Understanding reaction sequences and mechanisms during synthesis of nanocrystalline Ti2N and TiN via magnetically controlled ball milling of Ti in nitrogen

Abstract

Mechanically induced reactive synthesis of TiN via magnetically controlled ball milling of titanium under nitrogen gas was investigated using X-ray diffraction, advanced electron microscopy and Raman spectroscopy. Ball milling of titanium powder with nitrogen gas was performed in Uni-Ball-Mill, with external temperature of the vial and initial pressures of the nitrogen gas monitored, while the milled samples were taken out periodically, both before and after detection of an exothermic ignition point. Before ignition, nitrogen-enriched Ti, small proportions of TiN and very minor amounts of Ti 2 N are formed, in addition to the heavily deformed Ti. Raman spectroscopy revealed the pre-ignition products to include off-stoichiometric nitrides (TiN x ) and oxynitride skin (TiO x N y ). The formation of the new TiN and Ti 2 N products before ignition was attributed to the diffusion of highly polarized active N atoms into the mechanically activated clean surfaces of Ti, followed by local reaction. This local reaction is likely promoted by numerous cycles of induced local temperature rise and rapid quenching, large surface area and accumulation of deformation defects. After the exothermic ignition, there was rapid nucleation of new TiN crystals, and simultaneous growth of the pre-existing TiN and the newly formed TiN crystals. This understanding explains the reaction pathways leading to the formation of small proportions of TiN and very minor amounts of Ti 2 N before ignition and the thin-plated TiN after ignition. . (2018). Understanding reaction sequences and mechanisms during synthesis of nanocrystalline Ti2N and TiN via magnetically controlled ball milling of Ti in nitrogen. ABSTRACTMechanically induced reactive synthesis of TiN via magnetically controlled ball milling of titanium under nitrogen gas was investigated using X-ray diffraction, advanced electron microscopy and Raman spectroscopy. Ball milling of titanium powder with nitrogen gas was performed in Uni-Ball-Mill, with external temperature of the vial and initial pressures of the nitrogen gas monitored, while the milled samples were taken out periodically, both before and after detection of an exothermic ignition point. Before ignition, nitrogen-enriched Ti, small proportions of TiN and very minor amounts of Ti2N are formed, in addition to the heavily deformed Ti. Raman spectroscopy revealed the preignition products to include off-stoichiometric nitrides (TiNx) and oxynitride skin (TiOxNy). The formation of the new TiN and Ti2N products before ignition was attributed to the diffusion of highly polarized active N atoms into the mechanically activated clean surfaces of Ti, followed by local reaction. This local reaction is likely promoted by numerous cycles of induced local temperature rise and rapid quenching, large surface area and accumulation of deformation defects. After the exothermic ignition, there was rapid nucleation of new TiN crystals, and simultaneous growth of the pre-existing TiN and the newly formed TiN crystals. This understanding explains the ...

Discrete One-Stage Mechanochemical Synthesis of Titanium-Nitride in a High-Energy Mill

“…At this moment, a slight temperature jump (up to 10 • C) occurs on the surface of the mill wall due to the heat released during the formation of the final product. A similar temperature jump (less than 10 • C) during mechanosynthesis of titanium nitride in a high-energy mill was recorded by the authors of [34]. The authors denoted this temperature jump as the peak of combustion and synthesis.…”

“…The developed mathematical model was found to be suitable for the analysis of the macrokinetics of chemical transformations in the solid reagent-active gas systems. At the same time, it can be noted that the one-stage mechanochemical synthesis in the solid-gas systems (including the Ti-N system) is still poorly studied [2,[30][31][32][33][34][35].…”

Mechanocatalytic Ammonia Synthesis over TiN in Transient Microenvironments

“…Once a threshold concentration is reached (∼32 mol %), 45 a rapid, bulk change to TiN occurs. 46,47 The low ammonia yields at short milling times in N 2 indicate that the dissolved nitrogen is not reactive toward ammonia but must be incorporated into a nitride crystal lattice, supporting a MvK mechanism during catalytic ammonia formation. Thus, a critical amount of incorporated lattice nitrogen is required for relevant ammonia yields.…”

“…This significant increase in ammonia yield by increasing milling time in N 2 from 1.5 to 3 h aligns with the expected nitridation mechanism based on XRD (Figure 1a). 46,47 Initially, nitrogen gradually diffuses into the Ti metal, forming a solid solution. Once a threshold concentration is reached (∼32 mol %), 45 a rapid, bulk change to TiN occurs.…”

“…(a) X-ray diffractograms for unmilled Ti, unmilled TiN, and Ti milled for 1.5, 3, 4.5, and 6 h in 15 sccm N 2 . Peak assignment was performed according to the unmilled Ti and TiN samples and literature . (b) X-ray absorption spectra of the titanium K-Edge and (c) Fourier transform of the EXAFS region for Ti foil (black), Ti powder milled in N 2 for 4.5 h (blue), and unmilled TiN (green).…”