Imagine a world where smartphones, laptops, wearables and other electronics run without batteries. Researchers at the Massachusetts Institute of Technology have taken a step in this direction with the release of the first fully flexible device that can convert the energy from Wi-Fi signals into electricity to power electronics.
What is rectenna
A rectenna is a device that converts alternating current electromagnetic waves into direct current. Researchers described a new species in the journal Nature. It uses a flexible radio frequency antenna that captures electromagnetic waves, including Wi-Fi. It connects to a two-dimensional semiconductor several atoms thick. The alternating current flows into the semiconductor, which converts it to direct current, which allows you to power electronic circuits or charge batteries.
Thus, the device passively captures and converts Wi-Fi signals to DC. It is flexible and can be produced in rolls to cover a large area.
The New Way to Power the Internet of Things
“What if we create electronic systems that wrap around a bridge, or cover an entire highway or office walls and give electronic intelligence to everything around us? How do we power all this electronics? Asks co-author Thomas Palacios, professor in the Department of Electrical Engineering and Computer Science and director of the Center for Graphene Devices and 2D Systems in Microsystem Technology Laboratories. “We have come up with a new way to power the electronic systems of the future, harvesting Wi-Fi energy in a way that can be easily integrated over large areas so that all objects around us gain intelligence.”
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Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and IoT sensors. Flexible smartphones, for example, are a hot new market for big tech firms. The experimental device generates about 40 μW of power when exposed to typical Wi-Fi signal power levels (about 150 μW). This is more than enough to light up a simple mobile phone display or power chips.
Application in medicine
According to a researcher at the Technical University of Madrid, Jesús Grajal, one of the possible applications of the development is to provide data transmission for implantable medical devices. For example, pills that will transfer data about the patient's health to a computer for subsequent diagnosis.
“It is dangerous to use batteries to power these systems because if the lithium leaks, the patient will die,” Grahal says. "It is much better to harvest energy from the environment to power these small laboratories inside the body and transmit data to external computers."
Flexible rectifier
All rectennas rely on a component known as a "rectifier" that converts AC to DC. In traditional rectennas, the rectifier is made of silicon or gallium arsenide. These materials can cover Wi-Fi frequencies, but they are tough. Although they are relatively inexpensive to use to make small devices, covering large areas such as the surfaces of buildings and walls would be prohibitively expensive. Researchers have long been trying to solve these problems. But a few flexible rectennas that have been reported so far operate at low frequencies and cannot capture and convert gigahertz signals, which most cell phone and Wi-Fi signals are.
To create their rectifier, the researchers used a new two-dimensional material, molybdenum disulfide (MoS2), which, at a thickness of 3 atoms, is one of the thinnest semiconductor devices in the world. The team used the unusual behavior of MoS2: when exposed to certain chemicals, the atoms of the material rearrange in such a way that it acts as a switch, causing a phase transition from a semiconductor to a metallic material. This structure is known as a Schottky diode.
“By creating MoS2 in a 2D semiconductor-metal phase transition, we built a thin, ultra-fast Schottky diode that simultaneously minimizes series resistance and parasitic capacitance,” says project author Xu Zhang.
Parasitic capacitance is inevitable in electronics. Some materials build up a small electrical charge that slows down the circuit. Consequently, lower capacitance means higher rectifier speeds and higher operating frequencies. The parasitic capacitance of a Schottky diode is an order of magnitude less than modern flexible rectifiers, so it converts the signal much faster and allows you to capture and convert up to 10 GHz.
“This design has a fully flexible device that is fast enough to cover most of the radio frequency bands used by day to day electronics, including Wi-Fi, Bluetooth, cellular LTE and more,” Zhang says.
Effectiveness of flexible rectenna
In the described work, drawings of other high-performance flexible devices are proposed. The maximum output efficiency of the current device averages 40% and depends on the Wi-Fi power. The MoS2 rectifier has a typical efficiency of 30%. For reference, the efficiency of rectennas made from harder and more expensive silicon or gallium arsenide reaches 50-60%.
Now the development team plans to build more complex systems and improve the efficiency of the technology.
Author: Sergey Prots