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.
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.
“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.
