Mechanochemical preparation of nanocrystalline stannite/chatkalite composite: kinetics of synthesis and thermoelectric properties

In this study we demonstrate the use of Cu, Fe, Sn and S elemental precursors to synthesize chatkalite/stannite nanocomposite by high-energy milling in both lab-scale and industrial-scale mills. The products were characterized by XRD, SEM, HRTEM, EDX, Mössbauer spectroscopy and magnetometry. For the determination of optimum milling conditions, the magnetization data of the synthesized samples were used as output values using a Taguchi experimental design. The formation of the product proceeds via a multistep process comprising binary sulphides (SnS and CuS) formation at the beginning and chatkalite Cu6FeSn2S8/stannite Cu2FeSnS4 composite as the end products. The TEM images show tightly packed nanosized crystallites with the sizes in the range of tens of nanometers. Mössbauer study unveiled that iron is present in 3+ state instead of 2+ expected for stoichiometric phases. This discrepancy was explained by the presence of a small number of vacancies on Sn sites. Based on the thermoelectric measurements of the hot-pressed pellets, the calculated figure of merit reached a value of ZT = 0.27 at 600 K. The aim of this study is to prepare a representative of a promising class of environmentally acceptable thermoelectric materials in an environmentally sound solvent-free manner and characterize them while studying the kinetics of the reaction via magnetometry and the possibility to scale-up the solid-state synthesis process.