Effects of Ir Substitution and Processing Conditions on Thermoelectric Performance of p-Type Zr0.5Hf0.5Co1−xIrxSb0.99Sn0.01 Half-Heusler Alloys

A series of samples with the composition Zr 0.5 Hf 0.5 Co 1− x Ir x Sb 0.99 Sn 0.01 ( x = 0.0 to 0.7) were synthesized by high-temperature solid-state reaction at 1173 K. High-density pellets of the powders were obtained using hot press (HP) and spark plasma sintering (SPS) techniques. The thermoelectric properties of the pellets were measured from 300 K to 750 K. Independently of the pressing conditions, all Ir-containing samples ( x > 0) showed p -type semiconducting behavior. At 300 K, the electrical conductivity and thermopower of Zr 0.5 Hf 0.5 Co 1− x Ir x Sb 0.99 Sn 0.01 materials surprisingly increased with increasing Ir concentration. The largest electrical conductivity and thermopower values of 150 S/cm and 140 μ V/K, respectively, were observed at 300 K for x = 0.7. The thermal conductivity of the synthesized materials decreased with increasing Ir content, went through a minimum value ( x = 0.3), and increased thereafter with further addition of Ir. Pellets fabricated by SPS showed smaller thermal conductivity than pellets of the same composition obtained from uniaxial hot pressing. A thermal conductivity value of ∼2.0 W/m K was observed at 300 K for an SPS pellet with the com- position Zr 0.5 Hf 0.5 Co 0.5 Ir 0.5 Sb 0.99 Sn 0.01 . The thermal conductivity of Zr 0.5 Hf 0.5 - Co 1− x Ir x Sb 0.99 Sn 0.01 decreased with rising temperature, and the smallest value of ∼1.5 W/m K was observed at 750 K for the SPS specimen with x = 0.5.