טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSheskin Ariel
SubjectDefects and Interfaces in Ag-Alloyed PbTe Compounds for
Thermoelectric Applications
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Yaron Amouyal


Abstract

Conversion of waste heat into electrical power is performed using thermoelectric (TE) generators, which are based on the Seebeck effect. PbTe is considered one of the best compounds for TEGs operating at mid-temperature ranges owing to its high figure of merit, ZT, which denotes the thermodynamic conversion efficiency and can be improved by increasing electrical conductivity and the Seebeck coefficient while reducing thermal conductivity. A common way to improve TE properties is alloying with selected elements above their solubility limits to initiate the formation of second-phase precipitates, which reduce lattice thermal conductivity due to phonon scattering. Our approach is to enhance the TE performance of PbTe by co-doping with Ag (3.3 at. %) and Bi (0, 0.2, or 0.4 at. %), where Ag-atoms serve as Ag2Te-precipitate forming agents and Bi atoms act as electron donors. We track the microstructure evolution of both alloys for different aging times at 380 °C and investigate its effects on TE transport coefficients. Materials synthesis includes vacuum melting and homogenization, followed by grinding and hot pressing. Thermal conductivity is measured using laser flash analysis (LFA), and electronic transport coefficients are measured employing a dedicated four-point probe system equipped with micro-heating segments. We find that the thermal conductivity of the Bi-free samples decreases upon aging for 6 h, but increases for aging times as long as 48 h. Interestingly, electrical conductivity values decrease monotonously upon aging times. The key parameters determining these transport coefficients are the Ag2Te precipitate number density (Nv) and the matrix concentration. To elucidate these findings, we employ basic and advanced characterization techniques including scanning electron microscopy (SEM) as well as complementary transmission electron microscopy (TEM) and atom probe tomography (APT). Application of complementary TEM and APT enables us to perform a quantitative evaluation of the Ag iso-concentration surfaces surrounding the Ag2Te precipitates, which are critical for determining Nv.

 APT analysis yields Nv-values as high as 1024 m-3 together with an accurate evaluation of the Ag concentration in the inter-precipitate space (< 1.0 at. %). We account for the aging time-dependent transport coefficients in terms of interplay between the degree of PbTe-matrix super-saturation and the spatial distribution of the Ag2Te-precipitates.

This unique combination of experimental tools enables us, for the first time, to track the microstructure evolution of PbTe-based compounds and understand the implications for TE transport properties.