"New applications for a superconducting material called BSCCO (BIS-ko) are promising because of the material's interesting physical properties," says Greg Steinlage, a doctoral student in Purdue's School of Materials Engineering and a native of Englewood, Ohio .
"In the long run, for some applications, it may be more efficient and less expensive to operate than materials that have been around for decades."
BSCCO, a relatively new type of superconducting material, shows promise for uses such as more efficient power lines and better magnetic shields to prevent malfunctions of medical equipment.
Steinlage (STINE-lag-ee) is the first to use a technique called centrifugal slip casting to make superconducting tubes out of BSCCO, a process that involves spinning the material at very high speeds. The acronym BSCCO stands for the chemical elements that make up the material: bismuth, strontium, calcium, copper and oxygen.
Steinlage is working with Purdue's Office of Technology Transfer to obtain patent protection for this process and will present research on the properties of the BSCCO material at a conference of the American Ceramics Society in Los Angeles Oct. 19-22. His research is supported by the Purdue-based Midwest Superconductivity Consortium, a group of six universities funded by the Department of Energy to conduct research on high-temperature superconductors.
Steinlage cast his superconductor in the shape of a tube because tubular superconducting devices currently are used to protect sensitive electronic equipment against stray electromagnetic fields. Superconducting tubes also are being adapted by utilities to transmit electrical power more efficiently.
"Power applications are very important," Steinlage says. "A superconducting material is much more efficient at conducting electrical current than a metal wire. When current runs through the wires in power cables, it encounters resistance and loses some of its energy in the form of heat."
When a superconductor is cooled to very low temperatures, between -350 and -450 F, it has no resistance, so it can conduct current with no loss of energy.
"The tube shape allows you to flow liquid nitrogen through the center to keep it cool," Steinlage says. "There are companies now that are making longer tubes out of other superconducting materials and exploring the possibility of using them to replace the wires in underground cables."
The BSCCO material is relatively expensive, about $1 to $3 per gram. It takes about five grams to make one small tube three inches long with a dime-sized diameter (an average vitamin pill weighs about a gram). Steinlage says the cost may be high because the material is fairly new and in great demand by researchers. However, he says, devices made from BSCCO may actually be cheaper to use than other superconducting materials because it is much less expensive to cool them.
"The superconductors used today are mostly metallic, and to carry current with no resistance, they all have to be cooled to a lower temperature than the BSCCO material," Steinlage says. "Superconductors that have been around for about a decade need to be cooled to about - 450 F, which can only be done with liquid helium at a cost of about $28 a gallon. BSCCO only has to be cooled to -321 F. That can be done with liquid nitrogen, which is easier to get and has about the same cost as milk."
Superconducting tubes also are used to shield sensitive electronic equipment from interference from stray electromagnetic fields, such as those produced by cellular phones, radios and power lines. Stray fields are usually harmless to people, but certain electronic devices sometimes react to them, the way a television will pick up "snow" from a nearby hair dryer.
The results of the interference can be disastrous and even fatal. Random signals interfere with the electronic pulses that tell the microchips in computers what to do and when to do it. Delicate medical equipment, such as magnetic imaging machines, heart and breathing monitors, defibrillators, ventilators, and electronically controlled wheelchairs are particularly vulnerable. If not properly shielded, these devices can stop working or give false readings.
Superconductors make excellent shields because as current flows through the material it forms a very stable magnetic field around the outside of the tube. That field bends incoming external fields away from it or cancels them out entirely. Inside the tube, the magnetic field is zero, so electronics placed inside are safe and operate more efficiently.
Steinlage says the laboratory prototype of his superconducting tube can easily be scaled up to dimensions that will accommodate electronic equipment. Centrifugal slip casting already is used to manufacture alumina tubes for high-temperature furnaces, some iron pipes, and piston rings for high-performance cars. No new equipment would have to be developed to use this technique to produce magnetic shields, Steinlage says.
Steinlage credits two of his professors, Keith Bowman and Kevin Trumble, for first theorizing that tubes could be made from BSCCO, but Steinlage actually developed the process.
To make his superconducting device, Steinlage starts with a small amount of BSCCO material that has been crushed into a fine powder. Each tiny piece, called a grain, is shaped like a very rough-edged domino. The most current flows through BSCCO material when all the grains are lined up end to end like dominoes and stacked in layers like a brick wall.
"In other superconducting materials, the grains are symmetrical," Steinlage says. "Trying to orient those particles properly can be very difficult. It's like taking a bag of balls, drawing a dot on each one of them, then throwing them in a box to get all the balls to line up with their dots pointing the same direction. On the other hand, if we toss dominoes into a box one by one, they're likely to stack on top of one another like bricks. That's what we use to our advantage in processing this material."
To coax a greater proportion of the grains into lining up and stacking properly, Steinlage uses centrifugal slip casting, an approach never before applied to the BSCCO material. He mixes the superconductor grains with a liquid chemical called hexane and a special dispersant containing polymers, which are long chains of molecules, to make a slurry, or slip, about the consistency of vegetable oil. Using a syringe, he injects the slurry into a three-inch long tube-shaped plaster mold that has been lined with silver powder and that is spinning very rapidly in a lathe. The silver acts as a buffer between the superconducting material and the plaster.
As the slurry spins around, it is pushed outward by centrifugal force. As the hexane is slowly squeezed out through the pores in the plaster, the liquid helps pull the grains outward in such a way that most of them are lined up in the proper position. The polymer chains help the grains line up by attaching themselves to the grains' edges and interconnecting with each other like Velcro. The tube is then placed in a furnace to undergo a process called sintering, in which the superconducting grains are bonded together but do not melt.
One advantage of using centrifugal slip casting is that as a tube gets larger, it is easier to produce.
"The larger the radius of the tube, the more centrifugal force, or G-force, you get as you spin the materials around," Steinlage says. "If you double the radius, the weight of the particles in the fluid becomes twice as heavy, so they're apt to push each other farther outward, making the tube more dense and better aligned."
Source: Greg Steinlage, (765) 494-8751
Writer: Amanda Siegfried, (765) 494-4709; Internet, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com
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