Enhancement of thermoelectric properties of CuFeS2 through formation of spinel-type microprecipitates

CuFeS2 (chalcopyrite) is a promising n-type thermoelectric candidate for low-grade waste heat recovery. In this work, chromium-containing CuFeS2 materials of general formula Cu1-xCrxFeS2 (0.0 <= x <= 0.1) were prepared via solid-state synthesis. Efforts to substitute chromium in CuFeS2 leads to the preferential formation of a composite, in which lamellar precipitates of a Cr-rich, spinel-type [Cu,Fe,Cr]3S4 phase, are embedded in the unsubstituted CuFeS2 matrix. X-ray absorption near-edge spectroscopy (XANES) reveals that the electronic structure of copper, iron and sulfur in the principal CuFeS2 phase remains unaltered by chromium incorporation. However, the formation of [Cu,Fe,Cr](3)S-4 precipitates alters the Cu : Fe ratio of the CuFeS2 phase, producing a change in the net carrier concentration through reduction of a portion of Fe(3+ )ions to Fe2+. The chromium content of the spinel precipitates determines the extent of the change in the Cu : Fe ratio of the main CuFeS2 phase, and hence, indirectly affects the electrical properties. The micro/nanometre-sized [Cu,Fe,Cr](3)S-4 precipitates and nanoscale dislocations enable a broad spectrum of heat-carrying acoustic phonons to be scattered, resulting in a significantly reduced lattice thermal conductivity. Combined with an enhanced power factor, a maximum thermoelectric figure-of-merit, zT of 0.31 at 673 K is achieved for the x = 0.08 sample; a three-fold increase over that of the pristine phase.

Automated summary

CuFeS2 is a promising n-type thermoelectric candidate for low-grade waste heat recovery. In this study, chromium-containing CuFeS2 materials were synthesized, resulting in the formation of a composite with Cr-rich precipitates embedded in the CuFeS2 matrix. By reducing a portion of Fe(3+) ions to Fe2+, the Cu:Fe ratio of the CuFeS2 phase was altered, indirectly affecting the electrical properties. Additionally, the presence of nano-sized precipitates and dislocations significantly reduced the lattice thermal conductivity, resulting in an enhanced thermoelectric figure-of-merit.