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.     

Thermal Conductivity of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Affected by Grain Size and Pores

A fine measurement system for measuring thermal conductivity was constructed. An accuracy of 1% was determined for the reference quartz with a value of 1.411 W/m K. Bi_0.5Sb_1.5Te₃ samples were prepared by mechanical alloying followed by hot-pressing. Grain sizes were varied in the range from 1 μm to 10 μm by controlling the sintering temperature in the temperature range from 623 K to 773 K. The thermal conductivity was 0.89 W/m K for the sample sintered at 623 K, while a grain size of 1.75 μm was measured by optical microscopy and scanning electron microscopy. The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 K to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.     

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> solid solution

It is shown that the Seebeck coefficient α, the power factor α²σ, and the density-of-states effective mass m/m ₀ in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α²σ, and m/m ₀ is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.     

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.     

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 and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

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.     

Influence of Extrusion Temperature on the Formation of a Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Structure of <Emphasis Type="Italic">p</Emphasis>-Type Conductivity

The influence of competing processes of deformation, return, and recrystallization on the structure and properties of thermoelectric materials extruded at different temperatures is investigated. X-ray diffraction analysis, Harman’s method of measuring thermoelectric properties, and hydrostatic weighing are applied. The nonmonotonic dependence of the texture, electrophysical properties, and density on the extrusion temperature is revealed. In order to achieve the best thermoelectric properties of the material, the optimal extrusion temperature is established to be 400°C.     

Doping of the Bi<Subscript>1.9</Subscript>Sb<Subscript>0.1</Subscript>Te<Subscript>3</Subscript> solid solution with Sn impurity

In (Bi_1.9Sb_0.1)_{1 − x} Sn _x Te₃ solid solution with different contents of Sn, the electrical conductivity (σ₁₁) and the Hall (R ₁₂₃ and R ₃₂₁), Seebeck (S ₁₁ and S ₃₃), and Nernst-Ettingshausen (Q ₁₂₃ and Q ₃₂₁) coefficients have been measured. It is shown that doping with tin strongly modifies temperature dependences of the kinetic coefficients. The effect of tin on electrical homogeneity of the samples has been studied: with increasing number of Sn atoms embedded, crystals become more homogeneous. These features indicate the presence of the quasi-local states of Sn in the valence band of Bi_1.9Sb_0.1Te₃. Within a one-band model, we estimated the effective mass of the density of hole states (m _d ), the energy gap extrapolated to 0 K (E _g0 = 0.20–0.25 eV), the energy of impurity states (E _Sn ≈ 40–45 meV), and the scattering parameter (r ≈ 0.1–0.4). Numerical values of the scattering parameter indicate a mixed mechanism of scattering in the samples under investigation with dominant scattering at acoustic phonons. With increasing content of tin in the samples, the contribution of impurity scattering increases.     

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 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.     

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.     

Nernst-ettingshausen tensor in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystal

On one Sb₂Te₃ single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Q _ikl ) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (R _ikl ) and Seebeck (S _ij ) coefficients and the conductivity (σ _ii ) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c ₃. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.     

Effect of thallium doping on the mobility of electrons in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and holes in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Shubnikov–de Haas effect and the Hall effect in n-Bi_2–xTl_xSe₃ (x = 0, 0.01, 0.02, 0.04) and p-Sb_2–xTl_xTe₃ (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi_2–xTl_xSe₃ and increases the electron mobility. In p-Sb_2–xTl_xTe₃, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

Galvanomagnetic properties of Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films on different substrates

The results of studying the galvanomagnetic properties of Bi₈₅Sb₁₅ thin films on substrates with various thermal-expansion coefficients are presented. The significant effect of the difference between the thermal expansions of film and substrate materials on the galvanomagnetic properties of the films is revealed. An analysis of the film properties within the two-band model shows that the difference in the properties of films on different substrates is associated with changes in the charge-carrier concentration and mobility. It is shown that a decrease in the thermal-expansion coefficient of the substrate material results in a decrease in the carrier concentration in the film composition under study, which indicates a significant difference in changes in the film band structure in comparison with changes occurring with increasing antimony concentration.     

Segmented Generator Modules Using Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Materials

Previously, the Institute of Thermoelectricity has created Bi₂Te₃-based modules with an efficiency of ~7% in the temperature range of 30°C to 300°C, with legs that employed homogeneous thermoelectric materials. Herein, we present the results of development of such modules with legs made of inhomogeneous materials. Based on the theory of optimal control and object-oriented computer technology, programs to determine the requirements for material properties in the inhomogeneous legs were created. It was established that introduction of inhomogeneity in the form of continuous and step changes in three-segment n- and p-type legs yields almost identical efficiency increases of about 15%. Use of two segments reduces this value of 10% to 12%. Modules with two-segment legs encapsulated in thin-walled metal cases filled with inert gas have been built, yielding improved efficiency of 7.8% to 8%.     

Switching effects in the films based on Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>

Experimental results on the switching effects related to the phase transitions in Ge₂Sb₂Te₅ in the presence of external voltage or laser irradiation are presented. An electron model of the reversible switching is discussed.     

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).     

Preparation and properties of Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>-based thermoelectric material

The effect of synthesis conditions on the structure and thermoelectric properties of zinc-antimonide- based materials is investigated. The effects of Zn excess, the modes of spark plasma sintering, and In doping on the phase composition and the thermal stability of the properties of the obtained material are considered. The material is prepared by the method of the direct alloying of components and spark plasma sintering. It is shown that, at certain modes of spark plasma sintering, the introduction of an excess amount of Zn and In doping make it possible to obtain β-Zn₄Sb₃ with the thermoelectric efficiency ZT ≈ 1.47 at a temperature of 720 K, which shows the stability of characteristics under the performed tests.