New and innovative approaches for enhancing the repeatability and thermoelectric performance of Ag2X (X = S, Se, and Te) and SnTe

Abstract

Chalcogenide thermoelectric materials have gained significant attention in recent years due to their potential for high-performance thermoelectric energy conversion applications. While enhancing their thermoelectric performance has remained a core area of research, significant current interest has been shifting to address other important issues, including improving the reproducibility and reducing the toxicity of these materials. Chalcogenides offer numerous advantages over other classes of thermoelectric materials. These advantages include the fabrication of both n-type and p-type thermoelectric legs, often from the same parent material, through doping, low thermal conductivity, and high thermoelectric performance across a wide temperature range. For instance, Bi-based alloys exhibit high thermoelectric performance at room temperature; PbTe-based TE materials often reach their peak TE performance in mid-temperature range; and, Cu-based superionic compounds show excellent performance at high temperatures. However, it is important to note that sample stability under applied current and temperature gradients over several thermal cycles is an essential requirement for practical applications. This stability is crucial for achieving reproducibility and ultimately ensuring the success of thermoelectric devices.By successfully developing and implementing an efficient and optimized synthesis method, researchers can achieve a breakthrough in resolving the stability concerns of chalcogenide-based TE devices. This advancement would not only enable enhanced reproducibility but also pave the way for achieving superior TE performance in practical applications. This thesis addresses the issue of irreproducibility in the electronic transport properties of superionic thermoelectric materials of the Ag2X family (X = S, Se, and Te), and the metavalent alloys derived from SnTe. Ag2Te has long been recognized as a superionic compound with a decent figure of merit (zT) spanning from room temperature to the mid-temperature range. However, the sample-dependent transport properties of Ag2Te have hindered its potential as a thermoelectric (TE) material for practical applications. In Chapter 3 of this thesis, we investigate this issue in great details. We show that the primary reason for the observed irreproducibility is Ag metal ion migration during furnace/hot press or spark-plasma sintering at high temperatures. Above superionic transition, the Ag ions tend to migrate if the temperature or voltage gradient the material is subject to exceeds a temperature dependent threshold value. We then propose a novel synthesis method that operates at room temperature, resulting in the fabrication of highly dense and reproducible samples. This newly method also offers the advantage of controlling the formation hierarchy in particle size. As a result, the thermoelectric performance of the n-type Ag2Te is significantly enhanced by reducing the lattice thermal conductivity below its superionic limit. Thus, a high zT of 1.2 in n-type Ag2Te and close to 0.7 in p-type Ag2Te is obtained at 570 K. Through our research, we have also show that 570 K is a safe temperature limit for Ag2Te, below which reproducible properties can be obtained as the Ag metal ion migration under the applied voltage and temperature gradient remains local. Chapter 4 explores the thermoelectric properties of Ag2Se, which suffers large variations in transport properties for similar composition samples due to sample inhomogeneity and meta-stable phase formation. Such irreproducile behavior is reported in high temperature processed samples. In this chapter, our all-room-temperature synthesis approach, described in detail in chapter 3, is utilized to achieve high thermoelectric performance with excellent reproducibility and homogeneity. The role of Se excess in enhancing the zT is studied, revealing the suppression of Ag interstitial defects and reduction in carrier concentration, leading to improved thermoelectric properties compared to pristine Ag2Se. A high and reproducible zT value of 0.9 at 370 K is obtained without any extrinsic doping. In chapter 5, we employed our all room temperature synthesis technique to fabricate high density, nanostructured pellets of Ag2S. The thermal conductivity of our Ag2S sample is significantly suppressed compared to the ingot sample due to grain boundary scattering. By synthesizing anion excess samples, the thermal conductivity is further decreased, and a significantly improved zT value of around 0.9 at 670 K is obtained. The underlying reason of such reduction in thermal conductivity due to anion excess is studied via detailed differential scanning calorimetery (DSC) and temperature dependent electrical resistivity. In Chapter 6, the thermoelectric properties of SnTe are investigated, considering its low Seebeck coefficient and high thermal conductivity caused by Sn vacancies. A novel furnacesintering method is employed to fabricate nanostructured SnTe samples with significantly reduced thermal conductivity. We show that during the furnace sintering of a cold-pressed pellet, the nanoparticles of SnTe precipitates at the grain boundaries, leading to a nanostructured pellet which further enhances the thermoelectric properties. Band engineering with Ag doping is also explored, resulting in improved power factor and thermoelectric performance due to both valence band convergence and an increase in the band gap. Overall, this thesis successfully resolves the issue of irreproducibility in the electronic transport properties of Ag2Te and Ag2Se. High TE figure of merit is obtained in Ag2Te, Ag2Se, Ag2S superionics and SnTe metavalent compounds through innovative synthesis methods, including grain-size reduction, and hence nanostructuring, and band engineering techniques. The reproducibility of the observed properties is solidly established. The findings contribute to the understanding and development of thermoelectric materials with enhanced reproducibility and improved performance.