A novel class of hybrid organic thermoelectric materials is demonstrated for the first time for constructing flexible thermoelectric devices on polyimide substrates with high output power by using nanotechnology instead of conducting polymers such as poly(3,4-ethylenedioxythiophene). The hybrid organic thermoelectric materials are composed of nanoparticles of a polymer complex, carbon nanotubes, and poly(vinyl chloride), and show high performance (dimensionless thermoelectric figure-of-merit, ZT ≈ 0.3, based on the thermal conductivity through the film).

Effects of doping on the transport properties of CoSb3 have been systematically investigated using Ni, Pd, and Pt as donor impurities. It is shown that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on these donor impurities. Our theoretical analysis suggests that the electron effective mass and the conduction band deformation potential are significantly affected by both the doping levels and the donor impurities. These doping effects in CoSb3 can be attributed to (1) the changes in the electronic structure with doping and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band which can be explained in terms of a two-band Kane model. The observed changes in the electronic properties with doping are also consistent with the predictions of a recent band structure calculation of CoSb3. On the other hand, the lattice thermal conductivity decreases markedly with increasing carrier concentration, and is almost independent of the donor impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3. The effects of doping on the phonon scattering are also discussed on the basis of a model calculation as a function of the electronic properties and the impurity properties (atomic mass and size). As a result, it is found that the strength of the electron-lattice interaction (the electron-phonon coupling), which is closely related to the effective mass and the deformation potential, is an important factor affecting the scattering of phonons as well as charge carriers in heavily doped n-type CoSb3.

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We have prepared polycrystalline n-type Ba 8 Cu x Ga y Ge 46ÀxÀy (x ¼ 0{5, y ¼ 16 À 3x) clathrate compounds by arc melting and spark plasma sintering techniques and investigated the effect of Cu substitution for Ge on their thermoelectric properties. The Hall carrier concentration for Cu-substituted compounds is almost constant at the order of 10 20 cm À3 , which is comparable to that for Ba 8 Ga 16 Ge 30 stoichiometric compounds. The Seebeck coefficient for these compounds is comparable to that for n-Ba 8 Ga y Ge 46Ày compounds. From the analysis of the properties, the effective mass of the conduction band is estimated to be about 1.4 m 0 , which is equivalent to or slightly smaller than that of n-Ba 8 Ga y Ge 46Ày compounds. The Hall mobility increases as the Cu composition increases. Its temperature dependence obeys approximately T À1=2 dependence in the range of 80-300 K, indicating the dominance of the alloy disorder scattering. Two models are discussed to account for the reduction in the alloy disorder scattering by Cu substitution.

The structural and electronic transport properties of polycrystalline p-type CoSb3 with different grain sizes (about 3 and 3×102 μm) were investigated. The magnetic susceptibility was also measured. Samples were characterized by x-ray diffractometry, electron-probe microanalysis, and optical microscope observation. Samples were found to be stoichiometric and homogeneous. The Hall carrier concentration of the samples is of the order of 1018 cm−3 and weakly dependent on the temperature. The temperature dependence of the Hall mobility suggests that the predominant scattering mechanism drastically changes depending on grain size: for large grain size a combination of the neutral impurity scattering and the acoustic phonon scattering, and for small grain size the ionized impurity scattering. The magnetic susceptibility was found to be essentially diamagnetic independently of grain size, and to vary slightly with temperature. The weak temperature dependence of the susceptibility can be explained by taking into account the three contributions of ion cores, conduction electrons, and trace amounts of magnetic impurities. From the analysis of the susceptibility due to conduction electrons, the band gap energy was determined to be about 70–80 meV, consistent with a recent band structure calculation. Although the effects of nonmagnetic impurity phases segregated (Sb, etc.) on the scattering mechanism are not clear, the grain size is one of the key factors determining the transport properties of polycrystalline CoSb3.

The valence-band structure of CoAs 3 , CoSb 3 , and RhSb 3 with a skutterudite-type crystal structure has been investigated by x-ray photoelectron spectroscopy. The photoemission spectra are compared with recent density-of-states calculations. Our photoemission spectra results and theoretical results are in good agreement for the energy positions in the metal d states and the pnicogen p states, but relatively large differences are found for the positions in the pnicogen s states. Based on our photoemission spectra, the electronic bonding states and the chemical trends are explained qualitatively in terms of a simple tight-binding model. The double localized and itinerant nature of the metal d states is also discussed in relation to the properties of the skutterudites. The metal d-derived density of states feature is clearly observed at about 1.2-, 1.4-, and 2.4-eV binding energies for CoSb 3 , CoAs 3 , and RhSb 3 , respectively. From the point of view of the crystal-field effects, it can be considered that this d-character band originates predominantly from the d orbitals with T 2g symmetry, while d orbitals with E g symmetry hybridize strongly with the p orbitals forming the conduction band. Since the t 2g states are considered to be almost completely filled, corresponding to the zero spin Sϭ0 state ( 1 A 1 , t 2g 6 ), most of skutterudites exhibit diamagnetic properties. On the other hand, the slight chemical shifts of the core levels as compared with the pure elements indicate a small charge transfer from metal to pnicogen atoms in the skutterudites, leading to hybridization between metal d states and pnicogen p states. p-d hybridization causes not only a substantial screening of atomic Coulomb interactions at metal sites, but also a strong covalent bonding in these materials. Concerning a particular point of the band structure in skutterudites, our photoemission spectra near the valence-band edge show clear experimental evidence of a small density of states around the Fermi level due to a single band crossing the pseudogap.

Polycrystalline samples of Sr8GaxGe46−x clathrates with varied nominal Ga contents were prepared by powder metallurgy to produce n-type samples with carrier concentration n of 1.8×1020–1.1×1021cm−3. Their thermoelectric properties were measured in the temperature range from 300to900K. Their Seebeck coefficient and electrical conductivity were found to be typical for a heavily doped semiconductor, and varied systematically with their carrier concentration. The maximum value of the figure-of-merit ZT was 0.62 at 800K for n=5.6×1020cm−3. Other following properties were also measured or estimated, and their relationships with the thermoelectric properties are discussed. The band gap of the samples was estimated to be 0.4–0.5eV; their effective mass was an almost constant value of 3.1me at room temperature, and this value decreases with increasing temperature; and their electron Hall mobilities were 8–13cm2V−1s−1 at room temperature, increasing slightly with decreasing carrier concentration.

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