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

Abstract

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 Cu_1−xCr_xFeS₂ (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]₃S₄ precipitates alters the Cu : Fe ratio of the CuFeS₂ phase, producing a change in the net carrier concentration through reduction of a portion of Fe³⁺ 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]₃S₄ 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.