Is SrZn₂Sb₂ a Realistic Candidate for High-Temperature Thermoelectric Applications?

Abstract

The structure and thermal stability of thermoelectric (TE) SrZn2Sb2 have been probed by means of both in situ and ex situ powder X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetry, and inductively coupled plasma optical emission spectrometry. Densification of the sample during synthesis induced evolution of an increasing degree of impurity phases coming from the decomposition of SrZn2Sb2. Variable-temperature synchrotron powder X-ray diffraction experiments performed on an air-packed sample revealed a structural breakdown occurring between 500 and 600 K. DSC performed in an argon flow revealed the decomposition to be an exothermic and kinetically slow transition accompanied by a weight loss of zinc, showing the extent of decomposition to increase as a function of decreasing heating rate applied in the DSC experiments. Using the Kissinger and Ozawa analysis methods, activation energies of ≈1.9 eV were obtained. After five days at 850 K in vacuum, argon, and air atmospheres, cold-pressed samples showed a complete depletion of SrZn2Sb2, however, the zinc evaporation was strongly inhibited compared to the DSC experiments, which is likely to be because of the reduced surface area. These observations indicate that the structural decomposition is independent of both operating atmosphere and surface-to-volume ratio, making SrZn2Sb2 unsuitable for practical TE applications in intermediate- and high-temperature environments in spite of the promising TE properties reported previously.

Thermal Stability and Thermoelectric Properties of NaZnSb

“…For instance, SrZn 2 Sb 2 was previously reported to exhibit a maximum zT of 0.35 at 723 K [23]. This antimonide was further studied for its thermal stability in air, argon, and vacuum atmospheres, for both powdered and pelletized samples [8]. It should be noted that the studied samples contained only up to 90% of the target SrZn 2 Sb 2 phase.…”

“…The minor weight loss of ~0.5 wt % might be a result of Na/Zn subtle evaporation, although the mass loss is not coincident with a DSC peak. For comparison, in the case of SrZn 2 Sb 2 , the weight loss is considerable: ~15 wt % at 882 K with a very broad DSC peak at 882 K [8]. The PXRD pattern of the sample after the DSC-TG run corresponds to a parent NaZnSb compound with traces of an elemental Sb impurity, in line with negligible mass loss.…”

“…Beyond the intrinsic material performance, the evaluation of the thermal stability of these compounds is important for high-temperature applications. While Yb 14 MnSb 11 shows remarkable stability [6,7], recent studies of the SrZn 2 Sb 2 Zintl antimonide [8] have shown that it is not suitable for high-temperature applications due to structural instability at elevated temperatures.…”

Operando structural investigations of thermoelectric materials

“…During the past decade, increased focus on operational stability has resulted in an increasing number of stability studies, where powdered materials are exposed to elevated temperatures for a few thermal cycles (Pedersen & Iversen, 2008;Pedersen et al, 2007;Jørgensen et al, 2018;Leszczynski et al, 2011;Song et al, 2018). However, uniform heating of powders is not adequate to mimic the operational environment in a TE module for energy harvesting, which has a thermal gradient across a bulk sample and an accompanying potential gradient due to the Seebeck effect.…”

Operando X-ray scattering study of thermoelectric β-Zn4Sb3

“…Studies of the stability of TE materials typically use experimental conditions that do not resemble an operating module. This primarily involves powder X-ray diffraction (PXRD) studies at elevated temperatures, or thermal cycling, using various atmospheres (Jørgensen et al, 2018a(Jørgensen et al, , 2018bPedersen & Iversen, 2008;Pedersen et al, 2006Pedersen et al, , 2010Fischer et al, 2018;Reardon et al, 2017;. Some studies have exposed TE materials to (semi)realistic conditions but they have not simultaneously characterized the structural or TE properties (operando) (Reardon et al, 2017;Hung et al, 2017;Yin et al, 2014).…”