Enhancement of Ultrasonic Welding of Bulk Carbon Nanotube-Metallic and Graphene-Metallic Conductors

Nanostructured carbon sheets comprising carbon nanotube (CNT) or graphene are appealing for electrode and antenna applications. Physical connection to metal conductors requires enhanced mechanical strength, electrical performance, and thermal capability. Ultrasonic welding is a viable technique to fabricate stable and robust bonds but requires methods to further reduce electrical resistance and improve mechanical strength. The present dissertation has advanced ultrasonic welding between bulk CNT electrodes and Cu foil by utilizing chemically doped junctions at the bond interface. Specifically, a selective doping strategy using KAuBr4 at the bonding regions between bulk CNT electrodes and Cu lowers the electrical contact resistance measured utilizing novel Transfer Length Method (TLM) structures. This reduced electrical contact resistance at the CNT-metal weld interface also leads to a lower surface temperature measured by thermal imaging under high-applied current. A CNT interlayer was utilized during ultrasonic welding of graphene sheet to itself and Cu to improve adhesion. Optimized weld conditions were attained by varying the amplitude for a constant ultrasonic energy, altering the interface thickness between graphene and graphene/Cu. Mechanical analysis at optimal conditions demonstrated near-equivalent breaking forces to intrinsic graphene sheet. Welding of a chemically doped CNT adhesion layer reduced specific contact resistivity by 2X while retaining mechanical strength. Optical microscopy, coupled with elemental analysis from scanning electron microscopy (SEM), demonstrates failure in the graphene sheet layer limits the strength of the welded structures, while the CNT interlayer remains bonded to the graphene and Cu layers. Overall, the work has established the fundamental relationship for optimal bond properties in ultrasonically welded graphene-CNT-graphene and graphene-CNT-Cu structures.