Influence of Gaseous Environment on Reaction Behavior and Phase Formation in Ti/2B Reactive Multilayer Foils
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Influence of Gaseous Environment on Reaction Behavior and Phase Formation in Ti/2B Reactive Multilayer Foils Robert V. Reeves, Mark A. Rodriguez, Eric D. Jones Jr., and David P. Adams Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. ABSTRACT The effects of surrounding gaseous environment on the reaction behaviors and product formation for sputter-deposited Ti/2B reactive multilayers are reported. With the surrounding environment set to different air pressures, from atmospheric conditions to 10-4 Torr, Ti/2B samples were reacted in a self-propagating mode, and the average reaction wave velocities were determined through high-speed imaging. Propagation speeds for 3.0 μm-thick multilayers were in the range of 10.89 to 0.05 m/s depending on bilayer thickness (i.e., reactant layer periodicity) and ambient pressure. X-ray diffraction analysis showed that single-phase TiB2 forms within multilayers that have small bilayer thickness. Multilayers that have a large bilayer thickness developed a mixture of TiB2, TiB and TiO2. INTRODUCTION The concept of directly reacting elemental compositions of metals/non-metals and metals/metals to produce refractories, ceramics, and intermetallic compounds through Combustion Synthesis (CS) has been utilized and refined since its initial study in the 1960’s [13]. Utility is also found in the strong exothermic release that occurs during the reaction process in many of these systems. Possible applications that could benefit from the waste heat include propulsion, gasless igniters, and joining/brazing processes. One such exothermic system is the Ti/2B reactive system, with a heat of reaction of -5.52 kJ/g (-21.61 kJ/cc) [4]. Most work on Ti/2B CS has involved mixed powder compositions. However, mixed powder systems are typically porous, which creates additional complexity in the heat and chemical transfer mechanisms necessary to propagate a reaction unless samples are pressed to a high density [5,6]. More recently, high-purity, reactive multilayer films, produced by vapor deposition methods, have undergone extensive study [7-12], though few have been directly focused on the Ti/2B system [11,12]. These materials are formed by atomically depositing alternating, thin layers of the reactants to build μm-scale foils with nanometric periodicity [7-12]. This geometry provides several advantages over mixed powders for the researcher. First, the regular geometry of a multilayer foil eliminates the stochastic structure characteristic of compacted powders and the resulting existence of pores and trapped gases. Second, a single, and for all practical purposes, constant diffusion distance for the reaction can be defined and controlled through the material design. Finally, the interfacial area and interface structure between reactants, control of which is essential for reliable results, is much more consistent in multilayer foils than for pressed powders. In the Ti/2B system, both reactants have the potential to be oxidized, releasing large amounts of heat in the process. Despite this, little is p
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