Synthesis, Characterization, and Application of Nanothermites for Joining
Bohlouli Zanjani, Golnaz
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Thermite reactions were well studied in the past few decades; however, implementation of these reactions with nanoscale components is a new interest for today’s researchers both for military and civil industries. Nanothermites are mixtures of a metal fuel and a metal oxide, undergoing a redox reaction while heated, and generating a large amount of energy (heat/thrust) which can reach combustion temperatures above 3000K. Aluminum is commonly used as the fuel because of its abundance, easy handling, high reactivity and benign products. By using nano-sized components, the surface energy, contact area, and mixing homogeneity increase. These properties result in greatly improved reactivity and propagation rate as well as easier ignition compared to traditional thermites, which make them attractive as advanced propellants, pyrotechnics, and heat and thrust generators. They also find civil applications such as joining. Here, the application of nanothermites for joining metal to ceramic/glass is investigated. To approach this goal, composites of nanothermite modified by Copper powder were developed for the first time and their related properties were studied to find the best composition for joining. These energetic composites can be applied where a localized heat source is required. The advantage of using nanothermite for joining is its fast reaction, high energy density and liquid products that can wet surfaces. In this research, the reaction products were studied by X-Ray Diffraction spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy. The overall thermite reaction corresponding to the Al-NiO nanothermite was found producing the AlNi phase in a fuel-rich mixture. The microstructures of these reaction products showed the formation of a composite made from copper, AlNi and AlNi/Al2O3 spheres in an Al2O3 matrix. On the other hand, the influences of the fuel (Al) to oxidizer (NiO or CuO) mass ratio and the amount of Cu additive, on the ignition temperature and energy release were characterized using Differential Scanning Calorimetry. It was found that both parameters do not affect the ignition temperature significantly but change the energy release dramatically. Furthermore, according to these results, (Al-33%NiO)-50%Cu was selected and applied to join dissimilar materials such as copper, alumina-silica and glass. As a proof of concept, joint cross-sections were studied by SEM-EDAX revealing that the alumina phase produced by this reaction was joined to the glass/ceramic, while the metal phase wetted the metallic surfaces. Therefore, this composite was introduced as a good interlayer for dissimilar metal/ceramic surfaces.