Improving the Operational Stability of Thermoelectric Zn₄Sb₃ by Segmentation

Abstract

The mixed ionic–electronic conductor β-Zn4Sb3 is a cheap and high-performing thermoelectric material, but under operating conditions with a temperature gradient and a running current, the material decomposes as Zn readily migrates in the structure. Here, we report an improved stability of β-Zn4Sb3 by introducing ion-blocking interfaces of stainless steel to segment the sample, produced by a rapid one-step spark plasma sintering synthesis. The stability of the samples is tested under temperature gradients and electric currents, which reveals significantly improved stability of the segmented samples compared to unsegmented samples. The segmented samples are stable under a temperature gradient from 250 °C to room temperature with no external current, whereas the unsegmented sample decomposes into ZnSb and Zn under the same conditions. The thermoelectric figure of merit, zT, of the segmented sample is slightly reduced, mainly due to the increased thermal conductivity. In conclusion, a rapid one-step synthesis of segmented β-Zn4Sb3 is developed, which successfully improves the long-term operational stability by blocking the Zn ion migration.

Operando X-ray scattering study of segmented thermoelectric Zn4Sb3

“…Thus, the segmentation approach is expected to be more effective when the conditions are closer to steady state conditions, where the field generated from the Zn concentration gradient cancels the movement of additional Zn ions, consistent with the previously reported long term stability tests. 40 With the largest temperature gradient and highest current density, both the P-350-10 and S-350-10 samples break down in less than 10 min (Fig. S7 and S13c†).…”

“…Here, no thermal contact resistance across the stainless steel interfaces is observed, which matches well with the very low contact resistance of the stainless steel interfaces previously reported. 40 Having good thermal and electrical contact through the interfaces is essential for applications for the same reasons that they are important for contacts in module construction.…”

“…A great mechanical strength is obtained between the Zn 4 Sb 3 phase and the stainless steel interfaces. 40 The samples used for the ATOS setup are 10 mm long, and as the segments are introduced perpendicular to the pressing direction, samples above 10 mm in height are needed. The samples with a length of 10 mm are obtained by using tungsten carbide dies, as regular graphite dies cannot withstand the pressure required for producing these tall samples.…”

“…39 Numerous efforts have focused on improving their stability but they have so far been unsuccessful. However, recently it was shown that the stability of Zn 4 Sb 3 40 and Cu 2− δ S 31 can be improved by segmentation of the pellet with ion-blocking interfaces, which makes the materials able to withstand higher temperature gradients and current densities. By inserting ion-blocking interfaces, that still allow a flow of electrons, the voltage across each segment is reduced, and higher external potentials, from either a temperature gradient or an applied current, can be sustained before metal precipitation occurs.…”

“…34 We have recently shown that segmentation of Zn 4 Sb 3 with stainless steel as the ion-blocking interface effectively increases the stability of Zn 4 Sb 3 under temperature gradients with or without an applied current density, while maintaining its high thermoelectric properties. 40 In this study, we aim to get insight into the structural changes, kinetics, decomposition rates, and microstructure of both segmented and unsegmented Zn 4 Sb 3 under operating conditions, using X-ray scattering with the ATOS, together with ex situ scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The segmented samples of Zn 4 Sb 3 are shown to have signicantly reduced decomposition and Zn migration rates compared with unsegmented samples.…”