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Superconductor Ceramics

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Title: Superconductor Ceramics


1
Superconductor Ceramics
  • EBB443-Technical Ceramics
  • Dr. Sabar D. Hutagalung
  • School of Materials Min. Res. Eng.,
  • Universiti Sains Malaysia

2
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

3
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.

4
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.

5
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
    (Lenzs Law).

6
Non-superconductor
Bint Bext
7
Superconductor
Bext
Bint 0
8
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.

9
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10
Magnetic Levitation
11
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.

12
Type I Superconductors
13
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.

14
Type II Superconductors
15
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.

16
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.

17
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.

18
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.

19
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.

20
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.

21
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.

22
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.

23
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.

24
Table I Transition temperatures in inorganic
superconductors
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