טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDahan Meir Haim
SubjectMass Transport in Ag-alloyed PbTe Compounds for
Thermoelectric Applications
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Yaron Amouyal
Full Thesis textFull thesis text - English Version


Abstract

Thermoelectric (TE) devices attract extensive interest due to their ability to convert heat into electrical energy.  Among all TE properties, the thermal conductivity (κ) determines the conversion efficiency of TE devices, and can be described as a sum of two components, namely the lattice and electronic contributions, defined as κp and κe, respectively. Formation of second phases, as well as other nanometer-size features, plays a significant role in determination of the TE transport properties due to chiefly reduction of κp by phonon scattering, as well as variation of the Seebeck coefficient, S, and the electrical conductivity, σ. PbTe-based compounds establish an important class of TE materials due to their high melting point (1190 K) and relatively high zT at the mid-temperature range (600-800 K). The Pb-Te-Ag system is of prime interest due to its potential of forming Ag-rich precipitates dispersed in the PbTe-matrix. Investigation of the microstructure evolution of this system enabled us to predict the applicability regime of this system in service temperatures, and this involves both thermodynamic and kinetic aspects.

Herein, we apply first-principles calculations to evaluate the diffusion coefficients of silver atoms in a lead-telluride matrix. We determine both the activation energy for diffusion and the pre-exponential diffusion coefficient for the interstitial mechanism using the transition state theory (TST) to be 1.08∙10-5 cm2⸱s-1 and 52.9 kJ⸱mole-1, respectively. These values are compared with experimental data, and indicate that the characteristic times required for significant microstructure evolution in these compounds can be as short as 15 min. under service temperature (380 °C).

We synthesize materials applying vacuum melting and homogenization, followed by milling. We preform homogenization at 700 °C and aging heat treatments at 380 °C for duration up to 100 h to control nucleation and growth of Ag-rich phases. We track the microstructure evolution of Ag-doped PbTe alloys for different aging times at 380 °C using x-ray diffraction (XRD), and investigate the microstructure evolution rate, showing significant changes taking place during ca. 1 h.

Some of the powders are hot-pressed to create bulk samples. In-situ heating experiments were performed to measure the thermal conductivity using laser flash analysis (LFA) technique, and electronic transport coefficients employing a dedicated four-point probe system equipped with micro heating segments. We found that in the first few hours of the in-situ experiments, there were rapid changes for all samples in both electronic properties (Seebeck coefficient and electrical conductivity) and thermal conductivity.

This study provides us with predictive information on the thermal stability of PbTe-based compounds, thereby evaluating the kinetics of phase transformations in these compounds, and is useful to further improve our ability to control nucleation and growth of Ag-rich precipitates in PbTe.