Thermal joining of highly conductive bonds by using reactive Al-Ni nanofoils

In view of the demand for electrified long-range vehicles, the performance of tractionbatteries has to be improved. To achieve a high power density, the battery cells must beinterconnected with low electrical resistance within the joints. The joining process has to fulfill specific requirements, as the battery cells may only be exposed to very low mechanical and thermal impacts. Joining by using reactive aluminum-nickel nanofoils represents an innovative technology meeting the abovementioned requirements. These foils are multilayered systems consisting of several hundred alternating monolayers of aluminum and nickel, each with a thickness in the nanometer range. Their unique ability is that they can react with temperatures up to 1500 °C for a duration of a few milliseconds upon external ignition. This thermal reaction energy serves as a heat source in the joining process, melting the materials in the interfacial surface. Subsequently, the joining partners solidify and form an adhesive bond when compressed properly. However, these advantageous characteristics are contrasted by complex reaction mechanisms and an unknown interaction of the process parameters. For the industrial application of the joining technology, the requirements for initiating the exothermic reaction must be known. Therefore, the process window and the mechanism of ignition have to be scrutinized. For this purpose, an experimental test setup was developed to generate and monitor short circuit currents to ignite the nanofoils. The amperage as well as the layer composition of the nanofoils were varied within a parameter study. Two independent process windows for a stable ignition were identified for all analyzed nanofoils.