In this work is presented the application insertion techniques for the modification of the performance of thermoelectric materials. The results indicate that in cases as in Bi2Se3 compounds where insertion of foreign species in the lattice is possible due to its particular crystal structure, the insertion technique could be proved a valuable easy to apply and cost effective, alternative to doping technique. The techniques can produce homogeneous materials with electronic and thermoelectric properties finely tuned.

It is well known that the temperature dependence of Seebeck coefficient of β-iron disilicide has unusual shape that cannot be described by an ordinary theory. A theory describing such a shape is suggested. It is shown this shape can be explained by optical phonon drag effect. Very high value of ZT could be achieved in some material on the base of this effect.

In order to clarify the electrical properties of Cr-doped β-FeSi2(Fe1-xCrxSi2) and Mn-doped β--FeSi2(Fel-yMnySi2), the Hall effect and electrical resistivity of Fe1-xCrxSi2(0.01 ≦ x ≦ 0.05) and Fel-yMnySi2(0.01 ≦ y ≦ 0.10) have been measured in the temperature range between 80 and 300 K. fhe solid solutions Fel-xCrxSi2and Fel-yMnySi2are p-type over the measured temperature range. The observed features of RH of these solid solutions are explained by using the two-band model with the existence of two acceptor levels. The hole concentration of Fe1-xCrxSi2at 300 K ranges from 8.9×1018cm−3for x=0.01 to 1.1×1020cm−3for x=0.05, which is one order of magnitude higher than that of Fel-yMnySi2.

Epitaxial ReSi1.75 thin films of variable thickness between 15nm and 150nm have been prepared in an one step process by Facing Target Sputtering (FTS) onto heated (100) and (111)Si and SOS wafers. The epitaxial relations and film structure have been investigated by Xray diffraction and transmission electron microscopy. Epitaxial growth was found at a substrate temperature of 1070K. Thermoelectric properties were measured between 100K and 450K and compared to the transport behavior of bulk ReSi1.75 and polycrystalline films. A distinct dependence of both the conductivity and thermopower was found on the film thickness, on unintentional doping and on the film structure. The results show that epitaxial ReSi1.75 films prepared by FTS can be a basis for further investigations of thermoelectric silicide/silicon multilayers.

Single crystals of the known ternary cerium intermetallic Ce5Cu19P12were grown by Sn flux and I2transport methods. The long axis of the black hexagonal needles was confirmed to be the c-axis by single crystal X-ray diffraction. Electrical resistivity of both single crystals and a pressed pellet was measured from 4 - 300 K; the room temperature resistivity is about 400 μΩ-cm for the needle axis of the crystals and about 5 mΩ-cm for the pressed pellet. The thermopower of the pressed pellet was found to be 34 μV/K at room temperature.

Thin films of the hexagonal phase of Bi1-xTe1+x have been grown on CdTe(111) substrates using molecular beam epitaxy (MBE). Analysis of X-ray diffraction patterns (θ-2θ scans and rocking curves) of the films shows that their crystallinity depends upon the compositional deviation from stoichiometric BiTe. Measurements of the temperature-dependent thermoelectric power (TEP) of the films reveals that compositional changes cause the TEP to vary from electron dominant (n-type) to hole dominant (p-type), implying their possible application as a thermoelectric cooler or power generator. Measurements of the temperature-dependent resistivity of the films were conducted, and the analysis shows semimetallic behavior. These results demonstrate that Bi1-xTe1+xis an appropriate model system to study the dependencies of thermoelectric and structural properties on binary composition.

We have grown high quality Bi2Te3 thin films on CdTe(111)B substrates using MBE. Structural properties have been investigated using in-situ reflection high-energy electron diffraction (RHEED) and θ-2θ X-ray diffraction analysis. They show that Bi2Te3films on CdTe(111) grow along the (00.l) in the hexagonal cell with a layer-by-layer growth mode, resulting in a smooth surface, and an X-ray Bragg peak FWHM of 0.2°. The thermopower and electrical conductivity of the stoichiometric Bi2Te3 films were ∼200 μV/K and 103(Ωcm)−1, respectively, comparable to the single crystal bulk values. We have observed the antisite defect effect in Te-rich Bi2Te3films: excess Te occupies Bi lattice sites and behaves as an n-type dopant. Crystallinity and transport properties are strongly affected by non-stoichiometry.

We are continuing our synthetic investigations in the ternary A/Bi/Se systems and also expanded our interests into the quaternary A/M/Bi/Se (A = Rb, Cs, Sr and Ba; M = lanthanide or Pb) systems. We have synthesized several new ternary and quaternary bismuth selenides with band gaps <0.6eV such as Cs2Bi7.33Se12, A2Bi8Se13 (A = Rb, Cs), Ba4-xBi6+2/3xSe13, and Ba3±xPb3±xBi6Se15. The synthesis, crystal structures and charge transports properties of these new compounds are presented.

Reported are the temperature dependencies of the thermoelectric power and electrical resistivity of mixed rhenium chalcogenides Re6Te15-x,Sex (0 ≤ x ≤ 8) in the range 90–420 K. Influence of the partial chalcogen exchange on thermoelectric properties of these compounds is discussed. The samples are prepared by sintering elemental powders inside evacuated and sealed quartz ampoules at 1150 K for 170 hours. X-ray analysis reveals an orthorhombic lattice for samples with x < 8 and a tetragonal lattice for the Re6Te7Se8. sample. The lattice parameters and the unit cell volume decrease with increasing selenium concentration.

The measurements indicate p-type semiconducting behavior for all samples. The presence of the energy gap is observed at higher temperatures (T ≥ 180–220 K) for all x. Data suggest hopping conduction at lower temperatures. Room temperature resistivities increase non-linearly from 6.9 to 20.4 Ω m with the increasing selenium content. Initially, the thermoelectric power a increases with temperature for all samples, with the fastest increase in Re6Se8 Te7 and the slowest in Re6Te15. The temperature at which a reaches maximum decreases with the increasing Se content. Above this temperature, a decreases uniformly as the temperature increases, the slowest increase being for Re6Se8Te7 and the fastest for Re6Te15. Such α(T) dependence is also discussed. The temperature dependence of the power factor, α2/ρ, is presented. Comparison of ρ, α, and the power factor in Re6SexTe15-x with currently used state-of-the-art materials is given.

Temperature dependencies of the electrical resistivity, ρ, and the thermoelectric power, α, are reported for Re6 MxTe15 (M = Ga, In, Ag; x = 0, 1, 2) between 90–380 K. Theoretical discussion of the results is presented. The materials, synthesized by filling large voids in the Re6Te15 cluster system, may have potential thermoelectric applications around and below room temperatures. The samples are prepared by reacting 99.99% pure elemental powders in evacuated and sealed quartz ampoules at 1070 K for 170 hours. The resistivity data indicate semiconducting behavior for all samples. Possible hopping conduction is present at lower temperatures. The energy gap is observed at higher temperatures in all the samples.

Positive values of α in Re6(Ga,In)x Te15 (x = 0, 1, 2) indicate p-type semiconducting behavior in the studied temperature range. For these samples α increases initially with temperature, then levels off to a nearly constant value. The positions of the sharp peaks in a, observed at lower temperatures for x = 1, 2 only, depend on the Ga (In) concentration. High values of a (∼ 300 μV/K) are measured at room temperatures. In Re6AgTe15 α has small positive values (∼ 20–40 μV/K) between 185 K and 270 K. Outside this range α is negative. It reaches local maxima of -340 μV/K at 105 K and -350 μV/K at 370 K. In Re6Ag2Te15 α changes from positive to negative values above 295 K. A maximum positive value of +350 μV/K is reached at 250 K and maximum negative of -250 μV/K at 330 K. The power factor, α2/ρ, increases with temperature for all studied samples. Theoretical fits to α(T) for all samples are discussed. Also discussed is the effect of filling the voids in the rhenium-telluride system on the figure of merit.

The fabrication of a thermoelectric nanocomposite material consisting of nanometer scale bismuth (Bi) wires embedded in a porous mica template host is discussed in detail. In fabricating the mica templates, a positive ion accelerator is employed to irradiate 10 μm thick mica sheets with collimated beams of 15 MeV carbon ions at fluence levels of 1.5 × 1013 ions/cm2. The normally incident beam generates latent nuclear damage tracks in the direction oriented perpendicularly to the mica basal plane. Atomic Force Microscope (AFM) pictures of etched tracks verify that we have fabricated porous templates containing arrays of channels with densities up to 1013/cm2 and diameters as small as 5 nanometers, thus providing pores having an aspect ratio of about 2000:1. Scanning electron micrographs of 50 nm diameter tracks show that these are parallel channels with smooth, non-tapered walls. In addition, to fabricate the wires we have developed a solution-electrodeposition process employing a PC processor-driven potentiostatic/galvanostatic system. Currently, we are electrochemically embedding 10 μm long Bi wires through the nanochannel templates. The potentiostatic electrodeposition behavior of these wires is described. Current-voltage waveforms confirm that the wires are electrically uninterrupted through the mica template. Transmission electron micrographs (TEM) show these wires are single crystals, of well-defined orientation with diameters down to 50 Å. We observe bundles of 80 Å wires with a packing density of about 109/cm2. Energy Dispersive X-ray Spectroscopy (EDS) has corroborated the presence of Bi in the nanochannels. The observation of the bismuth crystal orientation in the porous mica template is examined.

The small effective mass of Bi, high anisotropy of its Fermi surface, and the high aspect ratio (length/diameter) of Bi nanowires make this an excellent system for studying quantum confinement effects of a one-dimensional (ID) electron gas in relation to electrical conductivity, thermoelectric power, and thermal conductivity. A theoretical model based on the basic electronic band structure of bulk Bi is suitably modified to describe 1D bismuth nanowires and is used to predict the dependence of these transport properties on nanowire diameter, temperature and crystalline orientation of the bismuth nanowires. Experiments have been carried out on ultra-fine single crystal Bi nanowires (10–120 nm diameter) with a packing density as high as 7 × 1010 wires/cm2 to test the quantum confinement assumptions of the model and the occurrence of a quantum confinement-induced semimetal-to-semiconductor transition as the wire diameter becomes less than 100 nm. Prospects for the use of bismuth nanowires for thermoelectric applications are discussed.

The resistance R of a high density array of 200 nm diameter wires in anodic alumina is studied experimentally. R decreases from 300 K down to 0.3 K. An anomalous drop in R was found at T<50 K. Possible explanations are considered, including one based on the onedimensional quantum size effect. Also, the large magnetoresistance is suggestive of weaklocalization effects. The implication for the thermoelectric properties is discussed.

Our exploratory research to identify new promising candidates for next generation thermoelectric applications has produced several interesting new materials which are briefly described here. We present their compositions, solid state structures, properties and charge transport behavior. The compounds CsBi4Te6, β-K2Bi8Se13, Ba4Bi6Se13, Eu2Pb2Bi6Se13, KBi6.33S10, Eu2Pb2Bi4Se10, Ba2Pb2Bi6S13 and K1.25 Pb3.5Bi7.25Se15 are particularly noteworthy.

A useful approach to identify materials with high thermoelectric figure of merit is to search for solids that offer great flexibility to modify and tailor the structure so as to achieve the optimal transport behavior. Among the most promising novel thermoelectric materials are solids with “open crystal structure”. They may be typified by structures with unfilled cages, crystals with an empty atomic sublattice, and by a network of polyhedral cages enclosing guest species. In this paper we present our latest results concerning transport properties in the above classes of solids. Specifically, we focus on the filled skutterudites, half-Heusler alloys, and clathrates.

The traditional approach to the search for improved materials, such as thermoelectric materials, involves the sequential synthesis and characterization of new compounds, followed by the investigation of their properties. Each single solid state compound has materials interaction parameters (e.g., degree of orbital overlap, orbital energy, band energies etc.) that determine its thermoelectric properties. These interaction parameters can be used to define a phase space, in which individual compounds are represented by a single point. Because this phase space can be rapidly explored with pressure, our efforts have focused on the synthesis of complex semiconductors and rare earth based materials, followed by pressure-tuning studies of their thermoelectric properties. This approach to exploring interaction parameter phase space potentially allows the thermoelectric power to be more rapidly and cleanly optimized than is possible with the traditional approach. Here we present some results for rare earth thermoelectric compounds that have been pressure-tuned. Demonstration of the existence of materials with improved ZT at high pressure can also provide insight into the structural and electronic parameters necessary to achieve high figures of merit at ambient pressure.

Equations describing the resistances to the transport of heat and electricity through boundaries of thin-film thermoelectrics are derived. We show that these boundary resistances obey a boundary form of the Wiedemann-Franz law and cause a new type of thermal instability for short thermoelectric devices. We consider boundary thermal resistances both for phonons (Kapitza resistance) and for electrons, the contact electrical resistance at the junctions, and the boundary thermoelectric effects. It is shown that the Kapitza resistance causes reduction of the effective thermal conductivity of the system only if the electron and phonon subsystems are out of equilibrium. In this case, the thermoelectric figure of merit Z can be increased by reducing the thickness of the film. The electrical contact resistance at the junctions is shown to degrade the performance of the device. However, according to the boundary Wiedemann-Franz law, electrical contact resistance is accompanied by a thermal boundary resistance for the electron subsystem, which can cause an additional enhancement of Z. In some cases, this can lead to a device with ZT as high as 3 at room temperatures.

The use of chemical solution routes to form inorganic thin films is a relatively new method which represents an alternative to vapor phase routes. The present study involves the use of a chemical solution route, the decomposition of metal carboxylates, to prepare bismuth thin films of controlled porosity. Such morphologies offer the opportunity to disrupt phonon transport without greatly affecting electrical conductivity and bismuth represents a well known system in which to investigate these effects. Porous bismuth thin films have been prepared using bismuth 2-ethylhexanoate (Bi[OOCCH(C2H5)C4H9]3) as the precursor in a solvent of 2-methyl- 1-propanol. The solution is deposited on glass, Kapton, silicon, alumina or magnesia substrates by spin coating and heated to between 250 – 300°C in hydrogen. Heat treatment temperature and time are important for controlling film microstructure as both pore volume (25 to 50%) and preferred orientation depend upon heat treatment conditions. Bismuth films (62 nm thick) with 32% porosity exhibit conductivities in the range of 150 S/cm with Seebeck coefficients comparable to that of bulk materials.

We have grown Bi1-xSbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE. We have observed several differences with the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum bandgap is shifted to a lower Sb concentration, from 15% in bulk to 9%. The power factor S2/ρ (where S is thermoelectric power(TEP) and ρ electrical resistivity) peaks at a significantly higher temperature (250K) than previously reported for the bulk alloy (80K). The magnetotransport properties of Bi1-x,Sbx thin films (x = 0, 0.09, and 0.16) and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis (QMSA) and multicarrier fitting to the magneticfield- dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The calculated S values are in good agreement with experimental results. The structural stability of bulk Bi is studied using the local density linear muffin-tin orbital method. It is shown that the internal displacement changes the Bi electronic structure from a metal to a semimetal, in qualitative agreement with a Jones-Peierls-type transition. The total energy is calculated to have a double well dependence on the internal displacement, and to provide a stabilization of the trigonal phase. We show that an increase of the trigonal shear angle leads to a semimetal-semiconductor transition in Bi.

We review thermoelectric effects in a magnetic field at a phenomenological level. The measurement problem is discussed in detail, and results are presented based on the time-dependent voltages developed by thermoelectric materials in zero field and in large magnetic fields. These time dependent voltages provide a direct and unambiguously accurate measurement of both the Peltier and Ettingshausen figures of merit, and can be used to obtain all the relevant transport including thermal conductivity. Discussion of the limiting performance of both Peltier and Ettingshausen coolers are discussed. Data are shown for Ettingshausen materials with part-per-million levels of Sn doping. Thought these materials exhibit high Ettingshausen figures of merit, Sn doping does not appear to improve performance.