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}.     

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.     

Microgenerator Using BiSbTe-Pt Thermopile and Pt-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramic Combustor

The performance of a microcombustor thermoelectric generator device based on a thermopile using p-type Bi_0.3Sb_1.7Te₃ (BST) and n-type Pt films has been investigated. The BST films were prepared by two different methods—pulsed laser deposition (PLD) and sputter deposition—on Si₃N₄/SiO₂ multilayers on Si substrate. The ceramic catalyst combustor was patterned on the thermopile end on a thin membrane fabricated by back-side bulk etching of the silicon substrate. At 138°C the thermoelectric power factors of the PLD and sputter-deposited films were 3.6 × 10⁻³ W/mK² and 0.22 × 10⁻³ W/mK², respectively. The power from the generator with the sputter-deposited film was 0.343 μW, which was superior to that of the device with the PLD film, which provided 0.1 μW, for combustion of a 200 sccm flow of 3 v/v% hydrogen in air.     

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.     

Electrodeposition of Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films from DMSO Solution

The electrochemical behavior of nonaqueous dimethyl sulfoxide solutions of Bi^III, Te^IV, and Sb^III was investigated using cyclic voltammetry. On this basis, Bi _x Sb_2−x Te _y thermoelectric films were prepared by the potentiodynamic electrodeposition technique in nonaqueous dimethyl sulfoxide solution, and the composition, structure, morphology, and thermoelectric properties of the films were analyzed. Bi _x Sb_2−x Te _y thermoelectric films prepared under different potential ranges all possessed a smooth morphology. After annealing treatment at 200°C under N₂ protection for 4 h, all deposited films showed p-type semiconductor properties, and their resistances all decreased to 0.04 Ω to 0.05 Ω. The Bi_0.49Sb_1.53Te_2.98 thermoelectric film, which most closely approaches the stoichiometry of Bi_0.5Sb_1.5Te₃, possessed the highest Seebeck coefficient (85 μV/K) and can be obtained under potentials of −200 mV to −400 mV.     

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.     

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.     

Development of (Bi,Sb)<Subscript>2</Subscript>(Te,Se)<Subscript>3</Subscript>-Based Thermoelectric Modules by a Screen-Printing Process

In major applications, optimal power will be achieved when thermoelectric films are at least 100 μm thick. In this paper we demonstrate that screen-printing is an ideal method to deposit around 100 μm of (Bi,Sb)₂(Te,Se)₃-based films on a rigid or flexible substrate with high Seebeck coefficient value (90 μV K⁻¹ to 160 μV K⁻¹) using a low-temperature process. Conductive films have been obtained after laser annealing and led to acceptable thermoelectric performance with a power factor of 0.06 μW K⁻² cm⁻¹. While these initial material properties are not at the level of bulk materials, the complete manufacturing process is cost-effective, compatible with large surfaces, and affords a mass-production technique.     

Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

Thermoelectric Properties of Ag-doped Mg<Subscript>2</Subscript>Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg₂Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg₂Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 × 10¹⁸ cm⁻³ for undoped films to 8.8 × 10¹⁸ cm⁻³ for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200–650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 μV K⁻¹, while it was 280 μV K⁻¹ for the most heavily doped film at ∼400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of ∼10⁻⁵ W cm⁻¹ K⁻² at 700 K.     

Conductivity of layers of a chalcogenide glassy semiconductor Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> in high electric fields

Effect of high electric fields on the conductivity of 0.5-1-μm-thick layers of a chalcogenide glassy semiconductor with a composition Ge₂Sb₂Te₅, used in phase memory cells, has been studied. It was found that two dependences are observed in high fields: dependence of the current I on the voltage U, of the type I ∝ U ⁿ , with the exponent (n ≈ 2) related to space-charge-limited currents, and a dependence of the conductivity σ on the field strength F of the type σ = σ₀exp(F/F ₀) (where F ₀ = 6 × 10⁴ V cm⁻¹), caused by ionization of localized states. A mobility of 10⁻³–10⁻² cm² V⁻¹ s⁻¹ was determined from the space-charge-limited currents.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

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.     

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.     

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.     

Enhancing Thermoelectric Properties of Polycrystalline Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> by Optimizing a Ball-Milling Process

Bismuth sulfide (Bi₂S₃) polycrystalline samples were fabricated by mechanical alloying (MA) combined with spark plasma sintering (SPS). The microstructure and electrical transport properties were investigated with special emphasis on the influence of the ball-milling process. Bi₂S₃ compound powders could be readily synthesized directly from elemental powders under all the investigated conditions, and highly dense n-type bulk Bi₂S₃ samples with high density (>95%) were fabricated by the subsequent SPS process. Changing the MA conditions had no apparent influence on the microstructure or phase structure of the MA-derived Bi₂S₃ powders, but the electrical properties and thermopower of the SPS-sintered Bi₂S₃ bulk samples were greatly dependent on the MA speed and time. The power factor of Bi₂S₃ was increased to 233 μW K⁻² m⁻¹ at 573 K by optimizing the ball-milling process. This power factor is higher than values reported to date for Bi-S binary samples without texture.     

Enhanced Thermoelectric Properties Obtained by Compositional Optimization in p-Type BixSb2?xTe3 Fabricated by Mechanical Alloying and Spark Plasma Sintering

Bi _x Sb_2−x Te₃ bulk alloys are known as the best p-type thermoelectric materials near room temperature. In this work, single-phase Bi _x Sb_2−x Te₃ (x = 0.2, 0.25, 0.3, 0.34, 0.38, 0.42, 0.46, and 0.5) alloys were prepared by spark plasma sintering (SPS) using mechanical alloying (MA)-derived powders. A small amount (0.1 vol.%) of SiC nanoparticles was added to improve the mechanical properties and to reduce the thermal conductivity of the alloys. The electrical resistivity decreases significantly with increasing ratio of Sb to Bi in spite of the weaker decreasing trend in Seebeck coefficient, whereby the power factor at 323 K reaches 3.14 × 10⁻³ W/mK² for a sample with x = 0.3, obviously higher than that at x = 0.5 (2.27 × 10⁻³ W/mK²), a composition commonly used for ingots. Higher thermal conductivities at low temperatures are obtained at the compositions with lower x values, but they tend to decrease with temperature. As a result, higher ZT values are obtained for Bi_0.3Sb_1.7Te₃, with a maximum ZT value of 1.23 at 423 K, about twice the ZT value (about 0.6) of Bi_0.5Sb_1.5Te₃ at the same temperature.     

Response of thermoelectric parameters of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> films to secondary recrystallization

Flash evaporation is used to grow Bi_0.5Sb_1.5Te₃ films 1200 nm thick on mica substrates. The average lateral crystallite sizes in the as-grown films are ~800 nm. The (0001) plane in the crystallites is preferentially parallel to the substrate plane. After heat treatment in an argon atmosphere, the effective lateral size of crystallites in which the third-order axis is perpendicular to the substrate plane increased by a factor of 3–5. The crystallites were preferentially oriented in the substrate plane as well. The thermoelectric-power parameter of Bi_0.5Sb_1.5Te₃ films after their heat treatment in an inert environment increased approximately twofold to values close to that of the corresponding single crystals.     

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.     

Deposition of Bismuth Telluride Thick Film by Solidification under Centrifugal Pressure

Bismuth telluride alloys—Bi_0.5Sb_1.5Te₃ and Bi_1.8Sb_0.2Te_3.33Se_0.17—have been deposited on polycrystalline zirconia via solidification under centrifugal pressure. The crystal growth under centrifugal pressure was a process in which the starting powders charged in the groove patterns of the substrates were first melted and then solidified under centrifugal acceleration of 10⁴ m/s². This new process offers c-axis-oriented films with a thickness of more than 100 μm. A mirror-like surface is another characteristic feature of these films. Owing to their orientation, reasonable power factors such as 4.2 mW/m K² and 2.7 mW/m K² (in plane) were obtained for p- and n-type films, respectively.