2D Janus AlSXY (X, Y = Cl, Br, I; X ≠ Y) monolayers with significant piezoelectric and high-temperature thermoelectric properties

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Carbon nanotubes (CNTs) have attracted much attention in developing high-performance, low-cost, flexible thermoelectric (TE) materials because of their great electrical and mechanical properties. Theory predicts that one-dimensional semiconductors have natural advantages in TE fields. During the past few decades, remarkable progress has been achieved in both theory and experiments. What is more important is that CNTs have shown desirable features for either n-type or p-type TE properties through specific strategies. Up to now, CNT‒polymer hybrids have held the record for TE performance in organic materials, which means they can potentially be used in high-performance TE applications and flexible electronic devices. In this review, we intend to focus on the intrinsic TE properties of both n-type and p-type CNTs and effective TE enhanced strategies. Furthermore, the current trends for developing CNT-based and CNT‒polymer-based high TE performance organic materials are discussed, followed by an overview of the relevant electronic structure‒TE property relationship. Finally, models for evaluating the TE properties are provided and a few representative samples of CNT‒polymer composites with high TE performance are highlighted.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Density functional theory and Boltzmann transport equations are used to investigate electronic band structure and thermoelectric (TE) properties of different two-dimensional (2D) materials containing Mo, S, Nb, Se, and Te. In MoS2-based monolayers (MLs) the substitution of S atoms by Te atoms up to the concentration of 12.5 at% leads to a more significant change of the band structure than in the corresponding case with Se atoms. In particular, the bandgap is reduced. At a high concentration of Se or Te the electronic structure becomes more similar to that of the SeMoS or TeMoS Janus layers, and the MoSe2 or MoTe2 MLs. It is found that local and random introduction of substitutional Se or Te atoms yields not very different results. The substitution of Mo by Nb, at the concentration of 2.1 at% leads to hole levels. The thermoelectric properties of the considered 2D materials are quantified by the Seebeck coefficient and thermoelectric figure of merit. The two characteristics are determined for different levels of p- or n-doping of the MLs and for different temperatures. Compared to the pristine MoS2 ML, Te substitutional atoms cause more changes of the thermoelectric properties than Se atoms. However, MLs with Se substitutional atoms show a high thermoelectric figure of merit in a broader range of possible p- or n-doping levels. In most cases, the maximum thermoelectric figure of merit is about one, both in p- and n-type materials, and for temperatures between 300 and 1200 K. This is not only found for MoS2-based MLs with substitutional atoms but also for the Janus layers and for MoSe2 or MoTe2 MLs. Interestingly, for MLs with one Nb as well as two or four Te substitutional atoms the highest values of the TE figure of merit of 1.2 and 1.40, respectively, are obtained at a temperature of 1200 K.

This work aims to study the effects of Co-doping in Cu-site of PrBa2(Cu1-xCox)3O7-y (PrBCO) ceramics (where x = 0.05, 0.10, 0.15, 0.17 and 0.20) on thermoelectric (TE) properties at high temperature. The result of solid-state reaction synthesis of Co-doped PrBCO compounds was optimally obtained at 900 °C for 16 h. Then, the powder was compacted into pellets by uniaxial press and sintered at 920 °C for 16 h and the crystalline structure was confirmed using X-ray diffractometry (XRD). The XRD data underwent Rietveld refinement employing the GSAS II program for a detailed analysis of phase composition and crystal structure. Microstructural characteristics and chemical composition were investigated through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The high-temperature thermoelectric properties, particularly electrical conductivity (), Seebeck coefficient () and thermal conductivity (ĸ) were also measured, and the figure of merit (ZT) was calculated to determine the thermoelectric performance of the compounds. The measured bulk density, determined through air and xylene displacement exceeded 90% of theoretical value. It was observed that Co-doping in the Cu-site of PrBCO led to an increased value of the Seebeck coefficient (), resulting in the ZT value tending to increase, particularly as the temperature rised. However, the electrical conductivity (σ) decreased, which was inversely related to  due to the reduction in carrier concentration. The power factor (PF) showed relatively high values over a broad temperature range for undoped and x=0.05 Co-doped samples. Furthermore, considering the thermal conductivity (κ), it was found that κ decreased after Co was added to PrBCO, due to the lattice structure being disturbed and causing increased phonon scattering. The sample with x = 0.05 exhibited the highest ZT value of 0.028 at 873 K, while the sample with x = 0.17 had the highest ZT value of 0.021 at 673 K.

Here we show the high-temperature thermoelectric (TE) properties of Cu2Ga4Te7 with the defect zinc-blende structure in which one-seventh of the cation sites are structural vacancies. Cu2Ga4Te7 exhibited relatively low electrical resistivity (ρ) and thermal conductivity (κ) and moderate positive Seebeck coefficient (S) at high temperatures, making this compound a promising high-performance p-type TE material. At 940 K, the S, ρ, and κ were +215 μV K−1, 10.1×10−5 Ω m, and 0.67 Wm−1 K−1, respectively, which resulted in the maximum dimensionless figure of merit ZT (=S2T/ρ/κ, where T is the absolute temperature) of 0.64.

Two-dimensional Al2I2Se2: A promising anisotropic thermoelectric material

Growth and thermoelectric characterization of chalcopyrite ZnSnSb2 with pseudocubic structure

The electronic structure and thermoelectric (TE) properties of Mg2GexSn1−x (x = 0.25, 0.50, 0.75) solid solutions are investigated by first-principles calculations and semi-classical Boltzmann theory. The special quasi-random structure (SQS) is used to model the solid solutions, which can produce reasonable band gaps with respect to experimental results. The n-type solid solutions have an excellent thermoelectric performance with maximum zT values exceeding 2.0, where the combination of low lattice thermal conductivity and high power factor (PF) plays an important role. These values are higher than those of pure Mg2Sn and Mg2Ge. The p-type solid solutions are inferior to the n-type ones, mainly due to the much lower PF. The maximum zT value of 0.62 is predicted for p-type Mg2Ge0.25Sn0.75 at 800 K. The results suggest that the n-type Mg2GexSn1−x solid solutions are promising mid-temperature TE materials.

Lead chalcogenides and their alloys belong at the heart of thermoelectrics due to their large thermoelectric figure of merit (zT). However, recent research has shown a limitation in the use of lead (Pb)-based materials due to their toxicity and efforts have been made to produce non-toxic analogues of lead chalcogenides. Tin chalcogenides have been predicted to be promising for this purpose due to their unique electronic structure and phonon dispersion properties. Here, we discuss the journey of tin chalcogenides in the field of thermoelectrics and topological materials with the main emphasis on the bonding, crystal structures, electronic band structures, phonon dispersion and thermoelectric properties. The thermal transport properties of tin chalcogenides are explained based on lattice dynamics, where resonant bonding and local structural distortion play an important role in creating lattice anharmonicity, thereby lowering the lattice thermal conductivity. Since thermoelectric and topological materials, especially topological insulators and topological crystalline insulators, share similar material features, such as a narrow band gap, heavy constituent elements and significant spin-orbit coupling, we have discussed the thermoelectric properties of several topological tin chalcogenides from a chemistry perspective. This feature article serves as a useful reference for researchers who strive to improve the properties of tin chalcogenides and advance the field of thermoelectric and topological materials.

A key factor improving the thermoelectric properties of Zintl compounds A5M2Pn6 (A = Ca, Sr, Ba; M = Ga, Al, In; Pn = As, Sb)

Three-dimensional tubular graphene/polyaniline composites as high-performance elastic thermoelectrics

We obtained a new material called monolayer 1T-Ag6S2 by replacing metal atoms in 1T phase transition-metal dichalcogenide sulfides (TMDs) with octahedral Ag6 clusters. Subsequently, the thermoelectric transport properties of monolayer 1T-Ag6S2 were systematically investigated using first-principles calculations and the generalized gradient approximation (GGA-PBE) exchange correlation functional. The findings demonstrate that monolayer 1T-Ag6S2 displays characteristics of a wide-bandgap semiconductor, with a bandgap of 2.48 eV. Notably, the incorporation of Ag6 clusters disrupts the structural symmetry, effectively enhancing the electronic structure and phonon properties of the material. Due to the flat valence band near the Fermi level, the extended relaxation time of the hole results in a greater effective mass compared to the electron, leading to a significant increase in the Seebeck coefficient. Under optimal doping conditions, the power factor of monolayer 1T-Ag6S2 can achieve 14.9 mW/mK2 at 500 K. The intricate crystal structure induces phonon path bending, reduces the overall frequency of phonon vibrations (<10 THz), and causes hybridization of low-frequency optical and acoustic branches, resulting in remarkably low lattice thermal conductivity (0.20 and 0.17 W/mK along the x and y axes at 500 K, respectively). The monolayer 1T-Ag6S2 demonstrates a remarkably high figure of merit ZT of 3.14 (3.15) on the x (y) axis at 500 K, significantly higher than those of conventional TMD materials. Such excellent thermoelectric properties suggest that monolayer 1T-Ag6S2 is a promising thermoelectric (TE) material. Our work reveals the deep mechanism of cluster substitution to optimize the thermoelectric properties of materials and provides a useful reference for subsequent research.

Aerogels are promising in the preparation of high‐performance thermoelectric (TE) materials due to their ultralow thermal conductivity. However, the TE performance of aerogels remains unsatisfactory. Herein, polyaniline/single‐walled carbon nanotubes (PANI/SWCNT) composite aerogel with high thermoelectric performance is synthesized through dynamic three‐phase interfacial electrochemical polymerization of aniline and subsequent physical mixing of PANI with SWCNTs followed by liquid nitrogen quenching and freeze‐drying. By adjusting the content of SWCNTs, the PANI/SWCNT aerogel (50 wt% SWCNTs) achieves a high power factor (PF) of 73.33 ± 2.03 µW m−1 K−2. The ultralow thermal conductivity of around 2.30 × 10−2 W m−1 K−1 for the composite aerogel is superior to that of most conventional organic TE materials, rendering an excellent figure of merit (ZT) value of ≈0.95 at room temperature and great potential in improving TE performance of organic and organic/inorganic composites. Thermal treatment of the composite aerogel further enhances the PF to 96.28 ± 1.89 µW m−1 K−2. This work provides an effective method in the formation of PANI/SWCNT TE composites and it puts forward insights into the preparation of other high‐performance TE composites.

Sulfide nanocrystals and their composites have shown great potential in the thermoelectric (TE) field due to their extremely low thermal conductivity. Recently a solid and hollow metastable Au2S nanocrystalline has been successfully synthesized. Herein, we study the TE properties of this bulk Au2S by first-principles calculations and semiclassical Boltzmann transport theory, which provides the basis for its further experimental studies. Our results indicate that the highly twofold degeneracy of the bands appears at the Γ point in the Brillouin zone, resulting in a high Seebeck coefficient. Besides, Au2S exhibits an ultra-low lattice thermal conductivity (∼ 0.88 W⋅m−1⋅K−1 at 700 K). At 700 K, the thermoelectric figure of merit of the optimal p-type doping is close to 1.76, which is higher than 0.8 of ZrSb at 700 K and 1.4 of PtTe at 750 K. Our work clearly demonstrates the advantages of Au2S as a TE material and would greatly inspire further experimental studies and verifications.