Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1-xBixTe alloys

Highly porous thermoelectric composites with high figure of merit and low thermal conductivity from solution-synthesized porous Bi2Si2Te6 nanosheets

Layer-structured Bi2Si2Te6 has garnered significant attention in the field of thermoelectrics due to its exceptional thermoelectric properties and unique structural characteristics. Enhancing the transport properties of composites by manipulating the thermal and electrical properties of materials through the fabrication of porous nanostructured materials has emerged as a promising strategy. This paper presents a study on enhancing the thermoelectric (TE) properties of Bi2Si2Te6 nanosheets (BST NSs) through nanostructuring and the fabrication of porous BST NSs (p-BST). The process involves Li intercalation and exfoliation to obtain BST NSs, followed by the creation of p-BST composites by introducing nanosized pores onto the surface of the NSs using high-power sonification for various durations. The incorporation of the porous structure effectively increases phonon scattering, leading to a decrease in the lattice thermal conductivity (κl) of the composite. The p-BST(2) composite demonstrates significantly low κ and enhanced thermoelectric figure of merit (ZT) values (∼0.63 W m-1 K-1 and ∼0.083) at room temperature. These results highlight the efficacy of porous structure preparation as a promising strategy for enhancing the thermoelectric performance of chalcogenide-based composites, offering potential solutions to environmental degradation and energy shortages.

Fine Tuning of Defects Enables High Carrier Mobility and Enhanced Thermoelectric Performance of n-Type PbTe

High carrier mobility is critical to improving thermoelectric performance over a broad temperature range. However, traditional doping inevitably deteriorates carrier mobility. Herein, we develop a strategy for fine tuning of defects to improve carrier mobility. To begin, n-type PbTe is created by compensating for the intrinsic Pb vacancy in bare PbTe. Excess Pb²⁺ reduces vacancy scattering, resulting in a high carrier mobility of ∼3400 cm² V^-1 s^-1. Then, excess Ag is introduced to compensate for the remaining intrinsic Pb vacancies. We find that excess Ag exhibits a dynamic doping process with increasing temperatures, increasing both the carrier concentration and carrier mobility throughout a wide temperature range; specifically, an ultrahigh carrier mobility ∼7300 cm² V^-1 s^-1 is obtained for Pb_1.01Te + 0.002Ag at 300 K. Moreover, the dynamic doping-induced high carrier concentration suppresses the bipolar thermal conductivity at high temperatures. The final step is using iodine to optimize the carrier concentration to ∼10¹⁹ cm^-3. Ultimately, a maximum ZT value of ∼1.5 and a large average ZT _ave value of ∼1.0 at 300-773 K are obtained for Pb_1.01Te_0.998I_0.002 + 0.002Ag. These findings demonstrate that fine tuning of defects with <0.5% impurities can remarkably enhance carrier mobility and improve thermoelectric performance.

Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further.

Enhanced Power Factor and Figure of Merit of Cu2ZnSnSe4-Based Thermoelectric Composites by Ag Alloying

The quaternary chalcogenide composites Cu₂ZnSn_1-xAg_xSe₄ (0 ≤ x ≤ 0.075) have been successfully synthesized by high-temperature melting and annealing followed by hot-pressing. The phase structure of the bulk sample has been analyzed by powder X-ray diffraction and Rietveld refinement combined with Raman spectroscopy to confirm Cu₂ZnSnSe₄ as the main phase with ZnSe and Cu₅Zn₈ secondary phases. The thermoelectric properties of all specimens have been investigated in the temperature range of 300-700 K. The replacement of Sn by Ag significantly enhances the electrical transport properties by providing extra charge carriers. The tremendous reduction in electrical resistivity enhances the power factor, and a maximum power factor of 804 μW K^-2 m^-1 is achieved at 673 K for the specimen with 5% Ag content. Furthermore, increased point defects increase phonon scattering, resulting in reduced thermal conductivity. The combined effect of improved power factor and suppressed thermal conductivity provides a good boost to the dimensionless figure of merit. The maximum figure of merit of zT = 0.25 has been achieved at 673 K for Cu₂ZnSn_0.95Ag_0.05Se₄, which is 2.5 times the value of the parent sample.

HPHT Synthesis: Effects of the Synergy of Pressure Regulation and Atom Filling on the Microstructure and Thermoelectric Properties of Yb x Ba8-x Ga16Ge30

Type-I clathrate compounds Yb _x Ba_8-x Ga₁₆Ge₃₀ have been synthesized by the high-pressure and high-temperature (HPHT) method rapidly. The effects of the synergy of atom filling and pressure regulation on the microstructure and thermal and electrical properties have been investigated. With the content of Yb atom increasing, the carrier concentration is improved, the electrical resistivity and the absolute Seebeck coefficient are decreased, while the thermal conductivity is reduced significantly. A series of extremely low lattice thermal conductivities are achieved, attributed to the enhancement of multiscale phonon scattering for the "rattling" of the filled guest atoms, the heterogeneous distribution of nano- and microstructures, grain boundaries, abundant lattice distortions, lattice deformations, and dislocations. As a result, a maximum ZT of about 1.07 at 873 K has achieved for the Yb_0.5Ba_7.5Ga₁₆Ge₃₀ sample.