Long‐Term Stability of (Ti/Zr/Hf)CoSb_1−xSn_x Thermoelectric p‐Type Half‐Heusler Compounds Upon Thermal Cycling

Abstract

The effect of thermal cycling upon the thermoelectric performance of state‐of‐the‐art p‐type half‐Heusler materials was investigated and correlated with the impact on the structural properties. We simulated a heat treatment of the material similar to actual applications in the mid‐temperature range, such as occurs during the energy conversion from an automotive exhaust pipe. We compared three different compositions based on the (Ti/Zr/Hf)CoSb1−xSnx system. The best and most reliable performance was achieved using Ti0.5Hf0.5CoSb0.85Sn0.15, which reached a maximum figure of merit ZT of 1.1 at 700 °C. The intrinsic phase separation and resulting microstructuring, which are responsible for the outstanding thermoelectric performance, were stable even after 500 heating and cooling cycles. The standard deviation of the obtained ZT values lies within 2–3 %, which is significantly smaller than the measurement errors.

Half‐Heusler materials as model systems for phase‐separated thermoelectrics

“…The as-cast samples undergo an eutectic demixing into two stable half-Heusler phases. The resulting micro-structure as well as the performance are amazingly stable under actual operation conditions [39]. The results of the calculations for T CoSb 0.8 Sn 0.2 with the pure Co:T =1:1 transition metal composition are summarized in Fig.…”

“…Based on these two concepts, a fine tuning of the Ti-to-Hf ratio for optimum phonon scattering in combination with optimum electronic properties, a maximum ZT of 1.2 at 710 • C in Ti 0.25 Hf 0.75 CoSb 0.85 Sn 0.15 has been achieved [38]. Very recently, the stability of the sub-microstructure and thermoelectric properties under thermal cycling conditions was proven [39]. Recently, high-performance ptype half-Heusler materials were synthesized on the FeSb base [24,40,41] that have a band structure similar to the CoSb-based materials.…”

Discovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency

“…Among the various thermoelectric materials, half-Heusler compounds, with large power factors 26 , robust mechanical properties 27 , and excellent thermal stabilities 28 , have been recently recognized as one of the most promising candidates for high temperature thermoelectric power generation 29 – 33 . However, due to the relative high lattice thermal conductivity (usually on the magnitude of ∼10 W m −1 K −1 for the pristine compounds), the ZTs (especially the average ZTs) of the state-of-the-art half-Heuslers remains relatively low comparing to other well-established material systems.…”

Thermoelectric Properties of the XCoSb (X: Ti,Zr,Hf) Half‐Heusler alloys

“…They are non‐toxic, environmentally friendly, and relatively cheap, having MgAgAs‐type cubic structures with the X, Co, and Sb atoms at the 4 b , 4 c , and 4 a Wyckoff positions, respectively . Both n‐ and p‐doped XCoSb alloys can be synthesized and are stable under thermal cycling . The thermoelectric figure of merit (with T being the temperature and the thermal conductivity) reaches values as high as 0.7 (n‐type) and 1.0 (p‐type) …”