Battery-free power technology for electronics

Battery-free technology enables the powering of electronic devices via ambient radiofrequency signals

Energy Harvesting Through Ambient Radiofrequency Signals

Understanding RF Signals and Their Potential

Radiofrequency (RF) signals are integral to wireless technologies like Wi-Fi, Bluethooth, and 5G. Researchers from the National University of Singapore (NUS) have innovated a prototype that can convert these ambient RF signals into direct current (DC) voltage, enabling small electronic devices to operate without the need for batteries. This breakthrough is poised to revolutionize the way we power low-energy devices.

Challenges in RF Energy Harvesting

Despite the potential of RF energy harvesting, significant challenges exist. Ambient RF signals typically have low power (less than-20 dBm), making it difficult for current rectifier technologies to efficiently convert these signals into usable energy. Enhancing antenna efficiency and impedance matching can improve performance, but this approach increases the size of the chip, complicating integration and miniaturization efforts.

Overcoming Obstacles with Spin-Rectifier Technology

Advancements in Spin-Rectifier (SR) Technology

To address the limitations of traditional rectifiers, NUS researchers, in collaboration with Tohoku University (TU) in Japan and the University of Messina (UNIME) in Italy, have developed a compact and highly sensitive spin-rectifier (SR) technology. This innovation effectively converts ambient RF signals at power levels below - 20 dBm into DC voltage, enabling the powering of small electronic devices without relying on batteries.

Performance and Efficiency of SR Arrays

The research Team enhanced SR devices, resulting in two configurations: a single SR-based rectenna operating between-62 dBm and -20 dBm, and an array of 10 SRs in series that achieved 7.8% efficiency and zero-bias sensitivity of approximately 34,500 mV/mW. This SR-array was successfully integrated into an energy harvesting module, capable of powering a commercial temperature sensor at-27 dBm. This development showcases the potential of SR technology for low-power RF applications.

Research Collaboration and Future Potential

This research was conducted experimentally in partnership with Professor Shunsuke Fukami's team at TU, with simulations performed by Professor Giovanni Finocchio of UNIME. The successful integration and scalability of SR technology suggest its potential for large-scale applications in low-powered RF and communication technologies.

Spin-Rectifier Technology for Efficient Low-Power Operation

Challenges in Low-Power Rectification

Current state-of-the-art rectifiers, such as Schottky diodes, tunnel diodes, and two-dimensional MoS₂, achieve efficiencies of 40%-70% at RF power levels of -10 dBm or higher. However, ambient RF power from sources like Wi-Fi routers is typically below -20 dBm. Developing rectifiers with high efficiency at these lower power levels is challenging due to thermodynamic limitations and parasitic effects at high frequencies.

Spin-Diode Effect and Nanoscale SR Technology

The spin-diode effect enables nanoscale spin-rectifiers to transform RF signals into DC voltage. Although SR technology has exceeded the sensitivity of Schottky diodes, its efficiency in low-power applications remains under 1%. To overcome these limitations, the research team optimized spin-rectifiers by analyzing factors such as perpendicular anisotropy, device geometry, and dipolar fields form the polarizer layer, as well as the dynamic response influenced by zero-field tunneling magnetoresistance and voltage-controlled magnetic anisotropy (VCMA).

Achievements in On-Chip Integration and Efficiency

By arranging the spin-rectifiers in an array and integrating small co-planar wave guides for RF power coupling, the researchers achieved a compact on-chip layout with enhanced efficiency. The self-parametric effect, driven by VCMA in magnetic tunnel junction-based spin-rectifiers, proved pivotal for effective low-power operation. This innovation positions SR technology as the most compact, efficient, and sensitive rectifier technology under ambient RF conditions.

Future Strategies and Industry Collaboration

Optimizing SR Technologies for On-Chip Applications

The NUS research team is currently focused on optimizing the efficiency and miniaturization of SR technologies by incorporating on-chip antennas, They are also developing series-parallel configurations to adjust impedance in extensive SR arrays, leveraging on-chip interconnections to link individual SR units. This strategy aims to significantly boost RF power harvesting, potentially producing rectified voltages in the range of several volts, thereby eliminating the need for a DC-to-DC converter.

Partnerships for Advancing SR Technology

The researchers are actively seeking partnerships with industry and academic institutions to advance the development of self-sustaining smart systems utilizing on-chip SR rectifiers. This collaboration could lead to the creation of compact on-chip technologies for wireless power transfer and signal detection applications, paving the way for the next generation of ambient RF energy harvesters and sensors based on spin-rectifiers.

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