As demands for energy increase throughout the world, the desire to create energy efficient technologies has emerged. While the field thermoelectricity has been around for well over a century, it is becoming increasingly popular, especially for automotive applications, as new and more efficient materials are discovered. Thermoelectricity is a technology in which a temperature difference can be applied to create a potential difference for the application of waste heat recovery or a potential difference can be used to create a temperature difference for heating and cooling applications.
Materials used in thermoelectric devices are semiconductors with high Figure of Merit, zT. The dimensionless thermoelectric Figure of Merit is a function of Seebeck coefficient, S, electrical resistivity, ρ, and thermal conductivity, κ. Experimental testing is used to determine the properties of these materials for optimized zT.
This thesis covers a new class of thermoelectric semiconductors based on rocksalt I-V-VI2 compounds, which intrinsically possess a lattice thermal conductivity at the amorphous limit. It has been shown experimentally that AgSbTe2, when optimally doped, reaches a zT=1.2 at 410 K.3 Unfortunately, there is a metallurgical phase transition at 417 K (144 °C). The phase transition at 417 K was identified to be a potential problem in thermal cycling, and is expected with all heavy chalcogenides of group Ib elements. This issue has to be resolved before I-V-VI2 can be considered practical. After examination, two routes to avoid the phase transition were planned: (1) alloying AgSbTe2 with Na, and (2) using off-stoichiometry formulations Ag1-xSb1+yTe2.
Experimental results show that with increased sodium concentration, resistivity increases substantially, causing this method to be impractical for optimizing zT, however it did eliminate the phase transition at 417K. Using off-stoichiometric silver antimony telluride shows promising results with high S, on the order of 250-400 μV/K, and maintains the low thermal conductivity on the order of 0.6-0.7 W/mK.