Thermoelectric property studies on nanostructured N-type Si-Ge Bulk Materials

Abstract

SiGe alloys are the only proven thermoelectric materials in power generation devices operating above 600 °C and up to 1000 °C in heat conversion into electricity using a radioisotope as the heat source. In addition to radioisotope applications, SiGe thermoelectric materials have many other potential applications, for example, solar thermal to electricity energy conversion and waste heat recovery. However, traditional SiGe alloy material shows low ZT values of about 0.93 at 900 °C, thus, 8% is the highest device efficiency for commercial SiGe thermoelectric devices. Recently, many efforts have been made to enhance the dimensionless thermoelectric figure-of-merit (ZT) of SiGe alloys. Among them, the nano approach has been recognized as an effective mechanism to obtain thermoelectric materials with good performance. In this approach, dense bulk samples with random nanostructures with high interface densities are synthesized through ball milling and a direct current hot press, leading to an enhancement ZT through reduced phonon thermal conductivity. Such a practical technique produced samples of nanostructured p-type dense bulk bismuth antimony telluride with a peak ZT of 1.4 at 1000 °C from either alloy ingot or elemental chunks. However, the generality of this approach has not been demonstrated. Here, we applied the same technique in SiGe system in order to fabricate a nanostructured n-type SiGe alloy with enhanced thermoelectric properties. In this thesis, numerous nanostructured n-type SiGe alloy samples were successfully pressed. The structure of these nanostructured samples was investigated via XRD, EDS, and TEM. It has been confirmed that many nano grains exist in our nanostructured samples.