Synthesis and thermoelectric properties of tantalum-dopedZrNiSn half-Heusler alloys
Degang Zhao*, Min Zuo, Zhenqing Wang, Xinying Tengand Haoran Geng
School of Materials Science and EngineeringUniversity of Jinan, Jinan 250022, P. R. China
Received 10 February 2014; Accepted 20 February 2014; Published 2 April 2014
The Ta-doped ZrNiSn half-Heusler alloys, Zr1�xTaxNiSn, were synthesized by arc melting and hot-press sintering. Microstructureof Zr1�xTaxNiSn compounds were analyzed and the thermoelectric (TE) properties of Zr1�xTaxNiSn compounds were measuredfrom room temperature to 823K. The electrical conductivity increased with increasing Ta content. The Seebeck coefficient ofZr1�xTaxNiSn compounds was sharply decreased with increasing Ta content. The Hall mobility was proportional to T�1:5 above673K, indicating that the acoustic phonon scattering was predominant in the temperature range. The thermal conductivity waseffectively depressed by introducing Ta substitution. The figure of merit of ZrNiSn compounds was improved due to the decreasedthermal conductivity and increased electrical conductivity. The maximum ZT value of 0.60 was achieved for Zr0:97Ta0:03NiSnsample at 823K.
Keywords: Thermoelectric; half-Heusler alloy; Sintering.
Thermoelectric (TE) materials have attracted worldwideattention for their application in electronic cooling, heatpump, power generation and thermal sensor, because of theirunique feature of mutual conversion between temperaturegradient and electricity.1–3 The conversion efficiency of TEmaterial is evaluated by its dimensionless figure of merit,ZT ¼ α2�T=κ, where α is the Seebeck coefficient, � is theelectrical conductivity, T is the absolute temperature and κ isthe total thermal conductivity. The total thermal conductivityis composed of an electron part (κE) and a phonon part (κph).
Therefore, to maximize ZT of TE material, large α and � aswell as low κ are required.
A class of intermetallics of formula MNiSn (M ¼ Ti, Zr,Hf) half-Heusler alloys has been reported to be narrowbandgap semiconductors with 0.1–0.5 eV indirect bandgapnear the Fermi level and to exhibit a high Seebeck coefficient(� 200 μV/K) and electrical conductivity (� 104 S/m), whichwould yield a relatively large power factor.4–6 However, oneof the problems of (Ti, Zr, Hf)NiSn half-Heusler alloys forTE applications is their relatively high κph, which is around
10Wm�1K�1 at room temperature. Generally, κE increaseswith the number of the carrier increasing in the material,while κph is reduced when the phonon is scattered by the
disorder in the material, for example, the grain boundary, theprecipitated impurities or the substituted atoms. Commonly,the degree of the phonon scattering depends on the density ofthe disorder; the more the disorder exists in the material, thestronger the phonon is scattered. Therefore, the materialswith more substituted atoms generally posses lower thermalconductivity. In an effort to reduce the κph, Hohl et al. studied
that the mass disorder in the (Ti, Zr, Hf)-site lattice of thesolid solutions causes additional phonon scattering, therebyreducing the thermal conductivity, and obtained about6Wm�1K�1 at room temperature for Zr0:5Hf0:5NiSn.6 Hir-oaki et al. investigated Nb doped ZrNiSn half-Heuslercompounds and achieved the low κ.7 Populoh et al. reportedthe Ti0:37Zr0:37Hf0:26NiSn alloys and the ZT reached 1.0 at725K.8 Yu et al. fabricated Hf0:6Zr0:4NiSn0:98Sb0:02 com-pounds and ZT of 1.0 at 1000K was attained, which wasmainly originated from the decreased κph due to the iso-
electronic substitution of Zr on Hf-site.9 Xie et al. proved thatalloying can be an effective approach for ZrNiSn half-Heusler system to improve the TE performance by enhancingpoint defect phonon scattering, while minimizing the dete-rioration of charge mobility due to the low alloy scatteringpotential.10 In this study, the Ta-doped ZrNiSn half-Heusleralloys whose Zr site is substituted by Ta was synthesizedand the transport properties including the carrier scattering*Corresponding author.
Functional Materials LettersVol. 7, No. 3 (2014) 1450032 (4 pages)© World Scientific Publishing CompanyDOI: 10.1142/S1793604714500325
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mechanism and the effective mass were discussed. Theresults are expected to be beneficial to the development ofZrNiSn half-Heusler TE materials.
Tantalum doped ZrNiSn ingots were prepared from ap-propriate quantities of zirconium metal chunk (99.9%),nickel metal ball (99.9%), tin metal shot (99.99%) and tan-talum metal sheet (99.9%) by arc melting in an arc furnaceunder an argon atmosphere. The resultant alloy ingot wasremelted two or three times to ensure homogeneity. Theingots were crushed by WC ball milling for 3 h. The obtainedpowders were sintered by hot-press sintering technique under80MPa at 1473K for 2 h under a flowing argon atmosphere.The polycrystalline pellets were sealed in an evacuated quartztube for annealing. The samples were annealed at 873K for72 h. The samples were cut using a diamond wheel andpolished for thermoelectric measurements.
The phase and microstructure of samples were analyzedusing X-ray diffraction (XRD, CuKα, Rigaku, Rint2000) andscanning electron microscopy (SEM, JEOL, JXA-8100), re-spectively. The � and α were measured with standard fourprobe (ZEM-2) from room temperature to 873K under argonatmosphere. The thermal diffusivity (λ) was measured bylaser flash method (Netzsch LFA427). The κ was calculatedfrom the relationship κ ¼ �λCp, where � is the density of the
sintered sample and Cp is the heat capacity. The Hall coef-
ficient (RH) was measured by the van der Pauw's method invacuum. The carrier concentration (n) and mobility (μH) wereestimated from RH and the electronic charge (e) from therelations of n ¼ 1=(eRH) and μH ¼ RH=�, based on the as-sumption of a single band model.
Figure 1 is the XRD patterns of Zr1�xTaxNiSn compounds(x ¼ 0, 0.01, 0.02 and 0.03). No impurity phase was identi-fied and all samples were single phase in MgAgAs structure.Figure 2(a) shows the SEM image of Zr0:99Ta0:01NiSn sam-ple. It can be seen that no impurity phase existed, which was
consistent with the XRD results. Similar results were alsoobserved in the Zr0:98Ta0:02NiSn and Zr0:97Ta0:03NiSn sam-ples. Figure 2(b) is the bright-field TEM image and selectedarea diffraction pattern of Zr0:98Ta0:02NiSn sample. The dif-fraction pattern was indexed as [1�10] incidence for half-Heusler structure. According to the XRD pattern, the latticeparameter value of pure ZrNiSn was evaluated to be0.6098 nm, which was in agreement with those reported inother literatures.11 Attributed to the different size of the ionicradii of Zr (0.145 nm) and Ta (0.134 nm) atoms, the latticeparameter decreased with increasing Ta content from 0.6098to 0.6078 nm (x ¼ 0:03).
Figure 3 is the TE properties of Zr1�xTaxNiSn compoundsas function of temperature. It can be seen from Fig. 3(a) thatthe � greatly increased with the Ta-concentration increasing,which should be caused by carrier electron-doping by the Tasubstitution since Ta has one more electron in its valenceshell than Zr. In addition, the nondoped ZrNiSn sampleshowed semiconductor conducting behavior, of which the �
increased with the temperature increasing and Zr1�xTaxNiSnsamples showed metal-like conduction behavior, indicating aFig. 1. XRD patterns of Zr1�xTaxNiSn.
(a)
(b)
Fig. 2. (a) SEM images of the sintered Zr0:99Ta0:01NiSn sample; (b) thebright-field TEM image and selected area diffraction pattern ofZr0:98Ta0:02NiSn sample.
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shift of the Fermi-level toward a higher position in the con-duction bands. Therefore, the doped Zr1�xTaxNiSn com-pounds became semi-metal or metallic behavior. Themaximum value of � for Zr0:97Ta0:03NiSn sample reached2:6� 105 Sm�1 at room temperature. The α of all sampleswas negative in the whole temperature range, indicatingthat the major carriers were electrons. It can be seen fromFig. 3(b) that the absolute value of α for Zr1�xTaxNiSnsamples was sharply decreased with the Ta-concentrationincreasing. The absolute value of α of Zr0:97Ta0:03NiSnsample at room temperature was 80 μV/K, which was rela-tively small in comparison with that of pure ZrNiSn sample.The α of Zr1�xTaxNiSn samples was proportional to thetemperature below 623K and the absolute values of αdecreased with the content of Ta increasing. The Seebeckcoefficient can be expressed as12:
α ¼ � �
3kBekBT
d ln �(E)dE E¼EF
� T
n(E)dn(E)dE
��������E¼EF
(1)
where kB, �(E), and n(E) are Boltzmann constant, electricalconductivity and value of density of states (DOS), respec-tively. As the Fermi level (EF) shifted to a higher position inthe conduction band, the DOS increased. Therefore, the αdecreased with the Ta-doping increasing.
The μH of Zr1�xTaxNiSn is shown in Fig. 4. The μH ofZr1�xTaxNiSn samples decreased with the Ta content in-creasing. The μH of Zr1�xTaxNiSn samples was in the orderof 10�3 m2V�1S�1 at room temperature, which was similarwith CoSb3-based skutterudites.13–15 It may be caused bythe similar carrier effective mass (3–5m0) of ZrNiSn andCoSb3 compounds. The m� is estimated using the followingequations:
m� ¼ h2
2kBTn
4�F1=2(η)
� �2=3
; (2)
α ¼ � kBe
(r þ 2)Frþ1(η)(r þ 1)Fr(η)
� η
� �; (3)
where Fr, h and r are Fermi integral, Planck's constant, and thescattering parameter of relaxation time, respectively. Theevaluated transport properties at approximately 300K are lis-ted in Table 1. It also can be seen from Fig. 4 that the Hallmobility showed temperature dependence of T�1:5 above673K, indicating that the acoustic phonon scattering wasdominant in the temperature range from 673K to 873K. Below673K, the Hall mobility had a weak temperature dependencerelationship and the relationship of μ / T�0:5 was observed,suggesting that a dominative mechanism of alloy scattering.
(a) (b)
(c) (d)
Fig. 3. Thermoelectric properties of Zr1�xTaxNiSn samples as function of temperature (a) �; (b) α; (c) κ; (d) ZT .
Synthesis and thermoelectric properties of Ta-doped ZrNiSn half-Heusler alloys
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Consequently, it can be concluded that there are multiplescattering mechanisms in the Zr1�xTaxNiSn system.
The total thermal conductivities (κ) of all Zr1�xTaxNiSncompounds are shown in Fig. 3(c). The κ of Zr1�xTaxNiSnsamples decreased in the whole temperature except for that ofZr0:98Ta0:02NiSn sample, which decreased up to 750K and thenincreased a little. In addition, the κ of Zr1�xTaxNiSn samplesdecreased with the content of Ta increasing and the κ ofZr0:97Ta0:03NiSn was about 8.8Wm�1K�1 at room tempera-ture. The κph is estimated by subtracting the electronic contri-
bution from the κ via the Wiedmann-Franz law (κE ¼ L0�T ,where the Lorenz number L0 has a numerical value of1:72� 10�8 V2/K2).16–18 For Zr1�xTaxNiSn samples, κE wasin the range of 0.1–1.1W/mK at 300K, but κ changed from8.8W/mK to 9.8W/mK. Therefore, κ came mainly from thelattice component contribution (κph).According to the theory of
Callaway,19 the reduction of κph can be attributed to the sub-
stitution of Ta atoms at the Zr site, which creates point defectcenter for phonons due to mass fluctuation (mass differences)and strain field fluctuation (size and interatiomic coupling forcedifferences) between the host atoms and impurity atoms.20
Similar results were also found in the TiNiSn and TiCoSbsystems.21–23 From the electrical and thermal transport prop-erties, the ZTof Zr1�xTaxNiSn samples is calculated and shownin Fig. 3(d). The ZT value increased monotonously with theincreasing temperature and was significantly improved for allZr1�xTaxNiSn samples compared with that of pure ZrNiSnsample. The increasedZTmainly originated from the decreased
κ and increased �. The maximum ZT of Zr0:97Ta0:03NiSn wasobtained as 0.60 at 823K.
In summary, n-type half-Heusler Zr1�xTaxNiSn were syn-thesized by arc melting and hot press technology. The � ofZr1�xTaxNiSn samples was greatly increased with increasingTa-content. The absolute value of α for Zr1�xTaxNiSn wassharply decreased with the Ta-doping increasing. The κ ofZr1�xTaxNiSn samples was effectively depressed by intro-ducing Ta substitution. The maximum ZT value was 0.60 at823K for Zr0:97Ta0:03NiSn.
Acknowledgments
The authors would like to thank for the help received fromShanghai Institute of Ceramics, Chinese Academy of Sci-ences and Shandong University for the measurement of TEproperties. Financial supports from the National NaturalScience Foundations of China (51202088), Shandong Pro-vincial Natural Science Foundation of China(ZR2013EML004), Promotive research fund for excellentyoung and middle-aged scientists of Shandong Province(BS2013CL004), Shandong Province Higher EducationalScience and Technology Program (J13LA53) and DoctoralFund of University of Jinan (XBS1232) are acknowledged.
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349 (2007).Fig. 4. Temperature dependence of Hall mobility of Zr1�xTaxNiSn samples.
Table 1. Temperature dependence of Hall mobility of Zr1�xTaxNiSnsamples.
ZrNiSnZr0:99Ta0:01-
NiSnZr0:98Ta0:02-
NiSnZr0:97Ta0:03-
NiSn
m � (m0) 3.6 3.9 4.1 4.2μH (cm2V�1s�1) 45 43 41 37n(1026m�3) 1.1 1.8 2.2 2.5
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