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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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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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

Superconductor Ceramics

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