Study on Half-Heusler Compounds for Thermoelectric Power Generation

Thermoelectric (TE) technique is unique in heat-to-power conversion due to its solid and non-moving nature. The efficiency of thermoelectric devices is related to the dimensionless figure-of-merit (ZT) of the material, defined as ZT=(S^2 σ)/κ T, where S, , , and T are the Seebeck-coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively, and the term S2 is called the power factor. The design of TE devices for power generation applications requires the knowledge of the mechanical properties of TE materials due to the externally loaded mechanical and thermal stresses. Therefore, this nanoindentation technique was adopted to test the mechanical properties of various TE materials applied in a moderate-temperature range (200-700 °C). The results show the half-Heusler (HH) compounds are more mechanically robust as compared with other materials and favorable for applications. However, the usage of hafnium (Hf) in the HH compounds is unfavorable for applications due to its ultrahigh price. Therefore, the TE performances of p-type HH compounds were investigated with decreased Hf usage. The optimized new compound (Hf0.19Zr0.76Ti0.05CoSb0.8Sn0.2) has ZT values similar to the previously reported best composition (Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2). But the specific power cost ($ W-1) of the new compound is much lower due to the suppressed usage of Hf. Similar suppressing of power cost was also obtained in the NbCoSn-based n-type HH compounds through the elimination of Hf usage. Furthermore, the study of TE performances of the NbFeSb-based p-type HH, another Hf-free compound, resulted in an extremely high power factor of ~106 μW cm-1 K-2 in Nb0.95Ti0.05FeSb due to the improved carrier mobility. This is the highest power factor among the semiconductor thermoelectric materials above room temperature. Subsequently, a single-leg device based on the high-power-factor material yielded a record output-power density of ~22 W cm-2 operating at between 293 and 868 K. Such a high output-power density greatly facilitates the large-scale power generation applications.