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This tiny silicon-based device developed at Sandia National Laboratories can catch and convert waste heat into electrical power. The rectenna, short for rectifying antenna, is made of common aluminum, silicon and silicon dioxide using standard processes from the integrated circuit industry. (Photo by Randy Montoya)

A team from the U.S. Department of Energy’s Sandia National Laboratories has created a small silicon-based device that harnesses waste heat and converts it to direct current (DC) power.

The new solid-state device—dubbed infrared rectenna—contains no moving parts that can jam, bend or break and does not have to directly touch the heat source and cause thermal stress.

“We have developed a new method for essentially recovering energy from waste heat,” Paul Davids, a physicist and the principal investigator for the study, said in a statement. “Car engines produce a lot of heat and that heat is just waste, right? So imagine if you could convert that engine heat into electrical power for a hybrid car. This is the first step in that direction, but much more work needs to be done.

“In the short term we’re looking to make a compact infrared power supply, perhaps to replace radioisotope thermoelectric generators [RTG].”

The new device—which is one-eighth of an inch by one-eighth of an inch—is comprised of common and abundant materials, including aluminum, silicon and silicon dioxide that are combined in unconventional ways. The top of the device is aluminum etched with stripes about 20 times smaller than the width of a human hair, which serves as an antenna to catch the infrared radiation.

Between the aluminum top and the silicon bottom is a layer of silicon dioxide that is about 20 silicon atoms thick. The patterned and etched aluminum antenna channels the infrared radiation into the silicon layer.

The infrared radiation trapped in the silicon dioxide creates very fast electrical oscillations, about 50 trillion times a second, which pushes electrons back and forth between the aluminum and silicon layers in an asymmetric manner, a process called rectification that generates net DC electrical current.

The device can currently produce eight nanowatts of power per square centimeter from a specialized heat lamp at 840 degrees.

The researchers believe that the device is easily scalable because it is developed using the same processes used by the integrated circuit industry.

“We’ve deliberately focused on common materials and processes that are scalable,” Joshua Shank, an electrical engineer and the paper’s first author, said in a statement. “In theory, any commercial integrated circuit fabrication facility could make these rectennas.”

The researchers did have difficulties doping other elements into the silicon so that it would reflect infrared light like a metal.

“There’s immense complexity under the hood and the devices require all kinds of processing tricks to build them,” Rob Jarecki, the fabrication engineer who led process development, said in a statement. “Typically you don’t dope silicon to death, you don’t try to turn it into a metal, because you have metals for that. In this case we needed it doped as much as possible without wrecking the material.”

Next, the researchers plan to make the device’s top pattern 2D x’s rather than 1D stripes, which would absorb infrared light over all polarizations.

They also want to redesign the rectifying later to be a full-wave rectifier rather than the current half-wave rectifier. The final plan to improve the device involves making the infrared rectenna on a thinner silicon wafer to minimize power loss due to resistance.

Through improved design and greater conversion efficiency, the power output per unit area will increase and could lead to an infrared camera that is a good alternative to RTGs for compact power supplies.

“We need to continue to improve in order to be comparable to RTGs, but the rectennas will be useful for any application where you need something to work reliably for a long time and where you can’t go in and just change the battery,” Shank said. However, we’re not going to be an alternative for solar panels as a source of grid-scale power, at least not in the near term.”  

  

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