The effect of spark plasma sintering on structure and phase stability in half‐Heusler thermoelectric alloys

Abstract

Thermoelectric (TE) materials have the capability to directly convert a temperature difference into an electric voltage across the material and vice versa. Half‐Heusler (HH) compounds are a promising group of TE materials. They are ternary intermetallic compounds, with the general formula XYZ – X and Y transition metals (e.g., X=Ti, Zr, Hf and Y=Ni, Co) and Z a metal or metalloid (e.g., Sn, Sb). The electronic structure and charge carrier concentration can be manipulated by atomic substitution on each crystallographic site to enhance the TE properties. However, further increase of efficiency of the alloys is limited by the relatively high thermal conductivity. Nanostructuring is reported to reduce the thermal conductivity as a result of an enhancement of the phonon scattering, thus increasing the efficiency of the TE materials [1]. Spark plasma sintering (SPS) is reported to give rapid densification of powders into bulk specimens and retain nanostructures (grain boundaries) from the starting powders. In the current study, n‐type XNiSn alloys with different (Ti, Zr, Hf) compositions on the X‐site were prepared by ball‐milling of arc‐melted and thermally annealed ingots, followed by SPS for 10 minutes, with temperatures between 850 and 1100 °C and applied pressures between 65 and 80 MPa. The SPS prepared alloys were investigated with a combination of X‐ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) techniques, supported by density functional theory (DFT) thermodynamic calculations. In all SPS samples, graphite is present in the surface regions. At the interface between the graphite and the XNiSn alloys, carbides are present as illustrated in figure 1 for the HfNiSn alloy – sintered at 1100 °C and 80 MPa. In agreement with previous reports on HH studies, single phase HH does not form in any of the alloy systems [2]. Secondary phases are distributed along compositional HH boundaries as illustrated in figure 2 from the TiNiSn alloy, sintered at 900 °C and 80 MPa. Phase separation of HH phases, with different composition X i and HH lattice parameter, is generally evident by splitting of the HH reflections in the XRD diffractograms. In the case of the TiNiSn alloy, such splitting of the HH reflection is not observed; however, we find compositional variations of the HH phase from the nominal composition – consistent with the variations in the shades of grey seen in the HH regions in figure 2. In addition to the HH and secondary phases seen in the figure, grains of full‐Heusler TiNi 2 Sn exist in the alloy.