Rapid, Sustainable, and Scalable Synthesis of Nanocrystalline Copper Sulfide via Intensive Mixing of Elements

This is the first-ever report of igniting a combustive process that we tentatively denote as a mixing-induced self-propagating reaction (MXSR) in an inorganic system by a simple intensive mixing of the educts. The occurrence of MXSR is proven by observing a spike during in situ temperature monitoring. We demonstrate this on the example of agitating copper and sulfur powders in a ball-free planetary mill jar, igniting a MXSR without the external heating or the mechanical impact. Intensive powder agitation yields a mixture of nanocrystalline covellite (CuS) and digenite (Cu1.8S) in under 2 min. MXSR ignition thresholds are precisely determined (jar filling ≥40%, mixing speed ≥700 rpm, Cu:S molar ratio of 0.625–1.00). If these thresholds are not respected, a gradual reaction partly proceeds and an unstable mixture of digenite and non-reacted sulfur is formed, which is transformed into covellite with time. In specific cases, MXSR can be ignited within a few minutes after the termination of mixing. In the end, scalability to 40 and 62.5 g scales in planetary and mixer mills, respectively, is demonstrated and the products' thermoelectric utility for waste heat conversion is showcased. The proposed MXSR pathway overcomes state-of-the-art limitations in mechanochemistry, including product contamination from milling media abrasion and energy-intensive activation, while achieving decent alignment with green chemistry (100% atom economy, 88% and 91% reaction mass efficiency for the experiments performed on a larger scale in a mixer and planetary mill, respectively).