The possibility of replacing iron with cobalt in iron disilicide is studied. It has been shown that, in a wide range of compositions, an attempt to replace iron with cobalt leads to the formation of cobalt silicides. Using the composition Fe0.98Co0.015Si2 as an example, it is shown that during the directional crystallization and subsequent annealing of samples, a regular microstructure is formed. Anisotropy of thermoelectric properties is observed along and across the crystallization axis of the sample. In the region of low cobalt concentrations, upon transition to pure β-FeSi2, the sign and magnitude of thermopower change sharply.

In this work, we study thermoelectric properties of GeTei-based alloys, doped with bismuth, with partial substitution of lead for germanium: Ge0.86Pb0.1Bi0.04Te. The aim of the study is to explore the possibility of increasing the thermoelectric efficiency of a compound by combining optimal doping and isovalent substitution to improve the electronic properties with a simultaneous decrease of the lattice thermal conductivity. We studied alloy samples prepared in two different research laboratories using similar, but not completely identical procedures. It is shown that the electronic (thermoelectric power and electrical conductivity) properties of the samples of the two groups are in good agreement with each other. The properties of alloys depend on the thermal history of the samples due to the presence at temperatures of 600–800 K of a phase transition from a low-temperature rhombohedral to a high-temperature cubic structural modification and missibility gap in GeTe–PbTe quasibinary system below 870 K. The thermoelectric figure of merit of alloys reaches a maximum value of 1.5 at a temperature of about 750 K.

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions.

In p-type thermoelectrics based on bismuth and antimony chalcogenides with a bismuth excess, the effective mass of the density of states m/m0 increases in nanocomposite and nanostructured solid solutions compared with the base material obtained by the direct crystallization method. It is shown that an increase in m/m0 is associated with an increase in the effective scattering parameter reff and strengthening of the dependence of the relaxation time on energy, which is typical for topological insulators. The material parameter β, proportional to the thermoelectric efficiency of ZT, at temperatures below room temperature increases greater in the nanostructured composition than in the nanocomposite with the inclusions of SiO2 due to the growth of m/m0 and a decrease in the lattice thermal conductivity κL. At high temperatures in the range of 300—500 K, the greatest growth in parameter β in the base material is associated with higher mobility.

In this work, we study the properties of GeTe -based alloys, doped with bismuth, with partial substitution of lead for germanium: Ge0.86Pb0.1Bi0.04Te. The aim of the study is to explore the possibility of increasing the thermoelectric efficiency of a compound by combining optimal doping and isovalent substitution to improve the electronic properties with a simultaneous decrease of the lattice thermal conductivity. We studied alloy samples prepared in two different research laboratories using similar, but not completely identical procedures. It is shown that the electronic (thermoelectric power and electrical conductivity) properties of the samples of the two groups are in good agreement with each other. The properties of alloys depend on the thermal history of the samples due to the presence at temperatures of 600–800 K of a phase transition from a low-temperature rhombohedral to a high-temperature cubic structural modification. The thermoelectric figure of merit of alloys reaches a maximum value of 1.5 at a temperature of about 750 K.

The temperature dependences of the Seebeck coefficient and conductance of the nanocomposite composed of polyvinylidene fluoride, multi-walled carbon nanotubes, and carbon nanofibers are measured. It is shown that the thermoelectric properties of the composite are very different from the properties of the initial carbon filler. In particular, the Seebeck coefficient of the nanocomposite at room temperature is almost two times higher than the thermoelectric power of the carbon filler.

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions.

In the p-type thermoelectrics based on bismuth chalcogenides and antimony with an excess bismuth, the density of states effective mass m/m0 increases in nanocomposite and nanostructured solid solutions compared with the base material obtained by the direct crystallization method. It is shown that an increase in m/m0 is associated with an increase in the effective scattering parameter reff and amplifying the relaxation time of energy, which is typical for topological insulators. The material parameter beta, proportional to the thermoelectric efficiency of ZT, at temperatures below room increases stronger in nanostructured composition than in a nanocomposite with the inclusions of SiO2 due to the growth of m/m0 and decrease the lattice thermal conductivity κL. At high temperatures in the range of 300-500 to the greatest growth of the parameter beta in the base material associated with higher mobility.

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.

Nanocomposite thermoelectrics based on Bi 0.45 Sb 1.55 Te 2.985 solid solution of p -type conductivity are fabricated by the hot pressing of nanopowders of this solid solution with the addition of SiO 2 microparticles. Investigations of the thermoelectric properties show that the thermoelectric power of the nanocomposites increases in a wide temperature range of 80–420 K, while the thermal conductivity considerably decreases at 80–320 K, which, despite a decrease in the electrical conductivity, leads to an increase in the thermoelectric efficiency in the nanostructured material without the SiO 2 addition by almost 50% (at 300 K). When adding SiO 2 , the efficiency decreases. The initial thermoelectric fabricated without nanostructuring, in which the maximal thermoelectric figure of merit ZT = 1 at 390 K, is most efficient at temperatures above 350 K.

AbstractAn 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.

AbstractNanocomposite thermoelectrics based on Bi_0.45Sb_1.55Te_2.985 solid solution of p -type conductivity are fabricated by the hot pressing of nanopowders of this solid solution with the addition of SiO_2 microparticles. Investigations of the thermoelectric properties show that the thermoelectric power of the nanocomposites increases in a wide temperature range of 80–420 K, while the thermal conductivity considerably decreases at 80–320 K, which, despite a decrease in the electrical conductivity, leads to an increase in the thermoelectric efficiency in the nanostructured material without the SiO_2 addition by almost 50% (at 300 K). When adding SiO_2, the efficiency decreases. The initial thermoelectric fabricated without nanostructuring, in which the maximal thermoelectric figure of merit ZT = 1 at 390 K, is most efficient at temperatures above 350 K.

Исследованы температурные зависимости концентрации и подвижности в термоциклах 300-700-300 K при разной длительности низкотемпературного отжига образца, предваряющего начало каждого термоцикла. Параллельно проведены 1500-часовый отжиг материала в перепаде температур 310-670 K и последующие измерения его термоэлектрических параметров. Результаты анализируются с учетом особенностей кристаллической структуры и ковалентного характера химической связи в ZnSb. Оценка рабочего диапазона и срока службы данного состава выполнена на основе модели взаимодействия примесных дефектов с понижением их акцепторной активности и ростом сечения рассеяния дырок при низких температурах. DOI: 10.21883/FTP.2017.08.44787.56

The temperature dependences of concentration and mobility are investigated in 300–700–300 K thermal cycles at different durations of low-temperature annealing of a ZnSb:0.1 at % Cu sample before the onset of every thermal cycle. Concurrently, the material is annealed for 1500 hours at a temperature difference of 310–670 K, and, then, its thermoelectric parameters are measured. The results are analyzed taking into account the features of the crystalline structure and the covalent nature of the chemical bonds in ZnSb. The operating range and lifetime of this composition are estimated on the basis of a model of the interaction of impurity defects with a decrease in their acceptor activity and an increase in the hole-scattering cross section at low temperatures.

Nanowire structures of zinc-doped indium antimonide are fabricated. These structures are formed in porous glass with characteristic pore sizes of ~7 nm. The temperature dependences of the electrical conductivity, Seebeck coefficient, and current–voltage characteristics are studied. An increase in the InSb Seebeck coefficient in porous glass in comparison with that of the bulk material is detected for the first time.

Получены нанопроволочные структуры антимонида индия, легированного цинком. Эти структуры были сформированы в пористом стекле с характерными размерами пор ~7 нм. Исследованы температурные зависимости электрического кондактанса, термоэдс и вольт-амперные характеристики. Впервые обнаружено увеличение термоэдс антимонида индия в пористом стекле по сравнению с термоэдс массивного материала. DOI: 10.21883/FTP.2017.06.44545.04

A model for describing the temperature dependence of the defect microstructure in high-efficiency ZnSb thermoelectric with a copper content of 0.1 at % is chosen. The temperature dependences of the chargecarrier density and mobility for thermal cycle I (300–700–300 K) are analyzed taking into account the features of the crystal structure and covalent chemical bond in ZnSb. The basic defect structure (at temperatures of T = 560–605 K) is the state when all Cu atoms are equally distributed between sites of both sublattices and behave as acceptors, and the number of intrinsic donor and acceptor defects is much smaller. The effect of the latter becomes noticeable when the temperature goes beyond the above-mentioned range. At T > 605 K, extra acceptors (antisite Zn Sb ) occur; upon cooling below 560 K, Cu 2 dimers arise and the electrical activity of the impurity lowers. Dimer decay upon heating leads to growth in the concentration with temperature up to saturation in the above-mentioned range. Additional thermal cycles II–VIII are performed; the observed changes in the temperature dependences of the hole concentration and mobility are discussed in the context of the investigated model.

Решается задача выбора модели для описания температурной зависимости микроструктуры дефектов в эффективном термоэлектрике ZnSb c 0.1 ат% Cu. Анализируются температурные зависимости концентрации и подвижности для термоцикла 300-700-300 K (термоцикл I) с учетом особенностей кристаллической структуры и ковалентного характера химической связи в ZnSb. Базовой дефектной структурой (температуры T=560-605 K) является состояние, когда все атомы Cu поровну распределены между узлами обеих подрешеток, являясь акцепторами, собственных дефектов акцепторного и донорного типа значительно меньше. Их действие становится заметным, когда температура выходит за рамки вышеупомянутого диапазона. При T>605 K появляются дополнительные акцепторы --- антиструктурный цинк ZnSb; при охлаждении ниже 560 K образуются димеры Cu2, электрическая активность примеси понижается. Распад димеров при нагревании вызывает увеличение концентрации с температурой вплоть до насыщения в указанном выше диапазоне. Были проведены дополнительные термоциклы II-VIII, обнаруженные изменения в температурных зависимостях концентрации дырок и подвижности обсуждаются в рамках упомянутой модели. DOI: 10.21883/FTP.2017.09.44878.8510

The technique for measuring the Hall coefficient and electrical conductivity in the thermal cycling mode is used to study the effect of the Sn impurity on the microstructure and properties of pressed ZnSb samples. Tin was introduced as an excess component (0.1 and 0.2 at %) and as a substitutional impurity for Zn and Sb atoms in a concentration of (2–2.5) at % The temperature dependences of the parameters of lightly doped samples are fundamentally like similar curves for ZnSb with 0.1 at % of Cu. The highest Hall concentration, 1.4 × 1019 cm–3 at 300 K, is obtained upon the introduction of 0.1 at % of Sn; the dimensionless thermoelectric figure of merit attains its maximum value of 0.85 at 660 K. The experimental data are discussed under the assumption of two doping mechanisms, which are effective in different temperature ranges, with zinc vacancies playing the decisive role of acceptor centers. In two ZnSb samples with SnSb and ZnSn additives, the charge-carrier compensation effect is observed; this effect depends on temperature and markedly changes with doping type. As in p-type AIV–BVI materials with a low Sn content, hole compensation can be attributed to atomic recharging Sn2+ → Sn4+. Types of compensating complexes are considered.