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Dielectric Materials
1. Dielectric Polarization
2. Polarization Charge Density
3. Permittivity
4. Electric Field in Dielectric Materials
5. Dielectric Materials and Applications
2
Dielectric polarization
- Negative/positive charges oriented toward the positive/negative electrode
- Charges do not move freely in the material but reoritented.
1. Dielectric Polarization
Fig. Polarization in materials [www.pcbdirectory.com]
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Polarization mechanisms in dielectric materials
- Electron displacements
- Ion displacements
- Dipole orientation
- Space (interfacial) charge displacements
Fig. Polarization mechanisms [Knowles]
4
Electronic polarization:
- Elecrons are shifted by external E-field force.
- Infinite dipoles (+ charge, − charge separated to a distance) are generated
in the material.
(dipole moment)
: polarizability
q
p d
p E
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Molecular polarization:
- Some materials have permanent dipole moments. They are called polar molecules.
(dipole moment)
(torque at the dipole center)
( )
(torque at points away from the dipole center)
(energy stored in the dipole)
q
U
p d
N p E
F p E
N p E r F
p E
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Ionic polarization:
- If the molecule contains ionic bonds, then the E-fields stretches the lengths of
these bonds.
- The effect of this change in length is to produce a net dipole moment in the
unit cell.
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Polarization charge density
- The effect of polarization is modelled using charge densities.
- Polarization charges cannot move around. → Bound charges
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20
0 0
polarization dipole moment per unit volume
ˆ1 (single dipole)
4
ˆ1 (distributed dipoles)
4
1 1 ( )
4 4
ˆ (polarized bound surface charge density)
V
S V
b
VR
V dVR
dV dV
R R
P
p R
P R
P S P
P n
(polarized bound volume charge density)b P
2. Polarization Charge Density
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Electric flux density in dielectric materials
0
0
0
(Gauss's law)
( )
(electric flux density)
b f
b f f
f
f
E P
E P
D E P
D
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Permittivity of dielectric materials
0 0 0 0
0
0
(1 )
(1 ) (permittivity)
1 (dielectric constant or relative permittivity)
e e
e
r e
D E P E E E E
3. Permittivity
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Example: a dielectric slab with uniform polarization
0
0
, on the top surface
, on the bottom surface
0 since is uniform.
/ , inside the slab
0, outside the slabe
0 everywhere
b
s
b
b
P
P
P P
PE
D E P
3. Electric Field in Dielectric Materials
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Example: dielectric in a parallel-plate capacitor
0
0| | | | Electric flux density is increased in the dielectric.
E E
D D
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Example: dielectric in an external field
0
00
0
(continuity of electric flux; Gauss's law)
| | | |
D D
E E
E E
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Example: a point charge above a dielectric interface
1 2 1 1 2 2
tan1 tan 2 2 1 2 1
1 1 2
2 1
( , , ) : cylindrical coordiantes
,
1, 0
4
1, 0
4
n n z z
x x y y
s z
D D E E
E E E E E E
q qz
R R
qz
R
E
1 2
1 2
2
1 2
2
q q
q q
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Properties
- Dielectric polarization: high permittivity, used in energy storage applications
(capacitor, supercapacitors)
- Small material loss: high-frequency electromagnetic materials (circuit boards,
radomes, antennas, waveguides)
- High resistivity: used as insulators
- High dielectric strength: used as insulators
4. Dielectric Materials and Applications
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Fig. Dielectric material applications thrusts [PennState Material Research Institute]
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Fig. Dielectric materials with special effects [Kemet]
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Dielectric material types
1) Ferroelectric materials (강유전체): 커패시터재료
- Have spontaneous electric polarization which can be reversed by external E-field
- Have very large dielectric constant
- All ferroelectric materials are pyroelectric.
- Have hysteresis effect
- Applications: ferroelectric capacitors, tunable capacitors, ferroelectric RAM
- Materials
Oragnic polymers: PVDF ferroelectric polymers, flexible electronics
Perovskite oxide ferroelectrics: PZT, BaTiO3, LiNbO3, sensors, actuators, SAW
devices
Hydrogen bonded radicals: KDP, TGS, Rochelle salt
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- Dielectric constant of ferroelectric materials
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- Ferroelectric hysteresis
- Hysteresis power loss per unit volume per cycle
HV
P PdE
Fig. Ferroelectric hysteresis curve [Cadence]
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2) Piezoelectric materials: 담배, 가스레인지라이터
- Electric charge accumulates by applied mechanical pressure
- Applications:
High voltage and power sources
Sensors
Actuators
Frequency standard
- Materials:
Crystalline materials: quartz
Ceramics
III-V and II-VI semiconductors
Polymers
Fig. Piezoelectricity [Electronic Design]
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Pyroelectric materials: 적외선센서
- When heated or cooled, they can generate a voltage by lectric polarization
- Applications: heat sensors, power generation, nuclear fusion
- Pyroelectric material types
Pyroelectric infrared sensors: motion, food, flame sensors
Fig. Pyroelectric effect [R. Kishore, Materials 11(8)]
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Thermoelectric materials: 전자식무소음냉장고
- Electric field generated with temperature change across the material
- Seebeck effect, Peltier effect, Thomson effect
- Applications: power generation, refrigeration
- Material types: bismuth telluride (Bi2Te3)
Fig. Thermoelectric effect [IDTechEx]
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Insulator: 전기절연체
- Electron densiy much smaller than conductors: ne = 106 /m3
- Current does not flow in insulators
- Resistivity: very high
- Dielectric strength or dielectric breakdown: Emax before breakdown
20 (vacuum), 30 (air), 100 (porcelain), 1000 (glass), 2000 (mica) kV/cm
- Atomic bond: shared electron pairs, like to catch electrons, no free electrons
Ionic bond: grid network with electric forces, no free electrons
- Insulating materials: ceramic (porcelain), plastic
- Power factor (= dissipation factor): accounts for power loss in dielectrics
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Solid insulator
- Uses: electrical devices, capacitors, cables
- Types: glass, polymer (PVC,DEHP, PTFE, PFA), porcelain, mica
Liquid insulator (= Liquid dielectric)
- Uses: electric power distribution equipments: transformer
- Types: mineral oil, silicone oil, synthetic esters, vegetable oils
- Dielectric strength: 30 - 90 kV/cm
Dielectic gas
- Uses: transformers, circuit breakers, switchgear, radar waveguides
- Types (dielectric strength relative to air): air, nitrogen (1.15), sulfur hexafluoride
(SF6) (3), R-114 (3.2), R-12 (2.9)
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Dielectric strength table
Material Dielectric strength (MV/m)
Air 3
Sulfur hexafluoride (SF6) 8.5–9.8
Alumina 13.4
Borosilicate glass 20–40
Silicone oil, mineral oil 10–15
Benzene 163
Polystyrene 19.7
Polyethylene 19–160
Neoprene rubber 15.7–26.7
Distilled water 65–70
High vacuum (200 μPa) (field emission limited) 20–40 (depends on electrode shape)
Fused silica 470–670
Waxed paper 40–60
PTFE (Teflon, extruded ) 19.7
PEEK (Polyether ether ketone) 23
Mica 118
Diamond 2,000
PZT (lead zirconate titanate), a piezoelectric ceramic material 10–25
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Insulators
- Electron densiy much smaller than conductors: ne = 106 /m3
- Ceramic (porcelain), plastic
- Dielectric materials
- Dielectric breakdown: 20 (vacuum), 30 (air), 100 (porcelain), 1000 (glass)
2000 (mica) kV/cm
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