High Thermoelectric Figure of Merit and Nanostructuring in Bulk p-type Ge-x(SnyPb1-y)(1-x)Te Alloys

1054Chem.Mater.2010,22,1054–1058DOI:10.1021/cm902009tHigh Thermoelectric Figure of Merit and Nanostructuring in Bulk p-type Ge x(Sn y Pb1-y)1-x Te Alloys Following a SpinodalDecomposition Reaction†Yaniv Gelbstein,*Boaz Dado,Ohad Ben-Yehuda,Yatir Sadia,Zinovy Dashevsky,andMoshe P.DarielDepartment of Materials Engineering,Ben-Gurion University of the Negev,Beer-Sheva84105,IsraelReceived July6,2009.Revised Manuscript Received August6,2009The use of thermoelectricity for direct energy conversion from thermal to electrical energy has a long-standing history.We report that the complex p-type Ge(Sn,Pb,Te)Te alloys upon suitable alloying with SnTe can be designed to obtain very high ZT values of up to1.2because of both optimal electronic transport properties and minimal lattice thermal conductivity.Such alloys follow a spinodal decomposition reaction,leading to an ordered periodic nano-structure that is effective in the scattering of phonons and reduction of the lattice thermal conductivity.IntroductionThermoelectricity is concerned with the interactionbetween thermal and electrical phenomena.The mostcommon applications are concerned with the conver-sion of thermal energy(or heat)into electrical powerand with the use of electrical current for cooling.Thedimensionless thermoelectric figure of merit(ZT),ex-pressed in eq1,is widely used as a measure of thethermoelectric efficiency with respect to the material’sproperties.ZT¼R2σK Tð1Þwhere Z is the thermoelectric figure of merit,R theSeebeck coefficient,σthe electrical conductivity,κthethermal conductivity,and T the absolute temperature.Materials with high ZT values are known as thermo-electric materials.Today’s commercial materials have aZT=1,leading to conversion efficiency on the order ofseveral percent(∼6%).Increasing ZT can be achievedby either increasing the numerator of ZT,P=R2σ, known as the power factor,or decreasing the thermalconductivity,κ(the denominator of ZT in eq1).κis asum of two major contributions,the lattice thermalconductivityκL(due to phonons flow)and the electronicthermal conductivityκe(due to electronic flow),as de-scribed in eq2.K¼K LþK eð2ÞCurrently,PbTe-based thermocouples,with an n-type leg (doped with PbI2for achieving maximal ZT values of about 1.1in the temperature range of400-600°C for carrier concentrations of2-6Â1019cm-3,respectively)and a p-type leg(alloyed with SnTe)have been employed in real applications.1,2The latter,in the form of Pb1-x Sn x-Te,unfortunately display much lower ZT values(e0.5) than those for the n-type PbTe component.Higher ZT values(∼0.8at450°C)for p-type legs were found in Na-doped PbTe;3however,because of the brittle nature of this material and the correspondent poor mechanical pro-perties,its usage in practical applications was eliminated.3,4 Therefore,further improvement of the ZT values of the p-type legs is a significant challenge that carries substantial benefit.Recently,optimally doped pseudobinary Pb1-x Ge x Te compounds were proposed as potential candidates to im-prove the thermoelectric performance of p-type legs.5,6 However,the ZT values of these pseudobinary alloys can be further increased by decreasing the lattice thermal con-ductivity due to the presence of additional structural defects. Recently,very low thermal conductivity values were ob-served in several nanostructured PbTe-based materials.7-12 One mechanism for nanopattern generation is based on the†Accepted as part of the2010“Materials Chemistry of Energy Conversion Special Issue”.*Corresponding author.E-mail:yanivge@bgu.ac.il.(1)Gelbstein,Y.;Dashevsky,Z.;Dariel,M.P.Physica B2005,363,196–205.(2)Gelbstein,Y.;Dashevsky,Z.;Dariel,M.P.Physica B2007,391,256–265.(3)Gelbstein,Y.;Gotesman,G.;Lishzinker,Y.;Dashevsky,Z.;Dariel,M.P.Scr.Mater.2008,58,251–254.(4)Gelbstein,Y.;Dashevsky,Z.;Dariel,M.P.J.Appl.Phys.2008,104,033702.(5)Gelbstein,Yaniv;Dashevsky,Zinovi;Dariel,Moshe P.Phys.Status Solidi2007,1(No.6),232–234.(6)Gelbstein,Y.;Ben-Yehuda,O.;Pinhas,E.;Edrei,T.;Sadia,Y.;Dashevsky,Z.;Dariel,M.P.J.Electron.Mater.2009,38(7),1478.(7)Poudeu,P.F.P.;D’Angelo,J.;Downey,A.D.;Short,J.L.;Hogan,T.P.;Kanatzidis,M.G.Angew.Chem.,Int.Ed.2006,45,1–5. (8)Sootsman,J.R.;Pcionek,R.J.;Kong,H.;Uher,C.;Kanatzidis,M.G.Chem.Mater.2006,18,4993–4995./cm Published on Web08/21/2009r2009American Chemical SocietyArticle Chem.Mater.,Vol.22,No.3,20101055spinodal decomposition.Spinodal decomposition is a me-chanism for phase separation that leads to a characteristicmodulated structure that can be exploited to control themicrostructure at the nanometer scale.13Nanostructures for spinodal decomposed systems werereported for TiO2-SnO2,13Mn-Cu,14and Cu-Ni-Sn15 alloys.Recently,a reduction of the lattice thermal con-ductivity and a corresponding ZT enhancement inPb1-x Sn x Te-PbS system was attributed to nano structuresresulting from the spinodal decomposition.12Alloying of(GeTe)x(PbTe)1-x compounds with SnTecan result in a spinodal decomposition with changedperiodicity that depends on the heat treatment.16To thebest of our knowledge,the nanostructure and the transportproperties of these alloys have not yet been investigated.The present communication is concerned with develop-ment of highly efficient nanostructured p-type Ge x-(Sn y Pb1-y)1-x Te alloys for thermoelectric applications,using spark plasma sintering(SPS).The thermoelectricfigure of merit was optimized by means of both alloyingand doping methods.Two alloys in this family,Ge0.5Sn0.25Pb0.25Te(undoped,Ag-doped and Bi2Te3-doped)and Ge0.6Sn0.1Pb0.3Te,were prepared and inves-tigated.The results were compared to a GeTe referencesample.Experimental DetailsSynthesis.GeTe,Ge0.5Sn0.25Pb0.25Te(undoped,3mol%Ag-doped,and3mol%Bi2Te3-doped),and Ge0.6Sn0.1Pb0.3Te alloyswere prepared according to the following procedure:(a)sealingthe source materials(purity of5N)at appropriate concentrationsin a quartz ampule under a vacuum of1Â10-5Torr,(b)meltingthe alloys in a rocking furnace at800°C for1h followed by waterquenching,(c)milling the compound to a maximal particle size of60mesh powder,and(d)Spark Plasma Sintering(SPS)(type HPD5/1FCT Systeme GmbH)at450°C for30min under amechanical pressure of32MPa for GeTe and at550°C for60min under a mechanical pressure of25MPa for the Ge x(Sn y-Pb1-y)1-x Te alloys.The different SPS parameters were optimizedin order to obtain dense samples(>96%)without any lateralcracks for the various investigated alloys.Electrical Properties.Seebeck coefficient and the electricalconductivity measurements were performed in a self-con-structed apparatus under an argon atmosphere up to∼450°Cat a heating rate of3°C/min.For Seebeck coefficient measure-ments,an auxiliary heater was used to maintain a temperature difference of10°C between the extremities of the samples. Electrical conductivity was measured by the“four-probe”method using an alternating power source of1V/50Hz.Thermal Conductivity.The thermal conductivity(κ)was determined as a function of temperature from room tempera-ture to500°C using the flash diffusivity method(LFA457, Netzsch).The front face of a disk-shaped sample(L=12mm, thickness≈1-2mm)is irradiated by a short laser burst and the resulting rear face temperature is recorded and analyzed.Ther-mal conductivity values were calculated using the equation κ=R3C P3F,where R is the thermal diffusivity,C P is the specific heat(measured using differential scanning calorimetry,STA 449,Netzsch),and F is the bulk density of the sample(calculated from the sample’s geometry and mass).Microscopy.The microstructure of Ge0.5Sn0.25Pb0.25Te (undoped and3mol%Bi2Te3-doped)was investigated by optical microscopy(Zeiss Axiovert25.Etching prior to optical microscopy by immersion for few seconds in a60mL solution composed of10mL of alcohol,10g of potassium hydroxide, 17.5mL of glycerol,and22.5mL of distilled water),high-resolution scanning electron microscopy(Jeol-7400F HRSEM), transmission electron microscopy(TEM,FEI TECNAI,G2), and high-resolution transmission electron microscopy(Jeol-2010HRTEM).Specimens used for the TEM and HRTEM were prepared as follows.Small square shape pieces with approximate sizes of 5mmÂ5mmÂ2mm were first cut from the spark plasma sintered disk using a low speed diamond wheel saw.The samples were then mounted inside a copper tube(L=3mm)and hand-polished with subsequently increasing grit(1000-1500)sand paper to about200-300μm in thickness.Samples were then thinned using a Gatan precision dimple grinder and ion milled to electron transparency using a Gatan precision ion polishing system(PIPS).Results and DiscussionAll the investigated Ge x(Sn y Pb1-y)1-x Te alloys dis-played a microscale quasi-ordered periodic fishbone structure,the orientation of which depended on the original high temperature cubic phase grain in which the demixing process took place.A typical structure of these alloys as observed in Ge0.5Sn0.25Pb0.25Te is shown in Figure1.From the optical microscope(a)and HRSEM (b)micrographs of this figure,the wavelength of the dissociation as determined from the side branches of the fishbone was about10μm.High-magnification TEM micrograph(c)revealed a finer periodic nanostructure with a wavelength of about100nm.In addition,electron-dispersive spectroscopy(EDS)analysis showed that the fishbone structure is Pb-rich,whereas the matrix is Ge-rich.The bright aspect of the Pb-rich regions(Pb is heavier than Ge)is obtained in the backscattered electron (BSE)examination mode of the HRSEM(b).Line scan EDS analysis crossing the fishbone patterns and the surrounding matrix revealed continuous and complemen-tary compositional modulations of Pb and Ge when scanning across the side-branches of the fishbone pattern (Figure1).It is noteworthy that the Sn composition remained constant while crossing the Pb-rich fishbone lamellas through the surrounding Ge-rich matrix.Higher magnification of HRTEM revealed that the Pb-rich(9)Poudeu,P.F.P.;D’Angelo,J.;Kong,H.;Downey,A.D.;Short,J.L.;Pcionek,R.;Hogan,T.P.;Uher,C.;Kanatzidis,M.G.J.Am.Chem.Soc.2006,126,14347–14355.(10)Androulakis,J.;Hsu,K.F.;Pcionek,R.J.;Kong,H.;Uher,C.;D’Angelo,J.;Downey,A.D.;Hogan,T.P.;Kanatzidis,M.G.Adv.Mater.2006,18,1170–1173.(11)Hsu,K.F.;Loo,S.;Guo,F.;Chen,W.;Dyck,J.S.;Uher,C.;Hogan,Tim;Polychroniadis,E.K.;Kanatzidis,M.G.Science 2004,303,818–820.(12)Androulakis,J.;Lin,C.-H.;Kong,H.;Uher,C.;Wu,C.-I.;Hogan,T.;Cook,B.A.;Caillat,T.;Paraskevopoulos,K.M.;Kanatzidis, M.G.J.Am.Chem.Soc.2007,129,9780–9788.(13)Chaisan,W.;Yimnirun,R.;Ananta,S.;Cann,D.P.J.Solid StateChem.2005,178,613–620.(14)Yin,F.;Ohsawa,Y.;Sato,A.;Kawahara,K.Acta Mater.2000,48,1273–1282.(15)Zhao,J.-C.;Notis,M.R.Acta Mater.1998,46,4203–4218.(16)Yashina,L.V.;Leute,V.;Shtanov,V.I.;Schmidtke,H.M.;Neudachina,V.S.J.Alloys Compd.2006,413,133–143.1056Chem.Mater.,Vol.22,No.3,2010Gelbstein et al. fishbone structure by itself composed from smaller scalemodulations in the10nm range(Figure2).These con-tinuous modulations without any sharp borders,as inprecipitates,are possibly consistent with the continuousnature of the spinodal decomposition generated struc-ture.The electrons diffraction patterns in this figureshow a doublet pattern(two nonconcentric periodicreflections),indicating the presence of two similar struc-tures with closely related lattice parameters providingadditional evidence to the decomposition of Ge x(Sn y-Pb1-y)1-x Te into Ge and Pb rich phases.In addition to the micro and nanoperiodic Pb-richmodulations,the3mol%Bi2Te3doped Ge0.5Sn0.25-Pb0.25Te sample showed Birich nano precipitates with atypical size of50nm(Figure3),which were not observedat any of the other investigated compositions.The effectof these precipitates will be discussed later.The temperature dependence of the Seebeck coeffi-cient,electrical conductivity and thermal conductivity of Ge0.5Sn0.25Pb0.25Te(undoped,3mol%Ag-doped, and3mol%Bi2Te3-doped)and Ge0.6Sn0.1Pb0.3Te alloys compared to GeTe are presented in Figures4-6.We discuss first the doping influence of Ge0.5Sn0.25-Pb0.25Te by Ag and Bi2Te3on the thermoelectric proper-ties.By comparing curves2,3,and5for Ag doping, undoped,and Bi2Te3doping,respectively,in Figures4-6,a consistent trend of decreasing Seebeck coefficient and increasing electrical and thermal conductivity values forFigure1.(a)Optical microscopy,(b)backscattered electrons HRSEM, and(c)TEM micrographs of Ge0.5Sn0.25Pb0.25Te.Figure2.HRTEM micrograph and typical electron diffraction patterns of Ge0.5Sn0.25Pb0.25Te.Figure3.HRSEM of scattered Bi-rich nanoprecipitates in3mol% Bi2Te3-doped Ge0.5Sn0.25Pb0.25Te.Figure4.Temperature dependence of the Seebeck coefficient for(1) GeTe,(2)Ge0.5Sn0.25Pb0.25Teþ3mol%Ag,(3)Ge0.5Sn0.25Pb0.25Te, (4)Ge0.6Sn0.1Pb0.3Te,and(5)Ge0.5Sn0.25Pb0.25Teþ3mol%Bi2Te3. Figure5.Temperature dependence of the electrical conductivity for the various prepared alloys.Notations are according to Figure4.Figure6.Temperature dependence of the thermal conductivity for the various prepared alloys.Notations are according to Figure4.Article Chem.Mater.,Vol.22,No.3,20101057Ag doping,as compared to the undoped condition,is apparent.This evidence indicates an Ag-acceptor behavior in the investigated alloys,leading to an increased carrier concentration.For Bi 2Te 3doping (curve 5in Figures 4-6),the opposite trend is observed,showing a donor behavior in the investigated alloys and leading to a decreased carrier concentration as compared to the undoped sample (curve 3).These trends show the potential of Bi and Ag to serve as active impurities (donors and acceptors,respectively)in Ge 0.5Sn 0.25Pb 0.25Te for tuning the optimal carrier concen-tration for thermoelectric applications.By comparing of these curves to the GeTe curve (curve 1in Figures 4-6),we observe that all the Ge 0.5Sn 0.25Pb 0.25Te curves (2,3,and 5)yield higher R and lower σand κvalues,as compared to the GeTe (curve 1),indicating a reduced carrier concentration at all the doping levels in Ge 0.5Sn 0.25Pb 0.25Te,as compared to pure GeTe.Pure GeTe shows a very high holes concen-tration of p ≈6Â1026m -3.It is well-known that for thermoelectric applications,much lower carrier concentra-tions are needed for optimal performance.This stands behind Bi doping of GeTe (p ≈2Â1026m -3),for increasing the thermoelectric performance.6Therefore,the reduced carrier concentrations obtained by using Ge 0.5Sn 0.25-Pb 0.25Te (in all of the investigated doping levels,curves 2,3,and 5)as compared to GeTe (curve 1),is desirable.Variation of the alloying of Ge x (Sn y Pb 1-y )1-x Te from undoped Ge 0.5Sn 0.25Pb 0.25Te (curve 3)to undoped Ge 0.6Sn 0.1Pb 0.3Te (curve 4)does not result in any dramatic changes of the investigated transport properties.This beha-vior can be attributed to the carrier compensation obtained by increasing the GeTe amount (from 50to 60at %)and decreasing the SnTe amount (from 25to 10at %),both p -type compounds with very high hole concentrations.The temperature dependence of the lattice thermal con-ductivity,κL ,for GeTe,Ge 0.5Sn 0.25Pb 0.25Te (undoped and 3%Bi 2Te 3doped),and Ge 0.6Sn 0.1Pb 0.3Te is presented in Figure 7.κL values were calculated as followed:1.Fermi Energy Calculation.Fermi energy (η)was calculated at low temperatures (<250°C),for assuring an extrinsic region,using eq 3and the measured Seebeck coefficient values (Figure 4)R ¼k e 52þr ÀÁF 3=2þr 32þr F1=2þr -η264375ð3ÞWhere,e ,k ,F r ,η,and r are the electron charge,Boltz-mann constant,Fermi integral,reduced Fermi energy(equal to [E F -E V ]/kT for p -type materials),and the scattering mechanism parameter (equal to -0.5for the case of scattering by acoustic phonons as the dominant scattering mechanism,as is the case of GeTe).Equation 3is valid for a quadratic dependence of the carrier energy on the crystal momentum and a power dependence of the mean free time between electron colli-sions,τon the kinetic energy ξ(τ=τo ξr ,where τo is the constant),as for the case of GeTe-based alloys.The heavy hole valence band,present in GeTe,was not taken into account,on account of its minor influence at low temperatures up to very high carrier concentrations of ∼7Â1026m -3.62.Electronic Thermal Conductivity Calculation.The electronic contribution to the thermal conductivity,κe ,was calculated using the Wiedemann -Franz relation (eq 4)and the measured electrical conductivity values (Figure 4).K e ¼L σTð4ÞWhere,L and T are Lorenz number and absolute tem-perature,respectively.Lorenz number,given by eq 5,was calculated using the Fermi energy (η)values obtained in section 1.L ¼k e2r þ72ÀÁr þ32F r þ5=2ðηÞF r þ1=2ðηÞ-r þ52ÀÁ2F 2r þ3=2ðηÞr þ32 F 2r þ1=2ðηÞ264375ð5Þttice Thermal Conductivity Calculation.Finally,the lattice contribution to thermal conductivity,κL ,was calculated by subtraction of k e from the meas-ured total thermal conductivity,κ,(Figure 6),accord-ing to eq 2.It is clear from the calculated lattice thermal conduc-tivity values of Figure 7that alloying of GeTe with both PbTe and SnTe to obtain nanostructured Ge 0.5Sn 0.25-Pb 0.25Te or Ge 0.6Sn 0.1Pb 0.3Te,which follow a spinodal decomposition reaction,results in a major reduction (∼50%)of the lattice thermal conductivity.The room temperature κL values for Ge 0.5Sn 0.25Pb 0.25Te or Ge 0.6Sn 0.1Pb 0.3Te are about 0.8W/mK,as compared to a value of ∼1.6W/mK that was found for pure GeTe.This reduction can be attributed to phonon scattering by the nano-size ordered modulations of the spinodal de-composition in Ge x (Sn y Pb 1-y )1-x Te.It can be also seen from Figure 6that Bi 2Te 3doping of Ge 0.5Sn 0.25Pb 0.25Te results in a further reduction of the lattice thermal con-ductivity (to a room temperature value of ∼0.55W/mK),which may be attributed to additional nano Bi precipita-tion in this alloy.The temperature dependence of the dimensionless thermoelectric figure of merit,ZT ,for the investigated alloys is presented in Figure 8.All the investigated alloysFigure 7.Temperature dependence of the lattice thermal conductivity for GeTe,Ge 0.5Sn 0.25Pb 0.25Te (undoped and 3%Bi 2Te 3-doped),and Ge 0.6-Sn 0.1Pb 0.3Te.1058Chem.Mater.,Vol.22,No.3,2010Gelbstein et al.display higher ZT values over most of the investigated temperatures as compared to pure GeTe.The highest ZT values were obtained for undoped Ge 0.5Sn 0.25Pb 0.25Te with a maximal value of ∼1.2at 450°C,underlining the high potential of this composition as a p -type leg in practical thermoelectric applications.ConclusionsGe 0.5Sn 0.25Pb 0.25Te (undoped,3mol %Ag-doped,and 3mol %Bi 2Te 3-doped)and Ge 0.6Sn 0.1Pb 0.3Te alloys,following a spinodal decomposition reaction,were pre-pared using spark plasma sintering,characterized by electronic microscopy and transport properties measure-ments,and compared to a reference GeTe sample.The spinodal decomposition of the matrix to both GeTe-and PbTe-rich phases was observed as periodic compositional modulations both in the micro-and nano-(down to 10nm)scales.The lattice thermal conductivity was calculated by reduction of the electronic thermal conductivity (calculated by Fermi -Dirac statistics,taking into account variations of Lorenz number with Fermi energy)from the measured total thermal conductivity.Major reduction (∼50%)of the lattice thermal conductivity was observed in undoped Ge 0.5Sn 0.25Pb 0.25Te and Ge 0.6Sn 0.1Pb 0.3Te alloys.Doping optimizations of Ge 0.5Sn 0.25Pb 0.25Te by Bi 2Te 3revealed a further reduction of the lattice thermal conductivity due to nanosized scattered Bi precipitates.Bi was found to serve as a donor,whereas Ag was found to serve as an acceptor in these alloys.Very high ZT values were obtained for un-doped Ge 0.5Sn 0.25Pb 0.25Te with a maximal value of ∼1.2at 450°C,indicating the high potential of this composition to serve as a p -type legs in practical thermoelectric applica-tions.Figure 8.Temperature dependence of the dimensionless thermoelectric figure of merit for the various prepared alloys.Notations are according to Figure4.。