Study of reaction sequences during MSR synthesis of TiC by controlled ball milling of titanium and graphite

Abstract

During reactive ball milling of Ti and C, mechanically induced self-propagating reaction (MSR) synthesis of TiC is reported to occur via a process including initial formation of small amounts of TiC prior to exothermic ignition, followed by MSR and the formation of stiochiometric TiC (Dorofeev et al., 2011; Oghenevweta et al., 2016 [1, 2]). However, both the chemical evolution and mechanisms of reaction and growth of TiC both prior to, during and after ignition are not fully understood. This research focuses on fundamental understanding of the reaction sequences during MSR synthesis of TiC from elemental Ti and C (graphite). Magnetically controlled ball milling of stoichiometric (1:1) powder mixtures of titanium (Ti) and graphite (C) was performed under He gas with the exothermic ignition point by determined via in-situ temperature measurement. X-ray diffraction, Scanning Transmission Electron Microscopy (STEM) with electron energy loss spectroscopy (EELS), Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS) were employed to interpret the reaction sequences. The reaction sequence involved the following: During milling prior to ignition, severely deformed Ti, spheriodised graphite and amorphous carbon forms, and minor amounts of off-stiochiometric nano-TiC 1−x crystals. In addition to the formation of TiC 1−x , XPS revealed the formation of oxide skins coating the surface regions of the milled powders. There is also an evolution towards stiochiometric TiC with increasing milling time. Immediately after ignition, a reacted product comprising a mixture of multi-layer stacks of thin TiC plates form, in addition to a matrix comprising large TiC particles which have been liquid phase sintered under the high local heat of reaction by thin layers of unreacted Ti. MSR occurs via a dual mechanism involving rapid growth of existing TiC, plus nucleation and growth of new TiC. High spatially resolved EELS combined with STEM revealed additional information about the inhomogeneous chemical environment of the milled powders.

Fast and efficient approach to synthesis of ultra-fine TiC powder

“…The results are well consisted with the 3-10 h of milling in [6,25,28,40,44]. After 30 min of milling comparing with 37 h in [28] or 19 h in [33,40], MSR was ignited and XRD pattern (figure 2 (30 min)) showed sharp peaks indexed with PDF card # 65-8808 which matched the fcc TiC phase (Fm-3m (225)) with a slight shift to lower Bragg angles, meaning the increase of TiC lattice parameters. All the Ti peaks disappeared, suggesting complete reaction obtained immediately.…”

“…In addition, the composition of final products is feasibly controlled by the ratio of raw materials. MA is usually performed by high-energy ball milling and often known as mechanically induced self-propagating reaction (MSR) or mechanically induced solid-state diffusion reaction (MDR) [24,28,29].…”

“…Delogu et al [29,30] reported that the mechanochemistry of Ti+C powder during ball milling depended on the chemical composition, the collision energy and volume of the powder charge. As the composition between Ti 20 C 80 and Ti 45 C 55 , or between Ti 72 C 28 and Ti 80 C 20 , a gradual kinetics was occurred. An MSR underwent while powder mixtures between Ti 45 C 55 and Ti 72 C 28 .…”

“…Wu et al [33] suggested a transitional bonded state (Ti···C) plays an important role in reducing the ignition temperature for a combustion reaction during the mechanosynthesis process. Oghenevweta et al [28,34] proposed a dual mechanism of MSR involving a rapid high temperature nucleation of new TiC with local melting of unreacted Ti and the growth of pre-existing nano-TiC.…”

“…Note that it usually required some tens of hours to obtain stoichiometric TiC depending on the type of mill [27] such as vibro-mill [30,35], magneto ball mill [28,31,32,34,36] and planetary ball mill [6,[37][38][39][40]. Specifically, the incubation period before a MSR was 37 h in [28,34], 70 h in [32] and 71 h in [31,36]. Up to now, minimum synthesis time in planetary ball mill is within 35 min [39].…”