With the goal of developing high-performance bulk thermoelectric materials, we have characterized ternary silver thallium tellurides. The ternary silver thallium tellurides exhibit extremely low thermal conductivity (<0.5 Wm−1K−1) and consequently their thermoelectric performance is excellent. Although the extremely low thermal conductivity materials, as typified by the ternary silver thallium tellurides, would be a new class of next-generation thermoelectric materials, thallium compounds are unsuitable for practical application because of their toxicity. Against such a background, we are currently exploring thallium-free thermoelectric materials with extremely low thermal conductivity. In this paper, we will briefly summarize the thermoelectric properties of ternary thallium tellurides obtained in our group. Further experiments aimed at improving the ZT of these materials will be presented. Finally, we will propose two candidates: Ag8GeTe6 and Ga2Te3 as thallium-free low thermal conductivity materials.

A new series of thallium-containing materials, Tl1-xPbmATem+2 (A= Sb, Bi; m=10, 18; 0≤x≤0.3), has been prepared. Diffraction data reveal that a rocksalt-type structure is adopted at all compositions. Transport property measurements indicate n-type behaviour for the bismuth-containing materials and p-type for the antimony analogues. The reduced power factors for the latter, compared to the bismuth-containing materials, are primarily due to their more resistive nature. The highest power factors occur at a thallium deficiency in the range 0.1≤x≤0.2. Measured thermal conductivities are reduced from that of the binary telluride, suggesting that the materials may have potential as n/p-type components in device applications.

Solid solutions of β-K2Bi8-xSbxSe13 are an interesting series of materials for thermoelectric investigations due to their very low thermal conductivity and highly anisotropic electrical properties. In this work, we aimed to synthesize solid solutions of β-K2Bi8-xSbxSe13 type materials using powder techniques. The synthesis was based on mechanical alloying as well as sintering procedures. The products were studied in terms of structural features, composition and purity with powder x-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Preliminary results on thermoelectric properties as well as IR reflectivity measurements are presented.

Electrode materials consisting of Cu, Ti and Ni were formed on Bi-doped n-type Mg2Si by means of a monobloc plasma-activated sintering (PAS) technique. Due to the difference in thermal expansion coefficients between Ti and Mg2Si, rather high residual thermal stresses gave rise to the introduction of cracks, which were mainly located in the Mg2Si layer, when Ti was used as the electrode material. In the case of the Cu electrodes, monobloc sintering could not be performed in a reproducible manner because Cu melts abruptly and effuses at around 973K, which is 100 K lower than the sintering temperature that is required for Mg2Si of good crystalline quality. When compared with the results for Cu and Ti, the monobloc PAS process for Ni was both stable and reproducible. The room-temperature I-V characteristics of Ni electrodes were considered to be adequate for practical applications, with durable Mg2Si-electrode junction properties being realized at a practical operating temperature of 600 K with ΔT = 500 K. The highest open circuit voltage (VOC) observed was 41 mV at ΔT = 500 K (between 873 K and 373 K) for Ni electrodes fabricated using the monobloc PAS process. The voltage (V) and current (I) values with a 10 Ohm load were ∼ 48 mV and ∼ 2 mA at ΔT = 500 K.

Very recently, we have found that the high density 2DEG (ne ∼1021 cm−3), which is confined within a unit cell layer thickness of SrTiO3, exhibits unusually large Seebeck coefficient (S2DEG/Sbulk ∼5)[1]. In the optimum, extremely high ZT2DEG of ∼2.4 can be obtained at room temperature, while the effective ZTeff. was only ∼0.24 because 9 unit cells of electrically insulating SrTiO3 layers were used to fabricate the 2DEG structure. Thus, high ZTeff can be realized if the insulating layer thickness is reduced significantly. We selected BaTiO3∼SrTiO3:Nb superlattice to reduce insulating layer thickness because dielectric constant of BaTiO3 is one order of magnitude large (∼3,000) as compared to that of SrTiO3 (∼300). We expected that the conduction electrons can be confined much strongly in the SrTiO3:Nb layer by sandwiching between highly dielectric BaTiO3 layers. As a result, we clarified that the critical BaTiO3 layer thickness is 1.2 nm, significantly small as compared to SrTiO3 layer (4 nm). The BaTiO3/SrTiO3:Nb superlattice films were fabricated by a pulsed laser deposition (PLD) method on (001)-face of LaAlO3 single crystal substrate at 900°C. During the film growth, we monitored RHEED intensity oscillation to control layer thickness precisely. Out-of-plane high-resolution X-ray diffraction measurements and cross sectional HAADF-STEM observations revealed that the resultant films were high quality BaTiO3/SrTiO3:Nb superlattice. Hall mobility of the SrTiO3:Nb layer was 0.4 cm2·V−1·s−1, while that of superlattice decreased gradually with increasing BaTiO3 layer thickness most likely due to that intra layer diffusion of Ba2+ ion occurred between BaTiO3 and SrTiO3:Nb layers[2], which was clearly observed by the EELS mapping. Seebeck coefficient |S|300K of SrTiO3:Nb layer was 57 μV·K−1, which corresponds carrier concentration ne of 5×1021 cm−3. The |S|300K value became large with decreasing the SrTiO3:Nb layer thickness (dSrTiO3:Nb) and it reached 305 μV·K−1, which is approximately 5 times larger than that of SrTiO3:Nb bulk. The slope of log |S|- log dSrTiO3:Nb plots was 1/2, suggesting that quantum size effect occurred. Critical BaTiO3 layer thickness for the quantum confinement of the electrons was 1.2 nm (3 unit cells of BaTiO3), which is significantly small as compared to SrTiO3 (4 nm). Thus, BaTiO3/SrTiO3:Nb superlattice would be a promising candidate to realize high ZTeff.

Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators in order to convert heat to electricity. These contacts must be stable to high temperatures and through thermal cycling. A ratio of the contact resistance to the leg resistance below 0.1 is the goal for fabrication of a high efficiency thermoelectric power generation device. Here we present the fabrication procedures and characterization of contacts of metal alloys to Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds. Contacts were fabricated and measured for both ingot and hot pressed materials. Stainless steel 316 has shown a low resistance contact to these thermoelectric materials when the proper bonding conditions are used. Different time-temperature-pressure conditions for bonding to n-type and to p-type legs are presented. Contact resistances below 10μΩcm2 have been measured. In addition, break tests have shown bond strengths exceeding the semiconductor fracture strength. One of the considerations used in selecting iron alloys for electrical interconnects is the similarity in the coefficient of thermal expansion to the LAST and LASTT materials which is 18 ppm/°C and relatively temperature insensitive. Contacts to the thermoelectric materials were accomplished by diffusion bonding in a furnace developed in our lab at Michigan State University. The furnace is capable of reaching temperatures of up to 1000°C with a controlled atmosphere of a reducing gas. Fabrication procedures and contact data are presented in this paper.

Three different materials crystallizing in the cubic Ir3Ge7 type are under investigation in our group, namely Mo3(Sb,Te)7, Nb3(Sb,Te)7, and Re3(E,As)7 (with E = Si, Ge, Sn). Our electronic structure calculations reveal a band gap to occur at 55 valence-electrons in all three cases, namely Mo3Sb5Te2, Nb3Sb2Te5, and Re3EAs6. Cubic holes exist in these structures that may be filled with small cations such as 3d transition metal atoms. Ni0.06Mo3Sb5.4Te1.6 is a degenerate p-type semiconductor that reaches ZT = 0.96 at 750°C, while Re3Ge0.6As6.4 is a degenerate n- type semiconductor with slightly lower ZT values. Preliminary results indicate that the Re3(Sn,As)7 system may be the most promising of the rhenium arsenides.

The Ca0.9Yb0.1MnO3/Ag composites(the ratio of Ag to Ca0.9Yb0.1MnO3 was 0, 4.7, 9.4, and 18.8 wt %) were prepared by wet milling various amounts of Ca0.9Yb0.1MnO3 and Ag2O powder mixtures followed by sintering in order to improve the mechanical properties of Ca0.9Yb0.1MnO3 for n-type legs of thermoelectric oxide devices. The obtained composites consisted of two phases such as Ca0.9Yb0.1MnO3 and metallic silver from the X-ray diffraction (XRD) analysis. The scanning electron microscope (SEM) analysis indicated that the Ag particles, the size of which was within 5 μm, were homogeneously dispersed in Ca0.9Yb0.1MnO3 matrix for all the composites. The σf of 18.8 wt% Ag composite became 251 MPa, which was 2 times larger value than that of Ca0.9Yb0.1MnO3. The power factor (S2ρ) was slightly improved by the addition of silver particles. The maximum S2ρ, i.e. 0.26 mWm-1K-2 at 573 K was obtained for 18.8 wt% Ag composite.

The preparation of partially filled n-type InxCo4Sb12 skutterudite compounds has been recently reported. The results were particularly promising, the materials exhibiting a ZT value far higher than one at moderated temperature. In this paper, we propose to investigate another way to tune the electrical and thermal properties by substituting Co atoms by Ni atoms in InxCo4Sb12. InxCo4-yNiySb12 polycrystalline samples have been prepared by a conventional metallurgical route. Structural analyses have been carried out by X-ray diffraction. The chemical composition and micro-homogeneity have been checked by electron probe microanalysis. Measurements of the electrical resistivity, thermoelectric power and thermal conductivity have been performed between 300 and 800 K. The influence of the presence of Ni on the thermoelectric properties of InxCo4Sb12 compounds is presented and discussed.

In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to the low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. The origin of these nanostructures has been studied using Monte Carlo (MC) simulation of an ionic model and ab initio studies of pair interaction energies. Effects of these substitutions on the band structure near the gap and their implications on transport properties are briefly discussed.

A Fe2VAl0.9Si0.1/Bi composite were fabricated using pulse-current sintering technique with mixed powder prepared by mechanical alloying. Since the raw powders were finely pulverized into nanometer sized crystallite and rapidly sintered to suppress grain growth during heat treatment, microstructural composite was obtained. The composite was consisted of Fe2VAl0.9Si0.1 grains with the size of 200-300 nm and nanometer-sized Bi particles. The size of the Bi particle decreased with the increase in the milling processing time, while the size of the Fe2VAl0.9Si0.1 grains showed no variation. The thermal conductivity of the composite decreased with the reduction of the Bi particle size, because of the increase in the number of hetero junction, at which phonons were scattered. On the contrary, the electrical conductivity was less affected by the incorporation of nanometer-sized Bi particles. This means that the hetero junction between Fe2VAl0.9Si0.1 and Bi has little effect on the carrier traveling or the dispersion of highly conductive Bi particles enhanced electrical conductivity. Although Seebeck coefficient was reduced by a negative effect of Bi incorporation on the Fe2VAl0.9Si0.1 crystal structure, the introduction of Bi nanoinclusions in the sintered Heusler material was effective to reduce the thermal conductivity without degradation of electrical conductivity and consequently improved the thermoelectric figure of merit.

The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.

We characterize several thin film thermoelectric micro-modules composed of 20 µm-thick elements and designed for cooling applications to identify factors that may limit device performance. Thermoelectric figure-of-merit measurements using the transient Harman technique are compared with maximum cooling data under no heat load. Correlation between the two measurements depending on the location of the parasitic joule heating in the module is analyzed. Near-infrared thermoreflectance is used to examine temperature non-uniformity in the module. The temperature distribution on the metal contacts due to the Peltier and Joule effects is obtained non-destructively through the silicon substrate of an active module.

We report the fabrication and characterization of segmented element power generator modules of 254 thermoelectric elements. The element is 1 mm × 1 mm in area, which consists of 300 μm thickness Bi2Te3 and 50 μm thickness ErAs:(InGaAs)1-x(InAlAs)x, so that each segment can work at different temperature ranges. Erbium arsenide metallic nanoparticles are incorporated to create scattering centers for middle and long wavelength phonons, provide charge carriers, and form local Schottky barriers for electron filtering. The thermoelectric properties of ErAs:InGaAlAs were characterized by variable temperature measurements of thermal conductivity, electrical conductivity and Seebeck coefficient from 300 K to 600 K. Generator modules of Bi2Te3 and ErAs:InGaAlAs segmented elements were fabricated and an output power over 5.5 W was measured. The performance of the thermoelectric generator modules can further be improved by improving the thermoelectric properties of the element material, and reducing the electrical and thermal parasitic losses.

Nanostructured thermoelectric materials have attracted lots of interest in recent years, due to their enhanced performance determined by their thermoelectric dimensionless figure of merit. However, because of equipment limitations, not much work has been done on combining simultaneous transport measurements and structural characterization on individual nanostructured thermoelectric materials. With an integrated TEM-STM system, we studied the structural behavior and electrical properties of bismuth (Bi) nanobelts and nanoparticles. Results showed that clean Bi nanostructures free of oxides can be produced by in-situ high temperature electro-migration and Joule annealing processes occurring within the electron microscope. Preliminary electrical measurements indicate a conductivity of two orders of magnitude lower for Bi nanoparticles than that for bulk Bi. Such in-situ studies are highly advantageous for studying the semimetal-semiconductor transition and how this transition could enhance thermoelectric properties.

We have recently found that the negative thermal expansion (NTE) materials, ZrW2O8 and HfMo2O8, show exceptionally low thermal conductivity. We surmise that the mechanism is the efficient coupling of the low-frequency optic phonons that give rise to negative thermal expansion with the heat-carrying acoustic phonons. Although neither ZrW2O8 nor HfMo2O8 has suitable electronic properties for thermoelectric applications, perhaps the principle of reduced thermal conductivity by low-frequency optic phonons in NTE materials can be used to develop more efficient thermoelectric materials.

Preliminary results from an investigation into the synthesis and characterization of silicon and germanium type II clathrates are reported. A series of NaxSi136 (0 < x < 24) clathrates was synthesized and characterized by powder X-ray diffraction and Rietveld analysis. The NaxSi136 lattice parameters are observed to first decrease, then increase with increasing Na content, indicating a non-monotonic structural response to Na filling. New type II Ge clathrate compositions Cs8Na16MyGe136-y (M = Cu, In) utilizing framework substitution are reported. Electrical transport measurements on a Cu substituted specimen indicate framework substitution modifies the transport properties of these materials. The potential type II clathrate phases possess for thermoelectric applications is discussed.

We performed comparative investigations of the Ag1-xPb18MTe20 (M = Bi, Sb) (x = 0, 0.14, 0.3) system to better understand the roles of Sb and Bi on the thermoelectric properties. In both systems, the electrical conductivity nearly keeps the same values, while the Seebeck coefficient decreases dramatically in going from Sb to Bi. Compared to the lattice thermal conductivity of PbTe, that of AgPb18BiTe20 is substantially reduced. The lattice thermal conductivity of the Bi analog, however, is higher than that of AgPb18SbTe20 and this is attributed largely to the decrease in the degree of mass fluctuation between the nanostructures and the matrix (for the Bi analog). As a result the dimensionless figure of merit ZT of Ag1-xPb18MTe20 (M = Bi) is found to be smaller than that of Ag1-xPb18MTe20 (M = Sb).

In order to evaluate the thermoelectric properties of n-type half-Heusler compound NbCoSn, single-phase NbCoSn alloy was prepared using directional solidification based on the phase diagram information of the Nb-Co-Sn ternary system which was investigated in this work. The isotherm at 1273 K and the reaction scheme together with the projection of liquidus surface were determined. Thermoelectric properties of NbCoSn based alloys were evaluated by Seebeck coefficient, electrical resistivity and power factor, which were measured in a temperature range from room temperature to 1073 K. NbCoSn has excellent values of Seebeck coefficient exceeding -250 μV/K at around 900 K and shows the metal-like temperature dependence of electrical resistivity.

We investigated the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. This material exhibits a wide range of electric behaviors, from a high-dielectric-constant tunable-paraelectric in its undoped form to a metallic superconductor when it is doped with a variety of elements.

The Srn+1TinO3n+1 and the yttrium-doped Srn+1TinO3n+1 Ruddlesden-Popper homologous series were prepared through the thermal decomposition of a metal citric acid complex. The starting solution consisted of the raw materials strontium acetate and titanium alkoxide. Citric acid was used as the chelating agent, and ethanol and distilled water were mixed and used as a solvent. Single phase Sr2TiO4 and Sr3Ti2O7 were produced without creation of SrCO3 by heat-treatment at 1073 and 1473 K, respectively, for three hours. We were unable to produce single phase Sr4Ti3O10. The samples consisted of Sr4Ti3O10 and SrTiO3 phases. Scanning electron microscopy showed that the grain shape of the Sr2TiO4 was plate-like, while those of the Sr3Ti2O7 and Sr4Ti3O10 were not and the grains had large numbers of pores. High-density Sr2-xYxTiO4 (x= 0 to 0.06) ceramic samples were produced under hot-press conditions (1823 K for one hour at a uniaxial pressure of 31.2 MPa). Although the undoped samples were insulators, the yttrium-doped samples showed high electrical conductivity (i.e. that of Sr1.98Y0.02TiO4 was ρ= 8.5×10−5 Ωcm). The Seebeck coefficient of Sr1.98Y0.02TiO4 was –160.7 μV/K, and the thermal conductivity was 3.38 W/mK.