We present first-principles calculation for the behavior of antimony telluride under stress. We focus on the calculation of transport properties for pure Sb2Te3 and the investigation of stress-induced defects. Our transport calculations were done within first-principles using the linearized-augmented plane-wave method, where we first calculated the transport distribution for several stress conditions. Next, using these stress dependent transport distributions, we derived the stress dependence of the electrical conductivity, the Seebeck coefficient, and the power factor. Our calculations for the stress-induced defects were done utilizing a pseu-dopotential method and confining ourselves to the antisite defect. In all cases, our results are in good agreement with experimentally obtained data. Furthermore, we found that hydrostatic pressure does not improve the power factor, while, on the other hand, a large increase under uniaxial stress can be observed. Also, both hydrostatic pressure and uniaxial stress are found to lower the formation energies of the antisite defects, suggesting a structural transition at high pressures.

Bismuth nanowires 200 nm in diameter were synthesized via electrochemical deposition into the pores of anodic alumina templates. A near neutral pH solution and a special sample holder were employed. Both polycrystalline and (012) oriented bismuth nanowire arrays were prepared. The electrical resistance of the samples versus temperature was measured in the range 2–300 K, and the results were fitted to a transport model. Despite the crystallographic alignment of the nanowire array, the model calculations suggest the dominance of a temperature independent scattering mechanism (such as grain boundary or nanowire boundary scattering). The results are compared to the electrical resistance of nanowires formed by impregnation techniques.

Thermoelectric performance of polycrystalline materials is greatly influenced by their microstructures including grain sizes, grain boundaries, grain orientations in anisotropic compounds, etc. The material microstructures are sensitive to the preparation processes and the starting materials. In the present study, n-type and p-type Bi2Te3-based sintered materials with highly preferred grain orientations have been fabricated through a spark plasma sintering (SPS) technique, by controlling the particle sizes of the starting powder and other sintering process parameters. The obtained textured Bi2Te3-based materials show a high mechanical strength as 80MPa in bending strength, which is 7 to 8 times as that of the melted ingot materials, and a significant anisotropy in thermoelectric transport properties. The optimal figure of merit (ZT) of the sintered materials in the direction perpendicular to the pressing direction (with c-axis preferred orientation) is comparable to that of the zone-melted ingots in the same crystallographic orientation.

The fabrication and characterization of SiGeC cantilever microcoolers are described. Silicon on insulator (SOI) was used as the substrate, and two layers of 3 μm p-SiGe0.07C0.0075 and 1.14 μm n-SiGe0.07C0.0075 lattice matched to silicon were grown using molecular beam epitaxy. The uni couple cooler was fabricated using conventional integrated circuit (IC) processing, and the cantilever structure was finally formed by removing the backside Si of SOI substrate by deep reactive ion etching. Devices with different n- and p-side length ratios were characterized. Cooling by 1.2K has been measured at room temperature. Modeling showed that the device performance was dominated by the smaller cooling temperature of the p-SiGeC leg of the cantilever structure. Parasitic heat conduction through the Si buffer layer is the main limitation to the device performance.

We propose the use of hetero-structured high ZT materials in thermally isolated thermoelectric couples for the purpose of improving overall device efficiency. Use of high ZT material incorporated in convective flow devices will yield a two-fold improvement in COP over isothermal TE devices. The analytical approach presented in this paper requires optimization of TE element aspect ratio according to available ΔT at location in the device. We discuss the limitations on thermoelectric element characteristics and geometry to achieve high COP. We also utilize moderate temperature differentials to meet the target of high COP. If space requirements are restrictive, or materials processing is limited, the materials described herein provide the unique ability to alter Seebeck coefficient, thermal conductivity, and electrical such that the required aspect ratio is suitable. Improvement in Seebeck coefficient and electrical resistivity independently increase COP by as much as 25% or more when applied to thermally isolated device considered.

AgPb10SbTe12 is one member of the cubic family of materials AgPbmSbTem+2, which adopts NaCl structure where Ag, Pb and Sb atoms occupy the Na site and Te atoms occupy the Cl site. Ingots of this compound were prepared by a solid state reaction for thermoelectric measurements. AgPb10SbTe12 is a narrow band gap semiconductor with Eg∼0.26 eV. In order to optimize the ZT of this member, compositions with deficiency of Ag and Bi-substitution were examined and found to exhibit enhanced power factor at 300 K. The Bi-substituted ingot had ZT∼0.39 at 300 K and ZT∼0.68 at 400 K. Carrier concentration and the mobility measurements are reported.

TiS2 belongs to a family of layered compounds that displays promise as a thermoelectric material. At room temperature the thermopower (a) of TiS2 displays an n-type behavior, with a magnitude of ≈ -200 μV/K. The electrical resistivity (ρ), is on the order of 1 mΩ-cm at room temperature and displays a “metallic-like” behavior with dR/dT > 0 from 300 K to 10 K. Thus, these compounds exhibit relatively large power factors (PF = α2/ρ) with a PF ∼30 μW/K2 cm at T = 300 K, which are comparable to the state-of-the art Bi2Te3 type materials, which have a PF ∼40 μW/K2, at T = 330K. These values suggest that further investigations of these systems could be profitable. Thin plate-like crystals of TiS2 are grown by the iodine vapor transport method with planar dimensions of 1 cm and thicknesses of 20 μm or more. In this synthetic approach some dopants can be integrated into the parent compound, effectively providing a route for the tuning of electronic properties. We present here some effects of elemental doping on the electronic properties in these TiS2 based materials.

Since the quite favorable thermoelectric properties of transition-metal oxide NaCo2O4 were first reported by Terasaki in 1997, extensive research work has been conducted, including the efforts to improve TE properties through doping or new synthesis approaches. In addition, theoretical investigations about the enhanced thermopower coupled with the small resistivity values for the metallic NaxCo2O4 have been investigated. The advent of the large thermopower and low resistivity appears not able to be explained via conventional free-electron theory. In this paper, thermoelectric and magnetic properties, including resistivity, thermopower, thermal conductivity, magnetic susceptibility and moment of single crystals and polycrystalline NaxCo2O4 are reported. The effect of Na concentration on the transport properties will also be discussed.

The successful fabrication of a nanocomposite in bulk form consisting of a randomly oriented assembly of nanoscale sized core-shell particles has required an increased understanding of the theoretical aspects of the thermoelectric transport properties. Our particular nanocomposite consists of a nanorod core in 1D quantum confinement coated with an outer conducting polymer shell. Upon fabrication of the bulk composite, these nanorods are embedded within a three-dimensional conducting polymer matrix. We address the nanorod thermoelectric components by assuming a single parabolic band within the one-dimension density of states. Both elastic and inelastic scattering of electrons or holes is accommodated by solving the variational form of the Boltzmann transport equation. An altered lattice thermal conductivity for the confined nanorods is calculated separately to account for boundary scattering that is important in low dimension structures. Exact expressions for the bulk effective electrical and thermal conductivities, Seebeck coefficient and figure of merit are then obtained through the field decoupling transformation, which is a special case of two-component composites. Our method is easily generalized to any two component composite of spherical or cylindrical microstructure and is independent of the materials bulk geometry. The results similarly apply to one, two or three dimensional transport regimes with or without quantum confinement merely by solving the appropriate fundamental transport equations for each constituent material. Comparison to experimental data is then presented which helps validate the nanocomposite theory.

Electronic structure calculations were performed on PrFe4P12 filled skutterudite, in order to provide more information regarding its properties. The energy bands yielded information regarding the metallic behavior as well as a mini gap of 0.02 eV. Total and projected Density of States (DOS) yielded information for the existence of a hybridization between Pr -f, Fe -d and P -p orbitals. Hence, this compound could be considered a likely candidate as a thermoelectric compound.

Complex quaternary chalcogenides (AgSb)xPbn-2xTen (0<x<n/2) are thought to be narrow band-gap semiconductors which are very good candidates for room and high temperature thermoelectric applications. These systems form in the rock-salt structure similar to the well known two component system PbTe (x=0). In these systems Ag and Sb occupy Pb sites randomly although there is some evidence of short-range order. To gain insights into the electronic structure of these compounds, we have performed electronic structure calculations in AgSbTe2 (x=n/2). These calculations were carried out within ab initio density functional theory (DFT) using full potential linearized augmented plane wave (LAPW) method. The generalized gradient approximation (GGA) was used to treat the exchange and correlation potential. Spinorbit interaction (SOI) was incorporated using a second variational procedure. Since it is difficult to treat disorder in ab initio calculations, we have used several ordered structures for AgSbTe2. All these structures show semimetallic behavior with a pseudogap near the Fermi energy. Te and Sb p orbitals, which are close in energy, hybridize rather strongly indicating a covalent interaction between Te and Sb atoms.

We present experimental and theoretical characterization of InP-based heterostructure integrated thermionic (HIT) coolers. In particular, the effect of doping on overall device performance is characterized. Several thin-film cooler devices have been fabricated and analyzed. The coolers consist of a 1μm thick superlattice structure composed of 25 periods of InGaAs well and InGaAsP (λgap ≈ 1.3μm) barrier layers 10 and 30nm thick, respectively. The superlattice is surrounded by highly-doped InGaAs layers that serve as the cathode and anode. All layers are lattice-matched to the n-type InP substrate. N-type doping of the well layers varies from 1.5×1018cm−3 to 8×1018cm−3 between devices, while the barrier layers are undoped. Device cooling performance was measured at room-temperature. Device current-versus-voltage relationships were measured from 45K to room-temperature. Detailed models of electron transport in superlattice structures were used to simulate device performance. Experimental results indicate that low-temperature electron transport is a strong function of well layer doping and that maximum cooling will decrease as this doping is increased. Theoretical models of both I-V curves and maximum cooling agree well with experimental results. The findings indicate that low-temperature electron transport is useful to characterize potential barriers and energy filtering in HIT coolers.

The dependences of the thermoelectric properties of (001)KCl/PbTe/SnTe/PbTe three-layer structures on the SnTe layer thickness (dSnTe = 0.5–6.0 nm) at a fixed thickness of PbTe layers were studied. It was established that the thickness dependences of the Seebeck coefficient, the Hall coefficient, electrical conductivity, charge carrier mobility, and the thermoelectric power factor are distinctly non-monotonic. Two possible reasons for this non-monotonic behavior of the thickness dependences of the thermoelectric properties are considered: the size quantization of the energy spectrum in a SnTe quantum well and / or the formation of edge misfit dislocations at the interfaces after reaching the critical thickness, which corresponds to the transition from a pseudomorphic growth to the introduction of misfit dislocations at the interfaces. It is suggested that the observed effect has a general character and should be taken into account when optimizing thermoelectric properties of superlattices.

A high-throughput screening technique has been developed and was utilized in the discovery of a new n-type oxide possessing good thermoelectric properties. Screening of metal binary systems consisting of 3d transition metals using this technique showed LaNiO3 to possess the desired n-type properties. Electrical resistivity (ρ) of this oxide is favorably quite low, however, the Seebeck coefficient (S) is as small as –25 μV/K. To enhance the thermoelectric properties of LaNiO3, high-throughput screening was employed to examine candidates from the metal ternary La1–xMxNiO3 and LaNi1–xNxO3 systems. Bi substitution in the La1–xMxNiO3 systems and Cu substitution in the LaNi1–xNxO3 systems were found to be effective for improvement of S and ρ respectively.

Using thermoelectric compatibility, efficient thermoelectric generators are rationally designed. With examples, compatible and incompatible systems are explained and materials proposed for targeted development. The compatibility factor explains why segmentation of TAGS with SnTe or PbTe produces little extra power, while filled skutterudite increases the efficiency from 10.5% to 13.6%. High efficiency generators are designed with compatible n-type La2Te3, and similar p-type material, while incompatible SiGe alloys actually reduce the efficiency. A refractory metal with high p-type thermopower (> 100 μV/K) is required for development. The Chevrel compound Cu4Mo6Se8 is a compatible p-type metal that provides a modest increase in efficiency. A fully segmented generator using Bi2Te3-type, PbTe, TAGS, Zn4Sb3, skutterudite, La2Te3, and Chevrel compounds between 25° C and 1000° C will achieve 18.1% conversion efficiency.

A phonon Boltzmann transport model is established to study the lattice thermal conductivity of nanocomposites with nanowires embedded in a host semiconductor material. Special attention has been paid to cell-cell interaction using periodic boundary conditions. The simulation shows that the temperature profiles in nanocomposites are very different from those in conventional composites, due to ballistic phonon transport at nanoscale. The thermal conductivity of periodic 2-D nanocomposites is a strong function of the size of the embedded wires and the volumetric fraction of the constituent materials. At constant volumetric fraction the smaller the wire diameter, the smaller is the thermal conductivity of periodic two-dimensional nanocomposites. For fixed silicon wire dimension, the lower the atomic percentage of germanium, the lower the thermal conductivity of the nanocomposites. The results of this study can be used to direct the development of high efficiency thermoelectric materials.

Electrical transport properties of NbCoSn1−xSbx (x =0, 0.01, 0.02 and 0.05), a half-Heusler compound and its alloys, have been studied in the temperature range from 80 K to 850 K. As-prepared samples exhibit metallic conduction and similar Seebeck coefficients near 300 K (S = –100 μVK−1). Except for NbCoSn0.95Sn0.05, both electrical resistivity, ρ, and the absolute value of S appreciably increase during the annealing for 6 days at 1123 K. Unusual increase in ρ of the annealed NbCoSn sample is found at about 200 K. ρ-T curves of the other annealed samples remain metallic over the measured temperature range and the ρ value noticeably decreases with increasing Sb content, x. Among the annealed samples, the high power factor, 25×10−4 Wm−1K−2 at 850 K, is obtained for NbCoSn0.95Sb0.05. The band structure of NbCoSn is calculated based on the determined crystal structure, indicating that NbCoSn is an indirect transition-type semiconductor with a band gap of approximately 1 eV. This is not consistent with the metallic behavior of ρ observed for the annealed NbCoSn sample above 400 K. Partial disordering of Nb and Co atoms is a conceivable answer.

We have investigated transition-metal substitutions into germanium clathrates, and describe the properties of two different Fe-substituted Ge clathrates, one with the chiral Ba6Ge25-type clathrate structure, and one based on the Ba6Ga16Ge30 clathrate with the type-I structure. In both cases Fe exhibits a high-spin local moment, with 5.5 μB and 5.6 μB per Fe. We observe ferromagnetic ordering, with Tc up to 278 K in the most heavily substituted Ba6Ga16Ge30 clathrate. X-ray powder diffraction and electron microprobe measurements confirmed the formation of these substituted phases. Ni-doped type I clathrates were found to be diamagnetic. Strong competition from Mn11Ge8 makes it difficult to produce pure Mn-doped clathrates, however a composite material with partially substituted chiral structure is described. Similar efforts to produce Co, Cr, and V-substituted clathrates have not been successful.

The thermoelectric properties as well as microstructure of binary and some ternary ReSi1.75 have been investigated. Binary ReSi1.75 exhibits a nice thermoelectric property as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along [001], although the ZT value along [100] is just moderately high. Mo substitution for Re in ReSi1.75 considerably increases the ZT value along [001] because of the decreased electrical resistivity, while the property improvement is not significant along [100]. On the other hand, Al and Ge substitutions for Si in ReSi1.75 considerably increase the ZT value along [100]. This is also because of the decreased electrical resistivity. When Al is added to ReSi1.75, the value of electrical resistivity is significantly reduced when compared to the binary counterpart and the temperature dependence of electrical resistivity changes from of semiconductor for the binary alloy to of metal for the Al-added alloys.

A series of cerium-containing CoSb3 samples were synthesized, with cerium quantities varying from 0 to 2 stoichiometric equivalents. These samples were annealed at low temperatures to crystallize the kinetically stable phases CexCo4Sb12 (x = 0–0.5). X-ray diffraction showed that these samples were phase pure, and Rietveld analysis on x-ray diffraction data from powder samples indicated that these samples were 25–88% crystalline. Electrical measurements showed that these samples are n-type, which was previously unknown in CexCo4Sb12. Magnetic measurements showed that the samples were paramagnetic due to the cerium being incorporated into the diamagnetic CoSb3 compound. In addition, they contained a ferromagnetic component that was attributed to the amorphous, cerium-containing phase.