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Superconductor Ceramics. EBB443-Technical Ceramics Dr. Sabar D. Hutagalung School of Materials & Min. Res. Eng., Universiti Sains Malaysia. What's a superconductor?. Superconductors have two outstanding features: 1). Zero electrical resistivity .

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Superconductor ceramics l.jpg

Superconductor Ceramics

EBB443-Technical Ceramics

Dr. Sabar D. Hutagalung

School of Materials & Min. Res. Eng.,

Universiti Sains Malaysia


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What's a superconductor?

Superconductors have two outstanding features:

1). Zero electrical resistivity.

  • This means that an electrical current in a superconducting ring continues indefinitely until a force is 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 the outer 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 effect experiment, a magnet floats above the surface of the superconductor


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What's a superconductor?

  • Most materials will only superconduct, at very low temperatures, near absolute zero.

  • Above the critical temperature, the material may have conventional metallic conductivity or may even be an insulator.

  • As the temperature drops below the critical point,Tc, resistivity rapidly drops to zero and current 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 nonsuperconductor metal.


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Meissner Effect

  • When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect.

  • This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance.

  • Zero resistance would imply that if we tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz’s Law).


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Non-superconductor

Bint = Bext


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Superconductor

Bext

Bint = 0


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Magnetic Levitation

  • Magnetic fields are actively excluded from superconductors (Meissner effect).

  • If a small magnet is brought near a superconductor, it will be repelled becaused induced supercurrents will produce mirror images of each pole.

  • If a small permanent magnet is placed above a superconductor, it can be levitated by this repulsive force.



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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.

Mat.

Tc (K)

Tc (K)

Gd*

Be

1.1

0

Rh

Al

1.2

0

Pa

W

1.4

0.015

Ir

Th

1.4

0.1

Re

Lu

1.4

0.1

Tl

Hf

2.39

0.1

In

Ru

3.408

0.5

Os

Sn

0.7

3.722

Hg

Mo

4.153

0.92

Ta

Zr

4.47

0.546

Cd

V

5.38

0.56

U

La

6.00

0.2

Ti

Pb

7.193

0.39

Tc

Zn

7.77

0.85

Nb

Ga

9.46

1.083

Type I Superconductors


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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 superconducting state.



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The Critical Field

  • An important characteristic of all superconductors is that the superconductivity is "quenched" when the 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 similar rare earth ceramic oxide, thulium-barium-copper-oxide, was reported to have a critical field of 36 Tesla at liquid nitrogen temperature and 100 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 = critical temperature.

  • In general, the higher Tc, the higher Bc.


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BCS Theory of Superconductivity

  • The properties of type I superconductors were modeled by the efforts of John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory.

  • A key conceptual element in this theory is the pairing of electrons 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 of superconductivity.


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JOSEPHSON EFFECT

  • JOSEPHSON EFFECT, the flow of electric current, in the form of electron pairs (called Cooper pairs), between two superconducting 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 the penetration ("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 called perovskites.

  • One of these superconductor is an yttrium (Y), barium (Ba) and copper (Cu) composition.

  • Chemical formula is YBa2Cu3O7.

  • This superconductor has a critical transition temperature around 90K, well above liquid nitrogen'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 publication of 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 powders including MgB2 or CaCuO2 or other ceramics while these ceramics still have significant disadvantages as compared to YBCO raw material.


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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.85Ce0.l5CuO4

22

YBa2Cu3O7

90

Tl2Ba2Ca2Cu3O10

125

HgBa2Ca2Cu3O8+d

133

Table I: Transition temperatures in inorganic superconductors