Thermoelectric properties of nanostructured bornite Cu5-xCoxFeS4 synthesized by high energy ball milling

Abstract

Cobalt-substituted bornite Cu5-xCoxFeS4 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) solid solutions were synthesized by high energy mechanical alloying (MA) followed by hot pressing. X-ray diffraction (XRD) spectra revealed bornite as the pure phase for substitution level up to x = 0.08, whereas a trace amount of the secondary phase Co3S4 was detected for x = 0.1. The surface morphology of hot-pressed samples confirmed their nanostructured nature, with grains sizes smaller than 50 nm. As expected, the electrical resistivity and Seebeck coefficient increased with an increase in the Co substitution level up to x = 0.04, as a results of a decrease in the hole concentration induced by substitution of Cu+ by Co+2. The combined effect of adjusting resistivity and Seebeck coefficient leaded to power factors up to 0.3 mWm−1K−2 at 590 K for Cu4.96Co0.04FeS4, which is 20% higher than the values measured from pure bornite Cu5FeS4. The total thermal conductivity decreased with increasing the substitution level up to x = 0.04, reaching a minimum thermal conductivity of 0.22 W/mK for x = 0.04 at 473 K, which is the lowest value reported for bornite in all temperature ranges. A maximum thermoelectric figure-of-merit ZT ≈ 0.5 was attained at 590 K for Cu4.96Co0.04FeS4.