Thermoelectric performance of CuFeS2+2x composites prepared by rapid thermal explosion

Abstract

Although many thermoelectric materials, such as Bi2Te3, PbTe and CoSb3, possess excellent thermoelectric properties, they often contain toxic and expensive elements. Moreover, most of them are synthesized by processes such as vacuum melting, mechanical alloying or solid-state reactions, which are highly energy and time intensive. All these factors limit commercial applications of the thermoelectric materials. Therefore, it is imperative to develop efficient, inexpensive and non-toxic materials and explore rapid and low-cost synthesis methods. Herein we demonstrated a rapid, facile and low-cost synthesis route that combines thermal explosion (TE) with plasma-activated sintering and used it to prepare environmentally benign CuFeS2+2x. The phase transformation that occurred during the TE and correlations between the microstructure and transport properties were investigated. In a TE process, single-phase CuFeS2 was obtained in a short time and the thermoelectric performance of the bulk samples was better than that of the samples that were synthesized using traditional methods. Furthermore, the effect of phase boundaries on the transport properties was investigated and the underlying physical mechanisms that led to an improvement in the thermoelectric performance were revealed. This work provides several new ideas regarding the TE process and its utilization in the synthesis of thermoelectric materials. Thermoelectric materials can be rapidly formed into efficient structural phases using controlled thermal explosions. Many thermoelectric materials contain toxic and expensive elements and are synthesized by energy intensive processes. ’Fool's gold‘, or chalcopyrite, a copper iron sulfide (CuFeS2) mineral, exhibits promising thermoelectric activity and a low toxicity. Xinfeng Tang from Wuhan University of Technology, China and co-workers have developed a quick process to arrange the atoms inside CuFeS2 for optimal electrical conductivity and heat retention. Exposing powdered reagents in a quartz tube to brief periods of intense temperatures initiated a series of exothermic, explosive reactions that synthesized target crystals in under a minute. The researchers adjusted the initial sulfur content to fine tune the chalcopyrite crystal phases generated during explosions, and, following sintering, recovered one compound with 130% times the normal thermoelectric conversion efficiency. In a typical thermal explosion process, a single-phase CuFeS2 is obtained in a very short time and the thermoelectric performance of the fully condensed bulk samples is better than that of the samples synthesized by the traditional methods. The presence of phase boundaries in the CuFeS2−x has an effect on the transport properties: phase boundaries scatter low-frequency phonons and reduce the thermal conductivity of a composite structure. Moreover, large differences in the carrier concentration in CuFeS2 and Cu1.1Fe1.1S2 drive a redistribution of electrons in the composite and lead to an enhancement in the electronic transport properties.