Improved thermoelectric performance of n-type half-Heusler MCo_1-xNi_xSb (M = Hf, Zr)

Highlights

• The MCoSb-based half-Heusler compounds were turned into n-type through Ni substitution at the Co site.

• The mechanisms of the observed anomalous variations of carrier concentration and lattice thermal conductivity were discussed.

• Peak ZT exceeding 1 was obtained in Hf_0.5Zr_0.5Co_0.9Ni_0.1Sb at 1073 K.

Abstract

The MCoSb-based (M = Hf, Zr) half-Heusler compounds were recognized as a promising p-type thermoelectric (TE) material for more than 2 decades although the base compound is intrinsically n-type. Here we investigate the TE properties of Ni-substituted n-type MCoSb. The anomalous changes of carrier concentration and lattice thermal conductivity with higher amount of Ni indicate the presence of atomic disorder. Peak power factor of ∼33 μW cm⁻¹ K⁻² and peak ZT of 0.6 are obtained in ZrCo_0.9Ni_0.1Sb. Further substitute Zr by Hf suppresses the lattice thermal conductivity and yields a peak ZT exceeding 1 in the composition Zr_0.5Hf_0.5Co_0.9Ni_0.1Sb at 1073 K. Thus the MCoSb compounds possess promising TE properties by both n- and p-type doping, which is unique among the half-Heusler based TE materials.

Introduction

The thermoelectric (TE) technique is promising in waste-heat recovery due to its solid-state and non-moving nature. The heat-to-power conversion efficiency of TE materials is governed by the figure-of-merit, ZT, which is defined as $ZT=[S^2\sigma /\left({\kappa }_e+{\kappa }_L\right)]T$, where S, σ, κ_e, κ_L, and T are the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, lattice thermal conductivity, and absolute temperature, respectively. The term S²σ is called the power factor, which governs the output power density with certain hot- and cold-side temperatures as well as fixed leg lengths [1]. The recent development of thermoelectric materials shows its great potential for heat-to-power conversion applications [2], [3], [4], [5], [6].

One group of interesting TE materials are the half-Heusler (HH) based compounds. The half-Heusler (HH) compounds are crystallized in the space group $F\overline{4}3m$ and typically possess the formula XYZ, where X, Y, and Z occupy the Wyckoff position 4b (0.5, 0.5, 0.5), 4c (0.25, 0.25, 0.25), and 4a (0, 0, 0), respectively, and leaving the position 4d (0.75, 0.75, 0.75) as voids. Among the hundreds of HH compounds, the ones with valance-electron-counts 8 and 18 are semiconductors, and suitable for TE application [7], [8]. Especially, good TE performances were observed in electron-doped MNiSn compounds (M = Hf, Zr, and Ti), hole-doped MCoSb compounds, and hole-doped NbFeSb compounds [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. In comparison to other TE materials, the half-Heusler compounds are very promising for power generation applications due to their good ZT at 700–1100 K [12], [14], [18], the mechanical robustness [22], [23], [24], low cost [9], [21], [25], and nontoxicity. More importantly, the thermoelectric generator devices based on half-Heusler compounds also showed a high thermal stability by thermal cycling the device 1000 times between 100 °C and 600 °C without degrading the TE performances [21]. In addition, the record high output power density of 22 W cm⁻² was also reported in the half-Heusler material [20].

Among half-Heusler compounds, the MCoSb-based compounds are particularly interesting because they are intrinsically n-type, while good p-type properties were realized through an overcompensating hole-doping, such as substituting Sn at the Sb site, and substituting Fe at the Co site, etc. [26], [27], [28]. This is different from the cases of the MNiSn- and NbFeSb-based compounds, where the favorable charge carrier (electrons or holes) is the same as the dominant carrier in the undoped compounds [20], [29]. On the other hand, previous work on n-type MCoSb suggested decent TE performance [30], [31], [32], [33], [34], [35], [36]. For example, Ouardi, et al. showed a high Seebeck coefficient of −367 μV K⁻¹ at 350 K in the composition Ti_0.99V_0.01CoSb [35]. Xie et al. obtained a peak ZT of ∼0.5 in Ti_0.5Zr_0.25Hf_0.25Co_0.95Ni_0.05Sb at 800 K [36]. A higher ZT of ∼0.7 was obtained in Ti_0.6Hf_0.4Co_0.87Ni_0.13Sb by Qiu et al. at 900 K [31]. These reports show that MCoSb-based compounds might also be promising n-type materials.

Here we investigate the composition- and temperature-dependent thermoelectric properties of n-type ZrCo_1-xNi_xSb, where x = 0, 0.05, 0.1, and 0.15. The undoped composition shows negative Seebeck coefficient. Ni substitution improves the TE performance, where a high power factor of ∼33 μW cm⁻¹ K⁻² and a peak ZT of ∼0.6 are obtained in ZrCo_0.9Ni_0.1Sb. Interestingly, we observe unexpected variations of the carrier concentration and the lattice thermal conductivity with increasing Ni concentration which should originate from atomic disorders, possibly the Zr/Sb antisite disorder. Further we investigate the TE properties of Zr_0.5Hf_0.5Co_1-xNi_xSb. The enhanced point defect scattering effectively suppresses the lattice thermal conductivity. On the other hand, the power factor is less affected. Therefore the peak ZT is boosted to higher than 1 in Zr_0.5Hf_0.5Co_0.9Ni_0.1Sb at 1073 K. The obtained peak ZT is comparable to the Sn-doped p-type MCoSb [9], [37]. Therefore the MCoSb-based half-Heusler favors both types of charge carrier for high TE performances, which is unique among the half-Heusler compounds.