Thermoelectric Materials Design via Microstructure and Composition Manipulations:
Experimental and Computational Approaches
Thermoelectric (TE) materials are involved in a variety of devices converting waste heat into electrical energy, as well as for solid-state refrigeration and are, therefore, of utmost technological importance. The energy conversion efficiency is determined by the dimensionless TE figure of merit, ZT. Attempts to optimize ZT require reducing the lattice thermal conductivity, while maintaining relatively high values of electrical conductivity. The PbTe- and AgSbTe2- compounds are two major derivatives of the promising lead-antimony-silver-tellurium class of materials, namely LAST, exhibiting good TE performance at the mid-temperature range. Introducing second-phase precipitates as well as lattice point defects to these base alloys is expected to reduce their lattice thermal conductivity, thereby increasing ZT. We, accordingly, focus our study on these two avenues.
We produce two phase systems comprising PbTe-based matrices that incorporate Ag2Te-precipitates and control their nucleation rate. Supplemented by theoretical calculations of phonon scattering, we predict that precipitate number density larger than ca. 1019 m-3 is demanded for reducing lattice thermal conductivity. This motivates investigation of the Ag2Te-precipitate temporal evolution, seeking for ways to reduce their average size down to the sub-micron length scale as well as increasing their number density. Optimized aging heat treatment conditions yield a maximum number density value of ca. 1020 m-3 due to aging at 380 °C.
Our attempts to finding the best substitution elements that act as phonon scattering centers include lattice dynamics density functional theory (DFT) calculations for the PbTe and AgSbTe2 compounds. For the AgSbTe2-phase, we predict that the formation of substitution defects at the Ag-sublattice sites should impede lattice vibrations, thereby reducing the lattice thermal conductivity. Calculations of the lattice vibrational modes of the La0.125Ag0.875SbTe2 compound indicate, indeed, reduction of the average sound velocity from 1684 to 1563 m•s-1 due to La-doping, which is manifested by ca. 14 % reduction in thermal conductivity. To validate the computational results, we produce two Ag-Sb-Te-based alloys: a ternary (without La additions), and a quaternary one alloyed by La below solubility limit. We measure the thermal conductivity of both alloys using the laser flash analysis (LFA) method, and observe reduction in thermal conductivity as a result of La-alloying from a value of 0.92 to 0.71 W•m-1•K-1 at 573 K due to La-alloying, as calculated from first-principles. Further attempts of second-phase nucleation in the AgSbTe2-based matrix are implemented, and its effecs on thermal conductivity are studied and compared to those of solute elements.
This project is supported by the Israel Science Foundation (ISF), the Nancy & Stephen Grand Technion Energy Program (GTEP), as well as the Russell-Berrie Nanotechnology Institute (RBNI).
