Vanadium-Doping-Induced Resonant Energy Levels for the

Article Cite This: ACS Appl. Energy Mater. 2018, 1, 757−764

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Vanadium-Doping-Induced Resonant Energy Levels for the Enhancement of Thermoelectric Performance in Hf-Free ZrNiSn HalfHeusler Alloys Nagendra S. Chauhan,†,‡ Sivaiah Bathula,*,†,‡ Avinash Vishwakarma,†,‡ Ruchi Bhardwaj,†,‡ Bhasker Gahtori,†,‡ Anil Kumar,‡ and Ajay Dhar*,†,‡ †

Academy of Scientific & Innovative Research (AcSIR), CSIR-National Physical Laboratory, New Delhi 110012, India Division of Advanced Materials and Devices, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India

‡

ACS Appl. Energy Mater. 2018.1:757-764. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/03/19. For personal use only.

S Supporting Information *

ABSTRACT: Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of cost-effective thermoelectric materials for power generation, owing to their thermal stability coupled with high cost of Hf, their figure-of-merit (ZT) still remains far below unity. We report a state-of-the-art figure-of-merit (ZT) ∼ 1 at 873 K in Hf-free n-type V-doped Zr1−xVxNiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The efficacy of V as a dopant on the Zr-site is evidenced by the enhanced thermoelectric properties realized in this alloy, compared to other reported dopants. This enhancement of ZT is due to the synergistic enhancement in electrical conductivity with a simultaneous decrease in the thermal conductivity, which yields ZT ∼ 1 at 873 K at an optimized composition of Zr0.9V0.1NiSn, which is ∼70% higher than its pristine counterpart and ∼25% higher than the best reported thus far in Hf-free n-type HH alloys. The enhancement of the electrical conductivity is due to the modification of the band structure by suitable tuning of the electronic band gap near the Fermi level, through optimized V-doping in ZrNiSn HH alloys. The reduction in the thermal conductivity has been attributed to the mass fluctuation effects and the substitutional defects caused by V-doping, which results in an abundant scattering of the heatcarrying phonons. The optimized V-doped ZrNiSn HH composition, therefore, strikes a favorable balance between cost and thermoelectric performance, which would go a far way in the realization of a cost-effective (Hf-free) HH based thermoelectric generator for power generation through waste heat recovery. KEYWORDS: thermoelectrics, half-Heusler alloys, thermoelectric performance, power factor, spark plasma sintering, figure-of-merit

1. INTRODUCTION In order to reduce the dependence on depleting fossil fuels, research is being globally focused on different analogues of renewable sources of energy.1,2 Thermoelectric generators (TEGs) are a convenient means of generating green energy by harnessing the waste heat, and their conversion efficiency is primarily governed by the thermoelectric figure-of-merit, ZT =

development of TEG devices which need to be addressed, before these devices can compete with other conventional source of renewable energy.3 Most of the efficient thermoelectric materials reported thus far contain toxic (Pb) and/or expensive elements (Ge, Ag, Co, Hf, rare-earths, etc.). Thus, the current focus of thermoelectric research is directed toward the development of thermoelectric materials, which are nontoxic, earth-abundant, and chemically stable at the operating temperatures. These include silicides,4,5 selenides,6,7

α 2σ κ

( )T , where α is the Seebeck coefficient, σ is the

electrical conductivity, T is the operating temperature, and κ is the total thermal conductivity arising from electronic (κe) and lattice (κl) contributions (κ = κe + κl). However, in the present scenario there are a quite few major challenges associated with © 2018 American Chemical Society

Received: November 29, 2017 Accepted: January 9, 2018 Published: January 9, 2018 757

DOI: 10.1021/acsaem.7b00203 ACS Appl. Energy Mater. 2018, 1, 757−764

Article

ACS Applied Energy Materials

Figure 1. (a) XRD patterns of Zr1−xVxNiSn (x = 0−0.2) half-Heusler alloys. (b) Schematic diagram of atomic positions in the lattice. (c) Lattice constant as a function of dopant concentration.

half-Heuslers,8−10 and other multielement solid solutions such as LAST and TAGS,11,12 which are currently being explored for TEG applications. Among these, half-Heusler (HH) alloys are considered to be one of the thermoelectric materials with the most potential owing to their excellent chemical stability and high power factor, and because they can be synthesized as both n- and p-type materials. However, the high prices of Hf (an inevitable constituent element in HH alloys) and the high thermal conductivity are currently the major bottlenecks for the development of HH-based alloys for TEG applications. The HH alloys are of considerable research interest for TEG applications owing to their superior mechanical strength, nontoxicity, and high thermal stability.13−15 Although HH alloys exhibit semiconducting behavior, their electronic structure can be modified by doping (substitution) and/or through partial filling of the vacant sites in their noncentrosymmetric face-centered-cubic MgAgAs-type crystal structure in order to obtain the desirable electronic properties.16,17 Among the numerous possible combinations that have been reported13 for HH alloys, the family of ZrNiSn alloys (ntype)18−20 has been most widely studied as they exhibit great potential as a thermoelectric material. Among all the strategies adopted, doping has been found to be the most effective in improving the thermoelectric performance of HHs.21−35 In particular, Hf has been found to be the most effective isoelectronic dopant22−24,36 in n-type ZrNiSn, while Sb is the most favorable dopant at the Sn-site.13,35,37,38 It is wellrecognized in thermoelectrics that, apart from ZT, low material cost and toxicity are the prime requirements, which need to be considered if TEGs have to compete with other existing sources of renewable energy. Despite Hf being an effective dopant, it is overwhelmingly expensive, and thus, reducing the usage of Hf is a key to achieving a nontoxic and low-cost HH material for TEG applications. Among the Hf-free HH alloys, a highest ZT ∼ 0.8 at 900 K has been reported in the n-type ZrNiSn HH alloy by creating intrinsic disorder via alloy scattering35 and phase separation.39 It has been established by both theoretical first principle calculations40 and experimental outcomes41−44 that the enhancement of thermoelectric properties in a material can be achieved by tuning the electronic structure in a way that

the electronic density of states resembles a Dirac delta function at the Fermi level, as this enables distribution of energy carriers in a narrow range, with high carrier velocity in the direction of the applied electric field. Enhancement of thermoelectric properties in half-Heusler alloys due to introduction of resonant states near the Fermi level was also observed in a half-Heusler system.45 Recently, in a systematic investigation of the role of V, Nb, and Ta as potential resonant dopant, it was found that vanadium introduces resonant states.46 The idea of implementing resonant states near the Fermi level in HH ZrNiSn by V-doping thus appears promising. In the present study, we investigate the influence of V-doping on the electrical and thermal transport properties of ZrNiSn HH alloys and report a state-of-the-art (ZT) ∼ 1 at 873 K in an optimized composition of Zr0.9V0.1NiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The synthesized HH alloys were characterized for their morphology, phase, and composition, on the basis of which the enhancement in their electrical and thermal transport properties has been discussed.

2. EXPERIMENTAL DETAILS The ingots (∼5 g) with nominal composition Zr1−xVxNiSn (x = 0− 0.2) were synthesized by arc-melting of Zr (99.97%), V (99.97%), Ni (99.97%), and Sn (99.98%) in stoichiometric proportions, under an argon atmosphere. The arc-melted ingots were repeatedly remelted to ensure compositional homogeneity. These pulverized powders were then loaded into a graphite die with an inner diameter of 12.7 mm and consolidated employing spark plasma sintering (SPS) at 1473 K under 50 MPa in a vacuum to obtain bulk dense pellets, which were annealed at 1023 K for 48 h. The density, as determined by the Archimedes principle, was found to be ∼98.7% of the theoretical density of all the synthesized HH alloys. The constituent phases were determined by Xday diffraction (XRD; Rigaku) using Cu Kα radiation (λ = 1.5406 Å). Morphology and elemental ratios of the samples are characterized by a field-emission scanning electron microscope (FE-SEM) (Zeiss; Model Supra 40VP). The Seebeck coefficient and electrical resistivity were measured simultaneously employing commercial equipment (ULVAC, ZEM3) on polished bar samples of dimensions 3 × 2 × 10 mm3. The thermal diffusivity was measured using the laser flash technique (Lineseis, LFA 1000) on disc-shaped samples of diameter 12.7 mm and thickness of 2.0 mm sprayed with a layer of graphite in order to 758


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Figure 2. FESEM image with elemental distribution of constituent elements and EDS analysis of Zr0.9V0.1NiSn HH alloys.

Figure 3. Electrical transport properties of Zr1−xVxNiSn half-Heusler alloys: (a) electrical conductivity, (b) Seebeck coefficient, (c) band gap variation with V-doping, and (d) Pisarenko plot (323 K). are ±6% for thermal diffusivity, ±7% for electrical conductivity, ±7% for Seebeck coefficient, ±2% for specific heat, and ±0.5% for density.

minimize errors due to emissivity. The specific heat was measured using a differential scanning calorimetry (DSC) instrument. The lattice conductivity (κE) was calculated using the Wiedemann−Franz law as κE = LσT (where L is the Lorenz number estimated using47 |α| ⎤ ⎡ L = ⎣1.5 + exp⎡⎣− 116 ⎤⎦⎦ × 10−8 where WΩK−2 and α is in in μV/ K). The carrier concentration and mobility were determined using a Hall effect measurement system (HEMS, Nanomagnetics), under a magnetic field of 0.5 T. The optical band gap was estimated using UV−vis−NIR spectroscopy (Agilent CARY-5000) using reflectance accessories in normal mode. The accuracies in transport measurement

3. RESULTS AND DISCUSSION 3.1. Microstructural Analysis. The XRD patterns of the synthesized Zr1−xVxNiSn HH alloys, with varying V composition, are shown in Figure 1a along with a schematic indicating the arrangement of atoms in the lattice (Figure 1b). All the major peaks could be indexed to the ZrNiSn phase (ICDDPDF-4+: 04-002-1779). The lattice constant is found to decrease with increasing V-doping (Figure 1c) although with 759


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ACS Applied Energy Materials Table 1. Electrical Transport Parameters of Half-Heusler Zr1−xVxNiSn at Room Temperature param

x=0

x = 0.05

x = 0.07

x = 0.10

x = 0.15

x = 0.20

electrical resistivity (Ohm m) Hall coeff RH (m3 C−1) carrier conc n (cm−3) Hall mobility μ (cm2 V−1 s−1 ) effective mass weighted mobility (μw)

5.02 × 10−5 8.24 × 10−8 7.57 × 1019 16.40 1.14me 19.99

3.07 × 10−5 4.25 × 10−8 1.47 × 1020 15.12 2.11me 46.38

2.67 × 10−5 3.23 × 10−8 1.93 × 1020 12.14 2.75me 55.28

2.60 × 10−5 2.82 × 10−8 2.21 × 1020 11.13 3.19me 63.41

2.18 × 10−5 1.76 × 10−8 3.54 × 1020 9.27 3.96me 73.02

2.1 × 10−5 1.2 × 10−8 5.21 × 1020 6.33 4.99me 70.54

interaction, which may result in bonding−antibonding splitting.46,58 However, in the case of Nb- and Ta-doping in ZrNiSn HH, the atomic levels lie in proximity with the Zr level; a consequence of this is that the antibonding levels that originate from Zr/(Nb and Ta) lie far away from the conduction band edge.28,60 Thus, introduction of these transition metal dopant levels in the vicinity of the band gap results in a significant reduction of α.26−28,60 However, in the case of V-doping45,46,58 a weaker hybridization between Zr and V leads to a localized nature of the V-doping-induced hybridized states near the conduction band edge resulting in distortions in the density of states (DOS) near the Fermi level (resonant states42). In resonant doping, the impurity states outside the energy gap are formed (deep defect states) instead of midgap states.61 Such states interact strongly with the lattice in a way that considerable distortions in the shape and energy distribution of the DOS can develop which results in decreasing mobility and increasing effective mass due to resonance scattering. Resonant states that induce Fermi level pinning have been observed in a number of narrow band gap semiconductors.42,43 In such selective scattering, the carriers exhibiting relaxation times and mean free paths (MFPs) within certain limits are scattered preferentially compared to the carriers whose energies lie outside these limits resulting in a significant influence on the electronic properties, which leads to an enhancement of α as a function of carrier concentration at a given temperature.26,45 Figure 3b shows the temperature dependence of α for different V-doped ZrNiSn where negative values of α indicate that electrons are the dominant charge carriers suggesting ntype conduction. It was observed that V-doping impacts the magnitude and the temperature dependence of α in a nonmonotonic fashion. The magnitude of α in all the doped samples was found to enhance with increasing temperature until 523 K, beyond which it exhibits a saturating behavior. A similar observation has been previously reported by Simonson et al.45 in V-doped HH alloys, which suggest that the V-doping introduces a local feature in the density of states near or within the Fermi level, which significantly contributes toward the enhancement in the α at room temperature, while at higher temperatures the promotion of n-type carriers into the conduction band and intrinsic excitation of carriers results in marginalizing this resonant state behavior. The temperature onset of saturation decreases with increasing V-doping, especially at higher doping levels, as observed in the present study (Figure 3a) and is in line with those reported earlier for pristine and doped ZrNiSn HH alloys (Ti,22 Pd,23 Nb,60 and Ta29). In order to further understand the resonant state phenomena in V-doped ZrNiSn, the band gap (Eg) was estimated using the relation62 αmax = Eg/2eTmax, where Tmax is the absolute temperature corresponding to αmax. The band gap, calculated from this relation, shows almost a linear decrease with

some deviation from Vegard’s law. Such deviations from Vegard’s law have also been previously reported in similar HH alloys26,27 owing to the differences in size, electrochemical potential, and thermal expansions of the constituent elements.48,49 The average crystallite size, calculated employing the Williamson−Hall method,10 was found to be in the range ∼30−40 nm for all the synthesized HH compositions, suggesting an intrinsic in situ nanostructuring, owing to SPS. Similar intrinsic nanostructuring employing SPS has also been previously reported in several alloys20,43,50−53 and has been attributed to a thermomechanical fatigue process.52 This SPSinduced nanostructuring is a more economical and efficient strategy than conventional time-consuming techniques of nanostructuring such as mechanical alloying and melt-spinning as it avoids the usage of multiple pieces of processing equipment. Figure 2 shows the morphology, composition, and elemental distribution for a typical synthesized HH sample (Zr0.9V0.1NiSn). This figure suggests an equiaxed microstructure with grain size of 20−50 nm, which is quite close to that estimated from the XRD data (Figure 1a). The energydispersive X-ray spectroscopy (EDS) results in this figure indicate that the composition of the synthesized HH alloys is very close to their initial stoichiometry. Figure 2 also shows the elemental distribution of the constituent elements, which clearly suggests that all the constituent elements in the synthesized HH alloys are uniformly distributed throughout the sample. 3.2. Thermoelectric Transport Properties. Figure 3a,b shows the temperature dependence of the electrical transport properties of Zr1−xVxNiSn with different V compositions. Figure 3a shows the temperature dependence of electrical conductivity (σ) in all samples and was found to increase nearly linearly with increasing temperature for all the compositions, which is typical of semiconducting behavior. Further, the magnitude of σ was found to increase with increasing V-doping throughout the temperature range of measurement, which can be attributed to the observed increase in the carrier concentration on V-doping in ZrNiSn HH alloys (Table 1). It is interesting to note that, with V-doping in ZrNiSn, the semiconducting behavior is preserved even at carrier concentration ∼1020 cm−3, which can be qualitatively understood in terms of its electronic band structure, which has been previously reported for ZrNiSn based HH alloys.45,54−57 In the ZrNiSn HH system, it has been reported that the band gap formation results in part from the d−d orbital repulsion between Zr and Ni, while Sn has no major contribution in the vicinity of the gap.45,58 In particular, the bands directly below the band gap were shown to be of strong Ni-d character, while those above were calculated to be of primarily Zr-d character, thus leading to hybridization of the d-orbitals of Ni and Zr.10,59 Doping at the Zr-site leads to hybridization of valence electrons between Zr and dopant atoms via their nearest neighbor 760


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Figure 4. Temperature dependence of Zr1−xVxNiSn half-Heusler alloys: (a) total thermal conductivity, (b) lattice thermal conductivity, (c) power factor, (d) figure-of-merit.

deviation from the single parabolic transport model and a modified band structure. Band engineering is a well-known strategy for enhancing the thermoelectric performance through modifications in the electronic band structure, which can be achieved by tuning the Fermi level through optimized doping.63 In the present studies, the V-doping significantly alters the electronic band structure close to the Fermi level by shifting of the Fermi level toward a higher position in the conduction band, similar to that reported for Nb-doping26,60 and Ta-doping28 in ZrNiSn. The estimated m* is directly related to the band curvature and valley degeneracy (Nv) as m* = Nv2/3mb*, where mb* represents the average (single valley) m* of the degenerate valleys. Overall, in Zr1−xVxNiSn a significant increase in α, m*, and n and a reduction in μ are observed with increasing V-doping. In a simplified single parabolic band model assumption, m* enhancement is attributed to an increased mb*, i.e., band flattening. However, the Pisarenko plot (Figure 3d) reveals a deviation from the single parabolic model, which suggest a possibility of an effective increase of degenerate valleys (Nv) contributing toward the enhancement of m*. Convergence of such multiple degenerate valleys effectively increases α with increasing n. Thus, simultaneous occurrence of V-induced resonant states and manipulation of the multiple valence bands in V-doped ZrNiSn results in an enhancement of α. In addition, the weighted mobility μw = μ(m*/me)3/2 , which primarily determines that the optimal electrical performance was also estimated, was found to increase with V-doping in ZrNiSn (Table 1), thus effectively enhancing the electrical performance. The temperature dependence of thermal conductivity of the synthesized Zr 1−x V x NiSn HH alloys with different V compositions is shown in Figure 4a. The corresponding

increasing V-doping in ZrNiSn HH alloys (Figure 3c), thereby suggesting that the band gap of ZrNiSn can be tailored by suitable V-doping. In addition, Figure 3c also shows the variation of experimental band gap, estimated using UV−vis spectroscopy, which also revealed a decreasing trend with Vdoping. Comparable band gap values have previously been reported in similar ZrNiSn31 based HH alloys. In order to better understand the dependence of α on V-doping in ZrNiSn, a Pisarenko plot (shown in Figure 3d) was constructed on the basis of the relationship between the Seebeck coefficient (α), effective mass (m*), and carrier density (n), which is described as α = (8π 2κB2/3eh2)m*T (π /3n)2/3, where kB is the Boltzmann constant, h Planck’s constant, e the carrier charge, n the carrier concentration, μ the mobility, and m* the effective mass of the charge carriers. This equation suggests that, assuming a single parabolic band and an energy-independent relaxation time approximation, the thermopower at a given temperature can be described by a unique value of m*. The calculated values of m* using this relation, as shown in Table 1, suggest that m* increases with increasing V-doping, which may be attributed to the modification of the density of states near the Fermi energy, owing to the V-doping. The V-induced resonant states at or below the Fermi level tend to get augmented by the surplus electrons contributed by V-doping, which increases m* as well as the carrier concentration of the n-type ZrNiSn (Table 1), leading to an enhancement of α as observed in the current study (Figure 3b). The observed increase in m* with doping concentration is also consistent with previous reported studies, where resonant states were observed.12,14,41,43,45,46 It is clear from Figure 3d that varying m* does not allow the fitting of all the data on a single parabolic curve, thereby suggesting a 761


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decrease in the thermal conductivity results in ∼70% enhancement in ZT for the optimized Zr0.9V0.1NiSn HH alloys over that of its pristine counterpart, when compared with ZrNiSn. This high thermoelectric performance in Hf-free HH alloys will go a long way toward realizing cost-effective and efficient Hf-free HH based thermoelectric power generators.

temperature dependence of specific heat capacity, experimental density, and thermal diffusivity is shown in Supporting Information, Figure S1a,b. It was observed that the thermal conductivity decreases with increasing temperature for all the compositions, but it decreases with increasing V-doping at all temperatures, although this decrease is more pronounced at lower temperatures. The lowest thermal conductivity of ∼2.9 W m−1 K−1 at 873 K was realized for the composition Zr0.9V0.1NiSn, which is ∼20% lower than its pristine counterpart and is the lowest reported thus far for Hf-free HH alloys.35,39,64 In order to better understand the phonon scattering process, we have calculated the lattice thermal conductivity by subtracting the electronic contribution from the total thermal conductivity, the temperature dependence of which is shown as Figure 4b. A direct comparison of Figure 4a,b suggests that the temperature dependence of both the total and lattice conductivity is similar for all the compositions. However, the dominant contribution to the total thermal conductivity originates from its lattice counterpart for all the compositions, although this contribution is much higher at lower temperatures and decreases with increasing temperature. This observed decrease in the thermal conductivity on Vdoping in ZrNiSn may be attributed to phonon scattering due to mass fluctuations owing to a large atomic mass difference between V and Zr. Further, the substitutional defects caused by V-doping could also contribute toward the phonon scattering process leading to lowering the thermal conductivity. Although the nanoscale grain size also contributes effectively to reducing the thermal conductivity of pristine as well as the doped HH samples, it is the mass fluctuation effects and the substitutional defects caused by V-doping which additionally contribute toward the phonon scattering process, thus leading to the lowering of the thermal conductivity. The temperature dependence of the power factor (α2σ) shown in Figure 4c suggests an enhanced power factor of ∼3.5 × 10−3 W m−1 K−1 at 873 K for Zr0.9V0.1NiSn HH composition and is similar to the values reported earlier for Nb-doping26,60 and Ta-doping28 in the ZrNiSn HH alloy. The calculated temperature dependence of ZT, after incorporating the values of electrical and thermal transport properties, is shown as Figure 4d. This figure suggests that a highest ZT ∼ 1.0, at 873 K, is realized in an optimized HH composition Zr0.9V0.1NiSn, which is ∼70% higher than that of its pristine counterpart and is the highest among all the Hf-free n-type HH alloys reported thus far.35,39

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00203. Thermal transport properties and Rietveld refinement plots of Zr1−xVxNiSn (x = 0−0.20) half-Heusler alloys (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +91 11 4560 9456. Fax: +91 11 4560 9310. ORCID

Nagendra S. Chauhan: 0000-0003-2579-6642 Sivaiah Bathula: 0000-0001-6093-6351 Ajay Dhar: 0000-0001-7494-0378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the Board of Research in Nuclear Sciences, Department of Atomic Energy, India, for the financial support under Scheme 37(3)/14/22/2016-BRNS. Author N.S.C. acknowledges the financial support from CSIRIndia in the form of a Senior Research Fellowship (SRF). The technical support rendered by Mr. Radhey Shyam and Mr. Naval Kishor Upadhyay is also gratefully acknowledged.

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REFERENCES

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4. CONCLUSIONS The current study reports a ZT of unity at 873 K for n-type half-Heusler alloys at an optimized composition of Zr0.9V0.1NiSn, synthesized using arc-melting followed by spark plasma sintering. This high thermoelectric performance has been achieved by rationally engineering the thermal and electrical transport properties by resonant doping of V at the Zr-site in ZrNiSn HH alloys. V-doping induces resonant states and contributes surplus electrons near the Fermi level, which increases the effective mass and carrier concentration simultaneously, thus favorably tuning the electronic transport. A significant reduction in thermal conductivity was also observed which owes its origin to the simultaneous occurrence of the multiple phonon scattering phenomenon involving substitutional defects, mass fluctuations, and the nanosized grains in the synthesized HH alloys. The synergistic enhancement in electrical conductivity along with simultaneous 762


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