In Situ TEM Studies of Nanostructured Thermoelectric Materials: An Application to Mg‐Doped Zn₄Sb₃ Alloy

Abstract

We demonstrate an advanced approach using state of the art in situ transmission electron microscopy (TEM) to understand the interplay between nanostructures and thermoelectric (TE) properties of high-performance Mg-doped Zn Sb TE systems. By using the technique, microstructure and crystal evolutions of TE material have been dynamically captured as a function of temperature from 300 K to 573 K. On heating, we have clearly observed precipitation and growth of a Zn-rich secondary phase as nanoinclusions in the matrix of primary Zn Sb phase. Elemental mapping by STEM-EDX spectroscopy reveals enrichment of Zn in the secondary Zn Sb nanoinclusions during the thermal processing without decomposition. Such nanostructures strongly enhances phonon scattering, resulting in a decrease in the thermal conductivity leading to a zT value of 1.4 at 718 K.

Stability and Thermoelectric Properties of Zn4Sb3 with TiO2 Nanoparticle Inclusions

“…Additionally, both the resistivity and the Seebeck coefficient plateaus above 500 K, which indicates some complex band behavior and potentially also the excitation of carriers, as it is clearly not a typical transition into bipolar transport, where one would expect the Seebeck coefficient to decrease as the resistivity starts to plateau. This trend has been observed before for β-Zn 4 Sb 3 , ,− ,− , and it remains constant throughout repeated heating cycles …”

“…The flattening and, in some cases, subsequent slight decrease in the resistivity above 500 K is indicative of some complex band behavior and potentially also the excitation of charge carriers; however, the dominant effect is related to the complex band structure as carrier excitation would typically give rise to a peak in the Seebeck coefficient (see Figure ). This trend is commonly observed for β-Zn 4 Sb 3 , ,− ,− , and it has been shown to be repeatable for at least 30 subsequent heating cycles …”

“…The optimal performance of β-Zn 4 Sb 3 is in the temperature range 473–673 K, − and this temperature range covers a lot of waste heat generated, for example, in the transportation and industrial sectors. , The performance of thermoelectric materials depends on different physical properties, which are concisely summarized in the thermoelectric figure of merit zT = S 2 T /ρκ, where S is the Seebeck coefficient, ρ is the electrical resistivity, T is the absolute temperature, and κ is the thermal conductivity, where the latter can be separated into contributions from the charge carriers and from the lattice . β-Zn 4 Sb 3 has an excellent zT mainly due to its low thermal conductivity, which can be ascribed to the scattering of phonons by the interstitial Zn atoms in the structure, resulting in a small lattice contribution. − A standing challenge with deploying Zn 4 Sb 3 in applications is the decomposition into ZnSb, Sb, Zn, and at times, ZnO, when exposed to the expected temperatures and thermal gradients used in applications. ,, This has led to a range of studies exploring ways to understand and combat this decomposition. ,− One of these studies investigated the effect of including TiO 2 or ZnO nanocomposites in the β-Zn 4 Sb 3 matrix, and significant improvements were observed in the thermal stability of powders containing TiO 2 nanoparticles, where 98 wt % of β-Zn 4 Sb 3 was intact after heating to 625 K in air compared with ∼30 wt % for pure β-Zn 4 Sb 3 . , The choice of TiO 2 nanoparticles in the present study is based on these results and their easy, cheap, and scalable synthesis with well-controlled size distribution …”

Improving the Operational Stability of Thermoelectric Zn4Sb3 by Segmentation

“…35 This is lower than Cu + in Cu 2 Se at 13.5 kJ mol −1 , 36 and even approaches the value of 9.6 kJ mol −1 for the fast ion conductor of α-AgI. 37 Attempts to suppress the Zn migration to improve the stability of Zn 4 Sb 3 have been manifold and include doping with, e.g., Mg, 30,38,39 Cd, 27 and Ag, 40 nanostructuring, 39,41,42 inclusions of ZnO 43 and TiO 2 nanoparticles, 43,44 and formation of amorphous grain boundaries, 45 but unfortunately all attempts so far have led to only insufficient improvements.…”

“…Attempts to suppress the Zn migration to improve the stability of Zn 4 Sb 3 have been manifold and include doping with, e.g., Mg, ,, Cd, and Ag, nanostructuring, ,, inclusions of ZnO and TiO 2 nanoparticles, , and formation of amorphous grain boundaries, but unfortunately all attempts so far have led to only insufficient improvements.…”