Nanomaterial to help turn heat into electricity

Physicists have demonstrated a new artificial material, or metamaterial, that glows in an unusual way when heated. The findings could drive a revolution in the development of cells that convert radiated heat into electricity, known as thermophotovoltaic cells.

Thermophotovoltaic cells have been predicted to be more than twice as efficient as conventional solar cells. They do not need direct sunlight to generate electricity; instead, they can harvest heat from their surroundings in the form of infrared radiation. They can also be combined with a burner to produce on-demand power or can recycle heat radiated by hot engines.

The metamaterial, made of tiny nanoscopic structures of gold and magnesium fluoride, radiates heat in specific directions. The geometry of the material can be tweaked to give off radiation in specific spectral range, in contrast to standard materials that emit their heat in all directions as a broad range of infrared wavelengths. This makes it suitable for use as an emitter paired with a thermophotovoltaic cell.

The key to the metamaterial’s remarkable behaviour is its novel physical property, known as magnetic hyperbolic dispersion. Dispersion describes the interactions of light with materials and can be visualised as a 3D surface representing how electromagnetic radiation propagates in different directions.

For natural materials, such as glass or crystals, the dispersion surfaces have simple forms: spherical or ellipsoidal. The dispersion of the new metamaterial is hyperbolic in form. This arises from the material’s remarkably strong interactions with the magnetic component of light.

The efficiency of thermophotovoltaic cells based on the metamaterial can be further improved if the emitter and the receiver have just a nanoscopic gap between them. In this configuration, radiative heat transfer between them can be more than 10 times more efficient than between conventional materials.

The metamaterial was created by researchers from the Australian National University, the ARC Centre for Ultrahigh bandwidth Devices for Optical Systems and the University of California, Berkeley. Their work has been published in the journal Nature Communications.

Image caption: A sketch of the metamaterial.