Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Influence of Sedimentation of Atoms on Structural and Thermoelectric Properties of Bi-Sb Alloys

Functionally graded thermoelectric materials (FGTMs) have been prepared by sedimentation of atoms under a strong gravitational field. Starting samples of Bi _x Sb_1?x alloys with different composition x were synthesized by melting of metals and subsequent annealing of quenched samples. The thermoelectric properties (Seebeck coefficient, electrical conductivity) of the starting materials were characterized over the temperature range from 300 K to 525 K. Strong gravity experiments were performed in a unique ultracentrifuge apparatus under acceleration of over 0.5 × 10⁶ G at temperatures of 538 K and 623 K. Changes of the microstructure and chemical composition were analyzed using scanning electron microscopy with energy-dispersive x-ray spectroscopy analysis. The distribution of the Seebeck coefficient of the Bi-Sb alloys was characterized by scanning thermoelectric microprobe. As a result of sedimentation, large changes in chemical composition (x = 0.45 to 1) were obtained. It was found that the changes in chemical composition were correlated with alterations of the Seebeck coefficient. The obtained experimental data allowed the development of a semiempirical model for the selection of optimal processing parameters for preparation of Bi-Sb alloys with required thermoelectric properties.     

Thermoelectric Properties of Polycrystalline SrZn2Sb2 Prepared by Spark Plasma Sintering

Compact polycrystalline samples of SrZn₂Sb₂ [space group $ P\overline{3} m1 $ , a = 4.503(1) Å, c = 7.721(1) Å] were prepared by spark plasma sintering. Thermoelectric performance, Hall effect, and magnetic properties were investigated in the temperature range from 2 K to 650 K. The thermoelectric figure of merit ZT was found to increase with temperature up to ZT = 0.15 at 650 K. At this temperature the material showed a high Seebeck coefficient of +230 μV K^?1, low thermal conductivity of 1.3 W m^?1 K^?1, but rather low electrical conductivity of 54 S cm^?1, together with a complex temperature behavior. SrZn₂Sb₂ is a diamagnetic p-type conductor with a carrier concentration of 5 × 10¹⁸ cm^?3 at 300 K. The electronic structure was calculated within the density-functional theory (DFT), revealing a low density of states (DOS) of 0.43 states eV^?1 cell^?1 at the Fermi level.     

Phase Stability and Thermoelectric Properties of CuFeS2-Based Magnetic Semiconductor

Recently, based on measurement results below 400 K, we suggested that chalcopyrite CuFeS₂-based alloys hold promise as thermoelectric materials. In this study, we have investigated the phase stability of such compounds and measured their thermoelectric properties at temperatures above 400 K. Thermogravimetric data indicate that the samples synthesized by a spark plasma sintering method were stable up to 700 K, above which sulfur deficiency becomes prominent. The electrical resistivity of the electron-doped samples showed metallic behavior up to 700 K. The Seebeck coefficients show large negative values of about ?300 μV/K above 400 K. As a result, the power factor of Cu_0.97Fe_1.03S₂ is ~1 mW/K²m in the temperature range of 400 K to 600 K.     

Low Thermal Conductivity in High-Z Thermoelectric Materials with Controlled Nanodispersions

We report wet chemical synthesis of a hierarchical nanocomposite thermoelectric material, (Bi,Sb)₂Te₃ + 2 vol.% Sb₂O₃, which exhibits a very high ZT value of 1.5 at 333 K. The key to such a high ZT value is to design the interfacial density (ID) of the nanodispersion and the mean diameter of the matrix (d) in the nanocomposite. To this end, (Bi,Sb)₂Te₃ with Sb₂O₃ nanodispersion was developed using in situ precipitation during solvothermal treatment. Nanocomposite structure was observed in sintered specimens. By evaluation of thermoelectric properties, it was confirmed that phonon scattering on the surface of Sb₂O₃ dispersion and κ _ph correspondingly decreased with ID. The formation of a well-controlled Sb₂O₃ dispersion (mean diameter of dispersion: D = 1.5 nm, ID = 0.06 nm^?1) and fine grains (d = 38 nm) led to an extremely low lattice thermal conductivity of 0.28 W m^?1 K^?1, while reducing the electrical conductivity moderately according to the conventional mixture rule.     

Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi2Te3 Thin Films

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10^?4 W/K² m and 35 × 10^?4 W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.     

Preparation by Poly(Acrylic Acid) Sol–Gel Method and Thermoelectric Properties of γ-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> Bulk Materials

γ-Na _x CoO₂ single-phase powders have been synthesized by a poly(acrylic acid) (PAA) sol–gel (SG) method, and γ-Na _x CoO₂ bulk ceramic fabricated using spark plasma sintering. The effects of the PAA concentration on the sample phase composition and morphology were investigated. The thermoelectric properties of the γ-Na _x CoO₂ bulk ceramic were also studied. The results show that the PAA concentration did not significantly affect the crystalline phase of the product. However, agglomeration of γ-Na _x CoO₂ crystals was suppressed by the steric effect of PAA. The Na _x CoO₂ bulk ceramic obtained using the PAA SG method had higher crystallographic anisotropy, better chemical homogeneity, and higher density than the sample obtained by solid-state reaction (SSR), leading to improved thermoelectric performance. The PAA SG sample had power factor (in-plane PF = σS ²) of 0.61 mW m^?1 K^?2 and dimensionless figure of merit (ZT) along the in-plane direction of 0.19 at 900 K, higher than for the SSR sample (in-plane PF = 0.51 mW m^?1 K^?2, in-plane ZT = 0.17). These results demonstrate that a simple and feasible PAA SG method can be used for synthesis of Na _x CoO₂ ceramics with improved thermoelectric properties.     

Low-Temperature,Solution-Based,Scalable Synthesis of Sb2Te3 Nanoparticles with an Enhanced Power Factor

Nanostructured thermoelectric (TE) materials, for example Sb₂Te₃, PbTe, and SiGe-based semiconductors, have excellent thermoelectric transport properties and are promising candidates for next-generation TE commercial application. However, it is a challenge to synthesize the corresponding pure nanocrystals with controlled size by low-temperature wet-chemical reaction. Herein, we report an alternative versatile solution-based method for synthesis of plate-like Sb₂Te₃ nanoparticles in a flask using SbCl₃ and Te powders as raw materials, EDTA-Na₂ as complexing agent, and NaBH₄ as reducing agent in the solvent (distilled water). To investigate their thermoelectric transport properties, the obtained powders were cold compacted into cuboid prisms then annealed under a protective N₂ atmosphere. The results showed that both the electrical conductivity (σ) and the power factor (S ² σ) can be enhanced by improving the purity of the products and by increasing the annealing temperature. The highest power factor was 2.04 μW cm^?1 K^?2 at 140°C and electrical conductivity remained in the range 5–10 × 10³ S m^?1. This work provides a simple and economic approach to preparation of large quantities of nanostructured Sb₂Te₃ with excellent TE performance, making it a fascinating candidate for commercialization of cooling devices.     

Thermoelectric Properties of Y1−x Ag x BaCo4O7+δ Ceramics

The thermoelectric properties of the Ag-doped ceramics Y_1?x Ag _x BaCo₄O_7+δ (x = 0.0, 0.05, 0.1, 0.15, and 0.2) were investigated from 373 K to 973 K. The results show that the doping of Ag can reduce the electrical resistivity. The Seebeck coefficients of the samples decrease when the Ag doping amount is small, but increase when the Ag doping amount is large. The activation energy of the electrical conductivity was calculated using Arrhenius plots, and it was found that the activation energy descends with increase of the Ag doping amount. According to the power factors, the optimum Ag doping amount is x = 0.15, which results in a higher power factor of 81 μW m^?1 K^?2 at 973 K, 72.7% higher than for the sample without Ag doping.