Sparklers in space

A scientist at Vanderbilt University is exploring the use of polycrystalline diamond as a replacement for the silicon solar cells.

A scientist at Vanderbilt University is exploring the use of polycrystalline diamond as a replacement for the silicon solar cells currently used in many space applications.

Timothy Fisher, an assistant professor of mechanical engineering at Vanderbilt University, is exploring the use of polycrystalline diamond as a replacement for the silicon solar cells currently used in many space applications.

Fisher maintains that diamond films would be ideal for use in outer space as they can withstand the high levels of radiation found in that environment. By contrast, the performance of silicon cells is said to degrade by about 50 percent after 10 years in orbit.

Diamonds also qualify for use in outer space because they can operate at high temperatures and have a potential conversion efficiency of 50 percent as compared to 10 to 15 percent for silicon solar cells.

Fisher’s diamond solar converters are not photovoltaic devices like silicon cells but solar thermal devices, which convert light into heat and heat into electricity.

Fisher’s device makes use of diamond films covered with millions of microscopic pyramids: about 10 million per square centimetre.

When heated, the tips of the pyramids emit streams of high-energy electrons. At the nanoscale, the laws of physics are said to differ from what they are at larger scales. In this instance, they favour the efficient production of high-energy electrons.

In Fisher’s design, the bottom of the diamond film is laminated to a metal sheet that acts as a cathode for the device. Another sheet of electrically conducting material is placed on top of the film with a very small gap in between from which almost all of the air has been removed.

The top sheet serves as the anode. A radiation absorber is attached to the bottom of the cathode. When sunlight is directed on the absorber plate, it heats up the device to about 1,000 degrees Celsius. When heated, the tips of the tiny pyramids emit streams of electrons that flow across the intervening vacuum to the anode, creating an electric current.

Because moving electrical current at low voltages causes high levels of line loss, Fisher has had to add a DC-to-DC converter that increases the voltage and reduces the current.

Fisher and his colleagues have been working on a small test device with a plain diamond film without the pyramids. ‘What we have seen increases our confidence that the converter will work,’ said Fisher.

Fisher’s ultimate goal is to produce a prototype cell that is a square centimetre in size and produces 10 watts of electrical power at 1,000 degrees Celsius.