GeTe Bi Te Composition - Hindawi Publishing Corporation(GeTe) 0.95 (Bi 2 Te 3)0.05 was prepared by...

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 Iñigo 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 © 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 𝑝-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 ∼1.6 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 𝑝-type legs in thermoelectric applications for the 50–500∘C temperature range, exhibiting high dimensionlessthermoelectric figure of merit (𝑍𝑇 = (𝛼2𝑇)/(𝜌𝜅), where 𝛼is the Seebeck coefficient, 𝜌 is the electrical resistivity, 𝜅 isthermal conductivity, and 𝑇 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 at427∘C 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 𝑝-type conductiv-ity (𝑝 ∼1020–1021 cm−3) [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

2 Journal of Nanomaterials

for obtaining quasi-binary (GeTe)𝑥(PbTe)

1−𝑥alloys 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 𝑝-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 10−5 Torr and meltingin a rocking furnace at 850∘C for 1 hour followed by waterquenching. Various milling conditions of the cast ingot toa maximal powder particle size of 50 and 250 𝜇m, 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 𝜇m maximal sieved powder were subsequentlyhot pressed (HPW5 Hot Press FCT Systeme GmbH) undera mechanical pressure of 35MPa at 450∘C and 650∘C for 30minutes, resulting in high density values of ∼95 and ∼97% ofthe theoretical density, respectively.

Seebeck coefficient (𝛼) and the electrical resistivity (𝜌)measurements were carried out in a self-constructed appa-ratus under an argon atmosphere up to ∼450∘C at a heatingrate of 3∘C/min. For the Seebeck coefficient measurements,an auxiliary heater was used tomaintain a temperature differ-ence of 10∘Cbetween the extremities of the samples. Electricalresistivity wasmeasured by the “four-probe”method using analternating power source of 1 V/50Hz.

The thermal conductivity (𝜅) was determined as a func-tion of temperature from room temperature to 500∘C usingthe flash diffusivitymethod (LFA457,Netzsch).The front faceof a disc-shaped sample (𝜙 = 12mm, thickness ≈ 1-2mm) isirradiated by a short laser burst, and the resulting rear facetemperature is recorded and analyzed. Thermal conductivityvalues were calculated using the equation 𝜅 = 𝛾 ⋅𝐶

𝑃⋅ 𝛿, where

𝛾 is the thermal diffusivity, 𝐶𝑃is the specific heat (measured

using differential scanning calorimetry, STA 449, Netzsch),and 𝛿 is the bulk density of the sample (calculated fromthe sample’s geometry and its mass). The room temperature𝐶𝑃value for all of the investigated synthesis conditions was

found to be equal to 0.23 ± 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, 𝑑, of the apparent submicron crys-talline domains was obtained by applying Scherrer equation,

𝑑 = (𝐾𝜆)/(𝜔 cos 𝜃), where 𝜆 represents the wavelength ofthe X-ray radiation, 𝜃 is the angle of the considered Braggreflection, 𝜔 is the peaks width on a 2𝜃 scale, and 𝐾 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 50𝜇m is shown in Figures1(a)–1(c).

It can be easily seen from these figures that the 50𝜇mpowder exhibits large microscale agglomerates composed ofless than 5 𝜇m 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 𝜇m, 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 cycles’ process.

Room temperature XRD analysis following the variousapplied synthesis conditions, including the high temperaturemelting (850∘C), and hot pressing (450 and 650∘C) condi-tions, obtained at higher temperatures than the rhombohe-dral to cubic phase transition (∼430∘C 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 𝜇mhand 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 𝜇m, relatively narrow peakswere observed, whichwere analyzed to correspond to averagegrain size of 40–60 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 20–25 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 𝜇m sieving, in which a higher level of crystallinitywas preserved following the powdering action.

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 𝜇mpowder, 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

2𝜃

Inte

nsity

(a.u

.)

250𝜇m

50𝜇m

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 250𝜇mand 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 650∘C were investigatedin the current research, yielding density values of ∼95 and∼97% of the theoretical density, respectively. The 650∘C(923K) temperature, which is about 90% of the meltingtemperature, 𝑇

𝑚, 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 (450∘C) was chosen to be inthe vicinity of the phase transition temperature (∼430∘Cin pure GeTe, [6]) from a low temperature rhombohedral

to a high temperature cubic structures. Both of these hotpressing temperatures, 650 and 450∘C,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 (∼585∘C [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,𝛼, electrical resistivity, 𝜌, and thermal conductivity, 𝜅, ofthe investigated (GeTe)

0.95(Bi2Te3)0.05

composition followinghand crushing and sieving to maximal powder agglomeratesof 50 and 250𝜇m and hot pressing at 450 and 650∘C areshown in Figures 3, 4, and 5, respectively.

Investigation of Figures 3 and 4 reveals that following650∘C hot pressing of both the 50 and 250𝜇m sievedpowder agglomerates, nearly identical low temperature 𝛼and 𝜌 values, with slightly higher values for the 50𝜇msample, were observed, indicating similar associated carrierconcentrations for both of the samples with slightly lowervalues for the 50𝜇m sample. This slightly lower overallcarrier concentration for the 50𝜇m 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

𝑥Sn1−𝑥

Te [9], Ge𝑥Pb1−𝑥

Te [10], and Bi2Te3[11]),

mechanical stresses can result in introduction of additionaldonor levels in the band gaps, and for the case of inherent𝑝-type compositions, in reduction of the overall carrierconcentration.

Although the general high similarity between the 𝛼 and 𝜌values obtained following 650∘C hot pressing for both of theinvestigated powder particle sizes, similar low temperature


220

180

160

140

120

100

80

60

40

0 100 200 300 400 500

GeTe (5% Bi2Te3)

250𝜇m/

450∘ C

250𝜇m/

650∘ C50𝜇

m/650∘ C200

Temperature T (∘C)

Seeb

eck

coeffi

cien

t𝛼(𝜇

V/K

)

Figure 3: Temperature dependence of Seebeck coefficient, 𝛼, for(GeTe)

0.95(Bi2Te3)0.05

following hand crushing for maximal pow-der particles size of 50 and 250 𝜇m and hot pressing at 650∘C(50 𝜇m/650∘C and 250 𝜇m/650∘C, resp.) and for 250 𝜇mhot pressedpowder at 450∘C (250𝜇m/450∘C).

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)

250𝜇

m/450∘ C

50𝜇m

/650∘ C

250𝜇

m/650∘ C


Elec

tric

al re

sistiv

ity𝜌

(mΩ

cm)

Figure 4: Temperature dependence of the electrical resistivity,𝜌, for (GeTe)

0.95(Bi2Te3)0.05

following hand crushing for maximalpowder particles size of 50 and 250 𝜇m and hot pressing at 650∘C(50 𝜇m/650∘C and 250 𝜇m/650∘C, resp.) and for 250 𝜇mhot pressedpowder at 450∘C (250𝜇m/450∘C).

Seebeck coefficient values accompanied by much higherlow temperature electrical resistivity values were obtainedfollowing hot pressing at 450∘C of maximal 250 𝜇m sievedpowder particles, as shown in Figures 3 and 4. This evidencecan be attributed to carrier concentration, 𝑝, values similar tothose obtained following the 650∘C hot pressing, as indicatedby the similar𝛼 values, but lower carriermobility,𝜇, values, asindicated by the higher 𝜌 values (𝜌 = (𝑝 ⋅ 𝑒 ⋅ 𝜇)−1) 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)

250𝜇m/450 ∘C

50𝜇m/650 ∘C

250𝜇m/650 ∘C

0


Ther

mal

cond

uctiv

ity𝜅

(W/m

K)

GeTe (e 5% Bi2TeT 3)

25500𝜇𝜇m/m/450 ∘CC

5500𝜇m/m/65500 ∘∘CC

25500𝜇𝜇m/m 665500 ∘CC

Figure 5: Temperature dependence of the thermal conductivity,𝜅, for (GeTe)

0.95(Bi2Te3)0.05

following hand crushing for maximalpowder particles size of 50 and 250 𝜇m and hot pressing at 650∘C(50 𝜇m/650∘C and 250 𝜇m/650∘C, resp.) and for 250 𝜇mhot pressedpowder at 450∘C (250 𝜇m/450∘C).

of the 650∘C hot pressing, and can be associated with lowerdiffusion rates of the involved elements.

Furthermore, both of the 𝛼 and 𝜌 temperature depen-dencies of the 450∘C hot pressed sample were totally differ-ent from those obtained following the 650∘C hot pressingprocedures. Following the 450∘C hot pressing conditions, athermal hysteresis was observed between the heating andcooling cycles while measuring both 𝛼 and 𝜌 values, with themost pronounced differences in the 100–400∘C temperaturerange. This abnormal 𝛼 and 𝜌 temperature dependent trendcan result from the second order phase transition, associatedwith GeTe based compounds, as was previously reported forsingle crystal grown Ge

𝑥Pb1−𝑥

Te and Ge𝑥Sn1−𝑥

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)

𝑥(Bi2Te3)1−𝑥

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 𝑥 values. As a result, a sequence of multiple(GeTe)

𝑥(Bi2Te3)1−𝑥

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 both𝛼 and 𝜌 values is expected. Such an effect can explain theabnormal trend observed in both 𝛼 and 𝜌 values following450∘C hot pressing. For the 650∘C (∼0.9 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 650∘C hot pressing conditions, lower


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)

250𝜇m/450 ∘C

50𝜇m/650 ∘C

250𝜇m/650 ∘C


𝜅L

(W/m

K)

Figure 6: Temperature dependence of the lattice thermal con-ductivity, 𝜅

𝐿, for (GeTe)

0.95(Bi2Te3)0.05

following hand crushing formaximal powder particles size of 50 and 250 𝜇m and hot pressing at650∘C (50𝜇m/650∘C and 250 𝜇m/650∘C, resp.) and for 250 𝜇m hotpressed powder at 450∘C (250𝜇m/450∘C).

values were obtained following pressing of the maximal50𝜇m powder agglomerates compared to the 250 𝜇m con-dition. Some reduction of the thermal conductivity of the50𝜇m powder is expected due to the slightly higher electri-cal resistivity values (Figure 4) and the consequently lowerelectronic thermal conductivity, 𝜅

𝑒, values, for this powder

condition, as evidenced from theWiedemann-Franz relation,𝜅𝑒= (𝐿/𝜌)𝑇, where 𝐿 and 𝑇 are Lorenz number and absolute

temperature, respectively. Yet, in order to explain the large(over than 20%) reduction of the thermal conductivityvalues of the “50 𝜇m” compared to the “250𝜇m” sample, thelattice contribution to the thermal conductivity, 𝜅

𝐿, was also

calculated for all of the investigated conditions as presentedin Figure 6. For this calculation the reduced Fermi energy(𝜂) was initially calculated at low temperatures (


GeTe (5% Bi2Te3)

250𝜇m/

450∘ C5

0𝜇m/650∘ C

250𝜇

m/650∘ 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


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 250𝜇m andhot pressing at 650∘C (50 𝜇m/650∘C and 250 𝜇m/650∘C, resp.) andfor 250 𝜇m hot pressed powder at 450∘C (250𝜇m/450∘C).

much higher ZT values, compared to the nonhomogenized450∘C hot pressing condition, were obtained, exhibiting veryhigh maximal values of ∼1.6, which are among the highestso far reported for any 𝑝-type thermoelectric material. Nopractical differences were observed between the “50𝜇m” and“250𝜇m” conditions following 650∘C hot pressing, probablydue to a compensation of the slightly non-optimized carrierconcentration, obtained for the “50𝜇m” 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 (


[9] Y. Gelbstein, Z. Dashevsky, andM. P. Dariel, “Powder metallur-gical processing of functionally graded p-Pb

1−𝑥Sn𝑥Te materials

for thermoelectric applications,” Physica B, vol. 391, no. 2, pp.256–265, 2007.

[10] Y. Gelbstein, Z. Dashevsky, and M. P. Dariel, “Highly efficientbismuth telluride doped p-type Pb

0.13Ge0.87

Te for thermoelec-tric applications,” Physica Status Solidi, vol. 1, no. 6, pp. 232–234,2007.

[11] N. Bomshtein, G. Spiridonov, Z. Dashevsky, and Y. Gelb-stien, “Thermoelectric, structural, andmechanical properties ofspark-plasma-sintered submicro- and microstructured p-TypeBi0.5Sb1.5Te3,” Journal of Electronic Materials, vol. 41, no. 6, pp.

1546–1553, 2012.[12] Y. Gelbstein, O. Ben-Yehuda, Z. Dashevsky, and M. P. Dariel,

“Phase transitions of p-type (Pb,Sn,Ge)Te-based alloys forthermoelectric applications,” Journal of Crystal Growth, vol. 311,no. 18, pp. 4289–4292, 2009.

[13] Y. Gelbstein, O. Ben-Yehuda, E. Pinhas et al., “Thermoelectricproperties of (Pb,Sn,Ge)te-based alloys,” Journal of ElectronicMaterials, vol. 38, no. 7, pp. 1478–1482, 2009.

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