A Simple, Inexpensive Device for Measuring the Critical Temperature of a High-Temperature Superconductor David 6. Green, Dijon Douphner, and Bennett Hutchinson Natural Science Division, Pepperdine University, Malibu, CA90263
The relatively small cost, ease of preparation, and novelty of demonstration of the YBa2C~307.r high-temperature superconductor has allowed a wide variety of students to participate in this exciting scientific advance.' This note describes a simule. inexuensive method of measurine the temperature at;uk;ich tKe Meissner effect exists in a-disk of YBa7Cu107.- - . - The Meissner effect can he demonstrated by floating a small magnet over a superconductor disk below its critical temuerature (TJ. The T, of a supercondudor depends on applied current and auulied external magnetic field. This method involves no appiied current and only a very small applied magnetic field: therefore, the temperature value obtained is an uppe; limit of T, . The 1eAtation of the magnet will vary with the magnetic field strength and its mametic geometry, as well i s the sample's porosity. As such, levitation of the magnet demonstrates that superconductivity is present. Since the flux exclusion is more a n intraaain rather than bulk effect, additional insight into the b u k properties and their auolications reauires other methods including resistivity measurement.' With these limitations recomized. the method described herein allows students to adapt a thermocouple system for use with a functionine high temperature commercial2 or student-prepared sup&&ducto< Students then measure an important property of a high-temperature superconductor, are introduced to the thermocouple method of temperature measurement, and handle a cryogenic liquid. Caution: Safety goggles and insulated gloves should be worn when performing this experiment, and eye protection should be worn by those observing it a t close range. .A
Schematic diagram of the apparatus. a. the plastic screw cap from 25 g reagent bottle C.1-2 rnm hole for thermocouple d. bonle neck cut offat bottle shoulder b. expanded polystyrene disk e. superconducting disk f. CuICu-Ni thermocouple junction
bottle off a t the shoulder (the dotted line in the figure, part B). The completed cell accommodates superconductor disks with diameters between 2.1 and 2.5 c i . The thermocouple system, consisting of a sample and reference junction, is prepared by twisting the ends of the constantan leads tiehtlv toeether and immersing the reference junction in a stirred deionized waterlice bath. Copperlconstantan (CulCu-Ni) thermocouples made from 0.005-in. diameter wire were purchased from Omeaa E n d neering, Inc. The copper leadof the sample junction is &tached to the positive input of a millivolt meter and the copper lead of the reference junction to the negative input. Any error in the thermocouple system is determined by measurine the voltaee obtained when the samule . "iundion is immersed directly in liquid nitrogen. Voltages are converted to temperatures using a conversion table (as found in the Omega Engineering, Inc. Temperature catalog, for example). Normally, voltage corrections do not have to be applied. The complete cell is shown in Figure C. The cap is laid on a dense closed-cellfoam pad and the sample junction of the thermocouple threaded through the hole and centered on the expanded polystyrene pad. The superconductor disk is then carefullv . ulaced in the cao on the thermoconule. The superconductor is fixcd in piace by screwing down the bushing snugly against the disk." The thermocouple is held in place by the superconductor. Liquid nitrogen is poured directly onto the superconductor until the Meissner effed is observed.' The superconductor is then allowed to warm while the voltage on the millivolt meter is monitored. The voltage is wcoried when the maenet falls. The Meissner effect can be reestablished by adding a small amount of liquid nitrogen to the cell. In this way the T, may be determined several times in only a few minutes so that errors due to nonuniform warming may be minimized. A high-temperature superconductor purchased from Flinn Scientific, Inc., described as having a T, of "about 90Kn,was used in the experiment. General chemistry students4 were able to assemble and use the device to determine the T,of the high-temperature superconductor in less than 2 h. ~ e n c r a l l ~ , s t u d e naveragcddata ts from fivc mcasurements to obtain a T, of 92 I2 K. Students also found as much as a 4 'C increase in superconductors with higher porosity than the purchased disks.
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'Harris, D. C.; Hills, M. E.; Hewston, T. A. J. Chem Educ 1987, 64, 847.
The suuercondudor holder is constructed from a common plaskc25 g screw cap reagent bottle that has a 21 mm i.d. (25 mm 0.d.) neck. The cap is removed and a 1-2 mm hole drilled in the side 3-4 mrn from the top (figure, part A) as the thermocouple feed-through. Athin, circular piece of expanded polystyrene from a foam coffeecup is cut to fit inside the cau and reulaces the waxed cardboard seal. The screw-in bushing that holds the superconductor disk in place is made by carefully cutting the neck of the
A SLperconduCtor k ~ that t nclLaes a superconoLnor w ~ t hor w thout a 0~111n tnermocoLple system can be p-rchasea from F Inn SCIentificlnc., Batavia, IL. The cost per student or group except forthe plastic cell holder described in the text and the millivolt meter is less than $30.00. If the suoerconductor disk twists when the bushino is screwed down. the diik mav ~,be held in dace while turnino the bishino 'G&&; hem stry ~aborai&r/itude"lia1 ~ e a v e r ~e&. o l Pep peroine Unwersty, assemalea rh~soev ce and measured me T, as part of the first-yearchemistry honors laboratory ~
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Volume 69 Number 4 April 1992
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