n-Bi2-xSbxTe3: A Promising Alternative to Mainstream Thermoelectric Material n-Bi2Te3-xSex near Room Temperature

Thermoelectric Materials and Devices for Advanced Biomedical Applications

Thermoelectrics (TEs), enabling the direct conversion between heat and electrical energy, have demonstrated extensive application potential in biomedical fields. Herein, the mechanism of the TE effect, recent developments in TE materials, and the biocompatibility assessment of TE materials are provided. In addition to the fundamentals of TEs, a timely and comprehensive review of the recent progress of advanced TE materials and their applications is presented, including wearable power generation, personal thermal management, and biosensing. In addition, the new-emerged medical applications of TE materials in wound healing, disease treatment, antimicrobial therapy, and anti-cancer therapy are thoroughly reviewed. Finally, the main challenges and future possibilities are outlined for TEs in biomedical fields, as well as their material selection criteria for specific application scenarios. Together, these advancements can provide innovative insights into the development of TEs for broader applications in biomedical fields.

Tailoring the Thermoelectric Properties of 3D-Printed n-Type Bi1.7Sb0.3Te3 with Incorporated Edge-Oxidized Graphene

Using three-dimensional (3D) printing technology to fabricate Bi2Te3-based thermoelectric (TE) generators opens a potential way to create shape-conformable devices capable of recovering waste heat from thermal energy sources with diverse surface morphologies. However, pores formed in 3D-printed Bi2Te3-based materials by the removal of the organic ink binder result in unsatisfactory performance compared to the bulk materials, which has limited the widespread application of the ink-based 3D printing process. Furthermore, managing the volatile Se element in the n-type materials poses significant technological challenges compared to the p-type counterparts, resulting in a scarcity of research on 3D printing of n-type Bi2Te3. Here, we synthesized edge-oxidized graphene (EOG)-incorporated Se-free n-type Bi1.7Sb0.3Te3 (BST) using a direct ink writing (DIW) process with a binder-free novel ink. The incorporated EOG provides connectivity between small BST grains separated by pores and induces a bimodal-like grain structure during the DIW and sintering process. The optimal EOG content of 0.1 wt % in 3D-printed n-type BST simultaneously achieved both carrier transport control and active phonon scattering, due to its unique microstructure. A maximum ZT of 0.71 was obtained in the 0.1 wt % EOG/BST materials at 448 K, comparable to commercial bulk n-type Bi2Te3-based materials. Further, a single-element device composed of the EOG-BST material exhibited a 2-fold improvement in performance compared to pure-BST. These results open a technological route for the application of 3D printing technology for ink-based TE materials.

Synergistic Tailoring of Electronic and Thermal Transports in Thermoelectric Se-Free n-Type (Bi,Sb)2Te3

Se-free n-type (Bi,Sb)2Te3 thermoelectric materials, outperforming traditional n-type Bi2(Te,Se)3, emerge as a compelling candidate for practical applications of recovering low-grade waste heat. A 100% improvement in the maximum ZT of n-type Bi1.7Sb0.3Te3 is demonstrated by using melt-spinning and excess Te-assisted transient liquid phase sintering (LPS). Te-rich sintering promotes the formation of intrinsic defects (TeBi), elevating the carrier concentration and enhancing the electrical conductivity. Melt-spinning with excess Te fine-tunes the electronic band, resulting in a high power-factor of 0.35 × 10-3 W·m-1 K-2 at 300 K. Rapid volume change during sintering induces the formation of dislocation networks, significantly suppressing the lattice thermal conductivity (0.4 W·m-1 K-1). The developed n-type legs achieve a high maximum ZT of 1.0 at 450 K resulting in a 70% improvement in the output power of the thermoelectric device (7.7 W at a temperature difference of 250 K). This work highlights the synergy between melt-spinning and transient LPS, advancing the tailored control of both electronic and thermal properties in thermoelectric technology.

Selective Charge Carrier Transport and Bipolar Conduction in an Inorganic/Organic Bulk-Phase Composite: Optimization for Low-Temperature Thermoelectric Performance

Abundant conducting polymers are promising organic substances for low-temperature thermoelectric applications due to their inherently low thermal conductivities. By introducing a conducting polymer filler (PEDOT:PSS─poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid)) into a representative inorganic thermoelectric matrix (Bi2Te3), a bulk-phase composite (i.e., inorganic matrix/organic filler) for low-temperature thermoelectric applications is proposed. This composite hosts an interfacial energy barrier between the inorganic and organic components, facilitating controlled carrier transport based on its energy level, known as the energy filtering effect, and thus the composite exhibits a highly improved Seebeck coefficient compared to pristine Bi2Te3. The composite also displays a completely different temperature dependence on the Seebeck coefficient from Bi2Te3 due to its distinct bipolar conduction tendency. By regulation of the energy filtering effect and bipolar conduction tendency, the composite undergoes noticeable variations in the Seebeck coefficient, resulting in a significantly enhanced power factor. Furthermore, the composite shows a substantially reduced thermal conductivity compared to Bi2Te3 because it has lower carrier/lattice thermal contributions, possibly attributed to its high carrier/phonon scattering probabilities. Owing to the superior power factor and reduced thermal conductivity, the composite exhibits markedly enhanced thermoelectric performance, achieving a maximum figure of merit of approximately 1.26 at 380 K and an average figure of merit of approximately 1.23 in the temperature range of 323-423 K. The performance of the composite is competitive with previously reported n-type Bi2Te3 binary or ternary analogues. Therefore, the composite is highly expected to be a promising n-type counterpart of p-type Bi2Te3-based alloys for various low-temperature thermoelectric applications.

The Synthesis and Thermoelectric Properties of the n-Type Solid Solution Bi2-xSbxTe3 (x < 1)

Commercial Peltier cooling devices and thermoelectric generators mostly use bismuth telluride-based materials, specifically its alloys with Sb2Te3 for the p-type legs and its alloys with Bi2Se3 for the n-type legs. If the p-type materials perform with zT well above the unity around room temperature, the n-type counterpart is lacking efficiency in this temperature range, and has the disadvantage of containing selenium. Indeed, despite the fact that selenium is not environmentally benign and that its handling requires precautions, the use of selenium does not facilitate the optimization of thermoelectric performance at or around room temperature, as the presence of selenium results in a larger band gap. In this study, we investigate the feasibility of a selenium-free n-type (Bi, Sb)2Te3 using a simple two-step process: mechanical alloying synthesis followed by spark plasma sintering. All the members of the solid solution Bi2-xSbxTe3 with x < 1 are n-type materials, with zTs between 0.35 and 0.6. The zT is maximized at lower temperatures with an increasing Sb content, which is proof that the band gap is reduced accordingly. We also show here that an edge-free sintering process considerably improves thermoelectric performance.

High Pressure Drives Microstructure Modification and zT Enhancement in Bismuth Telluride-Based Alloys

Manipulating and integrating the microstructures at different scales is crucial to tune the electrical and thermal properties of a given compound. High-pressure sintering can modify the multiscale microstructures and thus empower the cutting-edge thermoelectric performance. In this work, the high-pressure sintering technique followed by annealing is adopted to prepare Gd-doped p-type (Bi_0.2Sb_0.8)₂(Te_0.97Se_0.03)₃ alloys. First, the high energy of high-pressure sintering promotes the reduction of grain size, thus increasing the content of 2D grain boundaries. Next, high-pressure sintering induces strong interior strain, where 1D dense dislocations are generated near the strain field. More interestingly, the rare-earth element Gd with a high melting temperature is dissolved into the matrix via high-pressure sintering, thus promoting the formation of 0D extrinsic point defects. This concurrently improves the carrier concentration and density-of-state effective mass, resulting in an enhanced power factor. In addition, the integrated 0D point defects, 1D dislocations, and 2D grain boundaries by high-pressure sintering strengthen phonon scattering, thereby achieving a low lattice thermal conductivity of 0.5 Wm^-1 K^-1 at 348 K. Consequently, a maximum zT value of ∼1.1 at 348 K is achieved in the 0.4 at % Gd-doped (Bi_0.2Sb_0.8)₂(Te_0.97Se_0.03)₃ sample. This work demonstrates that high-pressure sintering enables microstructure modification to enhance the thermoelectric performance of Bi₂Te₃-based and other bulk materials.

Design of N-Type Textured Bi2 Te3 with Robust Mechanical Properties for Thermoelectric Micro-Refrigeration Application

Thermoelectric refrigeration is one of the mature techniques used for cooling applications, with the great advantage of miniaturization over traditional compression refrigeration. Due to the anisotropic thermoelectric properties of n-type bismuth telluride (Bi2 Te3 ) alloys, these two common methods, including the liquid phase hot deformation (LPHD) and traditional hot forging (HF) methods, are of considerable importance for texture engineering to enhance performance. However, their effects on thermoelectric and mechanical properties are still controversial and not clear yet. Moreover, there has been little documentation of mechanical properties related to micro-refrigeration applications. In this work, the above-mentioned methods are separately employed to control the macroscopic grain orientation for bulk n-type Bi2 Te3 samples. The HF method enabled the stabilization of both composition and carrier concentration, therefore yielding a higher quality factor to compare with that of LPHD samples, supported by DFT calculations. In addition to superior thermoelectric performance, the HF sample also exhibited robust mechanical properties due to the presence of nano-scale distortion and dense dislocation, which is the prerequisite for realizing ultra-precision machining. This work helps to pave the way for the utilization of n-type Bi2 Te3 for commercial micro-refrigeration applications.

High Thermoelectric Performance of p-Type Bi0.4Sb1.6Te3+x Synthesized by Plasma-Assisted Ball Milling

The exploration of new synthesis methods is important for the improvement of the thermoelectric property of a material for the different mechanisms of microstructure fabrication, surface activity modulation, and particle refinement. Herein, we prepared p-Bi₂Te₃ bulk materials by a simple synthesis method of the plasma-assisted ball milling, which yielded finer nanopowders, higher texture of in-plane direction, and higher efficiency compared to the traditional ball milling, favoring the simultaneous improvement of electrical and thermal properties. When combined with the Te liquid sintering, nano-/microscale hierarchical pores were fabricated and the carrier mobility was also increased, which together resulted in the low lattice thermal conductivity of 0.52 W·m^-1·K^-1 and the high power factor of 43.4 μW·cm^-1·K^-2 at 300 K, as well as the ranking ahead zT of 1.4@375 K. Thus, this work demonstrated the advantages of plasma-assisted ball milling in highly efficient synthesis of p-type Bi₂Te₃ with promising thermoelectric performance, which can also be utilized to prepare other thermoelectric materials.

Regulating the Configurational Entropy to Improve the Thermoelectric Properties of (GeTe)1-x(MnZnCdTe3)x Alloys

In thermoelectrics, entropy engineering as an emerging paradigm-shifting strategy can simultaneously enhance the crystal symmetry, increase the solubility limit of specific elements, and reduce the lattice thermal conductivity. However, the severe lattice distortion in high-entropy materials blocks the carrier transport and hence results in an extremely low carrier mobility. Herein, the design principle for selecting alloying species is introduced as an effective strategy to compensate for the deterioration of carrier mobility in GeTe-based alloys. It demonstrates that high configurational entropy via progressive MnZnCdTe₃ and Sb co-alloying can promote the rhombohedral-cubic phase transition temperature toward room temperature, which thus contributes to the enhanced density-of-states effective mass. Combined with the reduced carrier concentration via the suppressed Ge vacancies by high-entropy effect and Sb donor doping, a large Seebeck coefficient is attained. Meanwhile, the severe lattice distortions and micron-sized Zn_0.6Cd_0.4Te precipitations restrain the lattice thermal conductivity approaching to the theoretical minimum value. Finally, the maximum zT of Ge_0.82Sb_0.08Te_0.90(MnZnCdTe₃)_0.10 reaches 1.24 at 723 K via the trade-off between the degraded carrier mobility and the improved Seebeck coefficient, as well as the depressed lattice thermal conductivity. These results provide a reference for the implementation of entropy engineering in GeTe and other thermoelectric materials.

Tunable Electrical Conductivity and Simultaneously Enhanced Thermoelectric and Mechanical Properties in n-type Bi2 Te3

The recent growing energy crisis draws considerable attention to high-performance thermoelectric materials. n-type bismuth telluride is still irreplaceable at near room temperature for commercial application, and therefore, is worthy of further investigation. In this work, nanostructured Bi₂ Te₃ polycrystalline materials with highly enhanced thermoelectric properties are obtained by alkali metal Na solid solution. Na is chosen as the cation site dopant for n-type polycrystalline Bi₂ Te₃ . Na enters the Bi site, introducing holes in the Bi₂ Te₃ matrix and rendering the electrical conductivity tunable from 300 to 1800 Scm^-1 . The solid solution limit of Na in Bi₂ Te₃ exceeds 0.3 wt%. Owing to the effective solid solution, the Fermi level of Bi₂ Te₃ is properly regulated, leading to an improved Seebeck coefficient. In addition, the scattering of both charge carriers and phonons is modulated, which ensured a high-power factor and low lattice thermal conductivity. Benefitting from the synergistic optimization of both electrical and thermal transport properties, a maximum figure of merit (ZT) of 1.03 is achieved at 303 K when the doping content is 0.25 wt%, which is 70% higher than that of the pristine sample. This work disclosed an effective strategy for enhancing the performance of n-type bismuth telluride-based alloy materials.

Mediating Point Defects Endows n-Type Bi2 Te3 with High Thermoelectric Performance and Superior Mechanical Robustness for Power Generation Application

Bi₂ Te₃ -related alloys dominate the commercial thermoelectric market, but the layered crystal structure leads to the dissociation and intrinsic brittle fracture, especially for single crystals that may worsen the practical efficiency. In this work, point defect configuration by S/Te/I defects engineering is engaged to boost thermoelectric and mechanical properties of n-type Bi₂ Te₃ alloy, which, coupled with p-type BiSbTe, shows a competitive conversion efficiency for the fabricated module. First, as S alloying suppresses the intrinsic B i T e , antisite defects and forms a donor-like effect, electronic transport properties are optimized, associated with the decreased thermal conductivity due to the point defect scattering. The periodide compound TeI₄ is afterward adopted to further tune carrier concentration for the realization of an optimal ZT. Finally, an advanced average ZT of 1.05 with ultra-high compressive strength of 230 MPa is achieved for Bi₂ Te_2.9 S_0.1 (TeI₄ )_0.0012 . Based on this optimum composition, a fabricated 17-pair module demonstrates a maximum conversion efficiency of 5.37% under the temperature difference of 250 K, rivaling the current state-of-the-art Bi₂ Te₃ modules. This work reveals the novel mechanism of point defect reconfiguration in synergistic enhancement of thermoelectric and mechanical properties for durably commercial application, which may be applicable to other thermoelectric systems.

Isotropic Thermoelectric Performance of Layer-Structured n-Type Bi2Te2.7Se0.3 by Cu Doping

The lamellar structure of (Bi,Sb)₂(Te,Se)₃ alloys makes it difficult to achieve isotropic thermoelectric properties in the directions along and perpendicular to the c-axis, especially for n-type samples. In this work, by introducing Cu in polycrystalline n-type Cu_xBi₂Te_2.7Se_0.3 and applying the traditional synthesis process of high-energy ball milling and hot pressing, substantial enhancement of the thermoelectric figure of merit zT is obtained in both in-plane and out-of-plane directions. The intercalated Cu not only provides electron transport media for mobility improvement but also reduces the lattice thermal conductivity owing to the strain fluctuation. Typically, the van der Waals gap in the out-of-plane direction leads to relatively slower mobility and lower lattice thermal conductivity. Taking into account the same average density-of-state effective mass (m_avg^* ∼ 1.5m_e) predicted based on a single parabolic model, the obtained quality factor β is comparable in both directions. As a result, a peak zT ∼ 1.05 at 420 K and the average zT approaching to 1.0 in the temperature range 300-500 K are obtained in both directions for the Cu_0. 02Bi₂Te_2.7Se_0.3 sample. The simple synthesis process and isotropic thermoelectric properties in this work make n-type Bi₂Te₃ more convenient for potential production and application.

Identifying the Manipulation of Individual Atomic-Scale Defects for Boosting Thermoelectric Performances in Artificially Controlled Bi2Te3 Films

The manipulation of individual intrinsic point defects is crucial for boosting the thermoelectric performances of n-Bi₂Te₃-based thermoelectric films, but was not achieved in previous studies. In this work, we realize the independent manipulation of Te vacancies V_Te and antisite defects of Te_Bi and Bi_Te in molecular beam epitaxially grown n-Bi₂Te₃ films, which is directly monitored by a scanning tunneling microscope. By virtue of introducing dominant Te_Bi antisites, the n-Bi₂Te₃ film can achieve the state-of-the-art thermoelectric power factor of 5.05 mW m^-1 K^-2, significantly superior to films containing V_Te and Bi_Te as dominant defects. Angle-resolved photoemission spectroscopy and systematic transport studies have revealed two detrimental effects regarding V_Te and Bi_Te, which have not been discovered before: (1) The presence of Bi_Te antisites leads to a reduction of the carrier effective mass in the conduction band; and (2) the intrinsic transformation of V_Te to Bi_Te during the film growth results in a built-in electric field along the film thickness direction and thus is not beneficial for the carrier mobility. This research is instructive for further engineering defects and optimizing electronic transport properties of n-Bi₂Te₃ and other technologically important thermoelectric materials.

Influence of Nanoparticle Processing on the Thermoelectric Properties of (Bix Sb1-X )2 Te3 Ternary Alloys

The synthesis of phase-pure ternary solutions of tetradymite-type materials (Bix Sb1-x )2 Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties - Seebeck coefficient as well as electrical and thermal conductivity - is demonstrated.