Realization of Band Convergence in p-Type TiCoSb Half-Heusler Alloys Significantly Enhances the Thermoelectric Performance

Abstract

Band engineering is a promising approach that proved successful in enhancing the thermoelectric performance of several families of thermoelectric materials. Here, we show how this mechanism can be induced in the p-type TiCoSbhalf-Heusler (HH) compound to effectively improve the Seebeck coefficient. Both the Pisarenko plot and electronic band structure calculations demonstrate that this enhancement is due to increased density-of-states effective mass resulting from the convergence of two valence band maxima. Our calculations evidence that the valence band maximum of TiCoSb lying at the Γ point exhibits a small energy difference of 51 meV with respect to the valence band edge at the L point. Experimentally, this energy offset can be tuned by both Fe and Sn substitutions on the Co and Sb site, respectively. A Sn doping level as low as x = 0.03 is sufficient to drive more than ∼100% increase in the power factor at room temperature. Further, defects at various length scales, that include point defects, edge dislocations, and nanosized grains evidenced by electron microscopy (field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM)), result in enhanced phonon scattering which substantially reduces the lattice thermal conductivity to ∼4.2 W m −1 K −1 at 873 K. Combined with enhanced power factor, a peak ZT value of ∼0.4 was achieved at 873 K in TiCo 0.85 Fe 0.15 Sb 0.97 Sn 0.03 . In addition, the microhardness and fracture toughness were found to be enhanced for all of the synthesized samples, falling in the range of 8.3−8.6 GPa and 1.8−2 MPa•m −1/2 , respectively. Our results highlight how the combination of band convergence and microstructure engineering in the HH alloy TiCoSb is effective for tuning its thermoelectric performance.

Approaching the minimum lattice thermal conductivity in TiCoSb half-Heusler alloys by intensified point-defect phonon scattering

“…The energy offset of 51 meV in TiCoSb can be manipulated by alloying on the Co and Sb sites, which modifies the energy of the band edges at the L and Γ point, respectively. In contrast to Fe and Sn doping in TiCo 0.85 Fe 0.15 Sb that promotes band convergence, 7 Zr and Hf might tend to induce the opposite effect. This trend suggests that the Ti orbitals mainly control the energy of the valence band edge at the Γ point, as evidenced in the ZrNiSn, NbCoSn and NbFeSb HH alloys by electronic band structure calculations.…”

“…In order to facilitate a valuable comparison, the transport parameters of TiCo 0.85 Fe 0.15 Sb, which were determined in a previous experiment, have been included. 7 At 300 K, σ tends to decrease in samples alloyed with Zr and Hf with respect to TiCo 0.85 Fe 0.15 Sb. Further alloying with Bi (0 ≤ x ≤ 0.04) weakly affects σ due to the small concentration of Bi introduced on the Sb site.…”

“…2,3 The general strategy to enhance the power factor includes optimization of carrier concentration by doping, 4,5 and electronic band engineering. 6,7 Due to the independency of κ L on electronic transport, limiting the propagation of heat-carrying acoustic phonons can be realized by various approaches 8 that notably include point-defect (atomic scale alloying) scattering, 9,10 bonding effect, 11,12 edge dislocations, 13,14 grain boundaries, 15,16 nano-sized grains, and the formation of in situ inclusion of nano precipitates in the host material. 17,18…”

Transport phenomena of TiCoSb: defect induced modification in the structure and density of states

“…However, pristine TiCoSb should have shown positive S(T), i.e., p-type in nature. 79,80 Synthesized samples have shown an n-type nature as Fermi surfaces have been modified due to embedded phases and defects. The magnitude of S(T) values is different than the previously reported.…”

Investigation of the electronic structure, mechanical, and thermoelectric properties of novel semiconductor compounds: XYTe (X = Ti/Sc; Y = Fe/Co)

“…Thus they proposed and verified the minimal cross-plane thermal conductivity on some multilayer systems. 27 Very recently, Verma et al 28 applied the band engineering approach to enhance the TE properties of mechanically alloyed TiCoSb. Co-doping of Fe on the Co sites and Sn on the Sb sites significantly lowered the lattice thermal conductivity and enhanced its PF.…”