The intrinsic disorder related alloy scattering in ZrNiSn half-Heusler thermoelectric materials

Abstract

The intrinsic structural disorder dramatically affects the thermal and electronic transport in semiconductors. Although normally considered an ordered compound, the half-Heusler ZrNiSn displays many transport characteristics of a disordered alloy. Similar to the (Zr,Hf)NiSn based solid solutions, the unsubstituted ZrNiSn compound also exhibits charge transport dominated by alloy scattering, as demonstrated in this work. The unexpected charge transport, even in ZrNiSn which is normally considered fully ordered, can be explained by the Ni partially filling interstitial sites in this half-Heusler system. The influence of the disordering and defects in crystal structure on the electron transport process has also been quantitatively analyzed in ZrNiSn1-xSbx with carrier concentration nH ranging from 5.0×1019 to 2.3×1021 cm−3 by changing Sb dopant content. The optimized carrier concentration nH ≈ 3–4×1020 cm−2 results in ZT ≈ 0.8 at 875K. This work suggests that MNiSn (M = Hf, Zr, Ti) and perhaps most other half-Heusler thermoelectric materials should be considered highly disordered especially when trying to understand the electronic and phonon structure and transport features.

Enhancing the Figure of Merit of Heavy‐Band Thermoelectric Materials Through Hierarchical Phonon Scattering

“…Excess Fe may enter into the interstitial tetrahedral sites of the crystal structure to supply electrons, and make the hole concentration decrease. This phenomenon is similar to the scenario happened in another HH compound Ni 1+ y ZrSn, in which excess Ni had been proven to enter into the interstitial tetrahedral sites and generate electrons 28, 39…”

“…In contrast, high m b * of ≈2.5 m e are found for the p‐type Fe(V,Nb)Sb and ZrCoSb‐based HH compounds 29, 30. The room temperature μ of these p‐type HH compounds is usually several times lower than that of the typical n‐type counterparts,27, 28, 29, 30 which usually means smaller l c at the optimal carrier concentration. Hence, introducing more phonon scattering centers into the p‐type HH materials may be more feasible to improve their zT s, compared with the n‐type counterparts.…”

“…To reduce the κ L of HH compounds, isoelectronic alloying and nanostructuring are widely used,25, 26, 27, 28, 29, 30 which however may also cause a deterioration in μ , offsetting the reduction in κ L to some extent. For example, the average l c and l ph for n‐type Ni M Sn ( M = Hf, Zr, Ti) HH system are of the same order (≈10 −9 m) above room temperature, and the zT enhancement based on grain refinement is hence quite limited 18.…”

“…Similar result is also found for n‐type FeVSb‐based HH alloys 27. For these n‐type HH compounds the single band effective mass m b * are usually in the range of 1.0–1.3 m e 27, 28. In contrast, high m b * of ≈2.5 m e are found for the p‐type Fe(V,Nb)Sb and ZrCoSb‐based HH compounds 29, 30.…”

Ultrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)

“…In addition to the ultrahigh peak value, Nb 0.95 M 0.05 FeSb (M = Hf, Zr, Ti) system also showed remarkable enhancement in PF over a wide range of temperature in comparison with other high‐performance thermoelectric materials (see Figure 1), indicating the great potential for power generation over a wide temperature range. Furthermore, understanding the carrier scattering mechanism plays a crucial role in optimizing the electrical properties of thermoelectric materials, such as Mao et al demonstrated a significant enhancement in the mobility and PF of n‐type Mg 3.2 Sb 1.5 Bi 0.49 Te 0.01 by carefully tuning the hot pressing temperature to manipulate the variation of scattering mechanisms,61 while few studies focusing on the scattering mechanism in HH 63, 64. Therefore, further systematical investigation on the carrier scattering mechanism in the materials with ultrahigh PF is still necessary.…”