Microstructure Evolution and Thermoelectric Behaviour of Directionally Solidified Bi2te3 Based Multiphase Thermoelectric

Abstract

This study investigated the impact of unidirectional solidification on the Bi2Te3-Cu3Te2 based eutectic alloy. The crystals were grown using a 4 mm thick quartz tube placed vertically in a Bridgeman-type furnace that moved downwards towards cooled water.  The grain growth and alignment were significantly altered following solidification at different velocities: V = 10 µm/s (CBT1), 20 µm/s(CBT2), and 50 µm/s(CBT5). For the solidification velocity of 50 µm/s, a distinctive eutectic columnar alignment is observed. Scanning electron microscopy and X-ray diffraction analysis (XRD) were used to characterize the alloys. Solidified alloys constitute phases of trigonal (Bi2Te3) and tetragonal (Cu3Te2) crystal structure. In addition, changes in solidification velocities show drastic changes in the thermoelectric properties. In all the alloys, the electrical conductivity shows an increase with temperature variation suggesting a non-degenerate behavior. Among the investigated alloys, CBT2 shows the highest power factor of 8.9 mWm-1K-2 at 442 K resulting in an enhanced figure of merit (zT) to 0.93 at a temperature of 442 K.  Nevertheless, the presence of a high eutectic interface in CBT5 shows an increase in the resistance to electrical conductivity which results in highest Seebeck coefficient of -234.8 µVK-1 at a temperature of 345 K. In addition, thermal conductivity of CBT5 was found to be minimum (2.2 Wm-1K-1 at a 302 K). The electronic thermal conductivity contribution to the overall thermal conductivity is significantly reduced due to the strong scattering of carriers at the small eutectic surfaces in CBT5. The work provides novel perspectives on the utilization of unique processing route for optimizing thermoelectric properties of eutectic alloys.

Keywords: Thermoelectricity, Microstructure, Directional solidification, Transport properties, Eutectic alloy