Investigation of reactive bimetallic Ni-Al particles as a heat source for microwave-assisted joining

Thermal joining of materials with dissimilar thermo-physical properties requires an efficient and adaptable energy input that does not lead to thermal damage of the joining partners. Reactive particles provide a promising solution for this purpose as they represent a tailorable heat source by being able to undergo an exothermic, self-sustaining reaction. A sophisticated method of initiating the reaction is the use of microwaves, since they allow high heating rates, volumetric heating, and may influence the reaction even after the activation. In this way, particles consisting of nickel and aluminium can generate temperatures up to 1550 K within a few hundred milliseconds. However, high reaction rates, elevated temperatures, and still not fully understood microwave-specific phenomena complicate the derivation of cause-effect relationships. Therefore, an experimental set-up was developed, in which the reactive particles can be activated either in the predominant magnetic or electric field due to a customised resonant cavity. Elaborate high-temperature and several power measurement methods were implemented to characterise the activation and reaction behaviour of particles with a lamellar intrinsic structure. Both the electromagnetic field and the particle size influenced the resulting temperature-time profiles and arcing phenomena. In order to demonstrate the potential of reactive particles, they were also used as a new heat source in microwave-assisted adhesive bonding. An empirical study showed that microwaves and reactive particles interacted in an advantageous way with epoxy resins, promoted the cross-linking of the polymers, and allowed the fabrication of polypropylene joints in the microwave cavity.