High Thermoelectric Performance of Bornite through Control of the Cu(II) Content and Vacancy Concentration

Thermoelectric performance of the p-type semiconductor bornite, Cu5FeS4, is greatly enhanced through chemical substitution. Nonstoichiometric materials in which the Cu:Fe ratio and overall cation-vacancy content were adjusted are reported, and a figure of merit, ZT = 0.79, is achieved at temperatures as low as 550 K in CU4.972Fe0.968S4. All materials were synthesized mechanochemically and characterized by powder X-ray diffraction, differential scanning calorirnetry (DSC), and thermal and electrical transport property measurements. Single-phase behavior is retained in copper deficient phases, Cu5-xFeS4, for vacancy levels up to x = 0.1, while in materials Cu5+yFe1-yS4, in which the Cu:Fe ratio is varied while maintaining full occupancy of cation sites, single-phase behavior persists for y <= 0.08. Adjusting the Cu:Fe ratio at a constant cation-vacancy level of 0.06 in Cu4.94+zFe1-zS4, leads to single phases for z <= 0.04. DSC measurements indicate the temperature of the intermediate (2a) to high-temperature (a) phase transition shows a more marked dependence on the Cu:Fe ratio than the lower temperature 4a to 2a transition. The thermoelectric power factor increases almost linearly with increasing Cu(II) content. The maximum figures of merit are obtained for materials with Cu(II) contents in the range 0.10-0.15 (corresponding to 2.0-2.8% Cu(II)), which simultaneously contain ca. 1% of cation vacancies.