SYNTHESIS OF NICKEL SULFIDES BY MECHANICAL ALLOYING

The feasibility and kinetics of synthesizing various nickel sulfides b y milling of elemental mixtures of Ni and S in a high-energy shaker mi ll have been investigated. The phases Ni3S2 and the high-temperature p olymorph of NiS are formed readily via such processing. In distinction , it requires prolonged milling to obtain Ni7S6; NiS2 can only be obta ined as a minor reaction product; and Ni3S4 cannot be formed by millin g for the conditions of this study. Structural evolution during synthe sis and the kinetics of Ni3S2 formation are investigated in depth. S c oats the Ni particles and sulfide formation takes place at the interfa ce of the elements after a certain degree of microstructural refinemen t due to the plastic deformation accompanying milling. Ni3S2 forms rap idly at this stage. However, a stasis in the reaction is then observed . This is associated with NiS formation and a slight decrease in the a mount of Ni3S2. The stasis is of approximately 5-min duration and is f ollowed 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 dep endence of the initial and later stages of Ni3S2 formation and the eff ect of other parameters, such as mill atmosphere and use of premilled powder, on the reaction kinetics. The microstructures found in the int ermediate to later stages of milling are similar to those associated w ith self-sustaining reactions.