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Bi2Te3-based alloys are only commercial thermoelectric materials. While, currently, n-type Bi2Te 2. 7 Se 0. 3 -based alloys commonly have lower ZT values than those of p-type Bi 0. 5 Sb 1. 5 Te3 based alloys. In this paper, the influence of different amounts of dispersing honey on the thermoelectric performance of n-type Bi2Te 2. 7 Se 0. 3 alloys synthesized by the hydrothermal and hot-pressing methods was investigated. The carbon atoms contained in honey could increase lattice defects and grain boundary density effectively, and improved the Seebeck coefficient and suppress lattice thermal conductivity simultaneously. By adjusting the amount of honey, the maximum ZT value reached 0.82 at 423 K, which was approximately 65% higher than that of the pristine Bi2Te2.7Se0.3. [ABSTRACT FROM AUTHOR]

Bi2Te3-based alloys are currently the best room-temperature thermoelectric materials, and have been used widely in the field of thermoelectric refrigeration. Bi2Te3 ingots prepared by the zone-melting method provide convenience for large-scale commercial application. Currently, one trend is that the dimensionless thermoelectric figure-of-merit (ZT) value of the n-type Bi2Te2.7Se0.3 is slightly lower than that of the p-type Bi0.5Sb1.5Te3. In this paper, n-type Bi2Te2.7Se0.3 alloy ingots with large size were successfully prepared through the optimized zone-melting preparation process, and achieved a high ZT value of 1.16 at 360 K. This value is higher than all the n-type or p-type Bi2Te3-based ingots in current commercial applications. [ABSTRACT FROM AUTHOR]

Bi 2 Sr 2 Co 2 O y is a thermoelectric material with low thermal conductivity. The Bi 2 Sr 2 Co 2 O y /Si 80 Ge 20 composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si 80 Ge 20 alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si 80 Ge 20 in the Bi 2 Sr 2 Co 2 O y matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi 2 Sr 2 Co 2 O y + 0.2 wt% Si 80 Ge 20 sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi 2 Sr 2 Co 2 O y sample. [ABSTRACT FROM AUTHOR]

Theoretically, nanosecond phase dispersion is an effective method to optimize the performance of thermoelectric materials due to the energy filtering effect and strong phonon scattering. Because of the chemical stability of La2O3, it may be used as a second phase material in Bi2Te2.7Se0.3 matrixes. After the introduction of La2O3, its electric transport performance decreases slightly, but its thermal transport performance increases further. Finally, its thermoelectric performance is optimized. At 455 K, the maximum dimensionless thermoelectric figure of merit (ZT) value of Bi2Te2.7Se0.3 + 0.5 wt% La2O3 reaches 0.97. The experimental results demonstrate that La2O3 dispersion can improve the thermoelectric properties of Bi2Te2.7Se0.3 effectively. [ABSTRACT FROM AUTHOR]

Introducing the nanosecond phase into thermoelectric materials is an effective strategy to improve their thermoelectric properties. In this study, the influence of nano Y2O3 dispersion on the resistivity, Seebeck coefficient and thermal conductivity of Cu2Se + x mol% Y2O3 (x = 0, 0.5, 1.0, 2.0, 3.0) was investigated. Their resistivity and Seebeck coefficient have no obvious change under the combined effects of the microstructure and the energy filtering. Especially, Y2O3 dispersion can inhibit the recrystallization of Cu2Se and make the lamellar grains thinner, thus scattering phonon and reducing the thermal conductivity effectively. When x = 1.0, the thermal conductivity reaches a minimum of 0.65 W/K·m at 923 K. The dimensionless thermoelectric figure of merit (ZT) value of the Cu2Se + 1.0 mol% Y2O3 sample reaches 1.81 at 923 K. The study indicates that nano rare earth oxides are relatively effective second-phase that can reduce thermal conductivity, which may be further promoted for other thermoelectric materials. [ABSTRACT FROM AUTHOR]

Bi2Te3 is a classical thermoelectrical material and has been applied widely in commerciality. In this paper, the influence of La2O3 dispersion as the nanosecond phase on the thermoelectric properties of Bi2Te3 was investigated. The Bi2Te3 nanopowders were prepared by the hydrothermal method. After adding the La2O3 nanopowders according to Bi2Te3 + s wt% La2O3 (s = 0 , 0.5, 1.0, 1.5), they were hot pressed into bulks in vacuum. The experimental results showed that the dispersed La2O3 as the nanosecond phase could enhance phonon scattering and suppress the thermal conductivity of Bi2Te3 effectively. Although their electrical resistivity increased due to the deteriorated carrier mobility, as a combined effect, the thermoelectric merit value (ZT) of the Bi2Te3 + 0.5 (or 1.0) wt% La2O3 was optimized and reached 0.61 at about 455 K. [ABSTRACT FROM AUTHOR]

Cu2Se and Bi2Te3 nano-powders prepared by the hydrothermal method were mixed and hot-pressed according to the molar ratio Cu2Se + x mol% Bi2Te3 (x = 0, 1, 2, 3). Bi2Te3 doping as the second phase can influence the microstructure of Cu2Se greatly, increase the carrier mobility and concentration, and reduce the resistivity significantly. The power factor increases by 17–42% compared with the pure sample. However, the total thermal conductivity increases further, which is not our expectation. Finally, the dimensionless thermoelectric figure of merit ZT is not well optimized. [ABSTRACT FROM AUTHOR]

Misfit-layered oxides Ca 3 Co 4 O 9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) samples were synthesized using solid-state sintering method and the effects of SiC nanoparticles diapersion on the thermoelectric properties were investigated. Thermoelectric properties of Ca 3 Co 4 O 9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) were investigated up to 923 K. Compared with pure sample, the electrical resistivity of SiC-added samples reduces, for Ca 3 Co 4 O 9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) ceramic samples with x ≤ 0.05, the electrical resistivity decreases with increasing SiC nanoparticles adding amounts. And, the electrical resistivity exists transition from semiconductor to metal conduction mechanism. While, the thermal conductivity also decreases due to the addition of SiC nanoparticles. As a result, the Ca 3 Co 4 O 9+δ + 0.2 wt% SiC sample had the lowest thermal conductivity of 1.47 W/Km at 923 K, which was 18.4% lower than that of the Ca 3 Co 4 O 9+δ sample. The ZT value of Ca 3 Co 4 O 9+δ + 0.05 wt% SiC can reach 0.218, which is 40.9% higher than the pure Ca 3 Co 4 O 9+δ sample. [ABSTRACT FROM AUTHOR]

Lutetium-doped Lu x Bi 2- x Te 2.7 Se 0.3 ( x = 0–0.3) nanopowders were synthesized by the hydrothermal method, and then they were hot-pressed into bulk pellets at 773 K under a pressure of 60 MPa in vacuum. The doping effects of Lutetium on the thermoelectric properties were investigated. Compared with the undoped sample, the electrical resistivities of Lu doped samples increased, and the Seebeck coefficients increased slightly. The lower carrier mobility caused by scattering may be the reason for the increase of the electrical resistivities of the doped samples. The thermal conductivities were significantly reduced because the Lu doping introduces a number of point defects into the crystals, which can effectively enhance phonon scattering, and because smaller grain size caused by Lu doping can enhance interface scattering. As a result, all the Lu doped Lu x Bi 2- x Te 2.7 Se 0.3 samples show higher ZT values compared with the undoped samples, indicating that Lu doping is an efficacious way to improve the thermoelectric properties of the n-type Bi 2 Te 2.7 Se 0.3 alloys. [ABSTRACT FROM AUTHOR]

The Bi 2 Sr 2 Co 2 O y ceramic, known for its stability as a p-type thermoelectric material, has limited thermoelectric performance that necessitates enhancement. In this study, Bi 2- x Cu x Sr 2 Co 2 O y (x = 0, 0.025, 0.05, 0.1, 0.2) ceramic samples were synthesized using the conventional solid-state reaction method. Substitution of Cu2+ for Bi3+ in the samples increased carrier concentration, significantly reducing electrical resistivity. Utilizing the single parabolic band (SPB) model, it was determined that Cu incorporation led to an increased effective mass, thereby enhancing the Seebeck coefficient. Additionally, fine grains and lattice defects induced by Cu2+ substitution effectively scattered phonons, resulting in an ultra-low thermal conductivity of 0.59 Wm−1K−1 at 923 K for the Bi 1.8 Cu 0.2 Sr 2 Co 2 O y sample. Consequently, Cu substitution optimized both the electrical and thermal transport properties of Bi 2-x Cu x Sr 2 Co 2 O y samples. The dimensionless thermoelectric figure of merit (ZT) for Bi 1.95 Cu 0.05 Sr 2 Co 2 O y reached 0.31 at 923 K, representing a 63 % increase over the pristine Bi 2 Sr 2 Co 2 O y sample. [ABSTRACT FROM AUTHOR]

Ca 3 Co 4 O 9+δ is a typical p -type thermoelectric oxide material with a low thermal conductivity. In this study, double-layered oxide samples Ca(Ba,Sr) 3 Co 4 O 9+δ dispersed with different SiC contents were obtained via the traditional solid phase reaction method. The effects of different elemental substitutions and SiC dispersion contents on the microstructure and thermoelectric properties of the samples were studied. The double optimisation of partial substitution of Ca-site atoms and SiC dispersion considerably improved the thermoelectric properties of Ca 3 Co 4 O 9+δ. Through the elemental substitution, the resistivity of the Ca 3 Co 4 O 9+δ material was reduced. Conversely, introducing an appropriate amount of SiC nanoparticles enhanced phonon scattering and was crucial in reducing its thermal conductivity. After double optimisations, the dimensionless thermoelectric figure of merit (ZT) values of both Ca 2.93 Sr 0.07 Co 4 O 9+δ + 0.1 wt% SiC and Ca 2.9 Ba 0.1 Co 4 O 9+δ + 0.1 wt% SiC achieved an optimum value of 0.25 at 923 K. [ABSTRACT FROM AUTHOR]

Cu2Se has been considered as a promising thermoelectric material due to its unique structure and excellent performance. Herein, a series of Cu2-xNaxSe (x = 0, 0.01, 0.02, 0.03, and 0.04) polycrystalline samples were synthesized by combining hydrothermal synthesis and hot pressing to investigate the effects of Na-doping on the microstructure and thermoelectric properties of Cu2Se. Compared with the pristine Cu2Se, the Na-doping introduces numerous micro-pores that can optimize the thermal transport performance by strong phonon scattering effects of interfaces between the micro-pores and grains. The doped samples have excellent electrical properties and low thermal conductivity. The maximum value of ZT = 2.1 is obtained at 973 K for the Cu1.96Na0.04Se sample with nominal composition. The results confirm that introducing Na into Cu2Se is an effective and convenient strategy to improve the thermoelectric performance of the Cu2Se alloy by decreasing the lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 -based crystal ingots produced by zone melting (ZM) are the most widely used materials to manufacture commercial thermoelectric (TE) devices. However, the poor mechanical properties of such crystalline materials limit the production of small-scale devices. While processability can be improved by powder metallurgy (PM), it also leads to the deterioration of the TE properties due to the donor-like effect of the introduced defects and grain boundaries. Thus, additional efforts towards re-optimizing doping are required. Here, we report a PM method to improve the mechanical properties of Bi 2 Te 3 -based TE materials through the crushing and hot pressing of Bi 2 Te 3 ingots obtained by ZM. Besides, we demonstrate the TE properties of n-type Bi 2 Te 2.7 Se 0.3 (BTS) to be remarkably improved by Cu doping, which enables optimization of the carrier concentration and enhances phonon scattering at heterointerphases thus increasing the Seebeck coefficient and reducing thermal conductivity. Overall, optimized materials reach dimensionless TE figures of merit (ZT) up to 1.16 at 408 K, i.e. 45% higher than that of state-of-the-art BTS, and an average ZT value of 1.08 in the temperature range of 300–473 K. [ABSTRACT FROM AUTHOR]

Bi 2 Sr 2 Co 2 O y with excellent thermoelectric properties has received extensive attention from scientific researchers. In this study, Bi 2 Sr 2 Co 2 O y ceramic samples were prepared using the solid-phase sintering method. By Ag + substitution for Bi3+, the hole concentration and electrical conductivity were optimised. On this basis, nano-SiC particles were introduced to increase grain boundaries, improve phonon scattering, reduce the mean free path of phonons, and reduce the lattice thermal conductivity. A sample with a nominal composition of Bi 1.8 Ag 0.2 Sr 2 Co 2 O y + 0.025 wt% SiC had a maximum ZT value of 0.35 at 923 K, which was approximately 67 % higher than the ZT value of pure Bi 2 Sr 2 Co 2 O y. These results confirmed that under the combined action of Ag substitution and SiC doping, the thermoelectric performance of Bi 2 Sr 2 Co 2 O y was significantly improved. [ABSTRACT FROM AUTHOR]

As a new type of clean and environmentally friendly energy material that can mutually transform thermal energy and electric energy, thermoelectric materials have received extensive attention from researchers. In this study, the traditional high-temperature solid-phase reaction method was used to prepare Bi 1.9 Ba 0.1 Sr 2 Co 2 O y + g wt% SiC (g = 0, 0.025, 0.05, 0.1 and 0.15) polycrystalline samples. Ba element was used to substitute the Bi site of Bi 1.9 Ba 0.1 Sr 2 Co 2 O y , in an attempt to change the carrier concentration and raise its electrical transport performance. Then, based on Ba substituting, nano-SiC was introduced as the second phase to increase the grain boundary number, which would enhance phonon scattering and reduce its heat transport performance. Finally, the ZT value of Bi 1.9 Ba 0.1 Sr 2 Co 2 O y + 0.025 wt% SiC reaches 0.35 at 923 K, which is approximately 35 % higher than that of the pure sample Bi 2 Sr 2 Co 2 O y. [ABSTRACT FROM AUTHOR]

The Bi 2 Te 3 -based thermoelectric crystal ingots produced by the zone-melting (ZM) are the most widely used commercially at present. However, the poor mechanical properties caused by the ZM are unfavorable to the production of miniature devices. The mechanical properties can be improved by the powder metallurgy (PM), but the PM results in performance degradation of n -type Bi 2 Te 3 -based alloys, which could be attributed to the disappearance of intrinsic anisotropy and the increased carrier concentration originated from the "donor-like" effect of grain boundary. In this paper, the Bi 2 Te 2.7 Se 0.3 + 4 wt % Te (BTST) powder was from the ground ZM ingots, and the influence of CuO doping on the thermoelectric properties of hot-pressed BTST polycrystalline bulks with a large size was investigated. It can be found that the thermoelectric properties of BTST are improved remarkably by CuO doping due to the increased Seebeck coefficient and suppressed thermal conductivity, which is caused by the synthetic effects of optimised carrier concentration, increased effective mass, and enhanced phonon scattering by the Cu 2 Te/Bi 2 Te 2.7 Se 0.3 hetero-interfaces. Finally, the dimensionless thermoelectric figure of merit (ZT) of the sample (CuO) 0.1 (Bi 2 Te 2.7 Se 0.3 + 4 wt % Te) reaches 1.32 at 408 K, which is about 65 % higher than that of the pristine BTST, and its average ZT ave value reaches 1.27 in the temperature range of 300–473 K. The large size bulks produced by this method are not only commercialized directly, but also avoid the problem of poor mechanical properties. • Effects of CuO doping on the thermoelectric properties of Bi 2 Te 2.7 Se 0.3 + 4 wt% Te. • Simultaneous optimization of power factors (S2σ) and thermal conductivity (κ). • High peak ZT max of 1.32 at 408 K, and high average ZT ave of 1.27 from 300 K to 473 K. • Large size of 30 mm*30 mm*70 mm. [ABSTRACT FROM AUTHOR]

Nano-second-phase dispersion usually improves the thermoelectric properties of thermoelectric materials through energy filtering effects and phonon scattering effects. In this paper, MXene was introduced as a new nano second phase material into the Cu 2 Se alloy bulks prepared by the hydrothermal method and the vacuum hot-pressing method. The microstructure, electrical and thermal transport properties of the samples were studied, and the results showed that the introduction of MXene can not only optimize its electrical transport properties, but also significantly suppress its thermal conductivity, thereby effectively improving its thermoelectric properties. Overall, the maximum ZT value of the Cu 2 Se + 0.20 mol % MXene sample reached 1.77 at 923 K, which was 30 % higher than that of the pristine Cu 2 Se sample. • Mxene dispersant. • Simultaneous optimization of power factors (S2σ) and thermal conductivity (κ). • Improved ZT value by 33 % compared with the pure Cu 2 Se sample. [ABSTRACT FROM AUTHOR]

• Grain refinement effect of La 2 O 3 dispersant. • Ultra-low thermal conductivity (κ) caused by enhanced phonon scattering. • The optimized ZT value is improved by 21 % compared with the pristine Bi 2 Sr 2 Co 2 O y. Thermoelectric materials are functional materials that directly convert heat energy and electric energy into each other. Nano-second-phase dispersion is an effective strategy to improve the energy conversion efficiency of thermoelectric materials through the energy filtering effect and the phonon scattering effect. In this study, the solid-state reaction method was used to prepare Bi 2 Sr 2 Co 2 O y + l wt% La 2 O 3 (l = 0.0, 0.1, 0.25, 0.5, 1.75) thermoelectric composites. An ultra-low thermal conductivity of 0.58 W/Km at 923 K was obtained by dispersing a small quantity of La 2 O 3 as the nano-second-phase additive in the Bi 2 Sr 2 Co 2 O y matrix. This was attributed to the increased hetero-interfaces and improved phonon scattering through the grain refinement effect. The dimensionless thermoelectric figure of merit (ZT) values of Bi 2 Sr 2 Co 2 O y + 0.25 wt% La 2 O 3 reached 0.28 at 923 K when their power factors were combined with little change, which was 21 % higher than that of the un-optimised pristine Bi 2 Sr 2 Co 2 O y. [ABSTRACT FROM AUTHOR]

• Carbon nanodots dispersant. • Simultaneous optimization of power factors (S2σ) and thermal conductivity (κ). • A high ZT value of ~0.91 at 423 K and averaged ZT of ~0.81 at 320–520 K. With excellent performance at room temperature as well as being the most mature n -type thermoelectric material in commercialization, Bi 2 Te 2.7 Se 0.3 has been attracting increased attention. In this study, its carrier concentration and mobility were optimized by introducing excessive Te. When the excessive content of Te was 0.4, its power factor was the largest, and its thermal conductivity declined slightly. Based on Te excess, carbon nanodots were introduced as the dispersed nanosecond phase. The power factors were improved further due to the energy filter effect. In particular, the lattice thermal conductivity was depressed remarkably due to enhanced phonon scattering. Finally, at 423 K, the maximum ZT value of the sample reached 0.91, which was approximately 42% higher than that of the pure Bi 2 Te 2.7 Se 0.3. The experimental results demonstrate that carbon nanodots dispersion acting as a nanosecond phase is an effective method to improve the thermoelectric performance of n -type Bi 2 Te 2.7 Se 0.3 -based materials. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are widely used in space, solar energy, refrigeration, flexible materials and so on, because they can directly convert electric energy and heat energy into each other. In this paper, the polycrystalline ceramic materials Bi 2 Sr 2 Co 2 O y (abbreviated as BSCO) were prepared by the conventional solid-phase reaction sintering method, and their thermoelectric properties were optimized under the dual action of cationic Pb2+ substitution and nano-SiC dispersion. The change of Seebeck coefficients after Pb doping is different from that in theory, which does not accord with the coupling trend of the decrease of resistivity with the increase of Seebeck coefficients. This is because Pb doping opens a pseudogap in BSCO system, which leads to the increase of the Seebeck coefficient. Thus the electrical transportation properties of the materials are improved. Nano-SiC dispersion can increase phonon scattering, thus improve the thermal transport properties of the materials. Under the condition of 973 K and q = 0.025, the power factor of Bi 1.6 Pb 0.4 Sr 2 Co 2 O y + q wt% SiC (q = 0, 0.025, 0.05, 0.1 and 0.15) system reaches 307 × 10−6 Wm−1K−2, and the dimensionless thermoelectric figure of merit (ZT) value reaches 0.48, which is the maximum in the previously reported BSCO polycrystalline ceramics. • High electrical conductivity and low thermal conductivity are obtained simultaneously by Pb-doping and nano-SiC dispersion. • A high ZT value of 0.48 is achieved in Bi 1.6 Pb 0.4 Sr 2 Co 2 O y + 0.25 wt% SiC at 973 K. • Seebeck coefficients increase because Pb doping opens a pseudogap in BSCO system. [ABSTRACT FROM AUTHOR]

By embedding organic compound carbon nanodots (CDs) into the inorganic compound Cu 2 Se, a series of Cu 2 Se + x wt% CDs (x = 0, 0.2, 0.4, 0.6, and 0.8) nanopowders are obtained through hydrothermal synthesis and then carried out by hot-pressing. The effects of CDs dispersant on resistivity, Seebeck coefficients, and thermal conductivity are investigated. It is found that the resistivity deteriorates, but the Seebeck coefficient increases greatly due to the energy filtering effect of carbon nanodots in the matrix. Higher Seebeck coefficients also lead to higher power factors compared with the matrix. Importantly, carbon nanodots are well distributed in the matrix without obvious agglomeration, and suppress the re-crystallization of Cu 2 Se. With increasing of the content of CDs, phonon scattering increases due to the newly formed interfaces between CDs dispersant and Cu 2 Se matrix, which reduces thermal conductivity effectively. Correspondingly, the lowest total thermal conductivity reaches 0.45 W/Km at 973 K when x = 0.8. Because of the relatively high power factor and exceptional low thermal conductivity, the Cu 2 Se + 0.8 wt% CDs sample presents an excellent ZT value of 1.98. The present work indicates that CDs are novel and effective dispersants, and could be furtherly extended to use in other thermoelectric materials. • Cu 2 Se + x wt% CDs hybrid materials were synthesized successfully by combining hydrothermal synthesis and hot-pressing. • Exceptional low thermal conductivity is obtained by CDs scattering. • A high ZT value of 1.98 is achieved in Cu 2 Se + 0.8 wt% CDs at 973 K. [ABSTRACT FROM AUTHOR]