Unused Wi-Fi signals could be used to power electronics

Researchers from Tohoku University, the National University of Singapore and the University of Messina have developed a novel technology to efficiently harvest ambient low-power radiofrequency (RF) signals into direct-current (DC) power. This ‘rectifier’ technology can be integrated into energy harvesting modules to power electronic devices and sensors, enabling battery-free operation. The research findings have been published in the journal Nature Electronics.

Collecting and then converting ambient energy sources into usable energy is referred to as ‘harvesting’. Small devices can harvest the energy, which can reduce battery dependency, extend device lifetimes and minimise environmental impact. Instead of having to physically travel to devices in remote regions to replace batteries, the device can be powered remotely by ambient energy sources such as everyday RF wireless signals.

The downside of this method is that the source of the signal has to be in close proximity to the electronic device in question. Existing technologies, such as the Schottky diode, face challenges in terms of low RF-to-DC conversion efficiency for faint ambient RF signals (typically less than -20 dBm). To address these challenges, the researchers developed a compact and sensitive rectifier technology that uses a nanoscale spin-rectifier (SR) to convert ambient wireless RF signals that are less than -20 dBm to a DC voltage. The SR consists of a nanoscale magnetic tunnel junction made of CoFeB/MgO that is used in a non-volatile memory technology.

The researchers enhanced the SR devices, paying attention to the material’s magnetic anisotropy, device geometry and tunnelling barrier properties. Then, the RF-to-DC conversion performance was tested for two configurations: a single SR-based rectenna operational between -62 and -20 dBm, and an array of 10 SRs in series. Integrating the SR-array into an energy harvesting module, they successfully powered a commercial temperature sensor at -27 dBm.

The researchers are now exploring the integration of an on-chip antenna to improve the efficiency and compactness. The team is also developing series-parallel connections to tune impedance in large arrays of SRs, utilising on-chip interconnects to connect individual SRs. This aims to improve how RF power is harvested.

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