Physics
HSC Course
Stage 6
Ideas to implementation
Part 6: Superconductors for tomorrow today
Contents
Introduction ............................................................................... 2
The superconductor .................................................................. 4
Onnes ...................................................................................................4
Why is it a superconductor? ................................................................7
The BCS theory ...................................................................................9
Superconductor types .......................................................................13
Superconductor applications ............................................................14
Maglev ................................................................................... 23
The maglev train ................................................................................24
Using superconductors ........................................................... 28
Advantages and limitations ...............................................................28
Superconductivity – a new state? .....................................................30
Summary................................................................................. 33
Suggested answers................................................................. 35
Exercises – Part 6 ................................................................... 37
Student evaluation of module.................................................. 41
From ideas to implementation
1
Introduction
The development of our understanding of the nature of the electron has
taken us forward to a new level of technology. The theoretical discovery
and explanations for the behaviour of electrons within matter has driven
forward many of the advances that form the basis of that modern
technology.
Physicist doing basic research have consistently discovered new
phenomena, often by accident, that has been rapidly taken up by society.
The superconductors, especially those that operate at the relatively high
temperatures above the boiling point of liquid nitrogen (77K) offer the
potential to drive forward the next generation of technological miracles
society has come to expect as the product of scientific research.
In Part 6 you will be given opportunities to learn to:
•
identify that superconductors, while still having lattices, allow the
electrons to pass through unimpeded with no energy loss at
particular temperatures
•
explain current theory that suggests that superconductors are
conducting materials that, at specific temperatures, force electrons to
pair and, through interactions with the crystal lattice, are ultimately
able to form an unimpeded orderly stream
•
discuss the advantages of using superconductors and identify current
limitations to their use.
In Part 6 you will be given opportunities to:
2
•
process information to identify some of the metals, metal alloys and
compounds that have been identified as exhibiting the property of
superconductivity and the critical temperatures at which they operate
•
perform an investigation and gather first-hand information to
observe magnetic levitation and the way the magnet is held in
position by superconducting material
•
analyse information about magnetic levitation to explain why a
magnet is able to hover above a superconducting material that has
reached the temperature at which it is superconducting
Part 6: Superconductors for tomorrow today
•
gather and process information to describe how superconductors and
the effects of magnetic fields have been applied to develop the
maglev train
•
gather and process information to discuss possible applications of
superconductivity and the effects of those applications on computers,
generators and motors and transmission of electricity through power
grids
•
process information to recall the states of matter and their properties
and debate whether superconductivity is a new ‘state’.
Extracts from Physics Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus99/syllabus2000_list.html
From ideas to implementation
3
The superconductor
Onnes
The first superconductor was discovered in 1911 by Heike Kamerlingh
Onnes. Onnes was doing some basic research when he cooled mercury
with liquid helium to 4K then was surprised to find that the mercury lost
all electrical resistance. This is shown below.
Resistance (W)
0.16
0.12
0.08
At about -269r
mercury becomes
superconductive
0.04
0
-273
-271
-269
Temperature (rC)
-267
The resistance curve for mercury as it approaches 4K. Note the sudden decline
in resistance that begins around –268°C. Note the interchange of the Kelvin
and Celsius temperature scales.
The discovery of the superconductive properties of mercury encouraged
Onnes to continue his research on other metals to discover their
behaviour at very low temperatures. The question was whether or not
mercury, that is an unusual metal under normal circumstances, was also
unusual in its ability to show superconductivity.
What are the unusual properties of mercury, other than superconductivity,
that sets mercury apart from the other metallic elements.
_________________________________________________________
_________________________________________________________
Check your answer.
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Part 6: Superconductors for tomorrow today
Onnes research was successful when in 1913 he observed
superconductivity in lead and tin (also at 4K). This excited Onnes very
much because now he had established that other substances had the
ability to become superconductors. Not only that, these substances were
easily drawn into wires! Mercury could not be. The potential to produce
superconducting devices was immediately apparent to Onnes.
‘Tin and lead being easily workable materials, we can now
contemplate all kinds of electrical experiments with apparatus without
resistance.’
Onnes, 1913.
Superconductors. What’s the big deal?
The big deal was recognised immediately by Onnes who began dreaming
of applications. The first application he foresaw was the possibility of
producing intense magnetic fields without the need to use electromagnets
with iron cores.
Recall from the preliminary module Electrical energy in the home how you
made an electromagnet. What factors affected the strength of the
electromagnet you made?
_________________________________________________________
_________________________________________________________
Check your answer.
The strongest magnets are always electromagnets. The field strength is
controlled by the current in the coil surrounding the iron core and the
number of turns in the coil. Iron is a heavy element making it unsuitable
for many applications where portability is a problem. Additionally, iron
cores are limited in their ability to magnify the magnetic effect of the
current.
Superconductors offered a great improvement on the problem of
producing electromagnets. Those improvements were as follows.
•
The possibility of having the current so large in the conductor that
there would be no need for an iron core to magnify the magnetic
effect. The magnetic effect of the current alone would be able to
produce a big enough magnetic field.
•
The lower resistance of the super conducting wire would mean that
the length of the coil could be extremely long with no energy losses.
The result would be a coil with more windings hence a greater
magnetic effect.
From ideas to implementation
5
Superconductivity hits a snag
Onnes had visions of a ‘new world’. He foresaw a new technological
revolution. Unfortunately there was just one problem holding him up.
Tin and lead, though good workable metals, failed the crucial test.
They simply couldn’t carry the required electrical currents.
The problem seemed insurmountable. Superconductivity had been
discovered. The potential was recognised, but just like the transistor
where crystals of semiconductor material did not exist to enable the
construction of the real object, the material to realise the dream list of
applications for superconductors did not exist.
Fifty years passed before superconductors were made from a niobium-3tin or niobium titanium alloy that could carry the current size required.
Do Exercise 6.1 now.
Unfortunately a problem still existed. Niobium-3-tin alloy
superconductors still needed to be cooled to 4K. Cooling something to
4K is very expensive.
The search began for superconducting materials that would operate at
higher temperatures. The higher the temperature the better but a
particular goal was set. That goal was to find a superconductor material
that would behave as a superconductor at a temperature above the boiling
point of liquid nitrogen (77K). The reason for the search for higher
temperature superconductors is simple. Higher temperatures are cheaper
to produce and maintain. Liquid helium is at least ten times more
expensive to produce than liquid nitrogen.
In 1986 scientists, Dr. Johannes Georg Bednorz and Dr. Karl Alexander
Müller, in Zurich, Switzerland at IBM research laboratories, discovered a
group of ceramics that were superconductors. These ceramics were able
to operate as superconductors at a much higher temperature than any
other materials known. The range of possible uses for superconductors
had just expanded incredibly!
In 1988 Allen Herman discovered superconductors with even higher
transition temperatures. These superconductors were composed of
thallium, barium, calcium, copper and oxygen.
In all the high temperature superconductors discovered in the 1980s the
one thing that they all had in common was a crystal lattice that contained
planes of copper and oxygen atoms in their crystal lattice. The planes of
copper and oxygen are electronically active in these compounds. These
6
Part 6: Superconductors for tomorrow today
planes are sandwiched between other layers that act as spacers and as
reservoirs of positive and negative charge.
It is the electronic state of the layers that ultimately determines the
critical temperature of the superconducting material. That temperature is
related to the charge on the oxygen-copper planes.
1
Current research is seeking superconductors that will operate at room
temperature. Why is it desirable to make such superconductors that
operate at room temperature?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
What are some of the advantages a superconducting system used in
transmitting electricity would have if it could operate at room
temperature?
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Do a web search for the highest temperature superconductor currently
known. The development of this superconductor may have been as recent as
in the past few weeks. Write down your answer in the space below.
_________________________________________________________
_________________________________________________________
Do Exercise 6.2 now.
Why is it a superconductor?
Superconductors have resistances at least 10I7 times lower than copper
does at 0°C. Your knowledge of the resistance of copper should
immediately tell you that a superconductor must therefore be a special
device. The question you should now ask yourself is why are
superconductors so special?
From ideas to implementation
7
One of the things that makes a superconductor special is the behaviour of
electrons in the superconducting material.
In a superconductor an electron can detach itself from an atom and be
independent. In normal materials an atom can only be stable if it has the
same number of positive and negative charges. When an electron
detaches in normal material the atom becomes unstable. The atom has
more positive charges than negative charges. The atom becomes a
positive ion.
A normal metal conductor such as copper metal can be thought of as
being made up of a framework of ions. This framework is called a
lattice. The lattice is a network of connections between atoms of a
material. In superconductors, atoms form lattices too.
In the lattice of a superconducting material at a temperature above where
the material begins to lose all resistance, the ions making up the lattice
are vibrating about just like they do in non-superconducting materials.
While this is happening the free electrons are moving in random
directions. By lowering the temperature, the movement of the ions and
electrons slows.
If an electrical pressure (voltage) is applied to the metal conductor, the
free electrons begin to move in the direction pushed. Above the
superconducting temperature the electrons eventually hit the ions.
This is shown in the figure following.
atoms of lattice
Electrons bouncing off the vibrating ions.
Energy is converted to heat in these collisions
Vibrations of the lattice are large and
rapid above the superconducting state.
Collisions between electrons and ions cause the electrons to ricochet off
ions. In doing so some of the electron’s movement energy is converted
to heat. This lost electron energy is called electrical resistance.
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Part 6: Superconductors for tomorrow today
You will recall from the preliminary module Electrical energy in the
home that resistance produces heating. This occurs in all metals. The
table below shows the change in resistance for 1.024 mm diameter
copper wire per kilometre as temperature increases.
Temperature (rrC)
Resistance (W km-1)
0
19.3
20
20.9
50
23.4
75
25.4
The increase in resistance as temperature increases is symptomatic of the
increase in collisions between electrons and ions as the vibration energy
of the ions and electrons increases.
Superconductivity of metals occurs at lowered temperatures. At lower
temperatures atoms and electrons slow down, reducing the chance of the
electrons hitting the atoms. The temperature at which materials become
superconductive is their critical temperature.
The BCS theory
In 1957 John Bardeen, Leon Cooper, and John Robert Schrieffer
proposed a theory to explain why materials lose all resistance and
become superconductors at their critical temperatures. That theory for
superconductivity has come to be known as the BCS theory after the
initials of the surnames of the developers. John Bardeen, Leon Cooper,
and John Robert Schrieffer shared the Nobel Prize in physics in 1972 for
the theories development.
The main idea of the BCS model suggests that electrons in a
superconductor condense into a quantum ground state and travel together
collectively and coherently. The BCS theory states that single electrons
do not carry an electric current in a superconductor, but paired electrons
do. These pairs are called Cooper pairs.
Cooper’s idea was that the atomic lattice vibrations in the
superconducting material were directly responsible for unifying the entire
current into the one quantum state. In other words all the electrons have
the same energy level.
From ideas to implementation
9
The lattice vibrations force the electrons to pair up into teams that pass
all of the obstacles that cause resistance in the conductor at normal
conditions.
Cooper and his colleagues determined that electrons that normally repel
because of their similar electric charges must attract strongly in
superconductors. The answer to how this could occur was found to be
phonons.
Phonons are packets of inaudible sound waves (vibration energy) in the
crystal lattice as it vibrates. According to the BCS theory, as one
electron passes by positively charged ions in the lattice of the
superconductor, the lattice distorts. This is shown in the figure below.
electron passes through a gap
in the crystal lattice causing an
inward distortion of the lattice
electron
An electron causing distortion of the crystal lattice.
This distortion is due to an attraction of the positive ions of the lattice to
the negative electron. In distorting an area of increased positive charge
concentration forms and attracts a closely following negative electron
that will also pass along the relatively positive trough in the distorted
lattice area. The electron following is able to follow the path of the first
before the crystal lattice bounces back to its normal position.
The result is a pair of electrons travelling through the lattice that are
closely linked as shown in the figure following.
“Cooper pair”
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Part 6: Superconductors for tomorrow today
The Cooper pair. The second electron closely following the first gets an easy
passage through the crystal lattice. Cooper pairs constantly form, break down
and reform.
Effectively the electrons are paired and although separate, act as one unit
to cooperate in their passage through the superconducting material’s
crystal lattice. It is this process that links two electrons, that should
repel. The forces exerted by the phonons or vibrations in the crystal
lattice overcome the electrons' repulsion. The low temperatures of the
materials at superconducting temperatures make the effect of the passing
electron on the positive ions in the lattice more pronounced than in the
same material above the superconducting critical temperature.
The electron pairs produced in this way are coherent as they pass
through the conductor in unison. The electrons are screened by the
phonons and although paired are separated by some distance.
When one of the electrons passes close to an ion in the crystal lattice,
attraction between the negative electron and the positive ion occurs. This
causes a vibration that passes from ion to ion in the crystal lattice until
the other electron of the pair also absorbs the vibration. This means that
that the first electron of the Cooper pair has emitted a phonon and the
second electron has absorbed that phonon.
That exchange of energy somehow keeps the Cooper paired electrons
together and in the same quantum state. It is as though one of the
electron pairs had gained a little bit of positive charge. This little bit of
positive charge then aids in the electron pair’s navigation of the crystal
lattice of the material. Individual pairs are not stuck together forever.
They are constantly breaking and reforming. Individual electrons cannot
be identified so rather than consider them to be dynamically changing
pairs that are for most purposes identical, they can be considered as
permanently paired. A Cooper pair is shown in the figure following.
From ideas to implementation
11
The two electrons, entering the distortion
close together are called Cooper pairs.
They have become locked together and
will travel through the lattice unimpeded.
“Cooper pair”
area of distortion
A Cooper pair.
Cooper pairs pass through the superconductor more smoothly than
electrons on their own.
Electrons in a superconducting state are like a stream of rapidly moving
cars driving close together on the straight stretches of a freeway. The
slipstream turbulence of the lead vehicle drags the following car along
behind it so that car gets a bit of a free ride. The cars have the same
velocities. It takes an effort for the cars to break apart from this pattern.
The low pressure region between adjacent cars keeps them all into an
ordered stream. That energy is the equivalent of a quantum ground state.
BCS theory shows that electrons can be attracted to one another in much
the same way as the slipstream holds adjacent vehicles together. For the
electrons this occurs through interactions with the crystalline lattice
despite electrons having the same charge. (Like charges are supposed to
repel remember!)
It is the atoms of the lattice oscillating as positive and negative regions
that pulls and pushes the Cooper electron pairs without allowing a
collision on the path through the lattice. The electron pairing offers an
advantage because it is effectively putting the superconducting material
into a lower energy state.
As long as the superconductor is maintained below the critical
temperature the Cooper pairs are able stay together (or at least constantly
form) due to the reduced kinetic molecular motion of ions in the atomic
nucleus.
Cooper pairs do of course break as the temperature of the material rises.
When this occurs the superconductivity diminishes.
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Part 6: Superconductors for tomorrow today
Use the Internet or other sources available to you to research more fully the
way the formation of electron pairs enables superconductivity.
To see some websites that will enable you to get started on this activity see
sites on the physics websites page at:
http://www.lmpc.edu.au/science
Do Exercise 6. 2 now.
Superconductor types
Common elements that show the ability to become superconductors are
known as type I superconductors. The table below shows the critical
temperatures of some type I superconductors.
Element
Critical temperature (K)
aluminum
1.2
lead
7
mercury
4
tin
4
titanium
0.4
tungsten
0.015
zinc
0.85
In the 1950s and 1960s material scientists searched for superconductors
with higher critical temperatures. During that interval a group
superconductors were found based on the element niobium.
Later in the mid 1980s a new type of superconductor compound class
based on copper oxide was discovered. All these superconductors
composed of more than one element are called intermetallic
superconductors. Intermetallic superconductors are classified as type II
superconductors. They are often referred to as ceramic. The list below
From ideas to implementation
13
indicates the critical temperature of some of the hundreds of type II
superconductors known.
Superconducting material
Critical Temperature (K)
Bi2Sr2CuO6
KNb3Ge
La2BaCu O
0-110
23.2
20-40
Nb3Al
18.7
Nb3Ga
20.0
Nb3Sn
18.0
NbTi
10.0
HBaCaCu0
up to 130
YBaCuO Tl
90
Tl2Ba2Cu2O5
80-125
To see websites that show the structure of some superconducting
compounds see links on the physics websites page at:
http://www.lmpc.edu.au/science
Do Exercises 6.3 to 6.5 now.
Superconductor applications
Applications in transport
Superconductors have potential applications in transport.
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Part 6: Superconductors for tomorrow today
maglev car
A concept at this stage but possible in the future if high temperature
superconductor development proves possible. The cars would run on
tracks similar to the maglev train system. Such a system would require
massive infrastructure development.
maglev train
Prototype levitated trains have been constructed in Japan using
superconducting magnets. Velocities have been recorded in excess of
500 kmh-1. The train is constructed with a superconducting system
onboard the train designed to repel conventional rails below it. This lifts
the train producing greatly reduced friction running (air resistance still
limits speed).
Maglev train and guideway.
Medical applications
There are medical applications for superconductors.
Magnetic resonance imaging (MRI)
Common usage in hospitals and medical diagnostic centres. A noninvasive technique for determining soft tissue injury or disease in a
dynamic format. Provides more detail than ultrasound.
Magneto-encephalography
SQUIDS held against the head detect magnetic fields induced by nerve
electrochemical impulses in the brain.
From ideas to implementation
15
vacuum
liquid helium
squid
pick-up coils
magnetic flux
current dipole
magnetic field map
Magneto encephalograph.
Superconductors and electrical power
The electrical power generation and distribution industry can use
superconductors.
Motors
Superconducting motors could be made with a weight of about one tenth
that of conventional devices for the same output.
Superconducting magnetic storage (SMES)
These devices are designed to store DC electric current indefinitely and
to overcome the problem that power generating stations face of irregular
demand for electricity.
These systems consisting of large rings of superconducting material
could contain an electric current produced as surplus to demand until
required. The system could even out the demand cycle for existing
power stations enabling them to produce electricity at peak efficiency
levels.
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Part 6: Superconductors for tomorrow today
substation
The appearance of an SMES system from above. The complete system would
be a ring structure with the current continuing around the ring infinitely without
energy loss until required as long as the temperature was kept below the
superconducting critical temperature.
backfill
dolomite
coil
helium vessel
vacuum tank
A cutaway showing the multiple coil windings kept below ground in the SMES
system. The helium vessel is filled with liquid helium under pressure to ensure
the coils of superconducting material are kept below the critical temperature.
The vacuum tank acts in the same way as the vacuum does in a vacuum
thermos flask. It prevents heat gain by convection. The whole system is buried
for safety in case of current leakage.
From ideas to implementation
17
superconducting cable
aluminium brick
helium-vessel panels
A further cutaway of a coil of the SMES. The aluminium block serves assist in
keeping the coil cool by conducting any heat away from the coil to the liquid
helium vessel. It also acts as a potential earthing surface if catastrophic current
leakage was to occur. This enhances safety.
Generators
Superconducting generators could be made with a weight of about one
tenth that of conventional devices for the same output. This could be
crucial in applications where weight is critical.
Power transmission
Because 10% to 15% of generated electrical energy is dissipated in
resistive losses in transmission lines, the prospect of zero loss
superconducting transmission lines is commercially desirable.
Prototype superconducting transmission lines at Brookhaven National
Laboratory in the USA, carry 1000 MW of power within an enclosure of
diameter 40 cm. This is equivalent to the entire output of a large power
generation plant.
Fault current limiters
High fault-currents are caused by lightning strikes. These are dangerous
and cause expensive to repair damage in electric power grids. It may be
possible to reduce the fault current to a fraction of its peak value in less
than an AC cycle (1/50 sec).
Transformers and inductors
These are still largely experimental though the benefits have been
established in the USA, Japan and Europe.
The advantages experienced in test units include a decreased energy loss
from resistance effects when stepping voltages up and down.
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Part 6: Superconductors for tomorrow today
Commercial units of superconducting transformers are expected in 2001
from a number of companies.
Applications in electronics
The electronics industry has many applications for superconductors.
Josephson junctions
Consists of two superconductors separated by a thin insulating barrier are
used in fast electronic switches or sensitive magnetometers. A
magnetometer can be built that is able to detect incredibly small
magnetic fields. The device can be sensitive enough to measure the
magnetic fields in living organisms.
cu
rr
en
t
magnetic field
Josephson junction
vo
lta
ge
cu
rr
en
t
vo
lta
ge
superconductor
A Josephson junction
SQUIDS or superconducting quantum interference device
A SQUID is a superconducting loop interrupted in two places by
Josephson junctions. When sufficient electrical current is conducted
across the body of the SQUID a voltage is generated that is proportional
to the strength of any nearby magnetic field.
SQUIDS have application in such diverse areas as remote sensing for
minerals using highly sensitive magnetometers and MRI machines.
From ideas to implementation
19
Transistors
Superconducting transistors based on Josephson junctions could be used
to switch voltages very fast without the current requirements for power
that exist in the present computer design. These devices offer the
potential to speed up significantly the processing of signals. This is
critical in the Internet age.
In such devices a current of a particular size will flow across the insulator
barrier in a Josephson junction with no voltage between the
superconductors on either side of the barrier.
The current remains at practically zero for increasing low voltages across
the barrier until another threshold voltage is achieved whereupon the
current rises to the zero voltage level and then continues to climb almost
linearly from there. This enables the Josephson junction to act as a
transistor of extraordinary speed.
Circuitry connections
Use of superconductive films can result in more densely packed chips
that transmit information more rapidly by several orders of magnitude for
use in supercomputers
Supercomputers
Research has been conducted by numerous organisations since 1962 to
develop a superconducting computer. In 2000 one does not exist that has
significant advantages over the conventional and rapidly improving
semiconductor industry.
The USA government is seeking a faster computer based on hybrid
technologies that include the use of superconductor materials. The Jet
Propulsion Laboratory in the USA has a research project at present along
those lines with the aim being to produce a supercomputer that is
250 times faster than the fastest supercomputer available in 2000.
The project to build that computer is called the Hybrid technology multithreaded program (HTMT). The uses for this computer are anticipated to
include: nuclear stockpile stewardship, explosion simulation, fluid
dynamics modelling, climate modelling for long term, drug design,
economic modelling and weather forecasting.
Superconductors used in research
The research industry uses superconductors to aid in machines used to
study the nature of matter.
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Part 6: Superconductors for tomorrow today
Particle accelerators
Particle accelerators used in high energy physics studies are very
dependant on high-field superconducting magnets. A proton accelerator
at Fermilab uses 774 superconducting magnets in a ring of circumference
6.2 km.
Industrial applications of superconductors
There are many industrial applications of superconductors.
Separation
Magnets of great power used to remove impurities in food and raw
materials such as clay often require huge electric currents.
The huge electric currents required to run the coil and cool the coils in
conventional systems can be greatly reduced using a superconducting
system.
The first such commercial system was introduced in 1986 at a clay
separation facility in the USA.
A magnetic separator.
From ideas to implementation
21
Magnets
These are often required in separation and purification systems.
In addition materials that are paramagnetic or ferromagnetic align
themselves with magnetic fields as they drop through the Curie
temperature (temperature where natural magnetism falls away).
This overrides the effects of random thermal motion experienced by the
particles.
If such materials are placed in a strong magnetic field they heat up.
When the field is removed these substances cool down. This heating
cooling effect can be exploited in a heat pump.
Good performance can only be achieved if the magnet has a strength of
more than 10 teslas. In the laboratory temperatures as low as 10-6 K have
been achieved. This technology could be extremely important in the
food processing industry.
Magnetic shielding
This has application for high efficiency resolution of magnetic fields and
their measurement where it is desirable for the environment to be devoid
of external magnetic interference.
This applies in areas such as electronic measuring systems and medical
instruments eg. SQUID magnetometers.
To see sites that discuss some of the applications of superconductors
including superconducting computers, SQUIDS, SMES, Josephson
junctions, motors, generators, particle accelerators, and maglev vehicles see
pages on the physics websites page at:
http://www.lmpc.edu.au/science
Do Exercise 6.6 now.
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Part 6: Superconductors for tomorrow today
Maglev
A magnet will levitate above a superconductor (or a superconductor
above a magnet) because of the Meissner effect. This effect causes the
magnetic flux to be expelled from a superconductor.
A superconductor is an example of a diamagnetic material.
Diamagnetic objects are attracted toward regions of weak magnetic field,
whereas ferromagnetic (and paramagnetic) objects are drawn toward
regions of strong magnetic field.
A permanent magnet has field lines emerging from the north pole and
looping around the outside to re-enter at the south pole on the other end.
When it is hovering over a diamagnetic, superconducting material will
have those field lines repelled or pushed away from the superconducting
material because that material is diamagnetic. The field lines emerging
from the north pole of the permanent magnet cannot penetrate the
diamagnetic superconductor. The magnet is hence forced to rise above
the superconductor to give the magnetic field lines space to return into
the south pole. That lifting effect leads to magnetic levitation.
A normal situation with a ceramic disc magnet
N
S
magnetic field lines leave
the north pole and enter
the south pole
A bar magnet sitting on the surface of a superconducting
material above the critical temperature
N
S
From ideas to implementation
the magnetic field lines
penetrate the paramagnetic
surface and return into the
south pole
23
A bar magnet sitting on the surface of a superconducting
material at the critical temperature
the inability of magnetic field lines to enter
the south pole causes lift of the magnet
N
S
N
S
the magnetic field lines can
no longer penetrate the
surface – the surface is now
diamagnetic
the magnet is levitated to
enable the field lines to enter
the south pole
superconducting surface
below critical temperature
Magnetic levitation.
At your practical session with your teacher you will see and perform an
experiment to observe magnetic levitation and the way the magnet is held in
a levitating position by a superconductor material. This experiment requires
the use of liquid nitrogen so is hazardous.
To see a site that has a movie of a magnet levitating above a superconductor
go to a link on the physics websites page at:
http://www.lmpc.edu.au/science
The maglev train
There are two magnetic levitation systems currently operating around the
world: one in Germany; another in Japan.
The German system is called the Transrapid and uses a technology
known as known as electromagnetic suspension (EMS). The Japanese
system uses a different technology known as electrodynamics suspension
(EDS).
The differences in the two systems are shown in the table on the next
page.
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Part 6: Superconductors for tomorrow today
EMS (Germany)
EDS (Japan)
Uses conventional electromagnets mounted
at the ends of a pair of structures under the
train. No refrigeration problems.
Use superconducting magnets hence there
are refrigeration problems.
The electromagnet structures wrap around
and under either side of the guide way and
provide the undercarriage. The train is
constantly lifted (levitated) off the guideway.
System employs an undercarriage like
landing gear for aircraft for lift-off and landing.
The vehicle begins to levitate at speeds in
excess of 40 kmh-1 and achieves full levitation
at around 100 kmh-1. This may be of
advantage in case of power failure.
Uses a paramagnetic basis.
Uses diamagnetic basis to operate.
The magnets attract up toward the laminated
iron rails in the guide way and lift the train.
Makes use of attractive forces to lift the
vehicle and reduce friction.
Uses the opposing force between super
conducting magnets on the vehicle and
electrically conductive strips or coils in the
guide way to levitate the train. Makes use of
repulsive forces in the electrodynamics
suspension system, to lift the vehicle away
from the guide way
The distance between the electromagnets
and the guide way, which is about 10 mm,
must be continuously monitored and adjusted
by computer to prevent the train from hitting
the guide way. This system is inherently
unstable and would fail without this
intervention.
Is stable and does not require the continued
monitoring that the EMS system needs. The
distance between the train and guideway is
around 10 cm.
Magnetic propulsion
The maglev trains do not have an engine that provides the force to propel
the train forward. Instead the maglev train uses repulsion and attraction
of electromagnets to pull and push the train forward. This essentially
means the engine of the train is a magnet.
In the EDS maglev train the track is constructed similarly to the picture
shown below. The guideways on each side of the track contain
propulsion coils that act as electromagnets. The system works because
the propulsion coils are magnetised when an alternating electric current is
passed through them.
From ideas to implementation
25
beam
levitation and
guidance coil
propulsion coil
wheel support path
The guideway for a maglev system.
Why wouldn’t a direct current system work for a propulsion coil system in a
maglev train?
_________________________________________________________
_________________________________________________________
Check your answer.
The alternating current means the electromagnets have constantly
changing polarity so they will alternately be attracted then repelled by the
fixed magnets on the train. The frequency of the alternating current can
be carefully and precisely matched to the position and velocity of the
train. This means that the train is pushed on one cycle as the magnets in
the track and on the train are the same polarity and then the train is pulled
as the track and train magnets attract because they are of opposite
polarity.
Such a propulsion system has one major advantage over normal motors.
An absolute minimum number of moving parts!
Advantages of the maglev systems
26
•
Magnetic levitation trains in Germany and Japan are capable of
reaching speeds up to 500 kmh-1. They are faster than conventional
train systems.
•
Maglev systems do not use steel wheels on steel rails. Because
magnetic levitation trains do not touch the guide way therefore the
high cost of maintaining precise alignment of the tracks to avoid
excessive vibration and rail deterioration at high speeds is not a
problem.
Part 6: Superconductors for tomorrow today
•
Maglevs can provide sustained speeds greater than 500 kmh-1 limited
only by the cost of power to overcome wind resistance.
•
Maglevs do not touch the guide way. This confers advantages such
as: faster acceleration and braking, greater climbing capability;
enhanced operation in heavy rain, snow, and ice.
•
Maglev transportation offers an alternative to mass transit problems
in major metropolitan areas where traffic on ground and air has
become too congested.
•
Maglev systems are energy efficient. For long distance travel they
use about half the energy per passenger as a typical commercial
aircraft.
•
Maglev is an electrified transportation systems. They reduce the use
of petroleum, and pollute the air less than aircraft, diesel
locomotives, and cars.
In summary the major appeals of the maglev solution to provide mass
transit is high speed, it is environmentally cleaner and there is a reduction
in noise over those of aircraft at airports.
To see sites that describe the maglev train operating system see sites on the
physics websites page at:
http://www.lmpc.edu.au/science
To see a site that outlines how to build your own maglev train model go to
the physics websites page at:
http://www.lmpc.edu.au/science
From ideas to implementation
27
Using superconductors
Hopefully the learning you have done in this part has convinced you that
superconductors have a very important role to play in improving the
future quality of your life. Their potential is only limited by the lack of
our present understanding. The search to find even higher temperature
superconductors is going on right now as you read this. The rewards are
potentially as significant as those that came with the invention of the
commercial electric generator.
Advantages and limitations
Advantage: Since there is no loss in electrical energy when
superconductors carry electrical current, relatively narrow wires made of
superconducting materials can be used to carry huge currents.
Limitation: There is a maximum current that superconducting materials
can carry. Currents above that threshold change the superconductor to
being a normal conductor. The material will revert to the normal state
even though it may be below its transition temperature. The current
where this occurs is called the critical current density. The value of
critical current density is temperature dependent. The colder you keep
the superconductor, the more current it can carry.
In other words, the higher the temperature at which a material can
demonstrate superconductivity, the cheaper the superconductor is to
maintain. And the more efficiently it can run with cheap technologies
such as using liquid nitrogen as the refrigerant.
Advantage: Environmental benefits accrue from the higher efficiency of
generation, transmission, distribution and use of electric power using
superconductors.
Secondary benefits include the saving to the environment from less
pollution due to power generation in fossil fuel power plants. Lower
emissions result in a cleaner environment.
28
Part 6: Superconductors for tomorrow today
Another benefit may be the ability to use renewable energy sources in
areas distant from population or industrial centres and then to transport
that electricity over vast distances to where the demand is located using
superconducting cables with minimal energy losses. This could also
mean the requirement to build power stations of a conventional nature
often distant from energy resources is eliminated.
Limitation: The cost is prohibitive for immediate replacement of
existing technologies.
Advantage: The cost of generating electricity using conventional
technologies will decrease as the use of superconducting technologies in
generation and transmission increases. This means less fuel used to
produce the same amount of electrical energy consumed.
At the same time the increased efficiency of electric motor using
superconducting technology will be decreasing unit demand for the
electricity.
In countries where the population is increasing rapidly this will mean a
reduced demand for expensive new power plants to be constructed.
The net result is a social dividend through lower cost electricity.
Limitation: This is reliant on the technology developing rapidly and the
new technology actually being implemented, especially in the developing
world.
Advantage: superconducting power cables have increased capacity to
transmit power. Where underground power conduits into cities or urban
areas are of limited cross-section but the demand for power is rapidly
growing, the cross-section requirements for conventional power
transmission cables is also rapidly growing.
The lower cross-sectional demand of superconducting cables means that
the existing conduits will be big enough to cope with the increasing
demand. This saves the cost and interruption to infrastructure that occurs
when new cable is laid.
Limitation: The technology to insert and maintain the new
superconducting cable in the existing infrastructure pathways and pipes
may be an issue.
Do Exercises 6.7 and 6.8 now.
From ideas to implementation
29
Superconductivity – a new state?
Scientists generally study the properties of matter using the particle
theory. The particle theory is based on the idea that everything is made
up of particles. The particles have traditionally been found in one of
three states. You may be able to recall from earlier work in science what
those states of matter are.
If you recalled they were solids, liquids and gases you were correct.
The diagrams following show you how these states are traditionally
represented diagrammatically.
solid
liquid
gas
Note that the particles do not change size when a substance changes state. It
is simply the distance between the particles that changes as the substance
changes state.
1
Recall the properties of different states of matter using the figures
above. Consider the following.
•
Why solids have a fixed shape and volume.
•
Why liquids have a shape that fills the bottom of the container in
which the liquid is held.
•
Why gases fill the whole of their container.
•
Why compressibility of the three states varies.
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______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
30
Part 6: Superconductors for tomorrow today
2
Consider a substance that you are familiar with, water. How do you
get water to change state?
_____________________________________________________
_____________________________________________________
3
What are the physical properties of water that change as you
evaporate or freeze water? Consider density, volume and crystalline
structure of the material.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
4
Consider the properties of materials considered as superconductors.
What has to happen to the material to cause them to become
superconductors?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
5
Look back through your notes and recall what happens to normal
conductive materials such as copper when the temperature drops.
Describe the relationship.
_____________________________________________________
_____________________________________________________
_____________________________________________________
6
Is there any similarity in the pattern of change that occurs in
superconductors as they reach their critical temperature and the
change of state that occurs as a specific compound such as water as
it reaches a specific temperature such as 100rC or 0rC?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
From ideas to implementation
31
7
Do the physical properties of a substance alter when a change of
state occurs?
______________________________________________________
8
Do the properties of a superconducting material change in any
dramatic manner when the temperature drops below the critical
temperature? If so what properties change?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
The thing about superconductor materials when they reach their critical
temperature is that they do not seem to undergo a change of state. Their
appearance remains the same. Their properties with respect to two
features, diamagnetism and resistance change. The debate as to whether
these changes constitute a new state or not will continue for some time.
Do Exercise 6.6 now.
32
Part 6: Superconductors for tomorrow today
Summary
Complete the following summary. This will help you to recall the
learning in this module. It will also assist you when you come to revise
for your assessment tasks and exams.
•
The main features of the BCS theory used to explain how a
superconductor works are:
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
Four examples of type I superconductors are:
_____________________________________________________
_____________________________________________________
•
Four examples of a type II superconductor are:
_____________________________________________________
_____________________________________________________
•
The Meissner effect works because:
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
From ideas to implementation
33
•
Potential and existing applications of superconductors include:
______________________________________________________
______________________________________________________
______________________________________________________
•
The limitations to the use of superconductors include:
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Attempt to draw a concept map linking the things you have learned about
in this part in the space below. Link at least following concepts and
ideas on your map.
Superconductors, critical temperature, type I superconductors, type II
superconductors, Cooper pairs, BCS theory, magnetic levitation, maglev
trains, superconducting computers, SMES, superconductor advantages,
superconductor limitations.
34
Part 6: Superconductors for tomorrow today
Suggested answers
The superconductor
Mercury is a liquid at room temperature. It also evaporates over time if
left in air.
Superconductors. What’s the big deal?
The strength of the electric current, the number of turns of the coil and
the cross-sectional area of the coil all affect the strength of the
electromagnet.
Superconductivity hits a snag
1
The higher the critical temperature the cheaper superconductors are
to run. The current the superconductor can carry also increases when
the superconductor is at a temperature much reduced below the
critical temperature. If the refrigeration requirement could be
removed altogether the devices based on a superconducting coil
would be much easier to build.
2
Cheaper, higher currents on single wire, less complex infrastructure,
could easily transport electricity from remote regions to where
demand was peak, more efficient, less pollution from power stations.
Maglev propulsion
A direct current electromagnet would have only one polarity. It would
therefore only provide a single push or pull.
Superconductivity – a new state?
1
In a solid the particles are close together and have fixed positions.
The particles are lined up in rows and columns like eggs in an egg
carton or the oranges in a fruit box.
From ideas to implementation
35
In a liquid the particles have ‘empty space’ between one another and
move randomly at high speed. The particles bounce off the walls of
the sealed container. In a gas the particles are close together but can
move freely over one another.
The particle theory states that all matter is made up of particles is
very useful. It can explain properties or characteristics such as the
shape and the volume of solids, liquids and gases. It explains:
•
solids have a fixed shape and volume because the particles are
in fixed positions, the shape and volume do not change
•
liquids have a shape that fills the bottom of the container that the
liquid is held in because the particles can move freely over one
another
•
gases fill the entire container and escape when the lid is
removed because the particles are moving at high speed.
It is easy to reduce the space taken up by the particles of a gas into a
smaller volume. The particles do not change in size as this state is
compressed. Only the amount of space in which the particles move
becomes smaller.
Note that the particles do not change in size between solid, liquid
and gas. The amount of space taken up by all the particles can
change but not the size of each particle.
2
Change the temperature of the water and you can get the substance
to change state. Removal of energy from water changes the state
from liquid to a solid. Adding energy to water changes the state
from liquid to a gas.
3
Evaporation: density decreases, volume increases, noncrystalline
structure maintained.
Freezing: density decreases (water is a special case) then rapidly
increases as temperature continues to fall, crystalline structure
develops (think about a snow flake as a crystal), volume increases at
first then begins to decrease.
36
4
The temperature has dropped and the kinetic energy of the particles
decreases. The materials then become superconductors at a critical
temperature.
5
As the temperature decreases the resistance also decreases.
6
The behaviour of the materials all change at these temperatures. In a
way these temperatures are all critical temperatures or points where
changes occur.
7
Yes, they do change.
8
Conductivity changes. Resistance to the flow of an electrical current
drops to zero. There is no obvious change in other physical
properties.
Part 6: Superconductors for tomorrow today
Exercises – Part 6
Exercises 6.1 to 6.8
Name: _________________________________
Remember: You must submit your investigation into the heating
effects of current in a range of conductors with these exercises.
Exercise 6.1
What would be the advantages of using superconductor technology to
make a better electric motor or electromagnet?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 6.2
Describe the current theory of superconductors that allows for the
formation of Cooper pairs that interact with the crystal lattice to allow an
unimpeded orderly stream current stream through the crystal lattice at a
critical temperature.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
From ideas to implementation
37
Exercise 6.3
Use the information in the text or from other sources such as the Internet
to identify and name four superconductors that could operate in liquid
nitrogen and four superconductors that could only operate in liquid
helium.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 6.4
Why is the development of higher temperature superconductors of vital
importance if the technology is to come into everyday use in society?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 6.5
What are the differences between a type I supercondcutor and a type II
superconductor? Use examples of each type of superconductor and
consider their critical temperatures in your answer.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
38
Part 6: Superconductors for tomorrow today
Exercise 6.6
Use the information you have read about in the module and other
resources you have gathered from books, the media or the Internet to
prepare a time line that describes the history of the development of
superconductivity and the potential and actual effects of application of
the technology. Consider the areas of transport, computers, power
generation and transmission through power grids in your answer.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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From ideas to implementation
39
Exercise 6.7
Superconductivity has the potential to be used in many applications.
Chose one of those applications. Discuss the advantages and limitations
of using superconductors in that application.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 6.8
There has been significant debate in the scientific literature about
whether or not superconductivity is a new state of matter. Make a list
below of the features of superconductivity that would suggest to you that
superconductivity could be considered a new state of matter and a second
list of the features that suggest superconductivity is not a new state.
Consider things such as the relationship of the particles to each other and
changes in physical properties as temperature changes in your answer.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
40
Part 6: Superconductors for tomorrow today
Student evaluation of module
We need your input! Can you please complete this short evaluation to
provide us with information about this module. This information will
help us to improve the design of these materials for future publications.
1
Name: _______________________________________________
2
Location: ____________________________________________
3
Did you find the information in the module easy to understand?
_____________________________________________________
4
Were the instructions clear? ______________________________
5
What did you most like learning about? Why?
_____________________________________________________
_____________________________________________________
_____________________________________________________
6
Which sort of learning activity did you enjoy the most? Why?
_____________________________________________________
_____________________________________________________
_____________________________________________________
7
Did you complete the module within 30 hours? (Please indicate the
approximate length of time spent on the module.)
_____________________________________________________
_____________________________________________________
From ideas to implementation
41
8
Do you have access to the appropriate resources? eg. a computer,
the Internet, scientific equipment, chemicals, people that can provide
information and help with understanding science
______________________________________________________
______________________________________________________
______________________________________________________
Please return this information to your teacher, who will pass it along to
the materials developers at OTEN – DE.
42
Part 6: Superconductors for tomorrow today
Learning Materials Production
Open Training and Education Network – Distance Education
NSW Department of Education and Training