The effects of quaternary additions on thermoelectric properties of TiNiSn-based half-heusler alloys

Abstract

Half-Heusler-type compounds have gained increasing attention as promising thermoelectric materials. In the present work, a focus is placed on TiNiSn with additions of Hf, Zr, Si, or Pt. Nominally stoichiometric TiNiSn alloys were prepared using arc melting and subsequent annealing at 1,073 K for 2 weeks. The thermoelectric properties, such as thermoelectric power, electrical resistivity, and thermal conductivity, were measured in a temperature range from 300 K to 1,000 K. As-cast materials show metallic transport properties, while annealed ones exhibit semiconductor behavior. Microstructures of TiNiSn alloys basically consist of nonequilibrium four-phase; half-Heusler TiNiSn, Heusler TiNi 2 Sn, metallic Ti 6 Sn 5 , and Sn solid solution. The volume fraction of the half-Heusler TiNiSn phase significantly increases by annealing. It is revealed that coexisting metallic phases degrade the thermoelectric properties of halfHeusler TiNiSn. Alloy additions strongly affect not only thermoelectric properties but also phase stability. The thermal conductivity of TiNiSn alloys with alloy additions decreases because of the point-defect phonon scattering.

Thermoelectric and Thermophysical Properties of TiCoSb-ZrCoSb-HfCoSb Pseudo Ternary System Prepared by Spark Plasma Sintering

“…The of TiCoSb, ZrCoSb and HfCoSb are larger than those of as-cast samples reported in the previous study. 11) It is well known that the annealing condition 10,[13][14][15] and the deviation from the stoichiometric composition 6) have much effect on the thermoelectric properties of the half-Heusler compounds. For example, the of TiCoSb tends to increase with decreasing the Co/Sb ratio and with increasing the Ti/Sb ratio.…”

“…So, TiCoSb is expected to have a larger thermoelectric power S. From the literatures, [6][7][8][9][10][11] TiCoSb has a negative thermoelectric power. The improvement of the structural disorder 12) by annealing has been reported on MNiSn system [13][14][15] and TiCoSb system. 10) The thermoelectric properties of ZrCoSb and HfCoSb have been reported only for the electrical resistivity and thermoelectric power below room temperature.…”

Examining the thermal conductivity of the half-Heusler alloy TiNiSn by first-principles calculations

“…Surprisingly, the attempts made to reduce its lattice thermal conductivity are found to have less effect on its PF due to its low carrier mobility [8], and thus the possible improvement of ZT is expected. In 2003, Katayama et al [2] reported an upper limit ZT=0.  by alloying the ternary half-Heusler alloys.…”

“…The increase in thermal conductivity at high temperatures is mainly due to the remarkable increase in electronic contribution. It is well-known that TiNiSn-based half-Heusler alloys always exhibit maximum ZT around 700-800 K [2,3,7]. The reason is that the maximum power factor and lowest thermal conductivity occur simultaneous ly at high temperature.…”

Thermoelectric Properties of Half-Heusler Type LaPdBi and GdPdBi

“…The substitution in each site is very useful for improvement of thermoelectric properties, because the position of the Fermi level in the gap can be controlled and the lattice thermal conductivity reduces due to enhancement of the phonon scattering. Since the half-Heusler compounds with 18 valence electrons have the narrow band gap near the Fermi level and show the large thermoelectric power, (Ti, Zr, Hf)NiSn, [1][2][3][4][5][6][7] (Ti, Zr, Hf)CoSb [8][9][10][11] and other half-Heusler compounds have been studied as advanced thermoelectric materials. Although high performance n-type materials in the half-Heusler compounds have been reported, 4,6) such p-type thermoelectric materials have been scarcely reported.…”