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Tuning in to magnetic ink

An ink containing iron-oxide nanoparticles can be turned into fully printed and versatile components for cellular networks, says a team from KAUST.

According to the researchers, inkjet printing technology can be used to produce radio frequency devices, such as antennas, that can be magnetically reconfigured on demand. By tuning in to multiple cellular bands and standards, the belief is that this will boost prospects for inexpensive electronics that work worldwide.

A typical mobile antenna is made by depositing metallic patterns onto insulating silicon or glass wafers. These miniature aerials have excellent reliability, but only operate at fixed frequency bands. To fabricate devices that can adapt to different wireless settings, researchers have been turning to magnets. Replacing an insulating wafer with a magnetic one can, for example, achieve frequency tuning that can cover mega to gigahertz ranges.

Instead of the complex, multilayered ceramics currently used as magnetic substrates, the team investigated an approach based on printable electronics.

"If magnetic substrates can be printed, we can achieve huge cost savings and add new functionalities," says Atif Shamim. "There are a number of other metrics that can be optimised, such as thickness, that are impossible with fixed substrates."

By injecting iron-based reagents into a hot acetic acid solution, the researchers say they were able to synthesise magnetic iron-oxide nanoparticles, which dispersed into deionsed water to form an ink.

According to KAUST, when deposited as a thin film on a glass substrate, the new magnetic substrate acted as an energy-storing inductor device with an adjustable capacity of over 20%.

Printing the magnetic ink thicker than a few nanometers however, proved impossible due to its natural brittleness. To strengthen the ink, the team modified the nanoparticles' surfaces with hydrocarbon chains to help the tiny magnets mix evenly into an epoxy resin known as SU-8. The resulting paste was screen-printed and UV-cured into free-standing magnetic sheets of a few millimeters in thickness.

The team say that the fully printed magnetic wafer displayed promising antenna tuning of over 10% – a figure they aim to improve upon further.

"The surprise was that we got antennas with a good tuning range, even though we mixed in 50% SU-8," notes Shamim. "This means we could extend this tuning range further by adjusting this ratio and also move to more sophisticated roll to roll processes that print at meters per minute."

Bethan Grylls

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