Enhancement of thermoelectric properties in the Nb–Co–Sn half-Heusler/Heusler system through spontaneous inclusion of a coherent second phase

Abstract

Half-Heusler XYZ compounds with an 18 valence electron count are promising thermoelectric materials, being thermally and chemically stable, deriving from relatively earth-abundant components, and possessing appropriate electrical transport properties. The typical drawback with this family of compounds is their high thermal conductivity. A strategy for reducing thermal conductivity is through the inclusion of secondary phases designed to minimize negative impact on other properties. Here, we achieve this through the addition of excess Co to half-Heusler NbCoSn, which introduces precipitates of a semi-coherent NbCo2Sn Heusler phase. A series of NbCo1+xSn materials are characterized here using X-ray and neutron diffraction studies and electron microscopy. Electrical and thermal transport measurements and electronic structure calculations are used to understand property evolution. We find that annealing has an important role to play in determining antisite ordering and properties. Antisite disorder in the as-prepared samples improves thermoelectric performance through the reduction of thermal conductivity, but annealing during the measurement degrades properties to resemble those of the annealed samples. Similar to the more widely studied TiNi1+xSn system, Co addition to the NbCoSn phase results in improved thermoelectric performance through a decrease in thermal conductivity which results in a 20% improvement in the thermoelectric figure of merit, zT.

Improved Thermoelectric Performance in N-Type Half-Heusler NbCoSn by Heavy-Element Pt Doping

“…xPtxSn (x = 0.00-0.15) samples are shown in Figure 1a. The main phase is consistent with the cubic MgAgAs-type half-Heusler structure, and tiny impurity peaks are hardly observed in the range of binary phases (Nb3Sn, Co7Nb6) typically reported for this compound 41 . Although the impurity peaks are more noticeable for the samples with high Pt contents, the peak intensities are very low to enable precise identification of these secondary phases using the laboratory XRD analysis.…”

“…He et al 40 reported 80% enhancement in the power factor by improving the samples' phase purity, resulting in a maximum zT of 0.6 at 973 K in Sb-doped NbCoSn. Additionally, a material comprising the half-Heusler NbCoSn mixed with the full-Heusler NbCo2Sn was studied, and the antisite disorder was found to be suppressed by annealing 41 . Generally, half-Heusler alloys require a high content of chemical doping to realize the optimal electrical power factor and consequently, the dopants could also generate significant suppression on phonon transport 13 .…”

Thermoelectric performance and the role of anti-site disorder in the 24-electron Heusler TiFe2Sn

“…[7] The 24 VEC Heusler compounds such as TiFe 2 Sn and VFe 2 Al are of potential value as thermoelectrics due to relatively small band gaps. Anti-site disorder [8] and other structural defects [9] are characteristic to Heusler compounds, influencing the band structure [10] and electrical properties including the resistivity and Seebeck coefficient.…”

New frontiers for the materials genome initiative

“…Discrete dislocation dynamics (DDD) simulations are becoming more common but remain limited. While there have been insights gained in the area of small-scale plasticity and thin films, 13,90,[130][131][132][133][134][135][170][171][172][173][174] the challenge for the DDD simulations is to have the same impact on bulk plasticity. In the context of polymeric materials, models and characterization methods must be developed to understand welding and diffusion of polymeric molecules across interfaces, leading to entanglements and strengthening of complex threedimensional structures as they are produced.…”