Chapter 7 Electrical Properties Hong

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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 
electron
acceptor
Chloroanil, a
 electron
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.
Applications of
superconductors
• Zero electrical resistance  power
transmission over long distances
• Perfect diamagnetism  SQUID,
Levitation for transportation.
Levitation Car or Train
Superconducting wires
7.4 Semiconductivity
• Conductivity of semiconductivity is
given by :

 = n•e•
Number of
electron
Mobility
Charge
Semiconductivity
n = no exp (-E/kT)
Depend on
the dopant
level
Ionic Conductivity
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