Synergistically optimizing electrical and thermal transport properties of Bi2O2Se ceramics by Te‐substitution

Bi₂O₂Se oxyselenides, characterized with intrinsically low lattice thermal conductivity and large Seebeck coefficient, are potential n‐type thermoelectric material in the mediate temperature range. Given the low carrier concentration of ~10¹⁵ cm^?3 at 300 K, the intrinsically low electrical conductivity actually hinders further enhancement of their thermoelectric performance. In this work, the isovalent Te‐substitution of Se plays an effective role in narrowing the band gap, which notably increases the carrier concentration to ~10¹⁸ cm^?3 at 300 K and the electron conduction activation energy has been lowered significantly from 0.33 to 0.14 eV. As a consequence, the power factor has been improved from 104 μW·K^?2·m^?1 for pristine Bi₂O₂Se to 297 μW·K^?2·m^?1 for Bi₂O₂Se_0.96Te_0.04 at 823 K. Meanwhile, the suppressed lattice thermal conductivity derives from the introduced point defects by heavier Te atoms. The gradually decreased phonon mean free path reflects the increasingly intense phonon scattering. Ultimately, the ZT value attains 0.28 for Bi₂O₂Se_0.96Te_0.04 at 823 K, an enhancement by a factor of ~2 as compared to that of pristine Bi₂O₂Se. This study has demonstrated that Te‐substitution of Se could synergistically optimize the electrical and thermal properties thus effectively enhancing the thermoelectric performance of Bi₂O₂Se.     

Enhanced thermoelectric performance of n‐type Bi2O2Se by Cl‐doping at Se site

The n‐type polycrystalline Bi₂O₂Se_1?xCl_x (0≤x≤0.04) samples were fabricated through solid‐state reaction followed by spark plasma sintering. The carrier concentration was markedly increased to 1.38×10²⁰ cm^?3 by 1.5% Cl doping. The maximum electrical conductivity is 213.0 S/cm for x=0.015 at 823 K, which is much larger than 6.2 S/cm for pristine Bi₂O₂Se. Furthermore, the considerable enhancement of the electrical conductivity outweighs the moderate reduction of the Seebeck coefficient by Cl doping, thus contributing to a high power factor of 244.40 μ·WK^?2·m^?1 at 823 K. Coupled with the intrinsically suppressed thermal conductivity originating from the low velocity of sound and Young's modulus, a ZT of 0.23 at 823 K for Bi₂O₂Se_0.985Cl_0.015 was achieved, which is almost threefold the value attained in pristine Bi₂O₂Se. It reveals that Se‐site doping can be an effective strategy for improving the thermoelectric performance of the layered Bi₂O₂Se bulks.     

Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se

The Bi₂O₂Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 10¹⁵?cm^?3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi₂O₂Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03?S/cm in pristine sample to 133?S/cm in x?=?0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823?K for Bi_1.92Ge_0.08O₂Se, which is the highest ZT value for Bi₂O₂Se-based thermoelectric materials.     

Enhanced Thermoelectric Properties of Bi2O2Se Ceramics by Bi Deficiencies

Polycrystalline Bi_2?xO₂Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773 K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change of effective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is ?568.8 μV/K for Bi_1.9O₂Se at 773 K, much larger than ?445.6 μV/K for pristine Bi₂O₂Se. Featured with very low thermal conductivity [~0.6 W·(m·K)^?1] and an optimized electrical conductivity, ZT at 773 K is significantly increased from 0.05 for pristine Bi₂O₂Se to 0.12 for Bi_1.9O₂Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.     

Thermoelectric properties of Mo-doped bulk In2O3 and prediction of its maximum ZT

In a search for new thermoelectric materials, indium oxide (In₂O₃) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (<2 W/mK) and ZT values around 0.05. However, low electrical conductivity is a factor limiting the thermoelectric performance of this oxide, and was addressed in this study by Mo doping. It was found that Mo is soluble in In₂O₃ but forms secondary phases at a fraction near x = 0.06 and higher. Mo was found to be unsuitable for heavy n-type doping necessary to improve the thermoelectric performance of the oxide to the desired level (ZT = 1). However, the experimental data enabled us to analyze the electrical conductivity behavior and the Seebeck coefficient of doped In₂O₃ with different carrier concentrations, predicting a theoretically achievable maximum power factor value of 1.77 × 10^?3 W/mK² at an optimum carrier concentration. This estimation predicts the highest ZT value of 0.75 at 1073 K, assuming the lattice thermal conductivity value remaining at an amorphous level.     

Effects of sulfur substitution for oxygen on the thermoelectric properties of Bi2O2Se

In the present work, the thermoelectric properties of S-doped Bi₂O_2-xS_xSe at the temperatures from 320 to 793 K have been studied. The results show that the solubility limit of S is around x = 0.01 and S-doping is helpful to the sintering and grain growth of Bi₂O₂Se. Moreover, S-doping reduces the band gap of Bi₂O_2-xS_xSe remarkably as x rises. As a result, a thousand times promotion of electrical conductivity at x = 0.02 is obtained, leading to a nearly 3 times increase of power factor at 787 K. By virtue of the intrinsically low thermal conductivity, a peak ZT of 0.29 at 793 K with an average of 0.21 has been achieved for Bi₂O_1.98S_0.02Se, which is nearly 3 and 6 times larger than that of the pristine one. This study indicates that a small amount of S substitution for O could improve the thermoelectric properties of Bi₂O₂Se effectively.     

Exploring the potential of the mechanical/thermal properties and co-shielding ability of Bi2O3-doped aluminum borate ceramics against neutron/gamma radiation

The efficient optimisation of radiation shielding materials (RSMs), which protect people from potential radiant threats, is highly desirable; however, it remains challenging. This study addresses the low-cost fabrication of the ceramic-based RSMs, aluminium borate-based ceramics using Bi₂O₃ as a novel simultaneous shielding agent and sintering promoter. The phase compositions, microstructures, sintering kinetics, and performances of the as-prepared Bi₂O₃ doped aluminium borate ceramics (BDABCs) are systematically researched. Finally, co-shielding tests for neutron and gamma radiation are performed. The results demonstrate that Bi₂O₃ can positively influence the sintering densification process of BDABCs via the evident reduction in the sintering activation energy. The migration of the Bi₂O₃–B₂O₃ liquid phase affects the pore structure, crystal morphology, and thermal conductivity of the samples. The obtained BDABCs exhibited highly reliable mechanical properties with a maximum elastic modulus and modulus of rupture of 124.3 GPa and 54.9 MPa, respectively; controllable thermal conductivity from 1.32 to 6.16 W m^?1 K^?1; and 12 wt% Bi₂O₃-doped sample (1400 °C × 3 h, 1.5 cm) shows the best radiation shielding performance, including 58.6% neutron and 26.6% γ rays. The obtained results manifest the enormous potential of BDABCs as structural materials and functional RSMs.     

Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi₂Te_2.7Se_0.3) and n-type bismuth telluride (Bi₂Te₃) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi₂Te₃ matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK² and 3179 μW/mK² for pure and Se-doped Bi₂Te₃ compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi₂Te₃ was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.     

Thermoelectric properties of W1−xNbxSe2−ySy polycrystalline compounds

We investigate the thermoelectric properties of bulk polycrystalline samples of WSe₂-based compounds with partial substitutions in the cationic (W) and the anionic (Se) sublattices in the temperature range from 4.2 to 650 K. The substitution of W for Nb leads to a significant increase in the charge carrier concentration, however, deteriorates the charge carrier mobility. In contrast, the substitution of selenium for sulfur increases the charge carrier mobility, the thermal conductivity, and the Seebeck coefficient but conductivity changes non-monotonical. We show that the addition of sulfur in anionic sublattice affects the grain sizes in the polycrystalline material. Using substitutions in the anionic and cationic sublattices, we find the optimal ratio of the elements for better thermoelectric efficiency. The W_0.98Nb_0.02Se_1.7S_0.3 sample showed the best value of the figure of merit ZT = 0.26 (T = 650 K).     

Grain boundary engineering strategy for simultaneously reducing the electron concentration and lattice thermal conductivity in n-type Bi2Te2.7Se0.3-based thermoelectric materials

This study demonstrates atomic layer deposition (ALD) of an extremely thin Al₂O₃ layer over n-type Bi₂Te_2.7Se_0.3 to alleviate the adverse effects of multiple boundaries on their thermoelectric performance. Multiple boundaries reduce thermal conductivity (κ), but generate electrons, deviating from the optimum carrier concentration. Only one Al₂O₃ ALD cycle effectively suppresses Te volatilization at the grain boundaries, resulting in a decrease from 5.8 × 10¹⁹/cm³ to 3.6 × 10¹⁹/cm³ in the electron concentration. Concurrently, the one-cycle-Al₂O₃ coating produces fine grains, thus inducing numerous boundaries, ultimately suppressing the lattice κ from 0.64 to 0.33 W/m·K. A further increase in the number of Al₂O₃ cycles leads in a significant rise in the resistance, resulting in degradation of thermoelectric performance. Consequently, the ZT value is increased by 51 % as a result of Al₂O₃ coating with a single ALD cycle. Our approach offers new insights into the simultaneous reduction of the κ and electron concentration in n-type Bi₂Te₃-based materials.     

Enhanced thermoelectric performance of p-type sintered BiSbTe-based composites with AgSbTe2 addition

Bismuth telluride-based materials have been widely used in the field of thermoelectric cooling near room temperature. However, the material utilization and device conversion efficiency were limited by the low thermoelectric performance and poor mechanical properties of commercial zone-melting materials. With an aim to optimize the comprehensive properties, we prepared the composite samples of Bi_0.48Sb_1.52Te₃ (BST)-x wt% AgSbTe₂ (x = 0, 0.05, 0.1, 0.2) via the hot pressing method. It was found that the AgSbTe₂ addition can effectively increase the carrier concentration and improve the power factor to 46 μW cm^?1 K^?2 at 300 K. Due to the introduction of dislocations, stress and Te inhomogeneities, the lattice thermal conductivity of the composite was significantly reduced to 0.69 W m^?1 K^?1 at 325 K. As a result, a maximum ZT of 1.15 at 325 K is obtained for the x = 0.1 sample. Interestingly, BST-0.1 wt% AgSbTe₂ exhibits roughly isotropic thermoelectric performance perpendicular to and parallel to the pressing direction. Our study suggests that the BST-AgSbTe₂ composite is very promising for the application of thermoelectric refrigeration near room temperature.     

Synergistic optimization of electrical-thermal properties of dual vacancy Bi1−x-yPbyCu1−xSeO by improving mobility and reducing lattice thermal conductivity

We fabricate Bi_1?x-yPb_yCu_1?xSeO (x = 0, 0.03, 0.06, y = 0, 0.10) samples via 4 min-microwave synthesis combined with 5 min-spark plasma sintering. The phase composition, microstructure, valence, and electrical and thermal transport properties of the samples are investigated at 298–873 K. Pb doping provides impurity carriers and increases the concentration to 0.9–3.0 × 10²⁰ cm^-³ . Bi and Cu vacancy could provide a carrier transport channel to reduce carrier scattering probability, leading to improved mobility. Twin crystals, stacking faults, and grain boundary segregation are observed in Bi_0.87Pb_0.10Cu_0.97SeO on scanning transmission electron microscopy. Bi and Cu vacancy increase the sample point defects in Pb-doped or undoped samples which results in a decrease in lattice thermal conductivity. The lattice thermal conductivity of Bi_0.87Pb_0.10Cu_0.97SeO is decreased to an extremely low value of 0.13 Wm^?1 K^?1 and a maximum ZT value of 1.09 is achieved at 873 K.     

Enhanced thermoelectric properties of Cu2-xSe by coordinating carrier concentration to reduce thermal conductivity

Cu_2-xSe has been known as an ideal thermoelectric material for application in the middle-high temperature range due to the outstanding electric transmission conductivity and relatively low lattice thermal conductivity. However, its performance is significantly constrained by its high thermal conductivity and thermal stability issues, as well as its difficulty in element doping because of the influence of atomic size and atomic solid solubility. Here, we prominently reduced the thermal conductivity of Cu_2-xSe by addition different amounts of WS₂ to the Cu_2-xSe matrix, and successfully improved the power factor of the material because of the reduction of high Cu defects to coordinate the three mutually coupled parameters. The zT value of the WS₂-doping Cu_2-xSe sample was eventually enhanced by 56% compared with pristine Cu_2-xSe, and up to ca. 1 at 823 K. Further, we found that the symmetry of the Cu_2-xSe crystal had not been destroyed undergoing the doping of WS₂ into its crystal lattice, and the doped sample showed good thermal stability when cycle test was carried out twice between 315 K and 823 K.     

Enhanced thermoelectric properties of CNT dispersed and Na-doped Bi2Ba2Co2Oy composites

The thermoelectric properties of Bi₂Ba₂Co₂O_y and Bi_1.975Na_0.025Ba₂Co₂O_y+x wt% carbon nanotubes (CNT; x=0.00, 0.05, 0.10, 0.15, 0.5, and 1.0) ceramic samples synthesised by the solid-state reaction method were investigated from 300K to 950K. Na doping with a small amount played an important role in reducing resistivity and slightly reduced the Seebeck coefficients and the thermal conductivity. The CNT dispersant increased resistivity, but the thermal conductivity was reduced remarkably. In particular, the Bi_1.975Na_0.025Ba₂Co₂O_y+1.0wt% CNT sample exhibited an ultralow thermal conductivity of 0.39 W K⁻¹ m⁻¹ at 923K. This was attributed to the point defects caused by Na doping and the interface scattering caused by the CNT dispersant. The combination of Na doping and CNT dispersion had better effects on thermoelectric properties. The Bi_1.975Na_0.025Ba₂Co₂O_y+0.5wt% CNT sample exhibited a better dimensionless figure of merit (ZT) value of 0.2 at 923K, which was improved by 78.2%, compared with the undoped Bi₂Ba₂Co₂O_y sample.     

Enhancing the thermoelectric power factor of nanostructured SnO2 via Bi substitution

Tin dioxide (SnO₂) has recently proved to be a promising material for thermoelectric applications. We have investigated the influence of highly valence Bi doping as an electron donor in oxygenated SnO₂ materials on their thermoelectric properties. We have synthesized the pure and Bi doped SnO₂ nanoparticles (x = 0%, 5%, 10%, and 15%) through a simple hydrothermal approach. The Seebeck coefficient and Hall measurements have been used to determine thermoelectric behaviour. The measured value of the Seebeck coefficient increases from - 56 to - 83 μV/^°C as the Bi content increases. This improvement in the Seebeck coefficient has been attributed to the charge carrier localization (energy filtering effect) caused by the inclusion of the bismuth atoms and the presence of secondary phases based on BiO₂. However, the electrical conductivity measurements show an inverse relation with the Bi doping, increasing the impurities. The Sn_1-xBi_xO₂ sample with x = 15 has achieved the maximum Seebeck value, resulting in the upward trend in power factor of up to 1.97 × 10^?4 Wm^?1C^?2. Further, we have used X-ray diffraction and scanning electron microscopy to determine the effect of Bi on the SnO₂ crystal structure and surface morphology. Which also demonstrates the presence of composites with mixed phases.     

Effect of Ag+ doping and Ag addition on the thermoelectric properties of KSr2Nb5O15

The electron and phonon thermal transport behavior of Ag ⁺ doped KSr₂Nb₅O₁₅ were discussed by using the first-principles calculations. The band gap was reduced after Ag⁺ doping, and the electrons near the Fermi level had stronger transition capability, which effectively increased the carrier concentration and electrical conductivity and reduced the thermal conductivity, thereby improving the ZT of the doped KSr₂Nb₅O₁₅ from 0.6298 to 0.7214 (1200 K) under ideal conditions. In addition, the solid-state reaction method was used to prepare Ag nanoparticle added KSr₂Nb₅O₁₅ samples, and their thermoelectric performance was tested. The experimental results and the calculated results showed a good consistent trend in which Ag improved the thermoelectric properties of KSr₂Nb₅O₁₅. When the amount of addition of nanosized Ag was 20 wt%, the power factor and ZT of the material were the highest at 1073 K, which were 0.228 mW/(K²·m) and 0.1090, respectively. This research shows how to improve the thermoelectric performance of KSr₂Nb₅O₁₅ ceramics and broaden their temperature range for application.     

Enhanced thermoelectric properties of Na and Mg co−doped BiCuSeO

A series of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) was fabricated by a two?step solid?state reaction and spark plasma sintering (SPS), and the influence of Mg²⁺ doping on the thermoelectric properties of Bi_0.97Na_0.03CuSeO was systematically investigated. The SPS processed?Bi_0.97?xNa_0.03Mg_xCuSeO had a ZrSiCuAs?type tetragonal crystal structure (space group P4/nmm). The Mg²⁺ doping appreciably enhanced the electrical conductivity due to the increase in hole concentration. Furthermore, the Mg²⁺ doping increased the grain boundary areas and bulk porosity and induced the strain field and mass fluctuations, thereby reducing the phonon thermal conductivity. We significantly improved the thermoelectric performance of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) by enhancing the thermoelectric power factor and by reducing the thermal conductivity.     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.     

A thermoelectric generator comprising selenium-doped bismuth telluride on flexible carbon cloth with n-type thermoelectric properties

Carbon cloth was used as a flexible substrate for bismuth telluride (Bi₂Te₃) particles to provide flexibility and improve the overall thermoelectric performance. Bi₂Te₃ on carbon cloth (Bi₂Te₃/CC) was synthesized via a hydrothermal reaction with various reaction times. After over 12 h, the Bi₂Te₃ particles showed a clear hexagonal shape and were evenly adhered to the carbon cloth. Selenium (Se) atoms were doped into the Bi₂Te₃ structure to improve its thermoelectric performance. The electrical conductivity increased with increasing Se-dopant content until 40% Se was added. Moreover, the maximum power factor was 1300 μW/mK² at 473 K for the 30% Se-doped sample. The carbon cloth substrate maintained its electrical resistivity and flexibility after 2000 bending cycles. A flexible thermoelectric generator (TEG) fabricated using the five pairs of 30% Se-doped sample showed an open-circuit voltage of 17.4 mV and maximum power output of 850 nW at temperature difference ΔT = 30 K. This work offers a promising approach for providing flexibility and improving the thermoelectric performance of inorganic thermoelectric materials for wearable device applications using flexible carbon cloth substrate for low temperature range application.