1

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]

3

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

5

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

6

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.

7

Polarization charge density

- The effect of polarization is modelled using charge densities.

- Polarization charges cannot move around. → Bound charges

20

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

8

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

9

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

10

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

11

Example: dielectric in a parallel-plate capacitor

0

0| | | | Electric flux density is increased in the dielectric.

E E

D D

12

Example: dielectric in an external field

0

00

0

(continuity of electric flux; Gauss's law)

| | | |

D D

E E

E E

13

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

14

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

15

Fig. Dielectric material applications thrusts [PennState Material Research Institute]

16

Fig. Dielectric materials with special effects [Kemet]

17

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

18

- Dielectric constant of ferroelectric materials

19

- Ferroelectric hysteresis

- Hysteresis power loss per unit volume per cycle

HV

P PdE

Fig. Ferroelectric hysteresis curve [Cadence]

20

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]

21

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)]

22

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]

23

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

24

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)

25

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

26

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

27