Strategic control of excess tellurium to achieve high figure-of-merit in Te-rich Bi0.5Sb1.5Te3

Materials for Renewable and Sustainable Energy https://doi.org/10.1007/s40243-024-00293-4 (2025) 14:19 ORIGINAL PAPER Strategic control of excess tellurium to achieve high figure-of-merit in Te-rich Bi0.5Sb1.5Te3 Ranu Bhatt1,4 · Rishikesh Kumar1,2 · Pramod Bhatt3,4 · Pankaj Patro4,5 · Shovit Bhattacharya1,4 · Mani Navaneethan6 · Soumen Samanta1 · Ajay Singh1,4 Received: 4 September 2024 / Accepted: 29 December 2024 © The Author(s) 2025 Abstract Increasing the Te content in stoichiometric Bi0.5Sb1.5Te3 facilitates effective control over the anti-site defects and nanostructure; however, arresting excess Te in the host matrix is challenging. Herein, we report the success of a saturationannealing treatment in a vacuum, followed by air-quenching as a promising approach for synthesizing high figure-of-merit (zT) Bi0.5Sb1.5Te3+xTe (x = 0, 2, 5 and 10 wt%) materials. A remarkably high-power factor (α2σ ~ 6 mW at 300 K) is achieved in p-type Bi0.5Sb1.5Te3 + 5 wt% Te composition due to high carrier concentration (n) and good carrier mobility (µ). Microstructural analysis revealed the formation of densely interconnected polycrystalline grains featuring fine grain boundaries, planar/point defects, and strain field domains, contributing towards wide-length scale phonon scattering. The cumulative effect of drastically reduced thermal conductivity (κ ~ 0.8 W/m-K at 300 K), and enhanced power factor resulted in a record zT value ~ 2.2 at 300 K in Bi0.5Sb1.5Te3 + 5 wt% Te, with an average zT value up to 1.35 in temperatures ranging from 303 to 573 K. The COMSOL simulations predict a maximum conversion efficiency (ηmax) of ~ 15%, at a temperature gradient (∆T) of 270 K, for a single-leg thermoelectric generator (TEG) developed using this material. Keywords Transport properties · Microstructure · Thermoelectric generators · COMSOL Introduction With the depletion of fossil fuels, the escalation of greenhouse gases, and the surge in energy demand, an absolute shift towards renewable energy resources is crucial. Ranu Bhatt 1 Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 2 Department of Mechanical Engineering, Indian Institute of Technology, Patna 801103, India 3 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 4 Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India 5 Powder Metallurgy Division, Bhabha Atomic Research Centre, Vashi Complex, Navi Mumbai 400703, India 6 Nanotechnology Research Centre (NRC), Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur 603203, India Currently, in the power generation landscape, approximately one-third of the fuel is lost as heat into the environment [1–3]. Encountering this waste heat for useful power generation has envisaged great attention towards exploring alternate technologies, including thermoelectricity [4, 5]. Solid-state thermoelectric devices possess the excellent capability to harvest the heat energy (from waste heat or radioisotope heat sources such as Am-241, Pu-238, Sr-80, etc.), and directly convert it into electrical energy using the Seebeck effect [4, 6–8]. Considering the broad temperature distribution of waste heat, a variety of thermoelectric materials have been investigated, and commercially established for device fabrication for different temperature ranges like SiGe (1073–1273 K), PbTe (773 K), Bi2Te3 (RT573 K), etc. The market demand for Bi-Te TEGs is comparatively high, especially considering the abundant availability of waste heat below ) 600 K. However, the ( √ 1+(zT ) −1 avg P T √ =∆ η η =Q of commercially TC TH . 1+(zT )avg + T H available Bi-Te TEGs is typically ~ 5–6% only [8–10], which limits their broader commercial use. The η of TEG 13 19 Page 2 of 14 is( mainly governed by the thermoelectric figure-of-merit ) α 2σ zT = κ of the material, where α, σ, and κ are Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. For decades, Bi2Te3 and its derivatives (n-type Bi2Te2.7Se3 and p-type Bi0.5Sb1.5Te3) with zT~ 1, have been widely used for designing thermoelectric devices. Moreover, other factors such as device configuration, topology, electrode material selection, inter-diffusion barrier layers, thermoelement geometry, filler insulation materials for heat management, etc. also play a crucial role in determining the η of the thermoelectric devices [11–14]. The (Bi, Sb)2Te3 solid solution can be tuned to n/p-type by modulating the antisite defects, with the most promising p-type composition identified as Bi0.5Sb1.5Te3, exhibiting zT ~ 1.1 near 300 K [15–17]. Various strategies have been explored to further improve the zT of this composition, including defect engineering, band structure engineering, charge carrier optimisation, energy filtering, microstructural engineering, etc [18–23]. Among these approaches, microstructural engineering allows the selective manipulation of phonon scattering with different mean-free paths, without compromising the electrical conductivity of the material. For instance, the formation of coherent grain boundaries, combined with hierarchical length-scale features, represents a novel approach to substantially reduce the lattice contribution to κ without compromising the α2σ value [24–26]. The best η demonstrated at the lab scale using compatible high-zT n/p-type Bi-Te based material is ~ 8%, at ΔT of 240 K [19, 27]. In recent developments, unconventional nanocomposite approaches employing Te-excess composition have emerged as an effective means to achieve high zT Bi0.5Sb1.5Te3. For instance, Kim et al. reported a high zT (1.86 at 320 K) in p-type Bi0.5Sb1.5Te3 using 25 wt% excess Te as a sacrificial additive, during liquid-phase compaction. This significant enhancement in zT value is credited to the reduced κ value (0.35 W/m-K) resulting from dense dislocation arrays embedded in the grain boundaries of the sample [28]. However, replicating these results has proven challenging, with mixed outcomes reported. The addition of extra Te to Bi-Te may lead to adverse effects in the material, such as porosity, alterations in microstructure, hindered grain growth, lubrication of grain boundaries, preferred orientation growth, etc [28–31]. Crystallographically textured nanomaterials synthesised using a solution re-precipitation method, resulted in an improved zT value of 1.96 at 420 K [32]. Ettenberg et al. reported a zT value of 1.14 in p-type (Bi0.25Sb0.75)2Te3 alloy doped with 3 wt% excess Te in melt-grown samples [33]. In the majority of melt-growth (MG) cases, non-uniformity in composition along the growth direction of the 13 Materials for Renewable and Sustainable Energy (2025) 14:19 chunk is reported which leads to variations in the transport properties along the growth axis. Different approaches have been explored to stabilise the sublimation of Te from the matrix to achieve better mechanical strength and stable ther (...truncated)