Silver (Ag) doped Mg2Si1-xGex (x=0.1 to 0.4) samples were fabricated using a plasma activated sintering (PAS) method. The doping concentration of Ag was varied from 1 to 5 at.%. Undoped Mg2Si1-xGex exhibits n-type conductivity due to residual impurities in the Mg source material used and unintentionally process-induced impurities. The observed unstable behavior of the Seebeck coefficient of Ag-doped p-type Mg2Si1-xGex (x ≤ 0.3) in the region of 550 to 650 K, exhibiting a considerable drop in the value and occasional conduction type conversion, was correlated with the specific contaminants. For x∼0.4, the observed Seebeck coefficient varied from 0.2 mV/K at 823 K to 0.4 mV/K at room temperature, with no remarkable drop in the value with increasing temperature. An estimated ZT value of 5 at.% Ag doped Mg2Si0.6Ge0.4 was 0.18 at 844 K. It was found that both specific residual impurities and process-induced impurities affected the characteristics of the Seebeck coefficient of Mg2Si1-xGex.

Thermoelectricity in oxides, especially NaxCoO2 and related materials, is discussed from the point of view of first principles calculations and Boltzmann transport theory. The electronic structure of this material is exceptional in that it has a combination of very narrow bands and strong hybridization between metal d states and ligand p states. As shown within the framework of conventional Boltzmann transport theory, this leads to high Seebeck coefficients even at metallic carrier densities. This suggests a strategy of searching for other narrow band oxides that can be doped metallic with mobile carriers. Some possible avenues for finding such materials are suggested.

We report magnetic properties of [Ca2CoO3-δ]0.62CoO2 (Ca349) powders with various average size and the Bi- and Sr-doping effects on thermoelectric properties for the magnetically grain-aligned and densified Ca349 thick films. Magnetic anisotropy at 300 K depended on the initial average size of Ca349 powders and decreased with the decrease in the size. This presumably suggests that distortion of crystal structure was induced by a ball-milling process and led to the change of magnetic anisotropy. On the Bi- and Sr-doping effects, an obvious enhancement of thermoelectric properties did not emerge in the case of the Sr-doping, whereas the enhancement was observed for the Bi-doped Ca349 thick films. However, a drastic decrease of magnetic anisotropy was caused by the Bi-doping. For usage of the p-type layer in multilayered thermoelectric module, tuning of the Bi-doping levels in which both enhancement of thermoelectric properties and a certain level of magnetic anisotropy are achieved is required.

Doped lead telluride dimensional nanocomposites were prepared by densifying nanocrystals synthesized employing an alkaline aqueous solution-phase reaction. The nanocrystal synthesis procedure resulted in high product yields of over 2 g per batch. These nanocrystals were then subjected to Spark Plasma Sintering (SPS) for densification. Transport properties were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. The results for these lead telluride nanocomposites were compared to bulk polycrystalline lead tellurides with similar carrier concentrations.

Thermoelectric devices based on “n-type” oxide semiconductors and metal/oxide nanocomposites are being considered for high temperature thermocouples, heat flux sensors and energy harvesting devices. In terms of energy harvesting, preliminary 2D thermoelectric calculations indicated that enough electrical energy can be generated from the large thermal gradients that exist within a gas turbine engine to power active wireless devices. Several promising bi-ceramic junctions based on this concept were investigated in terms of their high temperature thermoelectric properties. The most promising bi-ceramic junction was based on indium tin oxide (ITO) and a NiCrCoAlY/alumina nanocomposite. The thermoelectric responses of these individual elements were evaluated relative to a platinum reference electrode. A maximum emf of 77 mV was achieved for a NiCrCoAlY/alumina nanocomposite/platinum thermocouple for an imposed temperature gradient of 1111 °C. The thermoelectric power for this couple was 78 μV/°C. When this NiCrCoAlY/alumina nanocomposite was combined with ITO to form a bi-ceramic junction, thermoelectric powers on the order of 700 μV/°C were obtained. A maximum electromotive force of 291mV was achieved for a hot junction temperature of 1100 °C. The thermoelectric response after repeated thermal cycling to 1200 °C was both repeatable and reproducible. The ITO was prepared in varying nitrogen, oxygen and argon partial pressures, which was used to control the charge carrier concentration, stability and thermoelectric response of the bi-ceramic junctions. The thermoelectric response decreased with increasing nitrogen partial pressure and increased with oxygen partial pressure in the plasma with the argon partial pressure constant. The relationship between the sputtering parameters and thermoelectric properties was investigated and the application of these bi-ceramic junctions as thermocouples and energy harvesting devices is discussed.

The thermoelectric properties of materials with compositions NaPb18-xSnxMTe20 (M=Sb, Bi, x=0, 3, 5, 9, 13, 16 and 18) were investigated in the temperature range 300-670K. All compositions exhibited p-type behavior over the measured temperature range. Electronic properties and transport were tuned through the manipulation of the Pb/Sn ratio. Increasing the Sn fraction results in an increase in electrical conductivity and a decrease in thermopower. The compositions NaPb13Sn5SbTe20 and NaPb9Sn9SbTe20 show a lattice thermal conductivity of ∼1 W/m/K at room temperature.

We report the physical characterization and thermoelectric properties of (Pb0.95Sn0.05Te)0.92(PbS)0.08 containing excess Pb and Sb prepared using the matrix encapsulation technique. Samples of (Pb0.95Sn0.05Te)0.92(PbS)0.08 : Pb 0.5 - 4 at. % rapidly quenched from the melt show microscale Pb inclusions that increase the thermal conductivity while slightly increasing the power factor, compared to (Pb0.95Sn0.05Te)0.92(PbS)0.08. Samples of (Pb0.95Sn0.05Te)0.92(PbS)0.08 : Pb 0.5%, Sb 2% prepared using the same technique show microscale Sb and Pb inclusions that upon heating cause rapid PbS and Sb segregation from the PbTe matrix. This behavior significantly alters the microstructure and degrades the transport properties of the material.

Indium in Pb1-xSnxTe alloys forms a resonant energy level in the conduction or valence bands, depending on x. In this study we investigate temperature dependence of the In level from 80 to 400K, complementing our previous work at 80 K. Measurements of electrical resistivity, thermopower, Hall and transverse Nernst-Ettinghausen effect are used to assess carrier mobility, Fermi level and scattering coefficient. Measurements are performed on a set of p and n type Pb1-xSnxTe:In with 0 < x < 30 at% and In up to 3 at%. We show that with increasing temperature the Fermi level crosses into the gap. It had been suggested theoretically that hybridization of the In level with one band at the Fermi level could have had a positive effect on the thermoelectric properties of materials, but the present results illustrate the need for temperature-dependent modeling and experimentation.

GeTe, a small bandgap semiconductor that has native p-type defects due to Ge vacancies, is an important constituent in the thermoelectric material known as “TAGS” [1]. TAGS is an acronym for alloys of GeTe with AgSbTe2, and compositions are normally designated as TAGS-x, where x is the fraction of GeTe. TAGS-85 is the most important with regard to applications, and there also is commercial interest in TAGS-80. The crystal structure of GeTe1+δ has a composition-dependent phase transformation at a temperature ranging from 430°C (δ = 0) to ∼ 400°C (δ = 0.02) [2]. The high temperature form is cubic. The low temperature form is rhombohedral for δ < 0.01, as is the case for good thermoelectric performance. Addition of AgSbTe2 shifts the phase transformation to lower temperatures, and one of the goals of this work is a systematic study of the dependence of transformation temperature on the parameter x. We present results on phase transformations and associated instabilities in TAGS compositions in the range of 70-85 at.% GeTe.

Bi2Te3-based thin films were fabricated on glass substrates by the pulsed laser deposition (PLD) method. The vapor pressures of Bi and Te are significantly different, so controlling the stoichiometric composition is difficult when using conventional physical vapor deposition techniques, and the thermoelectric properties of Bi2Te3 films are sensitive to the film composition. PLD is a promising technique for the fabrication of telluride-based films such as Bi2Te3 due to its superior capability for controlling the film composition. Another advantage of PLD is the flexibility that it allows in terms of atmosphere in the reaction chamber; high concentrations of gases such as oxygen or argon can be introduced. We have measured various compositions of Bi2Te3 based films, and have identified the optimal compositions for both n-type and p-type material. The thermal conductivities of these Bi2Te3 films were evaluated by an exact measuring system, and the results were twice as low as those of conventional bulk materials. These results suggest that PLD has significant advantages for the deposition of in-plane Bi2Te3-based thin films.

We report thermoelectric (TE) properties of mono-layer thick films of p-type [Ca2CoO3-δ]0.62CoO2 (Ca349) and n-type compounds of Ca0.9La0.1MnO3 (Mn113) and LaNi0.98Mo0.02O3 (Ni113) for the improvement of TE performance in multi-layered TE modules. In the case of magnetically aligned Ca349 film, the improvement of TE properties due to the reduction of resistivity (ρ) was achieved by optimization of hot-pressing temperature and ρ was sensitive to relative density of the Ca349 layer. For the decrease in ρ of the n-type layers, two different approaches, the choice of Ni113 as a n-type compound and usage of fine powders of Mn113, were attempted. Both approaches were effective for reducing ρ even in the sintering at ∼900°C.

Transition metal oxides and oxynitrides with perovskite-type structures are potential high temperature stable thermoelectric materials. With innovative soft chemistry synthesis procedures the crystallite size in complex oxide ceramics with perovskite-type structure can be decreased compared to classical synthesis routes and allows to lower the heat conductivity dramatically while a large power factor can be maintained at the same time. Submicron crystallites are beneficial to lower the lattice thermal conductivity without influencing the electron mobility.

In this work, perovskite-type cobaltates, molybdates and manganates are studied. The influence of heterostructures and nanocrystalline domains in perovskite oxides/oxynitrides and the correlation with transport properties and Seebeck coefficient will be discussed.

N-type conduction was observed in epitaxial Pb1-xSnxSe and Pb1-xSnxTe thin films grown by MBE with an increasing charge carrier concentration as well as an increasing thermopower value related to an increasing tin content reaching up to 12at%. The lattice thermal conductivity decreases as expected due to alloy scattering but the carrier mobility is almost stable. All these effects increase strongly the thermoelectric properties of Pb1-xSnxSe and Pb1-xSnxTe. The n-type conduction arises from a metal excess in the stoichiometry range. The same type of conduction has also been found in bulk samples. Bulk single crystals of Pb1-xSnxSe and Pb1-xSnxTe are grown by the unseeded vapor growth technique. A rather constant mobility and a decreasing thermal conductivity with increasing tin content are measured in the n-type bulk samples. A figure of merit of ZT=0.9 was measured at 325°C for a Pb0,965Sn0,035Se sample with a carrier concentration of about 9.3E18 cm-3, i.e. already better than known n-type PbTe-based materials for this temperature range.

In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κel,E) and for constant current (κel,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κel,J = LoσT, where Lo =2x10−8WΩ/K is the Lorentz number, overstimates κel,J by nearly 60% at 800K for carrier concentration n=5x1019/cm3. As a result, one tends to underestimate the lattice contribution κlatt = κexp - κel,J. We give theoretical values of effective Lorentz number L = κel.J/σT for different n and T.

Grain boundary scattering provides an effective avenue to lower the thermal conductivity in polycrystalline thermoelectric materials, but it is hard to do this without simultaneously degrading the power factor that is the product of electrical conductivity and thermopower. An immediate question arises as to whether one can fabricate a thermoelectrically favorable grain boundary?

In this paper we present a proof-of-principle grain boundary engineering study in the pulverized p-Bi2Te3 system. Utilizing the lately developed hydrothermal nano-coating technique, we fabricated an Alkali-metal(s)-containing surface layer with few tens of nm thick on the p-Bi2Te3 bulk reference grain, where it becomes part of the grain boundary upon hotpressing densification. The electrical resistivity, thermopower, thermal conductivity and Hall coefficient measurements constitute solid evidence that this heterogeneous layer helps decouple the otherwise inter-related resistivity, thermopower and thermal conductivity. To optimize the figure of merit ZT, we carefully varied the ratio between Na, K and Rb concentrations. It was found that the sample treated in the solution with Na/Rb =1:2 achieved a ZT comparable with that of the commercial ingot; in the mean time, the compatibility factor and robustness of device were considerably improved. In principle this technique can be applied to other existing polycrystalline thermoelectric materials as a new “tuning knob”.

Investigation of a family of bulk cubic compounds with general formula AgPbmMS2+m (M = Sb, Bi) is reported. These PbS-based cubic structured quaternary systems combine a set of desirable features for efficient ZT thermoelectric materials for solar thermal power generation. AgPbmMS2+m (M = Sb, Bi) possess an average NaCl structure (Fm-3m symmetry), high melting point (991 - 1095 °C for M = Sb; 865 - 1091 °C for M = Bi), relatively wide energy gap (0.7 > Eg (eV) > 0.39 for both Sb and Bi). The systematic variation of lattice parameters, energy gap and melting point is reported. Preliminary charge transport properties are reported along with variable temperature thermal conductivity data of selected members.

The electronic states of Ba8Ga16Ge30 and Sr8Ga16Ge30 are studied by soft x-ray photoelectron spectroscopy (XPS) at a high-energy facility. In Ba8Ga16Ge30, three bands are resolved in the valence band region. Resonance experiments together with theoretical calculations show that the three band structures in the valence band are mainly constructed by the Ge/Ga 4s and 4p wave functions with little contributions of Ba 5s, 5p and 5d. The valence band of Sr8Ga16Ge30 shows a similar feature to that of Ba8Ga16Ge30. It is clearly shown that the atomic orbitals of Ba and Sr make little hybridization with the orbitals made by the framework polyhedra. This provides the understanding of the thermoelectricity in clathrates, and assists the design in high performance thermoelectric materials in this family.

The Ag1-x(Pb1-ySny)mSb1-zTem+2 compounds have been found to exhibit high performance p-type thermoelectric properties and are now being considered for practical applications. In this paper, various m values and silver concentrations were investigated to determine the effect on thermoelectric properties. Also, the charge-compensated compounds were studied in order to eliminate the problem of excess tellurium evaporation at high temperature.

We present the synthesis, crystal structure, spectroscopic properties, and electronic structure of a new member of the Zintl family, Yb5Al2Sb6. The compound crystallizes in the Ba5Al2Bi6 structure type. A preliminary assessment of its thermoelectric properties including electrical conductivity, thermopower, and thermal conductivity are reported. Moreover, investigations of solid solutions of this phase, doping effects and chemical modifications will be presented. The room temperature conductivity, thermopower and thermal conductivity of Yb5Al2Sb6/0.5Ge were 1100 S/cm, 20 μV/K, and 3.8 W/m.K, respectively.

Recently, we have found that high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO3 exhibits extremely large Seebeck coefficient (|S|2D) [REF], approximately five times larger than |S|3D. Here we clarify the origin of giant |S|2D using [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]20 (x = 1∼60, y = 1∼20) superlattices. The |S|2D value increased proportionally to y{0.5, indicating that the density of states of the conduction band increases with decreasing y (quantum size effect). Superlattices of [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]z (x = 0∼60, y = 1∼20, z = 20) were fabricated on the (001)-face of LaAlO3 substrate by PLD using RHEED to count the number of SrTiO3 or SrTi0.8Nb0.2O3 layers. HR-XRD and HAADF-STEM studies revealed that high-quality [(SrTiO3)17/(SrTi0.8Nb0.2O3)y]z superlattices were successfully fabricated. A dramatic increase of |S|2D is seen with decreasing y-value of the [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]z superlattices. The slope for the plots of log |S|2D - log y is -0.5, and reached 290 μVK−1 (y = 1), which is ∼5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S|3D = 61 μVK−1). Further, the |S|2D value monotonically increases with x-value and is saturated when x-value > 16 (6.25 nm). We clarified that the critical barrier thickness for electron tunneling in [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]z superlattice is 6.25 nm (16 unit cell layers of SrTiO3). The present results give very important information to utilize the [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]z superlattices for practical application.