Dielectrics

Prof. V. KrishnakumarProfessor and Head

Department of PhysicsPeriyar UniversitySalem – 636 011

Introduction• Dielectric materials: high electrical resistivities, but

an efficient supporter of electrostatic fields.• Can store energy/charge.• Able to support an electrostatic field while

dissipating minimal energy in the form of heat.• The lower the dielectric loss (proportion of energy

lost as heat), the more effective is a dielectric material.

• Another consideration is the dielectric constant, the extent to which a substance concentrates the electrostatic lines of flux.

Capacitance• Two electrodes separated by a gap

define a capacitor.• When a bias is applied across the

capacitor plates, one charges positively, the other negatively.

• The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the ‘capacitance’ (C).

• The capacitance is related to the area of the plates (A), their separation (d), and the Dielectric Constant (εεo) of the dielectric between the plates

• Dielectric constant of vacuum; εo = 8.85x10-12 F/m=55.2 Me/(V*m)

d

AC o

eVm

mV

d

AQ mV

eo *

* 2*

Why does charge built up?There is generally not a built-in electric field between the plates of an unbiased capacitor.

When an electric field is applied, any charged carriers or species within the material will respond.

For a conductor or semiconductor, e- will flow to the + plate, and possibly also holes will flow to the - plate. Current is carried=no charge buildup.

For an insulator, there aren’t a significant number of free carriers. There are highly ionic species, however, but they aren’t very mobile at low temperatures. No appreciable current is carried=charge buildup.

Polarization in InsulatorsPositively charged species in insulators shift/rotate/align toward the negative electrode and negatively charged species shift/rotate/align towards the positive electrode; creating dipoles. The dipole moment density is termed the Polarization (P) and has the units of C/m2.

+

-

Electron CloudElectron Cloud

+

E

Electronic polarization, occurs in all insulators

-

+ +

+-

+ +

E

Ionic polarization occurs in all ionic solids: NaCl,

MgO…

-

- -

-

+ -

-

+

+

-

++

E

Molecular polarization, occurs in all insulating molecules;

oils, polymers, H2O…

A

q

V

pP

Electric Dipole Moment

Polarization

xqp

Dielectric Effects

Metal plates

Dielectric

d

AC

What makes e different from e0?

POLARIZATION

1

0

r

r

In electrostatics, the CONSTITUITIVE RELATION is

EP

PEED

0

0

Polarization

Susceptibility

Dielectric Effects

POLARIZATION arises from charge shifts in the material—there is a macroscopic separation of positive charge (e.g., the ions) and negative charge (e.g., the BONDING ELECTRONS).

Induced DIPOLE MOMENT

POLARIZATION is then

There are many sources of dipoles.

0xqdi Amount of charge shift

idipolesdNP

Definitions•Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium and is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. Permittivity relates therefore to a material's ability to transmit (or "permit") an electric field…The permittivity of a material is usually given relative to that of vacuum, as a relative permittivity, (also called dielectric constant in some cases)….- Wikipedia

DkDf

'r

"r

Permittivity and Permeability Definitions

•interaction of a material in the presence of an external electric field.

"'

0

rrrj

Permittivity (Dielectric Constant)

Permittivity and Permeability Definitions

•interaction of a material in the presence of an external electric field.

"'

0

rrrj

Permittivity (Dielectric Constant)

Dk

Permittivity and Permeability Definitions

•interaction of a material in the presence of an external electric field.

"'

0

rrrj

"'

0rr j

interaction of a material in the presence of an external magnetic field.

Permittivity (Dielectric Constant)

Permeability

Dk

Permittivity and Permeability Definitions

•interaction of a material in the presence of an external electric field.

"'

0

rrrj

"'

0rr j

interaction of a material in the presence of an external magnetic field.

Permittivity (Dielectric Constant)

Permeability

Dk

"'rrr j "'

rrr j

Electromagnetic Field Interaction

Electric Magnetic

Permittivity Permeability

FieldsFields

STORAGE

MUT

STORAGE

"'rrr j "'

rrr j

Electromagnetic Field Interaction

Electric Magnetic

Permittivity Permeability

FieldsFields

STORAGE

LOSS

MUT

STORAGE

LOSS

Loss Tangent

'

"

tanr

r

CycleperStoredEnergy

CycleperLostEnergy

QD

1tan

Dissipation FactorD

Quality FactorQ

r

'r

''r

Df

Relaxation Constant t

•t = Time required for 1/e of an aligned system to return to equilibrium or random state, in seconds.

cc f

2

11

11

10

100

10 100

Water at 20o C

f, GHz

most energy is lost at 1/t

'r

"r

j

s

1

)( :equation Debye

Dielectric Effects

ln(w)0

statics optical

PLO10-15 eV 30-50 meV

visibleinfrared

Major source of POLARIZATION is distortion of the bonding electrons around atoms. This leads to the normal semiconductor dielectric constant.

In POLAR materials, likeGaAs and SiC, the different charge on the A and B atoms can be polarized as well, leading to a difference between the optical and the static dielectric constants.

In Appendix C, the two values for GaAs are reversed!

Definition:

A photonic crystal is a periodic arrangementof a dielectric materialthat exhibits strong interaction with light

Piezoelectric Effect

In materials with NO REFLECTION SYMMETRY (like GaAs or many molecular species) the applied electric field produces a DISTORTION OF THE LATTICE (size change) and vice versa.

FORCE

ELECTRIC FIELD

A common piezoelectric is Poly-Vinylidene Flouride, which is used in a variety of stereo headsets. The most common is crystalline quartz used as frequency control crystals—pressure applied to the quartz has a resonance which can be used in a feedback loop to create a highly-stable oscillator—the quartz crystal oscillator.

Phase sensitive multimeter interfaced with impendence analyzer for dielectric

measurements