Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Alloys: Application Considerations

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type Pb_0.5Sn_0.5Te:Te and PbTe<Na> samples, both of which have a hole concentration of ∼1 × 10²⁰ cm⁻³. The ZT values of PbTe<Na> were found to be higher than those of Pb_0.5Sn_0.5Te:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for Pb_0.5Sn_0.5Te:Te at 450°C. However, the microhardness value of 49 H_V found for Pb_0.5Sn_0.5Te:Te was closer to that of the mechanically stable n-type PbTe (30 H_V) than to that of PbTe<Na> (71 H_V). Thus, although lower ZT values were obtained, from a mechanical point of view Pb_0.5Sn_0.5Te:Te is preferable over PbTe<Na> for practical applications.     

Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films for terahertz detectors

Liquid-phase epitaxy is used to fabricate Pb_0.8Sn_0.2Te films, undoped or doped with indium to different levels. The depth profiles of the carrier density and dopant concentration in the films are measured and examined. A uniform dopant concentration to a depth of 15 μm is obtained. Electrical-conduction inversion is observed at a temperature of 77.3 K as the doping level is varied. The liquid-phase epitaxial method is shown to be a more suitable technology for the reproducible manufacture of epitaxial films with a given carrier density, such as the ones used in terahertz detectors.     

Preparation of Mg<Subscript>2</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> by Induction Melting and Spark Plasma Sintering,and Thermoelectric Properties

Sn-doped, Mg₂Si_1−x Sn _x (x = 0 to 0.6) bulk alloys were prepared using Mg and Sn ingots as raw materials by suspended induction melting combined with the spark plasma sintering method, and the effects of Sn doping on thermoelectric transport properties were studied systematically. The results showed that Mg site vacancies caused by evaporation during the reaction process were filled by excess Mg addition (108 wt.% of the stoichiometric ratio of Mg₂Si). The resulting alloy samples were found to be single phase and relatively dense (above 98%). n-Type semiconducting characteristic of Sn-doped Mg₂Si_1−x Sn _x alloy was observed, and the electrical resistivity of all samples decreased with increasing temperature. The absolute Seebeck coefficient increased and the thermal conductivity was not changed significantly within the experimental Sn doping range. The dimensionless figure of merit (ZT) for Mg₂Si_0.4Sn_0.6 alloy reached its highest value of 0.25 at 400°C.     

Structure and Thermoelectric Properties of Te- and Ge-Doped Skutterudites CoSb<Subscript>2.875−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>0.125</Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

n-Type CoSb_2.875−x Ge_0.125Te _x (x = 0.125 to 0.275) compounds with different Te contents have been synthesized by a melt–quench–anneal–spark plasma sintering method, and the effects of Te content on the structure and thermoelectric properties have been investigated. The results show that all specimens exhibited n-type conduction characteristics. The solubility limit of Te in CoSb_2.875−x Ge_0.125Te _x is found to be x = 0.25. The solubility of Te in CoSb₃ is increased through charge compensation of the element Ge. The room-temperature carrier concentration N _p of CoSb_2.875−x Ge_0.125Te _x skutterudites increases with increasing Te content, and the compounds possess high power factors. The maximum power factor of 3.89 × 10⁻³ W m⁻¹ K⁻² was obtained at 720 K for the CoSb_2.625Ge_0.125Te_0.25 compound. The thermal conductivity decreases dramatically with increasing Te content due to strong point defect scattering. The maximum value of the thermoelectric figure of merit ZT = 1.03 was obtained at 800 K for CoSb_2.625Ge_0.125Te_0.25, benefiting from a lower thermal conductivity and a higher power factor. The figure of merit is competitive with values reported for single-filled skutterudites.     

Highly Efficient Ge-Rich Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te Thermoelectric Alloys

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type Ge _x Pb_1−x Te alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by Bi₂Te₃ doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping Pb_0.13Ge_0.87Te with 3 mol% Bi₂Te₃.     

Optical Properties of Epitaxial Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb Alloy Layers

Optical studies of unstrained narrow-gap Al _x In_{1 − x} Sb semiconductor alloy layers are carried out. The layers are grown by molecular-beam epitaxy on semi-insulating GaAs substrates with an AlSb buffer. The composition of the alloys is varied within the range of x = 0–0.52 and monitored by electron probe microanalysis. The band gap E _g is determined from the fundamental absorption edge with consideration for the nonparabolicity of the conduction band. The refined bowing parameter in the experimental dependence E _g (x) for the Al _x In_{1 − x} Sb alloys is 0.32 eV. This value is by 0.11 eV smaller than the commonly referred one.     

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.     

Local symmetry and electronic structure of tin atoms in (Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>x</Subscript>)<Subscript>1−<Emphasis Type="Italic">z</Emphasis></Subscript>In<Subscript>z</Subscript>Te lattices

Mössbauer spectroscopy of the ¹¹⁹Sn isotope has shown that the superconducting transition in (Pb_1?xSn_x)_1?zIn_zTe solid solutions is not accompanied by a change in the local symmetry of tin atoms or in their electronic structure.     

Thermoelectric Properties and Electronic Structure of Bi- and Ag-Doped Mg<Subscript>2</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Compounds

Mg₂Si_1−x Ge _x compounds were prepared from pure elements by melting in tantalum crucibles. The reaction was conducted under an inert gas in a special laboratory setup. Samples for thermoelectric measurements were formed by hot pressing. Structure and phase composition of the obtained materials were investigated by x-ray diffraction (XRD). Morphology and chemical composition were examined by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), respectively. Thermoelectric properties, i.e., the Seebeck coefficient, the electrical conductivity, and the thermal conductivity, were measured in the temperature range of 500 K to 900 K. The effect of Bi and Ag doping on the thermoelectric performance of Mg-Si-Ge ternary compounds was investigated. The electronic structures of binary compounds were calculated using the Korringa–Kohn–Rostoker (KKR) method. The effects of disorder, including Ge substitution and Bi or Ag doping, were accounted for in the KKR method with coherent potential approximation calculations. The thermoelectric properties of doped Mg₂Si_1−x Ge _x are discussed with reference to computed density of states as well as the complex energy band structure.     

Solution Growth and Thermoelectric Properties of Single-Phase MnSi<Subscript>1.75−<Emphasis Type="Italic">x</Emphasis></Subscript>

We have investigated the crystal growth of single-phase MnSi_1.75−x by a temperature gradient solution growth (TGSG) method using Ga and Sn as solvents and MnSi_1.7 alloy as the solute, and measured the thermoelectric properties of the resulting crystals. Single-phase Mn₁₁Si₁₉ and Mn₄Si₇ crystals were grown successfully using Ga and Sn as solvents, respectively. The typical size of a grown ingot of Mn₁₁Si₁₉ was 2 mm to 4 mm in thickness and 12 mm in diameter, whereas Mn₄Si₇ had polyhedral shape with dimensions in the range of several millimeters. The single-phase Mn₁₁Si₁₉ has good electrical conduction (ρ = 0.89 × 10⁻³ Ω cm to 1.09 × 10⁻³ Ω cm) compared with melt-grown multiphase higher-manganese silicide (HMS) crystals. The Seebeck coefficient, power factor, and thermal conductivity were 77 μV K⁻¹ to 85 μV K⁻¹, 6.7 μW cm⁻¹ K⁻² to 7.2 μW cm⁻¹ K⁻², and 0.032 W cm⁻¹ K⁻¹, respectively, at 300 K.     

On the Dielectric Study of Se<Subscript>80−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>20</Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 1 and 2) Glasses

In the present paper, the dielectric parameters such as the dielectric constant ε′(ω), dielectric loss ε″(ω) and alternating current (ac) conductivity have been investigated for bulk amorphous chalcogenide Se_80?x Te₂₀Pb _x (x = 0, 1 and 2) glasses in the frequency range 10 Hz to 500 kHz and within the temperature range from 300 K to 320 K. Dielectric constant ε′(ω) and dielectric loss ε″(ω) are found to be highly frequency (ω) and temperature dependent, and this behavior is interpreted on the basis of Guintini’s theory of dielectric dispersion. The ac conductivity (σ _ac) is found to be temperature independent and obey the power law ω ^s , where s < 1 and decreases as temperature rises. The obtained results are discussed in terms of the correlation barrier hopping model proposed by Elliot. The composition dependence of the dielectric constant, dielectric loss and ac conductivity are also discussed and reported here.     

Metal-insulator transition in chromium-doped Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te alloys

The electrical properties of chromium-doped n-Pb_1?x Ge _x Te alloys (x = 0.02–0.13) have been studied. A decrease in the free-electron concentration and a metal-insulator transition are observed as the germanium content of alloys increases. This is due to the Fermi level pinning by the chromium impurity level and to the flow of electrons from the conduction band to the impurity level. The experimental data obtained are used to calculate, in terms of the two-band Kane dispersion law, the dependences of the electron concentration and Fermi energy on the germanium content in the alloy. The motion rate of the chromium-related level with respect to the conduction band bottom is determined and a model of variation of the electronic structure with the matrix composition is suggested.     

High-Temperature Transport Properties of Yb<Subscript>4−<Emphasis Type="Italic">x</Emphasis></Subscript>Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>3</Subscript>

Polycrystalline L₄Sb₃ (L = La, Ce, Sm, and Yb) and Yb_4−x Sm _x Sb₃, which crystallizes in the anti-Th₃P₄ structure type (I-43d no. 220), were synthesized via high-temperature reaction. Structural and chemical characterization were performed by x-ray diffraction and electronic microscopy with energy-dispersive x-ray analysis. Pucks were densified by spark plasma sintering. Transport property measurements showed that these compounds are n-type with low Seebeck coefficients, except for Yb₄Sb₃, which shows semimetallic behavior with hole conduction above 523 K. By partially substituting Yb by a trivalent rare earth we successfully improved the thermoelectric figure of merit of Yb₄Sb₃ up to 0.7 at 1273 K.     

Photosensitivity of Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te:In films in the region of intrinsic absorption

The steady-state photocurrent in the fundamental absorption region of Pb_{1 ? x} Sn _x Te:In films is calculated with the field injection of electrons from the contact and their capture by traps in the bulk taken into account. The calculated and experimental current-voltage characteristics are compared at liquid-helium temperature. The represented experimental data on the dependence of the Hall effect on the injection level agree well with the considered model.     

Influence of Composition on the Thermoelectric Properties of Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films

Bi_1?x Sb _x solid solutions have attracted much attention as promising thermoelectric (TE) materials for cooling devices at temperatures below ～200 K and as unique model materials for solid-state science because of a high sensitivity of their band structure to changes in composition, temperature, pressure, etc. Earlier, we revealed a non-monotonic behavior of the concentration dependences of TE properties for polycrystalline Bi_1?x Sb _x solid solutions and attributed these anomalies to percolation effects in the solid solution, transition to a gapless state, and to a semimetal–semiconductor transition. The goal of the present work is to find out whether the non-monotonic behavior of the concentration dependences of TE properties is observed in the thin film state as well. The objects of the study are Bi_1?x Sb _x thin films with thicknesses in the range d = 250–300 nm prepared by thermal evaporation of Bi_1?x Sb _x crystals (x = 0–0.09) onto mica substrates. It was shown that the anomalies in the dependence of the TE properties on Bi_1?x Sb _x crystal composition are reproduced in thin films.     

Optical and thermal properties of CuAl<Subscript>x</Subscript>In<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>2</Subscript> solid solutions

The optical and thermal properties of crystals of CuAl_xIn_1?xTe₂ solid solutions grown by the Bridgman-Stockbarger method were studied for the first time. From the transmission and reflection spectra in the region of the intrinsic-absorption edge, the band gap (E _g ) was determined for the CuInTe₂ and CuAlTe₂ compounds and for their solid solutions; the concentration dependence of E _g was plotted. The E _g value was found to vary nonlinearly with x and can be described by the quadratic dependence. Dilatometry was used to study the thermal expansion of these solid solutions. The coefficient of thermal expansion (α_L) was shown to have a λ-shaped temperature dependence in the region of phase transitions. The isotherms are plotted for the concentration dependence of α_L. The thermal conductivity was investigated and its concentration dependence was plotted. The dependence of the thermal conductivity on x was established to have a minimum in the region of medium compositions.     

Optical properties of AgGa<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2</Subscript> alloys

Coarse-grained crystals of AgGaSe₂ and AgInSe₂ ternary compounds and their alloys are grown by planar crystallization of the melts. For the crystals produced in this way, the transmittance spectra near the fundamental absorption edge are studied. From the experimental spectra, the band gap (E _g) and its variation with composition are determined. It is established that E _g is a nonlinear function of the composition parameter x. The dependence E _g (x) is calculated theoretically in the context of the Van Vechten-Bergstresser model and Hill-Richardson pseudopotential model.     

Resonant gallium level in Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>x</Subscript>Te alloys under pressure

The effect of pressure on electrical properties of the n-Pb_1?x Sn_xTe alloy (x = 0.21) doped with gallium is studied. A decrease in the free-electron concentration with increasing temperature and an increase in this concentration with increasing pressure are observed; these observations indicate that the Fermi level is pinned at the resonant gallium level. The two-band Kane’s dispersion law and experimental data were used to calculate the pressure dependences of the electron concentration and Fermi level. A diagram of transformation of the charge-carrier energy spectrum in Pb_1?x Sn_xTe:Ga subjected to pressure is suggested; the rate of the shift of the gallium resonant level in reference to the midgap under the effect of pressure is determined.     

Photosensitive thin-film In/<Emphasis Type="Italic">p</Emphasis>-Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S Schottky barriers: Fabrication and properties

Thin Pb _x Sn_{1 − x} S films are obtained by the “hot-wall” method at substrate temperatures of 210–330°C. The microstructure, composition, morphology, and electrical characteristics of films are investigated. On the basis of the obtained films, photosensitive In/p-Pb _x Sn_{1 − x} S Schottky barriers are fabricated for the first time. The photosensivity spectra of these structures are investigated, and the character of interband transitions and the band-gap values are determined from them. The conclusion is drawn that Pb _x Sn_{1 − x} S thin polycrystalline films may be used in solar-energy converters.