间内的直流电（direct current, DC）电导率。所得Bi₂Te₃的电导率（σ）表明该样品具备半导体特性。自15分钟球磨生成Bi₂Te₃相后，直至10小时球磨，该相的电导率未出现显著变化，这是由于晶粒尺寸未发生明显改变。