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Abstract: A series of LaFeAsO1−x F x (x =0–0.225) oxyarsenides have been synthesized by a solid-state reaction method in order to optimize electrical transport properties through appropriate F doping. Both electrical resistivity and Seebeck coefficient of undoped LaFeAsO show an anomaly at about 150K, which is related to a structural phase and/or spin-density-wave (SDW) transition. Seebeck coefficient seems to be determined by two competitive factors: it is enhanced by suppressing the structural phase and/or SDW transition, and reduced by increasing carrier concentration. Seebeck coefficient is significantly enhanced just after suppressing the anomaly, and the maximum Seebeck coefficient reached −142μV/K for the sample with F doping x =0.075 from −58μV/K for undoped LaFeAsO, and then decreased with further increasing carrier concentration through F doping. Meanwhile, the electrical resistivity is decreased with increasing F doping, resulting in a maximum power factor value of 1.2mW/mK2 at 80K for polycrystalline LaFeAsO0.85F0.15 sample, this value is the same order as that of the best low temperature thermoelectric Bi88Sb12 compounds, and could be significantly higher in single crystals. [ABSTRACT FROM AUTHOR]

Tin sulfide (SnS), a typical IV–VI compound with low-cost, abundant-earth and eco-friendly elements, has aroused widespread attentions in the thermoelectric community due to its similar electron and phonon structures with SnSe. However, undoped SnS possesses the features of low carrier concentration and inferior band structures, which produces indecent thermoelectric performance. In this work, we successfully resolved these shortcomings of SnS through stepwise Na doping and Se alloying. Firstly, the carrier concentration of SnS was increased and optimized through Na doping, which leads to an enhancement both in electrical conductivity and Seebeck coefficients through activating multiple valance bands. Secondly, the electronic band structure of SnS was modified through Se alloying, both narrowing band gap and flatting valence band shape contribute excellent electrical transport properties, resulting in maximum power factor ∼6.0 μWcm−1K−2. After synergistically optimizing interdependent thermoelectric parameters through Na doping and Se alloying, a record high ZT of 0.70 at 873 K was obtained in polycrystalline SnS. Our work indicates that SnS is one of very promising earth-abundant thermoelectric materials for power generation in mediate-temperature range. • Power factor of p -type SnS was improved from 2.5 μWcm−1K−2 to 4.8 μWcm−1K−2 at 923 K through Na doping. • Power factor of p -type SnS was improved from 4.8 μWcm−1K−2 to 6.0 μWcm−1K−2 at 873 K through Se alloying. • ZT max of p -type SnS is distinctly enhanced from 0.25 to 0.70 at 873 K after Na doping and Se alloying. [ABSTRACT FROM AUTHOR]

Abstract In this work, we reported the thermoelectric performance of Sb doped n -type PbS can be improved through introducing different types of Cu inclusions (counter-doping, extra-adding and both of them). Firstly, Cu was selected to substitute Pb sites as counter-doping to tune the carrier density, resulting in maintaining power factor but a decreasing total thermal conductivity, thus in a maximum ZT (ZT max) ∼ 0.96 at 923 K in Pb 0.975 Sb 0.02 Cu 0.005 S. Then, we introduced extra Cu in n -type PbS to form interstitials to further reduce the lattice thermal conductivity to ∼0.77 W m − 1 K − 1 , and the ZT max was further enhanced to ∼1.06 at 923 K in Pb 0.98 Sb 0.02 S-0.02Cu. Finally, we combined both types of Cu (counter-doping and extra-adding) into n -type PbS, the power factor experiences slight reduction meanwhile the minimum lattice thermal conductivity can be reduced as low as ∼ 0.57 W m − 1 K − 1 at 923 K. Combining excellent power factor and dramatically reduced lattice thermal conductivity, we obtained a ZT max ∼1.23 at 923 K and the ZT ave ∼0.62 at 300–923 K in Pb 0.95 Sb 0.02 Cu 0.03 S-0.03Cu. This work provides a new idea to improve the thermoelectric performance in n -type PbS, indicating that PbS is one robust alternative of PbTe. Highlights • κ lat of n -type PbS is reduced from ∼ 1.33 Wm−1K−1 to ∼ 0.57 Wm−1K−1 at 923 K. • ZT max of n -type PbS is distinctly enhanced from 0.20 to 1.23 at 923 K. • ZT ave of n -type PbS is enhanced from 0.47 to 0.62 at 300–923 K. [ABSTRACT FROM AUTHOR]

Abstract In this work, we report that the high performance of SnTe could be realized by enhancing electrical transport properties (power factor) and reducing thermal conductivity through stepwisely introducing elements of Sb, In and Se. We found that Sb and In co-doping can enhance Seebeck coefficients in the broad temperature range via optimizing carrier density and introducing resonant state, respectively. Moreover, nanostructures could be formed when the Sb content beyond the solubility limit in SnTe matrix, which coupled with point defects from In and Se alloying result in a very low (lattice) thermal conductivity through the enhanced phonon scattering from defects and grain boundaries. A peak ZT ∼1.0 at 923 K can be achieved with the enhanced power factor and reduced thermal conductivity. Accordingly, the average ZT value increases from ∼0.20 for undoped SnTe to ∼ 0.60 for multiple-doped SnTe (Sn 0.823 Sb 0.17 In 0.007 Te 0.98 Se 0.02) over 300–923 K, generating a high calculated conversion efficiency ∼ 11%. Present results indicate the high performance of SnTe can be obtained through stepwisely optimizing strategy in both electrical and thermal transport properties. Highlights • A stepwisely enhancing in thermoelectric transport properties were achieved in p -type SnTe. • Power factor of p -type SnTe was enhanced from ∼3.4 μW cm−1 K−2 to ∼ 11.7 μW cm−1 K−2 at 300 K through Sb and In co-doping. • κ lat of p -type SnTe was reduced from ∼3.4 Wm−1K−1 to ∼ 1.4 Wm−1K−1 at 300 K through Sb, In co-doping, and Se alloying. • ZT ave of p -type SnTe is distinctly enhanced from ∼0.2 to ∼0.6 over 300–923 K. [ABSTRACT FROM AUTHOR]

Te-free PbS has been regarded as a promising alternative candidate for PbTe thermoelectric materials due to its advances of low-cost, earth-abundant. In this work, we report an enhanced thermoelectric performance in n -type PbS system arising from synergistically optimized carrier and phonon transport properties. The electrical transport properties are optimized by tuning carrier concentrations via Sb doping, resulting in a maximum power factor of ∼16.7 μWcm −1 K −2  at 623 K, which is approximately tenfold higher than that of undoped PbS (∼1.8 μWcm −1 K −2 ). PbSe and PbTe co-alloying in PbS was carried out to reduce lattice thermal conductivity through introducing point defects scattering and second phase, resulting in a minimum lattice thermal conductivity ∼0.71 Wm −1 K −1  at 923 K. Combined the enhanced power factor with suppressed thermal conductivity, a maximum ZT value ∼1.0 was obtained in n -type (PbS) 0.53 (PbSe) 0.25 (PbTe) 0.2 Sb 0.02 at 923 K. [ABSTRACT FROM AUTHOR]

SnTe, one lead-free analogue of PbTe, has been paid attention in thermoelectric community because it has the similar rock-salt crystal structure and electronic band structure. However, SnTe possesses a very high total thermal conductivity (∼10.0 W m −1 K −1 at room temperature) and a mediocre ZT (∼0.2 at 723 K). In this study, to reduce its thermal conductivity, we prepare SnTe samples by melting reaction followed by mechanical alloying and spark plasma sintering. We demonstrate that both total thermal conductivity and lattice thermal conductivity can be considerably reduced through reducing particle size from micro-scale to nano-scale via facially mechanical alloying method. Experimental results show that the total thermal conductivity at room temperature can be reduced from ∼10 W m −1 K −1 for SnTe ingot to ∼3Wm −1 K −1 for SnTe with mechanical alloying 25 h. Correspondingly, the lattice thermal conductivity at room temperature was reduced from ∼5Wm −1 K −1 to ∼2Wm −1 K −1 . Combination of reduced thermal conductivity and moderate power factor, average ZT ave can be increased by 33.3%, from 0.09 for SnTe ingot to 0.12 for SnTe with nano-grain size. [ABSTRACT FROM AUTHOR]

Abstract: We report on the effect of Mg doping on the thermoelectric properties of p-type BiCuSeO oxyselenide, with layer structure composed of conductive (Cu2Se2)2− layers alternately stacked with insulating (Bi2O2)2+ layers. The substitution of Bi3+ by Mg2+ leads to an enhancement of the electrical conductivity and a decrease of the thermal conductivity. Coupled to high Seebeck coefficients, ZT at 923K is increased from 0.45 for pristine BiCuSeO to 0.67 for Bi0.95Mg0.05CuSeO. However, the efficiency of Mg doping in the insulating (Bi2O2)2+ layer is low, and this doping only leads to a limited increase of the hole carriers concentration. Therefore Mg doped BiCuSeO has relatively low electrical conductivity which makes its thermoelectric figure of merit much lower than that of Ca, Sr and Ba doped BiCuSeO. [Copyright &y& Elsevier]

BiSbSe 3 is a promising medium-temperature thermoelectric material on account of its intrinsically low thermal conductivity. To further enhance the thermoelectric performance in n -type BiSbSe 3 , we conduct Br doping at Se sites to optimize its carrier concentration and electrical conductivity, finally a maximum ZT of ∼0.8 in nanostructured BiSb(Se 0.94 Br 0.06) 3 is obtained at 700 K through mechanical alloying. Based on the optimal BiSb(Se 0.94 Br 0.06) 3 composition, we perform percolation effect with mixed grain sizes in nanoscale and microscale to enhance carrier mobility. It is found that carrier mobility is favorably improved by the addition of microscale grains (∼30 μm) in nanostructured BiSb(Se 0.94 Br 0.06) 3 matrix due to the reduced grain boundaries to carrier scattering, which benefits high electrical conductivity and power factor. Simultaneously, the BiSb(Se 0.94 Br 0.06) 3 matrix with mixed grain sizes still maintains very low thermal conductivity due to its intrinsically strong lattice anharmonicity. Owing to the optimized electrical properties and maintained low thermal conductivity, we obtain a maximum ZT value of ∼1.0 at 700 K in BiSb(Se 0.94 Br 0.06) 3 –4C matrix with 40% microscale grains. This work provides a feasible method to optimize thermoelectric performance by designing microstructure with percolation effect. • Carrier concentration was improved to ∼1.91 × 1020 cm−3 in n -type BiSbSe 3 by Br doping. • PF max of n -type BiSbSe 3 was significantly enhanced to ∼6.0 μW cm−1 K−2 at 700 K through improving carrier mobility via utilizing percolation. • ZT max of n -type BiSbSe 3 was enhanced from ∼0.1 to ∼1.0 at 700K. [ABSTRACT FROM AUTHOR]

The n -type PbTe still maintains relatively low thermoelectric performance compared with its p -type counterpart. Thus, to promote its performance in n -type PbTe, we conduct successive strategies that firstly suppressing the lattice thermal conductivity (κ lat) through alloying with ternary compound AgSbSe 2 , and then optimizing carrier density with Sb, Bi or I doping. In AgSbSe 2 -alloyed PbTe samples, the κ lat can be evidently reduced, and the minimum κ lat decreases from ∼1.1 Wm−1K−1 in undoped PbTe to ∼0.8 Wm−1K−1 in PbTe–5%AgSbSe 2. Based on PbTe–5%AgSbSe 2 matrix, several dopants (Sb, Bi or I) are used to optimize its carrier density, and found that I element is verified to be the most effective dopant to enhance its electrical transport properties. In I-doped PbTe–5%AgSbSe 2 samples, the carrier density raises from ∼2.49 × 1016 cm−3 in PbTe–5%AgSbSe 2 to ∼1.15 × 1019 cm−3 in PbTe 0.992 I 0.008 -5%AgSbSe 2 , which contributes to the maximum power factor ∼14.8 μWcm−1K−2. Combining the reduced lattice thermal conductivity and improved power factor, the maximum ZT max and average ZT ave values in PbTe 0.992 I 0.008 -5%AgSbSe 2 are boosted to ∼1.0 at 723 K and ∼0.77 at 300–823 K, respectively. This work points out a valid avenue to improve thermoelectric transport property in n -type PbTe with separately successive strategies to adjust its thermal and electrical performance. • κ min ∼0.8 Wm−1K−1 was achieved in n -type PbTe through introducing AgSbSe 2. • PF max of n -type PbTe was significantly enhanced from ∼3.4 μWcm−1K−2 to ∼14.8 μWcm−1K−2 by I doping. • ZT max of n -type PbTe was enhanced from ∼0.45 to ∼1.00. [ABSTRACT FROM AUTHOR]

In this work, the thermoelectric performance of p -type PbS was boosted remarkably through Na doping and introducing Cu 2 S. Firstly, the electrical transport properties (power factor) of p -type PbS was optimized via Na doping, and the maximum ZT ∼ 0.67 was achieved in Pb 0.98 Na 0.02 S. Secondly, the total thermal conductivity of Pb 0.98 Na 0.02 S was suppressed through introducing Cu 2 S by means of significantly reducing the electronic thermal conductivity by 80% at 823 K through Cu counter-doping, meanwhile, the power factor of Pb 0.98 Na 0.02 S was further enhanced through optimizing carrier concentrations via introducing Cu. The combination of decreasing total thermal conductivities and the optimizing power factors leads to peak ZT ∼1.2 at 823 K in p -type Pb 0.98 Na 0.02 S–2%Cu 2 S. Present results illustrate that introducing Cu 2 S is an effective method to enhance the thermoelectric performance of p -type PbS through decreasing the electronic thermal conductivity which also has great potential in other thermoelectric materials system. • Thermoelectric performance of p -type PbS was remarkably boosted by reducing electronic thermal conductivity κ ele. • κ ele of p -type Pb 0.98 Na 0.02 S was reduced by 80% from ∼0.53 W m−1K−1 to ∼ 0.11 W m−1K−1 at 823 K with introducing Cu 2 S. • The maximum ZT ∼1.2 at 823 K was realized in p -type Pb 0.98 Na 0.02 S–2%Cu 2 S. [ABSTRACT FROM AUTHOR]

PbTe-based thermoelectric materials are considered to be a medium-temperature thermoelectric candidates and the high-performance n -type PbTe is urgently to be developed to match its p -type counterpart on account of practical application. Herein, we firstly optimized the carrier density of n -type PbTe through Sb doping, and obtained a maximum ZT value of ∼1.0. Then, based on the optimized carrier density, we introduced ternary compounds AgSb(Se, Te) 2 to suppress the lattice thermal conductivity. Minimum lattice thermal conductivity of n -type PbTe was reduced as low as ∼0.53 W m−1K−1 and ∼0.62 Wm−1K−1 through introducing AgSbSe 2 and AgSbTe 2 , respectively. Combining the maintained electrical properties with suppressed thermal conductivity, maximum ZT values ∼1.2 and ∼1.3 were obtained in PbTe+2%AgSbSe 2 and PbTe+2%AgSbTe 2 , respectively. This work provides an effective strategy of synergistically balancing charge and phonon transports in n -type PbTe with ternary AgSb(Se, Te) 2 compounds. • κ min of n -type PbTe was reduced to ∼ (0.53–0.62) Wm−1K−1 through introducing AgSb(Se, Te) 2. • Power factor ∼20.0 μWcm−1K−2 was realized in n -type PbTe through maintaining carrier mobility. • ZT max of n -type PbTe was enhanced from ∼1.0 to ∼ (1.17–1.25). [ABSTRACT FROM AUTHOR]