Self-Propagating Reaction of Reactive Multilayers Grown on Substrates with Designed Surface Topography

Reactive multilayer systems (RMS) consist of alternating layers of two or more components, which will react exothermically if the system is heated up locally, triggering a self-sustained and self-propagating reaction. Several studies were carried out with the aim of controlling the behavior of the reaction by changing the bilayer thickness or the atomic composition of the system. Moreover, recently the impact of the substrate and its surface topography on the propagation of the reaction has drawn attention. In previous studies, the influence of the surface topography of the substrate on the reaction behavior and on the phase transformation of the RMS during the reaction was revealed. However, the effect of surfaces with systematically distributed and defined structures needs to be further investigated.<br/>In order to produce substrates with designed surface topography, silicon chips with a defined number of line/valleys structures in the surface were produced by lithography, reactive ion etching process and KOH etching. The KOH etching time was adjusted in order to produce valleys with different depths in the surface of the substrate. Sequentially, a SiO₂ layer of 1 µm thickness was produced on the surface by thermal oxidation with the purpose of ensuring the self-propagation of the RMS reaction. Al/Ni multilayers of 5 µm total thickness with a bilayer thickness of 50 nm was deposited on the Si/SiO₂ substrates by magnetron sputtering. The RMS were characterized before and after reaction. The analysis of the morphology exhibits the influence of the substrate surface characteristics on the growth of the Al/ Ni multilayers and on the formation of defects, which are located in the slopes between lines and valleys. It was shown that varying the number and depth of the structures in the substrate allows us to tailor the size and the number of defects in the RMS.<br/>To investigate the impact of the produced defects and the new morphology on the behavior of the reaction propagation front, ignition experiments were carried out. The reaction was initiated by spark ignition, a high-speed camera and a high-Speed pyrometer were used to record the propagation front and to obtain the temperature/ time profile during the reaction. These results show the impact of the RMS morphology on the propagation front behavior, the velocity of propagation decreases exponentially in relation to the number of lines in the substrate surface, and the registered maximum temperature decreases as well for samples with a higher number of line structures. The calculated velocity varied from 11 m/s for the RMS deposited on flat substrates to 4.5 m/s for the RMS on substrates with line structures. Moreover, it was found that structures with a depth over 3 µm block the self-propagation of the reaction. Despite the difference in the propagation velocity and the temperature, the X-ray diffraction patterns of the reacted samples show the reflexes of the B2 phase AlNi in the different samples and no reflexes of Al or Ni can be observed, which suggests the complete reaction of the system.<br/>These results show the possibility of modifying the self-propagation characteristics of the RMS reaction by using substrates with designed surface structures, where the depth of the structures can be adjusted to define the path of the propagation front, while the line/ valley structures systematically distributed allows to control the velocity and maximum temperature during the reaction.