Levitating a Magnet Using a Superconductive Material Frederick H. Juemem.' .~ H. Dieckrnann? and Ronald I. Perklm'.3 - . Arthur 6. E l l i ~Gunther University of Wisconsin-Madison, Madison, WI 53706 The levitation of a magnet above a superconducting material (see Fig. 1) is known as the Meissuer effect. We have developed a technique, using an overhead projector, for demonstrating this effect to a large group of students.
Materials Overhead nroiector. olaced on its hack. with a mirror arraneed to moiect the imaee'dnto a vertical nroi&tion " . . screen (Fie.2): Plastic foam (coffee)cup. The base of such cups usually has a rim so that, when inverted, a small amount of liquid can be retained within the base. Pellet of superconducting material (yttrium barium copper oxide, YBazCusO7-,), about 13mm in diameter by 3 mm thick. (Details on preparation are given below.) 50-mL beaker Test-tube holder Approximately 0.2-L liquid nitrogen Insulated rubber-coated gloves Magnet about 1 mm X 1 mm X 3 mm. A samsrium-eohalt magnet (Edmund Scientific Co.) causes greater levitation than a chip obtained by breaking or core-drilling a typical ceramic-based magnet, because of its higher field strength. Plastic tweezers
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Figure 1. The Meissner effect: cobalt-samarium magnet levitated above superconducting ynrium barium copper oxide pellet.
Preparation of the "1-2-3" Superconductlng Pellet (1) Materials Dust mask and safety goggles Mortar and pestle, agate (e.g., VWR Scientific) 50 mL acetone 1.00 g yttrium oxide, Yz03(Alfa,99.99%) 2.11 g cuprie oxide wire, CuO (e.g., Baker, reagent grade) 3.50 g barium carbonate, BaC03 (e.g.,Baker, reagent grade) Alumina boat, 90 X 17 X 11.5 mm (e.g.,Thomas Scientific) Utility tongs Heat-proof gloves Ilent-prooi pad Tuhe furnnc~.1 in. diameter (Lindherg 54032 or equi\,nlent) Ppllet mess. of the kind used fur makme KBr el lets ~uartz'tuhe(24 mm o.d.), equipped withair-tight connections made at one end for attachment toan oxygen tank and at the other end for attachment to an oil-filled bubbler. The tuhe should extend about 20 cm from either end of the furnace. Tank of oxygen, with regulator and tubing to connect to quartz tube Krylon No. 1303, acrylic resin spray Procedure for Pellet Preparation Wear a dust mask and safety goggles a t all times, and cohduct grinding operations in a fume hood. In the mortar and pestle, grind together the yttrium oxide, cupric oxide, and barium carbonate with enough acetone t o make a thick slurry. Allow the acetone to evaporate completely in air; a t this point the mixed powder will flow freely. Place the powder in the alumina boat, and heat it in the tube furnace a t 950 O C in air for 1 h. Wearing heat-proof gloves and using tongs, remove the boat containing the mixed oxides from the tube furnace and place it on the heat-proof pad. Allow the mixture to cool to room temperature. It will be black or green-black.
' Institute of Chemlcal Education.
Deparlment of Chemistry. Greenwich High School, Greenwich CT 06830
When cool, re-grind the dry powder in the mortar and pestle (do not use acetone). Return the powder to the boat, and heat i t in the tube furnace for a 5-h period a t 950 O C in air. Again, cool it to room temperature. Grind the now black powder for a third time in t h e mortar and oestle.. this time using acetone. Allow the acetone to evaporate completely. L i e the pellet press to form pellets at about 30.000 Iblin.' pressure. The pellets a t this point are typically'somewhat fragile. Our pellets are -13 mm in diameter and twicallv-3 mm thick, iorresponding to the use of -2 g o f b e mixed oxide powder. Three pellets may be formed using the amount of material specified. Place the pellets in the alumina boat, and place the boat inside the quartz tube. Place the assembly in the tube furnace, and heat a t 950 "C for l h in order to sinter (heating just below the melting point to increase streneth and densitv ~~-~ i n d to promote interirinular bonding) thepilets. Allow the tube lurnare to cool to 500--600O C for the rrucial "sensitization" step, and pass pure oxygen gas through the quartz tuhe, over the pellets, a t a r a t e of about 10mLlmiu for 3 h. At the conclusion of the 3-h period, turn off the furnace, and allow i t to cool to room temperature while maintaining the flow of pure oxygen over the pellets. Spray coat the pellets with an acrylic resin such as K ~ y l o n No. 1303. This will protect the pellets from chemical decomposition, which can occur with prolonged exposure to water or water vapor. ~~
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The Demonstrallon Preparation Support the overhead projector on its "back", so that the staee is vertical. and arranee a mirror to -oroiect the staee " image onto the projection screen. (We use a special device formerlv sunnlied bv the 3M Comoanv that lets the overhead prijectdr rest in the approprke~position,provides a
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Number 10 October 1987
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Figure 2. Overhead projector set up to display the Meissner effect shown In Figure 1.
stage to support items to he projected, and supports the ~irror.A ) photograph of the device is given in Figure 2. A similar support could k$ constructed for other overhead projectors. A TOPS projector could also he used (2). Invert the foam cup and place i t adjacent to the stage of the overhead projector. Place the mixed-oxide pellet on the cup's hase. Turn on the projector and focus the image of the pellet onto the screen. Presentation Fill the 50-mL heaker with liquid nitrogen, and using the test tube holder to grasp the heaker pour liquid nitrogen over the oxide pellet into the shallow depression formed by the hase of the cup, filling it. Use care to avoid hitting the glass of the projector stage with liquid nitrogen; the glass could shatter. As the liquid nitrogen evaporates, replenish it in the ahove manner. After about a minute, the oxide pellet ~houldbe nxded helow its supwconducting temperature. Pick uo the magnet with the plastic tweezers, and place it about 2 Am abovethe center ofthe oxide pellet. ~ e l i a s the e maenet. It will he levitated approximately 3 mm ahove the peliet, and its image will be projected onto the screen. The magnet will remain suspended until the pellet warms to ahove its critical superconducting temperature, a t which time the magnet will no longer he levitated above the pellet; it may either settle to the pellet's surface or "jump" away from the pellet. Hazards Liquid nitrogen is extremely cold, having a boiling point of -196 OC. Skin contact with liquid nitrogen or with an object chilled by liquid nitrogen can result in severe frostbite. Protective gloves that will not absorb the cold liquid should be worn whenever liquid nitrogen is used (3).Excess liquid nitrogen should he disposed of promptly since, upon standing in an open container, i t can condense oxygen from the atmosphere, slowly enriching the liquid with liquid oxygen. This mixture of liquid oxygen and liquid nitrogen is a powerful oxidizer and may react violently with easily oxidizable substances. The toxicity of the pellet and magnet have not been fully characterized. They should not be ingested, and they should be kept away from small children. Care should he taken to avoid inhalation of powder particles should they he accidentally created. The levitation experiment should not he performed near flames, sparks, or flammable materials since powder particles from the rare-earthbased magnet can burn in air and create sparks. Disposal The only material to he disposed of is excess liquid nitrogen. I t may he allowed to evaporate from its container, or slowly poured onto the floor. Dispose of excess liquid nitrogen promptly; see the cautionary note ahove. 852
Journal of Chemical Education
Dlscusslon From experiments with magnets and iron filings, evidence can be seen of lines of force surrounding magnets. These lines of force compose a magnetic "field", the strength of which varies a t different locations relative to the magnet. Whenever an electricallv conductive material oasses through the magnetic field, a current is induced in the conductor. The maenitude of the current is affected hv the electrical resistance inherent in the conductor. ~ h i s ' p h e nomenon, announced in 1832 by both Joseph Henry in America and Michael Faraday in England, is now called electromaenetic induction. Hans Oersted in 1820 demonstrated t h i t a wire carrying an electrical current would deflect a compass needle placed nearby. The compass needle is of course 'magnet, and this showithat an electric current induces a magnetic field in the space surrounding the conductor. Commercial electric power generation exploits these effects by the use of conventional conductive materials such as copper wire. However, even these materials have some resistance to electrical current flow. For manv. vears . i t has been known that certain materials (such as mercury) lose all resistance to the flow of direct rlectrical current when rooled 1)elow a certain critical temperature. This phenomenon, together with the Meissner effect, is called superconducrivity. A manifestation of the hleissner effect is the Ievitnrion crf n magnet above u superwnducting material. M w h g a magnet near the superconductor induces n supercurrent (i.c., a current [hat does not decay) -xithin the superconducu)r. This supercurrent generates its own maenetic field such that the total magnetic field " inside the superconductor disappears. The two opposing fields, one from the magnet, and the one induced by the superconductor, cause the magnet to he repelled by the sunerconductor. iust as two like maenetic poles repel each otker. If the rep;lsive force at the sirface of the sGperconductor is greater than the force of gravity on the magnet, the magnet will he levitated at a distance from the superconductor such that the gravitational force downward is counterbalanced by the re&ive force at that distance. Until recently the critical temperature for all materials that exhibit superconductivity was extremely low, near the boiling point of liquid helium, which is 4 K. As a result, because of the expense of maintaining such low temperatures, applications of superconductivity were limited, althoueh the ~ o t e n t i a lbenefits of the phenomenon were a subject of wihe speculation. Rapid advinces have been made receutlv. in svnthesizine materials that exhibit the phenome. non of superconductiv%y a t much higher temperatures (4). I t is now possible to achieve superconductivity at the temperature of liquid nitrogen (77 K, -196 OC), a coolant that is much less expensive to prepare and store. This demonstration uses one of these materials, the "1-2-3" pellet first reported earlier this year ( I ) . We also use a samarium-cobalt magnet, which has a particularly high field strength. This provides a more effective demonstration, since the samarium-cobalt magnet will create more repulsive force and cause greater levitation from the oxide pellet than will a similarsized conventional magnet with lower field strength.
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Note If facilities for preparing the "1-23" superconducting pellets are unavailable ( 5 ) ,the Institute for Chemical Education of the University of Wisconsin-Madison has a limited supply of pellets and magnets ("levitation kits") and will be happy to supply them at cost. Details may he obtained by writing to the Institute. Acknowledgments We wish to thank Heidi Grant and B. Z. Shakhashiri for providing the samarium-cobalt magnet mentioned above. We are grateful to E. Hellstrom, J. Nordman, and D. Larhalestier for helpful comments, to T. Harkins for experimental
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reference 5.
~ i v ior~chemical i ~ ~ E ~ u c ~E ~~ ~ S ~o O PA, ~~ :,1965. 3. Shakhashiri, B. 2. Chemical Demonstrations; University of Wiseonain: Madison, WI, 1985: Vol 2, p 26. Literature CHed 4. For an oveniew of the recent sdvsnees and the significance of c u m n t aupermnductivity reaparch, SPO Dagani,R. Cham.Eng. News 1987,ffi (May 11). 7. 1. Our synthesis is hasod on those reported in: (a) Wu, M. K.:kshbum, J. R.;Tomg,C. J.; 5. Adeseriptionof a s y n f h ~ i s o1-2-3 f pelletausing faeilitieathatmsy heavailablein h'ih Hor,P.H.:Meng,R.L.:G~,L.;Huang.Z.J.:Wsng,Y,Q.;Chu,C.W.Phys.Reu.Letf. schmls may be found in Grant, P. M. New Sci. 1987,115 (July 301.36. 19R7.58.908; ibl Cava. R. J.; Batlogg, B.: uanDouer,R. B.; Murphy,D. W.;Sunahinc,
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