Ru/Al Multilayers Integrate Maximum Energy Density and Ductility for Reactive Materials

Nanometric reactive multilayers are capable of long-term chemical energy storage. The energy is released after local ignition during rapid self-propagating reactions (see reviews1,2,3). The reactions form intermetallic compounds within a micron-sized reaction front traveling along the multilayer (see Fig. 1(a)). This type of energy release makes reactive multilayers attractive as localized heat sources. Reactive multilayers of various chemistries have been explored to adjust the reaction properties to the requirements given by the application4,5,6,7,8,9,10,11,12,13. For example, in the NanoBondTM process, Ni/Al foil enables bonding of components at the smallest scale by building self-forming joints where only the joint interface exhibits heat-up. The need for external heat sources is eliminated, paving the way for accelerated production, e.g. in microelectromechanical systems (MEMS)13. However, the development of new joining strategies or the exploitation of novel applications requires three conflicting properties of the energetic systems14: 1.) reactions with maximal energy density (ED) leading to high reaction temperatures Tf, 2.) reactions to single-phase microstructures, and 3.) reaction products with room temperature (RT) ductility. Figure 1 Ru/Al reactive multilayers serve as a model material to integrate both maximum energy density and ductility. Currently, no system adequately combines these properties. The analysis of the most widely used systems Ni/Al and Pd/Al (e.g. for soldering) illustrates the conflict. Bonding at the micron scale needs high ED to produce stable reaction progress and high Tf. In this respect, equiatomic systems with maximal ED, such as Pd/Al (10.8 kJcm−3 15) are preferred. Theoretical calculations show that Tfs are around 2,800 °C15. However, there is no experimental evidence. When reacted as freestanding multilayers those foils lose mechanical integrity. The reasons for the loss in material integrity are currently an open research topic. Potential scenarios include processes like metal evaporation during the reaction, full melting due to the high heat of formation, and the formation of volatile precious metal oxides. The example reveals the drawback of too high EDs and Tf. However, motivated by the abovementioned joining or MEMS applications, reactions of optimal energetic materials should reach temperatures close to the maximal temperature of Pd/Al and simultaneously preserve foil integrity. High temperatures also cause composition variation during reaction. In this sense, equiatomic Ni/Al is a better choice than Pd/Al. The evolving B2-NiAl intermetallic is also stable for off-stoichiometric16,17 compositions guaranteeing robust fabrication of single-phase microstructures. By contrast, systems like Pd/Al or Pt/Al produce complex equilibrium or metastable phases around the equiatomic composition8,18,19. Destabilization of the reacted microstructures may occur. Consequently, optimal reactive materials should tolerate stoichiometric variations while forming the equilibrium compound. Whereas Ni/Al and Pd/Al each show advantages for one of the aforementioned key properties, they exhibit no RT-product-ductility. Thus, the systems are inefficient for applications requiring mechanical reliability. It is generally known that intermetallics are highly brittle16,20 eventually narrowing down the material selection substantially. However, there is a strong demand for systems producing RT-ductile intermetallics14. Their utilization would enable a technology expansion into fields requiring mechanical reliability at RT. In fact, researchers discovered a family of ductile B2 compounds20. However, self-propagating reactions of those systems produce multiphase microstructures because these phases are line compounds exhibiting no off-stoichiometry tolerance12. Furthermore, they have the lowest EDs (<6.6 kJcm−3). Hence, we are left to conclude: to date no energetic material combines high energy density with maximal ductility. Since Ni/Al is currently the only commercially available and already used metallic energetic material, we took its properties as a benchmark, theoretically explored various binary metallic systems, and identified the equiatomic Ru/Al system to show the best combination of aforementioned properties (see our reviews21,22). First, it combines high ED (8.0 kJcm−3) with the formation of the intermetallic RuAl also for off-stoichiometric compositions. Second, we calculated the adiabatic temperature Tad to infer the maximal possible Tf. Tad = 1,977 °C which is 339 °C higher than that of Ni/Al (1,638 °C1) enabling us to anticipate substantially higher Tfs for Ru/Al. Third, polycrystalline RuAl exhibits exceptional RT-ductility22. This is unusual for intermetallics (see Fig. 1(b)). We plot the mechanical anisotropy against Poisson’s ratio20. Ductile B2 compounds cluster in the indicated range (circular dotted line in Fig. 1(b)) also including RuAl leaving out brittle phases like NiAl, PtAl, and PdAl. The comparison of the EDs (data point diameter of every intermetallic in Fig. 1(b)) reveals the maximal value for the Ru/Al system among the ductile phases. We therefore hypothesize: Ru/Al is the system with maximal ED and maximal ductility.