Using a new optical magnetic metamaterial claimed to have revolutionary properties, physicists from the Australian National University (ANU) and the University of California Berkeley (UC Berkeley) have produced a prototype device that could be used in super-efficient thermophotovoltaic cells. These cells do not need direct sunlight to generate electricity, but instead absorb infrared radiation to convert to electric current and, unlike conventional photovoltaic cells, can do so even in the dark.
To create this new material, the researchers stacked twenty alternating nanomaterial sheets of gold and magnesium fluoride (with thicknesses of 30 nm and 45 nm, respectively), one on top of the other and sat them all on a 50-nm-thin silicon nitride base. Using focused ion milling, the researchers then cut a series of elongated holes in the material to produce cavities.
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All of this work endowed the material with properties designed to exploit a phenomenon known as "magnetic hyperbolic dispersion." Simple dispersion dictates how light behaves with various materials and is often seen as the a three-dimensional propagation of electromagnetic radiation in different directions. For example, a ground glass convex lens would have a spherical or ellipsoid shape to its dispersion.
With the new metamaterial, on the other hand, the incredibly strong interactions of the material with the magnetic component of light means that the dispersion characteristics take on a hyperbolic form – that is, a much more intense distribution pattern – and results in a directional, coherent and polarized thermal emission. In other words, it glows brightly and unusually when heated by infrared radiation.
The result of all this nanoscale topological manipulation means that the magnetic hyperbolic dispersion occurring in the metamaterial can be tuned to specific frequencies and intensities so that it may aid in vastly increasing thermal transfer efficiencies when paired as an emitter with thermophotovoltaic cells.
"Thermophotovoltaic cells have the potential to be much more efficient than solar cells," says Dr Sergey Kruk from the ANU Research School of Physics and Engineering. "Our metamaterial overcomes several obstacles and could help to unlock the potential of thermophotovoltaic cells."
The metamaterial devices were specifically created by scientists at UC Berkeley to Dr Kruk's specifications, and needed the expertise of UC Berkeley in producing such lilliputian structures and then precisely milling them to shape.
"The size of individual building block of the metamaterial is so small that we could fit more than twelve thousand of them on the cross-section of a human hair," says Dr Kruk.
According to the team, these new devices could also be amalgamated with a heater to produce on-demand power or attached to vehicle engines to recycle radiated heat into electrical power. The researchers also believe that the efficiency of thermovoltaic cells constructed from their metamaterial may be further improved by substantially reducing the gap between the emitter and the receiver to just a millionth of a millimeter or so. In this way, radiative heat transfer could improve more than tenfold over conventional materials.
The results of this research have been published in the journal Nature Communications.
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