Enhanced Thermoelectric Properties of La-Doped ZrNiSnHalf-Heusler Compound

RIZWAN AKRAM,1 QIANG ZHANG,1 DONGWANG YANG,1 YUN ZHENG,1

YONGGAO YAN,1,2 XIANLI SU,1 and XINFENG TANG1,3

1.—State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China. 2.—e-mail: yanyonggao@whut.edu.cn.3.—e-mail: tangxf@whut.edu.cn

The effect of La doping on ZrNiSn half-Heusler (HH) compound has beenstudied to explore the composition variation and structural modifications forimprovement of its thermoelectric performance. A series of LaxZr1�xNiSn(x = 0, 0.005, 0.01, 0.015, 0.02, 0.03) alloys were prepared by induction meltingcombined with plasma-activated sintering. Structural analysis using x-raydiffraction (XRD), scanning electron microscopy (SEM), and transmissionelectron microscopy (TEM) confirmed the resulting material to be a compositeof HH, NiZr, and La3Sn4-type phases. The volume fraction for the phasesother than HH ranged from 1.5% to 25% with increasing La content, as esti-mated by Rietveld analysis. The solubility of La in ZrNiSn is estimated to be1.5%. Point defects may play a significant role in carrier and phonon trans-port. Interestingly, the thermoelectric transport properties exhibited a con-siderable increase in electrical conductivity r with La doping and a significantdrop in thermal conductivity j, leading to a thermoelectric figure of merit (ZT)of 0.53 at 923 K, representing an improvement of about 37% compared withthe undoped sample.

Key words: Half-Heuslers, defects, composite materials

INTRODUCTION

Recent elevated interest in thermoelectric devicesbased on solid-state physics principles has beenmotivated by their significant advantages, such aslack of moving parts, no emissions, miniature size,and stability.1,2 The conversion efficiency of a ther-moelectric device is mainly determined by the per-formance of the material, which is expressed interms of a dimensionless figure of merit by therelation ZT = ra2T/j, where a, r, T, and j are theSeebeck coefficient, electrical conductivity, absolutetemperature, and thermal conductivity, respec-tively. High power factor (ra2) and low thermalconductivity j are required to achieve high ZT val-ues. Half-Heusler (HH) alloys have been studied bymany researchers as potential candidates for ther-moelectric power generation because of their

promising thermoelectric performance and goodchemical stability, and the low toxicity of theirconstituent elements.3,4 The band structures of HHalloys are strongly affected by the valence electroncount (VEC); For example, these compounds arenarrow-bandgap semiconductors when VEC = 8 or18, while deviation from VEC = 18 may lead tometallic conduction.5 The most intensively investi-gated HH alloys are MNiSn (M = Ti, Zr, Hf). MNiSnalloys are narrow-bandgap (0.1 eV to 0.5 eV) semi-conductors with good electrical properties, i.e., highpower factor.6–8 However, this system has intrinsichigh thermal conductivity, making it less favorablefor thermoelectric (TE) applications.

The thermoelectric performance of MNiSn alloyscan be optimized for practical TE power generationvia selective doping on the three sublattice sites;9

For example, doping of Sb or Bi on the Sn site cantune the carrier concentration.10,11 To reduce thelattice thermal conductivity, enhanced phononscattering at grain boundaries due to grain size

(Received February 12, 2015; accepted May 30, 2015;published online June 23, 2015)

Journal of ELECTRONIC MATERIALS, Vol. 44, No. 10, 2015

DOI: 10.1007/s11664-015-3882-6� 2015 The Minerals, Metals & Materials Society

3563

refinement and isoelectronic alloying on the M or Nisublattice causing additional mass or strain fluctu-ations have been successfully demonstrated.12–18

Mostly, the Zr site in ZrNiSn is substituted withisoelectronic elements. However, recently it hasbeen reported that doping at the Zr site with anyelement from the rare earths or group VB can alsooptimize the thermoelectric properties.19–21 The aimof this work is to investigate whether La, a rare-earth element, can substitute Zr in ZrNiSn andoptimize the thermoelectric performance. Compar-ing La with Zr, the atomic radius difference, DrLa/Zr

(1.87 A/1.60 A), is the largest among all the poten-tial dopant elements from the rare earths andgroups IVB and VB. This can intensify strain fieldsand mass fluctuations for point-defect scattering,thereby reducing the lattice thermal conductivity.Mass and strain fluctuations are also calculatedquantitatively using the model by Yang et al.12 Ourresults show the formation of in situ precipitatesfrom the lm to nm range, which may also play a rolein reducing the thermal conductivity besides point-defect scattering due to La doping.

EXPERIMENTAL PROCEDURES

Ingots of LaxZr1�xNiSn (x = 0, 0.005, 0.01, 0.015,0.02, 0.03) were prepared by high-frequency induc-tion melting under argon atmosphere. To ensurehomogeneity, all samples were melted repeatedly forthree times. The ingots were pulverized into powderand sintered by plasma-activated sintering (PAS) at1300 K for 7 min to obtain dense pellets. The phasecomposition of thesamples wasdeterminedby powderx-ray diffraction (XRD, Pro-PANalytical Empyrean,The Netherlands). Microstructural characterizationsof bulk samples were done by field-emission scanningelectron microscopy (FESEM, Hitachi SU-8020,Japan), electron-probe microanalysis (EPMA, JXA-8230, JEOL, Japan) equipped with wavelength-dis-persive spectroscopy (WDS), and high-resolution

transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan) equipped with energy-disper-sive spectroscopy. The electrical conductivity andSeebeck coefficient were measured simultaneously bythe standard four-probe method (ULVAC-RIKOZEM-3) in helium atmosphere. Densities of the sam-ples were measured using the Archimedes method.Thermal diffusivity was determined using the laserflash method (NETZSCH LFA-457). Thermal con-ductivity was calculated from the measured thermaldiffusivityD, specific heatCp, and densitydaccordingto the relationship j = DCpd. All measurements wereperformed in the temperature range from 300 K to923 K. The low-temperature Hall coefficient (RH) andlow-temperature electrical conductivity (r) werecharacterized using a Physical Property Measure-ment System (PPMS-9, Quantum Design, USA) in therange from 10 K to 300 K. The carrier concentration(n) and Hall mobility (lH) were determined as n = 1/eRH and lH = rRH. The errors in measurements of

Fig. 1. (a) XRD patterns of sintered Lax Zr1�xNiSn samples; (b) enlarged view of (a) ranging from 74� to 79�.

Fig. 2. Lattice parameter of Lax Zr1�xNiSn samples.

Akram, Zhang, Yang, Zheng, Yan, Su, and Tang3564

electrical conductivity, Seebeck coefficient, and ther-mal conductivity are estimated as ±5%, ±2%, and±5%, respectively.

RESULTS AND DISCUSSION

Figure 1a shows the XRD patterns of all thesamples. All the major peaks could be well indexedto the cubic crystal system (space group F 43m)

phase with MgAgAs-type structure, while a fewweak peaks due to the presence of a small fraction ofNiZr and La3Sn4 were also detected. Figure 1bpresents an enlarged view from 74� to 79�, wherethe diffraction peaks shift to low angle withincreasing La content, possibly due to the intro-duction of La with larger atomic radius into the Zrsublattice. The lattice parameter calculated usingthe Rietveld refinement method is shown in Fig. 2

Fig. 3. (a) Backscattered electron image of the polished surface of x=0.02 sample. (b) EDS profile collected on the whole surface. (c) Zrdistribution on the polished surface of x=0.02. (d) Ni distribution on the polished surface of x=0.02. (e) Sn distribution on the polished surface ofx=0.02. (f) La distribution on the polished surface of x=0.02.

Enhanced Thermoelectric Properties of La-Doped ZrNiSn Half-Heusler Compound 3565

.The lattice parameter is observed to increase withincreasing La content up to x = 0.015, after which itappears to saturate. From these XRD and latticeparameter results, the solubility limit of La inZrNiSn is estimated to be about 1.5%.

The elemental distribution on the polished sur-face of the x = 0.02 sample is presented in Fig. 3.Clearly, inhomogeneity can be seen, which is con-sistent with the XRD results. Rietveld analysis22

confirmed the presence of a composite materialwhere the volume fraction of other phases wasestimated to increase from 1.5% to 25% withincreasing La content. To further identify thesephases, EPMA was conducted on the polished sur-face of the samples. Figure 4a shows a backscat-tered electron image for x = 0.015 with the isolatedsecondary phases identified. Figure 4b shows the

EDS profile of the elements present and the chem-ical composition obtained from different regionsmarked in Fig. 4a, confirming the presence of NiZrand La3Sn4-type areas, consistent with the XRDresults. The SEM images (Fig. 4c) reveal a bentlamellar structure. In the HRTEM image (Fig. 4d),lamellar structures in the grain boundaries areagain exposed, consistent with the SEM images.Nanoprecipitates and defects in the grains are alsoquite obvious and can be seen in the inset of Fig. 4d.The inset magnified image shows a �30-nm-sizedprecipitate at the grain boundary. These crystaldefects and imperfections may have a significantinfluence on the thermoelectric performance.

a ¼ 8p2Tk2B

3qh2m� p

3n

� �2=3: (1)

Fig. 4. (a) Backscattered electron image of La0.015Zr0.985NiSn polished surface. (b) EDS profile collected on the whole surface (top) andchemical composition results for the encircled areas and the matrix in the left image (bottom). (c) SEM image of cracked surface ofLa0.005Zr0.995NiSn with enlarged area shown in the inset figure. (d) TEM image of La0.01Zr0.99NiSn with nanosized precipitate on grain boundary(enlarged in inset figure).

Akram, Zhang, Yang, Zheng, Yan, Su, and Tang3566

The Rietveld analysis results and room-tempera-ture electrical transport parameters of the samplesare given in Table I. The NiZr phase fractionincreased with increasing La content. The carrierconcentration and electrical conductivity wereobserved to increase with increasing La content,while the mobility decreased from x = 0 to x = 0.015,after which the fraction of impurities became sig-nificant, resulting in the rise of the mobility forx = 0.02 and x = 0.03. An unexpected increase of thecarrier concentration and electrical conductivitywas observed. Assuming that the alloys follow asingle parabolic band (SPB) model, the effectivemass of our samples was also calculated using Eq. 1.The effective mass showed an overall increasingtrend with La doping, indicating a change in bandstructure. It is also well known that incorporation ofdopants affects the lattice parameter of a solid.These volume deformations can induce deformationpotentials in the solid. Due to the deformationpotential, the conduction or valence band may alterits position. We believe that, in La-doped ZrNiSn, adeformation potential may have been induced due tothe change in the lattice parameter and the band-gap is possibly reduced. Moreover, this effect com-bined with the presence of metallic NiZr phase (forx = 0.02, 0.03) probably resulted in the increase ofthe carrier concentration as well as the electricalconductivity.23–26 From our low-temperature resis-tivity and Hall coefficient data, the carrier mobilitywas observed to obey a power-law dependence ofT�0.5, i.e., alloy-scattering-dominated transport, asshown in Fig. 5. For the 3% La content, the powerlaw was somewhat weaker. r was observed to in-crease with increasing temperature for all thesamples (Fig. 6a). The rise in r with increasing Lacontent can be attributed to the La doping and thepresence of metal-like NiZr phase.26 Figure 6b dis-plays the temperature dependence of the Seebeckcoefficient. The Seebeck coefficient decreased whilethe Lorenz number increased for all the samplesT

able

I.Carrierconcentration

n,electricalconductivity

r,mobility

lH,effective

mass

m*/m

0,Seebeck

coeffi

cienta,Lorenz

numberL,and

Rietv

eld

quantified

fractionsofth

esa

mplesatroom

temperatu

re

Sample

Carrier

Concentration,

n(1019cm

23)

Electrical

Conductivity,

r(104S/m

)CarrierMobility,

lH(cm

2V

21s2

1)

EffectiveMass

(m*/m

0)

Seebeck

Coefficient,

a(lV

K21)

Lorenznumber,

L(1028V

2K

22)

Rietv

eld

Quantified

ZrNiS

n/N

iZr

ZrN

iSn

11.7

2.8

15.7

2.3

�172.1

1.6

7–

x=

0.0

05

26.5

3.8

11.7

2.5

�128.3

1.8

098.6

/1.4

x=

0.0

128.9

4.2

9.1

2.7

�104.0

1.7

998.5

/1.4

x=

0.0

15

27.1

4.5

14.8

2.8

�47.3

1.8

998.4

/1.5

x=

0.0

2120.5

10.2

52.9

2.8

�41.9

2.1

972.6

/21.3

x=

0.0

352.3

15.1

179.2

2.9

�17.6

2.4

175.1

/24.8

Fig. 5. Carrier mobility of Lax Zr1�xNiSn samples.

Enhanced Thermoelectric Properties of La-Doped ZrNiSn Half-Heusler Compound 3567

with rising La content. The La doping impacted themagnitude and temperature dependence of a in anonmonotonic manner. All samples behaved as n-type semiconductors. The magnitude of a first in-creased with increasing temperature until reachinga maximum, then decreased due to intrinsic exci-tation. Leaving the magnitude of a aside, the max-imum of a systematically shifted toward highertemperature with increasing La content. Figure 6cshows that the maximum power factor was obtainedfor x = 0.005 due to the combination of a relativelyhigh electrical conductivity r and Seebeck coeffi-cient a. Figure 6d shows the temperature-depen-dent thermal conductivity, j, of the LaxZr1�xNiSnalloys. In general, j = jL + je, where jL and je arethe lattice and electronic thermal conductivity,respectively. je can be estimated from the Wiede-mann–Franz relationship, je = LrT, with the Lor-enz number L (see Eq. 2–4)) calculated based on aSPB under a relaxation-time approximation17,27–29

F gFð Þ ¼Z 1

0

xidx

1 þ exp x� gFð Þ; (2)

gF ¼ EF= kBTð Þ; (3)

L ¼ kBe

� �2 rþ 72

� �Frþ5

2gFð Þ

rþ 32

� �Frþ1

2gFð Þ

�rþ 5

2

� �Frþ2

2gFð Þ

rþ 32

� �Frþ1

2gFð Þ

" #28<:

9=;(4)

jL was thus calculated and is shown in the inset ofFig. 6d. The electronic thermal conductivities in-creased with increasing temperature, in agreementwith the increase in the electrical conductivity. Asignificant reduction in the lattice thermal conduc-tivity of all the doped samples was observed; Forinstance, the jL of the sample with x = 0.01 was5.9 W/m-K at 300 K compared with the jL value of7.8 W/m-K for x = 0 at 300 K.

Using the theory and models by Callaway, Slack,and Abeles, mass and strain fluctuations can also becalculated quantitatively.12 If we consider only the

Fig. 6. Temperature-dependent (a) Seebeck coefficient, (b) electrical conductivity, (c) power factor, and (d) thermal conductivity and latticethermal conductivity (inset).

Akram, Zhang, Yang, Zheng, Yan, Su, and Tang3568

Umklapp and point-defect phonon scattering pro-cesses, the jL ratio between crystal with disorder tothat without disorder, jL

P, is

jL

jPL

¼ tan�1ðuÞu

; u2 ¼ p2hDXhv2

jPLCexpt; Ccalc ¼ CM þ CS;

where u, X, h, v, Cexpt, CM, and CS are the disor-der scaling parameter, the average volume/atom,the Planck constant, the average lattice soundvelocity, and the experimental, mass, and strainfluctuation disorder scattering parameters,respectively. The results are presented inTable II. Compared with Zr0.5Hf0.5NiSn0.99Sb0.01,which has no contribution from strain field fluc-tuations,12 our results show that all samplesexcept that with x = 0.03 exhibited significantstrain field fluctuations in addition to mass fluc-tuations. Cexpt for x = 0.03 was very low comparedwith the CM calculated by the model, probably dueto the presence of a high percentage of impurities,eventually resulting in a high value of jL. Thetheoretical jL for x = 0.01 using the model withvalues corresponding to x = 0.005 was also calcu-lated, showing about 12% deviation from theexperimental value.

Hence, both mass fluctuations and strain fieldscontributed to the decrease of the lattice thermalconductivity by introducing point defects for La-doped ZrNiSn. The resulting ZT for all samples isshown in Fig. 7. The maximum ZT value achievedwas 0.53 for La0.01Zr0.99NiSn, mostly due to thedecreased lattice thermal conductivity. ZEM-3and LFA-457 remeasurements for La0.01Zr0.99-

NiSn gave repeatable results within ±3% devia-tion.

CONCLUSIONS

LaxZr1�xNiSn HHs were synthesized by inductionmelting combined with PAS. La can enter the Zrsublattice of HH, and high-concentration doping ofT

able

II.Latticeth

ermalconductivityjL,disordersc

alingparameteru,disordersc

atterin

gparametersCexpt,CM,and

CS,and

thestrain

field

-related

adju

stable

parametere 1

Sample

jL(W

m21K

21)

uCexpt

CM

CS

e 1

ZrN

iSn

7.8

––

––

–x

=0.0

05

6.3

0.9

30.0

09710

0.0

00469

0.0

09240

18

x=

0.0

15.9

1.1

00.0

13600

0.0

00932

0.0

12700

125

x=

0.0

15

6.4

0.8

90.0

08890

0.0

01420

0.0

07470

49

x=

0.0

26.2

0.9

80.0

10800

0.0

01840

0.0

08960

45

x=

0.0

37.7

0.2

30.0

00594

0.0

02720

0.0

00299e 1

–

jL-calc

(Wm

21K

21)

ucalc

Ccalc

CM

CS

e 1

x=

0.0

15.2

0.8

90.0

11138

0.0

00932

0.0

01818

18

Fig. 7. Temperature-dependent ZT.

Enhanced Thermoelectric Properties of La-Doped ZrNiSn Half-Heusler Compound 3569

La would introduce second phases such as NiZr andLa3Sn4. The electrical conductivity of the samplesincreased with increasing La content, whereas thethermal conductivity of the LaxZr1�xNiSn sampleswas effectively reduced by La substitution becauseof mass and strain field fluctuations due to pointdefects. The maximum obtained ZT was 0.53 at923 K for La0.01Zr0.99NiSn.

ACKNOWLEDGEMENTS

We acknowledge the support from the NationalBasic Research Program of China (973 program)under Project 2013CB632502, the Natural ScienceFoundation of China (Grant Nos. 51172174,51402222), and the 111 Project of China (Grant No.B07040).

REFERENCES

1. T.M. Tritt, Science 283, 804 (1999).2. J. Kim, J.H. Bahk, J. Hwang, H. Kim, H. Park, and W. Kim,

Phys. Status Solidi RRL 7, 767 (2013).3. C. Uher, J.H. Yang, S. Hu, D.T. Morelli, and G.P. Meisner,

Phys. Rev. B 59, 8615 (1999).4. S. Bhattacharya, A. Pope, R. Littleton, T.M. Tritt, V.

Ponnambalam, Y. Xia, and S.J. Poon, Appl. Phys. Lett. 77,2476 (2000).

5. Q. Shen, L. Chen, T. Goto, T. Hirai, J.H. Yang, G.P. Meisner,and C. Uher, Appl. Phys. Lett. 79, 4165 (2001).

6. S. Culp, S.J. Poon, N. Hickman, T.M. Tritt, and J. Blumm,Appl. Phys. Lett. 88, 042106 (2006).

7. P. Larson, D. Mahanti, and M. Kanatzidis, Phys. Rev. B 62,12754 (2000).

8. J. Yang, H. Li, T. Wu, W. Zhang, L. Chen, and J.H. Yang,Adv. Funct. Mater. 18, 2880 (2008).

9. G. Nolas, S.J. Poon, and M. Kanatzidis, MRS Bull. 31, 199(2006).

10. S.Kim,Y.Kimura,andY.Mishima, Intermetallics15,349 (2007).11. W.J. Xie, J. He, S. Zhu, X.L. Su, S.Y. Wang, T. Holgate, J.W.

Graff, V. Ponnambalam, S.J. Poonm, X.F. Tang, Q.J. Zhang,and T.M. Tritt, Acta Mater. 58, 4705 (2010).

12. J.H. Yang, G. Meisner, and L. Chen, Appl. Phys. Lett. 85,1140 (2004).

13. X. Yan, W.S. Liu, H. Wang, S. Chen, J. Shiomi, K. Esfarjani,H. Wang, D. Wang, G. Chen, and Z.F. Ren, Energy Environ.Sci. 5, 7543 (2012).

14. P.J. Lee and L.S. Chao, J. Alloys Compd. 504, 192 (2010).15. S. Bhattacharya, M.J. Skove, M. Russel, T.M. Tritt, Y. Xia,

V. Ponnambalam, S.J. Poon, and N. Thadhani, Phys. Rev. B77, 184203 (2008).

16. S. Bhattacharya, T.M. Tritt, Y. Xia, V. Ponnambalam, S.J.Poon, and N. Thadhani, Appl. Phys. Lett. 8, 43 (2002).

17. H.H. Xie, H. Wang, C.G. Fu, Y. Liu, G.J. Snyder, X.B. Zhao,and T.J. Zhu, Sci. Rep. 4, 6888 (2014).

18. P. Qiu, J. Yang, X.Y. Huang, X. Chen, and L.D. Chen, Appl.Phys. Lett. 96, 152105 (2010).

19. X.H. Liu, J. He, H.H. Xie, X.B. Zhao, and T.J. Zhu, Int. J.Smart Nano Mater. 3, 64 (2012).

20. D.G. Zhao, M. Zuo, Z.Q. Wang, and X.Y. Teng, Funct. Mater.Lett. 7, 1450032 (2014).

21. T.J. Zhu, K. Xiao, C. Yu, J.J. Shen, S.H. Yang, A.J. Zhou,X.B. Zhao, and J. He, J. Appl. Phys. 108, 044903 (2010).

22. D.K. Misra, A. Bhardwaj, and S. Singh, J. Mater. Chem. A 2,11913 (2014).

23. C.G. Van de Walle, Phys. Rev. B 68, 165209 (2003).24. G.S. Cargill III, J. Angilello, and K.L. Kavanagh, Phys. Rev.

Lett. 61, 1748 (1988).25. A. Janotti, B. Jalan, S. Stemmer, and C.G. Van de Walle,

Appl. Phys. Lett. 100, 262104 (2012).26. A. Amamout, R. Kuentzlert, Y. Dossman, P. Forey, J.L.

Glimois, and J.L. Feron, J. Phys. F 12, 2509 (1982).27. Z. Qiang, C. Long, W. Liu, Y. Zheng, X.L. Su, H. Chi, H.J.

Liu, Y.G. Yan, X.F. Tang, and C. Uher, Phys. Chem. Chem.Phys. 16, 23576 (2014).

28. Z. Du, T. Zhu, Y. Chen, J. He, H. Gao, G. Jiang, T.M. Tritt,and X. Zhao, J. Mater. Chem. 22, 6838 (2012).

29. H.H. Xie, H. Wang, Y.Z. Pei, C.G. Fu, X. Liu, G.J. Snyder, X.Zhao, and T. Zhu, Adv. Funct. Mater. 23, 5123 (2013).

Akram, Zhang, Yang, Zheng, Yan, Su, and Tang3570