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.     

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.     

Deposition of Nanocrystalline Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films Using a Modified MOCVD System

Nanocrystalline Bi₂Te₃ films were deposited on (100) GaAs substrates using a modified metalorganic chemical vapor deposition (MOCVD) system, and the effect of growth parameters on the structural properties were investigated. The modified MOCVD system employed a mixing room for the formation of nanoparticles of Bi₂Te₃ by gas-phase reaction and a graphite susceptor for growth of nanoparticles on the substrate. The grown films contained many crystallites of nanosize, and large crystallites consisted of small particles a few tens of nanometer in size. This nanostructured film approach can be an economical way of producing high-performance thermoelectric films with nanostructure compared with other top-down methods.     

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.     

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.     

Thermoelectric properties of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> ribbons prepared by melt spinning

The results of studying the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ alloy samples prepared by melt spinning quenching are presented. The material after melt spinning is shaped as thin ribbons and has a quasi-amorphous structure. The thermoelectric properties (thermoelectric power and electrical resistance) and crystallization processes of as-prepared melt-spun ribbons are studied at 300–800 K for the first time. The stability range of the initial state, the crystallization-onset temperature, and the effect of thermal annealing on the thermoelectric-power factor of the alloy are determined.     

Thermoelectric power of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> and Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi₉₂Sb₈ and Bi₈₅Sb₁₅ films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.     

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 Deformation Temperature on Texture and Thermoelectric Properties of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Prepared by Hot-Press Deformation

The effects of deformation temperature on texture and thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ sintered materials were investigated. The sintered materials were prepared by mechanical alloying and hot-press sintering. The hot-press deformation was performed at 723 K and 823 K by applying mechanical pressure in a graphite die. Then, the materials were extruded in the direction opposite to the direction of applied pressure. X-ray diffraction and electron backscattered diffraction patterns showed that the hexagonal c-plane tended to align along the extruded direction when the samples were deformed at high temperatures. The thermoelectric power factor was increased by high-temperature hot-press deformation because of the low electrical resistivity that originated from the c-plane orientation.     

Electrodeposition of <Emphasis Type="Italic">p</Emphasis>-Type Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films

Thermoelectric Sb _x Te _y films were potentiostatically electrodeposited in aqueous nitric acid electrolyte solutions containing different concentrations of TeO₂. Stoichiometric Sb _x Te _y films were obtained by applying a voltage of −0.15 V versus saturated calomel electrode (SCE) using a solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The nearly stoichiometric Sb₂Te₃ films had a rhombohedral structure, R[`3]m R\bar{3}m , with a preferred orientation along the (015) direction. The films had hole concentration of 5.8 × 10¹⁸/cm³ and exhibited mobility of 54.8 cm²/Vs. A more negative potential resulted in higher Sb content in the deposited Sb _x Te _y films. Furthermore, it was observed that the hole concentration and mobility decreased with increasingly negative deposition potential, and eventually showed insulating properties, possibly due to increased defect formation. The absolute value of the Seebeck coefficient of the as-deposited Sb₂Te₃ thin film at room temperature was 118 μV/K.     

Characterization of Ge<Subscript>22</Subscript>Sb<Subscript>22</Subscript>Te<Subscript>56</Subscript> and Sb-Excess Ge<Subscript>15</Subscript>Sb<Subscript>47</Subscript>Te<Subscript>38</Subscript> Chalcogenide Thin Films for Phase-Change Memory Applications

A phase-change memory device that utilizes an antimony (Sb)-excess Ge₁₅Sb₄₇Te₃₈ chalcogenide thin film was fabricated and its electrical properties were measured and compared with a similar device that uses Ge₂₂Sb₂₂Te₅₆. The resulting electrical characteristics exhibited I _reset values of 14 mA for Ge₂₂Sb₂₂Te₅₆ and 10.6 mA for Ge₁₅Sb₄₇Te₃₈. Also, the set operation time (t _set) for the device using Ge₁₅Sb₄₇Te₃₈ films was 140 ns, which was more than twice as fast as the Ge₂₂Sb₂₂Te₅₆ device. The relationship between the microstructure and the improved electrical performance of the device was examined by means of transmission electron microscopy (TEM).     

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.     

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.     

Thermoelectric Characteristics of <Emphasis Type="Italic">p</Emphasis>-Type (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript>/(Pb,Sn)Te Functional Gradient Materials with Variation of the Segment Ratio

The p-type (Bi,Sb)₂Te₃/(Pb,Sn)Te functional gradient materials (FGMs) were fabricated by hot-pressing mechanically alloyed (Bi_0.2Sb_0.8)₂Te₃ and 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te powders together at 500°C for 1 h in vacuum. Segment ratios of (Bi,Sb)₂Te₃ to (Pb,Sn)Te were varied as 3:1, 1.3:1, and 1:1.6. A reaction layer of about 350-μm thickness was formed at the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGM interface. Under temperature differences larger than 340°C applied across a specimen, superior figures of merit were predicted for the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs to those of (Bi_0.2Sb_0.8)₂Te₃ and (Pb_0.7Sn_0.3)Te. With a temperature difference of 320°C applied across a specimen, the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs with segment ratios of 3:1 and 1.3:1 exhibited the maximum output powers of 72.1 mW and 72.6 mW, respectively, larger than the 63.9 mW of (Bi_0.2Sb_0.8)₂Te₃ and the 26 mW of 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te.     

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.     

Features of the charge-transport mechanism in layered Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals doped with chlorine and terbium

The temperature dependences (T = 5−300 K) of the resistivity in the plane of layers and in the direction perpendicular to the layers, as well as the Hall effect and the magnetoresistance (H < 80 kOe, T = 0.5−4.2 K) in Bi₂Te₃ single crystals doped with chlorine and terbium, are investigated. It is shown that the doping of Bi₂Te₃ with terbium atoms results in p-type conductivity and in increasing hole concentration. The doping of Bi₂Te₃ with chlorine atoms modifies also the character of its conductivity instead of changing only the type from p to n. In the temperature dependence of the resistivity in the direction perpendicular to layers, a portion arises with the activation conductivity caused by the hopping between localized states. The charge-transport mechanism in Bi₂Te₃ single crystals doped with chlorine is proposed.     

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.     

Thermal Stability of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> in Contact with Ti and TiN

The thermal stability of a Ge₂Sb₂Te₅ chalcogenide layer in contact with titanium and titanium nitride metallic thin films has been investigated mainly using x-ray diffraction and elastic nuclear backscattering techniques. Without breaking vacuum, Ti and TiN have been deposited on Ge₂Sb₂Te₅ material using magnetron sputtering. Thermal treatments have been performed in a 10⁻⁷ mbar vacuum furnace. On annealing up to 450°C, the TiN metallic film does not interact with the chalcogenide film, but at the same time adhesion problems and instabilities in contact resistance arise. To improve the adhesion and eventually stabilize the contact resistance, an interfacial Ti layer has been considered. At 300°C, a TiTe₂ compound is formed by interacting with Te segregated from the Ge₂Sb₂Te₅ layer. At higher temperatures, the Ti layer decomposes the chalcogenide film, forming several compounds tentatively identified as GeTe, Ge₃Ti₅, Ge₅Ti₆, TiTe_2,, and Sb₂Te₃. It has been found that the properties of the Ge₂Sb₂Te₅ film can be retained by controlling the decomposition rate of the chalcogenide layer, which is achieved by providing a limited supply of Ti and/or by depositing a Te-rich Ge₂Sb₂Te₅ film.     

Effect of Electric Current on the Performance of Bi<Subscript>1.2</Subscript>Sb<Subscript>4.8</Subscript>Te<Subscript>9</Subscript> Thermoelectric Material Prepared by a Field-Activated Pressure-Assisted Sintering Process

Field-activated pressure-assisted sintering (FAPAS) was applied to sinter Bi_1.2Sb_4.8Te₉ thermoelectric materials under different conditions, including no-current sintering (NCS), low-density current sintering (LCS), and high-density current sintering (HCS). The effect of the current density on the final thermoelectric performance of the products was investigated. Applying a higher-density electric current and shorter dwell time can improve the thermoelectric performance of the sample by increasing its electric conductivity and decreasing its thermal conductivity. The maximum figure of merit ZT values of the NCS, LCS, and HCS samples were 0.46, 0.48, and 0.57, respectively. Therefore, applying a high-density electric current in the sintering process may be an effective way to obtain Bi_1.2Sb_4.8Te₉ thermoelectric material with high ZT value.     

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.