Preparation of Conductive Inks with Submicron Copper Salt Particles for Printed Electronics By Ultrafine Wet Grinding

Fabrication of electronic devices by printing conductive interconnections on plastic substrates is of growing interest. Several approaches for the sintering of metallic nanoparticles (NP), mainly silver, were recently developed. However, the high cost of silver limits commercial use and therefore inks with low cost metals, such as copper, are required. Inks containing copper NPs suffer from stability problems, as the NPs are quickly oxidized, and so lose their conductivity. Therefore, there is an unmet need for a stable copper ink with a low sintering temperature.

Our research is aimed at printing conductive patterns by inks composed of sub-micron copper formate particles. Unlike copper in its pure form, in this ink copper is present in a salt form, and so it is stable to oxidation, enabling a long shelf life. The ink can be easily screen-printed, and, upon thermal decomposition the salt is converted into conductive Cu° pattern. Two approaches were investigated to induce decomposition of copper formate patterns: by photonic curing, and by hot pressing.

Copper formate dispersion inks were formed by ultrafine wet grinding, which is a fast and simple top-down process for the production of sub-micron particles. A wet slurry / dispersion undergoes mechanical stress by stirring it with a hard media (usually zirconium oxide beads), in a mechanism somewhat like ball milling at a much smaller scale, thus producing finer particle sizes. By a short milling process, the particle diameter was reduced by three orders of magnitudes, to sub-micron. Milling of copper formate is performed with an organic solvent and a dispersing agent.

A hybrid ink of copper formate particles and carbon nanotubes (CNTs) was prepared by ultrafine wet grinding to improve results for samples treated by photonic curing. By adding as little as 0.5 wt % single-wall CNTs to the ink, the absorptance was enhanced by about 50%, and the threshold energy required to obtain a conductive pattern with by photonic curing was decreased by ∼25%.

1. Rosen, Y. S., Yakushenko, A., Offenhäusser, A., & Magdassi, S. (2017). Self-Reducing Copper Precursor Inks and Photonic Additive Yield Conductive Patterns under Intense Pulsed Light. ACS Omega, 2(2), 573-581.

2. Rosen, Y. S., Lidor, Y., Balter R., Szeskin A., Awadallaha A., Shacham-Diamand Y., and Magdassi S. "Copper interconnections and antennas fabricated by hot-pressing printed copper formate"- submitted

3. Rosen, Y., Grouchko, M. and Magdassi, S. (2015), Printing a Self-Reducing Copper Precursor on 2D and 3D Objects to Yield Copper Patterns with 50% Copper's Bulk Conductivity. Advanced Materials Interfaces. doi: 10.1002/admi.201400448