Abstract
Half-Heusler alloys, which possess the advantages of high thermal stability, a large power factor, and good mechanical property, have been attracting increasing interest in mid-temperature thermoelectric applications. In this work, extra Zr-doped TiZrxNiSn samples were successfully prepared by a modified solid-state reaction followed by spark plasma sintering. It demonstrates that extra Zr doping could not only improve the power factor on account of an increase in the Seebeck coefficient but also suppress the lattice thermal conductivity originated from the strengthened phonon scattering by the superlattice nanodomains and the secondary nanoparticles. As a consequence, an increased power factor of 3.29 mW m-1 K-2 and a decreased lattice thermal conductivity of 1.74 W m-1 K-1 are achieved in TiZr0.015NiSn, leading to a peak ZT as high as 0.88 at 773 K and an average ZT value up to 0.62 in the temperature range of 373-773 K. This work gives guidance for optimizing the thermoelectric performance of TiNiSn-based alloys by modulating the microstructures on the secondary nanophases and superlattice nanodomains.
Highlights
Thermoelectric (TE) materials can directly convert heat into electricity, i.e., the carrier density gradient causes the diffusion of the carriers from the hot to the cold end of a semiconductor, and an electric potential difference would be built up against this process, having bright prospects in the low-grade waste heat recovery [1 − 3]
It demonstrates that extra Zr doping could improve the power factor on account of an increase in Seebeck coefficient and suppress the lattice thermal conductivity originated from the strengthened phonon scattering by the superlattice nanodomains and the secondary nanoparticles
The energy conversion efficiency of thermoelectric materials is usually tailored by the dimensionless figure of merit (ZT), ZT = S2σ κ−1T, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, S2σ is the power factor (PF), κ is the total thermal conductivity, which is usually comprised of the electronic and lattice thermal conductivity
Summary
Thermoelectric (TE) materials can directly convert heat into electricity, i.e., the carrier density gradient causes the diffusion of the carriers (electrons and holes) from the hot to the cold end of a semiconductor, and an electric potential difference would be built up against this process, having bright prospects in the low-grade waste heat recovery [1 − 3]. The high thermal conductivity (e.g., ~ 10 W m− 1 K− 1 at room temperature [16]) is one of the biggest disadvantages for half-Heusler alloys as a thermoelectric material candidate [17]. Various strategies, such as boundary phonon scattering [18, 19], nanostructures [20], and multi-scale scattering [21], were employed to reduce the κl by strengthening the phonon scattering. Schrade et al. reported that both ultrafine grains (≤ 100 nm) and defects can play an important role in the reduction of κl, leading a low κl of 3.1 W m− 1 K− 1 at 625 K for TiNiSn [22]
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