Asymmetric Thermoelectric Performance Tuning in Low-Cost ZrFe<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>Sb Double Half-Heusler Materials
Joseph Ngugi Kahiu, Samuel Kimani Kihoi, Hyunji Kim, U. Sandhya Shenoy, D. Krishna Bhat, Ho Seong Lee
Abstract
The new paradigm for increasing the commercial viability of thermoelectric materials in the energy sector is the theoretical prediction and subsequent experimental validation and optimization of cheaper and inherently more efficient compositions. Herein, the experimental validation of the recently theoretically predicted ZrFe 0.50 Ni 0.50 Sb double half-Heusler and the ability to intrinsically tune this system to optimized p- or n-type materials by varying the Fe/Ni ratio in the synthesized ZrFe x Ni 1– x Sb ( x = 0.35–0.65) samples are demonstrated. The samples are synthesized by arc melting, hot pressing, and annealing. Subsequent microstructural analysis confirms the crystallization of the ZrFe x Ni 1– x Sb into the half-Heusler structure and reveals that the variation of the Fe/Ni ratio favors the Ni-rich side. Consequently, the best p-type x = 0.55 and n-type x = 0.35 samples exhibit higher power factor values stemming from an increased carrier concentration, higher density of state effective mass, and suppressed bipolar conduction, as indicated by the Hall data analysis and density functional theory simulations. The additional lattice disorders introduced by varying the Fe/Ni ratio suppress the thermal conductivity and increase the microhardness of the n-type samples. The ZrFe 0.35 Ni 0.65 Sb and ZrFe 0.55 Ni 0.45 Sb samples achieve maximum zT s of ∼0.43 and 0.06, respectively, which is a great improvement over the ∼0.001 value of the ZrFe 0.50 Ni 0.50 Sb sample. These results highlight the viability of tuning the performance of double half-Heuslers on the doubly doped site. They will be instrumental in demonstrating the feasibility of developing low-cost double half-Heusler materials with better intrinsic and highly tunable properties.