Solution-Chemical Syntheses of Nano-Structured Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe Thermoelectric Materials

In this work, nano-structured Bi₂Te₃ and PbTe thermoelectric materials were synthesized separately via solvothermal, hydrothermal and low-temperature aqueous chemical routes. X-ray diffraction (XRD), field-emission scanning-electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used to analyze the powder products. Results showed that the as-prepared Bi₂Te₃ samples were all single-phased and consisted of irregular spherical granules with diameters of ∼30 nm whereas the PbTe samples were mainly composed of well-crystallized cubic crystals with average size of approximately 100 nm. Some nanotubes and nanorods were found in Bi₂Te₃ and PbTe samples, respectively; these were identified as Bi₂Te₃ nanotubes and PbTe nanorods by EDS analysis. Possible reaction mechanisms for these syntheses are discussed in detail herein.     

Influence of Element Substitution on Corrosion Behavior of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Compounds

Atmospheric water may condense on the surface of Bi₂Te₃-based compounds constituting the Peltier module, depending on the operating environment used. In the stage of disposal, Bi₂Te₃-based compounds may come into contact with water in waste disposal sites. There are very few publications about the influence of condensed water on Peltier modules. Bi₂Te₃-Sb₂Te₃ or Bi₂Te₃-Bi₂Se₃ pseudo binary system compounds are used as p-type material or n-type material, respectively. The lattice distortion will be induced in the crystal of Bi₂Te₃-based compounds by element substitution due to the reduction in their thermal conductivity. However, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds remains unclear. In this study, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds with practical compositions has been investigated. Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 was prepared by the vertical Bridgman method. The electrochemical properties at room temperature were evaluated by cyclic voltammetry in a standard three-electrode cell. The working electrolyte was a naturally aerated 0.6 or 3.0 mass% NaCl solution. From the tendency for corrosion potential for all the samples, the corrosion sensitivity of ternary compounds was slightly higher than that of binary compounds. From the trend of current density, it was found that Bi_0.5Sb_1.5Te₃ had a corrosion resistance intermediate between Bi₂Te₃ and Sb₂Te₃. On the other hand, corrosion resistance was affected despite a small amount of Se substitution, and the corrosion resistance of Bi₂Te_2.85Se_0.15 was close to or lower than that of Bi₂Se₃. From the observation results of the corrosion products, the trends of morphology and composition of corrosion products for Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 were consistent with those of Sb₂Te₃ or Bi₂Se₃, respectively. From the results of x-ray photoelectron spectroscopy for the electrolyte after testing, the possibility that a corrosion product diffuses to the environment including the salt was suggested in Bi_0.5Sb_1.5Te₃. However, the amount of dissolved corrosion product was very low, and the chemical stability of the corrosion product was not changed or improved by element substitution.     

Phase Equilibria of the Sn-Bi-Te Ternary System

Bi₂Te₃ is one of the most promising thermoelectric materials, and Sn is the primary constituent of most electronic solders. Knowledge of phase equilibria of the Sn-Bi-Te ternary system is important for thermoelectric applications. Twenty-seven Sn-Bi-Te alloys were prepared and equilibrated at 160°C and 500°C. The equilibrium phases were determined, and the isothermal sections were constructed based on three binary constituent phase diagrams and ternary phase equilibria results. No ternary compounds were observed in the Sn-SnTe-Bi region. The SnTe phase is very stable and has tie-lines with the Sn, liquid, and Bi phases at 160°C. At 500°C, in addition to the already known SnBi₂Te₄ and SnBi₄Te₇ ternary phases, two new ternary compounds (Sn₂Bi₂Te₅ and SnBiTe₂ phases) were found. SnTe and Bi₂Te₃ have significant mutual solubilities, and the ternary compounds SnBi₂Te₄, SnBi₄Te₇, and Sn₂Bi₂Te₅ are all in the SnTe-Bi₂Te₃ pseudobinary section.     

Influence of Nanoinclusions on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Nanocomposites

n-Type Bi₂Te₃ nanocomposites with enhanced figure of merit, ZT, were fabricated by a simple, high-throughput method of mixing nanostructured Bi₂Te₃ particles obtained through melt spinning with micron-sized particles. Moderately high power factors were retained, while the thermal conductivity of the nanocomposites was found to decrease with increasing weight percent of nanoinclusions. The peak ZT values for all the nanocomposites were above 1.1, and the maximum shifted to higher temperature with increasing amount of nanoinclusions. A maximum ZT of 1.18 at 42°C was obtained for the 10 wt.% nanocomposite, which is a 43% increase over the bulk sample at the same temperature. This is the highest ZT reported for n-type Bi₂Te₃ binary material, and higher ZT values are expected if state-of-the-art Bi₂Te_3−x Se _x materials are used.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.     

Effects of SiC Nanodispersion on the Thermoelectric Properties of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Alloys

Polycrystalline p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se_0.3 thermoelectric (TE) alloys containing a small amount (vol.% ≤5) of SiC nanoparticles were fabricated by mechanical alloying and spark plasma sintering. It was revealed that the effects of SiC addition on TE properties can be different between p-type and n-type Bi₂Te₃-based alloys. SiC addition slightly increased the power factor of the p-type materials by decreasing both the electrical resistivity (ρ) and Seebeck coefficient (α), but decreased the power factor of n-type materials by increasing both ρ and α. Regardless of the conductivity type, the thermal conductivity was reduced by dispersing SiC nanoparticles in the Bi₂Te₃-based alloy matrix. As a result, a small amount (0.1 vol.%) of SiC addition increased the maximum dimensionless figure of merit (ZT _max) of the p-type Bi_0.5Sb_1.5Te₃ alloys from 0.88 for the SiC-free sample to 0.97 at 323 K, though no improvement in TE performance was obtained in the case of n-type Bi₂Te_2.7Se_0.3 alloys. Importantly, the SiC-dispersed alloys showed better mechanical properties, which can improve material machinability and device reliability.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Preparation and Thermoelectric Properties of Nanoporous Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Alloys

n-Type nanoporous Bi₂Te₃-based thermoelectric materials with different porosity ratios have been prepared by spark plasma sintering (SPS). The microstructure and phase morphology have been analyzed by x-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), and the thermoelectric properties of the SPS samples have been measured. Experimental results show that the nanoporous structures lying in the sheet layers and among the plate grains of the Bi₂Te₃ bulk material can lead to an increase in the Seebeck coefficient and a decrease in the thermal conductivity, thus leading to an enhanced figure of merit.     

Thermoelectric Properties of Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript> and Ni<Subscript>0.06</Subscript>Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript>

Mo₃Sb₇, crystallizing in the Ir₃Ge₇ type structure, has poor thermoelectric (TE) properties due to its metallic behavior. However, by a partial Sb-Te exchange, it becomes semiconducting without noticeable structure changes and so achieves a significant enhancement in the thermopower with the composition of Mo₃Sb₅Te₂. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the thermal conductivity by the so-called rattling effect. As part of the effort to verify this idea, we report herein the growth as well as measurements of the thermal and electrical transport properties of Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.     

Effects of Various Reductants and Surfactants on the Nanostructure of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Synthesized by a Hydrothermal Process

Various reductants and surfactants were used to investigate their effects on the structure of Bi₂Te₃ nanopowders prepared via hydrothermal synthesis at 150°C for 24 h. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were applied to analyze the phase distributions, microstructures, and grain sizes of the as-grown Bi₂Te₃. The results showed that the grain sizes and morphologies of the synthesized powders were mainly related to the effects of the reductants on the nucleation rate and the surface-adsorbed energy of the diverse surfactants on special crystal planes. Some surfactants could control the growth of the Bi₂Te₃ crystals and promote one-dimensional growth of nanorods during hydrothermal synthesis.     

Understanding Bulk Defects in Topological Insulators from Nuclear‐Spin Interactions

Non‐invasive local probes are needed to characterize bulk defects in binary and ternary chalcogenides. These defects contribute to the non‐ideal behavior of topological insulators. The bulk electronic properties are studied via ¹²⁵Te NMR in Bi₂Te₃, Sb₂Te₃, Bi_0.5Sb_1.5Te₃, Bi₂Te₂Se, and Bi₂Te₂S. A distribution of defects gives rise to asymmetry in the powder lineshapes. The Knight shift, line shape, and spin‐lattice relaxation are investigated in terms of how they affect carrier density, spin‐orbit coupling, and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi₂Te₂Se is the best topological insulator candidate material at the present time. These results, which are in good agreement with transport and angle‐resolved photoemission spectroscopy studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Effect of pH on Structural and Electrical Properties of Electrodeposited Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

Bi₂Te₃ thin films were electrodeposited at various pH values of a bismuth nitrate and tellurium oxide plating solution. Enhancement in pH results in a decrease in grain size. Transmission electron microscopy reveals the transformation of the film morphology from dispersed nanoparticles to connected chain-like nanostructures of Bi₂Te₃ as pH is increased. Electrical characterization for samples deposited in the temperature range of 300 K to 425 K shows a fourfold increase in Seebeck coefficient, S, between its maximum and minimum value as the solution pH changes from 1 to 3.5. Such enhancement of S is attributed to the increased connectivity of the nanostructures at higher pH.     

Features of reflection spectra of single crystals of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi₂Te₃-Sb₂Te₃ solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb₂Te₃ have been studied in the range of 400–4000 cm⁻¹ at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C ₃ (E ⊥ C ₃). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm⁻¹ corresponding to the optical band gap E _{g opt}. The features become more pronounced as the content of Bi₂Te₃ is increased to 80 mol % in the composition of the Bi₂Te₃-Sb₂Te₃ solid solution. A further increase in the content of Sb₂Te₃ is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in E _{g opt}.     

Effect of compositional dependence on physicochemical properties of Bi2Se3 doped system

Crystalline bulk compositions of Bi₂ (Se_1−xTe_x)₃ system with ( x=0.0–1.00) were prepared using the conventional melting method. The structural properties of bulk samples were studied with the aid of XRD and SEM analysis. Compositional element distribution and elemental ratios were estimated with EDX spectroscopy that attached with SEM. XRD patterns show that the prepared compounds are crystalline materials with single phases of Bi₂Se₃ and Bi₂Te₃ and/or a ternary phase. The grain size calculations were performed using the well-known Scherrer equation. The thermal studies analysis was carried out by using DSC. DSC studies revealed that the prepared samples are stable and none decomposable over the temperature range. Physicochemical properties such as compactness, molar volume and the free volume percentage were calculated for the concerned alloys based on the experimentally calculated densities of each compound. The measured parameters showed a strong dependence on the Te content.     

Effect of Vacancy Distribution on the Thermal Conductivity of Ga<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Our group has focused attention on Ga₂Te₃ as a natural nanostructured thermoelectric material. Ga₂Te₃ has basically a zincblende structure, but one-third of the Ga sites are structural vacancies due to the valence mismatch between Ga and Te. It has been confirmed that (1) vacancies in Ga₂Te₃ exist as two-dimensional (2D) vacancy planes, and (2) Ga₂Te₃ exhibits an unexpectedly low thermal conductivity (κ), most likely due to highly effective phonon scattering by the 2D vacancy planes. However, the effect of the size and periodicity of the 2D vacancy planes on κ has been unclear. In addition, it has also been unclear whether only the 2D vacancy planes reduce κ or if point-type vacancies can also reduce κ. In the present study, we tried to prepare Ga₂Te₃ and Ga₂Se₃ with various vacancy distributions by controlling annealing conditions. The atomic structures of the samples were characterized by means of transmission electron microscopy, and κ was evaluated from the thermal diffusivity measured by the laser flash method. The effects of vacancy distributions on κ of Ga₂Te₃ and Ga₂Se₃ are discussed.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.