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SUPERCONDUCTIVITY Ken Cheney 29 May 2006 PICTURES http://www.paccd.cc.ca.us/instadmn/physcidv/physics/teachers/cheney/lab% 20manuals/WEB%20Image%20Folders/Supercondicvity%20WEB/index.ht m ABSTRACT Some properties (Meissner Effect, Critical Temperature, Suspension, Energy Storage, Critical Current Density, Critical Magnetic Field, and – perhaps – AC Josephson Effect) of high temperature superconductors will be explored at liquid nitrogen temperatures. ABOUT THIS LAB CHAPTER I must apologies that this chapter will not derive the theory (actually as of May 2006 no one can) or give detailed instructions (the booklet that came with our kits from Colorado Superconductor Incorporated is very good) What I will attempt to do is give a little history, a very little theory, and the motivation, outline of the procedure, and the expected outcome for the parts of the experiment. Also I’ll give some warnings to protect the equipment and student! HISTORY It has long been known and understood that for most substances resistance decreases with temperature. It was a considerable surprise when Onnes found in 1911 that at a low enough temperature resistance could completely disappear. For seventy five years the highest temperature at which superconductivity could be achieved slowly (very slowly) inched up, all the way to 23K! D:\478180013.doc Ken Cheney 1 An excellent theory of superconductivity was developed by Bardeen, Cooper, and Schrieffer, the BCS theory. Yes, the same Bardeen! The theory has two flaws: It doesn’t tell you how to make room temperature superconductors and it seems impossible to explain in simple terms! The simplest attempt I’ve seen is at www.superconductors.org/oxtheory.htm. Electrons pair up (Cooper pairs) and move together through the metal. These pairs involve other pairs with the result that a collision (resistance) would have to change the energy of all these pairs, but this minimal energy change is greater than the thermal energy available??? Happily for our experiment in 1986 a new class of superconductors was discovered (not by theory, BCS doesn’t seem to work here). These rather complicated ceramics now (2006) include materials super conducting at temperatures up to 125K. The beauty of any temperature over 77K is that liquid nitrogen can be used for cooling and LN is cheaper than gasoline. Currently (2006) there is no theory that convincingly proves that there cannot be room temperature superconductors. Conversely no theory proves that there can be room temperature superconductors. The down side of ceramics is that they are brittle and not very amiable to being drawn into the wires desired for most applications. WARNINGS!! HANDLING LIQUID NITROGEN Mostly don’t get the LN on you! The teacher will supply LN in a wide mouthed Dewar. We will make a foam dipper (cutting up a foam soft drink container). Use this dipper to move the LN from the Dewar to your shallow foam container. PRESERVING THE SUPER CONDUCTING MAGNETS The magnets are ceramic hence brittle; please treat them gently! D:\478180013.doc Ken Cheney 2 The cold magnets will become covered with frost from water vapor in the air. Water is bad for the magnets so wipe off the frost and warm the magnets with a lamp when you are done. If available store the magnets in plastic bags with a drying agent. USE THE PLASTIC TWEEZERS PROVIDED TO HANDLE THE BLACK SUPERCONDUCTOR DISKS THE MEISSNER EFFECT The result of this effect is that a magnet will be stably suspended above a super-conducting object. The effect is easy to produce but a bit trickier to analyze! You do need a magnet with a great strength to weight ratio. The tiny cubical magnets supplied work well. Put a black superconductor disk in your foam container; add LN until it is just covered and the LN stops boiling. Use the tweezers to gently place the tiny magnet over the super-conducting disk and watch it float. You can probably make it spin very rapidly by gentile blowing. CRITICAL TEMPERATURE USING THE MEISSNER EFFECT Use the superconductor disk in the brass casing with two leads. These leads are for the thermocouple in thermal contact with the bottom of the super conducing disk. D:\478180013.doc Ken Cheney 3 See the section TEMPERATURE MEASUREMENTS below for instructions on using the thermocouple to measure temperature. CAREFUL WITH THE LEADS – FRAGILE! Immerse the case completely in LN, leave until the LN stops boiling. Remove the case from the LN –with the tweezers (!)— and set it on the table with the black superconductor on top. Float the tiny magnet cube. Record the voltage or temperature every five seconds until the magnet falls to the surface of the superconductor. The final temperature is the critical temperature. SUPER CONDUCTING ENERGY STORAGE This may be easier than outlined in the booklet although for the best effect you should probably follow the instructions in the booklet! The plan is to induce a current in a super-conducting ring by changing the magnetic flux through it. This current will last a very long time so long as the ring remains super conducting. The current can be detected by its magnetic field. Place the ceramic ring flat in your foam container. Put your strongest magnet near or in the center of the ring. Add LN until the LN stops boiling. Remove the magnet. This removal will induce a current in the ring, the current in turn will produce a magnetic field threading through the ring. To detect the magnetic field bring a compass near the ring. Putting the compass side by side with the ring will probably not show anything since the field there will be up or down! Explore with the compass a little above and to the side of the ring and it should be easy to see the compass needle pushed by the magnetic field. D:\478180013.doc Ken Cheney 4 MEASURING RESISTANCE VERSUS TEMPERATURE Measuring the resistance of something with zero resistance is not very promising! However we can measure the resistance from super conducting temperatures to non-super conducting temperatures. This time we use the superconductor in the brass case with five wires coming out. CAREFUL WITH THE LEADS – FRAGILE! One of the wires is actually a pair of thermocouple leads. The other four wires are for a four-wire resistance measurement. Yellow wires are voltage probes (2 and 3) Black wires are current probes (1 and 4) The plan is to separate the wires providing current (for the R=V/I measurement of resistance) from the wires measuring the voltage. With this arrangement negligible current goes through the voltage measuring wires and hence there is no significant voltage drop due to the current through these wires. These wires can be connected as shown in figure 2 on page 20 of the booklet (using a dc power supply and separate ammeters and voltmeters) or connected to a multiimeter with a four-point input. If you use the separate power supply it is vital to limit the current to no more than ½ amp. Several of our power supplies have current limiting facilities. You short them and set the current to the value you want. Then they will never permit more than that current to pass. Invaluable to avoid melting equipment such as superconductors. If you use the four-point input on our Keithley Model 2000 multimeters connect like this: D:\478180013.doc Ken Cheney 5 Use Superconductor Yellow wires 2 and 3 Voltage Black wires 1 and 4 Current Setting Thermocouple Single wire split at the end Keithly 2000 Sense Input Separate Voltmeter 4 volts for thermocouple CRITICAL CURRENT DENSITY As one might expect it is not possible to pass an infinite current even through a superconductor. Unfortunately the experiment to determine the critical current density appears to be complicated, time consuming, and not too satisfactory! Possible though if one is determined enough. CRITICAL MAGNETIC FIELD Super conducting magnets are used in particle accelerators, magnetic imaging machines etc. They save money on power and permit higher magnetic fields than can be obtained with conventional conductors under steady state conditions. Once again there are limits, here on the strength of the magnetic fields before the super conductor becomes a normal conductor. This could be very expensive if the huge current being carried by the super conductor melted the magnet when it stopped being super conducting and subject to P I 2 R ! The booklet describes how to measure this effect, complicated but doable with enough time. TEMPERATURE MEASUREMENTS WITH THERMOCOUPLES Thermocouples consist of a pair of metals connected at one end (the temperature sensing end) and separate at the other, reference, end. The thermocouple produces a voltage between the reference ends proportional to the temperature difference between the connected (temperature sensing) end and the reference ends. D:\478180013.doc Ken Cheney 6 The reference end can be assumed to be room temperature ( ), see the table on page 11 of the booklet. Or the reference ends can be held at some known temperature such as 0C for ice water, see page 43 of the booklet. Or, for the really lazy, a voltmeter with the tables built in can be used and read directly in temperature. Our Keithley Model 2000 multimeters have thermocouple options: Connect to the usual “Input” Press SHIFT and then TCOUPL Use the “arrow keys” to chose: UNITS --- C, K, or F TYPE --- J, K, or T we want T JUNC --- SIM to simulate a reference junction temperature. I’d expect room temperature but I’ll investigate. The thermocouples included in this kit are Copper-Constantan (type T) thermocouples. CRITICAL TEMPERATURES YBa 2Cu3O7 Bi2Sr2Ca 2Cu3O9 D:\478180013.doc Ken Cheney 92K 110K 7