A. I. Sorokin

The Influence of Anisotropy and Nanoparticle Size Distribution on the Lattice Thermal Conductivity and the Thermoelectric Figure of Merit of Nanostructured (Bi,Sb)2Te3

Two factors that are important for proper estimation of the thermoelectric figure of merit of bulk nanostructured materials based on bismuth telluride and its solid solutions have been investigated. First, the anisotropy of the thermoelectric properties of nanostructured (Bi,Sb)2Te3 fabricated by the spark plasma sintering (SPS) method was studied experimentally as a function of sintering temperature and pressure. Two measuring methods were used: (a) the Harman method and (b) separate measurements of electrical conductivity, Seebeck coefficient, and thermal conductivity (laser flash method). Anisotropy and transport property values obtained by these methods are compared. Secondly, the influence of the nanoparticle size distribution on the lattice thermal conductivity was taken into account theoretically for scattering of phonons both on nanoprecipitates with different compositions and orientations and on grain boundaries. The results of estimations based on different theoretical approaches (relaxation-time approximation, Monte Carlo simulations, and effective medium method) are compared using typical size distribution parameters from available experimental data.

On fabrication of functionally graded thermoelectric materials by spark plasma sintering

A process of fabrication of thermoelectrics by the spark plasma sintering (SPS) method to obtain materials with advanced properties is simulated. The effect of the die dimensions and geometry on the temperature field distribution inside the sintered sample is analyzed. The feasibility of fabrication of functionally graded materials by the SPS process using dies of special configuration is demonstrated.

Temperature and current density distributions at spark plasma sintering of inhomogeneous samples

We analyze the effect of inhomogeneity in the properties of a material on the conditions of obtaining thermoelectrics by spark plasma sintering. Inclusions localized and distributed over the volume of materials with different values of electric and thermal conductivities are considered. It is found that the presence of macroscopic inhomogeneities changes the current density distribution in the cross section of the sample being sintered. It is shown that inhomogeneity in the properties of materials during sintering do not substantially affect the temperature field in the sample at the macroscopic level, but change the current density distribution profile. The ranges of variation of the current density in the regions with inhomogeneous electric and thermal conductivities are determined for various types of macroscopically inhomogeneous inclusions and their distribution. The applicability of various models for describing spark plasma sintering is considered.

Improved mechanical properties of thermoelectric (Bi0.2Sb0.8)2Te3 by nanostructuring

Temperature-dependent strength of Bi-Sb-Te under uniaxial compression is investigated. Bi-Sb-Te samples were produced by three methods: vertical zone-melting, hot extrusion, and spark plasma sintering (SPS). For zone-melted and extruded samples, the brittle-ductile transition occurs over a temperature range of 200-350 °C. In nanostructured samples produced via SPS, the transition is observed in a narrower temperature range of 170-200 °C. At room temperature, the strength of the nanostructured samples is higher than that of zone-melted and extruded samples, but above 300 °C, all samples decrease to roughly the same strength.

Temperature fields control in the process of spark plasma sintering of thermoelectrics

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.

Structure of the Cu 2 Se compound produced by different methods

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.

On Improvement of Thermoelectric Properties of Bulk Bi-Sb-Te Nanostructures

Fabrication of bulk nanostructures based on bismuth and antimony chalcogenides, including ball milling and subsequent hot pressing or spark plasma sintering, are discussed. Sets of samples of different compositions were obtained using different technological conditions (pressure, sintering temperature). Structure and mechanical properties of consolidated samples were investigated. Thermoelectric parameters were measured using direct methods and Harman technique; measurement errors in the thermoelectric properties were determined. The thermal conductivity of bulk nanostructures based on Bi-Sb-Te was calculated taking into account real phonon spectrum and anisotropy; conditions that promoted the minimizing of thermal conductivity were determined.

Bulk Nanocrystalline Thermoelectrics Based on Bi-Sb-Te Solid Solution

A nanopowder from p-Bi-Sb-Te with particles ~ 10 nm were fabricated by the ball milling using different technological modes. Cold and hot pressing at different conditions and also SPS process were used for consolidation of the powder into a bulk nanostructure and nanocomposites. The main factors allowing slowing-down of the growth of nanograins as a result of recrystallization are the reduction of the temperature and of the duration of the pressing, the increase of the pressure, as well as addition of small value additives (like MoS2, thermally expanded graphite or fullerenes). It was reached the thermoelectric figure of merit ZT=1.22 (at 360 K) in the bulk nanostructure Bi0,4Sb1,6Te3 fabricated by SPS method. Some mechanisms of the improvement of the thermoelectric efficiency in bulk nanocrystalline semiconductors based on BixSb2-xTe3 are studied theoretically. The reduction of nanograin size can lead to improvement of the thermoelectric figure of merit. The theoretical dependence of the electric and heat conductivities and the thermoelectric power as the function of nanograins size in BixSb2-xTe3 bulk nanostructure are quite accurately correlates with the experimental data.

Simulation of SPS Process for Fabrication of Thermoelectric Materials with Predicted Properties

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.

Experimental and theoretical study of the thermoelectric properties of copper selenide

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.