We have previously reported the successful p-type doping of CsBi4Te6 which had a high figure of merit at temperatures below 300 K. In this study, several dopants were explored to make n-type CsBi4Te6. A program of measurements was performed to identify the optimum doping concentration for several series of dopants. The highest power factors occurred around 125 K for the 0.5% Sn doped CsBi4Te6 sample which had a power factor of 21.9 μW/cm•K2 and 1.0% Te doped CsBi4Te6 which had a power factor of 21.7 μW/cm•K2.

A thermoelectric device consisting of the porous part and the bulk part was proposed. Increase of heat exchange area due to porous medium will improve the efficiency of heat exchange between heating/cooling sources and the device. Estimation based on physical properties of FeSi2 indicated that generated power of the partially porous device per unit area can be several times higher than that of the bulk one in case of gas heating / cooling system. The partially porous thermoelectric devices were produced. In measurement of power, it has been confirmed that generated power per unit area of the partially porous FeSi2 devices was roughly 10 times higher than that of the conventional device. The proposed porous device will exhibit its advantage in the case of low heat transfer coefficient between the device and the heating / cooling sources.

Several new materials in the CsBi4Te6, A2Bi8Se13, (A = K, Rb, Cs), HoNiSb, Ba/Ge/B (B = In, Sn), and AgPbBiQ3 (Q = S, Se, Te) systems have shown promising characteristics for thermoelectric applications. New synthesis techniques are able to produce samples at much higher rates than previously possible. This has led to a persistent challenge in thermoelectric materials research of rapid and comprehensive characterization of samples. This paper presents a description of a new 4-sample transport measurement system and the related measurement techniques. Special features of the system include fully computer-controlled operation (implemented in LabView™) for simultaneous measurement of electrical conductivity, thermo-electric power, and thermal conductivity. This system has been successfully used to characterize several new thermoelectric materials (including some of the above-mentioned compounds) and reference materials exhibiting a wide range of thermal conductivities.

We present the structure and thermoelectric properties of the new quaternary selenides K1+xM4–2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22. The compounds K1+xM4-2xBi7+xSe15 (M= Sn, Pb) crystallize isostructural to A1+xPb4-2xSb7+xSe15 with A = K, Rb, while K1-xSn5-xBi11+xSe22 reveals a new structure type. In both structure types fragments of the Bi2Te3-type and the NaCl-type are connected to a three-dimensional anionic framework with K+ ions filled tunnels. The two structures vary by the size of the NaCl-type rods and are closely related to β-K2Bi8Se13 and K2.5Bi8.5Se14. The thermoelectric properties of K1+xM4-2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22 were explored on single crystal and ingot samples. These compounds are narrow gap semiconductors and show n-type behavior with moderate Seebeck coefficients. They have very low thermal conductivity due to an extensive disorder of the metal atoms and possible “rattling” K+ ions.

Our exploratory research in new thermoelectric materials has identified the ternary and quaternary bismuth chalcogenides β-K2Bi8Se13, K2.5Bi8.5Se14 and K1+xPb4-2xBi7+xSe15, to have promising properties for thermoelectric applications. These materials have needlelike morphology so they are highly anisotropic in their electrical and thermal properties. In order to achieve long and well-oriented needles for which, consequently, the best thermoelectric performance is expected, we developed a modified Bridgman technique for their bulk crystal growth. The preliminary results of our crystal growth experiments as well as electrical conductivity, Seebeck coefficient and thermal conductivity for the compounds obtained from this technique are presented.

The concept of carrier pocket engineering applied to Si/Ge superlattices is tested experimentally. A set of strain-symmetrized Si(20Å)/Ge(20Å) superlattice samples were grown by MBE and the Seebeck coefficient S, electrical conductivity σ, and Hall coefficient were measured in the temperature range between 4K and 400K for these samples. The experimental results are in good agreement with the carrier pocket engineering model for temperatures below 300K. The thermoelectric figure of merit for the entire superlattice, Z3DT, is estimated from the measured S and σ, and using an estimated value for the thermal conductivity of the superlattice. Based on the measurements of these homogeneously doped samples and on model calculations, including the detailed scattering mechanisms of the samples, projections are made for δ-doped and modulation-doped samples [(001) oriented Si(20Å)/Ge(20Å) superlattices] to yield Z3DT ≈ 0.49 at 300K.

Compounds with clathrate-hydrate type crystal lattice structures are currently of interest in thermoelectric materials research. This is due to the fact that semiconducting compounds can be synthesized with varying doping levels while possessing low, even ‘glass-like’, thermal conductivity. Up to now most of the work has focused on type I Si and Ge clathrates. Sn-clathrates however are viewed as having the greatest potential for thermoelectric cooling applications due to the larger mass of Sn and the expected small band-gap, as compared to Si and Ge clathrates. Transport properties on type I Sn-clathrates has only recently been reported [1–3]. In this report we present ongoing experimental research on both type I and II clathrates with an emphasis on the thermal transport of these novel materials. We present thermal conductivity data Si-Ge and Ge-Sn alloys as well as on a type II Ge clathrate for the first time, and compare these data to that of other clathrate compounds.

Resonant Ultrasound Spectroscopy and low-temperature ultrasonic attenuation measurements are reported for filled and unfilled skutterudites and for Ge-clathrates. These data reveal that an unusual elastic behavior complements the thermal properties of the filled skutterudites and indicate the presence of low-energy vibrational modes. The attenuation at low-temperatures in the single-crystalline Ge-clathrate is glasslike and can be described by the same phenomenological Tunneling Model that has been developed to describe the low-temperature properties of amorphous solids.

The thermoelectric figure of merit, ZT = α2σT/λ, has been measured for pentatelluride single crystals of HfTe5, ZrTe5, as well as Se substituted pentatellurides. The parent materials, HfTe5 and ZrTe5, exhibit relatively large p- and n- type thermopower, |a| > 125 μV/K, and low resistivity, ρ ≤ 1 mΩ•cm. These values lead to a large power factor (α2σT) which is substantially increased with proper Se substitution on the Te sites. The thermal conductivity of these needle-like crystals has also been measured as a function of temperature from 10 K ≤ T ≤ 300 K. The room temperature figure of merit for these materials varies from ZT “0.1 for the parent materials to ZT ≈ 0.25 for Se substituted samples. These results as well as experimental procedures will be presented and discussed.

Our research on ternary / quaternary chalcogenides for thermoelectric applications has lead to the identification of new interesting compounds and better understanding of the chemistry and physical properties of complex chalcogenides. The chemical, geometric, electronic diversity and flexibility has been well demonstrated in BaBiSe3 and Sr4Bi6Se13 type compounds. This presents both a challenge and more opportunity in controlling and optimizing the thermoelectric properties of these complex chalcogenides, compared with elemental and binary compounds. The importance of multivalley band structure in thermoelectric materials is emphasized. Only compounds with high crystal symmetry have the possibility of having a large number of degenerate valleys in the conduction bands or peaks in the valence bands, respectively. However, most of the complex chalcogenides crystallize in low crystal symmetry. An Edisonian method of exploratory synthesis and characterization may be the working approach to find good thermoelectric materials with ZT higher than 4.

Since few years, cerium filled and partially filled skutterudites are intensively studied because they show a wide variety of fundamental and applied properties. One of them consists in high values of thermal factors for rare earth atom in antimony skutterudites [1,2]. Slack suggests [3,4] a incoherent rattling of this ion in the oversized cage “Sb12” surrounding the cerium which affects highly the phonon motion and thus lowers the lattice thermal conductivity (kl). As a rule, the lattice thermal conductivity is decreased by a factor of 5 or greater by filling entirely the voids of the binary filled skutterudites with rare earth atoms [5]. Besides, kl decreases for partially filled compounds in respect with totally filled ones [6,7]. Mass fluctuation mechanism between cerium atom and vacancy is obviously involved as the origin of this last reduction. On that purpose, theoretical calculations [7] demonstrate that the reduction belonging to mass fluctuation mechanism is an order of magnitude lower than the measured decrease. As the mass fluctuation added to the “rattling” on the cerium site is not sufficient to explain such low values of thermal conductivity, another phonon scattering mechanism must exist. In order to find another mechanism we present the influence of the filling fraction of cerium on thermal factors and the temperature dependence of this factor for a partially filled compound.

We report theoretical analysis for the transient thermal response of thermoelectric (TE) element and the integrated thin-film devices. It is predicted that the TE element geometry and applied current pulse shape influences the transient response of the system. Analysis for the integrated systems shows that the transient response is affected by the effusivity of the attached mass. This analysis provides a means to examine the effectiveness of thermal management of the thin-film devices, particularly semiconductor lasers, using the transient mode operation of thermoelectric coolers, and also suggests geometry constraints and optimum pulse shapes for an integrated system.

Thermoelectric properties of low dimensional structures based on PbTe/PbSrTe-multiple quantum-well (MQW)-structures with regard to the structural dimensions, doping profiles and levels are presented. Interband transition energies and barrier band-gap are determined from IR-transmission spectra and compared with Kronig-Penney calculations. The influence of the data evaluation method to obtain the 2D power factor will be discussed. The thermoelectrical data of our layers show a more modest enhancement in the power factor σS2 compared with former publications and are in good agreement with calculated data from Broido et al. [5]. The maximum allowed doping level for modulation doped MQW structures is determined. Thermal conductivity measurements show that a ZT enhancement can be achieved by reducing the thermal conductivity due to interface scattering. Additionally promising lead chalcogenide based superlattices for an increased 3D figure of merit are presented.

We have fabricated Bi microwire array composites ranging in diameter from 10 to 50 micrometer using the method of high-pressure-injection (HPI) of the Bi melt into microchannel arrays (MCA) templates. The composites are dense, with Bi volume fraction in excess of 70 %. The parallel Bi nanowires, whose length appears to be limited only by the thickness of the host template (up to 2 mm), terminate at both sides of the composite in the Bi bulk. The individual Bi microwire crystal structure is rhombohedral, with the same lattice parameters as that of bulk Bi; the wires crystalline orientation is predominantly perpendicular to the (113) lattice plane. The transversal magnetoresistance and Seebeck effect of the wires has been measured in magnetic fields up to 0.8 Tesla and for temperatures ranging between 77 K and room temperature.

The effect of Ni doping on the Co site of the binary skutterudite CoSb3 is investigated. We measured resistivity, Hall effect, magnetoresistance, thermopower, thermal conductivity, and magnetization of a series of samples of the form Co1-xNixSb3 with x in the range x=0 to x=0.01. We find that Ni takes the tetravalent state Ni4+, assumes the d6 electronic configuration for the lower energy non-bonding orbitals, and gives an electron to the conduction band. Ni doping dramatically suppresses the thermal conductivity, changes the temperature dependence of the thermopower, and increases the carrier concentration. Low temperature anomalies in thermopower, Hall coefficient and magnetoresistance are found.

The compounds Tl9BiTe6 (TBT) and Tl9BiSe6 (TBS) crystallize in the tetragonal space group I4/mcm. Tl9BiTe6 has a thermopower of 185 μV/K and an electrical resistivity of 5.5 mΩcm at 300K, resulting in a power factor of S2/ρ = 0.6 mW/mK2. Compared to Bi2Te3 which is the state of the art material at this temperature this is about a factor of 7 lower. At 300 K TBS has a thermopower of 750 μV/K but a high resistivity of 130 Ωcm. To optimize the thermoelectric properties of TBT solid solutions have been formed with TBS. The resistivities and have been measured on Tl9BiTe1-xSex with x = 0.05, 0.08, 0.2 and 0.5. In addition to the electrical properties the lattice constants have been measured by X-ray diffraction. The dependence of the lattice constants on the Te/Se ratio clearly deviates from Vegard's law. Different affinities of Te and Se towards the two chalcogenide sites in the crystal can explain this behavior.

The resistance R(T), the magnetoresistance R(H) and the Seebeck coefficient S(T) of thin mono-crystalline glass-coated wires of pure and doped by acceptor impurities (Sn) bismuth are studied. The measurements are carried out in the temperature range 4.2 - 300 K, and the magnetic fields up to 14 T. A significant dependence of the resistance R(T) and thermoemf S(T) on doping, sample diameter and surface state is revealed. By recrystallization, annealing and etching the samples a change in the surface scattering character of the charge carriers was achieved. The influence of the strong (up to 3–4%) elastic stretch on the band structure and kinetic coefficients of thin wires was studied. The thermoelectric figure of merit in doped bismuth wires is estimated, and possible ways of its increase in the temperature range 77 - 300 K are regarded.

Recently, CsBi4Te6 has been reported as a high-performance thermoelectric material for low temperature applications with a higher thermoelectric figure of merit (ZT ∼ 0.8 at 225 Kelvin) than conventional Bi2-xSbzTe3-ySey alloys at the same temperature. First-principle electronic structure calculations within density functional theory performed on this material give an indirect narrow-gap semiconductor. Dispersions of energy bands along different directions in k-space display large anisotropy and multiple conduction band minima close in energy, characteristics of a good thermoelectric material.

We report measurements of the thermal conductivity on a potential high temperature thermoelectric material, the quasicrystal Al70.8Pd20.9Mn8.3. Thermal conductivity is determined over a temperature range from 30 K to 600 K, using both the steady state gradient method and the 3ω method. Measurements of high temperature thermal conductivity are extremely difficult using standard heat conduction techniques. These difficulties arise from the fact that heat is lost due to radiative effects. The radiative effects are proportional to the temperature of the sample to the fourth power and therefore can lead to large errors in the measured thermal conductivity of the sample, becoming more serious as the temperature increases. For thermoelectric applications in the high temperature regime, the thermal conductivity is an extremely important parameter to determine. The 3ω technique minimizes radiative heat loss terms, which will allow for more accurate determination of the thermal conductivity of Al70.8Pd20.9Mn8.3 at high temperatures. The results obtained using the 3ω method are compared to results from a standard bulk-thermal-conductivity-technique on the same samples over the temperature range, 30 K to 300 K.

The mean free path of phonons in superlattices is estimated. It is shown to be strongly dependent on the superlattice period due to the Umklapp scattering in subbands. It first falls with increasing the superlattice period until it becomes comparable with the latter after what it rises back to the bulk value. Similar behavior is expected of heat conductivity, which is proportional to the mean free path.