High thermoelectric performance by resonant dopant indium in nanostructured SnTe.

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 lead-containing 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 first-principles 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.

Figure-of-merit enhancement in nanostructured FeSb(2-x)Ag(x) with Ag(1-y)Sb(y) nanoinclusions.

We present the figure-of-merit (ZT) improvement in nanostructured FeSb(2-x)Ag(x) with Ag(1-y)Sb(y) nanoinclusions through a metal/semiconductor interface engineering approach. Owing to the interfaces between FeSb(2-x)Ag(x) and Ag(1-y)Sb(y) phases, as well as the identical work functions, both thermal conductivity and electrical resistivity of the nanocomposites were significantly reduced in the lower temperature regime compared with pure FeSb(2). Overall, an improvement of 70% in ZT was achieved for the optimized nanocomposite FeSb(1.975)Ag(0.025)/Ag(0.77)Sb(0.23) sample, in which Ag(0.77)Sb(0.23) is about 10% by molar ratio. The results of this approach clearly demonstrated the metal/semiconductor interface concept and confirmed the potential of strongly correlated material systems as promising thermoelectric materials.

Study of the thermoelectric properties of lead selenide doped with boron, gallium, indium, or thallium.

Group IIIA elements (B, Ga, In, and Tl) have been doped into PbSe for enhancement of thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity were systematically studied. Room-temperature Hall measurements showed an effective increase in the electron concentration upon both Ga and In doping and the hole concentration upon Tl doping to ~7 × 10(19) cm(-3). No resonant doping phenomenon was observed when PbSe was doped with B, Ga, or In. The highest room-temperature power factor ~2.5 × 10(-3) W m(-1) K(-2) was obtained for PbSe doped with 2 atom % B. However, the power factor in B-doped samples decreased with increasing temperature, opposite to the trend for the other dopants. A figure of merit (ZT) of ~1.2 at ~873 K was achieved in PbSe doped with 0.5 atom % Ga or In. With Tl doping, modification of the band structure around the Fermi level helped to increase the Seebeck coefficient, and the lattice thermal conductivity decreased, probably as a result of effective phonon scattering by both the heavy Tl(3+) ions and the increased grain boundary density after ball milling. The highest p-type ZT value was ~1.0 at ~723 K.

Heavy doping and band engineering by potassium to improve the thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe(1-y)Se(y).

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.

Enhancement of thermoelectric properties by modulation-doping in silicon germanium alloy nanocomposites.

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.