Development of High-Performance Nano-Grained Bulk Silicon-Germanium Thermoelectric Materials by Co-Sintering with Titanium

In this study, we developed a method for reproducibly fabricating high-performance nano-grained bulk Si-Ge thermoelectric materials free from severe oxidization. In our previous work, the oxidization of Si-Ge during mechanical alloying and sintering processes had led to poor reproducibility of the value of electrical resistivity. We found that co-sintering with Ti, which is more easily oxidized than Si and Ge near the sintering temperature, effectively reduces the oxygen concentration in the nano-grained bulk Si-Ge samples. The oxygen concentration in the sample co-sintered with Ti was found to be less than 2.4 at.%, and electrical resistivity was found to be less than 3.9 mΩ cm at 922 K with good reproducibility. High Seebeck coefficient (more than 400 μV K-1) and low thermal conductivity (less than 1 Wm-1K-1) were simultaneously achieved by constructive electronic structure modification via iron doping and nano-crystallization, respectively. As a consequence, we succeeded in obtaining a surprisingly large value of dimensionless figure of merit, ZT = 4 at 922 K, and the temperature range of ZT exceeding 1 extended at high temperatures above 700 K.