Microstructure-reactivity relationship of Ti + C reactive nanomaterials

The influence of short-term (≤10 min) high energy ball milling (HEBM) on the microstructure and reactivity of a titanium-carbon powder mixture is reported. It is proved that the mechanism of microstructural transformation in a Ti-C mixture during HEBM defines the reaction mechanism in the produced Ti/C structural energetic materials. More specifically, it is shown that after the first two minutes of dry milling (DM) in an inert (argon) atmosphere the initially crystalline graphite flakes were almost completely amorphized and uniformly distributed on the surface of the deformed titanium particles. A subsequent “cold-welding” leads to formation of Ti-(C-rich/Ti)-Ti agglomerates. TEM studies reveal that the (C-rich/Ti) composite layers consist of nano-size (20 nm) Ti particles distributed in the matrix of the amorphous carbon and thus are characterized by extremely high surface area contacts between the reagents. A rapid self-ignition of the material during DM occurs just after 9.5 min of mechanical treatment, resulting in formation of pure cubic TiC. Wet grinding (WG) of a Ti-C mixture in hexane, under otherwise identical parameters, provides more “soft” conditions, which do not allow the rapid amorphization of carbon during the first stage of grinding. As a result graphite and titanium form sandwich-like Ti/C composite particles, in which the reagents contact primarily along the boundaries of the layers. Such particles gradually transform to the TiC phase without a spontaneous reaction during the HEBM process. The reactivity, i.e., self-ignition temperature and ignition delay time, of different milling-induced microstructures, were also studied. It was found that the ignition temperature in Ti-C structural energetic material prepared under optimized HEBM conditions is ∼600 K, which is more than three times lower than that of the initial reaction mixture (Tig ∼ 1900 K). A significant decrease of the effective activation energy for interaction in the Ti-C system is observed (from 95 kcal/mol to ∼56 kcal/mol). It is explained by the fact that solely solid-state reactions are responsible for the ignition phenomenon in the produced structural energetic materials, whereas the dissolution of carbon in a melt is responsible for the reaction in non-mechanically treated mixtures. Analysis of the milling-induced microstructures and reaction kinetics of the Ti/C composite particles suggests that a combination of several factors is responsible for enhancement of their reactivity, with the carbon amophization on the first stage of HEBM playing a key role through formation of layers that provide intimate high surface area contacts between the reagents.