High-Performance and Stable (Ag, Cd)-Containing ZnSb Thermoelectric Compounds

Binary Zn-Sb-based compounds, ZnSb and Zn4Sb3, are promising thermoelectric (TE) materials because they are low-cost and earth-abundant. However, for a long time, their real applications have been limited by the low TE figure-of-merit (zT) of ZnSb and poor thermodynamic stability of Zn4Sb3. In this study, we successfully integrate both high zT and good stability in (Ag, Cd)-containing ZnSb compounds. Alloying Cd in ZnSb greatly suppresses the lattice thermal conductivity to a minimum value of 0.97 W K-1 m-1 at 300 K, while doping Ag significantly enhances the power factor to a peak value of 17.7 μW cm-1 K-2 at 500 K and reduces the bipolar thermal conductivity. As a result of the simultaneously optimized electrical and thermal transport, a peak zT of 1.2 is achieved for Zn0.698Ag0.002Cd0.3Sb at 600 K, which is comparable with the best values reported for Zn4Sb3-based compounds. Moreover, a current stress test confirms that introducing Ag and Cd does not degrade the good stability of ZnSb under an electric field. The phase composition and thermoelectric performance of Zn0.698Ag0.002Cd0.3Sb are not changed even under a high current density of 50 A cm-2, showing much better stability than Zn4Sb3. This study would accelerate the real application of ZnSb-based compounds in the field of waste heat harvesting.

In Situ TEM Studies of Nanostructured Thermoelectric Materials: An Application to Mg-Doped Zn4 Sb3 Alloy

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 Zn₄ Sb₃ 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 Zn₄ Sb₃ phase. Elemental mapping by STEM-EDX spectroscopy reveals enrichment of Zn in the secondary Zn₆ Sb₅ 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.

Fast direct synthesis and compaction of homogenous phase-pure thermoelectric Zn4Sb3

Zn4Sb3 is among the cheapest high performance thermoelectric materials, and it is made of relatively nontoxic elements. Strong activities are aimed at developing commercial power generation modules based on Zn4Sb3 making it vital to develop fast reliable synthesis processes for high-quality material. Here direct synthesis and compaction of homogeneous phase-pure thermoelectric Zn4Sb3 by spark plasma sintering (SPS) has been developed. Compared with the traditional quench and press method, the complexity and process time of the new method is very significantly reduced (order of magnitude), making large-scale production feasible. A composition gradient is observed in the pellet along the axis of applied pressure and current. The homogeneity of the pressed pellets is studied as a function of the SPS parameters: sintering time, applied current, sintering temperature and applied pressure, and the mechanism behind the formation of the gradient is discussed. The key finding is that pure and homogeneous Zn4Sb3 pellets can be produced by adding an extra layer of elemental Zn foil to compensate the Zn migration.

Recent advances in thermoelectric materials and solar thermoelectric generators – a critical review

Thermoelectric materials have been extensively used in space satellites, automobiles, and, more recently, in solar thermal application as power generators. Solar thermoelectric generators (STEGs) have enjoyed rapidly improving efficiency in recent years in both concentrated and non-concentrated systems. However, there is still a critical need for further research and development of their materials and systems design before this technology can deployed for large-scale power generation.

Synthesis and thermoelectric properties of Re3As6.6In0.4 with Ir3Ge7 crystal structure

The Re3As7- x In x solid solution was prepared for x ≤ 0.5 by heating the elements in stoichiometric ratios in evacuated silica tubes at 1073 K. It crystallizes with the Ir3Ge7 crystal structure, space group Im-3m, with a unit-cell parameter a ranging from 8.716 to 8.747 Å. The crystal structure and properties were investigated for a composition with x = 0.4. It is shown that indium substitutes arsenic exclusively at one crystallographic site, such that the As-As dumbbells with d As-As = 2.54 Å remain intact. Re3As6.6In0.4 behaves as a bad metal or heavily doped semiconductor, with electrons being the dominant charge carriers. It possesses high values of Seebeck coefficient and low thermal conductivity, but relatively low electrical conductivity, which leads to rather low values of the thermoelectric figure of merit.

Data analysis for Seebeck coefficient measurements

The Seebeck coefficient is one of the key quantities of thermoelectric materials and routinely measured in various laboratories. There are, however, several ways to calculate the Seebeck coefficient from the raw measurement data. We compare these different ways to extract the Seebeck coefficient, evaluate the accuracy of the results, and show methods to increase this accuracy. We furthermore point out experimental and data analysis parameters that can be used to evaluate the trustworthiness of the obtained result. The shown analysis can be used to find and minimize errors in the Seebeck coefficient measurement and therefore increase the reliability of the measured material properties.

Element-selective charge density visualization of endohedral metallofullerenes using synchrotron X-ray multi-wavelength anomalous powder diffraction data

An algorithm for determining the element-selective charge density has been developed using the maximum entropy method (MEM), Rietveld analysis and synchrotron X-ray multi-wavelength anomalous powder diffraction data. This article describes in detail both experimental and analytical aspects of the developed method. A structural study of yttrium mono-metallofullerene, Y@C82, 1:1 co-crystallized with toluene using the present technique is reported in order to demonstrate the applicability of the method even when only medium resolution data are available (d> 1.32 Å). Element-selective MEM charge density maps, computed from synchrotron X-ray powder diffraction data collected at three distinct wavelengths around the yttriumK-absorption edge (∼0.727 A), are employed for determining three crystallographic sites of the disordered yttrium.

Low-cost high-performance zinc antimonide thin films for thermoelectric applications

Zinc antimonide thin films with high thermoelectric performance are produced by a simple sputtering method. The phase-pure Zn(4)Sb(3) and ZnSb thin films fulfill the key requirements for commercial TE power generation: cheap elements, cheap fabrication method, high performance and thermal stability. In addition, two completely new meta-stable crystalline phases of zinc antimonide have been discovered.