Thin Copper Flakes for Conductive Inks Prepared by Decomposition of Copper Formate and Ultrafine Wet Milling

Fabrication of devices by printing conductive interconnections on plastic substrates is of growing interest. Currently, silver flakes are wildly used, however the high cost of silver prevents their wide use in many electrical devices. A new two-step process for synthesizing thin copper flakes, and their utilization in conductive inks, is reported. In the first step, sub-micrometer copper particles are formed by thermal decomposition and self-reduction of copper formate. These copper particles are then milled in a wet bead mill that results in their transformation into thin flakes with an average thickness of 48 nm. X-ray diffraction results indicate that the copper particles undergo plastic deformation in a mechanism similar to cold rolling. The effect of various process parameters and type of dispersing agents on the morphology and electrical performance is studied. The ink formulations result in printed patterns with 22% of bulk copper conductivity. The optimal ink is used to print functioning near field communication antennas on polyimide film, which is found to have a high bending durability.