Vol 53, No 5 (2019)

The results of investigations of the thermoelectric properties of In_0.2Ce_0.1Co₄Sb_12.3 compound prepared by the rapid-quenching technique are presented. In the process of rapid-quenching, amorphous and partially crystallized thin ribbons are formed. The thus obtained ribbons serve as the initial material for fabricating bulk samples. However, the thermoelectric properties of the fabricated ribbons are practically unstudied. In this study, the thermopower and electrical resistivity of amorphous and crystalline ribbons are investigated for the first time at temperatures of 100–750 K. The range of stability of the amorphous state is determined, and the crystallization kinetics of amorphous ribbons is investigated.

The effects of film thickness and block size on the Hall and Seebeck effects in bismuth films on mica substrates are analyzed using experimental data. A preferential decrease in the electron contribution with a decrease in the film thickness and a preferential decrease in the hole contribution with a decrease in the block size are established. The Hall and Seebeck coefficients are calculated using the classical size effect with regard to carrier scattering at block boundaries and anisotropy of the properties of carriers. In the calculation, the electron and hole mobility components and their concentration in a bismuth single crystal are used and the crystallographic orientation of the film crystal are taken into account. The results of the calculation are in good agreement with the experimental data. It is concluded that the value and sign of the Hall and Seebeck coefficients in bismuth films are determined by the competition of the classical size effect and scattering at block boundaries.

For p-Bi₂Te₃ crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse Nernst–Ettingshausen coefficient are obtained experimentally in the temperature range 77–450 K. The transmittance spectrum in the range 400–5250 cm^–1 is recorded at room temperature. It is shown that, to interpret the temperature dependences of the scattering parameter r and the ratio of the thermoelectric power to temperature (α/T), it is essential to take into account the complex valence-band structure and the contribution of heavy holes to transport phenomena. Estimations of the energy band parameters in the context of the two-band model give a hole effective mass close to the free electron mass and the energy gap between nonequivalent extrema at a level of several hundredths of eV. In the absorption spectrum derived from the transmittance spectrum, a sharp increase in absorption defined by indirect interband transitions with the band gap E_g ≈ 0.14 eV is observed in the region of frequencies ν ≥ 1000 cm^–1. The absorption spectrum calculated from the reflectance data using the Kramers–Kronig relations is in agreement with the experimental absorption spectrum.

The results of calculation of the charge-carrier concentration in bismuth–antimony films on substrates with different coefficients of thermal expansion are reported. The antimony content in the films is in the range 0–15 at %. Calculation is performed on the basis of experimental studies of the galvanomagnetic properties of the films. It is shown that the concentration is considerably higher in the case of substrates with a large coefficient of thermal expansion. The results of calculation of the position of the valence and conduction bands at 77 K in relation to the coefficient of thermal expansion of the substrate are reported. It is shown that, as the thin films experience in-plane strains defined by a difference between the coefficients of thermal expansion of the film and substrate materials, the position of the valence and conduction bands in the films changes relative to the position of the bands in the single crystal with the corresponding composition.

The thermoelectric properties in the temperature range 4.2–300 K and magnetoresistance oscillations in strong magnetic fields at low temperatures are studied in nanostructured layered films of topological thermoelectric materials n-Bi_{2 –x}Sb_xTe_{3 –y}Se_y. It is shown that the thermoelectric figure of merit in layered n-Bi_{2 –x}Sb_xTe_{3 –y}Se_y films is larger than that in the bulk material due both to an increase in the Seebeck coefficient below room temperature and to a decrease in the thermal conductivity and its weaker temperature dependence. Analysis of the magnetoresistance oscillations is used to determine the parameters of the topological surface states of Dirac fermions and estimate their influence on the thermoelectric properties.

The thermal resistances on the cold and hot sides substantially affect the output characteristics of thermoelectric devices. A dimensionless mathematical model of a thermoelectric cooler, which makes it possible to calculate device parameters, such as the optimal thermal resistance ratio on the cold and hot sides as well as the optimal current taking into account the influence of thermal resistances, is presented. The maximal temperature difference ΔT_max mode is considered. It is shown that the optimal cooler parameters are different for implementation of the ΔT_max and Q_max modes. The determining factor for the ΔT_max mode is the influence of the thermal resistance on the hot side, and the optimal current is 0.4–0.7 of the maximal current in most cases for the material with ZT = 1. It is shown that an additional increase in ΔT_max of a cooler is attained with a decrease in the thermal conductivity of the thermoelectric material due to a decrease in the influence of the thermal resistance on the hot side besides the effect from an increase in ZT. An increase in the length of thermoelectric legs has the same positive effect of an increase in ΔT_max of a cooler, while a decrease in the leg length negatively affects ΔT_max.

Antimony-telluride-based nanocomposite samples containing different weight fractions of graphite (Sb₂Te₃ + x% graphite, where x = 0.0, 0.5, 1.0, and 2.5%) are synthesized and studied. The samples are produced by a solid-state reaction with the use of a ball mill. X-ray diffraction measurements show that the Sb₂Te₃ phase is present in the nanocomposites. All of the diffraction peaks are identified as corresponding to the rhombohedral structure of symmetry \(R\bar {3}m\). No additional peaks related to graphite are observed because of its low content. Moreover, the X-ray data show the insolubility of graphite in Sb₂Te₃: the peaks related to Sb₂Te₃ remain unchanged upon the addition of graphite. The thermal conductivity, thermoelectric power, and resistivity of the samples are studied in the temperature range 80 K ≤ T ≤ 320 K. The thermal conductivity k of the nanocomposite decreases several times compared to the thermal conductivity of single-crystal Sb₂Te₃ and reaches k ≈ 0.95 W m^–1 K^–1 at x = 0.5%. The parameter k unsteadily depends on the content of graphite. The thermoelectric power of the nanocomposites with graphite at x = 1.0% is higher compared to that of nanostructured Sb₂Te₃.

In n-Bi₂Te₃ and n-Bi₂Te_{3 –y}Se_y thermoelectrics, the surface states of Dirac fermions of the interlayer Van der Waals surface (0001) are studied by scanning tunneling microscopy and spectroscopy. The surface morphology and modulated line profiles of the tunneling microscopy images are determined by local distortions of the surface density of electronic states and depend on the composition. The Dirac point E_D in the studied compositions is localized in the band gap and shifts to the top of the valence band with increasing Se content in n-Bi₂Te_{3 –y}Se_y alloys. The dependence between the parameters of the surface states of Dirac fermions (Dirac-point position, Fermi velocity, surface fermion concentration) and thermoelectric properties (Seebeck coefficient and power parameter) in the thermoelectrics under study is determined.

The results of experimental investigations into the thermoelectric properties (electrical conductivity, thermoelectric power, and thermal conductivity) of microtextured foils and single-crystal wires based on semimetal and semiconductor Bi_{1 –x}Sb_x alloys are presented in the temperature range of 4.2–300 K. It is found that the band gap ΔE in Bi–17 at % Sb wires increases with decreasing wire diameter d, which is a manifestation of the quantum-size effect. At low temperatures (T < 50 K), in the wires with d < 400 nm, the electrical conductivity increases due to the significant contribution of highly conductive surface states characteristic of topological insulators. It is found for the first time that the thermal conductivity of semimetal Bi–3 at % Sb foils at low temperatures is two orders of magnitude lower, and that of semiconductor Bi–16 at % Sb foils one order of magnitude lower, than that in bulk samples of the corresponding composition due to significant phonon scattering at grain boundaries and surfaces. This effect leads to considerable enhancement of the thermoelectric figure-of-merit ZT and can be used in miniature low-temperature thermoelectric energy converters.

Thermoelectric properties of cobalt monosilicide CoSi and (Co_{1 –x}M_xSi, M = Fe, Ni) alloys are studied. Alloy compositions with an iron content of up to 10 at % and nickel content of up to 5 at % are examined. The thermoelectric power and electrical resistivity are measured at temperatures in the range 100–800 K. Recent calculations of the band structure of cobalt monosilicide have revealed a number of significant differences from the standard semi-metallic model with the energy overlap of parabolic bands for electrons and holes. This requires the modification of previously employed models to describe the transport properties. The possibility of theoretical description of the experimental temperature and concentration dependences of the thermoelectric power and electrical resistivity with the use of different models for description of the electronic spectrum is analyzed.

One of the main energy characteristics of a cooling leg is the maximum available temperature difference (ΔT_max). Its increase indicates an increase in the coefficient of performance (COP) at all temperatures. The use of a segmented leg makes it possible to increase the ΔT_max value; however, the maximum available COP increases only at large temperature differences, whereas at small temperature differences the maximum available COP is even smaller than in a simple leg.

Extrusion methods have become widespread in the fabrication technology of thermoelectric materials with high strength characteristics. The extrusion method of thermoelectric materials based on Bi–Sb single crystals in a liquid medium under a high hydrostatic pressure is considered. It is shown that the proposed extrusion method makes it possible to fabricate Bi–Sb crystals of n-type conductivity with high thermoelectric characteristics at temperatures T ≲ 180 K.

An experimental setup is developed to measure the thermoelectric properties of semiconductor nanowires with diameters of up to 5 nm in dielectric matrices. This setup makes it possible to measure the electrical resistance and thermoelectric power of nanostructured samples in the temperature range of 77–400 K.

Two different procedures for estimating the electron affinity of SiC polytypes and the interrelation of these procedures with the results of ab initio calculations are discussed. Simple corrections to the Shockley–Anderson rules for the constructions of band diagrams of heterojunctions are proposed.

Type-I indirect-gap heterostructures are convenient objects for studying the spin dynamics of localized excitons, which are difficult to investigate in heterostructures of other types. It is shown that structures with such an energy spectrum can be formed from III–V binary compounds on substrates with the (110) orientation. The effect of the strain distribution and conduction-band structure in quasimomentum space on the energy spectrum of electronic states in the heterostructures is discussed.

The formation of antisite defects in Ge₂₀Te₈₀ and Ge₁₅As₄Te₈₁ vitreous alloys in the form of tin atoms in tellurium sites and tellurium atoms in germanium sites is shown by emission Mössbauer spectroscopy with the ^119mmSn(^119mSn), ^119mTe(^119mSn), ¹²⁵Sn(¹²⁵Te), and ^125mTe(¹²⁵Te) isotopes. It is shown that the isovalent substitution of germanium atoms by tin atoms does not vary the symmetry of the local surrounding of germanium sites, while tin and tellurium atoms reconstruct their local surrounding in sites unnatural for them.

Thermally activated hydrogen desorption from pentagraphane is studied by atomistic computer simulation. Pentagraphane is a recently predicted quasi-two-dimensional hydrocarbon compound that represents a pentagraphene single layer, in which both sides are covered with hydrogen and the C–C bonds form a network of adjacent pentagons, whereas the hexagons characteristic of carbon nanostructures are lacking. The effect of hydrogen desorption on the electronic structure, phonon density of states, and Young’s modulus are studied. The temperature dependence of the characteristic desorption time is determined by the molecular-dynamics method.