The Effects of Excess Co on the Phase Composition and Thermoelectric Properties of Half-Heusler NbCoSb

Abstract

NbCoSb with nominal 19 valence electrons, and is supposed to be metallic, has recently been reported to also exhibit the thermoelectric properties of a heavily doped n-type semiconductor. In this study, we prepared Co-rich NbCo1+xSb samples (x = 0, 0.2, 0.3, 0.4, 0.5), and their phase compositions, microstructures and thermoelectric properties were investigated. The Seebeck coefficient increased a great deal with increasing x, due to decreasing carrier concentration, and the total thermal conductivity reduced mainly because of declining κe. Finally, a peak thermoelectric figure of merit, ZT, was about 0.46 for NbCo1.3Sb at 973 K. This enhancement was mainly attributed to the reduction of electric thermal conductivity and the increase of Seebeck coefficient. The excess Co had effects on the carrier concentration, deformation potential Edef and DOS effective mass m*. Adding an excessive amount of Co leads to a very high Edef, which was detrimental for transport characteristics.

18-Electron half-Heusler compound Ti0.75NiSb with intrinsic Ti vacancies as a promising thermoelectric material

“…Comparing the ZT value of Ti 0.75 NiSb at 873 K with previous work, 26,[29][30][31][32][33][34][35][36] we found that it is below the average level, as shown in Fig. 11c.…”

“…Due to the introduction of many intrinsic Ti vacancies, a lattice thermal conductivity of about 1.08 W m À1 K À1 was obtained at 873 K for Ti 0.75 NiSb, which is much less than those of many other ternary HH materials. 26,[29][30][31][32][33][34][35][36] We have studied its phase composition, crystal structure and thermoelectric properties and provided explanations through theoretical calculations.…”

In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance

“…Further, Ge-doped samples exhibit high κ l at RT except for two samples having Ge concentrations of 1.5 and 2.0 atom %, however, at a high temperature of ∼873 K, 0.5 atom % Ge-doped sample also exhibits lower κ l along with 1.5 and 2.0 atom %, when compared with ZrNi 1.04 Sn. This kind of abnormal behavior has also been previously observed. , Here, it is worth noting that we have observed quite different results from that expected. In general, it is assumed that the inclusion of impurity atoms improves phonon scattering due to generated point defects.…”

“…However, in our observations referring to the entire measured temperature range, we found high κ l of samples with 3.0 and 4.0 atom % Ge-doping than that of ZrNi 1.04 Sn. Here, this anomalous behavior might be due to the variation in the existing volume fraction of secondary phases, which are metallic in nature. , Further, these results are not in good accordance with theoretical lattice thermal conductivity shown in Figure , as theoretical calculations have been carried out without introducing secondary phases.…”

Improving the Thermoelectric Properties of the Half-Heusler Compound VCoSb by Vanadium Vacancy

“…Unconventionally, experiments have confirmed that NbCoSb and VCoSb, with a cubic HH structure and 19 valence electrons per unit cell, exhibit moderate n-type TE properties and achieve a ZT value of about 0.4 and 0.5 at 973 K [23,24]. However, impurity phases always exist in a NbCoSb system and obstruct the improvement of its TE properties [25,26,27]. Moreover, Zeier et al presented that NbCoSb with stoichiometric composition is unstable, and the nonstoichiometric HH compound Nb 0.8 CoSb with 18-electron and Nb vacancies was pointed out to be a more stable semiconductor [28].…”