Defect Engineering for Enhancement of Thermoelectric Performance of (Zr, Hf)NiSn-Based n-type Half-Heusler Alloys

Abstract

Defect engineering of thermoelectric (TE) materials enables the alteration of their crystal lattice by creating an atomic-scale disorder, which can facilitate a synergistic modulation of the electrical and phonon transport, leading to the enhancement of their TE properties. This work employs a compositional nonstoichiometry strategy for manipulation of Nivacancies and Ni-interstitials through Ni-deficient and Ni-excess compositions of (Zr, Hf)Ni 1±x Sn-based half-Heusler (HH) alloys to realize a stateof-the-art TE figure-of-merit (ZT) of ∼1.4 at 873 K in 4 atomic % Ni-excess HH composition, which corresponds to a remarkable TE conversion efficiency of ∼12%, estimated using the cumulative temperature dependence model. These alloys are synthesized employing arc-melting followed by spark plasma sintering and are characterized for their phase, morphology, structure, and composition along with electrical and thermal transport properties to examine the implication of Niexcess and Ni-deficiency on the TE properties of the synthesized Zr 0.6 Hf 0.4 NiSn HH alloy. A significant enhancement (∼30%) of ZT is observed in the low doping limit of Ni-excess HH compositions over their stoichiometric counterpart due to Ni-interstitials and in situ full-Heusler precipitation, which enable a strong phonon scattering for a drastic reduction in lattice thermal conductivity and lead to an enhancement of ZT. However, Ni-deficient HH compositions exhibit a deterioration in the TE properties owing to the vacancy-induced bipolarity. The defect-mediated optimization of electrical and thermal transport, thus, opens up promising avenues for boosting the TE performance of HH alloys.

Ultra-High Thermoelectric Power Factors in Narrow Gap Materials with Asymmetric Bands

“…42,57−59 It is, however, important to notice that half-Heusler compounds are usually doped with a much higher carrier density than what is suggested here, typically ≥ 10 20 cm −3 , 60−63 whereas indications of best doping in the range around 10 19 cm −3 came recently for the (Hf, Zr)NiSn system in polycrystalline samples. 5 It is remarkable that an experimental characterization exists for polycrystalline HfNiSn at a low carrier concentration of ∼6 × 10 18 cm −3 , in the range of the suggested values. 64 The measured PF value at 900 K is around 3.6 mW/m•K 2 , lower than the computed value ∼15 mW/m•K 2 .…”

Interplay Between Electronic States and Structural Stability in Cation-Deficient VCoSb, NbCoSb, and TaCoSb Half-Heuslers

“…6 In half-Heusler (HH) thermoelectrics, based on semiconductor materials, the understanding and judicious selection of defects has also been beneficial to the performance of materials. [7][8][9][10] In thermoelectric materials, the presence of vacancy defects can help to tune the power factor or/and the lattice thermal conductivity to deliver a higher figure-of-merit (ZT). Thus, one finds reports of vacancies assisting the desired reduction of the lattice thermal conductivity in CuGaTe 2 , 11 while in SnTe-In 2 Te 3 , 12 Ge 9 Sb 2 Te 12 , 13 and -Zn 4 Sb 3 , 10 the presence of vacancies simultaneously improves the electrical properties and reduces the lattice thermal conductivity.…”

Scalable colloidal synthesis of Bi2Te2.7Se0.3 plate-like particles give access to a high-performing n-type thermoelectric material for low temperature application

“…The calculated CF of our TE material decreases with increasing temperature as a result of its direct correlation with ZT, and it is comparable to other state-of-the-art n-type TE materials in mid-temperature range. 69 This result suggests that n-type BTS consolidated from plate-like particles will also be a suitable TE material for segmentation with high-performance mid-temperature TE materials 22 , such as PbTe 70 and half-Heuslers, [71][72][73][74] for achieving efficient TE energy conversion. commercial applications, zone melting is a commonly used production technology for Bi2Te3based materials and devices.…”