Formation of novel core–shell and tadpole-like structures in the direct nitridation of aluminum powder by N2 and NH3

In this paper, the direct nitridation of Al powder was carried out under N2 and NH3 atmosphere in temperature range from 900 to 1400 °C, respectively. The structures, morphologies and compositions of the products were measured and analyzed by XRD, SEM and EDX. The SEM results showed core–shell structures of Al/AlN below 900 °C under N2 atmosphere, whereas tadpole-like and branches structures of AlN over 1200 °C under NH3 atmosphere. EDX results demonstrated that Al-rich AlN with extensive N/Al ratio could form in the direct nitridation process. The XRD results indicated that the pure AlN powders were obtained at 1400 °C under N2 atmosphere and at 1100 °C under NH3 atmosphere, respectively. The reaction analysis revealed that intensive passivation effect of Al occurred below 900 °C under NH3 atmosphere and hindered the nitridation, while the passivation effect was eliminated beyond 1000 °C and nitridation was boosted. In contrast, Al crust formed below 1100 °C under N2 atmosphere and ceased the nitridation, while the crust became unstable because of convection originated from intensive evaporation beyond 1200 °C and the nitridation was accelerated. So the reaction process played essential roles in preparing pure AlN by the direct nitridation of Al powder.

► Core-shell structures of Al/AlN, and tadpole-like structures of AlN are produced.
► Fully nitridation temperature using NH3 is 300 °C lower than that using N2.
► The molten Al layer and passivation effect are major resistances for nitridation.
► Al-rich AlN with extensive Al/N ratio could form in the direct nitridation process.