From an environmental perspective, lead-free SnTe would be preferable for solid-state waste heat recovery if its thermoelectric figure-of-merit could be brought close to that of the leadcontaining chalcogenides. In this work, we studied the thermoelectric properties of nanostructured SnTe with different dopants, and found indium-doped SnTe showed extraordinarily large Seebeck coefficients that cannot be explained properly by the conventional two-valence band model. We attributed this enhancement of Seebeck coefficients to resonant levels created by the indium impurities inside the valence band, supported by the firstprinciples simulations. This, together with the lower thermal conductivity resulting from the decreased grain size by ball milling and hot pressing, improved both the peak and average nondimensional figure-of-merit (ZT) significantly. A peak ZT of ∼1.1 was obtained in 0.25 atom % In-doped SnTe at about 873 K.

Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si(80)Ge(20))(70)(Si(100)B(5))(30) by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si(70)Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.

We present detailed studies of potassium doping in PbTe(1-y)Se(y) (y = 0, 0.15, 0.25, 0.75, 0.85, 0.95, and 1). It was found that Se increases the doping concentration of K in PbTe as a result of the balance of electronegativity and also lowers the lattice thermal conductivity because of the increased number of point defects. Tuning the composition and carrier concentration to increase the density of states around the Fermi level results in higher Seebeck coefficients for the two valence bands of PbTe(1-y)Se(y). Peak thermoelectric figure of merit (ZT) values of ~1.6 and ~1.7 were obtained for Te-rich K(0.02)Pb(0.98)Te(0.75)Se(0.25) at 773 K and Se-rich K(0.02)Pb(0.98)Te(0.15)Se(0.85) at 873 K, respectively. However, the average ZT was higher in Te-rich compositions than in Se-rich compositions, with the best found in K(0.02)Pb(0.98)Te(0.75)Se(0.25). Such a result is due to the improved electron transport afforded by heavy K doping with the assistance of Se.

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10Bismuth telluride (Bi 2 Te 3 ) and its alloys have been widely investigated as thermoelectric materials for cooling applications at around room temperature. We report a systematic study on many compounds in the Bi 2 Te 3 -Bi 2 Se 3 -Bi 2 S 3 system. All the samples were fabricated by high energy ball milling followed with hot pressing. Among the investigated compounds, Bi 2 Te 2 S 1 shows a peak ZT ~0.8 at 300 o C 15 and Bi 2 Se 1 S 2 ~0.8 at 500 o C. These results show that these compounds can be used for mid-temperature power generation applications. The leg efficiency of thermoelectric conversion for segmented elements based on these n-type materials could potentially reach 12.5% with cold side at 25 o C and hot side at 500 o C if appropriate p-type legs are paired, which could compete well with the state-of-the-art ntype materials within the same temperature range, including lead tellurides, lead selenides, lead sulfides, filled-skutterudites, and half Heuslers. 20 Broader ContextThermoelectric convertor has provided a new class of green energy from solar heat, terrestrial heat, waste heat from both automobile vehicles and industrial operations. Bi 2 Te 3 -based materials have distinguished themself in low-temperature power generation applications. For these applications, the hot side temperature is typically limited to less than 250 o C due to the declining ZT value. For the mid-25 temperature range, PbTe and skutterudite materials were being considered as the candidates. However, the toxicity or thermal stability issue is still the most worrying part for these materials. In this work, we proposed an alternative way by using a segmented leg made from Bi 2 (Te, Se, S) 3 -based materials, which shows a potential leg efficiency of 12.5% with cold side of 25 o C and hot side of 500 o C. It competes well with the state-of-the-art n-type materials within the same temperature range. Specifically, two new compounds, i.e., Bi 2 Te 2 S 1 and Bi 2 Se 1 S 2 , have been identified as the promising materials for the mid-temperature applications.