Thermoelectric Properties and n- to p-Type Conversion of Co-Doped ZrNiSn-Based Half-Heusler Alloys

Half-Heulser thermoelectric materials ZrNi_1?y Co _y Sn (y?=?0, 0.02, 0.04, 0.08, 0.12) were prepared by a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction analysis and electron probe microanalysis showed that single-phase half-Heusler compounds without compositional segregation have been obtained. The effects of Co doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity of ZrNiSn-based half-Heusler alloys have been investigated from 300?K to 900?K. The Seebeck coefficient displayed a change from negative to positive values above nominal Co doping content of y?=?0.02, indicating a transition in the conduction behavior from n-type to p-type. The maximum dimensionless figure of merit ZT of undoped ZrNiSn sample reached 0.5 at 870?K.