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Research ArticleInvestigation of the Microstructural and ThermoelectricProperties of the (GeTe)0.95(Bi2Te3)0.05 Compositionfor Thermoelectric Power Generation Applications
Lior Weintraub,1 Joseph Davidow,2 Jonathan Tunbridge,3 Richard Dixon,3
Michael John Reece,4 Huanpo Ning,4 Iรฑigo Agote,5 and Yaniv Gelbstein1,2
1 Unit of Energy Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel2 Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel3 Intrinsiq Materials Ltd., Cody Technology Park, Farnborough GU14OLX, UK4 School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK5 TECNALIA Research & Innovation, Mikeletegi Pasealekua, 2 20009 San Sebastian, Spain
Correspondence should be addressed to Yaniv Gelbstein; [email protected]
Received 12 June 2013; Accepted 5 December 2013; Published 19 January 2014
Academic Editor: Won-Seon Seo
Copyright ยฉ 2014 Lior Weintraub et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In the frame of the current research, the ๐-type Bi2Te3doped (GeTe)
0.95(Bi2Te3)0.05
alloy composed of hot pressed consolidatedsubmicron structured powder was investigated.The influence of the process parameters (i.e., powder particles size and hot pressingconditions) on both reduction of the lattice thermal conductivity and electronic optimization is described in detail. Very highmaximal ZT values of up to โผ1.6 were obtained and correlated to the microstructural characteristics. Based on the various involvedmechanisms, a potential route for further enhancement of the ZT values of the investigated composition is proposed.
1. Introduction
Thermoelectrics as a direct energy conversion methodbetween heat and electricity is mainly used for electricalpower generation and cooling applications. Germaniumtelluride based alloys are known as appropriate candidatesfor ๐-type legs in thermoelectric applications for the 50โ500โC temperature range, exhibiting high dimensionlessthermoelectric figure of merit (๐๐ = (๐ผ2๐)/(๐๐ ), where ๐ผis the Seebeck coefficient, ๐ is the electrical resistivity, ๐ isthermal conductivity, and ๐ is absolute temperature) values.GeTe based compounds are characterized by a continuousphase transition from a low temperature rhombohedral to ahigh temperature cubic rock salt structure that takes place at427โC in pure GeTe [1].
Germanium telluride based alloys are characterizedby a large deviation from stoichiometry toward telluriumrich compositions. The main nonstoichiometric defects are
doubly ionized metal vacancies. As a result of this deviationfrom stoichiometry, GeTe always exhibits ๐-type conductiv-ity (๐ โผ1020โ1021 cmโ3) [2]. To reduce the hole concentration,in order to obtain optimal thermoelectric properties, itis necessary to dope GeTe with donor type electroactiveimpurities. Bi
2Te3[3] and PbTe [4] act as donors while
dissolved in GeTe. A mechanism of doping GeTe by Bi2Te3
has been put forward by Gelbstein et al. [2], accordingto which the latter is implanted into the GeTe lattice inthe form of complexes consisting of a single electroneutralcation vacancy per each Bi
2Te3molecule implanted. The
introduction of Bi2Te3into the GeTe lattice changes the
cation-anion ratio and the vacancy formation is due to therequirement for charge equilibrium.The reduction of the holeconcentration in GeTe after introducing Bi
2Te3is associated
with the filling of the cation vacancies by germanium atomspresent in GeTe as a second phase. High thermoelectricperformance was recently reported by PbTe alloying of GeTe
Hindawi Publishing CorporationJournal of NanomaterialsVolume 2014, Article ID 284634, 7 pageshttp://dx.doi.org/10.1155/2014/284634
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2 Journal of Nanomaterials
for obtaining quasi-binary (GeTe)๐ฅ(PbTe)
1โ๐ฅalloys that can
be designed for achieving improved electronic properties andlower lattice thermal conductivities, as compared to PbTe andGeTe, separately, via phase separation reactions [5].
The present communication is concerned with the pos-sibility of increasing the thermoelectric figure of merit of ๐-type Bi
2Te3doped (GeTe)
0.95(Bi2Te3)0.05
alloy, by optimizingvarious powder metallurgy parameters (i.e., powder millingand hot pressing conditions) for obtaining both submicrofea-tures, resulting in reduced lattice thermal conductivity values,and optimal electronic doping. The combined effects on theresultant ZT values are discussed in detail.
2. Experimental
(GeTe)0.95
(Bi2Te3)0.05
was prepared by sealing the sourcematerials (purity of 5N) at appropriate concentrations in aquartz ampoule under a vacuum of 10โ5 Torr and meltingin a rocking furnace at 850โC for 1 hour followed by waterquenching. Various milling conditions of the cast ingot toa maximal powder particle size of 50 and 250 ๐m, usingagate mortar and pestle, and to smaller sized powder, usingenergetic ball milling (Fritsch-Pulverisette 7) at 700 RPM fol-lowing 10 and 20 cycles of 10 minutes each, were applied. The50 and 250 ๐m maximal sieved powder were subsequentlyhot pressed (HPW5 Hot Press FCT Systeme GmbH) undera mechanical pressure of 35MPa at 450โC and 650โC for 30minutes, resulting in high density values of โผ95 and โผ97% ofthe theoretical density, respectively.
Seebeck coefficient (๐ผ) and the electrical resistivity (๐)measurements were carried out in a self-constructed appa-ratus under an argon atmosphere up to โผ450โC at a heatingrate of 3โC/min. For the Seebeck coefficient measurements,an auxiliary heater was used tomaintain a temperature differ-ence of 10โCbetween the extremities of the samples. Electricalresistivity wasmeasured by the โfour-probeโmethod using analternating power source of 1 V/50Hz.
The thermal conductivity (๐ ) was determined as a func-tion of temperature from room temperature to 500โC usingthe flash diffusivitymethod (LFA457,Netzsch).The front faceof a disc-shaped sample (๐ = 12mm, thickness โ 1-2mm) isirradiated by a short laser burst, and the resulting rear facetemperature is recorded and analyzed. Thermal conductivityvalues were calculated using the equation ๐ = ๐พ โ ๐ถ
๐โ ๐ฟ, where
๐พ is the thermal diffusivity, ๐ถ๐is the specific heat (measured
using differential scanning calorimetry, STA 449, Netzsch),and ๐ฟ is the bulk density of the sample (calculated fromthe sampleโs geometry and its mass). The room temperature๐ถ๐value for all of the investigated synthesis conditions was
found to be equal to 0.23 ยฑ 0.01 J/grK.The structural characteristics of the various investigated
powder conditions were analyzed by high resolution scan-ning electron microscopy (Jeol-7400F HRSEM), transmis-sion electron microscopy (TEM-FEI TECNAI-G2), and X-ray diffraction (Rigaku DMAX 2100 powder diffractome-ter). The mean size, ๐, of the apparent submicron crys-talline domains was obtained by applying Scherrer equation,
๐ = (๐พ๐)/(๐ cos ๐), where ๐ represents the wavelength ofthe X-ray radiation, ๐ is the angle of the considered Braggreflection, ๐ is the peaks width on a 2๐ scale, and ๐พ is aconstant close to unity, to the experimental X-Ray diffractionpeaks.
3. Results and Discussion
3.1. Powder Structural Characteristics. Themicrostructure ofthe (GeTe)
0.95(Bi2Te3)0.05
powder hand crushed and sieved toa maximal powder particle size of 50๐m is shown in Figures1(a)โ1(c).
It can be easily seen from these figures that the 50๐mpowder exhibits large microscale agglomerates composed ofless than 5 ๐m powder domains, Figures 1(a) and 1(b), whichare composed of even finer particles of less than 300 nmparticles, as was observed by TEM analysis (Figure 1(c)). Avery similarmicrostructure was also observed for the powderobtained following hand crushing and sieving to a maximalpowder particle size of 250 ๐m, indicating that the basicminimal powder particles are much smaller in size thanthe actually sieved microscale powder agglomerates for thisspecific composition. Following 700 RPM, 10 and 20 cyclesof 10 minutes each, even finer particles than 100 nm wereidentified, as indicated in Figure 1(d). No finer details couldbe distinguished by the SEM for the 20-cycle ball millingcompared to the 10 cyclesโ process.
Room temperature XRD analysis following the variousapplied synthesis conditions, including the high temperaturemelting (850โC), and hot pressing (450 and 650โC) condi-tions, obtained at higher temperatures than the rhombohe-dral to cubic phase transition (โผ430โC for pure GeTe), exhib-ited the typical low temperature rhombohedral structure,being as expected in agreement with the equilibrium phasediagram.
Typical XRD patterns of the 50 and 250 ๐mhand crushedand sieved powder and ball milled at 700 RPM for 10 and 20cycles of 10 minutes each are shown in Figure 2. The figureillustrates a typical doublet, characteristic of the low temper-ature rhombohedral phase ofGeTe,which exhibits broadenedwidths with increasing the applied mechanical forces duringthe milling process. For the larger agglomerate powder, handcrushed and sieved to 250 and 50 ๐m, relatively narrow peakswere observed, whichwere analyzed to correspond to averagegrain size of 40โ60 nm, in agreement with the finest grains ofless than 300 nm observed by TEM (Figure 1(c)). Increasingthe applied mechanical forces during the 700 RPM energeticball milling procedure resulted in much broadened peaks,corresponding to finer average grain sizes of 20โ25 nm. Yet,due to the fact that these large peaks have broadening effect,evidenced in some amorphization of the originally synthe-sized crystalline GeTe phase, and therefore are expected todegrade the electronic transport properties of the investigated(GeTe)
0.95(Bi2Te3)0.05
composition, it was decided to focusin the next stages of the research on investigation of thetransport properties of the larger domains powder following50 and 250 ๐m sieving, in which a higher level of crystallinitywas preserved following the powdering action.
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Journal of Nanomaterials 3
(a)
(b)
(c)
(d)
Figure 1: SEM (a), higher magnification SEM (b), TEM (c) images of (GeTe)0.95
(Bi2Te3)0.05
hand crushed and sieved to maximal sized 50 ๐mpowder, and HRSEM image of similar matrix composition following 10-cycles ball milling ball at 700 RPM/10min (d).
160
120
80
40
040 41 42 43 44 45
2๐
Inte
nsity
(a.u
.)
250๐m
50๐m
700/10
700/20
Figure 2: Powder XRD of (GeTe)0.95
(Bi2Te3)0.05
following handcrushing for maximal powder particles size of 50 and 250๐mand following 10 and 20-cycle ball milling of 10 minutes at700 RPM/10min.
3.2. Transport Properties followingHot Pressing Consolidation.As was described above in the experimental section, two hotpressing temperatures of 450 and 650โC were investigatedin the current research, yielding density values of โผ95 andโผ97% of the theoretical density, respectively. The 650โC(923K) temperature, which is about 90% of the meltingtemperature, ๐
๐, of pure GeTe compound (998K [6]), was
chosen due to the associated high diffusion rates of theinvolved elements at this temperature and the expected highhomogeneity of the investigated composition. The lowerhot pressing temperature (450โC) was chosen to be inthe vicinity of the phase transition temperature (โผ430โCin pure GeTe, [6]) from a low temperature rhombohedral
to a high temperature cubic structures. Both of these hotpressing temperatures, 650 and 450โC,were chosen as slightlylower than the melting temperature and the phase transitiontemperatures of pure GeTe, respectively, due to the under-standing that the investigated (GeTe)
0.95(Bi2Te3)0.05
matrixcomposition, obtained by 5% alloying with the low meltingtemperature Bi
2Te3phase (โผ585โC [7]), which is stable from
low temperatures up to its melting, is expected to exhibitslightly lowermelting temperature and lower phase transitiontemperatures, respectively, compared to pure GeTe.
The temperature dependencies of Seebeck coefficient,๐ผ, electrical resistivity, ๐, and thermal conductivity, ๐ , ofthe investigated (GeTe)
0.95(Bi2Te3)0.05
composition followinghand crushing and sieving to maximal powder agglomeratesof 50 and 250๐m and hot pressing at 450 and 650โC areshown in Figures 3, 4, and 5, respectively.
Investigation of Figures 3 and 4 reveals that following650โC hot pressing of both the 50 and 250๐m sievedpowder agglomerates, nearly identical low temperature ๐ผand ๐ values, with slightly higher values for the 50๐msample, were observed, indicating similar associated carrierconcentrations for both of the samples with slightly lowervalues for the 50๐m sample. This slightly lower overallcarrier concentration for the 50๐m sample, in which slightlyhigher mechanical force was applied while crushing thesample to this smaller powder particles size sample, can beattributed to the strain induced compensation effect, inwhichas was already established formany telluride compounds (e.g.PbTe [8], Pb
๐ฅSn1โ๐ฅ
Te [9], Ge๐ฅPb1โ๐ฅ
Te [10], and Bi2Te3[11]),
mechanical stresses can result in introduction of additionaldonor levels in the band gaps, and for the case of inherent๐-type compositions, in reduction of the overall carrierconcentration.
Although the general high similarity between the ๐ผ and ๐values obtained following 650โC hot pressing for both of theinvestigated powder particle sizes, similar low temperature
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4 Journal of Nanomaterials
220
180
160
140
120
100
80
60
40
0 100 200 300 400 500
GeTe (5% Bi2Te3)
250๐m/
450โ C
250๐m/
650โ C50๐
m/650โ C200
Temperature T (โC)
Seeb
eck
coeffi
cien
t๐ผ(๐
V/K
)
Figure 3: Temperature dependence of Seebeck coefficient, ๐ผ, for(GeTe)
0.95(Bi2Te3)0.05
following hand crushing for maximal pow-der particles size of 50 and 250 ๐m and hot pressing at 650โC(50 ๐m/650โC and 250 ๐m/650โC, resp.) and for 250 ๐mhot pressedpowder at 450โC (250๐m/450โC).
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 100 200 300 400 500
GeTe (5% Bi2Te3)
250๐
m/450โ C
50๐m
/650โ C
250๐
m/650โ C
Temperature T (โC)
Elec
tric
al re
sistiv
ity๐
(mฮฉ
cm)
Figure 4: Temperature dependence of the electrical resistivity,๐, for (GeTe)
0.95(Bi2Te3)0.05
following hand crushing for maximalpowder particles size of 50 and 250 ๐m and hot pressing at 650โC(50 ๐m/650โC and 250 ๐m/650โC, resp.) and for 250 ๐mhot pressedpowder at 450โC (250๐m/450โC).
Seebeck coefficient values accompanied by much higherlow temperature electrical resistivity values were obtainedfollowing hot pressing at 450โC of maximal 250 ๐m sievedpowder particles, as shown in Figures 3 and 4. This evidencecan be attributed to carrier concentration, ๐, values similar tothose obtained following the 650โC hot pressing, as indicatedby the similar๐ผ values, but lower carriermobility,๐, values, asindicated by the higher ๐ values (๐ = (๐ โ ๐ โ ๐)โ1) which canresult from a lower homogeneity state of this sample, whichwas hot pressed at much lower temperature than for the case
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
100 200 300 400 500
GeTe (5% Bi2Te3)
250๐m/450 โC
50๐m/650 โC
250๐m/650 โC
0
Temperature T (โC)
Ther
mal
cond
uctiv
ity๐
(W/m
K)
GeTe (e 5% Bi2TeT 3)
25500๐๐m/m/450 โCC
5500๐m/m/65500 โโCC
25500๐๐m/m 665500 โCC
Figure 5: Temperature dependence of the thermal conductivity,๐ , for (GeTe)
0.95(Bi2Te3)0.05
following hand crushing for maximalpowder particles size of 50 and 250 ๐m and hot pressing at 650โC(50 ๐m/650โC and 250 ๐m/650โC, resp.) and for 250 ๐mhot pressedpowder at 450โC (250 ๐m/450โC).
of the 650โC hot pressing, and can be associated with lowerdiffusion rates of the involved elements.
Furthermore, both of the ๐ผ and ๐ temperature depen-dencies of the 450โC hot pressed sample were totally differ-ent from those obtained following the 650โC hot pressingprocedures. Following the 450โC hot pressing conditions, athermal hysteresis was observed between the heating andcooling cycles while measuring both ๐ผ and ๐ values, with themost pronounced differences in the 100โ400โC temperaturerange. This abnormal ๐ผ and ๐ temperature dependent trendcan result from the second order phase transition, associatedwith GeTe based compounds, as was previously reported forsingle crystal grown Ge
๐ฅPb1โ๐ฅ
Te and Ge๐ฅSn1โ๐ฅ
Te alloys [12].In case that as was explained referring to the higher lowtemperature electrical resistivity values, associatedwith lowercarrier mobility values, compositional inhomogeneity isapparent for this low temperature hot pressing process, com-positional variations in the investigated (GeTe)
๐ฅ(Bi2Te3)1โ๐ฅ
quasi-binary system, are expected to result is a decreasedphase transition temperature from the low temperaturerhombohedral to the high temperature cubic phases, withdecreasing of the ๐ฅ values. As a result, a sequence of multiple(GeTe)
๐ฅ(Bi2Te3)1โ๐ฅ
compositions is expected to result inextended temperature range inwhich in the vicinity of each ofthe individual phase transition temperatures, associated witheach of the apparent compositions, a continuous jump in both๐ผ and ๐ values is expected. Such an effect can explain theabnormal trend observed in both ๐ผ and ๐ values following450โC hot pressing. For the 650โC (โผ0.9 Tm) hot pressingtemperature, the much more homogenized sate, associatedwith the much higher diffusion coefficients of each of theinvolved components, suppresses this effect dramatically.
Regarding the thermal conductivity values (Figure 5)obtained following the 650โC hot pressing conditions, lower
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Journal of Nanomaterials 5
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
20 40 60 80 100 120 140 160 180 200
GeTe (5% Bi2Te3)
250๐m/450 โC
50๐m/650 โC
250๐m/650 โC
Temperature T (โC)
๐ L
(W/m
K)
Figure 6: Temperature dependence of the lattice thermal con-ductivity, ๐
๐ฟ, for (GeTe)
0.95(Bi2Te3)0.05
following hand crushing formaximal powder particles size of 50 and 250 ๐m and hot pressing at650โC (50๐m/650โC and 250 ๐m/650โC, resp.) and for 250 ๐m hotpressed powder at 450โC (250๐m/450โC).
values were obtained following pressing of the maximal50๐m powder agglomerates compared to the 250 ๐m con-dition. Some reduction of the thermal conductivity of the50๐m powder is expected due to the slightly higher electri-cal resistivity values (Figure 4) and the consequently lowerelectronic thermal conductivity, ๐
๐, values, for this powder
condition, as evidenced from theWiedemann-Franz relation,๐ ๐= (๐ฟ/๐)๐, where ๐ฟ and ๐ are Lorenz number and absolute
temperature, respectively. Yet, in order to explain the large(over than 20%) reduction of the thermal conductivityvalues of the โ50 ๐mโ compared to the โ250๐mโ sample, thelattice contribution to the thermal conductivity, ๐
๐ฟ, was also
calculated for all of the investigated conditions as presentedin Figure 6. For this calculation the reduced Fermi energy(๐) was initially calculated at low temperatures (
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6 Journal of Nanomaterials
GeTe (5% Bi2Te3)
250๐m/
450โ C5
0๐m/650โ C
250๐
m/650โ C
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 100 200 300 400 500
Figu
re o
f mer
itZT
Temperature T (โC)
Figure 7: Temperature dependence of the dimensionless thermo-electric figure of merit, ZT, for (GeTe)
0.95(Bi2Te3)0.05
following handcrushing for maximal powder particles size of 50 and 250๐m andhot pressing at 650โC (50 ๐m/650โC and 250 ๐m/650โC, resp.) andfor 250 ๐m hot pressed powder at 450โC (250๐m/450โC).
much higher ZT values, compared to the nonhomogenized450โC hot pressing condition, were obtained, exhibiting veryhigh maximal values of โผ1.6, which are among the highestso far reported for any ๐-type thermoelectric material. Nopractical differences were observed between the โ50๐mโ andโ250๐mโ conditions following 650โC hot pressing, probablydue to a compensation of the slightly non-optimized carrierconcentration, obtained for the โ50๐mโ powder due to thepreviously explained strain induced compensation effect, bythe lower lattice conductivity values of this sample, whichwere associated with the higher population of low (
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Journal of Nanomaterials 7
[9] Y. Gelbstein, Z. Dashevsky, andM. P. Dariel, โPowder metallur-gical processing of functionally graded p-Pb
1โ๐ฅSn๐ฅTe materials
for thermoelectric applications,โ Physica B, vol. 391, no. 2, pp.256โ265, 2007.
[10] Y. Gelbstein, Z. Dashevsky, and M. P. Dariel, โHighly efficientbismuth telluride doped p-type Pb
0.13Ge0.87
Te for thermoelec-tric applications,โ Physica Status Solidi, vol. 1, no. 6, pp. 232โ234,2007.
[11] N. Bomshtein, G. Spiridonov, Z. Dashevsky, and Y. Gelb-stien, โThermoelectric, structural, andmechanical properties ofspark-plasma-sintered submicro- and microstructured p-TypeBi0.5Sb1.5Te3,โ Journal of Electronic Materials, vol. 41, no. 6, pp.
1546โ1553, 2012.[12] Y. Gelbstein, O. Ben-Yehuda, Z. Dashevsky, and M. P. Dariel,
โPhase transitions of p-type (Pb,Sn,Ge)Te-based alloys forthermoelectric applications,โ Journal of Crystal Growth, vol. 311,no. 18, pp. 4289โ4292, 2009.
[13] Y. Gelbstein, O. Ben-Yehuda, E. Pinhas et al., โThermoelectricproperties of (Pb,Sn,Ge)te-based alloys,โ Journal of ElectronicMaterials, vol. 38, no. 7, pp. 1478โ1482, 2009.
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