LAB 68: Superconductivity

actorrattleUrban and Civil

Nov 15, 2013 (3 years and 10 months ago)

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Ken Cheney


1

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, Sus
pension, 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 CHAPT
ER

I must apologies that this chapte
r 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 resi
stance
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!

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Ken Cheney


2


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 temper
ature
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 to
gether 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 exp
eriment 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 7
7K 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 tem
perature 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!

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Ken Cheney


3


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
bag
s 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 t
he 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 w
ith two leads.


These leads are for the thermocouple in thermal contact with the bottom of
the super conducing disk.


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Ken Cheney


4

See the section
TEMPERATURE MEASUREM
ENTS

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 temp
erature every five seconds until the magnet falls
to the surface of the superconductor. The final temperature is the critical
temperature.

SUPER CONDUCTING ENE
RGY STORAGE


This may be easier than outlined in the booklet although for the best effect
you sh
ould 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 rin
g, 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 o
r 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.



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Ken Cheney


5

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 providin
g 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 wir
es.


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 limi
t 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
equip
ment such as superconductors.


If you use the four
-
point input on our Keithley Model 2000 multimeters
connect like this:




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Ken Cheney


6

Use

Superconductor

Keithly
2000

Separate
Voltmeter

Voltage

Yellow wires 2 and 3

Sense


Current

Black wires 1 and 4

Input


Setting


4


volts

Thermocouple

Single wire split

at the end


for thermocouple


CRITICAL CURRENT DEN
SITY

As one might expect it is not possible to pass an infinite current even
through a superconductor. Unfortunately the experiment to d
etermine the
critical current density appears to be complicated, time consuming, and not
too satisfactory! Possible though if one is determined enough.

CRITICAL MAGNETIC FI
ELD


Super conducting magnets are used in particle accelerators, magnetic
imaging m
achines 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 b
ecomes 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
2
P I R

!


The booklet describes how to measure thi
s effect, complicated but doable
with enough time.


TEMPERATURE MEASUREM
ENTS 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 produ
ces a voltage between the reference ends
proportional to the temperature difference between the connected
(temperature sensing) end and the reference ends.


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Ken Cheney


7

The reference end can be assumed to be room temperature (


), see the
table on page 11 of the book
let.



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
e
xpect room temperature but I’ll investigate.


The thermocouples included in this kit are Copper
-
Constantan (type T)
thermocouples.


CRITICAL TEMPERATURE
S



2 3 7
YBa Cu O 92K



2 2 2 3 9
Bi Sr Ca Cu O 110K