Compositions and thermoelectric properties of XNiSn (X = Ti, Zr, Hf) half-Heusler alloys

Abstract

Rietveld analysis of neutron powder diffraction data has been used to investigate the compositions of XNiSn (X = Ti, Zr, Hf) half-Heusler alloys prepared by solid state reactions. All samples containing Ti have 2-3% excess Ni, whereas the samples with X = Zr, Hf are almost stoichiometric.Samples with mixed X-metals are characterised by the presence of 3-4 distinct X 1-x X' x Ni 1+y Sn half-Heusler phases. Variable temperature and time dependent neutron powder diffraction for X = Ti and X = Ti 0.5 Hf 0.5 demonstrates that both the amount of excess Ni and the phase distribution are stable up to at least 600 °C. Debye temperatures of 367(2) K and 317 (2) K were obtained from the thermal displacement parameters. The samples containing Ti are characterised by a ~0.15 eV bandgap and a monotonously decreasing Seebeck coefficient. The compositions with Zr and Hf have similar bandgap values but show ambipolar transitions. Analysis of the thermoelectric transport data of degenerately doped Ti 0.5 Zr 0.5 NiSn 1-z Sb z samples using a single parabolic band model demonstrates that the transport is limited by alloy scattering and yielded an effective carrier mass of 2.5(1) m e .

Enhanced ideal strength of thermoelectric half-Heusler TiNiSn by sub-structure engineering

“…The substitution of Ti by Hf and Zr elements to form (Ti, Hf, and Zr)NiSn alloys is conrmed to be an effective way to enhance the thermoelectric properties of TiNiSn. [45][46][47][48][49][50][51] Herein, we investigate the effect of a mixture of Ti, Hf, and Zr on the ideal strength of (Ti, Hf, and Zr)NiSn. The most plausible slip system (111)/h 110i is chosen to impose the shear deformation on the doped systems (72 total atoms) of Ti 0.5 Hf 0.5 NiSn, Ti 0.5 Zr 0.5 NiSn, Ti 0.5 Hf 0.25 Zr 0.25 NiSn, and Hf 0.5 Zr 0.5 NiSn, respectively.…”

Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics

“…17 Other successful routes are to enhance boundary scattering through reduction of the average grain size [18][19][20] and by exploiting out-of-equilibrium approaches and segregation to create nanostructured materials. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] We have demonstrated that TiNiSn is unique in its ability to accommodate signicant amounts of interstitial Ni and Cu metals ($10% site occupancy), [41][42][43][44] whereas ZrNiSn and HfNiSn have far lower capacities (<2-3%) under the same preparation conditions. 45 These interstitials form highly effective phonon scattering centres, e.g.…”

Synthesis and Thermoelectric Properties of Ni-Doped ZrCoSb Half-Heusler Compounds

“…Half-Heusler compounds with 18 valence electron count (VEC) per unit cell have been widely studied as one of the promising thermoelectric materials working in the middle temperature range due to their narrow bandgap and high Seebeck coefficients, such as MCoSb, MNiSn (M = Ti, Zr, Nb, and Hf), et al [10][11][12][13][14] Among these compounds with 18 VEC, MCoSb based half-Heusler compounds (M = Ti, Zr, Nb, and Hf) have been extensively investigated including the substituting at M, A or B sites and many studies have achieved high thermoelectric properties. Yuan et al synthesized the ZrCoSb 1−x Sn x by substituting Sn on Sb sites and the maximum ZT value of 0.52 at 973 K was obtained [15].…”