Results of the temperature dependence of the thermoelectric power and fourprobe resistance of single wall carbon nanotube mats are presented. The data are interpreted in terms of the response of a percolating network of metallic nanotube bundles. To the best of our knowledge, the work represents the first systematic study of the transport properties of a series of samples prepared using different transition metal-Y growth catalysts. Although x-ray diffraction and Raman scattering data indicate these samples are nominally the same, we find that the identity of the catalyst has a pronounced effect on the electrical transport properties. The data are interpreted qualitatively in terms of a dilute Kondo system involving the coupling of the the localized magnetic moments of impurity atoms (derived from the catalyst), and the conduction electron spins in the nanotube walls.

We report calculations which show that the band structure of CoSb3 is typical of a narrow band-gap semiconductor. The gap is strongly dependent on the relative position of the Sb atoms inside the unit cell. We obtain a band gap of 0.22 eV after minimization of these position. This value is more than four times larger than the result of a previous calculation which reported that the energy bands near the Fermi surface are unusual. The electronic states close to the Fermi level are properly described by a two-band Kane Model. The calculated effective masses and band gap are in excellent agreement with Shubnikov de Haas and Hall effect measurements. Recent measurements of the transport coefficients of this compound can be understood assuming it is a narrow band gap semiconductor, in agreement with our results.

Starting to CeFe4Sb12 and trying to obtain low gap semiconductors with very low thermal conductivity, we have performed in this compound the substitution of Ni for Fe.This substitution induces a reject of cerium from the structure. In this paper we present synthesis and physical properties of CeyFe4-xNixSb12 in which a relationship is existing between x and y to retain the electronic account giving semiconducting character.

Bulk skutterudite phases based on the CoAs3 structure have yielded compositions with a high thermoelectric figure-of-merit (“ZT”) through the use of doping and substitutional alloying. It is postulated that further enhancements in ZT may be attained in artificially structured skutterudites by engineering the microstructure to enhance carrier mobility while suppressing the phonon component of the thermal conductivity. In this work the growth and properties of singlephase CoSb3 and IrSb3 skutterudite thin films are reported. The films are synthesized by pulsed laser deposition (PLD) where the crystallinity can be controlled by the deposition temperature. Powder X-ray diffraction (PXRD), Transmission electron microscopy (TEM) and Rutherford- Back Scattering (RBS) were used to probe phase, structure, morphology and stoichiometry of the films as functions of growth parameters and substrate type. A substrate temperature of 250°C was found to be optimal for the deposition of the skutterudites from stoichiometric targets. Above this temperature the film is depleted of antimony due to its high vapor pressure eventually reaching a composition where the skutterudite structure is no longer stable. However, when films are grown from antimony-rich targets the substrate temperature can be increased to at least 350°C while maintaining the skutterudite phase. In addition, adhesion properties of the films are explored in terms of the growth mode and substrate interaction. Finally, preliminary room temperature electrical and thermal measurements are reported.

Several compounds with the Cr3S4 structure type have been studied for their thermoelectric properties. All exhibit low lattice thermal conductivity of about 15 mW/cmK, independent of temperature. Many of the compounds, such as Co3Se4, Ni3Se4, Fe3Se4, Ti3Se4, FeNi2Se4, and FeCo2Se4, are metals with relatively low electrical resistivity and Seebeck coefficient. The Cr containing compounds, such as Cr3Se4, NiCr2Se4, CoCr2Se4, and FeCr2Se4, have the largest Seebeck coefficients and highest resistivity. thermoelectric applications is FeCr2Se4.

We investigate the effect of Fe substitution for Co on the lattice thermal conductivity of CoSb3 skutterudites. The polycrystalline materials are formed from uniaxially hot-pressed powders. Three alloys were prepared with 0, 3% and 10% Fe, respectively. Thermal conductivity measurements were made between 80 K to 450 K. The lattice thermal conductivity of 10%Fe:CoSb3 is approximately two times smaller than the lattice thermal conductivity of CoSb3 over the entire temperature range. This effect cannot be accounted for by the phenomenlogical theory considering only the mass difference and strain field due to the alloying (Fe) atom. Other phonon scattering mechanisms are discussed. Comparison is made with the partially-filled skutterudite alloy, La0.65Fe2.8Co1.1Sb12.

In this work we show that scanning probe electrostatic force microscopy (EFM) can be applied to low dimensional electronic nanostructures for imaging the density of states of quantum confined carriers. The results on EFM studies are presented for quasione- dimensional (ID) Bi quantum wire arrays and quasi-two-dimensional (2D) GaAs/AlxGa1-x As multiple quantum well structures.

We have fabricated ultra-fine Bi nanowire (10–120 nm) arrays with packing densities as high as 7.1 × 1010/cm2 by pressure injecting its liquid melt into the evacuated nano-channels of an anodic alumina template. Using this fabrication technique, we have also prepared Te-doped n-type Bi nanowires. Free-standing Bi nanowires with an aspect ratio (length/diameter) higher than 500 are obtained by etching away the anodic alumina matrix without attacking the Bi. The resulting Bi nanowires are shown to be single crystals (with the same crystal structure as bulk Bi) and all the wires of a nanowire array are similarly oriented along the wire axis. The small electron effective mass of Bi, the high anisotropy of its Fermi surface, and the large aspect ratio of the Bi nanowires make this a very promising material for low-dimensional thermoelectric applications and an excellent system for studying quantum confinement effects in a quasi-one-dimensional (1D) electron gas. A theoretical model based on the basic band structure of bulk Bi, suitably modified for the 1D situation, is constructed to explore the electrical transport properties of Bi nanowires, which are expected to be very different from those of bulk Bi.

Understanding phonon heat conduction mechanisms in low-dimensional structures is of critical importance for low-dimensional thermoelectricity. In this paper, we discuss heat conduction mechanisms in two-dimensional (2D) and one-dimensional (1D) structures. Models based on both the phonon wave picture and particle picture are developed for heat conduction in 2D superlattices. The phonon wave model, based on the acoustic wave equations, includes the effects of phonon interference and tunneling, while the particle model, based on the Boltzmann transport equation, treats the internal as well interface scattering of phonons. For 1D systems, both the Boltzmann transport equation and molecular dynamics simulation approaches are employed. Comparing the modeling results with experimental data suggest that the interface scattering of phonons plays a crucial role in the thermal conductivity of low-dimensional structures. We also discuss the minimum thermal conductivity of low-dimensional structures based on a generalized thermal conductivity integral, and suggest that the minimum thermal conductivities of low-dimensional systems may differ from those of their corresponding bulk materials. The discussion leads to alternative ways to reduce thermal conductivity based on the propagating phonon modes.

In bulk form, Si1-xGex is a promising thermoelectric material for high temperature applications. In this paper, we report results from an experimental study as well as theoretical modeling of the quantum confinement effect on the enhancement of the thermoelectric figure of merit. Si/Si1-xGex, multiple quantum well structures are fabricated using molecular beam epitaxy (MBE) on SOI (Silicon-on-Insulator) substrates in order to eliminate substrate effects, especially on the Seebeck coefficient. A method to eliminate the influence of the buffer layer on the thermoelectric characterization is presented. An enhancement of the thermoelectric figure of merit within the quantum well over the bulk value is observed.

A large enhancement in the thermoelectric figure of merit for the whole superlattice, Z3DT, is predicted for short period GaAs/AlAs superlattices relative to bulk GaAs. Various superlattice parameters (superlattice growth direction, superlattice period and layer thicknesses) are explored to optimize Z3DT, including quantum wells formed at various high symmetry points in the Brillouin zone. The highest room temperature Z3DT obtained in the present calculation is 0.41 at the optimum carrier concentration for either (001) or (111) oriented GaAs(20 Å)/AIAs(20 Å) superlattices, which is about 50 times greater than the corresponding ZT for bulk GaAs obtained using the same basic model.

The thermoelectric properties (resistivity and thermopower) of single crystals of the low-dimensional pentatelluride materials, Hffe5 and ZrTe5, have been measured as a function of temperature from 10K < T < 320K. Both parent materials exhibit a resistive transition peak, Tp ≈ 80K for HfTe5 and Tp ≈ 145K for ZrTe5. Each display a large positive (p-type) thermopower (α ≥ +125μV/K) around room temperature, which undergoes a change to a large negative (n-type) thermopower (α≤-125μV/K) below the peak temperature. The magnitude of this resistive anomaly is typically 3–7 times the room temperature value of ≈ 1 mΩ•cm. Through isoelectronic substitution of Zr for Hf (Hf1-xZrxTe5), a systematic shift is observed in Tp as the Zr concentration increases. Small Ti substitution for Hf and Zr affects the electronic properties strongly, producing a substantial reduction in Tp for either parent compound. However, the large values of thermopower and the magnitude of the resistive peak remain essentially unchanged. Substitutions of Se or Sb on the Te sites greatly affects the electronic behavior of the parent materials. Se doping increases the thermopower values by ≈20% while decreasing the resistivity by as much as 25%. These effects double the power factor, α2σT, of the parent materials. Small Sb substitutions appear to completely suppress the resistive anomaly. These features in the resistivity and thermopower signal a large degree of tunability in the temperature range of operation. The potential of these materials as candidates for low temperature thermoelectric applications will be discussed.

We present initial assessments of the thermoelectric properties of two ternary tellurides with known crystal structures, Tl2GeTeM5 and Tl2SnTe5. Tl2SnTe5 appears to have a p-type figure of merit about the same as that of Bi2Te3, the best thermoelectric material among binary compounds. A good figure of merit is possible because the lattice thermal conductivity is very low. Based on neutron diffraction data, we have calculated atomic displacement parameters and thermal expansion coefficients. The atomic displacement parameters give some understanding of the low lattice thermal conductivity.

An enhanced thermoelectric figure of merit, ZT, has been predicted for Bi2Te3 in the form of 2-dimensional quantum wells. A new approach to making multiple quantum well (MQW) structures for thermoelectric applications utilizing a chemical intercalation technique is proposed and investigated. It is proposed that by starting from Li intercalated Bi2Te3 and Bi2Se3, the layers of these materials can be separated by chemical means. The layers of Bi2Te3 or Bi2 Se3 can then be restacked, by self-assembly, forming a non-periodic array of quantum wells. These chemically prepared MQWs are characterized by X-ray diffraction, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) at various stages in the sample preparation to assess the degree to which the actual samples match the proposal. Experimental measurements of the Seebeck coefficient (S) and the electrical conductivity (σ) were performed over a range of temperatures for the initial bulk materials. It is found that some of the steps in the proposed fabrication have been achieved but still much improvement is needed before any practical thermoelectric 2D-system can be provided.

Recent measurements of the thermoelectric transport properties of a series of the half- Heusler compound ZrNiSn are presented. These materials are known to be bandgap intermetallic compounds with relatively large Seebeck coefficients and semimetallic to semiconducting transport properties. This makes them attractive for study as potential candidates for thermoelectric applications. In this study, trends in the thermoelectric power, electrical conductivity and thermal conductivity are examined as a function of chemical substitution on the various fcc sub-lattices that comprise the half-Heusler crystal structure. These results suggest that the lattice contribution to the thermal conductivity may be reduced by increasing the phonon scattering via chemical substitution. The effects of these substitutions on the overall power factor and figure-of-merit will also be discussed.

We have begun a systematic investigation the electrical transport properties of the AlPdMn quasicrystalline system. Resistivity and thermopower measurements have been performed over a temperature range between 5K and 320K. In the pure, single phase Al70Pd20Mn10 we have observed thermopowers as high as +80 μV/K around room temperature with resistivities of 1.5 mΩ-cm. Thermal conductivity measurements have been performed yielding values less than 2 W/m-K for all the samples investigated to date. We will discuss how the thermopower and resistivity vary as a result of differing sample composition as well as different processing and annealing conditions. Several different preparation techniques have been employed to further understand how various factors affect the thermal and electrical properties of this quasicrystalline system and how these may be adjusted to optimize these materials for possible thermoelectric applications.

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr1-xErx)Ni(Sn1-xSbx) and ErNiSb1-xPnx (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb1-xPnx (Pn=As, Sb, Bi) solid solutions, but for (Zr1-xErx)Ni(Sn1-xSbx) members vary from negative to positive values when 0 < x < 1. Some of these compounds show metallic conductivity while others exhibit thermally activated charge transport. Solid solutions of these materials have lower thermal conductivities than the pure members, RNiSb (R=Ho, Er, Yb and Y) and ZrNiSn. The electronic structures of RNiSb compounds, where R is Y, La, Lu, and Yb, have been studied with density functional theory. The results of the calculations for these systems, except for the Yb compound, indicate narrow gap semiconductors with large effective masses near the conduction band extrema. The Yb system is expected to show heavy fermion characteristics.

An investigation into the structural, chemical, electronic, and thermal properties of polycrystalline semiconductor clathrate compounds with different “guest” ions in the voids of these structures is presently underway by the author and his colleges. Despite the well-defined crystalline structure of these semiconductor materials, the thermal conductivity shows a magnitude and temperature dependence more typical of amorphous materials than crystalline materials. The localized low frequency vibrations of these “guest” ions is believed to interact with the acoustic phonons of the host lattice resulting in a very low thermal conductivity. Indeed in the case of Ge-clathrates a “glass-like” thermal conductivity was observed. The markedly low thermal conductivity displayed by these semiconductor materials, along with the high power factors, demonstrates the potential of this material system for thermoelectric applications.

We have calculated theoretically the vibrational modes of the type-I Ge46 clathrate and some related Zintl phase structures MxGayGe46-y using ab initio density functional theory. The vibrational modes of pure Ge46 (without guest species) are compared to the MxGayGe46-y structures, and it is found that small metal atoms (e.g. Na) have vanishing restoring force in the large cage, while the larger alkali-earth metal Ba has a restoring force constant less than 10% of that of the a framework atom. This makes them ideal “rattler” systems.

Thermionic emission current in heterostructures can be used to enhance thermoelectric properties beyond what can be achieved with conventional bulk materials. The Bandgap discontinuity at the junction between two materials is used to selectively emit hot electrons over a barrier layer from cathode to anode. This evaporative cooling can be optimized at various temperatures by adjusting the barrier height and thickness. Theoretical and experimental results for nonisothermal thermionic emission in heterostructures are presented. Single stage InGaAsP-based heterostructure integrated thermionic (HIT) coolers are fabricated and characterized. Cooling on the order of a degree over one micron thick barriers has been observed. Nonisothermal transport in highly doped tall barrier superlattices is also investigated. An order of magnitude improvement in cooling efficiency is predicted for InAlAs/InP superlattices.