TECH SPACETurning Diamond Film Into Solar Cells
Timothy Fisher is taking a Tiffany's approach to converting sunlight into electricity: with a $348,000 grant from National Reconnaissance Office, the assistant professor of mechanical engineering is exploring the use of polycrystalline diamond as a replacement for the silicon solar cells currently used in many space applications.
"Diamond has a number of potential advantages for use in outer space," says Fisher, who will be working on the project with Weng Poo Kang, an associate professor of electrical engineering and computer science.
Fisher maintains that diamond films:
That is because the system does not use natural, gem-quality diamonds. Instead, it uses thin films made up of millions of microscopic diamond crystals. Polycrystalline diamond films can be made artificially from methane, the main ingredient in natural gas, through a process called chemical vapor deposition.
So far, the advantages and costs of diamond solar systems are largely theoretical. No one has tried to make a diamond solar converter before. Fisher got the idea from the research of Vanderbilt colleagues Kang and "Diamond" Jim Davidson, professor of electrical engineering, who have been studying the use of polycrystalline diamond for electronics and sensor applications for a number of years.
"When I saw that they had found that diamond film emits electrons efficiently -- you don't have to use strong electric fields and a lot of energy to pull them from the surface -- I realized that it could be used for energy conversion," Fisher says.
Fisher's idea has a definite "back to the future" twist.
His diamond solar converters are not photovoltaic devices like silicon cells but solar thermal devices. That is, they do not convert light directly into electricity. Instead, they convert light into heat and heat into electricity. Diamond solar cells are very similar to thermionic emission devices that were developed more than 40 years ago.
In fact, they are a close cousin to the vacuum tubes that powered old-fashioned radios, televisions and even computers before the development of the transistor. In thermionic devices, electrons are released by heating. In diamond devices, however, electrons are extracted by combining heating and an electric field.
What gives the device a futuristic element is Fisher's use of diamond films covered with millions of microscopic pyramids: about 10 million per square centimeter.
When heated, the tips of these pyramids, which are only a few atoms across, emit streams of high-energy electrons. At this extremely small "nanoscale," the laws of physics can differ from what they are at larger scales. In this instance, they favor the efficient production of high-energy electrons.
"It is this nanoscale physics that makes the device work," Fisher says.
In the new design, the bottom of the diamond film is laminated to a metal sheet that acts as a cathode, or negative terminal, 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, or positive terminal. 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.
"This creates a large amount of current and a small voltage," Fisher says. Because moving electrical current at low voltages causes high levels of line loss, the engineer has to add another step: a DC-to-DC converter that increases the voltage and reduces the current. "This can be done with about 90 percent efficiency," he says.
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," he says.
The goal of the nine-month project is to produce a prototype cell that is a square centimeter in size and produces 10 watts of electrical power at 1,000 degrees Celsius.
Prof. Timothy S. Fisher's homepage
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TECH SPACEEntech-Led Team To Develop Advanced Solar Arrays For Spacecraft
Keller - Jan. 22, 2001
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