Demonstrating High Temperature Superconductivity in the Chemistry Lab through the Meissner Effect J. McHale, R. Schaeffer,and R. E. Salornon Temple University, Philadelphia, PA 19122 Superconductivity can be demonstrated by the observations of zero resistance or perfect diamagnetism (the Meissner effect). A consequence of the Meissner effect is the repulsive force between a magnet (which can be a permanent magnet) and a superconductor.The standard demonstration of this effect involves levitating a small permanent magnet over a superconductor cooled in liquid nitrogen. This, however, can be a difficult demonstration to perform with noncommercial phase impure samples. Even if levitation occurs, it is difficult to quantitate the effect for purity or comparative studies position. Recently, a pendulum method ( I ) was described for observing the Meissner effect in high temperature superconductors in which levitation was not possible due to the small volume fraction of the superconducting phase. In that method, the superconducting sample was affixed to a wall in a cell filled with liauid nitrogen. A small permanent magnet suspended firom a'thin fibcfconstitutedthe pcndulum. whose dlsdacement from equillbr~um(caused tv the of the high temperature superconductor^ was measured with a cathetometer. A new and simpler method to sensitively measure the Meissner effect with a top-loading balance is described here. The balance used in this work was a Fisher A-ZOODS although any top-loading balance, with a resolution of the order of 0.1-1 mg and which is enclosed is suitable. The permanent magnet should produce a magnetic field of a t least a few hundred gauss a t its surface. More consistent results are obtained if the diameter of the mamet is larger than thut of the superconducting pellet. The fieure illustrates thc anoaratus that is used. Twelve - - - ~ -stacked expanded polystyrene coffee cups support the magnet a t a position just below the upper lid of the balance that is then tared to negate the mass of the magnet and the cups. The sample is placed in another polystyrene coffee cup and cooled with liquid nitrogen. This cup is then
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placed on top of the balance directly above themagnet. The result, is a reading in grams of the strength of the repulsion between the superconductor and the magnetic field. If the sample is cooled after it is placed above the permanent magnet, the weight change is reduced relative to the case of a zero field cooled sample. This is due to trapping of a portion of the magnetic field in the interior of the superconducting sample (referred to as flux pinning), usually by impurities or crystal defects. Flux that is pinned does not contribute to the Meissner force and results in a smaller response. With samples of YBa2Cua07, weighing 1 g, the weight changes are of the order of 0.6 g. These results can be easily converted to units of force (dynes) by multiplying by grams, the gravitational constant (9.8 cm/sec2).This can vary depending on the distance from the superconductor to the magnet as well as the strength of the magnetic field. Weight changes of several grams can be demonstrated using larger masses of the 123 superconductor. Sensitivity is increased by reducing the distance between the sample and the magnet. An enclosed balance is advantageous for several reasons. First, the enclosure eliminates the downward convection of air caused bv the eva~oratinzliauid nitrogen. Second. static electriccharges, Ghich accumulate on tGe surfaces of insulating plastics, cause erroneous readinm The effects of these static charges are completely eliminated by the interposition of the plastic enclosure. The effect of any static electrical forces between the expanded polystyrene inside the balance and the upper lid of the enclosure are eliminated by taring of the system. Even without the use of a n enclosed halance, care should be taken to maintain a sufficient distance between the Dermanent magnet and the balance electronics. The magnet should not be placed on the balance pan. With a 2.54-cm diameter magnet having a mass of 40 g and a field at the surface of 150 gauss, a 43 g mass of 123 exhibited a weight change of 8.4 g. With pellets of 1.2 cm diameter and mass of 1.0 g, weight changes of the order of 0.5 g are easily observed. These pellets were pressed using a KBr pellet press a t 1000 psi. They were sintered at 940 "C for 20 h and then annealed at 500 "C for 5 h in flowing O2 with slow cooling to room temperature (1 'C per minute). Maintaining consistent pellet preparation is important since pellet density will effect the magnitude of flux exclusion and thereby alter the response. A simple demonstration of the dependence of the repulsive force on the lateral position of the sample, relative to the center of the magnet, can be achieved by sliding the liquid nitrogen containing cup on the enclosure surface. By so doing, it is not difficult to demonstrate that the repulsive force exibits a maximum at a certain radius and a minimumin the center. Beyond this radius, the force drops quickly. This well-studied phenomena ( 2 )is an important part of proposed levitation applications of superconductivity. The volcano shaped potential energy surface implies lateral stability, that is an important part of proposed levitation applications of superwnductivity. Volume 69 Number 12 December 1992
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An interesting experiment for which this device is well suited is observing the difference in magnetic response between samples which have been field cooled (cooled to below their transition temperature in the presence of a magnetic field) and zem field cooled (cooled to below T, while not exposed to a magnetic field). We have seen a 30% increase of the force in samples of 123 that have been zero field cooled as opposed to field cooled. Such studies can give insight into the density of pinning centers which is related to important physical properties of superconductors such as critical current density in the presence of magnetic fields. These properties are crucial for commercial applications such as the fabrication of powerful electromagnets for use in magnetic resonance spectrometers.
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Journal of Chemical Education
For students who have not previously been exposed to the Meissner effect, it may be more interesting to pour the liquid nitrogen over the sample while it is in place above the magnet. When this is done, there is a four to five second delay before the sample cools to below its transition temperature and the balance responds. This is a direct way of witnessing the change from the normal to the superconducting state. Literature Cited 1. Longo, A. S. and Kumar. B. J. S u ~ m o n d u c f i v i 1988,2,241 t~ 2. Davk.L. C.J A p p l . Phys. 1630.67,2631.
Acknowledgment We wish to acknowledge the support of the Ben Franklin Superconductivity Center, Project 89s-5045C-1.
