We explored the effect of K and K-Na substitution for Pb atoms in the lattice of PbTe, in an effort to test a hypothesis for the development of a resonant state that may enhance the thermoelectric power. At 300K the data can adequately be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range 1-15 × 1019 cm-3. A change in scattering mechanism was observed in the temperature dependence of the electrical conductivity, σ, for samples concurrently doped with K and Na which results in significantly enhanced σ at elevated temperatures and hence power factors. Thermal conductivity data provide evidence of a strong interaction between the light- and the heavy-hole valence bands at least up to 500K. Figure of merits as high as 1.3 at 700K were measured as a result of the enhanced power factors.

Metal nanoinclusions inside the bulk thermoelectric matrix have the potential to increase the power factor and reduce the lattice thermal conductivity. We have synthesized Bi2-xTe3+x (x=0, 0.05)compositions, to achieve better tenability in Seebeck and electrical conductivity. In this matrix phase, different volume fractions of Bi metal nanoinclusions were incorporated and its effect on thermoelectric properties is discussed. Ag metal nanoinclusions were incorporated into Bi2Te3(2:3) composition, and its effect on power factor is discussed here.

We have synthesized type-VIII and type-I Ba8Ga16Sn30 clathrates by using different annealing treatments, confirmed with XRD and electron microprobe measurements. NMR lineshape measurements identified a broad resonance corresponding to first-order-shifted satellites. Simulations for the type I structure based on first principles calculations provided an excellent fit to the data, with the best agreement provided by the calculated lowest-energy configuration, having no Ga-Ga bonds. These results allow us to address local configurations within the random type-I alloy, as well as atomic displacements and bond-length distributions, which we compare to experiment.

Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ or decreasing K. MeV ion bombardment caused defects and disorder in the film and the grain boundaries of these nano-scale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We have prepared 100 alternating layers of Si/Si+Ge nanolayered superlattice films using the ion beam assisted deposition (IBAD). The 5 MeV Si ions bombardments have been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the nanolayered superlattice films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. We have characterized the thermoelectric thin films before and after Si ion bombardments as we measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences

Nanocrystalline bulk silicon samples were fabricated using silicon nanoparticles from the gas phase, applying a spark-plasma sintering process. The mean diameter of the crystalline grains after sintering was 30 nm and smaller, the density above 97 % of that of crystalline silicon. Transmission electron microscopy showed a homogenous nanostructure. The thermal conductivity of such an n-type sample with a nominal doping level of 5×1020 cm-3 was around 11 Wm-1K-1 at room temperature. With Seebeck-coefficient α = -150 μV/K and specific conductivity σ = 290 S cm-1, the resulting efficiency ZT is approximately 0.02.

A membranous nanomaterial showing, for the first time, a hybrid thermal behavior between insulating and dissipative regimes is proposed with applications in both thermoelectrics (low thermal conductivity) and passive heat sinking (high thermal conductivity). While other compounds could be chosen, the nanomaterial is made up of a thin Si membrane covered by Ge quantum dots (QDs) with epitaxial facets. The QDs are voluntarily stretched in the direction [010] or y parallel to the membrane to form elongated islands. The broken symmetry induces an exalted phonon wave-guiding in y. Therefore, when hot and cold junctions are connected to the membrane following the stretching direction [010], the anisotropic thermal conductivity shows a significant exaltation with respect to the in-plane orthogonal direction [100] or x, where the Ge islands have the smallest average size. An example nanomaterial is obtained by repetition of molecular supercell slabs containing 4348 atoms each. The thermal conductivity shows a marked exaltation higher than 22 folds, from 1.5 to 33.5 W/m/K when the connection direction between the hot and cold junctions is rotated by 90° from x to y. Therefore, the nanomaterial presents a changing thermal behavior from insulation to passive dissipation when the heat propagation direction is modified from x to y. As a result, it could be used for the design of passive heat sinkers (from the phonons) when the two junctions are connected following [010]. In contrast, a thermal insulating behavior appears when the junctions are linked following [100]. This direction can be as well used for cooling applications. However, in this case, cooling is differently generated using the Peltier effect (from the electrons). Seebeck generation can be also envisioned in the direction [100].

Mg2Si1-xRx (R-Sb,Bi) compositions with x=0.025, 0.05 for Sb and x=0.01,0.025, 0.05 for Bi were synthesized by induction melting of the constituent elements followed by compaction by hot pressing. Phase identification by x-ray diffraction (XRD) indicate a biphasic nature for Bi substituted compositions for x≥.025 while solid solubility for the different Sb substitutions. Abundance distribution plots of the Seebeck (S) coefficient show asymmetry in the Sb compositions and is not observed in the Bi substituted samples. This indicates presence of vacancies only in the Sb substituted compositions. Electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ) values were measured from 300K to 773 K. The observed trends in the absolute values of α and S can be explained based on doping/second phase influence in Bi substitutions and due to effect of vacancies for Sb substitutions. The (κL) (phonon component) values show a significant decrease for the Sb substitutions due to the presence of vacancies. Calculation of the thermoelectric figure of merit (ZT) show a ZTmax of 0.56 for x=0.025 Bi composition compared to a ZTmax of ∼ 0.05 for Mg2Si.

CoSb3 is known as a skutterudite compound that could exhibit high thermoelectric figure of merit. However, the thermal conductivity of CoSb3 is relatively high. In order to enhance the thermoelectric performance of this compound, we tried to reduce the thermal conductivity of CoSb3 by substitution of Rh for Co and by Tl-filling into the voids. The polycrystalline samples of (Co,Rh)Sb3 and Tl-filled CoSb3 were prepared and the thermoelectric properties such as the Seebeck coefficient, electrical resistivity, and thermal conductivity were measured in the temperature range from room temperature to 750 K. The Rh substitution for Co reduced the lattice thermal conductivity, due to the alloy scattering effect. The minimum value of the lattice thermal conductivity was 4 Wm-1K-1 at 750 K obtained for (Co0.7Rh0.3)Sb3. Also the lattice thermal conductivity rapidly decreased with increasing the Tl-filling ratio. T10.25Co4Sb12 exhibited the best ZT values; the maximum ZT was 0.9 obtained at 600 K.