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Bulat L.P.,Saint Petersburg State University | Bublik V.T.,Moscow Institute of Steel And Alloys | Drabkin I.A.,GIREDMET Ltd. | Karataev V.V.,GIREDMET Ltd. | And 5 more authors.
Journal of Electronic Materials | Year: 2010

Bulk nanostructured polycrystals are prospective thermoelectrics. Fabrication and examination of nanostructured polycrystalline p-Bi-Sb-Te are discussed. The influence of fabrication conditions and the composition of the material on sizes of nanograins was investigated experimentally. Basic physical properties of fabricated nanothermoelectrics including nanocomposites were also studied. Theoretical estimation of the influence of phonon scattering on nanograin boundaries in bulk nanostructured thermoelectrics is presented. © 2010 TMS.


Lavrent'ev M.G.,GIREDMET Company | Osvenskii V.B.,GIREDMET Company | Pivovarov G.I.,Technological Institute for Superhard and Novel Carbon Materials | Sorokin A.I.,GIREDMET Company | And 3 more authors.
Technical Physics Letters | Year: 2016

We have studied the temperature dependence of the mechanical strength at uniaxial compression for solid solutions based on bismuth and antimony chalcogenides, which were prepared by three methods: (i) vertical zone melting (VZM), (ii) hot extrusion, and (iii) spark plasma sintering (SPS). In the samples of solid solutions obtained by VZM and extrusion, a brittle–ductile transition was observed in a wised temperature interval of 200–350°C. In nanostructured SPS samples, transition from brittle to plastic fracture was observed within 170–200°C. The room-temperature strength of nanostructured samples was eight to nine times as large as that of VZM samples, and the stress–strain curves of these materials were significantly different. At a temperature of about 300°C, the strength of nanostructured solid solutions decreases to nearly zero. © 2016, Pleiades Publishing, Ltd.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Osvenskii V.B.,Giredmet Ltd | Pshenay-Severin D.A.,Saint Petersburg State University
Journal of Electronic Materials | Year: 2014

A theoretical investigation of the lattice thermal conductivity of nanostructured materials based on Bi–Sb–Te is presented. The calculations were based on relaxation time approximation and took into account both the real phonon spectra, obtained from first-principles by use of density functional theory, and the anisotropy of phonon relaxation time. Phonon relaxation time data were determined from experimental values of the lattice thermal conductivity. The decrease of the thermal conductivity caused by the nanostructure was compared with results from calculations based on the linear Debye approach. Estimation showed that phonon boundary scattering can lead to a 55% decrease of thermal conductivity for a grain size of ~20 nm in the Debye approximation. Taking the nonlinearity of the acoustic phonon spectrum into account leads to a 20% larger decrease of the thermal conductivity because of boundary scattering. The reason is that consideration of the real phonon spectrum increases the relative contribution to thermal conductivity of acoustic phonons with low frequencies that are scattered more strongly at nanograin boundaries. Similarly, estimation of lattice thermal conductivity reduction as a result of phonon scattering by nanoinclusions gave an 8% larger decrease when the real phonon spectrum was used rather than the linear Debye approximation. For such a substantial decrease of lattice thermal conductivity, the effect of the optical phonons was estimated; it was shown that optical phonons can reduce the change of thermal conductivity as a result of grain boundary scattering by no more than 10%. Finally, the minimum lattice thermal conductivity was estimated to be 0.07 W/m K because of acoustic modes (0.09 W/m K in the Debye approach) and 0.14 W/m K when the contribution of optical modes was also taken into consideration. © 2014, TMS.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Pshenay-Severin D.A.,RAS Ioffe Physical - Technical Institute | Ivanov A.A.,GIREDMET Ltd. | Osvenskii V.B.,GIREDMET Ltd. | Parkhomenko Y.N.,GIREDMET Ltd.
Journal of Electronic Materials | Year: 2016

Copper selenide is a promising thermoelectric material. One of the reasons for its high efficiency is its low thermal conductivity that can be connected with the decrease of heat capacity with temperature. The possibility of the decrease of heat capacity with the increase of temperature in this material can be connected with the liquid-like behavior of copper ions. In order to reveal the influence of this factor, measurements of constant pressure heat capacity cp and calculations of constant volume heat capacity cV were performed for cubic β-Cu2Se at temperatures T = 450–1000 K. Both calculations and measurements made in the present work demonstrate only a small decrease of heat capacity with temperature. The temperature dependence of cp reasonably correlates with the literature data. But, cV values showed a similar trend only up to 770 K; at higher temperatures, the values obtained previously by other authors are considerably smaller. As the diffusion of copper atoms was taken into account in our calculations, the comparison suggests that small cV values obtained at T > 770 K previously are connected mainly with large thermal expansion of Cu2Se in this temperature range. © 2016 The Minerals, Metals & Materials Society


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Osvenskii V.B.,GIREDMET Ltd. | Pshenay-Severin D.A.,RAS Ioffe Physical - Technical Institute
Materials Today: Proceedings | Year: 2015

In the present work the influence of porosity on the thermoelectric figure of merit of bulk nanostructured n-SiGe solid solution is considered. The model for transport coefficients in the solid solution was based on that by Minnich et al. [1]. It took into account acoustic, nonpolar optical phonon scattering, alloy and ionized impurity scattering of electrons. The phonon transport was described taking into account both Umklapp and normal phonon-phonon scattering, defect scattering and scattering on electrons. Grain boundary scattering was taken into account using constant mean free path equal to the grain size both for electrons and phonons. The estimations based on this model correlate quite well with the experimental data and calculations in Ref. [1] and showed the increase of the thermoelectric efficiency by 25-35% at the grain size about 20 nm. The impact of porosity was estimated using the model by Lee et al. [2] with the following differences. The electron scattering in Born approximation was replaced by more accurate phase shift method. The scattering of phonons on pores was considered. Finally, corrections were added to the effective medium theory used in estimations. The results of estimations showed the possibility to improve the thermoelectric efficiency at the pore size less than several tens of nanometers. The increase of the thermoelectric efficiency due to porosity is comparable with the increase due to grain boundary scattering at the volume pore fraction greater than 10%. © 2015 Elsevier Ltd.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Osvenskii V.B.,GIREDMET Ltd. | Parkhomenko Y.N.,GIREDMET Ltd. | Pshenay-Severin D.A.,RAS Ioffe Physical - Technical Institute
Journal of Solid State Chemistry | Year: 2012

One of the possible ways to increase the thermoelectric figure of merit is the use of bulk nanostructured materials fabricated by melt spinning with subsequent hot pressing or spark plasma sintering. Among a variety of nanostructure types these materials contain regions of initial solid solution with nanometer sized inclusions of different compositions. In the present work the scattering of holes and phonons on nanoinclusions in such p-Bi xSb 1-xTe 3 based materials is considered. The change of transport coefficients due to this scattering mechanism is theoretically estimated. The estimations showed that the reduction of lattice thermal conductivity (about 12-13%) for nanoinclusions of Bi 2Te 3-Sb 2Te 3 solid solution with different compositions is much greater than the change in power factor. Therefore the corresponding increase of the thermoelectric figure of merit for this case is determined mainly by phonon scattering. Also it is shown that the results of estimations depend on phonon spectrum approximation, e.g. in the case of sine-shaped instead of linear phonon spectrum the estimations give two times higher thermal conductivity reduction. © 2012 Elsevier Inc.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Drabkin I.A.,GIREDMET Ltd. | Karatayev V.V.,GIREDMET Ltd. | Osvenskii V.B.,GIREDMET Ltd. | And 8 more authors.
Journal of Electronic Materials | Year: 2013

Ball milling with subsequent spark plasma sintering (SPS) was used to fabricate bulk nanothermoelectrics based on Bi x Sb2-x Te3. The SPS technique enables reduced size of grains in comparison with the hot-pressing method. The electrical and thermal conductivities, Seebeck coefficient, and thermoelectric figure of merit as functions of temperature and alloy composition were measured for different sintering temperatures. The greatest value of the figure of merit ZT = 1.25 was reached at the temperature of 90°C to 100°C in Bi0.4Sb1.6Te3 for sintering temperature of 450°C to 500°C. The volume and quantitative distributions of size of coherent dispersion areas (CDA) were calculated for different sintering temperatures. The phonon thermal conductivity of nanostructured Bi x Sb2-x Te3 was investigated theoretically taking into account phonon scattering on grain boundaries and nanoprecipitates. © 2013 TMS.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Drabkin I.A.,GIREDMET Ltd. | Karatayev V.V.,GIREDMET Ltd. | Osvenskii V.B.,GIREDMET Ltd. | And 3 more authors.
Journal of Electronic Materials | Year: 2014

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. © 2014 TMS.


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Osvenskii V.B.,GIREDMET Ltd. | Parkhomenko Y.N.,GIREDMET Ltd. | Pshenay-Severin .A.,RAS Ioffe Physical - Technical Institute | Sorokin A.I.,GIREDMET Ltd.
Journal of Electronic Materials | Year: 2015

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. © 2015 The Minerals, Metals & Materials Society


Bulat L.P.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Novotelnova A.V.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Asach A.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | Tukmakova A.S.,Saint Petersburg State University of Information Technologies, Mechanics and Optics | And 4 more authors.
Journal of Electronic Materials | Year: 2016

Spark plasma sintering (SPS) enables preparation of bulk homogeneous nanothermoelectrics with improved figure of merit, offering a potential route to production of segmented legs from layerwise powder in one sintering cycle. However, it is rather difficult to create appropriate thermal, electric, and pressure conditions to obtain optimal sample parameters. The sintering process of an example segmented leg with Bi2Te3, PbTe, and Mg2(Si0.4Sn0.6)0.993Sb0.007 parts using an SPS-511S installation has been simulated. The temperature distribution in the volume of a segmented sample with the stated composition has been obtained as a function of its geometry, as parameterized by the diameter-to-height ratio. The optimal ratio values, correlating with greater temperature difference along the sample length, have been determined. The results can be used to formulate recommendations for achieving better SPS processing conditions for fabrication of segmented thermoelectric legs. These legs have good performance due to the significant temperature difference in the leg during sintering. © 2016 The Minerals, Metals & Materials Society

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