טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGrossfeld Tom
SubjectMicrostructure Evolution and Enhancement of the
Thermoelectric Conversion Efficiency of PbTe-
Based Compounds for Renewable Energy
Applications
DepartmentDepartment of Energy
Supervisor Professor Yaron Amouyal
Full Thesis textFull thesis text - English Version


Abstract

A promising way of generating electrical power from heat flux is utilizing the thermoelectric (TE) effect. Application of TE generators  in daily life strongly depends on their thermodynamic conversion efficiency, which is determined by the dimensionless TE figure-of-merit, zT. Good TE materials possess high zT-values, which are obtained by combination of high electrical conductivity and low thermal conductivity, at the same time with high Seebeck coefficient.

Herein, we investigate the microstructure evolution of Ag-alloyed PbTe compounds with or without 0.04 at. % Bi additions. We control the nucleation and temporal evolution of Ag2Te-precipitates in the PbTe-matrix, aiming to achieve homogeneous dispersion of precipitates with high number density values, hypothesizing that they act as phonon scattering centers, thereby reducing lattice thermal conductivity. We measure the temperature dependence of the Seebeck coefficient and electrical and thermal conductivities, and correlate them with the  microstructure.

We manage to reduce thermal conductivity of PbTe by controlled nucleation of Ag2Te-precipitates, and obtain a number density value as high as 2•1020 m-3 after  6 h aging at 380 °C. This yields zT value of ca. 1.4  at 450 °C, which is one on the largest values reported for n-type PbTe compounds. Subsequent aging leads to precipitate coarsening and deterioration of TE performance. Interestingly, we find that doping with Bi atoms, besides their role as electron donors, improve the alloys’ thermal stability by suppressing microstructure evolution; thereby maintaining high TE performance that is stable at elevated service temperatures. The latter has prime technological significance.