Mechanism leading to the large thermoelectric power and metallic electrical conduction observed in NaxCoO2 was investigated by means of angle resolved photoemission spectroscopy and the Bloch-Boltzmann theory. As a result of thermoelectric power calculation using the experimentally determined electronic structure under the assumptions of rigid band and constant mean free path, NaxCoO2 were found to possess the Boltzmann-type electrical conduction over a wide carrier-concentration range. Analysis using the simplest tight-binding bands revealed that the two-dimensional hexagonal lattices including the crystalline structure of the present NaxCoO2 produce a characteristic spectral conductivity leading to the large thermoelectric power and metallic electrical conduction.

We propose herein that amorphous oxide semiconductor (AOS) superlattices, which can be deposited on various substrate including glasses or plastics without any substrate heating, are appropriate for the realization of superlattice thermoelectric devices. As an example, thermoelectric properties of AOS superlattices composed of a-In-Zn-O (well) and a-In-Ga-Zn-O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, were measured to clarify whether enhancement of Seebeck coefficient |S| occurs or not. The |S|2D value increases drastically with decreasing a-In-Zn-O thickness (dIZO) when the dIZO is < ∼5 nm, and reached 73 μV·K-1 (dIZO = 0.3 nm), which is ∼4 times larger than that of bulk |S|3D (19 μV·K-1), while it kept high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices.

PbTe-based thermoelectric (TE) materials exhibit promising thermoelectric properties and have potential applications in waste heat recovery from sources such as truck engines and shipboard engines. TE components designed for these applications will be subject to mechanical/thermal loading and vibration as a result from in-service conditions, including mechanical vibration, mechanical and/or thermal cycling, and thermal shock.

In the current study, we present and discuss the mechanical properties of several PbTe-based compositions with different dopants and processing methods, including n-type and p-type specimens fabricated both by casting and by powder processing. Room temperature hardness and Young's modulus are studied by Vickers indentation and nanoindentation while fracture strength is obtained by biaxial flexure testing. Temperature dependent Young's modulus, shear modulus, and Poisson's ratio are studied via resonant ultrasound spectroscopy (RUS).

It seems that no satisfactory TE property has been found in n-type oxide bulk materials even though Al-doped ZnO and La-doped SrTiO3 have high thermoelectric (TE) responses. Difficulty in developing high-performance TE materials seems to lie in finding low thermal conductivity in such oxides. The purpose of this study is to find a possibility to make an n-type TE oxide bulk material having low thermal conductivity and excellent TE properties as well. For this purpose, we fabricated and examined a series of composites constructed of TiO2 and Ln-doped SrTiO3 fine crystals. The composites were prepared via two processing steps: (1) precursor oxide preparation by wet processes; (2) sintering by using spark plasma sintering (SPS) apparatus. The microscopic structure was examined by using scanning electron microscope (SEM; HITACHI S-4500 model) attached with an energy dispersive x-ray spectroscopy. The electrical conductivities and the Seebeck coefficients were measured simultaneously using an ULVAC ZEM-1 instrument in helium atmosphere. The thermal diffusivities were measured by a laser flash method in vacuum. The composites obtained here were found to commonly have a mosaic type texture constructed of TiO2 and SrTiO3 fine particles with a typical size of 500 nm. The thermal conductivity values measured for three samples with different contents are ranged between 3 and 4 Wm-1K-1 in the temperature range from room temperature to 800 C. The values are apparently lower than the value for single crystal SrTiO3 samples presented in literature. Taking account the other TE data, e.g. Seebeck coefficient and electrical conductivity, we calculated dimensionless figure of merit, ZT, to be at maximum 0.15 at 800°C.

Thermoelectric power generation is a hopeful method harnessing waste thermal energy particularly covering a middle temperature range between 500 and 800K. A Zn4Sb3 compound is a promising “phonon glass electron crystal” material applicable to thermoelectric power generation around 700K. This material, however, has a problem of its brittleness. In this research, the silicon carbide whiskers were added into the Zn4Sb3 compound for overcoming the brittleness, and the fullerene nanoparticles were also added for improving the thermoelectric performance. The Zn4Sb3 compound was synthesized from mixture of pure zinc and antimony powders by a liquid-solid phase reactions method. Firstly, the synthesized compound powder was mixed with the fullerene nanoparticles. The planetary ball milling method was used in order to disentangle the fullerene agglomerate and to obtain a uniform mixture. Subsequently, the mixture was uniformly mixed with the SiC whiskers by the planetary ball milling. The final mixture was consolidated by the pulse discharge sintering. The synthesized phases were identified by XRD. The morphology of the whiskers after mixing was observed. The flexural strength and the thermoelectric properties of the sintered samples were measured. The length of SiC whiskers and the flexural strength were decreased with the mixing time. Though the addition of SiC whiskers lowered the thermoelectric performance, the combined addition of SiC whiskers and fullerene nanoparticles restored the performance by especially decrease of the thermoelectric conductivity owing to the phonon scattering.

Seebeck coefficient S(T), thermal conductivity κ(T) and electrical resistivity ρ(T) measurements on polycrystalline La1.85Sr0.15CuO4-δ (LSCO) compounds grown by solid-state reaction method were carried out in the temperature range between 100 and 290K. The obtained samples were submitted to annealing processes of different duration in order to modify their oxygen stoichiometry. The Seebeck coefficient is positive over the measured temperature range and its magnitude increases with the annealing time up to reach values close to 150 µV/K. The electrical resistivity exhibits a metallic behavior, in all samples, ρ(T) takes values less than 1mΩ-cm. As the annealing time increases, the total thermal conductivity increases up to values close to 3 W/K-m. From S(T), κ(T) and ρ(T) data, the thermoelectric power factor (PF) and the dimensionless figure of merit (ZT) were determined. These parameters reach maximum values around 25 µW/K2-cm and 0.18, respectively. The observed behavior in the transport properties become these compounds potential thermoelectric materials, which could be used in low temperature thermoelectric applications.

We studied the thermoelectric properties of BaSi2 and SrSi2. The polycrystalline samples were prepared by spark plasma sintering (SPS). The electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were measured above room temperature. The maximum values of the dimensionless figure of merit (ZT) were 0.01 at 954 K for BaSi2 and 0.09 at 417 K for SrSi2. We tried to enhance the ZT values of BaSi2 and SrSi2 by prepareing and characterizing La-doped BaSi2 and (Ba,Sr)Si2 solid solution.

Measurements of assembled thermoelectric (TE) modules commonly include investigations of the module output power versus load resistance. Using an AC electrical measurement for TE modules, a model of equivalent passive RC circuit has been developed and tested for both the thermal and electrical characteristics of the module. In this work we present and analyze data of a commercially available module, using equivalent passive RC circuit, to examine, explain and model the electro-thermal activity in the module. In addition data we analyzed by thermal quadrupole theory. Measurements were taken over the frequency range of 0,5mHz to 50 Hz and the device performance (ZT) was evaluated from measured data.

The presence of a high electron density of states in low-dimensional systems such as, nanowires and nanotubes, suggests that these 1D structures can be useful thermoelectric materials. Theoretical calculations predict that: (i) Semimetallic Bi nanowires exhibit semiconducting nature when their nanowire diameter is below 50 nm, and (ii) Semiconducting Bi nanowires with diameter < 10 nm are expected to have an enhanced figure of merit (ZT > 2), when Z=S2σ/κ (S: Seebeck coefficient, σ: electrical conductivity, and κ is the thermal conductivity). We report the synthesis of ∼10 nm diameter Bi nanorods using a pulsed laser vaporization method. The high resolution transmission electron microscopy images of our Bi nanorods show (i) crystalline <012> planes in the core of the nanorods, and (ii) coated with a thin amorphous Bi2O3 layer. The infrared absorption and the surface plasmon peaks in our Bi nanorods are blue-shifted in energy when compared to the corresponding spectra in bulk Bi.

Si1-xGex alloys show the high power generating efficiency as a thermoelectric- conversion material. We evaluate the thermoelectric power of the n-type SiGe system on the basis of the first principles calculations. The electronic-band-structure calculation is performed using all-electron full-potential linearized augmented-plane-wave (FLAPW) method within the local density approximation (LDA). The Seebeck coefficient is analyzed by the Bloch-Boltzmann equation. We find that the ordered rhombohedral SiGe has high Seebeck coefficient in comparison with zincblende SiGe. The efficiency of the thermoelectric power in Si1-xGex is gained by the local atomic configuration rather than the Ge concentration.

Low-dimensional thin-film thermoelectric materials including superlattices and quantum-dot heterostructures have shown significant potential for improving the thermoelectric properties. Highly structured thin film materials such as these would be useful for integrated microdevices such as power MEMS and thermally triggered MEMS actuators. For reasons including scalability and ease-of-integration, silicon is the choice material for the substrate, however there is a lattice and a thermal-expansion mismatch. In this work, a new approach for the synthesis of integrated PbSnSeTe based thermoelectric thin film materials on silicon is demonstrated by employing an epitaxial buffer layer of II-VI compound telluride materials (e.g., ZnTe, CdTe) to help bridge the lattice and thermal expansion mismatches with silicon. This multilayer can be used for subsequent growth of thick films having low-dimensionality structures. We report the initial results from studying the structural and thermoelectric properties of simple solid-solution alloy PbSnSeTe thin films on ZnTe/Si heterostructures. Data from transmission electron microscopy and in-situ electron diffraction will be presented that shows that despite the large lattice mismatch with silicon and the three different crystal structures, unusually high structural quality PbSnSeTe has been obtained by matching the (211) lattice symmetry and the lattice spacing along the [110] directions of the ZnTe and PbSnSeTe. The structural quality of the PbSnSeTe was studied by measuring the dislocation density through etch-pit counting and x-ray diffraction. Results presented will show that a dislocation density as low as 1.2 × 106/cm2 can be achieved by strategic lattice-matching between the buffer and thermoelectric layers. Electrical resistivity, doping density, and Seebeck coefficient values for solid-solution alloys without low-dimensional structuring will be shown to approach those of high-performance bulk.

Polycrystalline Mg2Si was fabricated from a reused-Silicon source, based on Si sludge, using Spark Plasma Sintering technique. The n-type and p-type dopants, bismuth (Bi) and silver (Ag), respectively, were incorporated into the Mg2Si. The thermoelectric properties were estimated from 300 to 873K. The power factors of undoped and Bi-doped samples from the reused-Si source were comparable to those from a solar grade Si source (99.99999%). The power factor was estimated to be 2.5 × 10-5 W/cmK2 for the Bi-doped sample from the reused-Si source. However, the power factor of the Ag-doped, p-type sample from the reused-Si source was lower than that from solar grade Si source. The dimensionless figures of merit of samples from the resused-Si source were slightly lower than those from a solar grade Si source. The dimensionless figure of merit was estimated to be 0.53 at 812 K for Bi-doped sample from the reused-Si source.

The ternary skutterudites AB1.5Q1.5 (A = Co, Rh, Ir, B =Ge, Sn and Q = S, Te) have been synthesized and structurally characterised. Powder diffraction data are consistent with the presence of anion ordering, which results in a lowering of the symmetry from cubic to rhombohedral. The ordered skutterudite structure contains rhomboidal four-membered B2Q2 rings, instead of the homonuclear rectangular rings found in binary skutterudites. The electrical transport properties of the AB1.5Q1.5 phases are consistent with semiconducting behaviour, and large values of the Seebeck coefficient have been observed for several of these materials. While their large electrical resistivities might make these materials unsuitable for thermoelectric applications unless significant improvements can be achieved by doping, these materials also exhibit significantly lower thermal conductivities than their binary counterparts.

The local electronic structure of phosphorus in the binary skutterudites CoP3 and NiP3, and in the filled skutterudite LaFe4P12 are studied using a combination of electron energy-loss spectroscopy and ab initio calculations. Relative to CoP3 we observe a filling of phosphorus s and d states in NiP3, while for LaFe4P12 increased EELS intensity indicates more empty s and d states close to the Fermi-level.

Magnesium silicide (Mg2Si) has been regarded as a candidate for advanced thermoelectric materials which is used in the temperature ranging from 500 to 800 K correspond to that of vehicle exhaust emission. Besides, Mg2Si has benefits such as abundance of constituent element of Mg2Si in the earth's crust and it's non-toxicity substances compared with other thermoelectric materials that operate in the conversion temperature range such as PbTe and CoSb3. The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S2ãT/Û, of its constituent thermoelectric material where S is the Seebeck coefficient, ã is the electrical conductivity, Û is the thermal conductivity, and T is the absolute temperature. For thermoelectric device operation, the use of a material with ZT more than unity is needed to realize a conversion efficiency of ∼10 %. The optimization of doping careers in Mg2Si is required in order to realize unity of ZT. In that way, we have grown Mg2Si crystals along with doping elements of Bi and Al using vertical Bridgman method.

Mg (99.99 %) and Si (99.99999 %) with a stoichiometric Mg : Si ratio of 67 : 33 were mixed congruently and melt into Mg2Si. Prior to the growth, Bi (99.999 %) powder at the ratio from 0.5 to 3 at % for Mg2Si and the pre-synthesized polycrystalline Mg2Si powder were mixed, and Mg2Si crystals were grown at a rate of 3 mm/h by vertical Bridgman method. Grown samples were characterized by x-ray diffraction (XRD) patterns and electron-prove microanalysis (EPMA), and the results indicated that Mg2Si crystals were reproductively grown due to use of polycrystalline Mg2Si as a source material of growth. Hall carrier concentrations were evaluated at room temperature. The electrical conductivity, the Seebeck coefficient, and the thermal conductivity were estimated in the temperature range from RT to 850 K. The grown crystals exhibited n-type conductivity in undoped and all Bi doped conditions. All the Bi doped crystals showed high electrical conductivity and high carrier concentration compared with that of the undoped crystal. On the other hand, the thermal conductivity was lowered in the proportion of the amount of Bi. Consequently, the thermal conductivity for the crystal that was Bi doped at 3 at % was 0.021 W/cmK at 842K, and its ZT reached 0.99 at 842 K, which is near the unity of ZT that is regarded as a standard of practical use for thermoelectric materials. The solid solubility limit of Bi to Mg2Si was assumed to be around 3 at % from our findings, and thus Al was codoped besides Bi in order further to improve the thermoelectric properties. We will discuss the results, additionally.

Thermoelectric modules are of great interest for power generation applications where temperature gradients of approximately 500K exist, and hot side temperatures near 800K. The fabrication of such modules requires optimization of the material compositions, low contact resistivities, and low thermal loss.

AgPbmSbTe2+m (LAST) and Ag(Pb1-xSnx)m SbTe2+m (LASTT) compounds are among the best known materials appropriate for this temperature range. Various measurement systems have been developed and used to characterize bulk samples in the LAST and LASTT systems within this operating temperature range. From the characterized data, modeling of modules based on these materials and segmented legs using LAST(T) with Bi2Te3 have been used to identify the optimal geometry for the individual legs, and the length of the Bi2Te3 segments. We have segmented LAST(T) with Bi2Te3 and achieved contact resistivities of less than 10 μΩ•cm2.

Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on LAST, LASTT, and segmented with Bi2Te3 materials. We also present the output data on these generators.

The melt spinning technique (MST) combined with post annealing processes is evaluated for the development of thermoelectric nanocomposites. The evaluated ones are based on two components almost immiscible in solid state but with crystallographic correlation. One is taken from the V-VI-components system and the other one from the IV-VI-components system. This concept was applied to p-(Bi0,2Sb0,8)2Te3 and to p-[(Bi0,2Sb0,8)2Te3]1-xPbTex composites. MST samples of all types were characterised for some structural and thermoelectric properties. All V-VI materials are clearly textured after MST and show no deterioration concerning the thermoelectric properties even after subsequent annealing processes. Structural analysis of p-[(Bi0,2Sb0,8)2Te3]1-xPbTex composites gave significant hints for oriented precipitates of a IV-VI-rich phase incorporated into the V-VI-rich matrix. The thermoelectric figure of merit of the evaluated composites could be enhanced by suitable annealing procedures of both the quenched bulk materials and the melt spin material.

Nanostructured materials have been shown to enhance phonon scattering and improve the figure of merit of thermoelectric materials. Solvothermal syntheses of nano-engineered CoSb3 skutterudite have been studied in the present work in which CoCl2 and SbCl3 were used as precursors and NaBH4 as a reductant. Elemental, structural and morphology characterization techniques including: XRD, SEM, TEM and EDAX were used to identify and characterize the products. NaBH4 was found to be a necessary factor in forming cobalt antimonide. It was detected the products' phase transited from Sb2Co to CoSb3 in conjunction with the amount increase of NaBH4. The change of synthesis time and temperature contributed little on the products' phases under the present experimental routes. Single-phased CoSb3 skutterudites was obtained from the solvothermal syntheses combined with an acid wash. The as-prepared CoSb3 powders consist of irregular nanoparticles with 15∼20 nm in size. A possible chemical mechanism was also discussed.

We report the synthesis of nanostructured composite PbTe with excess Pb and Sb metal inclusions. Scanning and transmission electron microscopy reveal these inclusions in both the nano- and macroscales. The electrical conductivity and Seebeck coefficient dependence on temperature show unusual trends which depend on the inclusion Pb/Sb ratio. Several ratios showed marked enhancements in power factor at 700 K. The thermal conductivity of these composites is reported.

Thermoelectric properties of Sr1-xYxTiO3 (x = 0-0.08) were calculated using a virtual crystal method. Using a Pechini's method, Sr1-xYxTiO3 (x = 0-0.06) precursor powder was prepared. Using this method, we prepared pure SrTiO3 at a lower temperature than would be using the conventional solid-state reaction method. The precursor solution was heated at 823 K for 5 h after drying at 353 K for 8 h to produce the precursor powder. The powder was sintered using a hot pressing technique. The relative densities of ceramics were more than 98%. The Seebeck coefficient and electrical conductivity of the samples were measured using the standard four-probe method in a flowing He gas atmosphere in a temperature range of 323 to 923 K. The conductivities of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3 at room temperature were 6.61 × 102, 5.61 × 103, and 1.58 × 104 S/m, respectively. The Seebeck coefficients of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3 at room temperature were -548, -264, and -196 μV/K, respectively.