Realizing p-type NbCoSn half-Heusler compounds with enhanced thermoelectric performance via Sc substitution.

N-type half-Heusler NbCoSn is a promising thermoelectric material due to favourable electronic properties. It has attracted much attention for thermoelectric applications while the desired p-type NbCoSn counterpart shows poor thermoelectric performance. In this work, p-type NbCoSn has been obtained using Sc substitution at the Nb site, and their thermoelectric properties were investigated. Of all samples, Nb0.95Sc0.05CoSn compound shows a maximum power factor of 0.54 mW/mK2 which is the highest among the previously reported values of p-type NbCoSn. With the suppression of thermal conductivity, p-type Nb0.95Sc0.05CoSn compound shows the highest measured figure of merit ZT = 0.13 at 879 K.

Introduction

Thermoelectric (TE) devices can directly convert waste heat into electricity via the Seebeck effect, which is a promising reliable alternative to mechanical converters if the efficiency can be improved [1]. This could become a pathway to more sustainable energy converters in times of energy consumption [2-5]. However, up to now the application of thermoelectric devices is limited by the low energy conversion efficiency η, which is essentially decided by the materials’ dimensionless figure of merit ZT: ZT = S2σT/κ (κ = κc + κL), where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity (κc and κL are the carrier and lattice components of κ, respectively), and T is the absolute temperature [6,7]. Thus, the combination of high S, σ and low κ is desirable for a large ZT. Unfortunately, these thermoelectric parameters are strongly coupled via carrier concentration and mobility [8-10], making it difficult to optimize one single parameter without altering the others. Therefore, it has been quite challenging to develop highly efficient thermoelectric materials. The strategic goals of TE research are to discover new materials with high TE performance and/or improve the performance of the existing well-known materials, such as SiGe alloy [10], PbTe [11], chalcogenides [12], skutterudites [13,14], clathrates [15,16], Zintl phases [17] as well as full/half-Heusler compounds [18,19] by band engineering and carrier filtering effect [20-22], phonon engineering [23,24], reducing the dimension of materials [25], or spin fluctuation [26].

Due to the excellent electrical and mechanical properties, high-temperature stability, and possibility to use non-critical elements, half-Heusler (HH) compounds have become attractive candidates for thermoelectric application [27-31]. This compound has a general formula XYZ (X, Y = transition metals, Z = main group elements), crystallizing in cubic C1 structure, space group [32,33]. An empirical rule proposed by Mahan and Sofo [34] states that the best thermoelectric performance is found with materials whose band gap is about 10kBT0, where kB is the Boltzmann constant and T0 is the operating temperature of the device. NbCoSn is one of HH thermoelectric compounds with a band gap of 0.987 eV [35], and such a band gap fulfils the ‘10kBT0ʹ rule. In addition, the small electronegative difference between Co (1.88) and Sn (1.96) ensures larger carrier mobility. Therefore, NbCoSn is a promising mid-high temperature thermoelectric material. Unsubstituted NbCoSn is an intrinsically n-type semiconductor. In 2006 Ono et al. [36] investigated the Sb and Ti substituted n-type NbCoSn and the highest ZT of 0.3 was achieved for the Nb0.99Ti0.01CoSn0.9Sb0.1 at 850 K. A decade later, He et al. [37] synthesized n-type NbCoSn1-Sb samples by arc melting combined with ball milling and hot-pressing processes, and the highest ZT reached ~0.6 at 1000 K for NbCoSn0.9Sb0.1. Generally, a TE device needs not only high ZT in n-type and p-type materials, but the n-type and p-type materials should have similar compositions and thus comparable mechanical properties and thermal expansion coefficient. For instance, n-type MNi(Sn,Sb) [38] and p-type MCo(Sb,Sn) [39,40] (M = Zr and Hf) compounds fulfil such criteria, and the thermoelectric module made of them reaches a record-high conversion efficiency of 12.4% with a temperature difference of 698 K [41]. To fabricate p-n NbCoSn couple, a p-type NbCoSn based compound is needed to match with the developed n-type NbCoSn compounds. However, few efforts have been devoted to investigating p-type NbCoSn. To the best of our knowledge, Ferluccio et al. [42] first realized p-type NbCoSn via substituting Ti and Zr at Nb site, and the maximum ZT value for p-type Nb0.8Zr0.2CoSn is only 0.03 at ~790 K. Obviously, the ZT value of p-type Nb0.8Zr0.2CoSn compounds is much inferior to those of n-type counterparts. Therefore, it is crucial to identify new p-type dopants and further improve the thermoelectric performance of p-type NbCoSn.

In this work, the NbCoSn compound has been prepared through arc melting followed by annealing processes, and then Sc, chosen as a p-type dopant, is substituted at Nb site to obtain Nb1-ScCoSn (z ≤ 0.1). There are two main reasons for choosing Sc as a p-type dopant: (1) The substitution of Nb (III B) by Sc (V B) can create more holes in this compound and further achieve a p-type NbCoSn. (2) The larger mass fluctuation between Nb (92.91 g/mol) and Sc (44.96 g/mol) can strengthen defect and alloying phonon scattering which can significantly suppress the lattice thermal conductivity. It is found that Nb1-ScCoSn compound possesses p-type conducting behaviour when Sc = 0.05. The highest power factor of this p-type compound is 0.54 mW m−1 K−2, which is 230% higher than the reported value of Nb0.8Zr0.2CoSn [42]. As a result, a peak ZT of ~0.13 is achieved.

Experimental details

Nb1-ScCoSn (z ≤ 0.1) compounds were prepared by arc-melting stoichiometric amounts of the elements Nb (wire, 99.999%), Sc (piece, 99.99%), Co (bulk, 99.999%), Sn (shot, 99.999%) in Ar atmosphere. The ingots were melted several times with flipping twice over each time to ensure homogeneity. The obtained ingots were sealed into evacuated quartz tubes, annealing at 1173 K for 7 days. And then the bars and pellets for measurements were prepared by cutting these annealed ingots.

The crystal structures of samples were investigated by powder X-ray diffraction (PXRD) on a Rigaku diffractometer (Rigaku, Japan) using Cu Κα radiation (λ0 = 1.5418 Å). The microstructures of polished samples were characterized by scanning electron microscopy (SEM; Zeiss Gemini, Germany), and the phase compositions were analyzed by energy-dispersive X-ray spectroscopy (EDX; Bruker, Germany). Electrical transport properties (Seebeck coefficient and electrical conductivity) were simultaneously measured by a ZEM-3 instrument (Ulvac-Riko, Japan) under He atmosphere from 300 to 900 K. The measurement errors for all samples are around ±3% (electrical conductivity) and ±5% (Seebeck coefficient). The Hall carrier concentration pH (nH) and mobility μH were calculated via pH = 1/eRH (nH = ﹣1/eRH) and μH = σRH, where e is unit charge and RH is the Hall coefficient measured by commercial Physical Properties Measurement System (PPMS; Quantum Design, USA) under magnetic fields from −5.2 T to 5.2 T. The thermal conductivity was calculated by the formula κ = DCpd, where D is thermal diffusivity measured by laser flash instrument (NETZSCH, LFA457, Germany) by coating all samples with a thin layer of graphite to minimize emissivity errors (the actual measurement error is about 3%), Cp is specific heat derived by temperature-dependent heat data using the differential thermal analyzer (NETZSCH, DSC204F1, Germany), and d is the samples’ density estimated by the Archimedes method. The relative densities of these samples are about 95%.

Results and discussion

Phase and microstructure

The cubic NbCoSn crystal structure is shown in Figure 1. The element Nb (4a site) and Co (4b site) frame the NaCl sublattice with octahedral coordination, leaving the all tetrahedral central sites (4c site) to the element Sn, but only half of the 4c site is occupied by Sn, forming this half-Heusler compound. Simply, by applying the Zintl chemistry concept [43], this crystal structure can be described into an anionic framework [CoSn]5- formed by the tetrahedral coordination of Co and Sn, and an electropositive Nb5+ filled in the octahedral voids formed by these tetrahedral frameworks.

Figure 1.

The crystal structure of NbCoSn (a) and the coordination environment of Nb and Sn (b)

PXRD patterns of Nb1-ScCoSn (z = 0, 0.01, 0.03, 0.04, 0.05, 0.06, 0.07, 0.10) samples are shown in Figure 2(a). The diffraction peaks of all the samples can be indexed to MgAgAs cubic crystal structure despite some minor Nb3Sn impurity phases, indicating all samples possess the HH phase. As it is visible from Figure 2(b), the unit cell parameter calculated via PowderCell [44] software increases with the increasing Sc content. Since the ionic radius of Sc3+ (0.87 Å) is larger than that of Nb5+ (0.74 Å) [45], the observed lattice expansion indicates Nb5+ is substituted by Sc3+.

Figure 2.

(a) PXRD patterns and (b) cell parameters of Nb1-ScCoSn samples

The phase purity and elemental distribution were further examined by SEM combined with EDX mapping. Figure 3 shows the secondary electron image and the EDX maps of a polished Nb0.95Sc0.05CoSn sample, which indicates no obvious phase segregation and uniform elemental distribution on this scale. Table 1 summarizes the actual chemical compositions of all prepared samples analyzed by EDX, and it shows that the actual chemical compositions are close to the nominal compositions.

Figure 3.

The secondary electron image and the elemental distribution maps of Nb0.95Sc0.05CoSn

Table 1.

Nominal, actual compositions by EDX and measured densities of Nb1-ScCoSn

Nominal	EDX results	ds (g/cm3)
Nb0.99Sc0.01CoSn	Nb0.98Sc0.01Co1.05Sn0.96	8.49
Nb0.97Sc0.03CoSn	Nb0.97Sc0.03Co1.06Sn0.94	8.46
Nb0.96Sc0.04CoSn	Nb0.98Sc0.04Co1.03Sn0.95	8.03
Nb0.95Sc0.05CoSn	Nb0.97Sc0.045Co1.03Sn0.95	8.39
Nb0.94Sc0.06CoSn	Nb0.94Sc0.053Co1.05Sn0.95	8.00
Nb0.93Sc0.07CoSn	Nb0.93Sc0.067Co1.05Sn0.96	8.37
Nb0.90Sc0.10CoSn	Nb0.95Sc0.09Co1.03Sn0.94	8.27

Electrical transport properties

Figure 4 shows the electrical transport properties of Nb1-ScCoSn at different temperatures. For comparison, the literature data of Nb0.8Zr0.2CoSn [42] are also plotted (black line). As displayed in Figure 4(a), the electrical conductivity (σ) of unsubstituted NbCoSn compound decreases with the rise of temperature, showing a metal-like conducting behaviour. While, after substituting Sc at Nb site, the σ gradually increases with increasing temperature, indicating a semiconductor behaviour. Besides, with the Sc concentration up to 0.05, the σ significantly decreases from 5.0 × 104 S/m (NbCoSn) to 0.07 × 104 S/m (Nb0.95Sc0.05CoSn) at room temperature and then goes up to ~0.6 × 104 S/m with further increasing Sc concentration to 0.1 (Nb0.9Sc0.1CoSn). In addition, the band gap can be obtained from the slope of lnσ vs. 1000/T curve (shown in Figure 4(b)) using the Arrhenius equation

Figure 4.

Temperature dependence of the electrical transport properties of Nb1-ScCoSn (a) electrical conductivity (b) lnσ vs. 1000/T plot

where ρ is a constant. The calculated band gap values are 0.35 eV, 0.33 eV, 0.29 eV and 0.20 eV for p-type Nb0.95Sc0.05CoSn, Nb0.94Sc0.06CoSn, Nb0.93Sc0.07CoSn and Nb0.90Sc0.10CoSn respectively. For the Nb1-ScCoSn compounds with z > 0.04, the slope of σ-T changes above 500 K and it can be explained by the impact of intrinsic conduction.

To further understand the conduction mechanism, the Hall coefficient at room temperature was measured and the calculated charge carriers concentration and mobility are presented in Figure 5(a,b). The electron concentration nH for NbCoSn is 17 × 1019 cm−3, which is on the same order of magnitude as compared to the value reported by He et al. (~24 × 1019 cm−3) [37]. Moreover, the nH decreases remarkably with the Sc content reaching 0.04. For z = 0.05, holes become the dominant carriers, and the hole concentration pH increases gradually from 0.8 × 1019 cm−3 to 3.7 × 1019 cm−3 with the Sc content increasing from 0.05 to 0.10. Therefore, Sc is obviously an effective hole (p-type) dopant since it generates acceptor level near the top of the valence band and shifts the Fermi level toward the valence band, resulting in an increase of hole concentration. As for the carrier mobility μH, the trend is similar to that of carrier concentration. Therefore, the σ decreases obviously and then increases slightly with increase of Sc content.

Figure 5.

The carrier concentration and carrier mobility of Nb1-ScCoSn samples at room temperature (a) n-type, (b) p-type

The temperature dependence of Seebeck coefficient (S) in Nb1-zSczCoSn is shown in Figure 6(a). The value of S for unsubstituted NbCoSn is −185 μV/K at room temperature, showing that NbCoSn is apparently an n-type semiconductor. With Sc content increasing, the S of Nb1-ScCoSn changes gradually from negative to positive with increasing of Sc content, which matches very well with the Hall measurements. The peak value of S reaches ~306 μV/K at 850 K for the Nb0.95Sc0.05CoSn sample, and it is much higher than that (150 μV/K) of Nb0.8Zr0.2CoSn. The single parabolic band (SPB) model is usually used to analyze the transport properties of half-Heusler compounds [46]. Assuming electron conduction occurs within an SPB, the Seebeck coefficient of a non-degenerate semiconductor is related to the effective mass m*, carrier concentration p and scattering parameter λ via

Figure 6.

Temperature dependence of the Seebeck coefficient (a) and the Seebeck coefficient versus the carrier concentration (p-type) (b) of Nb1-ScCoSn samples

where e is the elementary charge and h is the Planck constant [47]. For the NbCoSn compound, we assume that acoustic phonon scattering is the predominant scattering mechanism, thus λ = 0. According to the measured S and pH, the m* = 0.11me is obtained. With the m* = 0.11me and the Equation (2), we can plot S at 300 K as a function of p, a plot well-known as a ‘Pisarenko relation’. As shown in Figure 6(b), the red line is the calculated Pisarenko plot, and the blue dots represent measured data of Nb1-ScCoSn compounds. Most of the data lie on the calculated line, except for that of Nb0.93Sc0.07CoSn compound. The reason for such an exception is not clear yet. It is suspected that the second phase or/and the deviation of the composition may be responsible for such an exception.

Accordingly, the power factor (PF) was calculated via PF = S2σ and is shown in Figure 7. The highest PF of 0.54 mW/mK2 is achieved for Nb0.95Sc0.05CoSn compound mainly due to its high S, and it is 3 times higher than that of p-type Nb0.8Zr0.2CoSn (0.125 mW/mK2) [42]. However, the PF of Nb0.95Sc0.05CoSn is still much lower than the state of the art p-type HH compounds, such as (Ti/Hf)Co(SbSn) [39] and NbFeSb [48]. Therefore, much more effort must be devoted to optimizing the carrier concentration of p-type NbCoSn compounds.

Figure 7.

Temperature dependence of the power factor of Nb1-ScCoSn

Thermal transport properties

The temperature dependence of total thermal conductivity (κ) and lattice thermal conductivity (κL) are displayed in Figure 8. The κc is calculated by using the Wiedemann-Franz law: κc = LσT, where L is Lorenz number estimated by Fermi integral, and then κL is derived from the value subtracting the carrier component κc from the total thermal conductivity. Because of the low electrical conductivity, the calculated κc is much lower than κL. In other words, for Nb1-ScCoSn compounds κL ≈ κ. As shown in Figure 8(b), the κL of unsubstituted NbCoSn is ~10.8 W/mK at room temperature, and this value is similar as compared to the result reported by Ferluccio et al. [42]. After substituting Sc, the room temperature κL decreases dramatically to 4.2 W/mK for Nb0.9Sc0.1CoSn, where a reduction of 60% is achieved after Sc substitution. Such a significant reduction mainly ascribes to the point defect scattering due to the substantial atomic mass difference (mass fluctuation) and interatomic coupling force differences (strain field fluctuation) between Nb and Sc, thereby giving rise to the reduction of κL, especially at room temperature.

Figure 8.

The temperature dependence of total thermal conductivity κ (a) and lattice thermal conductivity κL (b)

To explain the reduction of κL in terms of the phonon scattering mechanisms, the lattice thermal conductivity of Nb1-ScCoSn can be evaluated via Debye-Callaway model [49]:

where x is the reduced frequency (x = hω/2kT), ω is the phonon angular frequency, v is the sound speed, is the Planck constant, θ is the Debye temperature, and is the combined phonon relaxation time. The literature data [42] of θ = 361 K and v = 3141 m/s for NbCoSn are used in Nb1-ScCoSn compounds as a good approximation. We assume all phonon scattering processes, including point-defect scattering, boundary scattering, Umklapp scattering, and phonon-free-electron scattering can occur in parallel and thus each adds to the process according to the Matthiessen’s rule, then the can be formulated in Equation (4)

where , , , and are phonon-point-defect scattering, phonon-boundary scattering, phonon-phonon Umklapp scattering and phonon-free-electron scattering relaxation times, respectively. The can be obtained through:

where and are relaxation times of the phonon-point-defect scattering processes due to strain and mass field fluctuations, V is the volume per atom, and are the disorder scattering parameters due to strain and mass field fluctuations [50]. The experimental disorder scattering parameters Γ ( can be obtained by

where u is the disorder scattering parameter, is the lattice thermal conductivity of the crystal with the disorder, and is the lattice thermal conductivity of the crystal without disorder [51]. The disorder scattering parameters of Nb1-ScCoSn compounds calculated according to Equation (6) are listed in Table 2.

Table 2.

The lattice thermal conductivity , disorder scattering parameter , disorder scattering parameters

Composition	κL	u	Γexpt
NbCoSn	10.8
Nb0.99Sc0.01CoSn	8.4	1.03	0.0025(4)
Nb0.97Sc0.03CoSn	7.2	1.44	0.0049(8)
Nb0.96Sc0.04CoSn	6.8	1.61	0.0062(7)
Nb0.95Sc0.05CoSn	6.1	1.93	0.0089(8)
Nb0.94Sc0.06CoSn	5.0	2.59	0.016(1)
Nb0.93Sc0.07CoSn	4.6	2.88	0.019(9)
Nb0.90Sc0.10CoSn	4.2	3.27	0.025(6)

For the phonon-boundary scattering, is independent of temperature and phonon frequency, and it can be described as , where d is the grain size of the bulk sample. For the Umklapp scattering, is dependent on temperature and phonon frequency and can be described as [52]

where is the Grüneisen constant and M is the average atomic mass of the crystal. For the phonon-free-electron scattering process [53], in the case of high carrier concentration can be described as

where is the deformation potential, is the sample’s density, and is the longitudinal sound velocity. With the reference values of physics parameters (Table S1 in Supplementary Information), the lattice thermal conductivity of Nb1-ScCoSn can be calculated by Equation (3) and the results are presented in Figure 9. In the calculation, we assume that Sc substitution does not significantly affect the basic physics parameters, such as θ, , E and , so the major variable parameter is . In such a case, Sc substitution mainly alters the . Generally, the calculated κL matches with the experimental values, implying that calculations based on Callaway-Debye model can give a rough prediction to the κL of NbCoSn system. In short, at room temperature the reduction of κL of Nb1-ScCoSn compounds is mainly due to that the Sc substitution induces strong point defect phonon scattering.

Figure 9.

Comparison of experimental and calculated lattice thermal conductivities at 300 K for Nb1-ScCoSn compounds

Figure of merit

Figure 10 shows the dimensionless figure of merit ZT of p-type samples. Due to the dramatic enhancement of power factor compared with p-type Nb0.8Zr0.2CoSn, and the significant suppression of thermal conductivity, the highest ZT of p-type Nb0.95Sc0.05CoSn achieves 0.13 at 879 K. It is much higher than that of Nb0.8Zr0.2CoSn [42], indicating Sc is an efficient p-type dopant for NbCoSn as compared to Zr.

Figure 10.

The figure of merit ZT for p-type NbCoSn and Nb0.8Zr0.2CoSn

Conclusions

In this work, homogenous Nb1-ScCoSn compounds were prepared by an arc-melting process followed by an annealing treatment. The p-type NbCoSn compounds were obtained by substituting iso-electronic Sc at the Nb site and the effects on the electrical and thermal properties were investigated. Generally, the substitution of Sc at Nb site can change the n-type NbCoSn to a p-type semiconductor by adjusting the Fermi level, indicating Sc is an appropriate p-type dopant. Also, the thermal conductivity is reduced. As a result, the highest ZT is 0.13 at 879 K in p-type Nb0.95Sc0.05CoSn sample, which is roughly 5 times higher than that of p-type Nb0.8Zr0.2CoSn.

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