Reactive Multilayers, Their Design and Their Applications: Bonding, Debonding, Repair, Recycle

While the redox-reaction-based thermite mixtures for exothermic reactions are well-known since a long time, systematic research on non-thermite reactive materials dates back to the 1960s in the USSR when Merzhanov and Borovinskaya did groundbreaking work on powder-based transition metal/carbon mixtures. Since then, the class of ingredients has widened to include metal/metal mixtures and with the progress in physical vapor deposition, precise nanoscale layering of the reactant has become possible. These reactive multilayer systems (RMS), comprising up to several hundred repetitions of individual layers, reach total film thicknesses of up to several tens of micrometers. The characteristic of these films is their capability of undergoing self-propagating exothermic reactions at up to ≈100 m/s and 2000 °C and more. Thermodynamics, reaction kinetics as well as thermal and chemical diffusion set the boundary conditions defining the reaction properties, thus providing means for reaction design.

The application of such self-propagating reactions is of great technological interest, e.g. for the joining of electronic components in electronics, the debonding of parts or even a repair of failure parts. However, the self-propagating reaction after multilayer ignition is hard to control in real technical systems. While the influence of the most prominent parameter, the bilayer thickness, has been studied extensively for various material combinations, many open questions exist regarding the effect of factors like surfaces, materials properties, roughness, morphology, thermo-mechanical stress, structuring or combined systems.

This special issue contains the latest research on such reactive multilayer systems based on Ni/Al and Ru/Al, the current understanding and their applications. The articles investigate the impact of the morphological characteristics and physical properties of the joining partners on the microstructural features of the fabricated RMS, as well as the role of thermophysical properties of the system and the kinetics of the reaction. The vision is the design of a “suitable” microjoining process with fitting electrical and thermal properties of a high-quality mechanical joining Figure 1.

Means to manipulate and tailor the reaction are explored for both improving the understanding of fundamental mechanisms as well as for application on a system level. Different approaches for multilayer modification by substrate patterning are investigated, i.e. photolithography, deep etching and direct laser interference patterning. The resulting structure shows more or less strong deviations from the well-known planar multilayer structure when depositing on flat substrates, like curved layers, pores and discontinuities, which unfold their effect on the scale of up to a few bilayers. Their effects on the self-propagating reactions are discussed with respect to thermal and chemical diffusivity and interface mixing. These modifications of the reactive multilayer foil (RMF) structure itself can be considered as intrinsic, while factors such as applied heat sinks (e.g. substrates) and stresses are extrinsic factors. Both aspects are covered in this special issue. Figure 2 shows exemplary RMF modifications derived from flat multilayers towards stronger interface curvature, thus changing surface energies, diffusion paths and interface area.

With an eye on applications like reactive joining, effects of multilayer modification on a larger scale are investigated. Femtosecond laser cutting is an effective way to structure a reactive multilayer without igniting it, opening the way to modifying the reaction front propagation in a joining scenario. The so far unknown role of oxide formation during the cutting process is investigated. Furthermore, substrate patterning by direct laser writing is used to promote distinct crack patterns in the RMF for improved liquid phase flow in dissimilar joining applications. These experimental works are complemented by simulations to develop reaction models, explore microstructure formation and to account for the limited access for measurement probes and the short time scales of the reaction. Furthermore, electrodeposition of Ni/Al and Pd-based reactive systems as a scalable alternative to PVD routes are explored.

This special issue will cover the most important aspects for reactive multilayer systems, from theory and simulation via preparation, structuring, interface, stress, and substrate effects to applications of reactive multilayer systems.

Yuile et al. are presenting in article number 202302283 a 2-D CFD simulation analysis of the assembly of LTCC/LTCC and Si/Si sandwiches by reactive bonding. Simulation of the reaction is also shown in article number 202302179, where Farshad Daneshpazhoonejad and coworkers report on an experiment-assisted approach to develop a numerical model for simulating the reaction propagation in reactive multilayers. Analogously, the heat transfer and propagation velocity for different heat loss conditions for guided propagation fronts are simulated by Farshad Daneshpazhoonejad, Deepshikha Shekhawat, et al. in article 202400523. Article 202302279 by Sascha Sebastian Riegler and coworkers report important thermodynamic data for Ni/Al reactive multilayers obtained by nano- and flash-calorimetry

The comparison of different sample preparation techniques for TEM Investigations of sputter-deposited Ni/Al reactive multilayers is presented by Juan Jesús Jiménez et al. in article 202302215.

Article 202302217 by a group effort presented by María del Carmen Mejia Chueca shows a new preparation process by Electrodeposition in Ionic Liquid Containing Ni Nanoparticles. Similarly, electroplating is used in article 202400400 by Klaus Vogel, Silvia Braun et al. to fabricate Palladium-based integrated Reactive Multilayer Systems.

Konrad Jäkel and coworkers show the influence of increasing density of artificial substrate microstructures on the self-propagating reaction of Al/Ni reactive multilayers in article 202302225. Similarly, article 202302254 by Marcus Glaser et al. shows the influence of surface structures on polymer-metal-joining based on reactive Al/Ni multilayer foils, whereas substrate thickness effects and substrate roughness influence are shown by Emina Vardo and coworkers in article 202302269. Similarly, the surface roughness and the influence on the morphology of reactive stacks sputtered onto ceramic (LTCC) substrates is investigated by Erik Wiss et al. in article 202302284. Beside the Ni/Al also Ru/Al reactive multilayers were deposited by magnetron sputtering and presented in article 202400258 by Vincent Ott and coauthors. Another approach by Konrad Jäckel and coworkers is presented in article 202400522, where the influence of additional intermediate thicker Al layers on propagation and heat flow of Al/Ni reactive multilayers is reported.

The jump over of self-propagating reactions between separated but thermally coupled reactive material elements is the topic of article 202400870, presented by Deepshikha Shekhawat and coauthors.

In extending the roughness effects, periodic 2D surface structuring is used by Yesenia Haydee Sauni Camposano et al. to control reaction velocity on a substrate as shown in article 202302272. External crack initiation by stress is used by Sebastian Matthes and group to tailor the reaction path, as presented in article 202302271. Even more structuring is done by Maria Martins and coworkers by Ultrashort Pulsed Laser Micromachining of reactive multilayers, but this also induces oxide formation and changes the reaction, as presented in article 202400215. In article 202400435, Christian Schäfer et al. are showing the fabrication of smooth, periodic deposition substrate surface structures by combining direct Laser Interference Patterning and Electropolishing

Finally, investigations on the possibilities of mechanical ignition in reactive Al/Ni multilayers are presented by Marcus Graske and group in article 202400479.