All devices that emit WiFi signals also produce high-frequency tetrahertz waves.
The concentrated power of these waves could serve as an alternative energy source, say researchers at the Massachusetts Institute of Technology. They've come up with designs for a device that could convert these waves into a direct current.
The scientists published their work earlier this month in the journal Science Advances.
When self-charging smartphone cases first arrived, they were a godsend. Most functioned like a portable battery pack that you charged before using, while others had their own solar panels that could harvest and convert light energy into a direct current.
Version 2.0 looks like it could take things a step further by using up what's already abundantly available. Think: smartphone cases that can soak up the leftover energy WiFi routers routinely produce.
Because WiFi-emitting devices, like routers, produce what are known as tetrahertz waves, or T-rays, it's theoretically possible to harness those electromagnetic waves to create energy to power up your iPhone.
Researchers at the Massachusetts Institute of Technology have actually found a way to change the behavior of graphene to induce a direct current of energy—perfect for a passive charger that only needs to be near a router. Their work appears in the March 27 issue of the journal Science Advances.
"We are surrounded by electromagnetic waves in the terahertz range," lead author Hiroki Isobe, a postdoctoral researcher at MIT's Materials Research Laboratory, said in a prepared statement. "If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now."
From A/C to D/C
There's a long history of scientists attempting to make ambient energy more practical. When left untapped, it's a potential waste of renewable energy. Perhaps the most obvious is light energy from the sun, which can be collected and transformed into electrical energy through solar panels.
To rein in these non-traditional energy sources, researchers typically rely on rectifiers, or devices designed to convert the alternating current of electromagnetic waves—which oscillate in every which direction—into a direct current. Basically, the machine ensures the current only flows in one direction.
Rectifiers usually convert low-frequency electromagnetic waves, like radio waves, through an electrical circuit fitted with diodes, or semiconductor devices that only allow the current to flow one way. This generates an electric field that escorts the waves through the circuit as a direct current, which may then power electrical devices. But because rectifiers are only reliable up to a given frequency, they can't service waves in the tetrahertz range.
To transform wayward T-ray energy into something more predictable and usable, Isobe and his fellow researchers wondered whether they could tinker with a material's atomic structure to force electrons to flow in one direction, rather than rely on rectifiers to induce a direct current. Their focus is on the atomic behavior of graphene, a wonder material that's commonly used in electronics, solar panels, and even asphalt.
Under normal conditions, graphene is inherently symmetrical. That means electrons feel equal force between them and incoming energy scatters the electrons in all directions equally. Isobe and his team began looking for ways to make this scattering asymmetrical.
The scientists discovered combining graphene with another material, boron nitride, forced graphene's electrons to skew in a common direction, meaning incoming tetrahertz waves would coax the graphene's electrons in one direction as a direct current. This overall effect is known as "skew scattering," or when electron clouds skew their motion in one direction.
One major caveat persists: Because graphene is made up of a single layer of carbon atoms, if it contains too many impurities, it will lose some of its properties, like conductivity. Impurities can act like a shield in the path of electron clouds, forcing the electrons to scatter.
"With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation," Isobe said. "So we want a clean sample to effectively get a skewed motion."
Strapped with this knowledge, Isobe and company created a blueprint for a tetrahertz rectifier; they've applied for a patent for what they're calling a "high frequency rectification" device.
Picture an antenna that collects and concentrates tetrahertz radiation from the environment. The researchers found that the stronger the tetrahertz energy, the better a device could convert it into a DC current. Inside the antenna is a small square of graphene atop a layer of boron nitride.
"This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy," said Liane Fu, coauthor and associate professor of physics at MIT, in the statement.
The team is currently working with experimental physicists at MIT to come up with a physical prototype. It should work at room temperature, which will open the door to tons of portable applications. In the near term, Isobe believes the tetrahertz rectifiers could be used to power implants in the human body, meaning surgery would no longer be required to change its batteries.
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