Download Superconductivity

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