Courtesy:
University of Arkansas
Researchers
build circuit that harnessed the atomic motion of graphene to generate an
electrical current that could lead to a chip to replace batteries.
A team of
University of Arkansas physicists has successfully developed a circuit capable
of capturing graphene’s thermal motion and converting it into an electrical
current.
“An
energy-harvesting circuit based on graphene could be incorporated into a chip
to provide clean, limitless, low-voltage power for small devices or sensors,”
said Paul Thibado, professor of physics and lead researcher in the discovery.
The
findings, published in the journal Physical Review E, are proof of a theory the
physicists developed at the U of A three years ago that freestanding graphene —
a single layer of carbon atoms — ripples and buckles in a way that holds
promise for energy harvesting.
The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman’s well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Thibado’s team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible.
Graphene
chip testing — A sample energy-harvesting chip under development.
Courtesy:
University of Arkansas
In the
1950s, physicist Léon Brillouin published a landmark paper refuting the idea
that adding a single diode, a one-way electrical gate, to a circuit is the
solution to harvesting energy from Brownian motion. Knowing this, Thibado’s
group built their circuit with two diodes for converting AC into a direct
current (DC). With the diodes in opposition allowing the current to flow both
ways, they provide separate paths through the circuit, producing a pulsing DC
current that performs work on a load resistor.
Additionally,
they discovered that their design increased the amount of power delivered. “We
also found that the on-off, switch-like behavior of the diodes actually
amplifies the power delivered, rather than reducing it, as previously thought,”
said Thibado. “The rate of change in resistance provided by the diodes adds an
extra factor to the power.”
The team
used a relatively new field of physics to prove the diodes increased the
circuit’s power. “In proving this power enhancement, we drew from the emergent
field of stochastic thermodynamics and extended the nearly century-old,
celebrated theory of Nyquist,” said coauthor Pradeep Kumar, associate professor
of physics and coauthor.
According
to Kumar, the graphene and circuit share a symbiotic relationship. Though the
thermal environment is performing work on the load resistor, the graphene and
circuit are at the same temperature and heat does not flow between the two.
That’s an important
distinction, said Thibado, because a temperature difference between the
graphene and circuit, in a circuit producing power, would contradict the second
law of thermodynamics. “This means that the second law of thermodynamics is not
violated, nor is there any need to argue that ‘Maxwell’s Demon’ is separating
hot and cold electrons,” Thibado said.
The team also discovered that the relatively slow motion of graphene induces current in the circuit at low frequencies, which is important from a technological perspective because electronics function more efficiently at lower frequencies.
Paul
Thibado, professor of physics, holds prototype energy-harvesting chips.
Courtesy:
Russell Cothren, University of Arkansas
“People
may think that current flowing in a resistor causes it to heat up, but the
Brownian current does not. In fact, if no current was flowing, the resistor
would cool down,” Thibado explained. “What we did was reroute the current in
the circuit and transform it into something useful.”
The team’s
next objective is to determine if the DC current can be stored in a capacitor
for later use, a goal that requires miniaturizing the circuit and patterning it
on a silicon wafer, or chip. If millions of these tiny circuits could be built
on a 1-millimeter by 1-millimeter chip, they could serve as a low-power battery
replacement.