The Effect of Severe Plastic Deformation on Thermoelectric ... · The Effect of Severe Plastic...

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The Eect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides Gerda Rogl 1,2,3 , Michael J. Zehetbauer 4 and Peter F. Rogl 1,3 1 Institute of Materials Chemistry, University of Vienna, Waehringerstr. 42, A-1090 Wien, Austria 2 Institute of Solid State Physics, TU Wien, Wiedner Hauptstr., 8-10, A-1040 Wien, Austria 3 Christian Doppler Laboratory for Thermoelectricity, Wien, Austria 4 Physics of Nanostructured Materials, University of Vienna, Boltzmanngasse 5, A-1090 Wien, Austria Although thermoelectric materials with a gure of merit, ZT, higher than 1.2 have gained considerable interest in electric power generation, little is hitherto known on the inuence of severe plastic deformation on thermoelectric performance. Severe plastic deformation is one of the elegant techniques to furnish ultrane grained microstructure with a high level of point, linear and surface defects. Particularly the phonon scattering on these defects is used as a main route to reduce thermal conductivity in thermoelectric materials and - as a consequence - increasing ZT. The present article provides an overview on the achievements of the various techniques of severe plastic deformation to gain high gures of merit in the thermoelectric materials, commonly used in energy conversion devices, such as skutterudites, clathrates, Heusler phases, and bismuth tellurides. For all skutterudites, high pressure torsion, as one of the major techniques of severe plastic deformation processes, is a great tool to either enhance ZT of hot pressed samples or to directly produce fast and easily high ZT thermoelectric bulks. A still unsurpassed highlight is the enhancement of ZT from 1.6 to almost 2 (Sr 0.09 Ba 0.11 Yb 0.06 Co 4 Sb 12 ) at 825 K. Whilst for thermoelectric clathrates so far little success was reported, high pressure torsion treatment of Heusler and Half-Heusler phases in some cases was able to boost ZT (VTa 0.05 Fe 2 Al 0.95 , rising ZT from 0.22 to 0.3) although the absolute ZT increases are still disappointing. For p- and n-type Half-Heusler alloys (NbFeSb- and TiNiSn-based) at least three temperature cycles are necessary to gain a thermally stable state, indicating that obviously the introduced defects and structure changes are more resistant against heat treatments than in case of skutterudites. Bismuth tellurides of type V 1 (VI) 1 and/or V 2 (VI) 3 (V, VI denote the group elements) have been already deformed by high temperature pressure or high temperature extrusion before the SPD methods were known, for the sake of improving ZT at least with temperatures 300-500 K, at most ghting with the condition to achieve a high electrical conductivity because of the strong anisotropy in these materials. By starting with ball milling followed by high temperature pressing at not too high temperatures, not only the electrical conductivity could be kept large but also the lattice thermal conductivity was diminished such that gures of merit up to ZT = 1.4 at T = 373 K were achieved. This value could be reached by many of the ECAP experiments published so far, although only across the sample long axis because of lattice anisotropy. First applications of HPT did not reach that ZT level as either the conditions of texture could be not fullled, or, above all, the processing rates and/or temperatures were too high. Recent investigations not having involved SPD found the importance of the lattice defectsspecic phonon scattering eciencies, especially that of dislocations, and by introducing them in suciently high densities, enhancements of p-type Bi- Tellurides up to ZT = 1.9 were possible. These ndings recommend the use of SPD methods here, not at least as they have been already applied very successfully by the authors of this review to both p- and n-type Skutterudites increasing the gure of merit up to ZT ³ 2. As concerns mechanical properties, the application of SPD methods signicantly raises the strength while leaving the elastic moduli unchanged unless new phases have been formed. [doi:10.2320/matertrans.MF201941] (Received March 12, 2019; Accepted June 14, 2019; Published August 30, 2019) Keywords: severe plastic deformation, high-pressure torsion, thermoelectrics, skutterudites, Half Heusler, clathrates, tellurides, mechanical properties 1. Introduction Thermoelectric (TE) materials are able to directly convert thermal energy into electrical energy and vice versa. To determine the quality of a TE material, the dimensionless gure of merit ZT = S 2 T/(μ-) is used, with S the Seebeck coecient or thermopower (S ³ ¦V/¦T), T the temperature, μ the electrical resistivity and - = - e + - ph the total thermal conductivity, consisting of the electronic part - e and the phonon part - ph . The electronic part of the thermal conductivity is linked to the electrical resistivity via the Wiedemann-Franz law, - e ³ L 0 T/μ, with the Lorenz number L 0 . Therefore one way to increase ZT is to decrease - ph , which is possible by enhancing the scattering of the heat carrying phonons, on various lattice defects like point defects including vacancies, dislocations, and grain boundaries. Not only a high ZT value at a certain temperature is important, but also a high average ZT, (ZT) a , over a wide temperature range, to maximize the so-called thermo-electric conversion eciency © ¼ T h T c T h ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þðZTÞ a p 1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þðZTÞ a p þ T c T h ð1Þ including the Carnot eciency, where T h and T c are the temperatures on the hot and cold side respectively. Bottom-upmethods like ball-milling (BM) followed by hot-pressing (HP) achieve materials with a grain size of the order of micrometers and (especially with high energy ball- milling, HBM) in the range of 100 nanometers and less. It was found that the smaller the grain size the higher is the thermoelectric performance due to a reduction in the phonon part of the thermal conductivity. 1) Another method to reduce the grain and/or crystallite size and concomitantly to increase the density of lattice defects in general and, thereby, further increase the scattering of heat carrying phonons, is to apply severe plastic deformation (SPD) in its various forms. 2-17) SPD methods produce materials with grains in sub- micrometer or even nanometer range. 18-20) Because of the presence of high hydrostatic pressure, SPD methods provide Materials Transactions, Vol. 60, No. 10 (2019) pp. 2071 to 2085 © 2019 The Japan Institute of Metals and Materials OVERVIEW

Transcript of The Effect of Severe Plastic Deformation on Thermoelectric ... · The Effect of Severe Plastic...

  • The Effect of Severe Plastic Deformation on Thermoelectric Performance ofSkutterudites, Half-Heuslers and Bi-Tellurides

    Gerda Rogl1,2,3, Michael J. Zehetbauer4 and Peter F. Rogl1,3

    1Institute of Materials Chemistry, University of Vienna, Waehringerstr. 42, A-1090 Wien, Austria2Institute of Solid State Physics, TU Wien, Wiedner Hauptstr., 8-10, A-1040 Wien, Austria3Christian Doppler Laboratory for Thermoelectricity, Wien, Austria4Physics of Nanostructured Materials, University of Vienna, Boltzmanngasse 5, A-1090 Wien, Austria

    Although thermoelectric materials with a figure of merit, ZT, higher than 1.2 have gained considerable interest in electric power generation,little is hitherto known on the influence of severe plastic deformation on thermoelectric performance. Severe plastic deformation is one of theelegant techniques to furnish ultrafine grained microstructure with a high level of point, linear and surface defects. Particularly the phononscattering on these defects is used as a main route to reduce thermal conductivity in thermoelectric materials and - as a consequence - increasingZT.

    The present article provides an overview on the achievements of the various techniques of severe plastic deformation to gain high figures ofmerit in the thermoelectric materials, commonly used in energy conversion devices, such as skutterudites, clathrates, Heusler phases, andbismuth tellurides.

    For all skutterudites, high pressure torsion, as one of the major techniques of severe plastic deformation processes, is a great tool to eitherenhance ZT of hot pressed samples or to directly produce fast and easily high ZT thermoelectric bulks. A still unsurpassed highlight is theenhancement of ZT from 1.6 to almost 2 (Sr0.09Ba0.11Yb0.06Co4Sb12) at 825K.

    Whilst for thermoelectric clathrates so far little success was reported, high pressure torsion treatment of Heusler and Half-Heusler phases insome cases was able to boost ZT (VTa0.05Fe2Al0.95, rising ZT from 0.22 to 0.3) although the absolute ZT increases are still disappointing. Forp- and n-type Half-Heusler alloys (NbFeSb- and TiNiSn-based) at least three temperature cycles are necessary to gain a thermally stable state,indicating that obviously the introduced defects and structure changes are more resistant against heat treatments than in case of skutterudites.

    Bismuth tellurides of type V1(VI)1 and/or V2(VI)3 (V, VI denote the group elements) have been already deformed by high temperaturepressure or high temperature extrusion before the SPD methods were known, for the sake of improving ZT at least with temperatures 300500K,at most fighting with the condition to achieve a high electrical conductivity because of the strong anisotropy in these materials. By starting withball milling followed by high temperature pressing at not too high temperatures, not only the electrical conductivity could be kept large but alsothe lattice thermal conductivity was diminished such that figures of merit up to ZT = 1.4 at T = 373K were achieved. This value could bereached by many of the ECAP experiments published so far, although only across the sample long axis because of lattice anisotropy. Firstapplications of HPT did not reach that ZT level as either the conditions of texture could be not fulfilled, or, above all, the processing rates and/ortemperatures were too high. Recent investigations not having involved SPD found the importance of the lattice defects’ specific phononscattering efficiencies, especially that of dislocations, and by introducing them in sufficiently high densities, enhancements of p-type Bi-Tellurides up to ZT = 1.9 were possible. These findings recommend the use of SPD methods here, not at least as they have been already appliedvery successfully by the authors of this review to both p- and n-type Skutterudites increasing the figure of merit up to ZT ³ 2.

    As concerns mechanical properties, the application of SPD methods significantly raises the strength while leaving the elastic moduliunchanged unless new phases have been formed. [doi:10.2320/matertrans.MF201941]

    (Received March 12, 2019; Accepted June 14, 2019; Published August 30, 2019)

    Keywords: severe plastic deformation, high-pressure torsion, thermoelectrics, skutterudites, Half Heusler, clathrates, tellurides, mechanicalproperties

    1. Introduction

    Thermoelectric (TE) materials are able to directly convertthermal energy into electrical energy and vice versa. Todetermine the quality of a TE material, the dimensionlessfigure of merit ZT = S2T/(μ) is used, with S the Seebeckcoefficient or thermopower (S ³ ¦V/¦T), T the temperature,μ the electrical resistivity and = e + ph the total thermalconductivity, consisting of the electronic part e and thephonon part ph. The electronic part of the thermalconductivity is linked to the electrical resistivity via theWiedemann-Franz law, e ³ L0T/μ, with the Lorenz numberL0. Therefore one way to increase ZT is to decrease ph,which is possible by enhancing the scattering of the heatcarrying phonons, on various lattice defects like point defectsincluding vacancies, dislocations, and grain boundaries. Notonly a high ZT value at a certain temperature is important,but also a high average ZT, (ZT)a, over a wide temperaturerange, to maximize the so-called thermo-electric conversionefficiency

    © ¼ Th � TcTh

    ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

    1þ ðZTÞap � 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

    1þ ðZTÞap þ Tc

    Th

    ð1Þ

    including the Carnot efficiency, where Th and Tc are thetemperatures on the hot and cold side respectively.

    “Bottom-up” methods like ball-milling (BM) followed byhot-pressing (HP) achieve materials with a grain size of theorder of micrometers and (especially with high energy ball-milling, HBM) in the range of 100 nanometers and less. Itwas found that the smaller the grain size the higher is thethermoelectric performance due to a reduction in the phononpart of the thermal conductivity.1) Another method to reducethe grain and/or crystallite size and concomitantly to increasethe density of lattice defects in general and, thereby, furtherincrease the scattering of heat carrying phonons, is to applysevere plastic deformation (SPD) in its various forms.217)

    SPD methods produce materials with grains in sub-micrometer or even nanometer range.1820) Because of thepresence of high hydrostatic pressure, SPD methods provide

    Materials Transactions, Vol. 60, No. 10 (2019) pp. 2071 to 2085©2019 The Japan Institute of Metals and Materials OVERVIEW

    https://doi.org/10.2320/matertrans.MF201941

  • extremely large strains, almost without changing the sample’sgeometry. Therefore SPD not only achieves very fine grainswith high and small angle boundaries, but also vacancies,dislocations and other defects.5,9,2125)

    There exist many techniques to sustain plastic strains asdescribed in an overview article by Valiev.18) Of course, theyall have advantages and disadvantages.

    A very effective method is high-pressure torsion(HPT).2629) Using HPT, the sample’s shape remains almostunchanged by the deformation, a finer grain structure isachieved but damage and fracture are suppressed. Thetechnique is essentially based on the use of a Bridgmananvil-type device: the thin sample (disk shaped) is subjectedto a torsional strain under a high pressure between two anvils.Additionally it is possible to process under various temper-atures, using cooling or heating with an inductive coil. Asin some cases it is necessary to prevent the sample fromoxidation, a modified version of the HPT equipment with atight “cage” for inert gas can be applied.

    Using HPT one has to take into account that the processedsamples are not completely homogeneous as the shear strain,£, is increasing from the center of the sample to the rimaccording to £ = (2³nr)/h, where n is the number ofrevolutions, r is the radius and h is the thickness of thesample; however, as Pippan et al.30) demonstrated, even inthe center (r = 0) the strain is not zero.

    For measuring the electrical resistivity as well as theSeebeck coefficient, the equipment requires samples withcuboid shape with the dimensions of at least 8 © 2 © 1mm3.The thermal conductivity is measured perpendicularly to theresistivity and the Seebeck coefficient. As the HPT-processedsamples are discs with 10mm in diameter and about 1mmin thickness, some inhomogeneities of the samples must beaccepted. More details of the measurement techniques can befound in Refs. 5, 6, 9, 15).

    The influence of the processing parameters (number ofrevolutions, applied pressure, temperature) as well as of thestarting conditions (grain size of the powder before HP)7) andvarious annealing processes5) on the structural and physicalproperties have been studied systematically for a p-typeskutterudite (DD0.60Fe3CoSb12, DD stands for didymium)by Rogl et al.10) It was found that processing at roomtemperature produced very brittle samples, almost unservice-able, therefore all further experiments were performed at600670K. Independent of all above-mentioned conditions,after HPT the lattice parameter was enlarged, the relativedensity decreased, the crystallite size became much smaller(up to fourty times), and the dislocation density increasedby about ten times. Independent of the applied pressure(28GPa) and the number of revolutions (15), the HPT-processed skutterudite samples were not completely homoge-neous and more or less strewn with microcracks.10) Theseobservations are in contrast to those of Masuda et al.,17)

    claiming that HPT-processing HP Heusler alloys at roomtemperature with 5GPa and 10 revolutions yielded homoge-neous and crack-free samples.

    Whilst the Seebeck coefficient, independent of thecomposition and processing parameters, in almost all casesis in the range of the materials’ state before HPT, theelectrical resistivity, due to introduced defects such as

    dislocations, vacancies and/or micro cracks, is dependenton the processing parameters and more or less enhanced. Thethermal conductivity, out of the above-mentioned reasons, isdecreased. The net effect usually is positive, i.e. as a result,ZT is higher for all HPT-processed samples.

    For a short introduction into the solid state physics ofthermoelectrics, into the various methods of severe plasticdeformation as well for a comprehensive summary on HPT-processed skutterudites the reader is referred to an earlierreview article by G. Rogl et al.9)

    2. Skutterudites

    Since 2010 the influence of HPT on structural, physical,mechanical and magnetic properties of p- and n-typeskutterudites has been investigated, and the results werepublished by the authors as a review article9) and in variousjournals.38,10,11,1316) It could be shown that after HPT-processing skutterudites do not exhibit any secondary phasesand/or impurities but microcracks and pores, an enhancednumber of dislocations and of other defects as well as smallergrains. These observations were confirmed via transitionelectron microscopy (TEM): as depicted in Figs. 1 and 2,where grain boundaries and dislocations are clearly visible.

    The lattice parameter is larger, the density lower. Duringmeasurement-induced heating or deliberate annealing, thegrains grow, the major part of the cracks fuse to pores ordisappear, the dislocation density becomes smaller, but allthese temperature induced changes do not restore the stateas it has been before HPT. These changes are reflected in the

    Fig. 1 Superimposed energy-filtered TEM image of HP+HPT-processedSr0.07Ba0.07Yb0.07Co4Sb12, showing grain boundaries and dislocationstructure.

    Fig. 2 TEM image of a selected area of CP+HPT-processed DD0.7Fe3-CoSb12, exhibiting a close up of dislocations and grain boundaries.

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2072

  • changes of the physical properties, mainly in the electricalresistivity and thermal conductivity right after HPT-process-ing and during the annealing process. Although the electricalresistivity even after annealing is higher for the HPT sample,the thermal conductivity is lower, resulting in a positivenet effect in respect of ZT, generally with enhancements of1328%.

    As the scheme is the same for all p-type (mainly DD filled,Fe/Co or Fe/Ni substituted) and n-type (multifilled)skutterudites, one of each was exemplarily selected fordemonstration.

    To evaluate the X-ray diffraction data in terms of lineprofile analysis, models for the crystallite size and itsdistribution as well as of the dislocations density wereapplied, using the CMWP-fit software.31,32) For example forp-type, DD0.6Fe3CoSb12, BM, the crystallite size was reducedby means of HPT from 152 nm to 53 nm, and the dislocationdensity of 2.8 © 1013m¹3 was ten times larger.5) Althoughfor the Sb/Sn substituted BM DD0.7Fe2.7Co1.3Sb11.8Sn0.2, thecrystallite size through HPT was reduced from 336 nm to95 nm, it grew back to 256 nm during the annealing process,but the residual strain was about 2.5 times higher afterHPT.16) For n-type Sr0.07Ba0.07Yb0.07Co4Sb12 the HPTinduced structural changes were less spectacular, showinga crystallite size reduction from 36.5 nm to 30 nm, and anenhancement of the dislocation density from 1.4 © 1014m¹2

    to 1.8 © 1014m¹2.5) Multiple filled p- and n-type skutter-udites, (Sr,Ba,DD,Yb)y(Fe1¹xNix)4Sb12, with ZTs ³ 1attracted interest because tuning the band structure and thelocation of the Fermi level adjusted both p- and n-typematerials to exhibit the same thermal expansion coefficientand mechanical properties. After HPT-processing, ZT wasenhanced by 20% for the p- and 38% for the n-typeskutterudite.8)

    For DD0.44Fe2.1Co1.9Sb12 the changes after HPT andannealing were investigated employing not only transitionelectron microscopy but also Raman spectroscopy andtexture measurements.15) Raman spectra of HP, HPT andHPT-annealed DD0.44Fe2.1Co1.9Sb12 showed almost the samepeak position, with a move to the lower energy side inthe higher frequency range, indicating a softening of thevibration modes. The spectrum for the HPT sample exhibitedadditional softening in the high-energy region as well as peakbroadening, denoting that the vibration modes related to theshorter SbSb bonds in the Sb4 rings are more affected thanthose with longer SbSb bonds. After annealing, thesefeatures were not visible anymore and the HPT-annealedskutterudite spectrum is undistinguishable from the HPspectrum. Anbalagan et al.14) gained the same results forRaman spectroscopy investigations of the double-element-substituted unfilled skutterudite Fe0.05Co0.95Sb2.875Te0.125.

    Texture measurements of DD0.44Fe2.1Co1.9Sb12, comparinga HP sample with a HPT-processed (for the latter performedon a plane perpendicular to the HPT pressing direction)did not show any changes in the orientation distribution ofthe crystallites. However, for Fe0.05Co0.95Sb2.875Te0.125Anbalagan et al.14) found changes in the crystallographictexture, which indicated strengthening of the (112), (102)poles and weakening of the 28 (123) pole of the HPT-processed sample.

    The temperature and phase stability of p-type skutteruditesDD0.7Fe3.1Co0.9Sb12, HP and HPT-processed, have beenstudied by means of thermal analysis (TA) and Knudseneffusion mass spectrometry (KEMS).33) The difference inthe mass loss per day at 800K, due to antimony evaporation(into vacuum), of the HP (34%) and the HPT-processedsample (56%) is marginal. For the sample, DD0.7Fe3CoSb12,for which commercial powder (TIAG, Austria) was simplyhot-pressed (labeled as HP), the lowest mass loss (

  • μ(T) but metallic behavior without anomalies. The temper-ature dependent thermal conductivity, (T), as displayed inFig. 4, due to the smaller grains and the defects discussedabove, introduced during HPT, is much lower for the HPTsample and hardly changes after annealing. The latticethermal conductivity in all cases is very low.

    Comparing the p- and n-type skutterudite in Figs. 3 and 4,one can also see for the HP samples that the lower μ(T), thehigher is (T). After HPT and after annealing, the relativeincrease in μ(T) is much higher for Sr0.09Ba0.11Yb0.05Co4Sb12than for DD0.6Fe3CoSb12 and this behavior is reflected in(T), with a much bigger downsizing for Sr0.09Ba0.11Yb0.05-Co4Sb12 than for DD0.6Fe3CoSb12. The illustration (Fig. 5)defines the temperature dependent Seebeck coefficient, S(T),which is positive for DD0.6Fe3CoSb12, indicating holes asmain carriers, but is negative for Sr0.09Ba0.11Yb0.05Co4Sb12, aselectrons are the main carriers. Figure 5 informs that neitherHPT-processing nor annealing for the p- as well as for then-type samples has an influence on S(T), as all S(T) curvesare, within the error bar, alike. With a more or less completely

    unchanged Seebeck coefficient after HPT and annealing, buta higher electrical resistivity in both cases, the power factor,pf = S2T/μ becomes lower, therefore both HP samplesexhibit the highest values (Fig. 6).

    Figure 7 summarizes the figure of merit, ZT. The increaseof ZT is higher for DD0.6Fe3CoSb12 by 41% for the HPTsample (from ZT = 1.1 to ZT = 1.6 at 823K) and 30%(to ZT = 1.5) than for Sr0.09Ba0.11Yb0.05Co4Sb12 with 19%for the HPT sample (from ZT = 1.6 to ZT = 1.9 at 838K)and 12% (to ZT = 1.8). These ZT enhancements can varyindividually: dependent on the starting material and thecomposition of the skutterudite, so far for all HPskutterudites, ZT was higher after HPT-processing. Thethermo-electric conversion efficiency increased for DD0.6Fe3-CoSb12 from © = 13.9% (HP) to © = 14.6% (HP+HPT), forSr0.09Ba0.11Yb0.05Co4Sb12 from © = 15.0% (HP) to © =16.0% (HP+HPT).

    Enhancing ZT via HPT-processing does not only work forfilled p- and n-type skutterudites, but also for unfilledskutterudites and filled skutterudites substituted at the Sb-site.

    Mallik et al.11) reported for Fe0.2Co3.8Sb11.5Te0.5 that ZTincreased from ZT = 1.06 to ZT = 1.3, which equals anenhancement of 23%.

    Fig. 4 Temperature dependent thermal conductivity, (T), (big symbols),and lattice thermal conductivity, ph(T), (small symbols) of HP, HP+HPTand annealed skutterudites, DD0.6Fe3CoSb12 and Sr0.09Ba0.11Yb0.05-Co4Sb12.

    Fig. 5 Temperature dependent Seebeck coefficient, S(T), of HP, HP+HPTand annealed skutterudites, DD0.6Fe3CoSb12 (right scale) andSr0.09Ba0.11Yb0.05Co4Sb12 (left scale).

    Fig. 6 Temperature dependent power factor, pf(T), of HP, HP+HPT andannealed skutterudites, DD0.6Fe3CoSb12 and Sr0.09Ba0.11Yb0.05Co4Sb12.

    Fig. 7 Temperature dependent ZT of HP, HP+HPT and annealedskutterudites, DD0.6Fe3CoSb12 and Sr0.09Ba0.11Yb0.05Co4Sb12.

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2074

  • Rogl et al.16) HPT-processed Sb-substituted DD0.54Fe2.7-Co1.3Sb11.9Ge0.1 and DD0.7Fe2.7Co1.3Sb11.8Sn0.2. Whilst ZT ofDD0.54Fe2.7Co1.3Sb11.9Ge0.1 after HPT was not much higher,for DD0.7Fe2.7Co1.3Sb11.8Sn0.2 ZT = 1.3 at 780K of the HPsample could be topped with ZT = 1.45 at 773813K of theHPT-processed sample. Both ZT values, for a processed anda HP p-type skutterudite are, to our knowledge, so far thehighest ZTs. The values of © ³ 14.6% before and after HPTare about the same.

    Recently, SPD via HPT at elevated temperatures and inprotective gas atmosphere was used to directly consolidateand plastically deform commercial p-type (DD0.7Fe3-CoSb12)34) and n-type ((Mm,Sm)yCo4Sb12)35) skutteruditepowder from TIAG, Austria, into a dense thermoelectricsolid. Applying this method, not only time and energyconsuming steps like ball milling and especially hot pressingcould be eliminated, but in addition this method is muchfaster as it takes about 15 to 25 minutes, dependent on thenumber of revolutions during HPT, instead of several hours(about 10). For both, the p- and n-type skutterudite, thestructural, physical and mechanical properties (see chapter 6)of the cold pressed (CP) and HPT-processed sample (referredto as CP+HPT 1r or CP+HPT 5r, dependent on thenumber of revolutions) were compared with those of areference sample (referred to as HP), which was traditionallyconsolidated in the hot press. It should be noted that coldcompacting (CP) of the powder is only necessary in order toeasily place the right amount of powder between the anvils ofthe HPT equipment.

    Whilst the calculated lattice parameters (Fig. 8, top) forboth, p- and n-type, are lower for the homogeneous HPsample than for the starting powder, they are higher afterCP+HPT. After measurement-induced annealing they“shrink” to the size of those of the starting powders.Interestingly, the lattice parameters for the n-type skutter-udite, HPT-processed by five revolutions are slightly lowerthan those processed with one revolution.

    Figure 8 (bottom) compares the measured relative densityin % with the calculated X-ray density (dX = (ZM)/(NV),where M is the molar mass, Z is the number of formula unitsper cell, N is Loschmidt’s number, and V is the volume of theunit cell). For the p- as well as for the n-type skutterudite theCP+HPT sample has a much lower density than the HPsample. The reason is that defects like vacancies and small

    cracks are introduced during HPT-processing. After anneal-ing the density further decreases for DD0.7Fe3CoSb12,whereas for (Mm,Sm)yCo4Sb12 a slight increase occurs.

    The crystallite size (Fig. 9) was evaluated from the X-raypattern via line profile analysis. The crystallite size of bothHP samples (p-type: 76 nm, n-type: 78 nm) is practically thesame and much smaller after HPT-processing (44 nm and45 nm, respectively, for one revolution, 39 nm for fiverevolutions). During measurement-induced annealing thegrains grow, but remain still smaller in comparison to theHP samples. The results of the profile analysis for thedislocation density is depicted in Fig. 9. Especially for thesample processed with five revolutions the dislocationdensity is ten times higher than for the HP sample, forthe other two samples about four times (3.6 © 1014m¹2 and3.3 © 1014m¹2). During the physical properties’ measure-ments many defects anneal out, but not completely so thatfinally for the annealed samples the dislocation density is stillabout three times higher in comparison to that of the HPsample.

    The temperature dependent electrical resistivity μ(T) forboth, p- and n-type CP+HPT (see Fig. 10), is much higherthan for the HP counterpart, descending from various defectsand cracks. For the p-type, after a maximum, μ(T) decreases,undergoes a low minimum and slightly increases.

    Fig. 8 Lattice parameter, a (top) and relative density, drel (bottom) ofDD0.7Fe3CoSb12 (left column) and (Mm,Sm)yCo4Sb12 (right column).

    Fig. 9 Crystallite size, cs (top) and dislocation density, dd (bottom) ofDD0.7Fe3CoSb12 (left column) and (Mm,Sm)yCo4Sb12 (right column).

    Fig. 10 Electrical resistivity, μ, of DD0.7Fe3CoSb12 and (Mm,Sm)yCo4Sb12prepared via HP and CP+HPT vs. temperature, T.

    The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides 2075

  • This behavior was backed by in situ synchrotron measure-ments for DD0.7Fe3CoSb12, which were performed in atemperature range of 300 to 825K in steps of 50K, in orderto evaluate the changes in grain size and dislocation densityof the CP+HPT samples before, during and after anneal-ing.34) With increasing temperature the size of the crystallites(³44 nm) decreases slightly (³38 nm) but above about 500Kit increases (³60 nm), but even at 800K the crystallite size issmaller than that of the HP sample (³78 nm). It wasconfirmed that at room temperature the grains at the rimare smaller than in the center, and that this proportion isindependent of the heat treatments and inversely proportionalto the dislocation density. Below about 600K the dislocationdensity (3.5 © 10144.2 © 1014m¹2) is not affected by theheat treatment but is decreasing almost linearly above 600K.Beyond that temperature, it is then bisected but still two timeslarger than the dislocation density of the HP sample. Furtherheat treatments do neither affect the crystallite size nor thedislocation density. These changes are in parallel with thechanges in the electrical resistivity for the first measurementwith increasing temperature.

    The thermally stable μ(T)-curve is higher than for the HPsample but has the same curvature. For the n-type sample forthe first measurement with increasing temperature, about thesame behavior occurs, but for decreasing temperature, μ(T)increases, instead of decreasing, especially for the sampleprocessed with five revolutions. This performance indicatesa change from metallic to semiconducting behavior duringmeasurement-induced annealing, which could be explainedvia the change of lattice distortion and its influence on theband gap.35) All further resistivity measurements confirmedsemiconducting behavior.

    Thermal conductivity (Fig. 11) for all three CP+HPTsamples is much lower than for the HP reference sample.After annealing, the thermal conductivity does not changemuch, but whilst the thermal conductivity becomes slightlylower for the annealed CP+HPT n-type sample it becomesslightly higher for the p-type. This feature is not surprising,as thermal conductivity acts reciprocal to the electricalresistivity. Lattice thermal conductivities are very low,

    indicating that the lower limit of the possible thermalconductivity reduction is reached.

    The temperature dependent Seebeck coefficient is dis-played in Fig. 12. For DD0.7Fe3CoSb12 and for(Mm,Sm)yCo4Sb12, the difference between HP and CP+HPTis marginal; also annealing hardly influences the Seebeckcoefficient. The power factor (Fig. 13) reaches the highestvalues for the HP samples, due to high and very highelectrical resistivities and because of almost no change inthe Seebeck coefficient.

    All ZT values (Fig. 14) are remarkable. With CP+HPT thegoal is, of course, to get high ZTs, but they should notcompete with the ZTs of the HP+HPT samples as one alwayshas to consider the sustainability and energy-savingpreparation procedure of the CP+HPT samples. The highestZT with ZT ³ 1.44 at 823K could be achieved for thethermally stable (Mm,Sm)yCo4Sb12 (5 revolutions).DD0.7Fe3CoSb12 has ZT ³ 1.3 at 783K. In addition thethermo-electric conversion efficiencies for the CP+HPTsamples are in the range of © = 12.215.7%.

    L. Zhang et al.36) compared the magnetic behavior ofPr0.67Fe3CoSb12 before and after HPT. Despite the fact that

    Fig. 11 Thermal conductivity, (big symbols) and lattice thermalconductivity, ph (small symbols) DD0.7Fe3CoSb12 and (Mm,Sm)yCo4Sb12prepared via HP and CP+HPT vs. temperature, T.

    Fig. 12 Seebeck coefficient, S, of DD0.7Fe3CoSb12 (right scale) and(Mm,Sm)yCo4Sb12 (left scale) prepared via HP and CP+HPT vs.temperature, T.

    Fig. 13 Power factor, pf, of DD0.7Fe3CoSb12 (right scale) and(Mm,Sm)yCo4Sb12 (left scale) prepared via HP and CP+HPT vs.temperature, T.

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2076

  • after HPT a long-range magnetic order for T < 5.6K couldnot be detected in the temperature dependent electricalresistivity curve, the susceptibility indicates anti-ferromag-netism, even though diminished, revealing that the magneticorder at T < 5.6K is just superposed by high residualresistivity. Before HPT the effective magnetic moment is4.18®B, after HPT it amounts to 4.07®B, accompanied by ametamagnetic transition in the isothermal magnetizationcurves. The field dependent transitions considerably smearout as a consequence of the nanograined structure.

    For all skutterudites, HPT-processing is a great tool toeither enhance ZT of HP samples or to directly produce fastand easily high-ZT thermoelectric materials.

    3. Clathrates

    Yan et al. investigated the influence of HPT on type Iclathrate Ba8Cu3.5Ge41In1.5, prepared via high frequencymelting followed by BM and HP (for details see Ref. 12)).All features, typical of HPT-processed samples (4GPa, 1revolution at 673K), like reduced grain size, but enhancedresidual strain, dislocation density, point defects and cracks,accompanied by a lower density, turned up. As aconsequence, compared with the BM+HP sample, theHPT-processed sample had a higher electrical resistivityand Seebeck coefficient but a lower charge carrierconcentration, lower Hall mobility and thermal conductivityso that finally no essential improvement of ZT in theinvestigated temperature range occurred (Fig. 15).

    Only one clathrate Ba8Cu3.5Ge41In1.5, was investigated.The enhanced electrical resistivity was balanced by a reducedthermal conductivity and slightly higher Seebeck coefficient,therefore ZTs of the HP and HP+HPT sample were alike.

    4. Heusler Alloys

    Kourov et al.2) HPT-processed a rapidly quenchedHeusler-type alloy Ni2.16Mn0.84Ga (2 and 5 revolutions,under a pressure of 3 and 5GPa), and studied the crystallinestructure and behavior of electrical resistivity, thermoelectricpower, thermal expansion, and magnetic properties. AfterHPT of the sub-microcrystalline alloy, a mixture of

    amorphous and nanocrystalline (90 vol%) phases wasformed, however, after annealing at T > 600K the structurechanged first to nanocrystalline and later back to a sub-microcrystalline one. SPD was made responsible for the factsthat (i) the temperature dependent electrical resistivityexhibits a negative temperature coefficient (ii) the long-rangemagnetic order underwent a transition to a magneticallyordered state accompanied with a significant drop in themagnitude of magnetization (iii) the temperature dependentSeebeck coefficient is proportional to the temperature in themagnetically ordered state, but that (iv) the absolute valueof S drops drastically when the magnetic order vanishes andthat (V) the thermal expansion coefficient is remarkablyincreased. All these observations indicate a rearrangement ofthe electronic band structure near the Fermi level caused bythe HPT treatment.

    Whilst the thermoelectric properties helped to get insightinto the electronic behavior of Ni2.16Mn0.84Ga, the inves-tigation of Heusler alloys from the system VFeAl focusedon the thermoelectric performance.

    Masuda et al.17) HPT-processed Heusler alloys,VTa0.05Fe2Al0.95 and V1.05Fe2Al0.95, at room temperaturewith 5GPa and 10 revolutions, and observed neither cracksnor differences in hardness between the center and the rimarea. The structural and physical properties of the soobtained samples were compared with the respective arc-melted samples before HPT and after it plus annealing at873K. They observed some grain size reduction afterHPT, which almost vanished after the annealing process.The lattice parameters of both processed alloys measuredas a function of temperature were enhanced right after HPTbut decreased with increasing temperature during themeasurement.

    The electrical resistivities after HPT are enhanced and showa rather sharp peak with a decrease due to recovery and graingrowth during measurement-induced heating, similar to thebehavior of HPT-processed skutterudites. After recurrence,above 800K, resistivities are in line with those of the arc-melted samples as well as with those of annealed ones. Theabsolute values of the Seebeck coefficient of the HPT sampleswere lower in comparison to the arc-melted ones butincreased after annealing. Thermal conductivity is almost

    Fig. 15 ZT vs. temperature, T, of Ba8Cu3.5Ge41In1.5, adapted fromRef. 12).

    Fig. 14 ZT of DD0.7Fe3CoSb12 and (Mm,Sm)yCo4Sb12 prepared via HPand CP+HPT vs. temperature, T.

    The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides 2077

  • bisected (about 5W/m) after processing and hardly changesduring annealing. Element mapping revealed that Ta atomssegregated at the grain boundaries, and it seems that they wereable to retard the grain boundary migration and this waysuppress grain growth during recrystallization. ThereforeVTa0.05Fe2Al0.95 even after annealing exhibited a grain sizeof about 80 nm, whereas in non-doped V1.05Fe2Al0.95 thegrains grew back to 270 nm, being a disadvantage for thethermal conductivity. Therefore ZT (Fig. 16) of the annealedV1.05Fe2Al0.95 is lower than that of the arc-melted counterpart,however, ZT of the annealed, HPT-processed VTa0.05Fe2Al0.95reaches ZT ³ 0.3 at around 500K because of the low thermalconductivity even after annealing in parallel with therestoration of the rather large power factor.

    For the Heusler-type Ni2.16Mn0.84Ga the comparison of thebehavior of the thermoelectric power, electrical resistivity,and magnetic properties indicated a significant rearrangementof the electronic band structure near the Fermi level due tothe HPT treatment of the alloy. Whilst for VTa0.05Fe2Al0.95 ahigher ZT after HPT was detected, this was not the case fornon-doped V1.05Fe2Al0.95 because during annealing the grainsgrew back to original size.

    5. Half-Heusler Alloys

    Although Half-Heusler (HH) alloys reveal increasinginterest as thermoelectric materials, severe plastic deforma-tion has been applied only recently. Particularly the group ofRogl et al.37) investigated the influence of HPT on p-type(NbFeSb and Ti0.15Nb0.85FeSb) and n-type (Ti0.5Zr0.5NiSn,Ti0.5Zr0.5NiSn0.98Sb0.02 and Ti0.5Zr0.5NiSn + DA, where DAstands for densification aid) Half-Heusler (HH) alloys. AllHP samples were processed at 650K, applying 4GPa and 1revolution under argon to prevent oxidation. X-ray diffractionpatterns revealed peak broadening for all HPT samples,indicating finer grains and/or an enhanced dislocationdensity. After measurement-induced heating the peaksbecame slimmer. Lattice parameters became larger afterHPT, whereas the relative density became lower. Forskutterudites, the second measurement for increasing temper-ature showed all physical properties being similar to those

    measured in the first measurement with decreasing temper-ature; for HH alloys, however, at least three times cyclingis necessary to gain a thermally stable, annealed sample,indicating that for HH alloys the introduced defects andstructure changes seem to be more resistant against heattreatments.

    Both samples NbFeSb and Ti0.15Nb0.85FeSb were preparedas described in detail in Ref. 38). ZTs of processed andannealed samples were enhanced (Figs. 17 and 18,respectively).

    Interestingly, the HPT-processed NbFeSb exhibits anelectrical resistivity lower than that of the HP sample,already for the first measurement; at room temperature theannealed sample has a resistivity of about five times lowerthan that of the HP; but at 823K the values are about thesame. Seebeck coefficient of the HP sample exhibits a cross-over from negative to positive values at around 455K andafter undergoing a maximum (at 623K), it changes back tonegative values at around 505K (insert in Fig. 17). Thesechanges are also present after severe plastic deformation viaHPT. As the Seebeck coefficient of the HPT annealed sampleis slightly lower in comparison with the HP sample, and thethermal conductivity is almost three times lower, ZT at 623Kreaches ZT = 0.00043. This is, as absolute value, very low,but as relative value double the ZT-value of the HP sample.

    Fig. 16 ZT vs. temperature, T, of VTa0.05Fe2Al0.95 and V1.05Fe2Al0.95,adapted from Ref. 17).

    Fig. 17 ZT vs. temperature, T, of NbFeSb. Insert: S(T) of NbFeSb.

    Fig. 18 ZT vs. temperature, T, of Ti0.15Nb0.85FeSb.

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2078

  • Ti0.15Nb0.85FeSb behaves like a typical HPT-processedalloy. Electrical resistivity is - right after HPT - much higher,shows a maximum and decreases. After annealing theresistivity is only slightly higher than after hot-pressing.The Seebeck coefficient does not change within the error bar,but thermal conductivity is much lower, so that ZTs of theHPT sample at 823K are higher than the ZT of the HP(ZT = 0.68) sample with ZT = 0.71 and ZT = 0.74 (for thefirst measurement and the annealed sample, respectively).

    Figures 1922 summarize the thermoelectric propertiesfor all three investigated n-type HH alloys, Ti0.5Zr0.5NiSn,Ti0.5Zr0.5NiSn0.98Sb0.02 and Ti0.5Zr0.5NiSn + DA, however,not all measured temperature-dependent curves are presented:only the graphs of the HP (the reference), of the HPT-processed (first measurement, temperature increasing) and theadditionally annealed sample (after cycling) are presented.All samples, arc-melted, annealed, BM and HP, wereprepared as described in detail in Ref. 39). For all threeHH alloys (Fig. 19), the electrical resistivity after HPT isvery high, presents a maximum between 500 and 600K andis furtheron decreasing with increasing temperature. ForTi0.5Zr0.5NiSn0.98Sb0.02 and Ti0.5Zr0.5NiSn + DA after threetimes cycling, μ(T) of the annealed and of the HP sample areshowing the same curvature with higher values for theannealed one. For these two compounds, the absolute values

    of the Seebeck coefficient after HPT and cycling are lowerthan those for the HP sample. The undoped sampleTi0.5Zr0.5NiSn without DA, displays an extremely highresistivity right after HPT-processing, but after measure-ment-induced heating it is even lower than for the HP sample.The absolute values of the Seebeck coefficient ofTi0.5Zr0.5NiSn after HPT and annealing are about 60% lowerthan those for the HP sample (Fig. 20).

    For all three HH alloys, the thermal conductivity (Fig. 21)is decreased although not by the same extent. The highestenhancement of ZT (see Fig. 22), almost 20%, is exhibitedby Ti0.5Zr0.5NiSn with ZT = 0.81 at 773K. For Ti0.5Zr0.5-NiSn + DA, however, the HP sample has a higher ZT thanthe as-processed one, mainly because of a rather low Seebeckcoefficient.

    For p- and n-type HH alloys, at least three temperaturecycles are necessary to gain a thermally stable state,indicating that obviously the introduced defects and structurechanges are more resistant against heat treatments than incase of skutterudites. For both p-type HH alloys investigated,as well as for two of the three n-type HH alloys, ZT wasenhanced after HPT in comparison to the HP counterparts.

    SEM images (Fig. 23), exemplarily shown for the HHalloy system Ti0.15Nb0.85FeSb, compare the microstructure ofthe fracture surfaces of the HP, HP+HPT and the HP+HPT

    Fig. 19 Electrical resistivity, μ, vs. temperature, T, for n-type HH alloys.

    Fig. 22 ZT vs. temperature, T, for n-type HH alloys.Fig. 20 Seebeck coefficient, S, vs. temperature, T, for n-type HH alloys.

    Fig. 21 Thermal conductivity, , vs. temperature, T, for n-type HH alloys.

    The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides 2079

  • annealed samples. Grain sizes of the HP sample ranging from0.1 to about 10 µm are reduced to less than 2 µm after HPT.However, during annealing the grains grow and show a ratherhomogeneous size. TEM images (Fig. 24) confirm thesefindings and show that the dislocation density in the HPTsample is enhanced by almost two orders of magnitude(for details on the thermoelectric HH phase TixNb1¹xFeSb,please see Ref. 37)).

    6. Bismuth Tellurides

    This type of non-skutterudite alloys belongs to the groupof semiconductors and shows its maximum of the figure ofmerit ZT typically around T = 400500K, mainly due tothe maximum of thermoelectric power and the minimum ofphonon thermal conductivity at this temperature.

    Attempts to improve ZT mainly concern macroscopicdeformation processes. Already with elastic deformation,some tuning of the band gap is possible especially when thedeformation induces a phase transition. The paper ofOvsynnianikov et al.40) is an example the effects of whichcan follow from applying elastic hydrostatic pressuresbetween 225GPa in Bi2Te3 and Sb2Te3: whilst pressurestill 4GPa already enhance the power factor PF due to atransition from semiconductor to metal characteristics, therhombohedral R�3m phase of these compounds undergoes

    a phase transition into a monoclinic C2/m lattice. Thistransition, however, deteriorates PF in case of p-type Bi-tellurides, while it increases it continuously till pressures of25GPa in case of Se-doped n-type tellurides because of acontinuous increase of electrical conductivity. Biswas et al.also present a simple technical model with diamond anvils fora compact thermoelectric high-pressure module, whichprovides both a permanent pressure and a heat sink.40)

    First ideas for applying plastic deformation for the sakeof improvement of ZT in those alloys were to re-orient theTEs’ crystal lattice by plastic deformation for the sake ofincreasing the electrical conductivity i.e. achieve a texturealong (00ℓ) and thus maximize conductivity. The first realexperiments with application of plastic deformations goback to the nineteen-nineties when Seo et al.41) applied hotextrusion (HE) to p-type Te-doped Bi0.5Sb1.5Te3 and n-typeSbI3-doped Bi2Te2.85Se0.15 thermoelectric compounds, within573713K. Besides the improvement by doping with Te andSb, the figure of merit could be improved by a factor 3, byapplying HE and increasing the HE temperature, arriving atZT = 0.87 at RT, probably mainly due to the increase ofelectrical conductivity.

    One of the first works trying generation of lattice defects(i.e. grain boundaries by nanocrystallization) in thesesemiconductor materials was that by Yu and colleagues in200942) who reached a ZT = 0.94 at 398K and/or at least

    Fig. 23 SEM micrographs (images of secondary electrons) of the fracturesurfaces of Ti0.15Nb0.85FeSb, HP (a), HPT (b) and HPT after annealing (c).

    Fig. 24 TEM micrographs of Ti0.15Nb0.85FeSb, HP (a), HPT (b) and HPTafter annealing (c).

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2080

  • ZT = 0.7 between 325525K, through BM, cold pressingand sintering between 473773K. This ZT was markedlyhigher than the former record-figure-of-merit achieved byZone Melting (ZM)43) being ZT ³ 0.75 only.

    A significant increase of ZT in these materials waspresented also in 2008 by Poudel et al.44) and Ma et al.45)

    who reached a peak ZT = 1.4 at 373K in p-type BixSb2¹xTe3bulk ternary alloys through reductions in thermal con-ductivity by means of nanocrystallization and defectgeneration. The latter was achieved by suitable ball millingand high pressure consolidation.

    The first real use of an SPD method has been done by Imet al.46) in 2004 who chose a multi-pass ECAE (called alsoECAP) method at a temperature of 773K. However, the ZTvalues reached by Seo et al.41) could not be overcome. Thesame is true with the ECAE experiments performed by Fanet al.47) in 2008, as well as by Hayashi et al. carried out in200648) and 201049) although in the latter work they tried tooptimize the ECAE route with regard to a (00ℓ) texture forthe sake of maximizing the electrical conductivity. In 2008another treatise concerning ECAE processed Bi0.5Sb1.5Te3compound was undertaken by Lim et al.50) at temperatures653733K arriving at a still considerable grain size of about10 µm which explains that the figure of merit reached thesame values as did the previous ECAP works i.e. ZT = 0.9at RT.

    A much better ZT value could be achieved by the groupof Sun et al.,51) Fig. 25 reaching ZT = 1.16 at RT afterusing a modified ECAP method also at 773K but with arotary die, after having sintered the powder of (Bi,Sb)2Te3alloys. Although the Seebeck coefficient was decreased byECAP, the electrical conductivity was strongly increasedwhat led to an overall increase of ZT by 10% compared withthe ball milled & pulse discharge sintered material.

    The second SPD method applied to V-VI and/or V2-VI3alloys has been High Pressure Torsion (HPT), by M. Ashidaet al.52,53,55) and T. Hamachiyo et al.54) Investigations havebeen done with the stoichiometric alloy Bi0.5Sb1.5Te3.0produced by ball milling (BM) or Vertical Brigdman Method(VBM), sintered by hot pressing at 673K, and then HPT-processed at 473K under a pressure of 6GPa and rotated byspeeds of 0.11 rpm. A combination of VBM and HPT usinghigh HPT rates yielded power factors (PFs) as low as 1 to4 © 10¹3Wm¹1 K¹2.52) However, modifying the processingroute by starting with BM followed by a low-rate HPT of0.1 rpm provided a power factor being around 6 © 10¹3

    Wm¹1 K¹2 within a temperature interval from 320470K55)

    (see also Fig. 26). This increase of PF could be attributedpartially to the low-rate HPT’s texture evolution whichproceeded along (00ℓ), and partially to the enhanced Seebeckcoefficient,55) Fig. 26. Unfortunately, the works5255) did notpresent the thermal conductivity, which is necessary toreliably estimate the ZT-value resulting from these experi-ments. According to the authors’ promising experiencesfrom Skutterudites (see this article) and from the results fromother techniques of plastic deformation applied to thetellurides mentioned above,44,45,51) one can hope that HPTcould be very successful in further enhancing the ZT valuethrough the generation of lattice defects acting as additionalscattering centers for further decreases of the thermal

    Fig. 25 Increases of the electrical conductivity (upper graph) and of thepower factor (lower graph) through increasing numbers x of ECAP passesEPx, by measurements in transversal (T, full symbols) and longitudinal(L, empty symbols), with increasing measuring temperature, in a(Bi,Sb)2Te3 alloy (from Sun et al., Ref. 51)).

    Fig. 26 BM and HPT-processed BiSbTe showing an increase in electricconductivity (upper graph) and an enhancement of power factor(lower graph) due to low-rate HPT-processing (from Ashida et al.,Ref. 55)).

    The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides 2081

  • conductivity and thus increases of ZT. Among the numerousmethods of plastic deformation, HPT seems to provide thehighest number of advantages: (1) HPT is the most powerfulmethod achieving a maximum of lattice defects, not at leastthanks to the enhanced pressure which allows for very largestrains without developing cracks and failures; (2) HPTcan achieve bulk nanocrystalline samples directly frompowders; (3) HPT does not introduce a strong texture or under certain conditions can achieve (00ℓ) textures atelevated processing temperatures and low processing rateswhich maximize the electrical conductivity in those Te-composites.

    There have been done also some but only few treatises toimprove ZT in PbTe, e.g. by Biswas et al. in 2012,56) whodoped this alloy with Na, or with SrTe and Na, and appliedprocessing by Spark Plasma Sintering (SPS) where thesample is quickly molten and sintered under pressure. Withthis processing technique, small scale second phase particleswere formed with both phase boundaries for small particleswith strain fields around them, as well as with misfitboundary dislocations at the larger ones could be generated.Such defects have been intentionally produced in order toprovide additional scatterers for phonon scattering thusgiving a minimum of lattice thermal conductivity. In thebest case, values of figure of merit till ZT = 2.2 at 900Kwere reached.

    In what follows, more recent developments are describedwhich concern the specific scattering properties of the variouslattice defects. A marked step herein was given by the paperof Kim et al.57) which emphasizes the high scattering powerof dislocations (especially closely neighboured ones): In

    contrast to point defects as well as grain boundaries whichpreferably scatter high and low frequency phonons,respectively, the dislocations cover the broad intermediaterange of phonons with medium frequency (see Fig. 27). Byaimed introduction of such dislocations through liquid phasecompacting process of melt-spun eutectic compound ofstoichiometric Bi0.5Sb1.5Te3 with pure Te, Kim et al. showedthat the ZT value can be increased up to ZT = 1.89 (Fig. 27)compared to ZT = 0.91 of the ingot or BM initial materialand to ZT = 1.4 reported by the works of Poudel44) and Ma45)

    using BM and Hot Pressing (HP) for these p-type BiSbTecompounds. It has been shown by the authors of this article16)

    that a nanostructure with a regular dislocation array in thegrain boundaries indeed gives the lowest lattice thermalconductivities possible (near to the theoretical minimum) thusenabling records in ZT of 1.5 for p- and almost 2.0 for n-typeskutterudites. That arrangement has been achieved by hightemperature HPT. In a paper of Park & Lee,58) published oneyear after that of Kim,57) they recommend to apply this SPDtechnique also to BiSeTe compounds. Recently an extensivework on n-type V2VI3 alloys was published by Hu et al.59)

    who for the first time report the optimization of TE propertiesby applying a special way of hot deformation (HD, called“progressive”). By choosing a large number of HD countsbut not too high HD temperatures (at maximum 723K) toaccount for a low electrical conductivity as well as for asufficient number of lattice defects including dislocations andgrain boundaries, at a grain size of only 60 nm, they reachedrecord values of ZT = 1.3 in Sb-doped n-type Bi, at T =470K (Fig. 28).

    Fig. 27 A: Spectral lattice thermal conductivity ¬s(f ) with different contributions of phonon scattering. Dislocation scattering is operativeover the whole frequency range, in contrast to boundaries and point defects which mainly scatter phonons with low or high frequencies,respectively. B, C: Lattice thermal conductivity ¬lat (left bottom, B) and figure of merit zT (right, C) for melt-solidified (BM), solid-phasecompacted (S-MS), and liquid-phase compacted (Te-MS) Bi0.5Sb1.5Te3 alloys. Only the latter contains dislocations embedded in grainboundaries which cause a large overall phonon scattering, thus minimizing the lattice thermal conductivity and maximizing the figure ofmerit (from Kim et al., Ref. 57)).

    G. Rogl, M.J. Zehetbauer and P.F. Rogl2082

  • 7. Mechanical Properties of Skutterudites and Half-Heusler Alloys

    So far for many HPT-processed materials, mechanicalproperties have been measured. The details about theequipments, measurements and evaluations of the data aredescribed in Refs. 3, 60, 61). Here only room temperaturedata are discussed, and only examples, typical of the generalbehavior, are presented here.

    The values of the elastic moduli, independent of thepreparation steps (HP+HPT, HP+HPT-annealed or CP+HPT, CP+HPT-annealed) of HPT-processed skutteruditesare not much different from those of the respective HP ones.Also a slight change in the samples’ density has not muchinfluence. For example, all Young’s moduli (E) for HP,CP+HPT and CP+HPT-annealed p-type DD0.7Fe3CoSb12are in the range of 136GPa ¯ E ¯ 144GPa;34) for ball-milled and HP DD0.6Fe3CoSb12, E = 150GPa, after HPT onegets E = 153GPa, which is practically the same valueconsidering 3% uncertainty. Also for (Mm,Sm)yCo4Sb12there is no change in E within the error bar, independentof the number of revolutions, with all values 142GPa ¯E ¯ 145GPa.35)

    The values of hardness of HP skutterudites and of HHalloys are, besides the composition, strongly dependent onthe density. As shown in Fig. 29, generally n-type HPskutterudites are harder than p-type skutterudites for a largevariation of density. After HPT-processing, the grains aremuch smaller and, as a consequence of the Hall-Petchrelation, hardness must be higher, which is indeed the casefor the p- and n-type skutterudites, independent of thepreparation method HP+HPT or CP+HPT. Hardness of HPTsamples also depends on the processing conditions, i.e.samples processed with more than one revolution have muchhigher values, especially prominent for (Mm,Sm)yCo4Sb12prepared by five revolutions (sample CP+HPT-5r). As HPT-processed samples exhibit a gradient in strain along theradius, the hardness depends on the distance from thesample’s center as depicted in Fig. 30. For example,DD0.6Fe3CoSb12 with an overall density of 97% after HPT(4GPa, 1 revolution, 623K) exhibited a hardness of HV =498 in the center section of the sample and HV = 522 at therim.5) It is interesting to note that (i) the hardness of HH

    alloys generally is much higher than that of skutterudites and(ii) the difference in HV between center and rim area is largerfor the HH alloys. These observations are in contrast to thoseof Masuda et al.,17) as they claimed that no difference in thehardness between rim and center occurred when theymeasured their HP+HPT-processed (5GPa, 10 revolutions)Heusler alloys.

    Besides all these facts, for HP+HPT samples the startingconditions play a role. Hand milled (HM) and ball milledsamples of DD0.44Fe2.1Co1.9Sb12 were HPT-processed underthe same conditions (4GPa, 1 revolution, 623K). TheHM+HP sample had a density of 95.2%, and HV = 393,which changed to 94% and a much higher hardness HV =479 (rim) after HPT resulting in an enhancement of 22%. TheBM+HP sample had a density of 98.3% and HV = 410.After HPT, HV was increased to 562 (rim), which equals37%, Ref. 7).

    Generally after HPT and HPT annealing, no mentionablechanges in the elastic moduli occurred. Due to the changingstrain from the center to the rim of a circular sample, hardnessvalues are higher in the rim than in the center area but in anycase higher than those of the HP sample although theirdensity is lower.

    Fig. 29 Hardness, HV of various skutterudites vs. relative density, drel, HPand HPT-processed. Most data of HP skutterudites are adopted fromRef. 61).

    Fig. 30 Hardness, HV (left scale) and in GPa (right scale) of HP and HPT-processed (measured in the center, middle and rim area of the circularsample) skutterudites and Half-Heusler alloys.

    Fig. 28 Effect of Sb-alloying, and of progressive hot deformation (HD) tothe zT-value in the n-type BiSbTe compounds indicated (after Hu et al.,Ref. 59)).

    The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides 2083

  • 8. Thermal Expansion of Skutterudites

    Thermal expansion exhibits a quite unusual behavior afterHPT-processing, related to the behavior of the temperaturedependent electrical resistivity.6,7,9,15) As all temperaturedependent curves of the thermal length change exhibit thesame features, exemplarily the thermal expansion ofDD0.6Fe3CoSb12 is shown (Fig. 31) and discussed.

    In the low temperature range (4.2300K), no anomaliesfor the HPT sample are visible, however, above 300K thecurve is not strictly linear anymore, and at ³443K instead offurther expanding, the sample contracts. At about 470K theexpansion continues. The anomaly occurs in about the sametemperature window in which the electrical resistivity startsto decrease after a maximum; obviously the ‘shrinking’ ofthe sample is connected with the annealing of defectsand/or disappearance of cracks. As can be seen in Fig. 31,with decreasing temperature as well as for a second run withincreasing temperature, no anomalies appear. It should beadded that thermal expansion was not only measured in thepressing direction as imaged in Fig. 31, but also perpendic-ular to it,6) showing the same behavior. All thermal expansioncoefficients of HPT-processed samples hardly differ fromthose of the HP samples and are listed in Ref. 9).

    9. Conclusions

    This article is to show that SPD-processing of thermo-electrics can provide values of figure of merit (ZT) higherthan 1.2, and thus gets highly attractive for materials whichenable sustainable generation of electric power. These highvalues arise from the ultrafine grained microstructure incombination with a high level of point, linear and surfacelattice defects which significantly enhances the scatteringof the phonons, thus leading to a minimum of thermalconductivity and therefore to exceptional values of ZT. Theextent of SPD-induced enhancement depends on the specialthermoelectric materials. So far, the following thermo-electrics have been tried to get improved by SPD processing:(1) From all thermoelectrics, n- and p-type skutterudites

    benefit most from SPD-processing especially if oneapplies high pressure torsion (HPT) to already hot-

    pressed or still powdered materials thus producingthermoelectric bulks. Significant enhancements offigure-of-merit ZT for n-type skutterudites from 1.6 toalmost 2, and for p-type skutterudites from 1.15 toalmost 1.5 at 825K could be achieved.

    (2) HPT-processing of Heusler and Half-Heusler phasesyields about the same relative improvement of ZT (0.22to 0.3) as skutterudites, although the absolute valuesstill appear limited. However, SPD-induced increasesof ZT achieved in Clathrates were found to be nearlynegligible.

    (3) Low temperature thermoelectrics like Bismuth Tellur-ides of type V1(VI)1 and/or V2(VI)3 (with V, VIrepresenting the group elements) show a considerablepotential in SPD-induced increases of ZT. Already withhigh temperature pressure after ball milling, ZT could beenhanced from ZT ³ 1.0 up to ZT = 1.4 at T = 373K,at least by ensuring a high electrical conductivity whileconsidering the strong anisotropy in these materials.Therefore the choice of the best SPD-processingtechnique includes the careful optimization of texture,but also that of the processing rate and temperature.Recent investigations underline the importance of thelattice defects’ specific phonon scattering efficiencies -especially those of dislocations - and their generation insufficiently high densities, thus reaching enhancementsof p-type Bi-Tellurides up to ZT = 1.9.

    As concerns the mechanical properties of SPD-skutteruditesand Half-Heusler-alloys, the application of SPD methodssignificantly raises the strength while the elastic moduliremain unchanged unless new phases have been formed.

    Acknowledgements

    The authors thank O. Eibl and J. Bursik for contributingthe SEM and TEM images.

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