Superconductor Ceramics

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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
  (Lenzs Law).

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Non-superconductor
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Superconductor
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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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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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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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Type II Superconductors
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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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Table I Transition temperatures in inorganic
superconductors