Major advantage of extruded Bi2Te3 based thermoelectric materials is high mechanical strength compared with that of melt-crystallized materials. Mechanical properties are of special importance for thermogenerator module applications where thermogenerator branches may undergo elevated thermal stresses due to large temperature differences at the modules. Since extrusion is typically a high-temperature process the structure of extruded materials is controlled by the plastic deformation in multiple slip systems resulting in the formation of a final deformed structure. The grain orientations are predominantly such that the most probable cleavage plane orientation is parallel to the extrusion axis. Recovery processes occur simultaneously and different recrystallization stages may take place. In the latter case the deformed texture may be destroyed. Structure evolution along the extruded rod of Bi2Se0.3Te2.7 ternary solid solution was studied with metallography and X-ray diffraction. Extrusion was interrupted for the study and so the specimen was a whole rod the initial part of which was the extrusion billet and the final part was the as-extruded material. The structure of the material is formed by competitive processes of dislocation generation and annealing. The plastic deformation energy is the highest in the extruder zone of the rod. Both the hardening processes and the texture are controlled by the plastic deformation mechanism. Plastic deformation is accompanied by generation of defects that are most likely vacancy type ones.

This work is a complex experimental and theoretical study of the extrusion process by equal-channel angular pressing in the production of chalcogenides based on bismuth telluride, which are of interest for the creation of new functional thermoelectrics.

Abstract—The Cu2 – xSe (0.03 ≤ x ≤ 0.23) powders fabricated by mechanochemical synthesis have been studied by X-ray diffraction. The in situ study has been carried out for the temperature dependences of the lattice parameters, the structures, and the phase compositions of the powders in the temperature range 25–350°C. The powder compositions are shown to differ from the charge compositions and are shifted to lower copper concentrations. The estimation of peak half-widths of the cubic β phase indicates an increase in the structure imperfection after the phase transition from the α phase to the β phase of Cu2 – xSe at ~140°C. It is shown that the superpositions of the subtraction solutions (copper vacancies) and interstitials solutions (copper atoms in interstitial sites), whose proportion is changed as a function of temperature and the deviation from stoichiometry, are in the thermodynamic equilibrium in the copper selenide solid solution at room temperature. The change in the slope of the dependence of the lattice parameter of the powder Cu2 – xSe samples on the composition (0.03 ≤ x ≤ 0.23) in the temperature range 25–350°C enables the suggestion that interstitial copper atom concentration increases with temperature and deviation from stoichiometry.

Abstract —The Cu_2 – _ x Se (0.03 ≤ x ≤ 0.23) powders fabricated by mechanochemical synthesis have been studied by X-ray diffraction. The in situ study has been carried out for the temperature dependences of the lattice parameters, the structures, and the phase compositions of the powders in the temperature range 25–350°C. The powder compositions are shown to differ from the charge compositions and are shifted to lower copper concentrations. The estimation of peak half-widths of the cubic β phase indicates an increase in the structure imperfection after the phase transition from the α phase to the β phase of Cu_2 – _ x Se at ~140°C. It is shown that the superpositions of the subtraction solutions (copper vacancies) and interstitials solutions (copper atoms in interstitial sites), whose proportion is changed as a function of temperature and the deviation from stoichiometry, are in the thermodynamic equilibrium in the copper selenide solid solution at room temperature. The change in the slope of the dependence of the lattice parameter of the powder Cu_2 – _ x Se samples on the composition (0.03 ≤ x ≤ 0.23) in the temperature range 25–350°C enables the suggestion that interstitial copper atom concentration increases with temperature and deviation from stoichiometry.

The process of field-assisted sintering of nanostructured thermoelectrics for the formation of effective materials for alternative power engineering has been numerically simulated. Functionally graded thermoelectrics and segmented thermoelement branches have been sintered in a temperature-gradient field. Modified die tooling elements are proposed that allow the desired thermal conditions to be created for the sintering of inhomogeneous effective materials.

We used mathematical modeling to compare the stress and deformation in a Bi0.4Sb1.6Te3 solid solution base thermoelectric material for extrusion through different diameter dies. The results show that extrusion through a 20mm diameter die produces a more inhomogeneous deformation compared with extrusion through a 30mm diameter die. Extrusion through a die of a larger diameter produces a structure that is coarser but has a more homogeneous grain size distribution. The degree of preferential grain orientation is higher for extrusion through a larger diameter die. We found a change in the lattice parameter of the solid solution along the extruded rod, correlating with detect formation during extrusion. The concentration of vacancies is higher for extrusion through a smaller diameter die. This difference between the structures results from a more intense dynamic recrystallization for a smaller diameter die. Increasing the die diameter and lowering the extrusion temperature allow retaining the thermoelectric properties of the material due to a better texture.

Проведено экспериментальное и теоретическое исследование температурных зависимостей теплоемкости, теплопроводности, коэффициента теплового расширения и кинетических коэффициентов (электропроводности, коэффициента термоэдс) селенида меди в интервале температур 300-873 K. Соответствие результатов расчета и эксперимента наблюдается до температуры ~ 773 K. Величина максимальной термоэлектрической эффективности в наноструктурированном селениде меди находится на уровне ZT~1.8. Продемонстрирована корреляционная зависимость ZT от величины теплопроводности во всем исследованном температурном интервале. DOI: 10.21883/FTP.2017.07.44635.21

The temperature dependence of the thermal conductivity of nanostructured samples of copper selenide prepared by mechanochemical synthesis from initial pure components in a planetary ball mill followed by spark plasma sintering has been studied. The thermal conductivity of nanostructured samples was measured in the temperature range 410–860 K. At 410–780 K, the thermal lattice conductivity κ ph varies insignificantly in the range 0.35–0.37 W/(m K). At a higher temperature T > 780 K, κph decreases to 0.19 W/(m K). To analyze the influence of mobile copper ions on the thermal conductivity of the lattice, molecular-dynamic calculations were performed using a classical interatomic potential obtained from ab initio calculations for the cubic β-Cu2Se modification. The simulation results demonstrate a high mobility of copper ions, and the calculated temperature dependence of the lattice thermal conductivity agrees with the experiment to 780 K. At a temperature T > 780 K, κ ph deviates from the calculation results, and this deviation is most pronounced in the nanostructured material. As a result, at the maximum measurement temperature, the lattice thermal conductivity decreased to ~0.19 W/(m K), which agrees with available data for nanostructured Cu 2 Se samples produced by various methods.

Методом компьютерного моделирования рассмотрен процесс активированного полем спекания наноструктурированных термоэлектриков для получения материалов с повышенной эффективностью. Спекание функционально-градиентных термоэлектриков и составных ветвей термоэлементов производилось в градиентном температурном поле. Предложена модификация элементов оснастки, позволяющая создавать необходимые температурные условия для спекания неоднородных эффективных материалов. DOI: 10.21883/PJTF.2017.14.44832.16436

Spark plasma sintering (SPS) is a promising method for fabrication of thermoelectric materials. The electric and thermal fields in the SPS process have been simulated by using the finite element method to model an SPS-511S experimental setup. Investigation of thermoelectric materials based on Bi2Te3 solid solutions revealed that the temperature measured close to the sample during application of the electric current could be reproduced by the simulation. Modification of the compression mold configuration could be used to alter the electric and thermal conditions, adjust the Joule heat released in the setup elements, and create a gradient temperature field during the SPS process. The temperature–time dependence in the sample was also studied, revealing that the temperature difference along the vertical axis may reach hundreds of degrees. Prediction of the sintering temperature in each layer may allow further prediction of the thermoelectric properties of the sample. More accurate modifications of the SPS process based on such computer simulations may help to form structures with macroscopically inhomogeneous and functionally graded legs.

Copper selenide is a promising material for power generation in a medium-temperature range 600–1000 K. A number of features of the Cu–Se system, namely, the existence of phase transition in a Cu2Se compound, the high speed of the diffusion of Cu ions, and the high vapor pressure of Se at elevated temperatures, make it necessary to carry out a series of experimental investigations to develop and optimize the methodology for obtaining the bulk material based on copper selenide. The influence of mechanochemical synthesis regimes and subsequent compaction method on the thermoelectric properties and structure of copper selenide is studied. The source material is obtained by mechanochemical synthesis. The methods of hot pressing (HP) and spark plasma synthesis (SPS) are used to obtain the bulk samples. The investigation of the structure and phase composition is performed by the X-ray diffraction and scanning electron microscopy. It is shown that increasing the duration of the mechanochemical synthesis up to 5 h leads to the depletion of copper in powders and to the formation of nonstoichiometric β-phase Cu1.83Se, which persists after SPS. A comparison of the structure and properties of the material obtained by SPS and HP showed that the material obtained by HP has a greater degree of grain defects. The highest thermoelectric efficiency ZT = 1.8 at a temperature of 600°C is achieved for the material obtained by SPS. It is shown that low thermal conductivity is the main factor affecting the value of the thermoelectric efficiency ZT of the studied materials. The difference in the values of thermal conductivity of the materials obtained by different methods is related to the electronic component of thermal conductivity.

Исследована температурная зависимость теплопроводности наноструктурированных образцов селенида меди, полученных методом механохимического синтеза из исходных чистых компонентов в планетарной шаровой мельнице с последующим искровым плазменным спеканием. Измерение теплопроводности наноструктурированных образцов проводилось в диапазоне температур 410-860 K. При 410-780 K теплопроводность решетки kappaph слабо изменяется в диапазоне 0.35-0.37 W/(m·K). При более высокой температуре T>780 K kappaph снижается до 0.19 W/(m·K). Для анализа влияния подвижных ионов меди на теплопроводность решетки проведены расчеты методом молекулярной динамики с использованием классического межатомного потенциала, полученного из ab initio расчетов для кубической модификации beta-Cu2Se. Результаты моделирования демонстрируют высокую подвижность ионов меди, а расчетная температурная зависимость решеточной теплопроводности согласуется с экспериментом до 780 K. При температуре выше 780 K наблюдается отклонение kappaph от результатов расчета, которое особенно сильно выражено в наноструктурированном материале. В результате при максимальной температуре измерения решеточная теплопроводность снижается до уровня ~ 0.19 W/(m·K), что согласуется с литературными данными для наноструктурированных образов Cu2Se, полученных различными методами. DOI: 10.21883/FTT.2017.10.44983.091

Приведены результаты расчета теплопроводности решетки в теллуриде висмута и оценки ее снижения в наноструктурированном материале за счет граничного рассеяния. Расчет проводился с использованием метода решеточной динамики с учетом реального спектра фононов и фонон-фононного взаимодействия. Его результаты хорошо согласуются с экспериментальными данными для кристаллического материала. Оценки для наноструктурированного материала дали снижение теплопроводности на 30% при размерах зерен 20 нм. Проводится сравнение данного метода расчета с расчетами, использующими приближенные методы описания спектра и процессов рассеяния. Показано, что наименьшее отличие в оценках (около 10%) может быть получено при использовании приближения постоянного матричного элемента фонон-фононного рассеяния с коррекцией частотной зависимости для акустических мод. DOI: 10.21883/FTP.2017.06.44544.03

The field-activated sintering of segmented thermoelement branches is considered within a timedependent computer model. Modified asymmetric tooling with an electrical insulating layer is proposed. This tooling facilitates the formation of a temperature difference as high as several hundreds of kelvins in the sample. The influence of the insulating-layer thickness on the axial and radial temperature differences in the samples is analyzed. The possibility of reducing the radial temperature gradient is demonstrated.

Поступило в Редакцию 30 августа 2016 г

The temperature dependences of the specific heat, thermal conductivity, coefficient of thermal expansion (CTE), and transport coefficients (electrical conductivity and thermoelectric power) of copper selenide are experimentally and theoretically investigated in the temperature range of 300–873 K. The calculation results correlate with the experimental data up to a temperature of ~773 K. The maximum thermoelectric figure of merit of nanostructured copper selenide is ZT ~ 1.8. The correlation dependence between ZT and the thermal conductivity within the entire temperature range under consideration is shown.

The results of calculations of lattice thermal conductivity in bismuth telluride are presented and its decrease in a nanostructured material due to boundary scattering is estimated. Calculation is carried out using the lattice-dynamics method with the real phonon spectrum and the phonon–phonon interaction taken into account. Its results well agree with experimental data for the crystalline material. The estimates for the nanostructured material give a decrease in the thermal conductivity by 30% for grain sizes of 20 nm. Calculations by this method are compared with those obtained using approximate methods of description of the spectrum and scattering processes. It is shown that the least difference in the estimates (about 10%) can be obtained when using the approximation of a constant matrix element of phonon–phonon scattering with correction of the frequency dependence for acoustic modes.

A precise method for calculating segmented legs for thermoelectric generators in a one-dimensional approximation by the method of thermal balance with effective values of the thermoelectric parameters is proposed. This method allows one to accurately take into consideration the temperature dependences of the electrical conductivity, the Seebeck coefficient, and thermal conductivity. The effect of contact resistance on the resulting power-conversion efficiency is also taken into account. The procedure of segmented leg calculation is demonstrated, and a method of transitioning from leg calculation to thermoelement calculation is proposed.

The process of creating thermoelectrics by spark plasma sintering of nanostructured powders in order to obtain materials with improved thermoelectric properties has been modeled. The factors that influence the distribution of thermal field in the sintering process have been analyzed. The influence of geometric parameters of tooling on the formation of temperature gradient field required for effective sintering of functionally gradient materials and segmented branches of thermo-elements has been considered. The results can be used to determine the conditions and modes of sintering of functionally gradient materials in installations of spark plasma sintering and hot pressing.

С использованием нестационарной компьютерной модели рассмотрен процесс активированного полем спекания составных ветвей термоэлементов. Предложена модификация оснастки. Оснастка асимметричной формы, содержащая изолирующий слой, способствует формированию в образце перепада температур, достигающего нескольких сотен градусов. Проанализировано влияние толщины электроизоляционного слоя на величину осевого и радиального перепадов температур в образцах. Показана возможность понижения радиального температурного градиента. DOI: 10.21883/FTP.2017.06.44551.11