The use of first principles methods based on density functional theory to investigate novel thermoelectric materials is illustrated for several empty and filled skutterudite compounds, including CoSb3, COP3, La(Fe,Co)4Sb12 and La(Fe,Co)P12. Band structures and their relationship to transport properties especially as regards optimization of thermoelectric properties is discussed. Phonon models constructed from calculations and existing experimental data for CoSb3 are presented. These have been extended to the filled skutterudites, particularly LaFe4Sb12 using additional first principles calculations to fix the La related parameters in the model. This model allows an interpretation of neutron scattering data as well as an understanding of the low frequency phonon modes that transport heat in these compounds.

The atomic displacement parameters (ADPs) measure the mean-square displacement amplitude of an atom about its equilibrium position in a crystal. It is demonstrated that the ADPs can be used to identify crystalline solids with unusually low lattice thermal conductivties. A low lattice thermal conductivity is essential in the design of thermoelectric materials with improved efficiencies.The atomic displacement parameters (ADPs) have been measured using powder neutron diffraction as a function of temperature for several clathrate-like compounds (RxCo4-yFeySb12, where R= La, Ce, Yb or TI, x=0.22, 0.8, 1, y=0, 1;Tl2SnTe5 and Tl2GeTe5). The ADP data show that in each of the compounds one of the atoms is weakly bound and “rattles” within its atomic cage. This atomic “rattling” severely reduces the ability of these crystals to conduct heat and in some cases the lattice thermal conductivity approaches the theoretical minimum value. In many clathrate-like compounds, the ADP can also be used to estimate the Einstein frequency of the “rattler”, and to predict the existence of localized vibrational modes.

There is considerable current effort to discover new thermoelectric materials with a high figure of merit Z. Some of these new materials are narrow-gap semiconductors with rather complex crystal structures. In this paper we discuss the results of electronic structure calculations in two classes of such systems. The first class consists of BaBiTe3, a structural and chemical derivative of the well-studied Bi2Te3. Similarities and differences in the band structures of these two systems are discussed. The second class consists of half-Heusler or “stuffed”-NaCl compounds MNiX, where M is Y, La, Lu, Yb, and X is a pnictogen; As, Sb, Bi. To understand the physical reason behind the energy gap formation, we compare the electronic structure of YNiSb with that of an isoelectronic system ZrNiSn, another isostructural compound of thermoelectric interest. These calculations were carried out within density functional theory (in generalized gradient approximation) using self-consistent full-potential LAPW method. Energy gaps and effective masses associated with the conduction band minimum and valence band maximum have been calculated and these quantities have been used to estimate transport properties. Large room temperature thermopower values in Bi2Te3 and BaBiTe3 can be understood in terms of multiple conduction and valence band extrema whereas similar large values in ZrNiSn and other half-Heusler compounds can be ascribed to large electron and hole effective mass.

The synthesis by controlled crystallization of elementally modulated reactants of a series of kinetically stable, crystalline skutterudites (M′1−xM4Sb12 where M′ = vacancy, RE, Hf, Al, Sn, Pb, Bi, In, Ga, Al and Y; M = Fe, Co) is discussed. Low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150°C. Nucleation of the skutterudite structure occurs exothermically on annealing at temperatures near 200°C regardless of the ternary metal. The metastable ternary compounds and the new binary compound were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 300°C for the binary compound FeSb3 to 550°C for Eu′1−x;Fe4Sb12 to higher temperatures for the filled cobalt end members. The occupation of the lattice site for the ternary cation was found to depend on the composition of the initial reactant. Samples up to a half gram in size have been synthesized and hot-pressed into pellets for measurement of both electrical properties and thermal conductivity. Preliminary evidence is presented for the formation of skutterudite superlattices, again synthesized by the controlled crystallization of elementally modulated reactants.

Since the proposal of skutterudites as possible Phonon-Glass, Electron-Crystal materials, a lot of work has been done trying to fill the structural voids with foreign “rattling” atoms. In order to keep the electronic count per unit cell constant (and thus, the semiconducting properties of most of the compounds under study) partial replacement of either the cation or the anion in the original formula by an appropriate neighbor in the periodic table is an option. In the case of antimonides, replacing part of the Sb with Ge or Sn in order to compensate the extra charge introduced by void fillers has proved useful for compounds based on rare-earth filled IrSb3. In the case of RhSb3, we found that large quantities of Sn can be incorporated into the skutterudite structure of RhSb3 without either filling the voids or producing charge carriers.

We have analyzed the stability of several cross-sections of the Rh-Sb-Sn ternary system and have found a wide range of compositions with the basic skutterudite structure as we vary the Sn content. In all the cases, the tin goes substitutionally into the antimony sites. The voids remain empty. Density measurements suggest the existence of metal vacancies, confirmed by Rietveld refinement of the powder X-ray diffraction patterns. The possibility of Sn-induced mixed-valence of Rh on the anion sites is being investigated.

Modulated elemental reactants have been used to synthesize metastable ternary compounds with the skutterudite crystal structure. The initial reactants are made up of multiple repeats of a unit containing elemental layers of a ternary metal, iron or cobalt (or a combination of these two) and antimony. The elemental layers interdiffuse upon low temperature annealing and form amorphous reaction intermediates. In this paper we target the compounds PbxFe4−yCoySb12. On annealing at temperatures between 12° and 150° C (depending on the composition) crystallization of the skutterudite structure occurs. The compounds are only kinetically stable, decomposing into a mixture of binary compounds upon annealing past a temperature of about 600°C. Preliminary data for the Seebeck coefficient and the electrical conductivity was collected The thermoelectric properties of the lead cobalt antimony skutterudite - films were measured as a function of lead occupancy.

The thermoelectric properties near ambient temperature of half-Heusler alloys based on HoPdSb, DyPdSb, and ErPdSb are reported. The Seebeck coefficients are between 60 and 250 μV/K. The resistivities range between 0.6 and 20 mΩcm, and the majority carriers are p-type. Thermal conductivities are smallest in intentionally disordered materials. The highest ambient temperature ZT obtained is 0.06. Band structure calculations are presented, and are compared to those for ZrNiSn. It is suggested that half-Heusler alloys with 18 electrons per formula unit may represent a large class of thermoelectric materials. The thermoelectric properties of another family of cubic symmetry antimonides, based on Ho3Au3Sb4 and Sm3Au3Sb4, are also reported.

We present the synthesis and structure of the new chalcogenide compounds, Rb0.5Bi1.83Te3, APb2Bi3Te7 (A = Cs, Rb), K1.25Pb3.50Bi7.25Se15 and A1+xPb4−2xSb7+xSe15 (A = K, Rb). The layered structures of the first two telluride compounds are related to each other in a very interesting fashion. These compounds are n-type metallic conductors and solid solutions with Se and Sb have also been synthesized. K1.25Pb3.50Bi7.25Se15 and its Sb analogs have a complex three-dimensional structure composed of NaCl- and Bi2Te3-type building units. The thermal stability, melting behavior, electrical conductivity and thermopower of these compounds are reported.

In previous investigations we have introduced a variety of new chalcogenide-based materials with promising properties for thermoelectric applications. The chalcogenide CsBi4Te6 was previously reported to have a high ZT product with a maximum value at 260K. In order to improve this value, a series of doped CsBi4Te6 samples has been synthesized. Current doping studies have been very encouraging, with one sample found to have a maximum power factor of 51.5 μW/cm·K2 at 184 K. This paper reports on material characterization studies through the usual transport measurements to determine optimum doping concentration for various dopants.

Chevrel phases have an open lattice with large voids in which cations can be inserted. These cations are good scattering centers for phonons. The thermal conductivity of these compounds must be low. Chevrel phases are generally metallic, but we calculated the density of states of several compounds and we found that Zn2Mo6Se8, Cd2Mo6Se8, TiMo6Se8 are semiconducting whereas SnMo6Se8, Cu2Mo6Se8 and CrMo6Se8 are metallic. We are currently trying to synthesize Zn2Mo6Se8 by different ways.

Bismuth as a semimetal is not a good thermoelectric material in bulk form because of the approximate cancellation between the electron and hole contributions. However, quantum confinement can be introduced by making Bi nanowires to move the lowest conduction subband edge up and the highest valence subband edge down to get a one-dimensional (1D) semiconductor at some critical wire diameter dc. A theoretical model based on the basic band structure of bulk Bi is developed to predict the dependence of these quantities on wire diameter and on the crystalline orientation of the bismuth nanowires. Numerical modeling is performed for trigonal, binary and bisectrix crystal orientations. By carefully tailoring the Bi wire diameter and carrier concentration, substantial enhancement in the thermoelectric figure of merit is expected for small nanowire diameters.

It is well known that bismuth telluride (Bi2Te3), its isomorphs (Bi2Se3 and Sb2Te3) and their alloys have the optimum bandgap (0.13 eV to 0.21 eV) for efficient solid state cooling applications around 300 K. Recently interesting work argued that the use of quantum well structures can enhance the figure of merit ZT as a result of the improvement of carrier charge density of state and the reduction of the thermal conductivity. However, for the production of such structures it is necessary to establish the optimum growth conditions and the doping levels of thin films based on Bi2Te3 and its isomorphs.

In this paper we report on the growth characteristics of Bi2Te3 ternary alloys (even quaternary) thin films elaborated by the Hot Wall Epitaxy (HWE) technique. Ternary alloys based on bismuth telluride have been deposited as thin films on silicon and silica substrates. Hot Wall Epitaxy have been demonstrated to be a suitable technique in chalcogenides growth. These films are formed in a closed chamber, that make possible to keep substrates at relatively high temperature Ts without selective loss of individual components from condensate. Experimental procedures, such as substrate and source materials preparations, have been described in our previous publications. Thin films obtained are well oriented (001) and have block single-crystal structure. These films were studied by microstructural investigations and electrical measurements (electrical conductivity σ, Hall coefficient RH and Hall mobility μn) in the temperature range from liquid nitrogen to 570 K.

The electrical conductivity, Seebeck coefficient, and thermoelectric power factor of PbTe/Pb1−xEuxTe quantum well structures are investigated theoretically. The variational method is employed. The anisotropy of effective masses, the multivalley character of the bulk semiconductors and also the dependence of effective masses in dimensional quantization subbands on the well width are taken into account. The carrier scattering both on optical and acoustical phonons is considered for structures with (111) and (100) crystallographic orientation. It is found that the power factor is larger in (100) oriented quantum wells. The results of recent experiments are discussed.

We prepared composite materials of AgBiTe2 with several contents of Ag2Te small-size grains for applications to thermoelectric devices. By enhancing long-wavelength phonon scattering at the grain boundaries, lattice thermal conductivity (thermal conductivity due to lattice vibrations) decreased 30% and thus the thermoelectric characteristics were significantly improved.

We report data of Raman study of Si/Ge and Si/SiGe superlattices designed for thermoelectric applications. The obtained Raman spectra clearly indicate the presence of folded doublets from longitudinal acoustic phonons. Due to the significant difference in the sound velocities for Si and Ge the position of the doublets strongly depends on the superlattice layer thickness and ordering. Comparison of the Raman data for different samples with measured thermal and thermoelectric properties allows us to determine a correlation between the strength of the phonon confinement in these structures and their thermoelectric properties. Our experimental results are consistent with recent theoretical prediction of increased thermoelectric figure of merit in semiconductor quantum wells.

The ternary compound, SrBiTe3, was prepared by the molten flux method. It crystallizes in the orthorhombic space group Pmmn with a=4.665(2)Å, b=4.517(2)Å, c=16.129Å, and Z=2. The compound is composed of a NaCl-type [Bi2Te4]2- layer and a flat square (Te2)2- net with equal Te-Te distances of 3.24Å. The Sr2+ ions are stabilized between the layers in monocapped square antiprismatic sites. X-ray and electron diffraction studies reveal a 12-fold superstructure due to subtle distortions within the Te net of the compound. The superstructure was refined in the monoclinic space group C2 with a=27.923(5)Å, b=32.228(6)Å, c=4.5069(9)Å, β=90.000(3)°. The compound is a semiconductor with a band gap value of 0.14 eV. It melts incongruently at 613 °C.

The AgPbBiQ3 class of compounds and their solid solution members are related to the NaCl structure type, where Ag, Pb and Bi atoms are statistically disordered on the Na site and Q atoms occupy the Cl site. These compounds were synthesized by combining the elements in the appropriate ratio and heating under static vacuum at 900° C for 3 days. They are narrow gap semiconductors with band gaps in the range of 0.6 to 0.28 eV. The charge-transport properties were measured on ingots as a function of temperature. The compounds AgPbBiTe3, AgPbBiSe3, AgPbBiTe2.75Se0.25and AgPbBiTe2Se, undoped, possess an electrical conductivity in the range of 70 S/cm to 400 S/cm. These materials exhibit negative thermopower ranging from -40 μV/K to -160 μV/K at room temperature and thermal conductivity less than 1.30 W/mK.

B4C ceramics doped with various content of Si (0 to 2.03 at%) are prepared via hot pressing. The composition and microstructure of the ceramics are characterized by means of XRD and EPMA. Their electrical conductivity and Seebeck coefficient of the samples are measured from room temperature up to 1500K. The electrical conductivity increases with temperature, and more rapidly after 1300K; the Seebeck coefficient of the ceramics also increases with temperature and rises to a value of about 320μVK−1. The value of the figure of merit of Si-doped B4C rises to about 4 × 10−4K−1 at 1500K.

We have recently developed a device to measure resistivity and Seebeck values of a material as a function of temperature over a range of 80K < T < 700K. These measurements overlap in temperature with our existing apparatus (4K < T < 320K). These measurements are necessary for the investigation of potential thermoelectric materials at elevated temperatures, where power generation applications are important. The probe design allows for various types of samples to be evaluated from needle like samples ( L ≈ 2mm, D ≈ 50μm ) to larger pressed pellet samples ( ≈ 2mm × 2mm × 10mm). Separate high temperature cartridge heaters are used to stabilize both the absolute temperature, T, as well as the temperature difference, ΔT. Thermocouples measuring T+ΔT/2 and T-ΔT/2 are also employed to measure sample voltages for resistance and thermoelectric power. Design details and measurement specifics will be discussed. Data taken on standards a well as some research samples will be presented.

Polymer based thermoelectric pastes have been developed by dispersing metal and metal alloy powders into a polymer binder system. Thermoelectric materials and elements are formed by depositing the pastes into a mold or onto a substrate and subsequently sintering them to form a conductive network with thermoelectric properties. A Seebeck coefficient of 65 μV/°K, and a resistivity of 0.05 Ω-cm have been achieved in the early stage of material development. A novel processing method has also been developed for the fabrication of small size thermoelectric elements and thermoelectric modules with a multitude of thermoelectric couples using these pastes. Thermoelectric elements as small as 250 μm in diameter and 25 μm in thickness have been achieved. Thermoelectric modules with 275 couples/in2have been fabricated using the thermoelectric pastes and the fabrication technique. Further improvement in the material properties and reduction in the sizes of the thermoelectric elements are underway.