Study and analysis of composition variation influence for chemical reactions sequence and thermal effects in Al-Ni multilayered thermite materials

Over the recent years а reactive joining technology that uses nanostructured multilayer foils as local heat sources have attracted increasing attentions. Multilayer structures replaced the traditional powder thermite... [ view full abstract ]

Over the recent years а reactive joining technology that uses nanostructured multilayer foils as local heat sources have attracted increasing attentions. Multilayer structures replaced the traditional powder thermite mixtures and opened up new possibilities of nano- and micro-scale surfaces and materials joining processes, based on self-propagating reactions in these foils providing rapid bursts of energy that can heat and melt the surrounding solder or braze layers and join materials at room temperatures and without additional external heating.
The authors completed a detailed thermodynamic analysis of Al-Ni thermite system. As a result, the sequence of phase transformations and chemical reactions has been predicted, which allowed to optimize the stoichiometric ratio of the thermite mixture components. Mathematical modeling of the heat release process was allowed to estimate the speed of a chemical reaction front propagation, and to determine the minimum value of the structure total thickness for its initiation. Experimental samples of thermite multilayer Al/Ni foils of different total and bilayer thickness (from micro to nanosize) were fabricated by alternating magnetron sputtering of Al and Ni targets. For TGA and DSC measurements multilayer structures separated from the substrates by dissolving the sacrificial layer. The individual layers and the total structure thicknesses were controlled by cross-sections SEM measurements. Structural and elemental analysis was performed by XRD and XEDS measurements. The speed of chemical reactions propagation were evaluated using high-speed video camera.
It was shown that the start temperature of the reaction can be varied in wide range changing the component layer thicknesses from micro- to nanosizes. Thus using this size effect it is possible to increase the sensitivity of the material to the initiation. Changing the atomic proportions of thermite structure components allows to change intensity and character of heat release. Thus, the multilayered thermite materials can be tailored to specific applications and tasks.
X-ray phase composition studies was given the opportunity to make corrections in the thermodynamic calculations and to determine the sequence of phase transformations that allows to optimize the thermite material composition.