Structure, microstructure and thermoelectric properties of germanite-type Cu22Fe8Ge4S32 compounds

Abstract

This paper describes the influence of the powder synthesis and densification techniques on the structure, microstructure and thermoelectric properties of Cu22Fe8Ge4S32, a synthetic derivative of the naturally occurring germanite mineral. Two powder synthesis approaches are compared, namely mechanical alloying and conventional sealed tube synthesis, combined with two densification methods: spark plasma sintering and hot pressing. Structural analyses by Le Bail refinement of X-ray powder diffraction patterns and transmission electron microscopy confirmed the high crystallinity and the absence of structural defects in the samples. It is especially highlighted that mechanical alloying combined with low sintering temperature allows to reach high purity and to limit the formation of secondary phases due to sulfur volatilization in the bulk specimens. The changes in the electrical resistivity and Seebeck coefficient with the sample preparation methods evidence the high sensitivity of the material to slight stoichiometric deviations. Conversely, the thermal conductivity is less influenced by stoichiometric variations and microstructural changes. This investigation draws attention to the significant impact of powder synthesis and sintering methods on the electrical transport properties of complex quaternary Cu-based sulfides specifically designed to present intrinsically low thermal conductivity for potential thermoelectric applications.