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 TiCo0.85Fe0.15Sb0.97Sn0.03. 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.