Self-completed Joule heat welding of ultrathin Pt wires

Two Pt wires with diameters of about 800 nm were successfully welded by Joule heating in a scanning electron microscope. Melting and solidification at the point contact of the thin wires occurred continuously under a constant current supply and the welding process was completed within several seconds. This rapid and self-completed welding is due to the geometrical features of the nanocontact and the heat transport properties of the thin wires. The current required for joining wires in a scanning electron microscope was much lower than that in air. This is remarkable for longer wires and was found to be due to greater insulation under high-vacuum conditions. The difference in the thermal boundary conditions, in other words the difference in the melting conditions at the nanocontact, was evaluated experimentally and a parameter, comprising the applied current, the geometrical quantities of the wires, and a function for calibrating the thermal boundary conditions, with which the conditions for welding thin wires under different vacuum levels could be determined, is presented.