Ajay Singh

Thermoelectric generators (TEGs) enable direct conversion of waste heat into electrical energy. Although Bi2Te3-based alloys have long been the conventional thermoelectric materials, concerns over their toxicity and limited availability have encouraged the development of sustainable alternatives. This study presents a tellurium-free TEG module based on optimized n-type Mg3(Sb, Bi)2, a solid solution of Mg3Sb2 and Mg3Bi2, and p-type MgAgSb as promising alternatives to conventional Bi2Te3-based thermoelectric materials. Both materials were synthesized using scalable vacuum-melting methods and exhibited enhanced thermoelectric performance, which was attributed to improved weighted mobility and reduced lattice thermal conductivity. Low-resistance metallic contacts with a specific contact resistivity of approximately 15–20 μΩ·cm2 were successfully fabricated on both materials using a “monobloc” sintering method. An 11 p−n couple TEG module fabricated from these materials achieved a conversion efficiency of approximately 7.1% at a temperature difference of 250 K, in good agreement with simulation predictions. Moreover, at a temperature difference of 300 K and a hot end temperature of approximately 603 K, the TEG module achieved a conversion efficiency of approximately 8.5%, surpassing the performance of state-of-the-art Bi2Te3-based modules under comparable operating conditions.

In the present study, we have modified multi-walled carbon nanotube (MWCNT) fibers using a high-power visible laser beam (∼532 nm) with a pulse energy density of ∼0.48 mJ/cm2 under ambient conditions. The laser-irradiated fibers exhibited significant improvement in electrical conductivity with a minimal impact on the Seebeck coefficient. The optimally irradiated fibers (50 s exposure time) showed the highest power factor of ∼65 μW/mK2 near room temperature, representing an enhancement of nearly 56% compared to pristine fibers. Detailed characterization revealed that the rapid temperature rise of samples on laser exposure leads to defect annihilation, removal of amorphous carbon, and improved alignment of MWCNTs within the fiber. These modifications collectively contribute to the improvement in the thermoelectric performance and fracture strength of laser-irradiated samples. The present scalable approach of improving the thermoelectric and mechanical properties of MWCNT fibers through laser beam exposure holds great promise for wearable thermoelectric applications.

Defect engineering in thermoelectric materials is a key approach for controlling lattice vibrations. Introducing defects in the lattice brings disorder, which facilitates scattering of phonons and suppresses heat transport. Although this can be beneficial for enhancing the material’s thermoelectric performance, increased disorder may also reduce charge carrier mobility, thereby lowering the electrical conductivity and overall thermoelectric efficiency. Therefore, optimizing the efficiency of thermoelectric materials requires careful management of the delicate interplay between structural order and disorder. In our recent work, controlled disorder was achieved in Mg2(Si0.3Sn0.7)0.98Sb0.02 through Zn doping. This approach simultaneously enhanced carrier mobility (∼90 cm2/Vs) and weighted mobility (∼660 cm2/Vs) at 300 K while suppressing lattice thermal conductivity. In the present work, we investigate the nature of this disorder by employing a combination of experimental techniques including extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), positron annihilation lifetime spectroscopy (PALS), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). EXAFS analysis reveals that Zn occupies Mg-sites (ZnS) and non-Mg (ZnNS) sites in nearly equal proportions. XANES results confirm that ZnNS sites promote p-d orbital hybridization, resulting in broader electronic bandwidth and enhanced charge transport. XRD and TEM analyses confirm that the dislocation density (ND), predominantly associated with edge-type dislocations, increases monotonically with Zn doping, thereby enhancing phonon scattering. PALS measurements further support these findings by revealing the evolution of defects and the formation of vacancy clusters. Thus, controlled disorder provides an effective strategy for designing high performance thermoelectric materials.

Free-standing multiwalled carbon nanotubes (MWCNTs) sheet exhibits low thermoelectric power factor due to poor inter-junctions’ connectivity. The present work demonstrates a simultaneous improvement of two anti-correlated thermoelectric parameters-Seebeck coefficient and electrical conductivity of free-standing MWCNTs sheet through dual doping with PEDOT: PSS and SnTe. The sequential energy dependent scattering of charge carriers at MWCNT/PEDOT-SnTe interfaces leads to a significant improvement of Seebeck coefficient. The enhanced carrier concentration along with the facile transport of the high energy charge carriers through PEDOT-SnTe composites due to improved connectivity between MWCNTs renders improvement of electrical conductivity. Such a synergetic combination of improved Seebeck coefficient and electrical conductivity resulted in a 7-fold enhancement in power factor of MWCNT/PEDOT: PSS-SnTe composite films.

This research describes a portable power source that harvests energy from waste heat using a thermoelectric generator (TEG). The output voltage and power of TEG depends on the temperature gradient across it. This paper describes an experimental investigation to keep the TEG's cold end temperature low by using phase changing material (PCM) in the heatsink. Using this setup for a 102°C heat source results in a temperature gradient of 35°C, allowing the energy harvesting device to generate 502 mW of power with a voltage of 410 mV. The output voltage of the energy harvesting device is made suitable for any electronics load or for battery charging by dc-dc boost converter. The proposed oscillator driven self-startup MPPT (maximum power point tracking) controlled dc-dc converter operates over a wide input voltage and power range of 210 mV (47 mW) to 1.5 V (1.48 W). The circuit uses a unique technique for resampling of open circuit voltage which is determined by the change of duty cycle value from steady state. It eliminates the need for periodic sampling. The maximum efficiency of 76.5% at an input voltage of 300 mV and an input power of 242 mW demonstrates that the present work has the potential to become a future portable power bank.

We report nanostructured Te thin film exhibiting a high figure-of-merit (ZT) of ~ 0.34 at 493 K. The films exhibit temperature enhanced electrical conductivity ~ 43 Scm⁻¹, very high Seebeck coefficient of ~ 385 µV⁻¹K⁻¹ contributed by non-degenerate valence bands and low thermal conductivity ~ 0.9⁻¹ Wm⁻¹K⁻¹ through suppression of bipolar effect. The ZT of nanostructured films is ~ 142% higher than the annealed films having larger grain size. The high Seebeck coefficient (~ 405 µV⁻¹K⁻¹) of nanostructured Te films near room temperature prompted us to make a device, which was subjected to a very small temperature difference of ~ 4 K by touching with finger and produced an open circuit voltage of ~ 18 mV. The periodic generation of this open circuit voltage with fast response when repeatedly touched with finger, suggest the utility of these films in real time human touch applications and temperature sensing.

Tin selenide (SnSe) has emerged as a surprising new p-type thermoelectric material with exceptionally low thermal conductivity, leading to impressive thermoelectric performance. However, the presence of thermally conducting & electrically insulating SnO₂ at the grain boundaries of SnSe supresses its desirable thermoelectric properties. In this work we report temperature dependence of Seebeck coefficient for polycrystalline SnSe which exhibits an unusual reversible change in conduction from n- to p-type at ~ 565 K. The n-type conduction near room temperature is attributed to the Se deficiency as well as SnO₂ present at grain boundaries. At high temperature formation of Sn vacancies leads to p-type conduction. The Zn doping at Sn site (Sn_1−xZn_xSe) establishes the usual p-type conduction in the entire temperature range along with the lowering of thermal conductivity, resulting in a figure-of-merit of ~ 0.15 at 700 K, which is nearly 275% higher than that for pristine SnSe. Graphical abstract

The majority of the primary energy in any process is wasted as low-grade heat (< 373 K). Thermoelectric power generators are ideally suited for conversion of such low-grade waste heat into electricity. So far Bi₂Te₃-based alloys based thermoelectric generators are commercially ruling such energy conversion applications. From long term commercial sustainability point of view the scarcity of elemental Te in earth crest and the presence of heavy element Bi are issues of concern. Here we report environment friendly and Te free n-Ag₂Se and p-MgAgSb combination as an alternative to the Bi₂Te₃-based alloys. The mechanically strong and low specific contact resistivity electrical contacts based on Ni and Ag were directly bonded on Ag₂Se and MgAgSb respectively using deformation bonding method. For a temperature difference (∆T) of ~ 82 K, the uni-leg device displayed a conversion efficiency of ~ 1.9% that is comparable to the state-of-the-art Bi₂Te₃-based thermoelectric module for a similar ∆T. Finally, a thermoelectric power generator module comprising of 18 p-n legs was coupled with a DC-DC boost converter for demonstration an application by harvesting low grade waste heat. Graphical Abstract

Carbon nanotubes are promising candidates for thermoelectric power generation because of their one-dimensionality mediated high Seebeck coefficient, high electrical conductivity with added advantages of flexibility, light weight, and scalability. We report the temperature-dependent thermoelectric properties of single-walled carbon nanotube (SWCNTs) films. The SWCNTs films exhibit p-type metallic conduction with high Seebeck coefficient (∼69.5 μVK−1) and moderate electrical conductivity (∼76 Scm−1). The films exhibit low thermal conductivity (∼0.1 Wm−1 K−1) due to phonon scattering at the interjunction region. The synergetic combination of thermoelectric properties resulted in a high figure-of-merit of ∼0.11 at 305 K. A flexible thermoelectric generator based on SWCNTs films mounted on a curved hot surface exhibited an output of 17 mV and 54 μA under a small temperature gradient of 10 K. The present work provides possible avenues for developing wearable SWCNTs based thermoelectric power generation modules for harvesting body heat.