Tiny tungsten beams lord over light

Lattices of microscopic tungsten rods can act as heat shields, researchers have found. Such structures may dramatically boost the efficiency of incandescent light bulbs and of thermophotovoltaic devices, which convert radiated heat into electricity, says research leader Shawn Yu Lin of the Sandia National Laboratories in Albuquerque, N.M.

WHAT A MESH! This four-tiered stack of tungsten microrods can selectively block radiated heat. Such structures might make light bulbs more efficient. Fleming et al./Nature

For about 15 years, scientists have been developing orderly microstructures called photonic crystals that block electromagnetic radiation within certain bands of wavelengths (SN: 1/26/02, p. 61: Metallic materials made to order). Expecting to use the crystals for room-temperature applications, such as circuits that would conduct photons instead of electrons, the developers haven’t worried about creating devices that can withstand high temperatures.

Knowing that photonic crystals could be useful in hot environments, Lin, Sandia colleague James G. Fleming, and their coworkers at the Ames (Iowa) Laboratory turned to a mesh of tungsten bars, each about 1 micrometer thick. Tungsten’s melting temperature tops 3,400C. To build the photonic crystal out of the metal, the scientists first made a template from silicon and then replaced the silicon with tungsten.

Measurements of the new photonic crystal’s properties show that it almost completely blocks radiated heat in the band of infrared wavelengths between 8 and 20 micrometers, the team reports in the May 2 Nature. Using a finer version of the tungsten mesh with thermophotovoltaic devices might improve their performance. For example, the mesh could filter the thermal radiation from a heat source so that only the optimal wavelengths reach the device.

The researchers also found that the structure absorbs more light than expected at the 8-micrometer end of the band and transmits more light than expected at certain wavelengths below 8 micrometers. This suggests, they say, that such a photonic crystal–if its features were miniaturized tenfold and it were used as the filament of a light bulb–might shift much of the bulb’s infrared emissions into shorter, visible wavelengths. If so, Lin and his colleagues claim, the incandescent bulb’s electricity-to-light conversion efficiency might leap from today’s 5 percent to a whopping 60 percent.

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