There is great scientific, economic and environmental interest in the development of thermoelectric materials capable of direct thermal-to-electric energy conversion with high efficiency. Recent theory predicts that in materials with a fine-tuned electronic density of states, electrons can be placed in energy-specific equilibrium, and the efficiency of thermoelectric power generation can approach the fundamental Carnot limit. Here we review the relevant theory of energy-specific equilibrium. We describe a concept for a proof-of principle demonstration of near-Carnot efficient power conversion involving a single, ballistic nanowire at low temperatures, and we discuss the potential for room-temperature applications in diffusive materials.

We investigated the effects of doping of rare-earth ions (Ce, Pr, Nd, Sm, and Eu) on the thermoelectric properties of ZnO, especially Seebeck coefficient in the temperature range from 100 to 800 °C. The data were scattered more or less depending on the doping species, but it was commonly found that the rare-earth doped samples showed higher Seebeck coefficients with compared to those for Al-doped samples. The highest values were observed for Nd-doped one ranging from -360 to -400μVK−1 in the temperature range observed, the lowest ones for Pr-doped one ranging from -250 to -310 μVK−1, whereas those for Al-doped one ranging from -80 to -140 μVK−1. It should also be pointed out that the power factor at 100 °C for Pr-doped ZnO was twice or more as much as that for Al-doped one. From the results it seemed that rare earth doping is effective for enhancement of thermoelectric power of ZnO.

Over a decade ago it was predicted that nano-scaled thermoelectric (TE) materials might have superior properties to that of their bulk counterparts. Subsequently, a significant increase in the figure of merit, ZT (ZT > 2), has been reported for nano-scaled systems such as superlattice and quantum dot systems constituently based on those more commonly used bulk TE materials (e.g., Bi2Te3 and PbTe). However, the challenge remains to achieve these higher performance results in bulk materials in order to more rapidly incorporate them into standard TE devices. Recent theoretical work on boundary scattering of phonons in amorphous materials indicates that micron and submicron grains could be very beneficial in order to lower the lattice thermal conductivity and yet not deteriorate the electron mobility. The focus in this paper will be to highlight some of our new directions in bulk thermoelectric materials research. Thermoelectric materials are inherently difficult to characterize and these difficulties are magnified at high temperatures. Specific materials will be discussed, especially those bulk materials that exhibit favorable properties for potential high temperature power generation capabilities. One potentially fruitful research direction is to explore whether hybrid TE materials possess possible enhanced TE properties. These “engineered” hybrids include materials that exhibit sizes from on the order of a few nanometers to hundreds of nanometers of the initial materials. These initial materials are then incorporated into a bulk structure. A discussion of some of the future research directions that we are pursuing is highlighted, including some bulk materials, which are based on nano-scaled or hybrid composites. The synthesis techniques and the synthesis results of many of these nano-scale precursor materials will be a primary focus of this paper.

Difficulty in preparing (Bi2Te3)x(Sb2Te3)y superlattices due to interdiffusion of Sb and Bi led to the study of interduffusion barriers. TiTe2 has been explored as an interdiffusion barrier to minimize the interdiffusion of Sb and Bi, as TiTe2 is not soluble in either Bi2Te3 or Sb2Te3. Preparation of (TiTe2)3(Sb2Te3)y(TiTe2)3(Bi2Te3)z superlattices has been achieved with varying x, y and z. The formation of the superlattices was studied as a function of annealing temperature and time. TOF-SIMS depth profiles were used to study the extent of interdiffusion in the samples. Unit cell control was achieved allowing for the preparation of an array of superlattices with varying periods with very good reproducibility.

The synthesis of (Bi2Te3)x(TiTe2)x and (Bi2Te3)x(TiTe2)3 superlattices using modulated elemental reactants was successfully accomplished. This required the calibration of the deposition parameters to achieve both the desired atomic compositions of the constituent layers and the deposition of the absolute amounts of each of the components to yield the title compounds. Proper annealing conditions were determined from an investigation of the x-ray diffraction patterns of a superlattice sample as a function of annealing temperature. The change in lattice parameters as a function of x showed the expected linear behavior with slopes consistent with values expected from the published lattice parameters of the binary components. Rietveld refinement showed that the characteristic structure of the binary components is maintained in the superlattices studied.

The nanocrystals of PbTe and related compounds (Pb1−xSnxTe, PbTe1−xSex), differing by size and composition, have been synthesized using a chemical vapor transport (CVD) technique. The size–selective precipitation mechanism relying on the variation of heating temperatures, Ar flow rates and admixture with Au particles, enables a relatively good control of particle size distribution. In addition, the doping ratios of those nanocrystals are readily modified by changing the atomic ratio in the raw starting materials. Subsequently, a yield of hundreds of milligrams of nanocrystals which exhibit narrow size distributions at 100 nm, 300 nm and 600 nm and controllable composition have been obtained. XRD patterns taken on the PbTe samples show sharp, which indicate good crystallinity of samples. According to the shift of the Bragg reflections, the lattice constants of (Sn / Se) doped PbTe change with the variation of the doping ratios.

Ternary compounds LnPdX (Ln = lanthanide elements of La, Gd, Er; X = Sb, Bi) were prepared by a spark plasma sintering (SPS) technique. The crystal structure of LaPdSb and GdPdSb was confirmed to be a hexagonal ZrBeSi-type structure and different from the other compounds with a MgAgAs-type structure. The electrical resistivities ρ of LaPdSb and GdPdSb indicate the metallic or semimetallic characteristics, while those of ErPdSb and LnPdBi indicate semiconductor characteristics. From the ln ρ − 1/T plot, the band gap energies E g were estimated to be 0.28, 0.053, 0.081, and 0.049 eV for ErPdSb, LaPdBi, GdPdBi, and ErPdBi, respectively. All the samples have positive thermoelectric powers S above room temperature. The largest power factor S 2/ρ was obtained as 49.5 μW/K2 cm at 327 K for LaPdSb. From the Hall effect measurements on ErPdX, the carrier concentration n of ErPdSb and ErPdBi were obtained as 5.9×1018 and 3.21×1019 cm−3 at room temperature, respectively. It is considered that the difference of n at room temperature is mainly due to the magnitude of the band gap energy.

Thermoelectrics have always been attractive for power generation and cooling because of power reliability and environmentally friendly issues. However, this concept remains non-competitive due to the limitation in the efficiency of available thermoelectric materials and device designs [1]. In the 1990s, Hicks and Dresselhaus predicted the possibility of a dramatic enhancement in thermoelectric performance based on the special behavior of low dimensional materials [2, 3]. This enhancement is in part due to the increase in quantum confinement effects, the increase in electronic density of states at specified energies, and the increase in the phonon interface scattering for low dimensional structures.

Nanowires and core-shell nanowires can be considered to be model systems to illustrate representative behavior in low dimensional thermoelectric materials. It is expected that a system made out of nanowires or core-shell nanowires would have a higher thermoelectric performance than its bulk counterpart due to an increase in the number of interfaces. The interfaces that are introduced must be such that phonons are scattered more strongly than are electrons. Theoretical studies have been carried out to better understand the transport properties of Si-Si1−xGex ordered nanowire composites. The composite is modeled as having Si wires embedded in a Si1−xGex host matrix. Thus, core-shell Si/Si1−xGex nanowires can be considered as a building block of the composite. The effect of the wire diameter and the shell alloy composition on ZT is presented.

Thin films of clathrate material have been grown using the laser ablation technique on a variety of substrates. Films deposited on silicon substrates exhibited a significant deficiency of Ga and Ge while the stoichiometry was preserved in films deposited on quartz, sapphire, and glass substrates. Ablation characteristics of the clathrate target for laser radiation at three different wavelengths in the UV, visible and IR were studied. The UV and IR laser wavelengths produce low ablation thresholds and high growth rates. Crystalline films were obtained above a growth temperature of 400°C. Conductivity measurements on the films at low current injection (<40 µA) showed semiconducting behavior. At high currents the films demonstrated a semiconductor-to-metal transition for temperatures lower than 150 K.

Measurements of Seebeck coefficient, S(T) and electrical resistivity, ρ(T) on polycrystalline La2−xSrxCuO4+d(LSCO) (0<x≤0.2) samples are reported. The Seebeck coefficient is positive in whole measured temperature range (77K and 300K) and it decreases with Sr content. At room temperature S(T) changes from 400 μ/K for the samples with the lowest levels of Sr to 30 μV/K for the samples with the highest Sr levels. The behavior of S(T) fits to Heikes model, which describes the behavior of Seebeck coefficient in systems where the correlated hopping is present. With the Sr content, the electrical resistivity changes its behavior from semiconducting to metallic and it took values from 2.4 to 10−3 Ωcm. From S(T) and rho(T) measurements the thermoelectric power factor, PF was obtained. The maximum values for PF were about 5 μW/K2cm in the samples where x= 0.03, which are comparable to the typical values for conventional thermoelectric semiconductors. The structural and morphological properties of the samples were studied by x-ray diffraction analysis and Scanning Electron Microscopy (SEM) respectively. The behavior of transport properties opens de possibility of considering this family of perovskite-compounds as a thermoelectric material which works below room temperature.

Recent development in thermoelectric conversion, especially in the area of quantum well (QW) thin film materials, have demonstrated the potential to achieve the high efficiency and power density to fabricate future power supplies. In this study, we develop the large area QW films of N-type Si/SiC integrated with P-type B4C/B9C, which can be used as thermoelectric devices for waste heat recovery. The approach is to fabricate thick large area film stacks (up to 11 μm) deposited by sputter deposition technique on 6” n-type (100) silicon substrates, which might be proven to be a suitable method for potentially manufacturing large area thermoelectric devices in a cost effective manner. These more basic studies are being carried out to better understand variables such as film thickness, deposition rate and other important parameters of these ∼10 nm films. The resulting as deposited and annealed multilayer stacks were characterized in terms of thin film uniformity, thickness, growth rate, composition, and thermoelectric performance, by Spectroreflectometry, atomic force microscopy (AFM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), and electrical measurements. Issues, which could cause film stack degradation, such as interface layer formation, film delamination, and crack formation lowering the device performance will be presented and correlated to device efficiency.

Long single crystalline whiskers (10-200 µm diameter) were synthesized using tellurium-doped precursors. The length of these whiskers varies from less than 1 mm up to 9 mm. The thermopower and resistivity were approximately 150 µV/K and 5 mΩ-cm respectively at 325K. The thermopower was measured using a differential technique, while the resistivity was measured using a standard four-probe method. The thermal conductivity of these small samples was measured using our parallel thermal conductance technique. The total thermal conductivity was on the order of 2 Wm−1K−1.

Complex crystals such as skutterudites have interstitial voids in the lattice that can be filled by various impurity atoms. The filling fraction limit (FFL) for the intrinsic voids in the lattice of CoSb3 is studied by density functional methods. The FFL is shown to be determined not only by the interaction between the impurity and host atoms but also by the formation of secondary phases between the impurity atoms and one of the host atoms. A model is proposed to quantitatively explain the phenomenon. The predicted FFLs for Ca, Sr, Ba, La, Ce, and Yb in CoSb3 are in excellent agreement with reported experimental data. Detailed analysis reveals the existence of a quantitative relationship between the repulsive interaction of impurity atoms and their charge state. A correlation between the FFL of an impurity atom and its charge state and electronegativity is discovered.

Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators. These contacts must be stable to high temperatures and through thermal cycling. Here we present the fabrication procedure and characterization of several contacts to Pb-Sb-Ag-Te (LAST) compounds. Contact materials investigated include tungsten, antimony, tin, nickel, and bismuth antimony based solder. The contacts were typically deposited by an electron beam evaporation method after careful preparation of the sample surface. The resistances were measured by using the transmission line model, and ohmic behavior was verified through current vs. voltage measurements. The best contact resistivities of less than 20 µΩ·cm2 have been measured for annealed antimony to n-type LAST samples. We present these procedures for fabricating low resistance contacts and the use of these procedures towards the fabrication of high efficiency thermoelectric generator modules.

The preparation and characterization of the AgPbmSbTem+2 family of compounds with m=2, 4, 6, and 8 is reported. Phase segregation was observed in all of these materials. The lattice thermal conductivity of these samples is low (<1.1 W/m-K). Powder x-ray diffraction, thermal analysis, and electron microscope investigations of these systems show that ideal solid solutions are not formed. The transport properties of these composite materials are presented and suggest that they could have promising thermoelectric properties when optimized.

We report analogs of the homologous family CsPbmBi3Te5+m, namely Cs0.74K0.76Bi3.5Te6, CsNa0.98Bi4.01Te7, Cs0.69Ca0.65Bi3.34Te6, Rb0.82Pb0.82Bi3.18Te6, Rb0.19K1.31Bi3.50Te6, RbSnBi3Te6, Rb0.94Ca0.94Bi3.06Te6 and RbYbBi3Te6. They all crystallize in the orthorhombic space group Cmcm and are isostructural to CsPbBi3Te6 the first member of the family except for CsNa0.98Bi4.01Te7 which is isostructural to CsPb2Bi3Te7, the second member of the family. Preliminary thermoelectric measurements on Cs0.76K0.74Bi3.5Te6 show electrical conductivity values in the range of 600-1000 S/cm. The thermopower shows n-type charge transport with room temperature values of ∼ -35 µV/K. By comparison, the electrical conductivity at room temperature of Rb0.82Pb0.82Bi3.18Te6 was ∼1400S/cm, while the thermopower was ∼ -20 µV/K.

We have synthesized the type II silicon clathrates Na1Si136 and Na8Si136, and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600°C to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type II sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na8Si136 exhibits a clear dip in the range from 50 to 70 K. These data suggest that the “rattling” behavior observed in type I clathrates may also be present in type II clathrates.

Recent efforts at Teledyne Energy Systems, Inc. have lead to significant improvements in thermoelectric (TE) materials and power conversion devices. A number of improved formulations have been identified for standard n-type PbTe and p-type TAGS alloys. The preliminary test results appear to show nearly a 50% increase in the dimensionless figure-of-merit, ZT, for these two alloys. The new TE material formulations also exhibit a lower sublimation rate which is likely to allow use of these materials at higher heat source temperatures and thus result in higher Carnot efficiencies. The combination of higher ZT and higher Carnot efficiency allows achievement of exceptionally high thermoelectric conversion efficiencies through small changes to proven technology. For example, a thermoelectric conversion device which uses segmented BiTe/modified PbTe/modified TAGS thermoelectric couple design and operates between 300 °K and 900 °K will have a conversion efficiency of approximately 16.2%. Effort is continuing to confirm the preliminary test results and validate the materials fabrication processes. A number of improvements have also been made in the TE device fabrication processes and assembly techniques which have reduced the internal thermal and electrical losses, increased conversion efficiency, enhanced reliability and reduced manufacturing cost for the thermoelectric devices.

Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena with both bulk samples containing nanoscale constituents and nanoscale materials exhibiting enhanced thermoelectric performance in their own right. Prior theoretical and experimental proof of principle studies on isolated quantum well and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents. In this review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications. A review of some of the results obtained to date are presented.

Derivatives of β-K2Bi8Se13 are an interesting series of materials for thermoelectric investigations due to their very low thermal conductivity and highly anisotropic electrical properties. Up to now substitutions on the Bi and alkali metal sites have been studied in order to tune the thermoelectric properties. In this work, the thermoelectric properties of the sulfur-substituted K2Bi8Se13−xSx (0<x<13) are presented with respect to Seebeck coefficient, the electrical and thermal conductivity as a function of temperature. Seebeck coefficient measurements showed the n-type character of all members while electrical conductivity shows higher values compare to the other solid solution series of the same type. The lattice thermal conductivity is affected due to the Se/S disorder. The temperature dependence of the figure-of-merit ZT shows that these materials have potential for high temperatures applications with promising thermoelectric performance.