The 18-electron rule is a widely used criterion in the search for new half-Heusler thermoelectric materials. However, several 19-electron compounds such as NbCoSb have been found to be stable and exhibit thermoelectric properties rivaling state-of-the art materials. Using synchrotron X-ray diffraction and density functional theory calculations, we show that samples with nominal (19-electron) composition NbCoSb actually contain a half-Heusler phase with composition Nb 0.84 CoSb. The large amount of stable Nb vacancies reduces the overall electron count, which brings the stoichiometry of the compound close to an 18-electron count, and stabilizes the material. Excess electrons beyond 18 electrons provide heavy doping needed to make these good thermoelectric materials. This work demonstrates that considering possible defect chemistry and allowing small variation of electron counting leads to extra degrees of freedom for tailoring thermoelectric properties and exploring new compounds. Here we discuss the 18-electron rule as a guide to find defect-free half-Heusler semiconductors. Other electron counts such as 19-electron NbCoSb can also be expected to be stable as n-type metals, perhaps with cation vacancy defects to reduce the electron count.

Here we report the thermoelectric properties of NbCoSn-based n-type half-Heuslers (HHs) that were obtained through arc melting, ball milling, and hot pressing process. With 10% Sb substitution at the Sn site, we obtained enhanced n-type properties with a maximum power factor reaching ∼35 μW cm−1 K−2 and figure of merit (ZT) value ∼0.6 in NbCoSn0.9Sb0.1. The ZT is doubled compared to the previous report. In addition, the specific power cost ($ W−1) is decreased by ∼68% comparing to HfNiSn-based n-type HH because of the elimination of Hf.

No abstract

A B S T R A C TWe surprisingly made a new n-type thermoelectric compound NbCoSb with half-Heusler (HH) structure having valence electron count of 19, different from the traditional 18, which opens up a new route to develop new half-Heusler thermoelectric materials not following the traditional valence electron count of 18. The samples are made by arc melting followed by ball milling and hot pressing. The effect of hot pressing temperature on the thermoelectric properties of NbCoSb samples has been studied. A maximum thermoelectric figure-of-merit (ZT) of $0.4 is achieved at 700 C in NbCoSb sample that is hot pressed at 1000 C. This work add a new member to HH compounds for thermoelectric applications, although the peak ZT of $0.4 is still lower than that of the traditional HHs. Moreover, it is very interesting to see that a traditionally thought of valence electron counts of 18 is not required.

No abstract

This paper mainly investigates a class of almost periodic Nicholson's blowflies model involving a nonlinear density-dependent mortality term and time-varying delays. Combining Lyapunov function method and differential inequality approach, some novel assertions are established to guarantee the existence and exponential stability of positive almost periodic solutions for the addressed model, which generalize and refine the corresponding results in some recent published literatures. Particularly, an example and its numerical simulations are given to support the proposed approach.

Half-Heusler compounds with valence electron count (VEC) of 19 were not believed to have good thermoelectric properties because it was theoretically predicted as metallic. However, this work demonstrates experimentally that half-Heusler compound VCoSb we synthesized is in fact a good thermoelectric material. As-made samples show single half-Heusler phase and negative Seebeck coefficient with a peak value around -130 V/K at 600 °C, which indicates the semiconductor-, not metallic-, like behavior. The VCoSb samples were made by arc-melting the elements to first form ingots, then ball-milling the ingots, and finally hot-pressing the fine powder to form the bulk materials. Different hot-pressing temperatures at 750 °C, 800 °C, and 900 °C were carried out and the results are discussed. A maximum thermoelectric figure-of-merit (ZT) around 0.5 is achieved at 700 °C for all the samples.

No abstract