Silver is the primary conductive ink used in printed electronics today for products such as UHF antennas for RFID, membrane keyboards, flexible displays and battery testers, but as the cost of silver is predicted to increase exponentially over the years copper could be seen as a suitable alternative for many of these applications.
Copper has excellent conductivity properties however many problems such as purity, oxidation, particles' size, dispersion quality and deposition at low temperature in air have been areas of research to date.
The Polymer Chemistry Research Group at the University of Helsinki, Finland, has succeeded in producing nano-sized metallic copper particles that when reduced to nano-scale, the properties of the material undergo substantial changes.
In nanoparticles the number of surface atoms is considerably greater than the number of atoms inside the material, which, among other things, makes the melting temperature of nanomaterials very low. With suitable heat treatment (sintering), the particles manufactured by the team could be made into electricity-conducting layers and patterns on paper.
The researchers wanted to test the ability of polymeric and small-molecule compounds that contain amine groups to protect copper nanoparticles during their manufacturing stage - Polymers are large-molecule compounds that, as functional materials, change their properties according to the changing ambient conditions.
The particles were manufactured with either poly(ethylene imine) (PEI) or tetraethylenepentamine (TEPA) used as protecting compounds. The average size of the particles at room temperature was 8.5 nm (with PEI as the protecting agent) or 19.4 nm (with TEPA as the protecting agent). Slightly oxidised at their surface, the particles were sintered to the paper surface, and the electrical conductivity of the layer thus formed was measured. Particles manufactured using PEI released the protective agent during sintering at relatively low temperatures (150-200 °C). At these temperatures, the size of the particles increased rapidly.
The researchers claim that the electrical conductivity of the sintered particles was so good that they made promising materials for use in printed electronics on paper which may one day replace components such as electronic boards.
Further developments in copper nanoparticles
Further industry developments come from American company, NanoDynamics who have developed a range of nanoscale metals for printed electronics which include copper powders and flakes that provide a high performing economically efficient option for larger-scale circuit production. These powders offer excellent repeatability and processing characteristics, and enable a direct additive circuit printing process that is more environmentally sound than traditional etching methods. The copper powders and flakes are ideal for printed conductors, dipped capacitor terminations and conductive polymers. They provide exceptional purity and particle size control, and are available in sizes from 200 nm to 6 microns.
Applied Nanotech Holdings Inc have produced a copper nanoparticle ink that can be rapidly sintered by electro-magnetic irradiation, by using a laser, in air, at room temperature to consistently achieve metallic conducting traces with low resistivity in the micro-ohm-cm range. This process involving Aerosol Jet printing of the inks followed by simple sintering offers manufacturers a rapid and inexpensive method for printing electronic circuits onto a wide range of flexible substrates.
For more read Replacing printed silver with copper.
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