Study of half-Heusler and related structures as high-temperature thermoelectrics

Abstract

Motivated by recent advancement in half-Heusler thermoelectric materials, here we studied the phase stability and thermoelectric properties of some half-Heuslers (HH) and their derivatives. From within the HH family, the HH alloys and compounds with a valence electron count (VEC) 18 per formula unit are of particular interest in the thermoelectric research as they exhibit a semiconducting behavior due to a finite energy gap between their filled bonding orbitals and empty antibonding orbitals. Recently, it was shown that some HH compounds with VEC 19 (for example, NbCoSb) also exhibit a semiconducting behavior. This is puzzling since the extra electron is expected to give rise to a metallic behavior. Later, Xia et al. found that NbCoSb cannot be formed in a phase pure HH structure in the 111 stoichiometry but only in the stoichiometry Nb0.8CoSb, i.e., with 20% cationic vacancy [205]. These vacancies contribute to the lowering of the lattice thermal conductivity; however their presence also affects the carrier mobility adversely. After giving an overview of the recent thermoelectric research on HH materials in chapter 1 and the experimental techniques used in this thesis in chapter 2, in chapter 3 we study Nb0.8+δCoSb (δ = 0.03) doped with Sn at the Sb site (a slight Nb excess - that is, 0.8 + δ instead of 0.8 helps in electron doping and hence tuning the carrier density). Sn doping for Sb in Nb0.83CoSb plays two vital roles: (i) with increasing Sn doping, excess Nb concentration δ in the Nb0.8+δCoSb can be increased at least upto 0.1, which is more than 0.05 without Sn-doping, (ii) since Nb is strongly electropositive, small variations in Nb results in large fluctuations in the carrier concentration (i.e., the tuning is coarse). We show that Sn doping helps fine tune the carrier concentration to obtain an optimum concentration for maximizing the power factor. In the Sn-doped samples we obtained a zT exceeding little over 1 near 1100 K, which is 20% higher than the value previously reported for the undoped or Ni doped Nb0.8+δCoSb. Detailed electronic structure calculations are done to understand the role of Sn-doping in the observed zT enhancement. We also performed Raman spectroscopy and HRTEM to investigate the short-range vacancy ordering in these compounds. In chapters 4 and 5, we study TiNiSi structure-type compounds. These compounds have an orthorhombic structure but with Heusler-like tetrahedral coordination [100, 12, 18]. In chapter 4, we investigated five such orthorhombic ternary compounds, namely, ZrNiSi, ZrNiGe, NbCoSi, HfNiSi, and ZrNiSb [162]. Based on a theoretical study, Guo et al. [58] had predicted these compounds to stabilize with the HH structure at room temperature. However, the arc-melted samples crystallize with the TiNiSi-type orthorhombic structure. We studied the phase stability and thermoelectric properties of these samples. We show the presence of a pseudo-gap like feature in their electronic density of states except ZrNiSb where there is finite density of states at the Fermi energy. Detailed electronic structure and density of states were obtained to understand their bonding mechanism and semi-metallic behavior. In chapter 5, we have investigated the effect of doping ZrNiSi with Sb at the Si site. The solubility of Sb in ZrNiSi at the Si site is as low as 5 %, beyond which it undergoes a spinodal-like decomposition. The higher doping leads to an enhanced power factor and lower thermal conductivity. We show that zT of ZrNiSi increases by more than an order of magnitude by a small Sb doping. Effect of annealing in the Sb doped samples undergoing spinodal-like decomposition is also studied. We show that annealing leads to a small reduction in the thermoelectric performance. Recently, double Heuslers (DH) have gained interest in thermoelectric research. Two or more elements randomly occupying the same Wyckoff position create a highly disordered structure which helps in reducing the thermal conductivity [59, 6]. In chapter 6, we study some new double Heuslers (DH) that were synthesized by mixing orthorhombic ZrNiSb (VEC 19) and cubic TiFeSb (VEC 17). Both these end-members by themselves do not exist in the pure phase. The new compounds obtained have compositions (ZrNiSb)1−x(TiFeSb)x where (x = 0.4, 0.45, 0.5, 0.55, 0.6, and 0.7). The sample x = 0.5, was nearly pure with a cubic HH structure, XYZ, with X site randomly occupied by Zr and Ti, and Y site by Ni and Fe. We synthesized composition around x = 0.5 and found that compositions with x > 0.5 show a p-type behavior and those with x < 0.5 n-type. We studied these samples, for their phase purity, microstructure, and thermoelectric properties. They exhibit a very low thermal conductivity due to the presence of a high level of atomic disorder inherent to the DH alloys. For these samples, the highest thermoelectric figure of merit zT turned out to be 0.125 for the p-type and 0.2 for the n-type samples. In chapter 7, the summary of the work carried out and future directions that this work has opened up are presented.