Vanadium-Doping-Induced Resonant Energy Levels for the

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Vanadium Doping Induced Resonant Energy Levels for the Enhancement of Thermoelectric Performance in Hf-free ZrNiSn half-Heusler Alloys Nagendra Singh Chauhan, Sivaiah Bathula, Avinash Vishwakarma, Ruchi Bhardwaj, Bhasker Gahtori, Anil Kumar, and Ajay Dhar ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Energy Materials

Vanadium Doping Induced Resonant Energy Levels for the Enhancement of Thermoelectric Performance in Hf-free ZrNiSn half-Heusler Alloys Nagendra S. Chauhan1,2, Sivaiah Bathula1,2,*, Avinash Vishwakarma1,2, Ruchi Bhardwaj1,2, Bhasker Gahtori1,2, Anil Kumar2 and Ajay Dhar1,2,∗ 1

Academy of Scientific & Innovative Research (AcSIR), CSIR-National Physical Laboratory, New Delhi110012, India. 2 Division of Advanced Materials and Devices, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India.

Abstract Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of costeffective 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 heat-carrying phonons. The optimized V-doped ZrNiSn HH composition, therefore, strikes a favourable balance between cost and thermoelectric performance, which would go a far way in the realization of a costeffective (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.

∗

Corresponding author: [email protected]; [email protected] Tel.: +91 11 4560 9456; Fax: +91 11 4560 9310 1 ACS Paragon Plus Environment

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Introduction

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

, where is the

Seebeck coefficient, is the electrical conductivity, T is the operating temperature and is the total thermal conductivity arising from electronic ( ) and lattice ( ) contributions ( = + ). However, in the present scenario there are a quite few major challenges associated with 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 towards the development of thermoelectric materials, which are non-toxic, earth-abundant and chemically stable at the operating temperatures. These include, Silicides4-5, Selenides6-7, half-Heuslers8-10 and other multi-element solid solutions such as (LAST, TAGS)11-12, which are currently being explored for TEG applications. Among these, halfHeuslers (HH) alloys are considered to be one of the most potential thermoelectric materials owing to their excellent chemical stability, high power factor and because they can be synthesized as both, nand 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 HHbased alloys for TEG applications. The HH alloys are of considerable research interest for TEG applications owing to their superior mechanical strength, non-toxicity and high thermal stability13-15. Although HH alloys exhibit semiconducting behaviour but their electronic structure can be modified by doping (substitution) and/or through partial filling of the vacant sites in their non-centrosymmetric 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 (n-type)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 favourable dopant at the Sn-site.13, 35, 37-38

It is well recognised in thermoelectrics that apart from ZT, low material cost and toxicity are the 2 ACS Paragon Plus Environment

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prime requirements, which need to be considered if TEG 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 in achieving a non-toxic and low-cost HH material for TEG applications. Among the Hf-free HH alloys, a highest ZT ~ 0.8 at 900K has been reported in 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 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 fermi level was also observed in half-Heusler system.45 Recently, in a systematic investigation on 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, based on 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 re-melted 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 obtained bulk dense pellets, which were annealed at 1023 K for 48 hours. 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 X-day 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 3 ACS Paragon Plus Environment


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40VP). The Seebeck coefficient and electrical resistivity were measured simultaneously employing commercial equipment (ULVAC, ZEM3) on polished bars samples of dimensions 3 × 2 × 10 mm3. The thermal diffusivity was measured using the laser flash technique (Lineseis, LFA 1000) on diskshaped samples of diameter 12.7 mm and thickness of 2.0 mm sprayed with a layer of graphite in order to 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, = 1.5 + exp −

||

∗ 10!" where WΩK-2 and α in µV/K). The carrier concentration and mobility

were determined using Hall Effect Measurement System (HEMS, Nano-magnetics), under a magnetic field of 0.5T. The optical band gap was estimated using UV-Vis-NIR spectroscopy (Agilent CARY5000) using reflectance accessories in normal mode. The accuracies in transport measurement are: ± 6% for thermal diffusivity, ± 7% for electrical conductivity, ± 7% for Seebeck coefficient, ± 2% for specific heat and ± 0.5% for density.

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 Fig. 1(a) along with a schematic indicating the arrangement of atoms in the lattice (Fig 1(b)). All the major peaks could be indexed to the ZrNiSn phase (ICDD- PDF-4+: 04-002-1779). The lattice constant is found to decrease with increasing V-doping (Fig. 1(c)) although with some deviation from the Vegard’s law. Such deviations from the 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 method10, was found to be in the range of ~ 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 earlier reported in several alloys20, 43, 50-53 which has been attributed to thermo-mechanical fatigue process.52 This SPS-induced nanostructuring is more economical and an efficient strategy than conventional time-consuming techniques of nanostructuring such as mechanical alloying and melt-spinning as it avoids the usage of multiple processing equipment. Fig 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 4 ACS Paragon Plus Environment

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nm, which is quite close to that estimated from the XRD data (Fig 1(a)). The Energy-dispersive X-ray spectroscopy (EDS) results in this figure indicate that the composition of the synthesized HH alloys is very close to their initial stoichiometry. Fig. 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 3 (a) and (b) shows the temperature dependence of the electrical transport properties of the Zr1−xVxNiSn with different V compositions. Fig. 3(a) 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 behaviour. 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 behaviour is preserved even at carrier concentration ~ 1020 cm-3, which can be qualitatively understood in terms of its electronic band structure, which has been earlier reported for ZrNiSn based HH alloys.45, 54-57 In 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 Zr-site leads to hybridization of valence electrons between Zr and dopant atoms via. their nearest neighbour 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, as a consequence of which 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 dopants levels in the vicinity of band gap results in a significant reduction of α.26-28, 60 However, in case of V-doping45-46, 58 a weaker hybridisation 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 mid-gap 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 5 ACS Paragon Plus Environment


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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 Fig 3(b) shows the temperature dependence of α for different V-doped ZrNiSn where negative values of α indicates that electrons are the dominant charge carriers suggesting a n-type 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 till 523K, beyond which it exhibits a saturating behaviour. Similar observation has been previously reported by Simonson et al.45 in V-doped HH alloys, which suggest that the Vdoping introduces a local feature in the density of states near to or within the Fermi level, which significantly contributes towards 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 (Fig 3(a)) and is in-line with those reported earlier for pristine and doped ZrNiSn HH alloys (Ti22, Pd23, Nb60 and Ta29). In order to further understand the resonant state phenomena in V-doped ZrNiSn, the band-gap (#$ ) was estimated using the relation62, %&' = #$ /2*%&' , where %&' is the absolute temperature corresponding +, %&' . The band-gap, calculated from this relation shows almost a linear decrease with increasing V-doping in ZrNiSn HH alloys (Fig3(c)), thereby suggesting that the band gap of ZrNiSn can be tailored by suitable V-doping. In addition, Fig 3(c) also shows the variation of experimental band gap, estimated using UV-Vis spectroscopy, which also revealed a decreasing trend with V-doping. Comparable band-gap values have earlier 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 Fig 3 (d)) was constructed based on the relationship between the Seebeck coefficient (α) , effective

mass

(-∗ )

and

the

carrier

density

(.),

which

is

described

as

:

= /81 2 32 ⁄3*ℎ2 )-∗ /1⁄3.)7 , where 83 is the Boltzmann constant, h the Planck’s constant, * the carrier charge, n the carrier concentration, µ the mobility, and m* is the effective mass of the charge carriers. This equation suggests that assuming a single parabolic band and an energy-independent 6 ACS Paragon Plus Environment

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relaxation time approximation, the thermo-power 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 (Fig. 3(b). 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 Fig 3(d) that varying m* does not allow the fitting of all the data on a single parabolic curve thereby suggesting a 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 doping63.

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 Nb26, 60 and Ta28 doping in ZrNiSn. The estimated m* is directly related to the band curvature and valley degeneracy /9: ) as (-∗ = 2/;

9: -

Vanadium-Doping-Induced Resonant Energy Levels for the

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