Thermoelectric transport enhancement of Te-rich bismuth antimony telluride (Bi0.5Sb1.5Te3+x) through controlled porosity

Alloys of Bi2Te3 and Sb2Te3 are the best performing p-type thermoelectrics near room temperature and have been the subject of extensive engineering efforts. Dramatic improvement is achieved by introducing defects that effectively scatter phonons and reduce thermal conductivity. Unfortunately, outstanding results are often difficult to reproduce as the process variables involved are difficult to control or possibly unknown. Here, a reproducible and controllable method of fabricating porous Bi0.5Sb1.5Te3+x is presented. While effective medium theory (EMT) predicts no benefit, improvements in the thermoelectric quality factor, B (which determines the maximum zT of a materials), were as high as 45% parallel to the pressing direction for a sample of roughly 20% porosity. The study of microstructural evolution with increasing porosity is facilitated by a combination of Scanning/Transmission Electron Microscopy (S/TEM) and Electron Backscattered Diffraction (EBSD). This study reveals a statistically significant shift in the distribution of grain boundaries favoring lower energy twins, which coincides with an increase in the presence of stepped twin boundaries. This work demonstrates the potential benefits of careful grain boundary engineering and the need for further detailed studies of the dependence of thermal and electrical transport on grain boundary structure and orientation in these alloys.