|M.Sc Student||Joseph Elad|
|Subject||A First-Principles Study of the Role of Substitutional|
Elements in PbTe - Based Thermoelectric Compounds
for Renewable Energy Applications
|Department||Department of Energy||Supervisor||Professor Yaron Amouyal|
|Full Thesis text|
The main reason for the energy crisis and growing pollution today is the large amount of waste energy. One of the possibilities to harvest some of the waste energy is via thermoelectric (TE) devices. These devices can convert heat flux into electric current, and vice-versa. Better TE materials will allow us to increase the efficiency of these devices and exploit waste heat.
To optimize the power generation efficiency of a TE material, we require a high values of Seebeck coefficient, which is the ratio of voltage to the temperature difference across the material. Also, combination of high electrical conductivity with low thermal conductivity values is demanded to produce high electrical power, maintaining a relatively large temperature gradient across the TE device.
This study focuses on lead-telluride (PbTe), which is a promising TE compound employed for power generation at the mid-temperature range. Our goal is to optimize the chemical composition of PbTe-based compounds by introducing substitutional point defects, to improve both vibrational and electronic properties, thereby the resulting TE performance. For this purpose we perform ab-initio calculations, based on the density functional theory (DFT). Our approach combines calculations of both vibrational and electronic properties, as follows. We derive the lattice sound velocity and heat capacity from the elastic moduli, which are determined from total-energy calculations. Electronic transport properties are calculated applying the Boltzmann transport theory. In addition, we use DFT to examine thermodynamic interfacial properties of the two-phases system PbTe/Ag2Te, in an effort to understand the precipitation process and predict the influence of Bi additions.
We find the dependence of the TE properties on the doping concentration and chemical identity of doping elements, and demonstrate how we can use it to find the optimum doping concentration. As a general trend, we find that dopants belonging to the 4th-5th columns of the periodic table are the most effective in reducing lattice thermal conductivity, whereas dopants of the 2nd, 14th, and 16th columns are the most effective ones in increasing the TE power factor. Furthermore, we predict that Bi will prefer to segregate to the (100)||(100) and (100)||(110) PbTe/Ag2Te interfaces, reducing their interfacial free energies. These predictions allow us to compare our findings to experimental data.
We, finally, discuss how this methodology could be generalized for dopant selection in other materials as well, oriented toward improving TE performance.