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Bi2Te3‐based materials have drawn much attention from the thermoelectric community due to their excellent thermoelectric performance near room temperature. However, the stability of existing n‐type Bi2(Te,Se)3 materials is still low due to the evaporation energy of Se (37.70 kJ mol−1) being much lower than that of Te (52.55 kJ mol−1). The evaporated Se from the material causes problems in interconnects of the module while degrading the efficiency. Here, we have developed a new approach for the high‐performance and stable n‐type Se‐free Bi2Te3‐based materials by maximizing the electronic transport while suppressing the phonon transport, at the same time. Spontaneously generated FeTe2 nanoinclusions within the matrix during the melt‐spinning and subsequent spark plasma sintering is the key to simultaneous engineering of the power factor and lattice thermal conductivity. The nanoinclusions change the fermi level of the matrix while intensifying the phonon scattering via nanoparticles. With a fine‐tuning of the fermi level with Cu doping in the n‐type Bi2Te3–0.02FeTe2, a high power factor of ~41 × 10−4 Wm−1 K−2 with an average zT of 1.01 at the temperature range 300–470 K are achieved, which are comparable to those obtained in n‐type Bi2(Te,Se)3 materials. The proposed approach enables the fabrication of high‐performance n‐type Bi2Te3‐based materials without having to include volatile Se element, which guarantees the stability of the material. Consequently, widespread application of thermoelectric devices utilizing the n‐type Bi2Te3‐based materials will become possible. [ABSTRACT FROM AUTHOR]

Half‐Heusler compounds are promising materials for thermoelectric applications due to their high zT at elevated temperatures. However, their intrinsic high thermal conductivity limits their efficiency. Doping with Hf or Zr can improve the zT of these materials. Recently, a high zT of 1.5 at 1200 K achieved in p‐type Nb1‐xHfxFeSb has attracted much attention. While the effect of doping Hf in thermal conductivity is studied thoroughly, the effect of Hf doping on band parameters is not fully evaluated. This study investigates the effect of Hf and Zr doping on the electronic band parameters and thermoelectric properties of NbFeSb using the Single Parabolic Band model. The results show that Hf doping increases the weighted mobility of the samples, while Zr doping has no significant effect. Hf doping with x = 0.14 is predicted to improve the zT of NbFeSb by 35% at 300 K (0.19 → 0.26). These results show the intricate effects of Hf and Zr doping on the electronic and thermal properties of NbFeSb. [ABSTRACT FROM AUTHOR]

Recent studies have revealed the outstanding thermoelectric performance of Bi-doped n-type SnSe. In this regard, we analyzed the band parameters for Sn1−xBixSe (x = 0.00, 0.02, 0.04, and 0.06) using simple equations and the Single Parabolic Band model. Bi doping suppresses the carrier-phonon coupling while increasing the density-of-states effective mass. The n-type SnSe is known to have two conduction bands converge near 600 K. Bi doping changes the temperature at which the band convergence occurs. When x = 0.04, its weighted mobility maximized near 500 K, which indicated the possible band convergence. The highest zT of the x = 0.04 sample at mid-temperatures (473–573 K) can be attributed to the engineered band convergence via Bi doping. [ABSTRACT FROM AUTHOR]

Recent discovery of new p‐type Ta1‐xTixFeSb‐based half‐Heusler thermoelectric alloy has drawn much attention due to its high thermoelectric performance, zT of ∼1.52 near 970 K. However, the electronic band parameters of TaFeSb nor Ti‐doped TaFeSb have not been studied so far. Here we report the band parameters of Ta1‐xTixFeSb (x=0–0.16) calculated by the Single Parabolic Band model. Ti doping (x=0.12) both increases the density‐of‐states effective mass and non‐degenerate mobility by 27 and 29 times compared to those of the pristine TaFeSb (x=0). This simultaneous increase results in weighted mobility improvement by a factor of 4000 with Ti doping of x=0.12. Based on the estimated weighted mobility and the lattice thermal conductivity, the 300 K zT of Ta0.88Ti0.12FeSb (x=0.12) can be further increased by 10% once the carrier concentration is appropriately tuned. [ABSTRACT FROM AUTHOR]

Band engineering is an effective strategy to improve the electronic transport properties of semiconductors. In thermoelectric materials research, density‐of‐states effective mass is an undoubted key factor in verifying the band engineering effect and establishing a strategy for enhancing thermoelectric performance. However, estimation of the effective mass is demanding or inaccurate depending on the methods taken. A simple equation is proposed, valid for all degeneracy: Log10 (md*T/300) = (2/3) Log10 (n) − (2/3) [20.3 − (0.00508 × |S|) + (1.58 × 0.967|S|)] that utilizes experimentally determined Seebeck coefficient (S) and carrier concentration (n) to determine the effective mass (md*) at a temperature (T). This straightforward equation, which gives an accurate analysis of the band modulation in terms of md*, is indispensable in designing thermoelectric materials of maximized performance. [ABSTRACT FROM AUTHOR]

Summary: Excess Cu has been reported as an effective way to enhance the thermoelectric performance of n‐type Bi2Te3‐based alloys as well as to secure the reproducibility of their electronic properties. However, the effect of Cu doping into Bi2Te3 lattice is also known to be complex since Cu can occupy either interlayer or cation/anion sites, depending on conditions. Herein, Cu doping behavior in a binary Bi2Te3 prepared by a conventional melt‐solidification process was demonstrated, and corresponding changes in electronic and thermal transport properties were investigated. We found that the mechanism behind electronic transport properties improvements was different depending on Cu doping behavior: (a) power factor enhancement (especially at elevated temperatures) at low Cu concentrations (x ≤ 0.008 in CuxBi2Te3) is mainly due to interlayer intercalation of Cu, which optimizes electron concentration and increases the effective mass and (b) power factor enhancement at higher Cu concentrations (0.012 ≤ x ≤ 0.02 in CuxBi2Te3) is due to the substituted Cu at Bi‐site, which increases the weighted mobility ratio, as well as intercalated Cu. Enhanced room temperature zT ~ 0.68 and average zT ~ 0.53 were obtained in Cu0.02Bi2Te3 due to synergetic effect of intensified point defect (intercalated and substituted Cu) phonon scattering. [ABSTRACT FROM AUTHOR]

Influence of secondary phases of PdTe2 in n-type thermoelectric Cu0.008Bi2Te2.7Se0.3 alloys on their thermoelectric properties was investigated through in situ phase separation by melt spinning process. The phase segregation of PdTe2 in the Cu0.008Bi2Te2.7Se0.3 matrix were successfully induced by using the melt spinning process. Since heterointerfaces of PdTe2 and Cu0.008Bi2Te2.7Se0.3 form potential barriers, low energy carrier filtering effect could be expected; however, the power factor and effective mass decreased with the PdTe2 addition, which implies that the carrier filtering effect did not occur. The potential barrier of 0.06 eV expected at the interfaces between PdTe2 and Cu0.008Bi2Te2.7Se0.3 matrix seems not to induce carrier energy filtering effect. However, further investigation based on single parabolic band model shows that the non-degenerate mobility and quality factor B are enhanced by PdTe2 addition, suggesting that the further optimization of the carrier concentration would increase zT further in PdTe2-added Cu0.008Bi2Te2.7Se0.3 samples. This implies that the favorable carrier filtering effect could be attained by further controlling the optimal carrier concentration even with small barrier heights. [ABSTRACT FROM AUTHOR]

The addition of Cu becomes essential in polycrystalline n -type Bi 2 (Te,Se) 3 -based alloys since it is known to enhance stability of carrier transport properties as well as thermoelectric performance. However, a way to further optimize is necessary owing to the limited controllability of transport parameters by Cu addition. Herein, we present improved carrier transport properties via I-doping in Cu 0.008 Bi 2 Te 2.7 Se 0.3. Weighted mobility and effective mass are increased simultaneously by small amount doping of I at Te/Se-site. As a result, ~15% increased power factor and high average thermoelectric figure of merit (zT) of 0.79 were obtained in a wide temperature range. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf0.35Zr0.35Ti0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf0.35Zr0.35Ti0.3NiSn1−xSbx (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m−1 K−1 is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf0.35Zr0.35Ti0.3NiSn0.99Sb0.01. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nanoparticles in thermoelectric alloys has been considered as one of the most important ingredients to enhance their thermoelectric figure of merit zT mainly by reducing the lattice thermal conductivity due to intensified phonon scattering. However, the scattering mechanism of phonon with respect to wavelengths, which provides the comprehensive design rules for nanocomposites with enhanced zT , has not been fully understood. Here, we report a critical role of nanoparticles for the lattice thermal conductivity reduction from the theoretical and experimental analysis of the temperature-dependent thermal and electronic transport properties of p -type Ag/Cu nanoparticles-embedded Bi 0.5 Sb 1.5 Te 3 with respect to their electronic, bipolar, and lattice thermal conductivities. It was found that the introduction of the Ag/Cu nanoparticles reduced the lattice thermal conductivity through the additional phonon scattering based on the changeover between the Rayleigh and geometrical scatterings, indicating the indispensability of nanoparticles to scatter phonons that cannot be scattered effectively by either point defects or grain boundaries. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Substitutional doping is known to be effective when used to enhance the thermoelectric figure of merit zT , and this is generally explained as resulting from a reduction in the thermal conductivity caused by an additional atomic-scale defect structure. However, a comprehensive analysis of the substitutional doping effect on the electrical and thermal properties together has not been undertaken, especially when the bipolar thermal conductivity becomes serious. A previous study by the authors also showed that the zT of Bi 0.4 Sb 1.6 Te 3 thermoelectric alloys was enhanced by indium (In) doping due to the reduction of the total thermal conductivity. Here, we more closely analyze the electrical and thermal transport properties of a series of indium (In)-doped p-type Bi 0.4 Sb 1.6-x In x Te 3 (x = 0, 0.003, 0.005, 0.01) using both the single-parabolic-band model and the Debye-Callaway model in an effort to investigate the origin of the observed thermal conductivity reduction more closely. The bipolar contribution to the total thermal conductivity was estimated exclusively based on a two-band model based on a single-parabolic-band model. Furthermore, the lattice thermal conductivity was calculated using the Debye-Callaway model while taking additional In substitutional defects into consideration. The calculations indicated that the significant suppression of bipolar thermal conductivity was achieved as a result of the increased bandgap in Bi 0.4 Sb 1.6 Te 3 caused by In doping. Additional point defects from In doping also reduced the lattice thermal conductivity, but not as much as the bipolar thermal conductivity did. The study suggests that the suppression of bipolar conduction by means of a bandgap modification can be an effective approach for enhancing zT further via a simple In-doping process in Bi 0.4 Sb 1.6 Te 3 . [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides have gained renewed attention as promising thermoelectric materials primarily because of their unique structures and superior electronic properties. In this study, we investigate the electrical transport properties of a series of Hf(Se,Te) 2 samples (Hf(Se x Te 1- x) 2 , x = 0, 0.025, 0.25, 0.375, 0.5, and 1). All the samples form a single phase of a complete solid solution, displaying a wide variety of electrical properties based on the evolution of the composition. HfTe 2 exhibits metallic conduction with very high electrical conductivity of ~1600 S/cm, whereas HfSe 2 exhibits semiconducting conduction with very low electrical conductivity of 0.33 S/cm at room temperature. In contrast, the Seebeck coefficient of the HfTe 2 sample is as low as 17 μV/K, whereas that of HfSe 2 approaches 730 μV/K. Therefore, the power factor for HfTe 2 represents a much higher value of 0.24 mW/mK2 when compared with 0.01 mW/mK2 for HfSe 2 at 600 K. The superior electrical transport properties of HfTe 2 are primarily caused by its high carrier concentration of 1021 cm−3 and high density-of-state effective mass of 0.91 m 0. A comparison of the electronic band dispersion between HfSe 2 and HfTe 2 calculated using the density functional theory indicates that the dispersions of the conduction and valence bands seem to remain unchanged: HfSe 2 exhibits a finite band gap, whereas HfTe 2 exhibits no or a very small band gap. • A series of Hf(Te 1−x Se x) 2 (x = 0, 0.025, 0.25, 0.375, 0.5, and 1) was investigated. • Complete solid solution is formed throughout whole HfSe 2 -HfTe 2 composition. • HfSe 2 -HfTe 2 system provides a wide variety of electrical transport properties from semiconducting to metallic. • Band dispersions were calculated for HfSe 2 and HfTe 2 using the density functional theory. • Power factor for HfTe 2 exhibits 0.24 mW/mK at 600 K. [ABSTRACT FROM AUTHOR]

One means of enhancing the performance of thermoelectric materials is to generate secondary nanoprecipitates of metallic or semiconducting properties in a thermoelectric matrix, to form proper band bending and, in turn, to induce a low-energy carrier filtering effect. However, forming nanocomposites is challenging, and proper band bending relationships with secondary phases are largely unknown. Herein, we investigate the in situ phase segregation behavior during melt spinning with various metal elements, including Ti, V, Nb, Mo, W, Ni, Pd, and Cu, in p-type Bi0.5Sb1.5Te3 (BST) thermoelectric alloys. The results showed that various metal chalcogenides were formed, which were related to the added metal elements as secondary phases. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the BST composite with various secondary phases were measured and compared with those of pristine BST alloys. Possible band alignments with the secondary phases are introduced, which could be utilized for further investigation of a possible carrier filtering effect when forming nanocomposites. [ABSTRACT FROM AUTHOR]

• Bipolar contribution to thermoelectric properties in Cu 0.008 Bi 2 (Te 1- x Se x) 3 was investigated by tuning Te/Se ratio. • Weighted mobility and bipolar thermal conductivity was calculated using two-band model. • Weighted mobility for hole carriers gradually decreased with increasing Se content. • Bipolar thermal conductivity was remarkably reduced from 0.4 to 0.06 W m−1 K−1 in Cu 0.008 Bi 2 Te 2.1 Se 0.9. • Peak zT was enhanced to 0.92 in Cu 0.008 Bi 2 Te 2.1 Se 0.9 at 440 K. Bipolar conduction in Bi-Te-based alloys is a critical barrier to high conversion efficiency in thermoelectric power generation applications. Herein, we investigate the effect of Te/Se ratio control on bipolar conduction in n -type Cu 0.008 Bi 2 (Te,Se) 3. Based on the two-band model and Callaway model, we found that electronic and thermal transports of minority carriers (holes) were gradually decreased with an increase in Se content. The high-temperature power factor of Se-rich Cu 0.008 Bi 2 Te 2.1 Se 0.9 was higher in value compared to reference Cu 0.008 Bi 2 Te 2.7 Se 0.3 due to the weighted mobility ratio increase, and its bipolar thermal conductivity was significantly reduced simultaneously. As a result, a peak figure of merit (zT) of 0.92 was obtained at 440 K in Cu 0.008 Bi 2 Te 2.1 Se 0.9. [ABSTRACT FROM AUTHOR]

Compositional tuning is one of the important approaches to enhance the electronic and thermal transport properties of thermoelectric materials since it can generate point defects as well as control the phase evolution behavior. Herein, we investigated the Ti addition effect on the grain growth during melt spinning and thermoelectric transport properties of Hf0.5Zr0.5NiSn0.98Sb0.02 half-Heusler compound. The characteristic grain size of melt-spun ribbons was reduced by Ti addition, and very low lattice thermal conductivity lower than 0.27 W m−1 K−1 was obtained within the whole measured temperature range (300–800 K) due to the intensified point defect (substituted Ti) and grain boundary (reduced grain size) phonon scattering. Due to this synergetic effect on the thermal transport properties, a maximum thermoelectric figure of merit, zT, of 0.47 was obtained at 800 K in (Hf0.5Zr0.5)0.8Ti0.2NiSn0.98Sb0.02. [ABSTRACT FROM AUTHOR]

A significant obstacle to obtaining enhanced thermoelectric performance (defined by a thermoelectric figure of merit, zT) in commercial p -type Bi-Sb-Te alloys is bipolar transport originating from their intrinsic narrow-band-gap semiconducting characteristics. Cation-site doping is commonly used to suppress the bipolar conduction. However, zT enhancement occurs often only at elevated temperatures since the electronic thermal conductivity mainly increases at low temperatures due to the increase of hole concentration. Herein, the substitution of isovalent S ions in the anion Te-site of Bi-Sb-Te is explored to obtain a high zT over a wide temperature range by simultaneously increasing the density-of-states effective mass and weighted mobility. The zT of Bi 0.49 Cu 0.01 Sb 1.5 Te 3 is enhanced by ~10 % for all measured temperatures, and the average zT increases beyond 1.0 between 300 and 520 K, benefitting from the synergetic control of band structure and deformation potential via S substitution. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Introduction of nanophases is known to be effective in improving thermoelectric performance as it allows the simultaneous engineering of electronic and thermal transports. In this study, we synthesized Fe and Co nanoparticle-embedded MnSi 1.787 Al 0.01 nanocomposites through a simple nanometal-decoration technique and spark plasma sintering. The nanoparticles introduced in the matrix caused energy band bending at the matrix–nanoparticle interface, which induced charge transfer and energy filtering effects. These two seemingly opposing effects were combined so as to increase the power factor of the nanocomposites by enhancing the electronic transport. Moreover, we found that the lattice thermal conductivity decreased owing to intensified phonon scattering. Hence, a maximum ZT of 0.53 (at 773 K) was achieved in 0.6 vol% Fe nanoparticle-embedded nanocomposites, which is 25% higher than that of the pristine sample. Image 1 • We synthesized Fe and Co nanoparticles-embedded nanocomposites. • Enhanced electronic transport properties by a well-controlled band alignment. • Improved power factor by the optimized charge transfer and energy filtering effect. • Suppressed lattice thermal conductivity due to the intensified phonon scattering. • Increased thermoelectric performance by 25% achieved. [ABSTRACT FROM AUTHOR]

Significant bipolar conduction of the carriers in Bi2Te3-based alloys occurs at high temperatures due to their narrow bandgaps. Therefore, at high temperatures, their Seebeck coefficients decrease, the bipolar thermal conductivities rapidly increase, and the thermoelectric figure of merit, zT, rapidly decreases. In this study, band modification of n-type Cu0.008Bi2(Te,Se)3 alloys by sulfur (S) doping, which could widen the bandgap, is investigated regarding carrier transport properties and bipolar thermal conductivity. The increase in bandgap by S doping is demonstrated by the Goldsmid–Sharp estimation. The bipolar conduction reduction is shown in the carrier transport characteristics and thermal conductivity. In addition, S doping induces an additional point-defect scattering of phonons, which decreases the lattice thermal conductivity. Thus, the total thermal conductivity of the S-doped sample is reduced. Despite the reduced power factor due to the unfavorable change in the conduction band, zT at high temperatures is increased by S doping with simultaneous reductions in bipolar and lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

Doping is known as an effective way to modify both electrical and thermal transport properties of thermoelectric alloys to enhance their energy conversion efficiency. In this project, we report the effect of Pd doping on the electrical and thermal properties of n-type Cu0.008Bi2Te2.7Se0.3 alloys. Pd doping was found to increase the electrical conductivity along with the electron carrier concentration. As a result, the effective mass and power factors also increased upon the Pd doping. While the bipolar thermal conductivity was reduced with the Pd doping due to the increased carrier concentration, the contribution of Pd to point defect phonon scattering on the lattice thermal conductivity was found to be very small. Consequently, Pd doping resulted in an enhanced thermoelectric figure of merit, zT, at a high temperature, due to the enhanced power factor and the reduced bipolar thermal conductivity. [ABSTRACT FROM AUTHOR]

Summary: Excess Cu has been reported as an effective way to enhance the thermoelectric performance of n‐type Bi2Te3‐based alloys as well as to secure the reproducibility of their electronic properties. However, the effect of Cu doping into Bi2Te3 lattice is also known to be complex since Cu can occupy either interlayer or cation/anion sites, depending on conditions. Herein, Cu doping behavior in a binary Bi2Te3 prepared by a conventional melt‐solidification process was demonstrated, and corresponding changes in electronic and thermal transport properties were investigated. We found that the mechanism behind electronic transport properties improvements was different depending on Cu doping behavior: (a) power factor enhancement (especially at elevated temperatures) at low Cu concentrations (x ≤ 0.008 in CuxBi2Te3) is mainly due to interlayer intercalation of Cu, which optimizes electron concentration and increases the effective mass and (b) power factor enhancement at higher Cu concentrations (0.012 ≤ x ≤ 0.02 in CuxBi2Te3) is due to the substituted Cu at Bi‐site, which increases the weighted mobility ratio, as well as intercalated Cu. Enhanced room temperature zT ~ 0.68 and average zT ~ 0.53 were obtained in Cu0.02Bi2Te3 due to synergetic effect of intensified point defect (intercalated and substituted Cu) phonon scattering. [ABSTRACT FROM AUTHOR]

Substitutional doping is known to be effective in reducing lattice thermal conductivity in order to enhance the thermoelectric efficiency. However, the effect of co‐doping of two different substituents has not been investigated exclusively. Here, the effect of Ag and Ga co‐doping in p‐type Bi0.42Sb1.58Te3 alloys is examined with respect to the electronic and thermal transport properties, and the results are compared to cases of Ag‐doping and Ga‐doping separately. When the Ag and Ga are individually doped, the Ag‐doping increases the hole concentration, and the Ga‐doping reduces it. When both Ag and Ga are co‐doped, their opposite effects on the carrier concentration cancelled each other while maintaining the optimal concentration of the pristine Bi0.42Sb1.58Te3. An analysis of the lattice thermal conductivity reduction by the Ag and Ga co‐doping confirms that the co‐doping is as effective as the cumulative effect of each single doping. As a result, the co‐doped Bi0.42Sb1.58Te3 alloys have power factors comparable to that of the pristine Bi0.42Sb1.58Te3, and a drastically reduced lattice thermal conductivity owing to cumulative influences from the two independent dopants. Consequently, the co‐doping provides a beneficial effect in enhancing the thermoelectric efficiency by effectively suppressing the lattice thermal conductivity while maintaining high power factors. [ABSTRACT FROM AUTHOR]

Cu doping has been known to enhance thermoelectric performance of p -type (Bi,Sb) 2 Te 3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb) 2 Te 3 alloys by means of Bi-rich compositions of Bi 0.6 Sb 1.4 Te 3 and Bi 0.7 Sb 1.3 Te 3 , which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi 0.6 Sb 1.4 Te 3 composition from 0.25 to 0.98. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Herein, thermoelectric transport properties of lightly Pb-doped Bi 2- x Pb x Te 3 (x = 0, 0.0025, 0.005, and 0.01) alloys are investigated. Negative Seebeck coefficient (−169 μV/K) of the pristine Bi 2 Te 3 increases to −120 μV/K for x = 0.0025 at 300 K. As Pb further doped, positive Seebeck coefficients of 191 and 157 μV/K are measured for x = 0.005 and 0.01, respectively. P -type power factor increased to 2.32 mW/mK2 by the Pb doping for x = 0.01 at 300 K, compared with n -type power factor of 1.81 mW/mK2 of the pristine Bi 2 Te 3. The decrease in κ lat was observed for x = 0.01 due to additional phonon scattering by point defects. As results, p -type thermoelectric figure of merit zT of 0.42 for x = 0.01 is achieved compared with n -type zT of 0.35 for Bi 2 Te 3 at 300 K. The maximum p -type zT of 0.56 was obtained for Bi 1.99 Pb 0.01 Te 3 (x = 0.01). Therefore, a relative high p -type zT value was successfully achieved by the slight Pb doping in Bi 2 Te 3 by carrier type change from n -type to p -type. [ABSTRACT FROM AUTHOR]

Transition metal chalcogenides have received significant attention as electronic materials owing to their tunable electronic transport properties and unique crystal structures. In this work, the electrical, thermal, and thermoelectrical transport properties of FeTe 2 -CoTe 2 solid solution system were investigated by synthesizing a series of (Fe 1– x Co x)Te 2 polycrystalline alloys with x = 0, 0.25, 0.5, 0.75, and 1. FeTe 2 and CoTe 2 exhibited identical orthorhombic structures and formed a complete solid solution. (Fe 1– x Co x)Te 2 system exhibits wide range of electronic transport characteristics; CoTe 2 exhibits metallic conduction with high electrical conductivity of ∼8000 S/cm with electron carriers, whereas FeTe 2 exhibits semiconducting conduction with relatively low electrical conductivity of ∼300 S/cm with hole carriers at room temperature. The optimum carrier transport for high power factor of 1.53 mW/cmK2 at 600 K is observed for (Fe 0.5 Co 0.5)Te 2 composition. The electronic band dispersions of FeTe 2 , (Fe 0.5 Co 0.5)Te 2 , and CoTe 2 were calculated by using the density functional theory and it was found that a distinct flat band is present near the Fermi level for Fe 0.5 Co 0.5 Te 2 , supporting the high power factor of Fe 0.5 Co 0.5 Te 2. As the lattice thermal conductivity is reduced for the solid-solution samples with additional point defect scattering, the thermoelectric figure of merit (zT) increased significantly to 0.18 for Fe 0.5 Co 0.5 Te 2 compared to 0.001 for FeTe 2 or 0.08 for CoTe 2 at 600 K. [Display omitted] • A series of (Fe 1– x Co x)Te 2 alloys (x = 0, 0.25, 0.5, 0.75, and 1) was investigated. • Complete solid solution is formed for (Fe 1– x Co x)Te 2 (x = 0, 0.25, 0.5, 0.75, and 1). • FeTe 2 -CoTe 2 system provides a wide variety of electrical transport properties from semiconducting to metallic. • Band dispersions were calculated for FeTe 2 , Fe 0.5 Co 0.5 Te 2 and CoTe 2 using the density functional theory. • Power factor and thermoelectric figure of merit are optimized for Fe 0.5 Co 0.5 Te 2. [ABSTRACT FROM AUTHOR]

Cr 2 Se 3 is a layered narrow-bandgap semiconductor with a low lattice thermal conductivity of ∼1 W/mK, which can be considered as a potential thermoelectric material. A solid-solution alloy system with Cr 2 Te 3 can be designed to manipulate the electrical and thermal transport properties. In this study, a series of Cr 2 Se 3 -Cr 2 Te 3 solid-solution alloys (Cr 2 (Se 1- x Te x) 3 , x = 0, 0.33, 0.5, 0.67, 0.83, and 1) were synthesized, and their electrical and thermal transport properties were investigated. Regarding the electrical transport properties, the power factor (∼0.35 mW/mK2) of Cr 2 Se 3 gradually and significantly decreased as the Te content (x) increased owing to the significant decrease in the Seebeck coefficient that resulted from the large increase in the carrier concentration. Regarding the thermal transport properties, the total thermal conductivity increased with x as a result of the large increase in the electrical conductivity, despite the continuous decrease in the lattice thermal conductivity. Consequently, the thermoelectric figure of merit (zT ∼ 0.20) of Cr 2 Se 3 gradually and significantly decreased to 0.009 for Cr 2 Te 3 at 700 K; thus, no enhancement of zT was observed from the experimental results of simple solid-solution alloying. On the other hand, the thermoelectric quality factors of the solid-solution compositions for x = 0.33–0.83 were found to be enhanced compared to that of the Cr 2 Se 3 sample, implying that zT could be further enhanced by optimizing the carrier concentration. Indeed, enhanced zT values were estimated based on a single parabolic band model for the solid-solution compositions with x = 0.33–0.83 if the carrier concentration was optimized to ∼1019 cm−3, which was found to be owing to the increased nondegenerate and weighted mobility (inferring weaker carrier–phonon interaction). Therefore, it is suggested that the solid-solution alloying approach could provide a feasible strategy to enhance the thermoelectric performance by reducing carrier-phonon interaction when further combined with carrier concentration optimization. • A series of Cr 2 (Se 1- x Te x) 3 alloys (x = 0, 0.33, 0.5, 0.67, 0.83, and 1) was investigated. • Solid solution alloy is formed for the Cr 2 (Se 1- x Te x) 3 compositions. • Cr 2 (Se 1- x Te x) 3 showed the degraded thermoelectric properties with x increasing. • However, thermoelectric quality factors for x = 0.33–0.83 were found to be enhanced. • Enhanced thermoelectric properties are analyzed when carrier concentration optimized further. [ABSTRACT FROM AUTHOR]

Sb 2 Te 3 -based alloys exhibit decent thermoelectric transport properties in the mid-temperature range above 550 K. However, the study on Sb 2 Te 3 alloys has been limited in simple doping approach. Herein, evolution in the thermoelectric transport properties associated with the phase formation of solid solution alloys of Sb 2 (Te 1− x Se x) 3 (x = 0, 0.25, 0.33, 0.5, 0.75, and 1.0) compositions is investigated to widen the strategy to solid solution alloying. A single phase of the Sb 2 Te 3 rhombohedral structure formed from x = 0 to 0.5, whereas mixed phases with an orthorhombic structure of Sb 2 Se 3 was observed at x = 0.75. The electrical conductivity weighted mobility were gradually decreased as Se content increases to x = 0.75. Consequently, the power factor was decreased gradually and significantly to 0.076 mW/mK2 for x = 0.75 (Sb 2 (Te 0·25 Se 0.75) 3) compared with 2.6 mW/mK2 of the pristine sample. The total thermal conductivity of 2.0 W/mK for the Sb 2 Te 3 was significantly reduced gradually to 0.64 W/mK for x = 0.75 owing to simultaneous decrease in electrical and lattice thermal conductivities. The reduction in lattice thermal conductivity is mainly owing to the addition point defect scattering caused by the Se addition. Nevertheless, thermoelectric figure of merit zT of Sb 2 Te 3 is gradually decreased from 0.64 to 0.11 for x = 0.75 at 650 K due to the degradation in electrical transport properties. Furthermore, the theoretical zT was estimated for all samples with varying carrier concentration based on single parabolic band model. [ABSTRACT FROM AUTHOR]

Metal chalcogenide alloys are considered as electronic materials owing to their adjustable electrical properties either by doping or alloying. In this study, the electrical and thermal properties of the NiS 2 –NiSe 2 system are investigated by synthesizing Ni(S 1- x Se x) 2 (x = 0, 0.25, 0.5, 0.75, and 1.0) compositions. All samples exhibit pyrite-type cubic structures and form complete solid solution alloys. It is found that the NiS 2 –NiSe 2 system exhibits a wide spectrum of electrical characteristics; NiS 2 exhibits semiconducting conduction with low electrical conductivity σ of 7.8 S/cm and a carrier concentration of ∼1019 cm−3 at room temperature, whereas NiSe 2 exhibits metallic conduction with high σ of 11,600 S/cm and ∼1021 cm−3 carriers. As x of Ni(S 1- x Se x) 2 increased, the σ is gradually increased. On the other hand, the magnitude of Seebeck coefficient S is gradually decreased with x at 700 K. Thus, the optimized power factor (S 2∙ σ) of 0.10 mW/mK2 at 700 K is achieved for Ni(S 0.75 Se 0.25) 2 composition. Lattice thermal conductivities of the solid solution samples (x = 0.25, 0.5 and 0.75) range from 2.0 to 3.8 W/mK at 300 K, representing a reduction compared to those of NiS 2 and NiSe 2 (5.5 and 6.1 W/mK, respectively). Consequently, an enhanced thermoelectric performance was achieved in Ni(S 0.75 Se 0.25) 2 benefiting from an optimized solid solution alloying, with a maximum thermoelectric figure of merit of 0.020 at 700 K. [Display omitted] • A series of Ni(S 1- x Se x) 2 (x = 0, 0.25, 0.5, 0.75, and 1.0) was investigated. • NiS 2 and NiSe 2 exhibits semiconducting and metallic conduction, respectively. • The optimized power factor of 0.10 mW/mK2 is achieved for Ni(S 0.75 Se 0.25) 2. • Lattice thermal conductivities of the solid solution samples were reduced. • Optimized zT of 0.02 was obtained for Ni(S 0.75 Se 0.25) 2. [ABSTRACT FROM AUTHOR]

Herein, we report the enhanced thermoelectric performance of Cu incorporated Bi 2 Te 2.7 Se 0.3 prepared by melt spinning. The electronic transport properties were significantly improved by Cu incorporation due to the synergetic effect of carrier tuning, band modification, and electron-phonon transport control. The steady dimensionless thermoelectric figure of merit higher than 0.90 was achieved in the wide range of temperature range of 350–450 K for Cu 0.008 Bi 2 Te 2.7 Se 0.3 , which is ~30% enhancement in comparison with pristine Bi 2 Te 2.7 Se 0.3. This result can lead to high thermoelectric power generation efficiency 4.2% at Δ T = 180 K. Unlabelled Image [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 -based alloys are studied as most suitable thermoelectric alloys at room temperature. However, the lower zT values of Se-doped n -type Bi 2 Te 3- x Se x than those of p -type Bi x Sb 2- x Te 3 still hinder provision of high device performance. In this study, in effort to search for n -type Bi 2 Te 3 -based alloys other than Se-doped Bi 2 Te 3 , electrical, thermal, and thermoelectric transport properties of Bi 2- x Co x Te 3 (x = 0, 0.03, 0.06, 0.09, and 0.12) alloys were examined systematically. By Co doping, power factor of Bi 2 Te 3 alloys was increased by 65% to 2.89 mW/mK2 at 300 K. The enhanced electric transport properties were analyzed by experimental phenomenological parameters deduced from two-band model. It was also found that Co doping reduced bipolar and lattice thermal conductivities. Consequently, zT at 300 K was improved to 0.48 for Bi 1.91 Co 0.09 Te 3 (x = 0.09) by 37% compared to that of 0.35 for Bi 2 Te 3. The maximum zT of 0.62 was obtained for Bi 1.91 Co 0.09 Te 3 at 400 K. The two-band model predicts that the z T of Co-doped Bi 2 Te 3 can be further improved by optimizing carrier concentration. [ABSTRACT FROM AUTHOR]

The phase formation and thermoelectric transport properties of FeSe 2 –CoSe 2 systems were investigated by synthesizing a series of (Fe 1–x Co x)Se 2 polycrystalline alloys (x = 0, 0.25, 0.5, 0.75, and 1). It was observed that as x increased from 0 to 0.5, lattice parameters increased while the orthorhombic structure of FeSe 2 was retained. The bipolar conduction behavior of FeSe 2 (x = 0) changed to n -type conduction at all measured temperatures. The electrical conductivity is greatly increased with x , and the largely enhanced power factor of 1.37 mW/mK2 at 600 K is seen for Fe 0·5 Co 0·5 Se 2 (x = 0.5) compared to 0.37 mW/mK2 for FeSe 2. As x is further increased to 0.75 (Fe 0·25 Co 0·75 Se 2), the cubic phase of CoSe 2 started to form and no further enhancement of power factor is seen. Lattice thermal conductivity is gradually decreased to 3.18 and 1.79 W/mK Fe 0·75 Co 0·25 Se 2 (x = 0.25) and Fe 0·5 Co 0·5 Se 2 (x = 0.5) at 600 K, compared to 4.36 W/mK for x = 0 (FeSe 2). Consequently, the thermoelectric figure of merit zT at 600 K for Fe 0·75 Co 0·25 Se 2 (x = 0.25) and Fe 0·5 Co 0·5 Se 2 (x = 0.5) is greatly enhanced to 0.14 and 0.16, respectively, compared to 0.05 for x = 0 (FeSe 2). [Display omitted] • A series of (Fe 1– x Co x)Se 2 alloys (x = 0, 0.25, 0.5, 0.75, and 1) was investigated. • Orthorhombic structure of FeSe 2 was retained to x = 0.5. • Lattice thermal conductivity of FeSe 2 is gradually decreased to x = 0.5. • Power factor is maximized for x = 0.5 • Thermoelectric figure of merit is optimized for Fe 0·5 Co 0·5 Se 2 (x = 0.5). [ABSTRACT FROM AUTHOR]

Bi-Te based materials have been used for near-room-temperature thermoelectric applications. However, their properties dramatically decrease at high temperatures (over 100 °C), limiting their use in power generation. In this study, we investigated the enhanced thermoelectric properties of Bi-Te based materials by Cu doping and employing the melt-spinning (MS) process that can be utilized especially at elevated temperatures. By changing the doping amount, we could modulate the temperature dependence of thermoelectric properties, where the maximum ZT temperature could be shifted from room temperature to 450 K. The highest ZT value, 1.34, was achieved at 400 K for 2% Cu-doped Bi 0.5 Sb 1.5 Te 3 , which is due to the enhancement in power factor and reduction in lattice thermal conductivity. The average ZT value between room temperature and 530 K was 1.17 for 2% Cu-doped Bi 0.5 Sb 1.5 Te 3 , which is 46% higher than that of pristine Bi 0.5 Sb 1.5 Te 3 . Consequently, the synergetic effect of MS process and Cu incorporation can be a promising method to widen the application of Bi-Te based thermoelectric materials for mid-temperature power generation. [ABSTRACT FROM AUTHOR]

Filled skutterudites are promising mid-to-high temperature range thermoelectric materials for power generation, however, a traditional melt-solidification process followed by annealing (TMA) and powder metallurgical sintering requires a long processing time more than 10 days to ensure the structural and compositional homogeniety of materials with a high thermoelectric conversion efficiency zT . To address this, we herein report a simple and efficient synthesis of high-performance n- and p-type filled skutterudites that successfully produces a complete single phase from single to multiple filled materials in a day. The nanograin (∼440 nm) structured bulks are prepared from the combined process of temperature-regulated melt spinning (MS) using ingots and short-time spark plasma sintering (SPS). The controlled phase evolution and transformation by adjusting rapid solidification and densification conditions are demonstrated by a comprehensive analysis including structure refinement and atomic-scale observation, verifying the desired occupancy and random distribution of filling elements, respectively. The maximum zT values of filled skutterudites fabricated here were 1.48 ± 0.17 at 800 K for n-type In 0.12 Yb 0.20 Co 4.00 Sb 11.84 and 1.15 ± 0.13 at 750 K for p-type Ce 0.91 Fe 3.40 Co 0.59 Sb 12.14 , which are comparable to the highest zT values reported for filled skutterudites fabricated by TMA-based processes. Superior reproducibility achieved in shortened processing time enables the present synthetic process to be utilized for commercial manufacturing process that can be readily applied to massive production of bulk filled skutterudites for high-performance thermoelectric power generators. [ABSTRACT FROM AUTHOR]

Ti 2 FeNiSb 2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti 2 FeNiSb 2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti 2 Fe 0.9 Co 0.1 NiSb 2 is achieved, which is approximately six times higher than that of the pristine Ti 2 FeNiSb 2. The thermoelectric performance of Ti 2 FeNiSb 2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping. • Atomic site-targeted doping can fine-tune band parameters of double half-Heusler. • Doping at different atomic sites has a different impact on the phonon scattering. • Doping Co at Fe-sites of Ti 2 FeNiSb 2 improves zT of pristine Ti 2 FeNiSb 2 by six times. [ABSTRACT FROM AUTHOR]