For release: Aug. 21, 2000
Contacts:
Karl Gschneidner Jr., Metallurgy and Ceramics, (515) 294-7931
Vitalij Pecharsky, Metallurgy and Ceramics, (515) 294-8220
Susan Dieterle, Public Affairs, (515) 294-1405
AMES, Iowa -- A set of alloys developed at the U.S. Department of Energy's Ames Laboratory could significantly boost the cooling power of cryocoolers, devices that cool medical devices and scientific equipment to extremely low temperatures.
Ames Lab scientists Karl Gschneidner Jr., Alexandra (Sasha) O. Pecharsky and Vitalij K. Pecharsky say the alloys are 10-40 percent more efficient than lead -- the material the alloys would replace -- in reaching temperatures between 60 Kelvin (minus 352 F) and 10 K (minus 442 F). Temperatures in that range are needed for such medical equipment as magnetic-resonance imaging units. Some equipment aboard space satellites also must operate at these low temperatures.
The scientists add that the new alloys are also more environmentally friendly than lead and could provide a cost-effective alternative to using liquid helium to cool equipment on earth.
"We think there will be a lot of interest in these alloys because they support energy-efficient cooling, they're environmentally clean and they could eventually replace liquid helium in cooling down research and medical equipment," said Gschneidner, a senior metallurgist. The three scientists recently shared their findings about the alloys at the 11th annual International Cryocooler Conference in Keystone, Colo.
Cryocoolers are small refrigeration devices in which a heat-transfer gas, typically helium, is pumped back and forth by a compressor. As the compressor piston depresses, it pushes the gas through the regenerator -- an arrangement of material that absorbs much of the heat from the gas. The gas expands, cools and then flows through heat exchangers that remove more heat, cooling the equipment. As the compressor piston retracts, the gas is drawn back through the system. When it passes back over the regenerator material, some of the absorbed heat is released back into the gas to moderate its temperature. This cycle is repeated at a high rate, often as fast as 30 times per second.
Although the regenerator is only 2-3 inches in length and about a half-inch in diameter, it is an integral part of cryocooler technology because of its effectiveness in maintaining a constant low temperature. The Ames Lab scientists have developed a set of 36 alloys to serve as regenerator materials for cryocoolers that reach temperatures of 10-60 K. Erbium, a rare-earth metal, is the main component of the alloys. Differing amounts of other metals control the magnetic ordering temperature and thus the heat capacity of each alloy. This gives manufacturers flexibility in choosing the best alloy for a specific use.
Scientist Vitalij Pecharsky said regenerator materials must be capable of absorbing large amounts of heat without changing their temperature. "At temperatures close to absolute zero, which is 0 Kelvin, normal materials have a negligible heat capacity," said Pecharsky, who is also a professor of materials science and engineering at Iowa State University. "Hence, even the smallest energy input will disrupt the equilibrium, causing the temperature to rise. The erbium-based materials prevent that from happening because they have an outstanding heat capacity."
They must also be configured to provide a large surface area -- in forms such as screens, foils, wire mesh, small spheres, tightly packed powders or a dimpled surface rolled up in jelly-roll fashion -- that will soak up heat without needlessly impeding the gas flow. "You can arrange the regenerator material in a variety of ways as long as the gas can flow through," said Gschneidner, who is also an Anson Marston distinguished professor of materials science and engineering at ISU.
Most 10-60 K cryocoolers use lead regenerators. However, lead is a soft metal and can only be used in the form of small spheres. The erbium-based alloys can absorb 25-175 percent more heat than lead in that temperature range and are much harder than lead, making them versatile enough to be used in a variety of forms, Gschneidner said.
"A cryocooler using an erbium-based regenerator would be more efficient because it could cool the load to a lower temperature, or it could cool a bigger load without needing additional power," he said.
Unlike lead, a known toxin, the erbium-based alloys pose no health or environmental hazards, he added. Although lead is cheaper than erbium, the scientists don't consider the price difference a problem.
"The erbium alloys might add $100 to the price of a cryocooler, which typically costs between $20,000 and $100,000," Gschneidner said. "The slight difference in cost won't make that much of a difference, especially considering the greater cooling power possible with erbium-based regenerators."
Moreover, the efficiency of the erbium-based alloys could enable laboratories and medical facilities to switch to cryocoolers instead of using liquid helium to cool instruments. Gschneidner noted that liquid helium costs about $3.50 per liter in the United States and up to $15 a liter in Europe and Japan. "If you're spending between $50,000 and $80,000 a year on liquid helium, you'd repay the cost of the cryocooler pretty fast," he said.
The scientists have applied for a patent on the alloys. Atlas Scientific, a California-based company that designs and builds cryocoolers, recently signed two licensing agreements to use the materials and is gearing up to conduct tests of the alloys using a pulse-tube cryocooler. Additionally, the University of Victoria in British Columbia is testing the alloys in a Gifford-McMahon cryocooler.
Ames Laboratory is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
Last revision: 8/21/00 sd
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