Hot isostatic pressing of wires made of magnesium diboride, or MgB2, significantly increased the amount of electrical current the wires can carry without electrical resistance. Wires made from MgB2 would reduce the costs of such products as MRIs and electrical generators, say the researchers: Adriana Serquis, Leonardo Civale, Xiaozhou Liao, J. Yates Coulter, Duncan Hammon, Yuntian Zhu, Dean Peterson and Fred Mueller from Los Alamos' Superconductivity Technology Center; and Vitali Nesterenko from the University of California, San Diego. They presented their findings on Dec. 3 at the Materials Research Society meeting in Boston.
"This material will likely serve as a bridge to the energy future in a variety of cost-driven applications, because potentially this is the lowest-cost superconducting material," said Peterson, who leads the Los Alamos Center. "There's nothing to prevent making this material into wires that are many miles long."
Since 1911, researchers have known that certain materials can conduct electricity without resistance at temperatures near absolute zero. However, few commercial applications for superconducting materials have been developed due to high costs of materials, manufacturing and cooling. Coated conductor tapes based on deposition of high temperature superconducting films on flexible metal tapes offer the highest potential for broad scale applications; however, alternative materials also have potential uses.
The discovery of superconductivity in MgB2 in early 2001 generated high interest because this material is several times cheaper to make than other superconductors.
"Initially, everybody was excited about this material because of the lower manufacturing costs, but the performance wasn't good enough for most applications," Serquis said. After two years of research into the properties of MgB2, Serquis and her fellow researchers devised a way to improve the amount of electricity that wires made of the material could carry.
Using commercially available powders and the so-called powder-in-tube method common to the manufacture of many superconducting wires, the scientists initially found MgB2 wires were too porous to achieve high critical current density, or maximum current-carrying capacity proportional to the wire size.
They treated the material in a commercial high-pressure furnace with hot isostatic pressing and prepared wires approximately 80 feet long and one-sixteenth of an inch thick, which they wrapped into a coil. Hot isostatic pressing reduced the porosity of the wires and increased their current carrying capacity by 45 percent as compared to the best MgB2 wires prepared with traditional annealing methods.
"After hot isostatic pressing, we found that the materials had more of the structural defects that are beneficial in increasing the current carrying capacity," Serquis said.
Higher carrying capacity means that smaller wires can carry more current using the same amount of material.
"With a smaller conductor, the cost of wire used in MRI magnets could be reduced from $3-10 per kiloampere of current per meter of wire to only $1-2," Peterson said. "Because many miles of wire are wound into the coils in a typical MRI, the development of less expensive wires by the private sector could put an MRI machine in every doctor's office and even in veterinary hospitals."
Today's MRI machines use niobium-alloy wires that are much more expensive than MgB2 wires and require costly liquid helium refrigeration to maintain superconducting properties. Los Alamos scientists fabricated a prototype MgB2 coil that generated magnetic fields in the range useful for MRI applications (above 1 tesla) operating at a temperature of 25 degrees above absolute zero. This temperature can be reached using commercially available refrigeration units at much lower operating costs.
The researchers are now working on further improvements to the MgB2 wires.
The Department of Energy's Office of Electric Transmission and Distribution supported the work.
Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.
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