MS3 of Ti2AlN powder at 700 °C with controllable morphology and its formation mechanism analyzed by DFT-TST-SD

Abstract

Ti2AlN powder with ∼97 wt% purity has been synthesized at 700 °C (far below all the previously reported temperatures) via molten salt shielded synthesis (MS3) in the air. The effects of sintering temperature, holding time, and salt content on its purity and particle size are investigated. The reaction mechanism from reactants to products is disclosed through density functional and transition state theories (DFT and TST). The regulation mechanism in morphology is illustrated by the spinodal decomposition (SD) and phase separation model. When the holding time or salt content improves, the purity increases. Ti2AlN powder synthesized at 700 °C for 2 h with a salt content of 80 wt% exhibits a purity of 97.7 wt%, an average particle size of 13.8 μm, and a uniform layer structure. Ti2AlN is reacted via Ti + Al + TiN→Ti2AlN, which occurs when the free energy of Ti–Al–TiN exceeds that of Ti2AlN by the energy barrier of 4.08 eV, and it is a necessary condition for achieving the result that the synthesis is completed at the critical temperature of 700 °C. Grain growth and the surface tension of molten salt regulate the morphology. The above mechanism has been revealed for the first time in this field to date.