Electronic Structure and Thermoelectric Properties of Pseudoquaternary $$\hbox{Mg}_{2}\hbox{Si}_{1-x-y}\hbox{Sn}_{x}\hbox{Ge}_{y}$$ Mg 2 Si 1 - x - y Sn x Ge y -Based Materials

Abstract

A theoretical study is presented on complex pseudoternary Bi-doped Mg 2 Si 1ÀxÀy Sn x Ge y materials, which have recently been revealed to reach high thermoelectric figures of merit (ZT) of $1.4. Morphological characterization by scanning electron microscopy and energy-dispersive x-ray spectroscopy indicated that the investigated samples were multiphase and that the alloy with nominal composition Mg 2 Si 0:55 Sn 0:4 Ge 0:05 contained three phases: Mg 2 Si 0:35 Sn 0:6 Ge 0:05 (Sn-rich phase), Mg 2 Si 0:65 Sn 0:3 Ge 0:05 (Si-rich phase), and Mg 2 Si 0:15 Sn 0:5 Ge 0:35 (Ge-rich phase). The electronic structure of all these phases was calculated in the framework of the fully charge self-consistent Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) to treat chemical disorder. Electron transport coefficients such as the electrical conductivity, thermopower, and the electronic part of the thermal conductivity were studied by combining the KKR-CPA technique with Boltzmann transport theory. The two-dimensional (2D) plots (as a function of electron carrier concentration and temperature), computed for the thermopower and power factor, well support the large thermoelectric efficiency detected experimentally. Finally, employing the experimental value of the lattice thermal conductivity as an adjustable parameter, it is shown that ZT % 1.4 can be reached for an optimized Bi content near T % 900 K in case of the nominal composition as well as the Sn-rich phase. The question of the effect of disorder on the convergence of the conduction bands and thus the electron transport properties is addressed through detailed examination of the Fermi surfaces.

Nanostructure and doping stimulated phase separation in high-ZT Mg2Si0.55Sn0.4Ge0.05 compounds

“…The aim of the present work is to make quantitative measurements of the structural constituents in various length scales of the compounds, in order to assess the effect of their microstructure in an attempt to help optimize specific length-scale effects towards further improving the figure of merit. The same topic was dealt with previously from a theoretical point of view by Kutorasinski et al [33,34].…”

Thermoelectric Properties ofMg2(Ge,Sn): Model and Optimization ofZTSun1 2016Phys. Rev. Applied452191View full textAdd to dashboardCite

“…On the other hand, the p type material is inferior to n type on the performance with maximum ZT value of 0.4 at 1000 K. Jiang and co-workers28 reported ZT of 0.38 at 650 K for Mg 2 Ge 0.4 Sn 0.6 samples, which agree well with our calculated result at corresponding carrier concentration if composition difference is considered. High ZT values up to 1.4 at 900 K was seen in both experiments and theory calculations of n-type Mg 2 (Si-Ge-Sn) pseudo-quaternary solid solutions54,55 . This results from very low thermal conductivity (∼ 2 W/mK) due to multi-phase alloying.…”

Thermoelectric performance of n-type Mg2Ge

“…Amongst the binary compounds, n -type Mg 2 Si 9, 10 and Mg 2 Sn 11 exhibit inherently higher thermoelectric performance than Mg 2 Ge 12 . Following the high z T of 1.1 reported for the pseudo-binary Mg 2 Si-Mg 2 Sn system 8 , originating from mass-difference phonon scattering and conduction band convergence 8 , considerable efforts have been devoted to fabricating and characterising complex ternary 2, 13, 14 and quaternary 2, 15, 16 compounds. Whereas, binary Mg 2 Ge has received very limited attention, with only two experimental reports on n -type Sb-doped samples 12, 17 and a single report on p -type Ag-doped Mg 2 Ge thin films 18 .…”