High temperature waste heat recovery has been gaining attention in recent years as it forms one of the largest sources of available energy. A rapid development of thermoelectric (TE) materials that can directly convert heat into electricity through the Seebeck effect, opens promising pathway for harvesting the thermal energy from the surroundings. In order to harvest the high-quality waste heat at elevated temperature, excellent thermal and mechanical stability of the TE materials is critical for a sustainable energy harvesting. In this respect, half-Heusler (hH) alloys are one of the promising high-temperature TE materials due to their high dimensionless thermoelectric figure of merit (zT) along with excellent mechanical and thermal stability. This dissertation demonstrates novel hH compositions and microstructures for the waste heat recovery systems. Focus in the thesis is on development of high performance hH TE materials with excellent in-air thermal stability at high temperatures (>700K). This will allow manufacturing of high efficiency and durable high temperature thermoelectric generators (TEGs).

In chapter 3 and 4, a comprehensive optimization of n-type MNiSn and p-type MCoSb (M = Hf, Zr, and Ti) compounds is investigated through systematic control of processing parameters during melting and sintering. The synthesis conditions were controlled to achieve the phase purity, desired microstructure and the enhanced charge-carrier transport. Optimized n-type and p-type compositions are found to exhibit zTmax ~ 1 at 773 K.

Chapter 5 describes breakthrough in decoupling of TE parameters in n-type half-Heusler (hH) alloys through multi-scale nanocomposite architecture with tungsten nanoinclusions. The tungsten nanoparticles not only assist electron injection, thereby improving electrical conductivity, but also enhance the Seebeck coefficient through energy filtering effect. The microstructure comprises of disordered phases with feature sizes at multiple length scales, which assists in effective scattering of heat-carrying phonons over diverse mean-free-path ranges. Cumulatively, these effects are shown to result in outstanding thermoelectric performance of zTmax ~ 1.4 at 773 K and zTavg ~ 0.93 between 300 and 973 K.

In order to deploy TE materials into a thermal energy conversion device, it is essential to understand the transformation behavior under thermal cycling at high temperatures. In-air thermal stability of the hH compositions is demonstrated in chapter 6. All the optimized compositions are found to be stable below 673 K in-air condition. The n-type MNiSn and p-type NbFeSb compounds were found to show good thermal stability even at higher temperatures (>773K), whereas MCoSb compounds did not exhibit similar level of stability.

Building upon the improved material performance and thermal stability, uni-coupled TE generators are demonstrated that exhibit high power density of 13.81 W⸱cm-2 and conversion efficiency of 10.9 % under a temperature difference of 674 K. The uni-couple TEG device shows stable performance for more than 150 hours at 873 K in air. These results are very promising for deployment of TE materials in waste heat recovery systems.