Superconductor Ceramics
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Transcript of Superconductor Ceramics
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What's a superconductor?
Superconductors have two outstanding features:1). Zero electrical resistivity.
This means that an electrical current in asuperconducting ring continues indefinitely until a forceis applied to oppose the current.
2). The magnetic field inside a bulk sample is zero(the Meissner effect).
When a magnetic field is applied current flows in theouter skin of the material leading to an induced magnetic
field that exactly opposes the applied field. The material is strongly diamagnetic as a result.
In the Meissner effectexperiment, a magnet floatsabove the surface of the superconductor
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What's a superconductor?
Most materials will only superconduct, at very lowtemperatures, near absolute zero.
Above the critical temperature, the material mayhave conventional metallic conductivity or mayeven be an insulator.
As the temperature drops below the criticalpoint,Tc, resistivity rapidly drops to zero andcurrent can flow freely without any resistance.
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What's a superconductor?
Linear reduction in resistivity as temperature is
decreased:
= o
(1 + (T-To
))
where : resistivity and : the linear temperature coefficient of
resistivity.
Resistivity: s ~ 4x10-23 cm for superconductor.
Resistivity: m ~ 1x10-13 cm for nonsuperconductormetal.
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Meissner Effect
When a material makes the transition from the normal tosuperconducting state, it actively excludes magnetic fieldsfrom its interior; this is called the Meissner effect.
This constraint to zero magnetic field inside asuperconductor is distinct from the perfect diamagnetismwhich would arise from its zero electrical resistance.
Zero resistance would imply that if we tried to magnetize asuperconductor, current loops would be generated to
exactly cancel the imposed field (Lenzs Law).
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Non-superconductor
Bint = Bext
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Superconductor
Bint = 0
Bex
t
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Magnetic Levitation
Magnetic fields are actively excluded fromsuperconductors (Meissner effect).
If a small magnet is brought near asuperconductor, it will be repelled becausedinduced supercurrents will produce mirrorimages of each pole.
If a small permanent magnet is placed above asuperconductor, it can be levitated by thisrepulsive force.
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Magnetic Levitation
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Types I Superconductors
There are 30 pure metals which exhibit zero
resistivity at low temperature.
They are called Type I superconductors (Soft
Superconductors).
The superconductivity exists only below their
critical temperature and below a critical magnetic
field strength.
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Mat. Tc (K)
Be 0
Rh 0
W 0.015
Ir 0.1
Lu 0.1
Hf 0.1
Ru 0.5
Os 0.7
Mo 0.92
Zr 0.546
Cd 0.56U 0.2
Ti 0.39
Zn 0.85
Ga 1.083
Mat. Tc (K)
Gd* 1.1
Al 1.2
Pa 1.4
Th 1.4
Re 1.4
Tl 2.39
In 3.408
Sn 3.722
Hg 4.153
Ta 4.47
V 5.38La 6.00
Pb 7.193
Tc 7.77
Nb 9.46
Type ISuperconductors
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Types II Superconductors
Starting in 1930 with lead-bismuth alloys, were
found which exhibited superconductivity; they are
called Type II superconductors (Hard
Superconductors).
They were found to have much higher critical fields
and therefore could carry much higher current
densities while remaining in the superconductingstate.
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Type II
Superconductors
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The Critical Field
An important characteristic of all superconductorsis that the superconductivity is "quenched" whenthe material is exposed to a sufficiently high
magnetic field. This magnetic field, Bc, is called the critical field.
Type II superconductors have two critical fields.
The first is a low-intensity field, Bc1, which partially
suppresses the superconductivity. The second is a much higher critical field, Bc2,
which totally quenches the superconductivity.
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The Critical Field
Researcher stated that the upper critical field of
yttrium-barium-copper-oxide is 14 Tesla at liquid
nitrogen temperature (77 degrees Kelvin) and at
least 60 Tesla at liquid helium temperature.
The similarrare earth ceramic oxide, thulium-
barium-copper-oxide, was reported to have a critical
field of36 Tesla at liquid nitrogen temperature and100 Tesla or greater at liquid helium temperature.
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The Critical Field
The critical field, Bc, that destroys the
superconducting effect obeys a parabolic law of the
form:
where Bo = constant, T = temperature, Tc = criticaltemperature.
In general, the higher Tc, the higher Bc.
2
1
c
oc
T
TBB
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BCS Theory of Superconductivity
The properties of type I superconductors were modeled by
the efforts ofJ ohn Bardeen, Leon Cooper, andRobertSchrieffer in what is commonly called the BCS theory.
A key conceptual element in this theory is the pairing ofelectrons close to the Fermi level into Cooper pairs through
interaction with the crystal lattice.
This pairing results from a slight attraction between the
electrons related to lattice vibrations; the coupling to the
lattice is called a phonon interaction.
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BCS Theory of Superconductivity
The electron pairs have a slightly lower energy and leave
an energy gap above them on the order of .001 eV which
inhibits the kind of collision interactions which lead to
ordinary resistivity. For temperatures such that the thermal energy is less
than the band gap, the material exhibits zero resistivity.
Bardeen, Cooper, and Schrieffer received the Nobel
Prize in 1972 for the development of the theory ofsuperconductivity.
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JOSEPHSON EFFECT
JOSEPHSON EFFECT, the flow of electric current, in theform of electron pairs (called Cooper pairs), between twosuperconducting materials that are separated by an
extremely thin insulator. A steady flow of current through the insulator can be
induced by a steady magnetic field.
The current flow is termed Josephson current, and thepenetration ("tunneling") of the insulator by the Cooper
pairs is known as the Josephson effect. Named after the British physicist Brian D. Josephson, who
predicted its existence in 1962.
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Superconductor Ceramics The ceramic materials used to make
superconductors are a class of materials calledperovskites.
One of these superconductor is an yttrium (Y),barium (Ba) and copper (Cu) composition.
Chemical formula is YBa2Cu3O7.
This superconductor has a critical transitiontemperature around 90K, well above liquidnitrogen's 77K temperature.
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High Temperature Superconductor (HTS)
Ceramics
Discovered in 1986, HTS ceramics are working at 77 K,
saving a great deal of cost as compared to previously
known superconductor alloys.
However, as has been noted in a Nobel Prize publicationof Bednortz and Muller, these HTS ceramics have two
technological disadvantages:
they are brittle and
they degrade under common environmental influences.
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HTS Ceramics HTS materials the most popular is
orthorhombic YBa2Cu3O7-x(YBCO)ceramics.
Nonoxide/intermetallic solid powdersincluding MgB2 or CaCuO2 or otherceramics while these ceramics still havesignificant disadvantages as compared toYBCO raw material.
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Table I: Transition temperatures in inorganicsuperconductors
Compound Tc
(K)
PbMo6S8 12.6
SnSe2(Co(C5H5)2)0.33 6.1
K3C60 19.3
Cs3C60 40 (15 kbar applied pressure)
Ba0.6K0.4BiO3 30
Lal.85Sr0.l5CuO4 40
Ndl.85
Ce0.l5
CuO4
22
YBa2Cu3O7 90
Tl2Ba2Ca2Cu3O10 125
HgBa2Ca2Cu3O8+d 133