Boundary Engineering for the Thermoelectric Performance of Bulk Alloys Based on Bismuth Telluride

Impact of Crystallite Size and Phase Boundaries on Magnetic and Thermoelectric Properties of Cu-Added BiFeO3

In this study, bismuth ferrite (BFO) and copper-added BFO were synthesized using the coprecipitation method. The incorporation of copper into the BFO lattice led to a reduction in the phase percentage of BFO due to the early formation of CuBi2O4. X-ray diffraction analysis revealed a decrease in crystallite size up to 0.1 CBFO, followed by an increase. This reduction in crystallite size causes an imbalance between the spins of the sublattices, resulting in an antiferromagnetic core/ferromagnetic shell (AC/FS) structure. The uncompensated spins generated by the decreasing crystallite size weaken the ferromagnetic properties with the addition of Cu. Additionally, the reduction in crystallite size leads to decreased electrical conductivity due to carrier scattering, with the maximum conductivity observed in BFO attributed to its volatilization. The Seebeck coefficient enhancement in 0.1 and 0.15 CBFO indicates an energy filtering effect caused by barriers at the phase boundaries. The introduction of Cu into the BFO matrix also results in reduced lattice thermal conductivity due to active centers for phonon scattering created by Cu-induced defects. The lowest lattice thermal conductivity was observed in 0.1 CBFO, which is attributed to the significant reduction in crystallite size and the presence of phase boundaries enhancing phonon scattering. The highest thermoelectric figure of merit (zT) was achieved in thermally unstable BFO due to Bi3+ volatilization, which was mitigated by the formation of CuBi2O4 in CBFO.

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO₃ perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti₃ C₂ T_x MXene in a matrix of SrTi_0.85 Nb_0.15 O₃ (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38 mW is obtained, which is higher than the reported values for oxide TEG.

Tailoring the phase transition of silver selenide at the atomistic scale

Thermoelectric materials provide promising solutions for energy harvesting from the environment. Silver selenide (Ag₂Se) material attracts much attention due to its excellent thermoelectric properties under superionic phase transition. However, the optimal thermoelectric figure of merit occurs during the phase transition at high temperatures, making low-temperature devices unable to benefit from their best thermoelectric performance. Here, we tailored the phase transition process of Ag₂Se materials with various sizes, and probed the phase transition temperature by in situ transmission electron microscopy. By tuning the motion of the atoms near the surface using size-dependent surface energy, the phase transition-induced process is tailored towards low temperatures. This work paves the way for future phase transition engineering to enhance thermoelectric performance.

Microscopic origin of the extremely low thermal conductivity and outstanding thermoelectric performance of BiSbX3 (X = S, Se) revealed by first-principles study

In this paper, we performed comprehensive investigations of both the thermal and electrical transport properties of BiSbSe₃ and BiSbS₃ by using first-principles calculations and Boltzmann transport theory. Due to the repulsion between the lone-pair electrons of Sb and the p orbital of Se(S), both BiSbSe₃ and BiSbS₃ show strong anharmonicity with Grüneisen parameters of 1.90 and 1.79, respectively. As a result, these two materials possess extremely low lattice thermal conductivities. Meanwhile, both BiSbSe₃ and BiSbS₃ exhibit similar anisotropic thermal transport behaviors, which is due to the smaller phonon group velocities along the a axis. The predicted highest ZT values at 750 K are 2.9 for n-type BiSbSe₃ and 1.2 for p-type BiSbS₃. Our calculations provide insights into the origin of the extremely low thermal conductivity in BiSbSe₃ and BiSbS₃, which is meaningful for exploiting high performance thermoelectric materials with low thermal conductivity.

Hf-Doping Effect on the Thermoelectric Transport Properties of n-Type Cu0.01Bi2Te2.7Se0.3

Polycrystalline bulks of Hf-doped Cu0.01Bi2Te2.7Se0.3 are prepared via a conventional melt-solidification process and subsequent spark plasma sintering technology, and their thermoelectric performances are evaluated. To elucidate the effect of Hf-doping on the thermoelectric properties of n-type Cu0.01Bi2Te2.7Se0.3, electronic and thermal transport parameters are estimated from the measured data. An enlarged density-of-states effective mass (from ~0.92 m0 to ~1.24 m0) is obtained due to the band modification, and the power factor is improved by Hf-doping benefitting from the increase in carrier concentration while retaining carrier mobility. Additionally, lattice thermal conductivity is reduced due to the intensified point defect phonon scattering that originated from the mass difference between Bi and Hf. Resultantly, a peak thermoelectric figure of merit zT of 0.83 is obtained at 320 K for Cu0.01Bi1.925Hf0.075Te2.7Se0.3, which is a ~12% enhancement compared to that of the pristine Cu0.01Bi2Te2.7Se0.3.

Practical High-Performance (Bi,Sb)2Te3-Based Thermoelectric Nanocomposites Fabricated by Nanoparticle Mixing and Scrap Recycling

Bi₂Te₃-based compounds are the most mature and widely used thermoelectric materials. However, industrial device fabrication will inevitably produce a lot of Bi₂Te₃ scraps, which results in wastes of expensive material resources. In this work, we recycled p-type (Bi,Sb)₂Te₃ scraps and reprocessed them by making nanocomposites with nano-SiC. The thermoelectric performance was enhanced, and a high ZT value of 1.07 was achieved, which is a significant improvement compared with commercial p-type (Bi,Sb)₂Te₃ ingots. Also, the hardness showed a notable increase, which is beneficial for device fabrication. In addition, we adjusted the proportion of Bi/Te of the commercial p-type (Bi,Sb)₂Te₃ scraps, thereby improving the thermoelectric performance and obtaining a higher ZT value of 1.2.

Promising and Eco-Friendly Cu2 X-Based Thermoelectric Materials: Progress and Applications

Due to the nature of their liquid-like behavior and high dimensionless figure of merit, Cu₂ X (X = Te, Se, and S)-based thermoelectric materials have attracted extensive attention. The superionicity and Cu disorder at the high temperature can dramatically affect the electronic structure of Cu₂ X and in turn result in temperature-dependent carrier-transport properties. Here, the effective strategies in enhancing the thermoelectric performance of Cu₂ X-based thermoelectric materials are summarized, in which the proper optimization of carrier concentration and minimization of the lattice thermal conductivity are the main focus. Then, the stabilities, mechanical properties, and module assembly of Cu₂ X-based thermoelectric materials are investigated. Finally, the future directions for further improving the energy conversion efficiency of Cu₂ X-based thermoelectric materials are highlighted.

Phase Formation Behavior and Thermoelectric Transport Properties of P-Type YbxFe3CoSb12 Prepared by Melt Spinning and Spark Plasma Sintering

Formation of multiple phases is considered an effective approach for enhancing the performance of thermoelectric materials since it can reduce the thermal conductivity and improve the power factor. Herein, we report the in-situ generation of a submicron-scale (~500 nm) heterograin structure in p-type Yb-filled (Fe,Co)₄Sb₁₂ skutterudites during the melt spinning process. Mixed grains of Yb_xFe_3−yCo_1+ySb₁₂ and Yb_zFe_3+yCo_1−ySb₁₂ were formed in melt spun ribbons due to uneven distribution of cations. By the formation of interfaces between two different grains, the power factor was enhanced due to the formation of an energy barrier for carrier transport, and simultaneously the lattice thermal conductivity was reduced due to the intensified boundary phonon scattering. A high thermoelectric figure of merit zT of 0.66 was obtained at 700 K.

Powering the Hydrogen Economy from Waste Heat: A Review of Heat-to-Hydrogen Concepts

Ever-increasing energy demands and environmental concerns require new and clean energy supplies, many of which are intermittent and do not correlate with demand. To balance supply with demand, a universal energy vector should be employed such that intermittent renewable energy can be stored and transported and then used when needed. Hydrogen is the perfect universal energy vector and a possible solution that ensures environmental cleanliness, maximum utilization of renewable energy sources, and high efficiency, whereby the combustion of the fuel yields only water. One abundant and freely available energy source-both anthropogenic and natural-is heat. Heat can be obtained from industrial processes and is indeed often viewed as a waste product with a premium to remove but is notoriously difficult to capture, store, and transport. Capturing and storing low-grade heat therefore provides a significant opportunity and can be achieved by coupling thermoelectric generators and water electrolyzers. A thermoelectric generator is placed within a thermal energy gradient and produces a flow of current that is fed to the electrolysis unit with which it produces hydrogen and oxygen as the final products. The hydrogen can be stored for long periods and transported for "on-demand" use in fuel cells for electricity from hydrogen burners for a return to thermal energy. This Review summarizes the current state-of-the-art research into implementing thermoelectric generators and utilizing heat as a primary energy source to produce hydrogen, which could replace the need for extra electric power to run hydrogen production units. Furthermore, suitable requirements, modifications, and other related aspects associated with such a new and novel method of hydrogen generation are discussed. Hydrogen produced from otherwise-wasted energy sources can be considered to be green.

High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis, functional homogeneity, and thermal stability

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed.

Thermoelectric Performance of Sb2Te3-Based Alloys is Improved by Introducing PN Junctions

Interface engineering has been demonstrated to be an effective strategy for enhancing the thermoelectric (TE) performance of materials. However, a very typical interface in semiconductors, that is, the PN junction (PNJ), is scarcely adopted by the thermoelectrical community because of the coexistence of holes and electrons. Interestingly, our explorative results provide a definitively positive case that appropriate PNJs are able to enhance the TE performance of p-type Sb₂Te₃-based alloys. Specifically, owing to the formation of the charge-depletion layer and built-in electric field, the carrier concentration and transport can be optimized and thus the power factor is improved and the electronic thermal conductivity is decreased. Meanwhile, PNJs provide scattering centers for phonons, leading to a reduced lattice thermal conductivity. Consequently, the p-type (Bi₂Te₃)_0.15-(Sb₂Te₃)_0.85 composites comprising PNJs achieve a ∼131% improvement of the ZT value compared with the pure Sb₂Te₃. The increased ZT demonstrates the feasibility of improving the TE properties by introducing PNJs, which will open a new and effective avenue for designing TE alloys with high performance.

Enhanced Thermoelectric Performance in n-Type Bi2Te3-Based Alloys via Suppressing Intrinsic Excitation

Currently, the application of thermoelectric power generators based on Bi₂Te₃-based alloys for the recovery of low-quality waste heat is still limited because of the aggravated intrinsic excitation of the material at elevated temperatures. In this study, excessive Te and dopant I are introduced to the n-type Bi₂Te_2.4Se_0.6 material with the purpose of suppressing its intrinsic excitation and improving the thermoelectric performance at elevated temperatures. These Te and I atoms act as electron donors to effectively reduce the density of minority carriers (holes) and weaken their negative contribution to the Seebeck coefficient. Likewise, the initial band structure and the carrier scattering mechanism are scarcely altered. Similar to the p-type Bi₂Te₃-based alloys, we found the "conductivity-limiting" mechanism is also well obeyed in the present n-type Bi₂Te_2.4Se_0.6-based materials. The reduced minority carrier partial electrical conductivity in these Te-excessive and I-doped Bi₂Te_2.4Se_0.6 samples significantly decreases the bipolar thermal conductivity, leading to lowered total thermal conductivity at elevated temperatures. Finally, the peak zT is successfully shifted up to higher temperatures for these Te-excessive and I-doped Bi₂Te_2.4Se_0.6 samples. A maximum zT of 1.0 at 400 K and an average zT of 0.8 at 300-600 K have been realized in Te-excessive Bi₂Te_2.41Se_0.6.

High-performance shape-engineerable thermoelectric painting

Output power of thermoelectric generators depends on device engineering minimizing heat loss as well as inherent material properties. However, the device engineering has been largely neglected due to the limited flat or angular shape of devices. Considering that the surface of most heat sources where these planar devices are attached is curved, a considerable amount of heat loss is inevitable. To address this issue, here, we present the shape-engineerable thermoelectric painting, geometrically compatible to surfaces of any shape. We prepared Bi₂Te₃-based inorganic paints using the molecular Sb₂Te₃ chalcogenidometalate as a sintering aid for thermoelectric particles, with ZT values of 0.67 for n-type and 1.21 for p-type painted materials that compete the bulk values. Devices directly brush-painted onto curved surfaces produced the high output power of 4.0 mW cm^-2. This approach paves the way to designing materials and devices that can be easily transferred to other applications.