Graphene at room temperature could provide a clean source of limitless low-voltage energy for sensors and small devices. Physicists at the University of Arkansas developed a circuit that can draw power from the material’s thermal motion, then convert it into an electrical current. An energy-harvesting graphene circuit could be incorporated into a chip – which can be installed into electronics to provide continuous power.
The research builds upon previous findings that freestanding graphene ripples and buckles in a way that could be manipulated for energy harvesting. They harnessed both the Brownian motion (random movement of particles) and the nanometer-sized rippling to generate an electric current. The ripples appear to stem from subatomic particle interactions in the graphene, but their origin is still unknown.
The team built their circuit with two diodes for converting an alternating current (AC) into a direct current (DC). They knew that a single diode wouldn’t work thanks to a landmark refuting by physicist Léon Brillouin in the 1950s. But having two diodes in opposition allowed the current to flow both ways along separate paths. The approach produced a pulsing DC that performed work on a load resistor.
The two-diode design also helped boost the amount of power delivered. Paul Thibado, a physics professor and lead researcher in the discovery, said:
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. The rate of change in the resistance provided by the diodes adds an extra factor to the power.
To prove the diodes raised the circuit’s power, the team used a reasonably new field of physics. Pradeep Kumar, a physics associate professor, and coauthor said:
In proving this power enhancement, we drew from the emergent field of stochastic thermodynamics and extended the nearly century-old, celebrated theory of Nyquist.
They also discovered that graphene’s minimal motion induces a low-frequency current in the circuit – an essential factor for efficiency. Electronics function more thoroughly at lower frequencies.
Next, the physicists will determine if DC can be stored for later use in a capacitor. To do this, they have to miniaturize the circuit and pattern it onto a silicon wafer or chip. If millions of tiny circuits like that were arranged on a 1×1 millimeter chip, they believe that it could serve as a low-power battery replacement. The energy generated isn’t much, but it’s enough to substitute low-power batteries – and it wouldn’t ever need to be replaced.
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