Self-Propagating High-Temperature Reactions in Heterogeneous Reactive Nanocomposites

Self-propagating high temperature reactions are of interest due to the potential for fabrication of unique materials, including metastable phases and super refractory alloys and ceramics. Reactions occurring in the TiN/B, Ni/Al, and Ti/BN systems were investigated and characterized using mechanically-induced nanostructured composites through high energy ball milling (HEBM). A wide range of diagnostic equipment was applied, and experimental techniques were developed to characterize the reaction mechanisms, reaction kinetics, and diffusion behavior of these systems with particular focus on TiN/B, which was investigated here for the first time.

TiN/B belongs to a subset of combustion reactions, known as solid flame systems, where the adiabatic combustion temperature does not exceed any phase transition temperature or eutectic point of the reactants, intermediates, or products, but for which no ternary phases or substantial solid solutions form. In situ time-resolved x-ray diffraction, infrared video recording, and thermal gravimetric analysis/differential scanning calorimetry were employed to determine the reaction pathway of this system, which is one of the primary goals of this dissertation. Cubic boron nitride, the high-pressure BN polymorph, was produced by shock-induced reactive synthesis using the TiN/B system.

High speed microscope video analysis identified two propagation modes for Ni/Al nanostructured composites: a micro-heterogeneous mode limited by heat transfer between composite particles, and a nano-quasi-homogeneous mode limited by reaction kinetics within single particles. An in situ TEM technique that couples energy dispersive spectroscopy mapping with a heated transmission electron microscope stage was used to measure diffusivity in the temperature range of 623 K – 723 K, which is relevant for understanding solid-state ignition behavior of Ni-Al.

The development of mechanical processing methods for fabricating nanostructured composites presents an opportunity to prepare novel reactive mixtures from systems for which thermodynamic calculations indicate substantial heats of formation but were not previously identified as feasible due to solid-solid diffusion limitations.