Thermoelectric Properties of Doped Half-Heuslers NbCoSn1-xSbx and Nb0.99Ti0.01CoSn1-xSbx

The thermoelectric properties of doped half-Heuslers, NbCoSn1-x Sb x and Nb0.99Ti0.01CoSn1-x Sb x , have been studied in the temperature range from 320 to 850 K. These half-Heuslers are synthesized by the arc melting of pure metals followed by annealing for one week at 1123 K in vacuum. The samples with x=0, 0.02, 0.04, 0.06, 0.08, and 0.10 are prepared by spark plasma sintering (SPS) using fine powders of crushed arc-melted buttons. The sintered samples are of a nearly single phase. All the samples exhibit metal-like temperature-dependent electrical resistivity ρ of the order of 10-6–10-5 Ω·m in the measured temperature range. For both NbCoSn1-x Sb x and Nb0.99Ti0.01CoSn1-x Sb x , the ρ values at 320 K rapidly decrease by partial substitution in the range between x=0 and 0.02, and finally reach 5 µΩ·m at x=0.10. The Seebeck coefficient S is appreciably enhanced by partly substituting Nb sites with 1 at. % Ti. The highest power factor is 22×10-4 W·m-1·K-2 for Nb0.99Ti0.01CoSn0.9Sb0.1 at 700 K. The thermal conductivity κ weakly depends on temperature and does not show any peculiar trend upon Sb partial substitution. The dimensionless figure of merit Z T=S 2 T/ρκ for Nb0.99Ti0.01CoSn0.9Sb0.1 reaches 0.3 at 850 K, which is fourfold that of pure NbCoSn.