K2Bi8 Se13 belongs to a class of complex Bi-Te-Se systems which show great potential for thermoelectric performance. This compound forms in two distinct phases α-K2Bi8 Se13 (triclinic with space group P-1) and β-K2Bi8 Se13 (monoclinic with space group P 21/m). In the β-phase, there is substantial disorder at four sites in the unit cell, occupied by two K and two Bi atoms. To understand the electronic properties of these two different phases we have carried out band structure calculations 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. Spin-orbit interaction (SOI) was incorporated using a second variational procedure. The α. phase is found to be a semiconductor with an indirect band gap of 0.47eV. For the β-phase we have chosen two different ordered structures. The system is a semi-metal for one of the structures whereas for the other, it is a narrow gap semiconductor with a gap of 0.38eV in the absence of SOI, but the gap collapses and the system becomes a semimetal with low density of states at the Fermi energy when SOI is included.

The tailoring of the sign and magnitude of the absolute thermoelectric power was achieved in structural composites by the choice of the reinforcing fibers and of the particulate filler between laminae. The resulting thermoelectric structural composites included continuous carbon fiber polymer-matrix composites and short fiber cement-matrix composites. In addition, it resulted in thermocouples in the form of structural composites. The fibers and interlaminar filler impacted the thermoelectric behavior in the longitudinal and through-thickness directions respectively.

We measured the temperature dependence of electrical resistance (R) and thermopower (S) of Cs8Zn4Sn42 under high pressure up to 1.2 GPa. Both R and ∣S∣ at room temperature increased with pressure. We observed gap widening, irreversible ∣S∣ increasing under high pressure, which were similar to the behaviors of Cs8Sn44. However, the relaxation e.ect of R for Cs8Zn4Sn42 was negligible in contrast with that of Cs8Sn44. We found that the power factor S2σ (σ: electrical conductivity) near room temperature decreased linearly with pressure. The results suggest that the defects in different forms played an important role in transport properties for tin clathrates under high pressure.

Resistivity and thermopower data is presented on Eu-doped CoSb3 skutterudites over the temperature range of 10 K to 700 K. Thermal conductivity is also presented from 10 K to 300 K in order to determine the figure of merit. Eu0.43Co4Sb11.59Ge0.31 and Eu0.42Co4Sb11.37Ge0.50 exhibit an enhanced figure of merit as compared to CoSb3 and Eu0.2CoSb3. Data on a series of Yb-filled CoSb3 is underway and preliminary data is presented.

Single crystals of Ge1−xSix alloys in the composition range 0 < x < 1 non-doped and doped with boron, gallium, and phosphorus as an impurity were grown by the Czochralski method. The measurements of the thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (α) of the grown crystals were performed in the temperature range 300 – 1000 K. The thermal conductivityκ shows a minimum (κ ∼ 3.5 W/K.m) around the Si content x = 0.5 - 0.7, which can be explained by the phonon scattering due to a distortion of the crystal lattice. The Seebeck coefficientα was 300 - 400 μV/K at 600°C in the impurity-doped GeSi alloys. An ∣α∣ vs ln (σ) plot of highly impurity-doped GeSi alloys obeys a linear relation. The slope of the straight line is 1.2 k/e, indicating the coorporation of the charged impurity into the alloy scattering mechanism.

The transition metal pentatellurides HfTe5 and ZrTe5 exhibit a broad resistive anomaly as a function of temperature. This behavior is also reflected in the thermopower as it changes from a large positive value below room temperature to a large negative value at lower temperatures with the zero crossing corresponding well with the peak temperature of the resistive anomaly. The large values of the thermopower at low temperatures (T ≈ 150 K) have made these materials attractive for investigation for potential low temperature thermoelectric applications. The magnitude of the resistive peak and the peak temperature are highly sensitive to doping as well as external influences such as magnetic field and pressure. In this study we examine the effect of doping with various rare earth elements (RE = Ce, Sm and Dy) and the subsequent effects on the electrical resistivity and the thermopower. These results will be discussed in relation to the thermoelectric performance of these materials.

Filled skutterudites exhibit properties that comply with the concept of a “phonon-glass electron-crystal”, as proposed by Slack. The optimal filled skutterudite would have filler atoms that exhibit large thermal vibration amplitudes in the voids of the crystal structure. It is desirable that these loosely bound atoms give rise to strong phonon scattering without greatly affecting the essential part of the band structure of the skutterudites. This criterion is difficult to meet. Most attempts have employed charge compensation for filling fractions above 50 %. In this report we present the use of a high-pressure technique for the synthesis of new filled skutterudites. By using our high-pressure synthesis technique CoSb3-based skutterudites filled with group-14 elements (Ge, Sn, and Pb) have been synthesized with up to 100 % filling without charge compensation of the host lattice. The structural analysis reveals that the Sn atoms exhibit very large thermal vibration amplitude, indicative of a large “rattling” motion. The Sn-filled specimens exhibit a low thermal conductivity, lower than that of any previously reported filled skutterudite, while the favorable semiconducting nature of the host lattice is not substantially changed by Sn filling. Tin atoms may therefore be better “rattlers” in the CoSb3 host lattice than lanthanide or actinide atoms.

Electronic structure of CoSb3 is calculated by means of full-potential linearlized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA). The calculated band gap of CoSb3 with the consideration of spin-orbit (SO) interaction is 110meV, which is about a half of that without SO interaction. It is found that simple four bands model with the Kane's nonparabolic valence and conduction bands, two parabolic conduction bands describe calculated electronic structure near band edge very well. Using the simple four bands model, thermoelectric properties are calculated and are discussed. Larger band gap, e.g., Eg=200meV with the microscopic mechanism such as phonon scattering yields a fair agreement with the experiment in a wide range of temperature.

SiB6 has proved to a potentially useful material because of its excellent thermoelectrical properties above 700°C, low specific gravity, high degree of hardness, and moderate melting point. SiB6, which has poor sinterability with a conventional sintering technique due to the covalent characteristic, has been successfully densified fully using a spark plasma sintering(SPS) method. The SPS-processed specimens consisted of SiB6, SiB4 and SixBy phases. Pure SiB6 powder were densified fully at the sintering temperature of 1600°C. In particular, it was found that the rare earth element was very effective in evolving the microstructure of SiB6 phase, resulting in reducing the sintering temperature and controlling grain growth. These effects were discussed in details in terms of microstructure evolution during the SPS process.

The electronic structure of a crystalline boron carbide has an energy forbidden gap of ∼ 3 eV and is hence a good insulator. But, on the other hand, the electrical conductivity of boron carbide is measurable. It is therefore believed that the defects formation in boron carbide is responsible for its electrical conductivity and a theory of hopping conduction of bipolaron through localized defects were developed, accordingly. Although the bipolaron electrical conductivity model does not rely on any specific type of defect, the bipolaron formation in boron carbide is believed to be a defective CBB intraicosahedral chain in connection with an B11C icosahedron. The current study examined the existing theory of bipolaron electrical conductivity by performing a systematical study on the formation energies of the defects in boron carbide using a state-of-the-art ab-initio electronic structure method. The studied defects cover a) stoichiometric variations of carbon concentration, b) missing boron atoms, and c) distribution of carbon atoms in the materials. It is found that the ground state of a fully carbonated boron carbide consists of B11C icosahedra connected by CBC intraicosahedral chains, i.e. consistent with the reported structural model of B4C. When carbon concentration is reduced, however, the population of CBC chain is found to be intact, while the population of B11C icosahedron is reduced by the replacements of B12 icosahedron. This observation is fundamentally different from the existing model of boron-rich boron carbide. The localized states associated with missing boron atoms are identified and the electrical conductivity through these localized defects states is studied.

In previous studies we have investigated the thermoelectric properties of undoped and doped compositions of β-K2Bi8Se13. The attempt to substitute Rb for K resulted in a different structure type, but with potentially useful thermal properties. In this paper the results of measuring the thermoelectric properties of small crystalline samples for K2−xRbxBi8Se13 solid solutions are reported for the compositional ranges with 0 ≤ × ≤ 1 and with 1.8 ≤ × ≤ 2. The presence of rubidium provides a significant improvement in the thermopower but has much less of an effect on the electrical conductivity. The combined results are reflected in the power factor behavior as a function of temperature.

We prepared a series of (AgBiTe2)1−x(Ag2Te)x (0≤×≤1) composite materials by melt and cool down [1]. The Hall coefficient and the electrical conductivity were measured by the standard van der Pauw technique over the temperature range from 93K to 283K from which the Hall carrier mobility was calculated. Ag2Te had the highest mobility while the mobility of AgBiTe2 was the lowest of all samples at 283K. However the mobility of the (AgBiTe2)0.125(Ag2Te)0.875 composite material was higher than the motility of Ag2Te below 243K. It seems that a small second phase dispersed in the matrix phase is effective against the increased mobility.

Skutterudites are a class of materials that show promise for thermoelectric applications due to their high power factor and the ability to “tune” the thermal conductivity due to the addition of “rattling” atoms into the novel structure of these materials. Thermopower and resistivity is measured and reported for a series of Eu and Yb-doped skutterudites over a temperature range of approximately 100 K – 700 K using the High Temperature Thermoelectric Probe. Sample measurement techniques are briefly discussed. Data from various dopings is presented and compared in hope of showing trends that point towards improvements in these skutterudites.

Lead telluride thin films have been grown on BaF2(111) substrates by pulsed laser deposition from a Nd:YAG laser (λ = 532 nm) at very low temperature (150°C). The chemical composition, the morphology and the crystallographic structure of the layers depend strongly on the deposition conditions. Post-annealing treatments of the films also affect their microstructure. Preliminary electrical properties, conducted in the temperature range of 5 to 350 K, revealed that all the samples were n-type with Hall mobility values greater than 104 cm2/V.s at low temperatures.

Experiments demonstrating thermal to electrical energy conversion using thermal diodes have shown an enhancement of the open circuit voltage over the thermoelectric open circuit voltage. Two different physical mechanisms are proposed to be responsible for the effects seen: (1) Thermionic injection from the emitter can occur when a temperature gradient is present, which induces an increased ohmic return current under zero-current conditions. (2) Blockage of the ohmic return current leads to a voltage increase for both thermoelectric and thermionic forward currents. Both effects increase the efficiency of energy conversion. Experiments show enhancements of the figure of merit in the range of 5-8 over the thermoelectric values. The best results are consistent with a single-side conversion efficiency in excess of 30% of the Carnot limit.

This paper reviews the connections between the transport properties underlying the thermoelectric performance of a material and microscopic quantities, particularly as they may be obtained from first principles calculations. These are illustrated using examples from work on skutterudites. The results are used to suggest yet to be explored avenues for achieving higher thermoelectric performance within this class of materials.

We developed a method for making 4-point contacts to Bi nanowires with a thick oxide coat using a combination of lithographic and focused ion beam (FIB) techniques. The resistivity of Bi nanowires with diameters in the range 70-200nm is found to increase with decreasing wire diameter. In contrast to bulk Bi, the temperature dependence of the resistivity is found to decrease monotonically with increasing temperature. The results are explained on the basis of increased scattering in the nanowire and the known temperature dependence of the electronic properties of bulk Bi. A large magneto-resistance was also measured, indicating a high crystalline quality of the nanowires. A large spread in the measured values of the resistivity indicates significant systematic error in the measurement technique. Possible sources for error are discussed.

Thin films of p-type Bi0.5Sb1.5Te3, n-type Bi2Te2.7Se0.3 and n-type with SbI3 doping were deposited on mica substrates using Nd-YAG pulsed laser ablation at temperatures ranging from 300°C to 500°C. These films were characterized using X-ray diffraction, SEM and TEM. X-ray mapping and EDS were used to determine the composition. The films showed uniform thickness and high crystalline quality with a preferred (00n) alignment with the substrates. The film quality in terms of composition and crystal perfection is studied as a function of growth temperature. It was found that films deposited at 350°C gave improved crystallinity and thermoelectric characteristics. The Seebeck coefficient, electrical resistivity and Hall mobility were measured as a function of temperature and compared with the measurements on the bulk. Correlation of thermoelectric properties with microstructure is discussed.

Thin films of PbTe were deposited on Si (111) wafers and glass substrates at a constant power for different times and at a constant time at various power levels. In some cases substrate heating to a temperature of ∼673K was performed during sputtering. Structural analysis by Xray diffraction (XRD) and high-resolution electron microscopy (HRTEM) were performed. The composition of the PbTe film was evaluated by Auger depth profile. At an appropriate combination of power and deposition time only (200) and its higher order peaks were observed in the PbTe film. It is expected that it is feasible to obtain epitaxial PbTe film by RF magnetron sputtering.

Transient thermoelectric cooling effect has been known since 1960s. Most of the previous studies focus on the minimum temperature achievable for free standing thermoelectric elements. Practical thermoelectric coolers have a passive mass load, which affect the minimum temperature achievable. This is particularly true for thermoelectric micro coolers because the object to be cooled is comparable to the micro coolers in size. In this paper, we present the performance analysis of the integrated system made of the cooling object and thermoelectric element, and establish the criterion for the utilization of transient cooling effect based on the analysis of the time constants. Effects of thermoelectric element shape on the transient thermoelectric cooling effect are also studied.