Journal of Alloys and Compounds xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Nonmonotonic change in the structural grain size of the Bi0.4Sb1.6Te3

thermoelectric material synthesised by spark plasma sintering

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.04.087

⇑ Corresponding author. Tel.: +7 9166471954.E-mail address: ntabachkova@gmail.com (N.Yu. Tabachkova).

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2013.04.087

V.B. Osvenskiy a, V.P. Panchenko a, Yu.N. Parkhomenko a, A.I. Sorokin a, D.I. Bogomolov b, V.T. Bublik b,N.Yu. Tabachkova b,⇑a State Scientific-Research and Design Institute of Rare-Metal Industry ‘‘Giredmet’’, B.Tolmachevsky lane, Building 5-1, Moscow 119017, Russiab National University of Science and Technology ‘‘MISIS’’, Leninsky pr. 4, Moscow 119049, Russia

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Nanostructural thermoelectric materialsSpark plasma sinteringTransmission electron microscopy

a b s t r a c t

A nonmonotonic variation of crystallite size in thermoelectric materials depending on spark plasma sin-tering (SPS) temperature was found in this work by using X-ray diffraction. The crystallite size growswith increasing temperature from 250 �C to 400 �C and decreases at temperatures from 400 �C to500 �C. The transmission electron microscopy results suggest that the decrease in the average crystallitesize is associated with an intense formation of fine grains at an SPS temperature of 450 �C as a result ofrepeated recrystallisation. New grains in the structure precipitate faster than the old grains grow. Thetotal density of the defects including the twins decreases. The initiation of new repeated recrystallisationcentres occurs on the grain boundaries, on the dislocation defects and in the grain bulk, most likely on thesubgrains. The pattern that has been discovered solves the problem of nano state preservation at elevatedSPS temperatures.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction Thus, researchers have turned their attention to the synthe-

Semiconductor thermoelectric materials are widely used ingenerators, refrigerators, air conditioners and other appliances.The main energy parameters for the thermoelectric devices, i.e.,the temperature difference at the radiators, the refrigeration effi-ciency of the thermostats, air conditioners and refrigerators andthe efficiency of thermal electric generators are determined bythe thermoelectric efficiency. Attempts to increase the efficiencyof the thermoelectric materials have been made for many years,and an increase in the efficiency from ZT = 1 by 15–20% wouldyield a large economic effect. However, it is now clear that conven-tional methods are not helpful in solving this problem. One of thenew fields currently drawing much attention is the synthesis ofbulk nanostructural materials [1].

The first proof of a large increase in the efficiency of the nano-structural materials was obtained for small-sized nanostructureswith quantum wells and quantum dots. However, the best experi-mental results proved to be poorly reproducible, and diffusion pro-cesses could impair the durability of such structures. Furthermore,the fabrication of small-sized nanostructural thermoelectric materi-als with quantum wells and quantum dots is a very complex andexpensive process that cannot be considered as a potential basisfor a commercial technology of highly efficient thermoelectric mate-rials [2].

sis of bulk nanostructural thermoelectric materials [3–5]. Agood thermoelectric figure of merit (ZT > 1) for (Bi,Sb)2Te3 bulknanostructural thermoelectric materials was achieved in previ-ous studies [6–8], with a common feature being the use ofnanopowders ground in planetary ball mills followed by hightemperature, high pressure compaction. Our approach to thesynthesis of high efficiency thermoelectric materials is far moreeconomically viable and shows good promise for commercialapplication. Studies on the synthesis of bulk nanostructuralthermoelectric materials have been reviewed elsewhere [9].In our work, we used similar technical methods for synthesis-ing (Bi,Sb)2Te3 bulk nanostructural thermoelectric materials[10,11].

A common viewpoint in the current literature is that an in-crease in the thermoelectric figure of merit of nanostructuralthermoelectric materials is caused by a decrease in the latticeheat conductivity due to higher phonon scattering at grainboundaries and in-grain structural defects [9]. An earlier study[12] made progress with a theoretical analysis of the effectof phonon scattering on the lattice heat conductivity in aBi0.4Sb1.6Te3 base nanostructural bulk material, the results ofwhich were in a good agreement with the experimental heatconductivity vs. grain size curve.

In the present work, we synthesised the Bi0.4Sb1.6Te3 base bulknanostructural thermoelectric material using the spark plasma sin-tering (SPS) method for nanopowders produced in a high-energyball mill.

Fig. 2. X-ray diffraction pattern of bulk thermoelectric materials after SPS.

2 V.B. Osvenskiy et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx

2. Specimens and methods

The nanostrucural specimens were obtained from synthetic nanopowders ofpreset composition. The raw materials were Bi (99.999), Sb (99.999) and Te(99.999). The specimens were synthesised by direct melting of the components insoldered quartz ampoules. The ingots were mechanically activated in a protectiveatmosphere in a high-energy planetary ball mill AGO-2U (Activator Co., Russia) orPM400 (Retsch, Germany), with a milling time of 2 h. Compact nanostructural spec-imens were obtained using the SPS method in an SPS-511S instrument (SPS Syntex,Japan). The specimens sintered in the graphite mould were 3–10 mm in thicknessand 20 mm in diameter. The specimens were sintered at temperatures ranging from250 �C to 550 �C, with the current density varying from 100 to 225 A/cm2, at a pres-sure of 50 MPa for 5 min, current pulse duration 3 ms. Thermoelectric propertieswere measured using the Harman method [13]. The thermoelectric material struc-ture was studied using X-ray diffraction (Bruker D8 Discover, Germany) and trans-mission electron microscopy (JEM 2100, Japan) with an accelerating voltage of200 kV. The crystallite size was determined from the broadening of the diffractionlines by approximating the profile shape with Voigt functions (Cauchy and Gaussfunction convolution) for the instrumental profile (reference) and for the profiledistorted by the small crystallite size and microstrains. The reference materialwas annealed powder from the Bi0.4Sb1.6Te3 solid solution. Substructure parameterswere studied using profiles for two different reflection orders that allow separatingthe effects of microstrains and small crystallite sizes [14].

3. Results

3.1. Bi0.4Sb1.6Te3 structural study

X-ray diffraction and transmission electron microscopy (TEM)studies showed that the powder particles retained the Bi0.4Sb1.6Te3

composition. The average crystallite size in the powder particlesafter mechanical milling was approximately 10 nm as determinedfrom X-ray line broadening. Transmission electron microscopyshowed that the powder is agglomerated, with the agglomeratesconsisting of 5–15 nm size particles. Fig. 1a and b shows the lightfield and dark field images of an agglomerate consisting of fine-grained particles. Fig. 1c shows a high resolution image of a singleparticle in an agglomerate. Thus, the crystallite size after themechanical milling was equal to the grain size, and each powderparticle consisted of a single domain.

After SPS, we also studied the structure of the bulk thermoelec-tric material using X-ray diffraction and transmission electronmicroscopy. In an earlier study [10], we showed a correlation be-tween crystallite size and the properties of the material. If grainsizes are within several decades of nanometres, crystallite size isthe same as the grain size, while larger grains include several crys-tallites separated by subboundaries.

3.2. Structural study of bulk thermoelectric materials after SPS

An X-ray diffraction study showed that the specimens after SPSare single-phase at different temperatures: the X-ray diffraction

Fig. 1. TEM images discrete agglomerates of powder (a) bright field, (b) a dar

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013)

patterns contained only the lines corresponding to the Bi0.4Sb1.6Te3

ternary solid solution (Fig. 2). We determined the average crystal-lite size from diffraction line broadening depending on SPS temper-ature (Table 1).

Table 1 suggests that crystallite size is a nonmonotonic functionof SPS temperature. At temperatures below 400 �C, the crystallitesize grows, while at temperatures above 400 �C, the crystallite sizedecreases, with the minimum crystallite size occurring at 500 �C.At 550 �C, the crystallite size shows another abrupt period ofgrowth, and the former small size of the crystallite no longer con-tributes to diffraction line broadening. To understand the mecha-nism of these structural transformations, we studied specimenssintered at different temperatures using transmission electronmicroscopy.

Fig. 3 shows thermoelectric material structures sintered at dif-ferent temperatures (250–550 �C). The fact that the as-sinteredgrain size for 250 �C (Fig. 3a) is one order of magnitude greaterthan the initial grain size suggests that collective recrystallisationoccurred under low defects mobility. Consequently, the grainsgrown from the initial powder (approximately 10 nm) have inter-nal dislocations. The grain boundaries are generally straight. Dislo-cations and twins occur in some grains, and the grain size varies.No structural pores are observed. The crystallite size is smallerthan the grain size, and grain fragmentation by dislocations isclearly observed. The structure contains discrete nanosized parti-cles. Fig. 4 shows an example of such a particle. The atomic planeorientation in the particle differs from the atomic plane orientationof the parent matrix. Energy dispersion analysis suggests that thenanoparticle solid solution has the same composition as the parentmatrix (accurate to within the composition measurement error),i.e., the particle is not an excess component precipitate. Aftersintering at 300 �C, the grain size grows further (Fig. 3b). A large

k field, (c) high-resolution images of a single particle in the agglomerate.

, http://dx.doi.org/10.1016/j.jallcom.2013.04.087

Table 1Changes fine structure and values of the thermoelectric properties depending on SPS temperature for Bi0.4Sb1.6Te3 solid solution.

SPS temperature(�C)

Average crystallite size (nm) Electrical conductivity(X�1 cm�1)

Seebeck coefficient(lV/K)

Thermal conductivity(W/mK)

ZT

250 120 355 250 0.95 0.70300 150 435 240 1.0 0.75350 180 485 235 1.0 0.80400 Crystallite size does not contribute to line broadening 680 220 1.1 0.9450 120 770 217 1.07 1.0500 70 785 216 1.03 1.05550 Crystallite size does not contribute to line broadening 680 220 1.02 0.95

Fig. 3. TEM images of structure thermoelectric material obtained at the different SPS temperatures (a) 250 �C, (b) 300 �C, (c) 400 �C, (d) 450 �C, (e) 500 �C and (f) 550 �C.

V.B. Osvenskiy et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx 3

number of equilibrium (120�) junctions appear. However, disloca-tions are still clearly visible in some grains. Discrete nanoparticlesstill occur, but their number is not greater than the number of dis-crete nanoparticles that are obtained from 250 �C sintering. Thesediscrete nanoparticles are possibly the grains that have the samesize as the initial powder and that have not grown during sintering.After 350 �C, sintering the thermoelectric material structure doesnot change significantly compared to the 300 �C sintering. Thegrain size varies, and the structure contains a large number oftwins. There are still discrete nanosized grains, and the numberof these particles does not grow compared to that at the lowerSPS temperatures. After sintering at 400 �C, the grain size growsslightly but becomes more homogeneous. A large number of twinspersists. The inner structure of the grains becomes more perfect, asindicated both by the TEM results (Fig. 3c) and by an abrupt in-crease in the crystallite size. The grain junctions are close, beingin equilibrium. Almost no nanoparticles are observed: their num-ber is far smaller than at lower SPS temperatures. This dependenceof the number of nanoparticles on sintering temperature suggests

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013)

that these nanoparticles are inherited from the initial nanopowder.After sintering at 450 �C, the TEM patterns change dramatically(Fig. 3d). The structure contains a large number of small (nano-sized) grains located inside the grains and on the grain boundaries.However, the total volume of these new grains is not large. Thecrystallite size decreases. Fig. 5 shows a diffraction pattern of thearea containing fine grains inside a large initial grain. All of thereflections in the diffraction pattern correspond to the latticeparameter of the initial solid solution of Bi0.4Sb1.6Te3. Fig. 6a andb shows direct resolution images of nanosized grains. As the com-position of the nanosized grains does not differ from the specimenbulk, one can attribute those grains to repeated recrystallisation.After sintering at 500 �C, the structure of the material contains alarge number of new, well-faceted grains that have grown further(Fig. 3e). The total volume of these grains grows. Fig. 7 shows dis-crete grain within grain particles that are grown to occupy the en-tire grain volume. Sintering at 500 �C produces pores, suggestingthat the repeated recrystallisation is interrelated with point defecttransformations. Point defect coagulation leads to pore formation.

, http://dx.doi.org/10.1016/j.jallcom.2013.04.087

Fig. 4. High-resolution image of discrete nanoparticles in material structureobtained at the temperatures 250 �C.

Fig. 5. Diffraction pattern of the area containing fine grains inside a large initialgrain.

4 V.B. Osvenskiy et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx

This structure is characterised by a far smaller crystallite size closeto the size of the new grains. Thus, the structural changes observedby TEM seem to account for the nonmonotonic dependence of thecrystallite size on SPS temperature by the superimposition of thegrain growth during the collective recrystallisation or the repeatedrecrystallisation that occurs intensely at SPS temperatures above450 �C. After sintering at 550 �C, the structure corresponds to a la-ter stage of repeated recrystallisation (Fig. 3f) when the formerlysmall new grains grow to occupy the entire volume and form anew structure consisting of relatively large grains. By analogy withthe previous SPS temperature, the specimen obtained from sinter-ing at 550 �C contains a large number of pores.

Comparison of the values of the thermoelectric figure of merit(Table 1) and data on the structure of materials obtained at differ-ent SPS temperatures show that the maximum value of the ther-moelectric figure of merit is observed at SPS temperatures in the

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013)

range of 450–500 �C, where the numbers of ultrafine particles areformed by repeated recrystallisation.

The main questions thus include the nature of the new grainsand the cause of their nucleation. Unlike collective recrystallisa-tion, the motive force of which is a decrease in the length of theboundaries, the cause of the formation of small grains in the bulkof large ones is most likely a decrease in their volume energy.The similarity of the elemental compositions for the new and oldgrains suggests that the cause of the repeated recrystallisation isa decrease in the energy of the intrinsic defects in the grain bulk.In the absence of dislocations, these intrinsic defects can be intrin-sic point defects. One can then assume that the new precipitatinggrains have a lower concentration of intrinsic point defects com-pared to that of the old grains.

The high concentration of intrinsic point defects in the bulk ofthe material can be due to the nucleation of these intrinsic pointdefects in the initial powder particles as a result of milling in ahigh-energy ball mill. The repeated recrystallisation then requiresa nonuniform distribution of intrinsic point defects on the nanosizescale. These local inhomogeneities can have different origins in theSPS specimens. First, the nanosize particles of the powder that dif-fer in size may initially have different concentrations of intrinsicpoint defects, which will be inherited during their consolidation.Second, local inhomogeneities of pulsed current density in thespecimen during sintering in a spark plasma discharge will inevita-bly lead to a nonuniform temperature distribution. In this case, theintrinsic point defects will be annealed at different temperaturesduring sintering, and as a result, there will be local regions withdifferent concentrations of intrinsic point defects. During repeatedrecrystallisation, the intrinsic point defects will redistribute in thegrain bulk. As the process has a diffusive nature, the recrystallisa-tion occurs intensely at high sintering temperatures due to the in-creased mobility of the intrinsic point defects. Excessive intrinsicpoint defects may either move to the grain boundaries or formagglomerations in the bulk of the grains. The pores that are visibleunder the transmission electron microscope in the specimens sin-tered at SPS temperatures of 450–550 �C suggest that the intrinsicpoint defects have a vacancy nature. It is therefore most probablethat all of the abovementioned structural changes are due to aredistribution of the intrinsic point defects during specimen sinter-ing, leading to a decrease in the average concentration of theintrinsic point defects in the nanostructured material. However,one cannot ignore another possible cause of the repeated recrystal-lisation in a heavily anisotropic material like the Bi2Te3—Sb2Te3 so-lid solution. In this material, the boundary energy depends stronglyon the boundary orientation. The grain boundary that is closer tothe cleavage plane will have the lower energy. Because of the heav-ily anisotropic growth of discrete grains, the grain boundaries thatform during recrystallisation may have such a configuration forwhich the tendency to reduce their total energy will cause onegrain to grow at the expense of another in the lower growth ratedirection. With an increase in the sintering temperature, therecrystallisation may occur not only due to an increase in the graingrowth rate in the lower growth rate direction but also due to theformation of differently oriented precipitates having a lowerboundary energy.

We will now analyse the possible relationship between thethermoelectric properties of materials obtained by SPS at differenttemperatures and the respective structural changes. The increasein the electrical conductivity with increasing temperature can beaccounted for by two factors. First, at a low SPS temperature (atleast below 350 �C), the specimens are insufficiently sintered,which is suggested by the low relative density that increases asSPS temperature increases to 400 �C (Fig. 8). With an increase inSPS temperature to 350–400 �C, the percentage of high electricalresistivity grain boundaries decreases. Second, the annealing of

, http://dx.doi.org/10.1016/j.jallcom.2013.04.087

Fig. 6. High-resolution images of discrete nanoparticles arising due to repeated recrystallization at SPS temperature 450 �C: (a) The particle inside the bulk of grain, (b) theparticle at the grain boundary.

Fig. 7. TEM image of discrete grain with in-grain particles grown to occupy theentire grain volume.

Fig. 8. Specimen density vs. SPS temperature.

V.B. Osvenskiy et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx 5

donor intrinsic point defects that form during the milling of thematerial also leads to an increase in the electrical conductivity.

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013)

The defects are annealed most intensely as temperatures increaseto above 350 �C. At sintering temperatures below 400 �C, the heatconductivity of the material increases due to an increase in thecontribution of the electrons to the heat conductivity. However,at sintering temperatures above 400 �C where the repeated recrys-tallisation occurs, the heat conductivity decreases. This decrease inthe heat conductivity can only be due to a decrease in the lattice(background) heat conductivity. The new nanosized grain nucle-ations that are most likely centred at these temperatures are asso-ciations of intrinsic point defects, and phonon scattering at theseassociations can be the cause of the decrease in the lattice heatconductivity.

4. Conclusion

A nonmonotonic dependence of the crystallite size on SPS tem-perature has been revealed. Transmission electron microscopy datasuggest the following mechanism of structural changes. Sinteringat 250 �C leads to collective recrystallisation under low defectsmobility. After sintering at 300–400 �C, the grains continue to growand their structure exhibits large twins. As the defect mobilitygrows, a new structural change – repeated recrystallisation –occurs (450 �C). New grains in the structure precipitate faster thanthe old grains grow. The total density of the defects including thetwins decreases. The initiation of new repeated recrystallisationcentres occurs on the grain boundaries, on the dislocation defectsand in the grain bulk, most likely on the subgrains. This regularityallows synthesising structures with different grain sizes at SPStemperatures of 450–500 �C, depending on the degree of repeatedrecrystallisation.

Acknowledgements

We are indebted to the Materials Science and Metallurgy JointUse Center, Moscow Institute of Steel and Alloys (National Univer-sity of Science and Technology), for the use of their instruments.This work was supported by the RF Ministry of Education and Sci-ence (State Contract No. 16.552.11.7073)

References

[1] A.V. Dmitriev, I.P. Zvyagin, UFN 180 (2010) 821–838.[2] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001)

597–602.[3] G. Chen, M.S. Dresselhaus, J.P. Fleurial, T. Caillat, Int. Mat. Rev. 48 (2003) 45–

48.

, http://dx.doi.org/10.1016/j.jallcom.2013.04.087

6 V.B. Osvenskiy et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx

[4] M.S. Dresselhaus, G. Chen, M.Y. Tang, Y. Ronggui, L. Hohyun, D. Wang, R.Zhifeng, J.-P. Fleurial, P. Gogna, Adv. Mater. 19 (2007) 1043–1053.

[5] L.P. Bulat, I.A. Drabkin, V.B. Osvensky, G.I. Pivovarov, J. Thermoelectricity 4(2008) 27–33.

[6] B. Poudel, Q. Hao, Y. Ma, X.Y. Lan, A. Minnich, X. Yan, D.Z. Wang, A. Muto, D.Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z.F. Ren, Science 320(2008) 634–637.

[7] Y. Ma, Q. Hao, Y.C. Lan, X.W. Wang, G.H. Zhu, D.Z. Wang, R.W. Gould, D.C.Cuff, M.Y. Tang, M.S. Dresselhaus, G. Chen, Z.F. Ren, Nano Lett. 8 (2008)4670–4673.

[8] Y. Lan, B. Poudel, Y. Ma, M.S. Dresselhaus, G. Chen, Z. Ren, Nano Lett. 2009 (9)(2009) 1419–1422.

[9] Y. Lan, A.J. Minnich, G. Chen, Z. Ren, Adv. Funct. Mater. 20 (2010) 357–376.

Please cite this article in press as: V.B. Osvenskiy et al., J. Alloys Comp. (2013)

[10] L.P. Bulat, V.T. Bublik, I.A. Drabkin, V.V. Karatayev, V.B. Osvensky, G.I.Pivovarov, D.A. Pshenai-Severin, E.V. Tatyanin, N.Yu. Tabachkova, J.Thermoelectricity 3 (2009) 67–72.

[11] L.P. Bulat, V.T. Bublik, I.A. Drabkin, V.V. Karatayev, V.B. Osvensky, Yu.N.Parkhomenko, G.I. Pivovarov, D.A. Pshenai-Severin, N.Yu. Tabachkova, J.Electron. Mater. 39 (2010) 1650–1653.

[12] L.P. Bulat, I.A. Drabkin, V.V. Karataev, V.B. Osvenskiy, D.A. Pshenai-Severin,Phys. Solid State 9 (2010) 1836–1841.

[13] A. Abrutin, I. Drabkin, V. Osvenskiy, Corrections used when measuringmaterials thermoelectric properties by Harman method, in: Proceedings ofEuropean Conference on Thermoelectrics, second ed., Kraków, Poland, 2004,pp. 3–7.

[14] E.V. Shelekhov, T.A. Sviridova, Met. Sci. Heat Treat. 42 (2000) 309–313.

, http://dx.doi.org/10.1016/j.jallcom.2013.04.087