SYNTHESIS OF NICKEL SULFIDES BY MECHANICAL ALLOYING

The feasibility and kinetics of synthesizing various nickel sulfides by milling of elemental mixtures of Ni and S in a high-energy shaker mill have been investigated. The phases Ni3S2 and the high-temperature polymorph of NiS are formed readily via such processing. In distinction, it requires prolonged milling to obtain Ni7S6; NiS2 can only be obtained as a minor reaction product; and Ni3S4 cannot be formed by milling for the conditions of this study. Structural evolution during synthesis and the kinetics of Ni3S2 formation are investigated in depth. S coats the Ni particles and sulfide formation takes place at the interface of the elements after a certain degree of microstructural refinement due to the plastic deformation accompanying milling. Ni3S2 forms rapidly at this stage. However, a stasis in the reaction is then observed. This is associated with NiS formation and a slight decrease in the amount of Ni3S2. The stasis is of approximately 5-min duration and is followed by a recurrence in the formation of Ni3S2 and a disappearance of the NiS phase. The kinetics can be mimicked through a model of the mechanical alloying process. The model is able to predict the time dependence of the initial and later stages of Ni3S2 formation and the effect of other parameters, such as mill atmosphere and use of premilled powder, on the reaction kinetics. The microstructures found in the intermediate to later stages of milling are similar to those associated with self-sustaining reactions.