Chapter 7 Electrical Properties by ewghwehws

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									Chapter 7
Electrical
Properties
Hong-Wen Wang
Basic of electrical properties
• What is characteristics of metallic
  conductivity ?
• What is characteristics of
  superconductivity ?
• What is semiconductivity ?
• What is ionic conductivity ?
• What is dielectrics ? Ferroelectrics ?
  Piezoelectric ? Pyroelectrics ?
  7.2 Metallic conductivity:
       organic metals
• Characteristics of organic metals
  – Flexibility
  – Easy fabrication
  – High conductivity as metal
• Two main categories:
  – Conjugated system
  – Charge transfer complexes
     Conjugated systems
• Organic solid are ususlly insulators
• Polymers such as polyethylene are
  insulators - only C-C single bonds.
• However, polymers have conjugated
  could be electrical conductive such as
  polyacetylene.
• There are cis and trans for
  polyacetylene.
     Doped polyacetylene
• The polyacetylene has the
  conjugated long-chain which is
  potential for electrical conductivity
• Doping suitable inorganic compounds
  – Acceptor : Br2, SbF5, WF6 and H2SO4
  – Donor: alkali metals
  – Conductivity as high as 103 ohm-1cm-1 in
    trans-polyacetylene can be achieved.
  – Synthetic metal.
  Doped
with FeCl3,
0.3 S/cm
  at R.T.




                        Oxidized
                         to 102
                         S/cm




              聚對伸苯基   聚砒硌
 Charge transfer complexes
• Two-component organic system in
  which one is a electron donor and
  the other an electron acceptor
  – Donor – acceptor form separate,
    alternating stacks
  – Electron transfer take place
  – conducting behavior
TCNQ, a    Chloroanil, a
 electron     electron
 acceptor     acceptor
               TTF, a  electron
                     donor




Paraphenyle
nediamine,
a  electron
   donor


                      BEDT-TTF, a 
                      electron donor
        Superconductivity
• At the end of 1986, superconductivity oxide
  La2-xBaxCuO4-x and YBa2Cu3O7 were
  discovered.
• YBa2Cu3O7 can be superconducting at
  Tc=92 K, which is easily achieved by liquid
  N2.
• Superconductor are characterized by two
  phenomena.
  – Zero resistivity
  – Perfect diamagnetic.
Superconductivity
- phenomenon 1,
 zero resistance
     The properties of zero
          resistance
• Superconductor are zero resistance to the
  flow of electrical current below Tc (90 K,
  for YBaCuO)
• Above Tc (92 K, for YBaCuO), materials
  resistance gradually rises with increasing
  temperature and is normal metallic state.
• Resistance is from electron-phono
  collisions
     The properties of zero
          resistance
• Superconductivity could be explained by
  BCS theory but need modification.
• A loose associated electron pairs (Cooper
  pairs) more cooperatively through the
  lattice in such a way that electron-phonon
  collisions are avoided.
• More works need to be done to
  understand ceramic superconductors.
What is diamagnetic ?
Phenomenon 2, perfect diamagnetic
    Perfect Diamagnetic :
     The Meissner Effect
• Superconductor exhibits “perfect
  diamagnetism” and expel a magnetic
  field (< Hc)
• The is called the Meissner effect.
• Fig. 7.5 (a) ~ (f)
  Critical temeprature Tc, critical
magnetic field Hc, and critical current
  density Jc for superconductivity

• The superconductivity is lost when
  either following happened:
  – Heating above Tc
  – Appling the magnetic field higher than
    Hc.
  – Increasing the electrical current beyond
    Jc.
  – These are called critical temperature,
    critical magnetic field, and critical
    current density.
        High-temperature
        superconductor –
     Ceramic superconductor
• All high-temperature superconductor are
  ceramics
  – Challenges to produce wires, tapes….
• Four categories of ceramics:
  – YBa2Cu3O7    93 K
  – Bi2Sr2Ca2Cu3O10   110 K
  – HgBa2Ca2Cu3O10    134 K
  – Tl2Ba2Ca2Cu3O10     125 K
• Under high pressure, Tc might increase to higher
  temperature.
      Type I, and Type II
       superconductors
• Type I superconductor
  – With increasing H or T, an abrupt
    change from a superconducting to a
    non-superconducting state occurs.
• Type II superconductor
  – There is a transition state, so called
    vortex state, or mixed state, between
    superconductor and normal metallic
    region.
    Type II superconductor
• In vortex (mixed) state
  – Magnetic line are bunched together through
    vortex regions.
  – No lateral displacement
  – Levitation , non-contact vehicle is possible.
定子為超導材,轉子為永磁.轉子利用超導材料與永久磁鐵間
之作用力,懸浮, 旋轉於空氣中. 高效率,壽命長,低維修
轉速可達520,000rpm, 振幅僅5μm.
Levitation Car or Train
Superconducting wires
         Applications of
        superconductors
• Zero electrical resistance  power
  transmission over long distances

• Perfect diamagnetism  SQUID,
  Levitation for transportation.
Semiconductivity
Ionic Conductivity

								
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