The formation of Cu2S from the elements: I. Copper used in form of powders

The synthesis of Cu2S from copper and sulfur from various sources has been studied in the DTA from 25 to 600°C. The educts and products were characterised by X-ray diffraction. Mixtures of copper and sulfur were investigated in form of powders and of pellets. In the beginning of the reaction sulfides with high sulfur content are formed. In powders the main reaction takes place immediately after melting of sulfur whereas in pellets the larger exothermic effects appear at higher temperatures. In most samples the formation of Cu2S is completed above 450°C. The difference in behaviour of powders and pellets is mainly based on the sample-crucible geometry, but also on different educt concentrations. Similar investigations were performed by mechanical alloying in a planetary ball mill with total milling times in the range 1–120 min. CuS, Cu1.8S, Cu1.95S, Cu1.96S, and α-Cu2S are formed in this order with increasing milling times. Almost no overlap in their formation was observed, because the brittle copper sulfides are removed by milling from the surface of the particles. The formation of further products is thus not hindered. Particle size, pretreatment and source of copper but not of sulfur influences significantly the reaction. The behaviour of copper powder of various manufacturers differed, despite the same pretreatment and particle size ranges, due to differences in particle size distribution, particle shape, grain size or oxygen content. Ageing experiments revealed that less metastable products are formed than in milling experiments or in quenched samples. Ageing in inert atmosphere produces CuS and does not exceed this stage even after several months. Different intermediate products were formed with a higher rate from the beginning, when the samples were stored in vacuum after short irradiation with intense light.