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Control of bipolar conduction is essential to improve the high-temperature thermoelectric performance of materials for power generation applications. Recently, Hf(Te1−xSex)2 alloys have gained much attention due to their potential use in thermoelectric power generation. Increasing the Se alloying content significantly increases the band gap while decreasing its carrier concentration. These two factors affect bipolar conduction substantially. In addition, the weighted mobility ratio is estimated from the experimental electronic transport properties of Hf(Te1−xSex)2 alloys (x = 0.0, 0.025, 0.25, 0.5, 1.0) by using the Two-Band model. From the bipolar thermal conductivity also calculated using the Two-Band model, we find that it peaks near x = 0.5. The initial bipolar conductivity increase of x < 0.5 is mostly due to the decrease in the weighted mobility ratio and carrier concentration with increasing x. For x > 0.5, the drop in the bipolar conductivity can be understood with significant band gap enlargement.

Control of bipolar conduction is essential to improve the high-temperature thermoelectric performance of materials for power generation applications. Recently, Hf(Te1−xSex)2 alloys have gained much attention due to their potential use in thermoelectric power generation. Increasing the Se alloying content significantly increases the band gap while decreasing its carrier concentration. These two factors affect bipolar conduction substantially. In addition, the weighted mobility ratio is estimated from the experimental electronic transport properties of Hf(Te1−xSex)2 alloys (x = 0.0, 0.025, 0.25, 0.5, 1.0) by using the Two-Band model. From the bipolar thermal conductivity also calculated using the Two-Band model, we find that it peaks near x = 0.5. The initial bipolar conductivity increase of x < 0.5 is mostly due to the decrease in the weighted mobility ratio and carrier concentration with increasing x. For x > 0.5, the drop in the bipolar conductivity can be understood with significant band gap enlargement. [ABSTRACT FROM AUTHOR]

Bi2Te3-based thermoelectric (TE) materials have been demonstrated to be a potential candidate for mainly thermoelectric cooling/refrigeration applications. However, minority charge carriers excitation at high temperature reduces thermopower which restricts these materials for the use in power generation. In present work, substitution of Ni on Sb site in Bi0.5Sb1.5-xNixTe3 (x = 0, 0.01, 0.04 and 0.08) actuates the system to supress the intrinsic excitation leading to shift in highest ZT to higher temperature regime. The Density functional theory (DFT) calculations and experimental results reveal that Ni in Bi0.5Sb1.5Te3 provides the extra holes and slightly reduces the band gap Eg which enhances the σ of Ni-doped Bi0.5Sb1.5-xNixTe3 samples and α at elevated temperature. Moreover, Ni-doping in Bi0.5Sb1.5Te3 also reduces κL which is attributed to the phonon scattering due to mass fluctuations and microstructural features such as grain boundary and strain field domain observed from HRTEM investigation. These favourable condition leads to maximum ZT∼1.38 at 433K for Bi0.5Sb1.46Ni0.04Te3 and ZTavg ∼1.1 between 300K and 503K. Interestingly the calculated theoretical TE conversion device efficiency η of Bi0.5Sb1.46Ni0.04Te3 (η∼5.5%) was achieved to be nearly twice than the efficiency of matrix Bi0.5Sb1.5Te3 (η∼3%). Experimental electronic transport is well corroborated with theoretically estimated DFT results.

Bi2Te3 is a good candidate to be used in thermoelectric generators. For a higher efficiency of the generators, shifting the temperature at which Bi2Te3 performs best to higher temperatures is required. Bipolar thermal conductivity suppression is the most effective approach to improve high-temperature thermoelectric performance. However, characterization of the bipolar thermal conductivity is challenging because it is related to individual contribution to Seebeck coefficient and electrical conductivity from the majority and minority carriers. Two-band model calculations are performed using reported band parameters of n-type Bi2Te3 to estimate the effects of band gap and minority to majority carrier concentration ratio in the bipolar thermal conductivity suppression. Individual Seebeck coefficient, electrical conductivity, and carrier concentrations from the majority and minority carriers are evaluated while varying chemical potential with a different band gap. It was demonstrated that increasing the band gap and chemical potential increased the individual Seebeck coefficient from minority carriers while decreasing the individual electrical conductivity and concentration from minority carriers. As a result, it was shown that the band gap increase and especially the magnitude of the minority to majority carrier concentration ratio decrease was effective in the bipolar thermal conductivity suppression.

Herein, we report a reproducible improvement of the thermoelectric performance in BiSbTe alloy via kinetically and dynamically regulating excitation behavior in the intrinsic excitation regime. Doping with Pb increases the carrier concentration to the optimal value, kinetically pushing the onset temperature for the intrinsic excitation from 350 K for Bi 0.46 Sb 1.54 Te 3 sample to 450 K for Pb 0.01 (Bi 0.46 Sb 1.54) 0.995 Te 3 sample. Moreover, the refined grains, nanostructures as well as dense dislocations induced by the melt-spinning process not only lower the lattice thermal conductivity via intensifying the interfacial phonon scattering but also selectively block the migration of minority carrier, modifying the dynamic process of the charge carrier and resulting in a smaller ratio of ( μ e / μ h ). This improves the power factor in the measured temperature range and suppresses the bipolar thermal conductivity in the intrinsic excitation regime. Thus, a maximum power factor of 4.44 mW m−1 K−2 and the lowest thermal conductivity of 1.06 W m−1 K−1 are attained for the sample with refined grains and nanostructures. All these contribute to the highest ZT value of 1.31 at 380 K and the highest average ZT value of 1.17 from 300 K to 500 K for Pb 0.002 (Bi 0.46 Sb 1.54) 0.999 Te 3 sample. This improved thermoelectric property was further verified in a thermoelectric module with the conversion efficiency of 4.38 % under a temperature gradient of 175 K, which is 20 % higher than that of the commercial ZM-based module under the same temperature gradient. [ABSTRACT FROM AUTHOR]

Because the intrinsic Ge vacancies in GeTe usually lead to high hole concentration beyond the optimal range, many previous studies tend to consider Ge vacancies as negative effects on increasing the figure of merit ZT of GeTe‐based alloys, and consequently have proposed various approaches to suppress Ge vacancies. However, in this work, it is demonstrated that the Ge vacancies can have great positive effects on enhancing the ZT of GeTe‐based alloys when the hole concentration falls into the optimal range. First, hole concentration of GeTe is reduced close to the optimal range by co‐alloying of Pb and Bi, and then the Ge vacancies are increased by adding excess Te into the Ge0.8Pb0.1Bi0.1Te1+x. The Ge vacancies can cause lattice shrinkage and promote rhombohedral‐to‐cubic phase transition. As revealed by first‐principle calculations, theoretical simulations, and experimental tests, Ge vacancies can facilitate the band convergence, suppress the bipolar transport at higher temperature range, and reduce the lattice thermal conductivity. Combining these effects, a peak ZT of 1.92 at 637 K and an average ZT of 1.34 within 300–773 K in Ge0.8Pb0.1Bi0.1Te1.06 can be obtained, demonstrating the great significance of utilizing vacancy‐type defects for enhancing ZT. [ABSTRACT FROM AUTHOR]

Abstract Cation substitutional doping has been shown to be an effective method to modify both the electronic and thermal transport in p -type (Bi,Sb) 2 Te 3 -based thermoelectric alloys. However, there are not many studies that have attempted a quantitative analysis on the influence of cation substitution on the electronic and thermal properties of n -type Bi 2 (Te,Se) 3 -based alloys. In this work, we report a comprehensive analysis of the influence of substitutional Nb doping on the electrical and thermal conductivity in n -type Cu 0.008 Bi 2 Te 2.7 Se 0.3 alloys. First, we found that Nb doping increases the carrier concentration of both the electrons and holes, whereas the weighted mobility of the electrons and holes is only slightly modified based on a single parabolic band model. As a result, the bipolar thermal conductivity was increased as the Nb was doped. Next, the contribution of point defect scattering by the Nb substitution on the thermal conductivity of the lattice was quantitatively analyzed using a Debye-Callaway model, and it was concluded that the influence of cation substitutional doping in n -type Bi 2 (Te,Se) 3 is as effective as that in p -type (Bi,Sb) 2 Te 3. Highlights • Electronic properties of Nb-doped n -type Cu 0.008 Bi 2 Te 2.7 Se 0.3 alloys was analyzed. • Carrier concentrations of electrons and holes were increased by the Nb doping. • Reduction of lattice thermal conductivities was analyzed based on Callaway model. • Doping in n -type Cu-Bi 2 Te 2.7 Se 0.3 is effective in reducing thermal conductivity. [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]

A unique method taking advantage of a synergistic effect at (LaVO 3)/Cu 0.07 Bi 0.5 Sb 1.5 Te 3 interfaces to boost the thermoelectric figure of merit is demonstrated. The unusual electrical conductivity and energy filtering effect largely help to improve power factor and reduce thermal conductivity of the composites. As a result, a high ZT of 1.26 and ZT avg of 1.13 are successfully achieved. [Display omitted] • Mott insulator LaVO 3 (LVO) was added into Cu 0.07 Bi 0.5 Sb 1.5 Te 3 (BST) to boost the ZT. • LVO/BST interface produces an unusual high σ and enhanced energy filtering effect. • Addition of LVO helps to reduce both the bipolar and lattice thermal conductivity. • A high ZT of 1.26 and ZT avg of 1.13 are successfully achieved. • Mechanical properties of the composites are greatly enhanced by the addition of LVO. The enhancement of thermoelectric (TE) figure of merit, ZT, is of great essence to improve the efficiency of TE devices since the commercial applications of TE devices have been constrained due to low performance. Here, we report a novel approach for ZT improvement via a synergistic effect of unusual conducting behavior and energy filtering at the (LaVO 3)/Cu 0.07 Bi 0.5 Sb 1.5 Te 3 incoherent interfaces. In this work, the incoherent interfaces have been engendered by the incorporation of LaVO 3 (LVO) nanoparticles into Cu 0.07 Bi 0.5 Sb 1.5 Te 3 (BST) matrix, inducing a remarkable enhancement in electrical conductivity due to defect formations and an insulator-to-metal transition. This induced conducting behavior also eminently suppressed the adverse intrinsic excitation thanks to increases in hole density and band turning in composites. Meanwhile, the low-work function in the BST matrix establishes an interface potential with a height of 2.03 eV that efficiently filters low energy carriers, and consequently alleviates the decrement of Seebeck coefficients. Upon LVO addition, not only does the bipolar thermal conductivity decrease but the lattice thermal conductivity approaches the amorphous limit due to enhanced phonon scattering mechanisms at incoherent interfaces. A significant thermoelectric figure of merit, ZT of 1.26 and ZT avg = 1.13 are achieved for the sample with 5 wt% of LVO in BST, which is about 26% and 20% higher than the pristine BST, respectively, thanks to its optimized power factor and low thermal conductivity. Additionally, LVO dispersion strengthens the mechanical properties of the composites, demonstrated in their enhanced hardness and compressive strength. [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]

Here, we report a peak ZT of 1.85 at 873 K for sulfur and Pb codoped polycrystalline SnSe by boosting electrical transport properties while suppressing the lattice thermal conductivity. Compared with single sulfur doped samples, the carrier concentration is improved 1 order of magnitude by Pb incorporation, thereby contributing to improved electrical conductivity and power factor. Moreover, the introduction of sulfur and Pb suppresses the bipolar thermal conductivity by enlarging the band gap. The lattice thermal conductivity significantly decreased as low as 0.13 W m -1 K -1 at 873 K due to the synergic approach involving suppressing bipolar thermal conductivity, large mass fluctuation induced by sulfur incorporation, and nanoprecipitates. We demonstrate that the combination of tailoring band structure, suppressing bipolar thermal conductivity, and introducing large mass fluctuation contributes to the high thermoelectric performance in SnSe. The high performance was achieved through boosting electrical transport properties while maintaining ultralow thermal conductivity. Our findings offer a new strategy for achieving high performance thermoelectric materials.

Solid solutions of Mg 2 Si 0.6 Sn 0.4-x Bi x with 0 ≤ x ≤ 0.03 were prepared by a one-step synthesis and consolidation method, using MgH 2 as a starting material. The thermoelectric properties of these samples were evaluated over the temperature range 300-775 K. Figure of merit, zT, values were determined over this range for all compositions and found to increase with temperature reaching a value of 1.36 at 775 K for samples with x = 0.02. Examination of the components of the total thermal conductivity showed that the bipolar thermal conductivity is suppressed by an increase in the band gap, resulting from solid solution formation, and by low minority carrier mobility. The suppression of the bipolar thermal conductivity is believed to be the consequence of charged grain boundaries.