The role of hydrogen and iron in silicon nitridation by ball milling

We have studied in detail the α-Si3N4 formation during ball milling of Si in NH3 and N2 environments and subsequent annealing. α-Si3N4 can be formed upon annealing after Si is milled in NH3, but no such phase is found for milling in N2. This difference is attributed to the role of H in embrittlement of the powder particles and to the subsequent introduction of Fe contamination into the Si powder. Our studies also show that the presence of Fe greatly enhances the nitridation reaction of as-milled Si powder. Fe silicide phases can be found after milling in NH3 and annealing, and the extent of silicide depends on the N content in the powder. In favorable cases, Fe can be removed from the α-Si3N4 powder by an acid leach.