Vol 51, No 7 (2017)

An array of identically oriented single-crystal bismuth islands formed on mica plates is used as a substrate to improve the structure of bismuth thin films grown by vacuum thermal evaporation. The array of islands is formed by the chemical etching of a single-crystal bismuth film 1 μm thick grown by floating-zone recrystallization under a coating. The structure of the obtained films is studied by X-ray diffraction, atomicforce microscopy, and electron backscatter diffraction scanning electron microscopy.

The optical absorption spectra of n-Bi₂Te₃ in the range 40–300 meV are studied at room temperature in relation to the electron concentration and sample thickness in order to determine the parameters of the additional subband in the conduction band of Bi₂Te₃ and to clarify the possible effect of this subband on charge-carrier transport. It is shown that bismuth telluride is a direct-gap semiconductor with an additional subband in the conduction band. These data are consistent with the results of studies of quantum oscillations in n-Bi₂Te₃ in high magnetic fields at temperatures below 20 K.

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi₂Te₃ heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.

A model of a microthermoelectric battery consisting of series connected thermoelectric cells of one-dimensional nanostructures formed into bundles by external contacts is proposed. The electrochemical processes of forming Bi and Sb nanowire arrays with the required dimensions and physicochemical properties in porous anodic alumina templates with large length-to-diameter ratios are studied and developed. The design and basis of the fabrication technology of microthermoelectric battery branches are developed, whose heat flow density will be controlled by the Seebeck coefficient measured at external contacts connecting the nanowire ends. It is shown that a set of parallel branches in the battery can be fabricated due to a high density of nanowire packing in a porous matrix, which will make it possible to achieve a fundamentally higher increase in the microheat meter efficiency.

It is necessary to take into account the thermal resistances of structural components located between a material and a heat emitting medium (on the cold side) and between a material and a heat absorbing medium (on the hot side) when simulating thermoelectric cooling devices. A dimensionless mathematical model taking into account the mentioned thermal resistances and describing the cooling and heating capacity, voltage, and coefficient of performance of the devices, depending on current, is proposed in this study. Using this model, the optimal values of the current and thermal resistances on the hot and cold side of the devices can be found for implementation of the maximum cooling capacity mode and other operating conditions.

Copper-selenide (Cu₂Se) samples are produced by mechanochemical synthesis and compaction by spark plasma sintering and hot pressing. The structure and phase composition of the samples before and after heat treatment are studied by the X-ray diffraction technique and electron microscopy. The character of changes in the shape and size of structural elements of the samples is shown. Variations in the phase composition of copper selenide in the temperature range from 25 to 500°C are studied in situ.

A significant difference in the thermal conductivity of the gradient-inhomogeneous branches of thermoelements at a difference in the operating temperature in the cases of parallel and antiparallel directions of temperature and composition gradients is experimentally established. The results obtained should be taken into account in using gradient-inhomogeneous materials for the optimization of thermoelectric energy converters.

The effect of modification of the surfaces of bismuth thin films with an antimony layer of 10 nm thick was studied. Three types of structures deposited onto mica substrates are compared: a bismuth film, an antimony layer as a sublayer for the bismuth film, and an antimony layer as the coating of the bismuth film. A substantial difference in the galvanomagnetic properties of the obtained structures is revealed in the dependence on the position of the antimony layer in the structure and on the bismuth-film thickness.

The reason why manganese monosilicide inclusions are formed during the growth of higher manganese silicide (HMS) crystals has not been studied in detail so far. Changes in the amount and density of these inclusions are greatly influenced by dopants. In this study, the structure of HMS crystals with various contents of germanium as a doping element is examined. It is found that raising the germanium content to 1 at % results in the fragmentation of layered manganese monosilicide inclusions and, simultaneously, leads to significant changes in the thermoelectric properties of HMS crystals. The results of the microstructural analysis of HMS crystals in relation to the germanium concentration may be of use for understanding the mechanism by which the manganese monosilicide phase is formed during the growth of higher manganese silicide crystals.

The crystal structures of layered crystals, i.e. ternary alloys [(Ge,Sn,Pb)Te(Se)]_m[(Bi,Sb)₂(Te,Se)₃]_n (m, n = 0, 1, 2…) (space groups \(R\overline 3 m\), \(P\overline 3 {m_1}\), and P2₁/m) are analyzed. The hierarchical relations between various space groups of ternary alloys are determined. The structural code of crystals of the family is determined, which takes into account their composition, crystal structure, and thermodynamic stability of layer stacks forming the superstructures.

The band gap E_g of a number of new thermoelectric materials, such as skutterudites, clathrates, Heusler phases, (Ge,Sn,Pb)(Te,Se)]_m[(Bi,Sb)₂(Te,Se)₃]_n (m, n = 0, 1, 2…) ternary alloys, and others is estimated. Estimations are performed using Goldsmid-Sharp’s formula E_g = 2e|α_max|T_max, Vegard’s rule E_g = (mE_g^A + nE_g^B)/(m + n), and the empirical dependence T_max = f(E_g) (A and B denote the corresponding binary alloys, T_max is the temperature corresponding to the thermoelectric-power maximum |α_max| or the thermoelectric figure of merit (ZT)_max, and e is the elementary charge). It is shown that the empirical dependence E_g = f(T_max) gives the most accurate estimates of E_g for difference classes of thermoelectric materials.

Based on the analysis of current scientific publications, materials for thermoelectric devices operating at different temperatures within 100–1300 K are reviewed. The main attention is paid to the fabrication of nanostructured thermoelectric materials. It is established that the most promising techniques for the synthesis of such materials are melt spinning, spark plasma sintering, and extrusion.

The influence of the conditions for preparing samples from granules of Bi_0.5Sb_1.5Te₃ solid solution obtained by melt solidification in liquid on the mechanical and thermoelectric properties of these samples is investigated. The microstructure and surface morphology of sample cleavages are analyzed by optical and scanning electron microscopy. The mechanical properties of the samples are investigated by compression tests at temperatures from 300 to 620 K. The thermoelectric characteristics (Seebeck coefficient and electrical and thermal conductivities) are measured both at room temperature and in the temperature range of 100–700 K. The samples with the highest thermoelectric figure of merit, (ZT)_max ≈ 1.3 at 370 K, are obtained.

The causes of the “charge-carrier density collapse”, i.e., a sharp increase in the equilibrium charge-carrier density n, p = 1 × 10¹⁹ → (2–5) × 10²⁰ cm^–3 in going from binary alloys such as GeTe and Bi₂Te₃ to the family of ternary alloys [(Ge,Sn,Pb)(Te,Se)]_m[(Bi,Sb)₂(Te,Se)₃]_n [m, n = 0, 1, 2…) are discussed. The phenomenon is associated with the positional disordering of heterovalent cations (Ge⁺², Sn⁺², Pb⁺² ↔ Bi⁺³, Sb⁺³) in the cation sublattice of ternary alloys. The phenomenon is not observed during the disordering of isovalent cations (Bi⁺³ ↔ Sb⁺³) or anions (Te^–2 ↔ Se^–2).

The methodological framework for assessing the environmental and economic efficiency of Bi₂Te₃-based thermoelectric cooling modules is considered taking into account resource consumption and the environmental impacts of their life cycle. It is shown that this approach makes it possible to determine the environmental aspects of thermoelectric modules and, if needed, to find ways for environmental improvements, as well as to obtain necessary information for potential investors and other interested parties.

Variations in the tensoresistance, tensomagnetoresistance, and magnetotensoresistance are experimentally and theoretically studied in wide ranges of magnetic-field strengths, 0 kOe ≤ H ≤ 100 kOe, and mechanical stresses, 0 GPa ≤ X ≤ 0.7 GPa, at 77 K under conditions of nondegenerate statistics of the electron gas in n-Ge crystals with different crystallographic orientations.

The rate of spontaneous emission of an emitter placed in a disordered one-dimensional photonic crystal is analyzed. It is shown that the spontaneous emission of a dipole placed in a disordered photonic crystal can either be enhanced (if the frequency corresponds to an optical eigenmode of the structure) or suppressed (if the frequency lies in the band gap or the position of the emitter corresponds to a node in the electric- field profile of the eigenmode). It is demonstrated that, for high levels of disorder, the photonic band gap is narrowed and the emission rate at the center of the band gap becomes significantly different from zero. Furthermore, for high levels of disorder, localized states featuring significantly enhanced spontaneous emission may appear within the photonic band gap.

Composite varistors based on polar and nonpolar polymers (the matrix) and inorganic phase (dispersant) ZnO are studied. It is shown that composite varistors with a high breakdown electric field and a high nonlinearity can be developed by varying the physical structure of the polymer–ZnO heterogeneous structure. Nonpolar polyethylene and nonpolar polyvinylidene fluoride are chosen as the polymeric matrix.

A review is given on some investigations to enhance ZT through very effective selective scattering of phonons from bottom-up nanostructuring methods. Novel concepts are also being utilized in an attempt to overcome the conventional tradeoff in the power factor.