Power/Performance Bits: July 16

Bacterial solar; infrared rectenna; boron arsenide’s thermal conductivity.


Bacterial solar
Researchers at the University of British Columbia developed a solar cell that uses bacteria to convert light to energy. The cell worked as efficiently in dim light as in bright light, making solar a potential option in areas of the world that frequently have overcast skies.

Called biogenic cells, they work by utilizing the natural dye that bacteria use for photosynthesis. Previous efforts have focused on extracting the dye, a costly and complex process that involves toxic solvents and can cause the dye to degrade.

The team’s solution was to leave the dye in the bacteria. They genetically engineered E. coli to produce large amounts of lycopene, which gives tomatoes their red-orange color and is particularly effective at harvesting light for conversion to energy. The researchers coated the bacteria with a mineral that could act as a semiconductor, and applied the mixture to a glass surface.

With the coated glass acting as an anode at one end of their cell, they generated a current density of 0.686 milliamps per square centimeter.

“We recorded the highest current density for a biogenic solar cell,” said Vikramaditya Yadav, a professor in UBC’s department of chemical and biological engineering. “These hybrid materials that we are developing can be manufactured economically and sustainably, and, with sufficient optimization, could perform at comparable efficiencies as conventional solar cells.”

The cost savings are difficult to estimate, but Yadav believes the process reduces the cost of dye production to about one-tenth of what it would be otherwise. The holy grail, Yadav said, would be finding a process that doesn’t kill the bacteria, so they can produce dye indefinitely.

Infrared rectenna
Researchers from Sandia National Laboratories are working on building a new infrared rectifying antenna that can turn waste heat into DC power.

“In the short term we’re looking to make a compact infrared power supply, perhaps to replace radioisotope thermoelectric generators,” said Paul Davids, a physicist at Sandia. Called RTGs, the generators are used for such tasks as powering sensors for space missions that don’t get enough direct sunlight to power solar panels.

The new device is about 1/8 inch by 1/8 inch, half as thick as a dime and metallically shiny. The top is aluminum, etched with stripes that serve as an antenna. Between the aluminum top and the silicon bottom is a very thin layer, about 20 silicon atoms thick, of silicon dioxide. The patterned and etched aluminum antenna channels the infrared radiation into this layer.

The rectenna, short for rectifying antenna, is made of common aluminum, silicon and silicon dioxide using standard processes from the integrated circuit industry. (Source: Randy Montoya / Sandia National Laboratories)

Because the team makes the infrared rectenna with the same processes used by the integrated circuit industry, it’s readily scalable, said Joshua Shank, electrical engineer at Sandia. “We’ve deliberately focused on common materials and processes that are scalable. In theory, any commercial integrated circuit fabrication facility could make these rectennas.”

One of the biggest fabrication challenges was doping the silicon so that it would reflect infrared light like a metal, said Rob Jarecki, the fabrication engineer who led process development. “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.”

The latest version of the infrared rectenna produces 8 nanowatts of power per square centimeter from a specialized heat lamp at 450 degrees C. For context, a typical solar-powered calculator uses about 5 microwatts, so they would need a sheet of infrared rectennas slightly larger than a standard piece of paper to power a calculator. So, the team has many ideas for future improvements to make the infrared rectenna more efficient.

The team plans to make the device more efficient. Ideas include making the rectenna’s top pattern 2D x’s instead of 1D stripes, in order to absorb infrared light over all polarizations; redesigning the rectifying layer to be a full-wave rectifier instead of the current half-wave rectifier; and making the infrared rectenna on a thinner silicon wafer to minimize power loss due to resistance.

The team thinks that within five years, the infrared rectenna may be efficient enough for it to be a good alternative to RTGs for compact power supplies.

Thermally conductive material
Researchers from the University of Texas at Dallas, the University of Illinois at Urbana-Champaign, and the University of Houston created highly thermally conductive crystals of the semiconductor boron arsenide, a material they point to as having the potential to boost heat dissipation in electronics.

While diamond is an excellent choice for heat dissipation, having a thermal conductivity of around 2,200 watts per meter-kelvin, structural defects in man-made diamond and high cost of natural diamond limits its use.

In 2013, researchers at Boston College and the Naval Research Laboratory published research that predicted boron arsenide could potentially perform as well as diamond as a heat spreader. In 2015, a University of Houston team successfully produced such boron arsenide crystals, but the material had a fairly low thermal conductivity, around 200 watts per meter-kelvin.

“We have been working on this research for the last three years, and now have gotten the thermal conductivity up to about 1,000 watts per meter-kelvin, which is second only to diamond in bulk materials,” said Bing Lv, assistant professor of physics at UT Dallas.

A crystal of boron arsenide, imaged with an electron microscope. (Source: University of Texas at Dallas)

“The boron arsenide crystals were synthesized using a technique called chemical vapor transport,” said Illinois postdoctoral researcher Qiye Zheng. “Elemental boron and arsenic are combined while in the vapor phase and then cool and condense into small crystals. We combined extensive materials characterization and trial-and-error synthesis to find the conditions that produce crystals of high enough quality.”

The Illinois team used electron microscopy and a technique called time-domain thermoreflectance to determine if the lab-grown crystals were free of the types of defects that cause a reduction in thermal conductivity.

“We measured dozens of the boron arsenide crystals produced in this study and found that the thermal conductivity of the material can be three times higher than that of best materials being used as heat spreaders today,” Zheng said.

“I think boron arsenide has great potential for the future of electronics,” Lv said. “It’s semiconducting properties are very comparable to silicon, which is why it would be ideal to incorporate boron arsenide into semiconducting devices.”

Lv also said that while arsenic by itself can be toxic to humans, once it is incorporated into a compound like boron arsenide, the material becomes very stable and nontoxic.

The researchers next plan to try other processes to improve the growth and properties of the material for large-scale applications.

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