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Dielectric Materials: Properties and Applications

Content 1. Dielectrics : Properties

2. Fundamental definitions and Properties of electric dipole

3. Various polarization mechanisms involved in dielectric : 3.1 Electronic polarization, 3.2 Ionic polarization, 3.3 Orientation polarization, 3.4 Space charge polarization; 3.5 Total polarization

4. Active and Passive Dielectrics

5. Frequency and Temperature on Polarization of Dielectrics : 5.1 Frequency Dependence, 5.2 Temperature Dependence,

6. Internal field or Local field : 6.1 Definition, 6.2 Derivation, 6.3 Clausius – Mosoti Equation

7. Dielectrics and Loss Tangent; 7.1 Loss in purified gas; 7.2 Loss in commercial dielectric ; 7.3 Power loss

8. Dielectric Breakdown: 8.1 Types of dielectric breakdown; 8.2 Remedies for breakdown mechanisms

9. General Applications

10. Applications of dielectric materials : 10.1 Dielectrics in capacitors, 10.2 Insulating materials in transformers

11. Ferro-electrics : 11.1 Properties, 11.2 Applications

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Understanding Dielectric:

• Solids which have an energy gap of 3eV or more are termed as insulators.

• In these materials, it is almost not possible to excite the electrons from the valence band to conduction band by an applied field.

• Generally dielectrics are also called as insulators, thereby poor conductors of electricity. However they allow movement of some electrons at abnormally high temperatures, causing a small flow of current.

• Dielectrics are non-metallic materials of high specific resistance ρ, negative temperature coefficient of resistance (-α), large insulation resistance.

• Insulation resistance will be affected by moisture, temperature, applied electric field and age of dielectrics.

Understanding Dielectric:

• Dielectric materials are electrically non-conducting materials such as glass, ebonite, mica, rubber, wood and paper.

• All dielectric materials are insulating materials.

• The difference between a dielectric and an insulator lies in their applications.

• If the main function of non-conducting material is to provide electrical insulation, then they are called as insulator. On the other hand, if the main function of non-conducting material is to store electrical charges then they are called as dielectrics.

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Properties

• Generally, the dielectrics are non-metallic materials of high resistivity.

• They have a very large energy gap (more than 3eV).

• All the electrons in the dielectrics are tightly bound to their parent nucleus.

• As there are no free electrons to carry the current, the electrical conductivity of dielectrics is very low.

• They have negative temperature coefficient of resistance and high insulation resistance.

Fundamental Definitions And Properties Electric Dipole

• A system consisting of two equal and opposite charges n(+q, -q) separated by a distance (d) is called an electric dipole.

DIPOLE MOMENT (P) • The product of the magnitude of the charge (q) and distance

between two charges (d) is called as dipole moment.

• Dipole moment P = qd (coulomb-metre)

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Fundamental Definitions And Properties Electric Dipole

PERMITTIVITY (ε) • The permittivity represents the dielectric property of a medium. It

indicates easily polarizable nature of material. Its unit is farad/metre

DIELECTRIC CONSTANT (εr ) • A dielectric characteristic of a material is determined by its

dielectric constant. It is a measure of polarisation of the dielectrics. Definition • It is the ratio between absolute permittivity of the medium (ε) and

permittivity of free space (εo). Dielectric constant = Absolute permittivity (ε) / Permittivity of free space (εo )

εr = ε / εo

Fundamental Definitions And Properties Electric Dipole

POLARIZATION Definition • The process of producing electric dipoles inside the dielectric by the

application of an external electrical field is called polarization in dielectrics.

POLARISABILITY (α) It is found that the average dipole moment field (E).

μ = α E

Where (α) is the polarisability.

α = μ / E

Polarisability is defined as the ratio of average dipole moment to the electrical field applied. Its unit is farad m2 .

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Active and Passive Dielectrics

The dielectric materials can be classified into active and passive dielectric materials. i. Active dielectrics When a dielectric material is kept in an external electric field, if it actively accepts the electricity, then it is known as active dielectric material. Thus, active dielectrics are the dielectrics, which can easily adapt themselves to store the electrical energy in it. ii. Passive dielectrics Passive dielectrics are the dielectrics, which restrict the flow of electrical energy in them. So, these dielectrics act as insulators. Examples: All insulating materials such as glass, mica, rubber etc.,

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Basically, there are four mechanisms of polarization:

Electronic or Atomic Polarization

This involves the separation of the centre of the electron cloud around an atom with respect to the centre of its nucleus under the application of electric field (see (a) in figure below).

Ionic Polarization

This happens in solids with ionic bonding which automatically have dipoles but which get

cancelled due to symmetry of the crystals. Here, external field leads to small displacement of ions from their equilibrium positions and hence inducing a net dipole moment (see (b)).

Dipolar or Orientation Polarization

This is primarily due to orientation of molecular dipoles in the direction of applied field which would otherwise be randomly distributed due to thermal randomization (see (c and d)) and finally

Interface or Space Charge Polarization

This involves limited movement of charges resulting in alignment of charge dipoles under

applied field. This usually happens at the grain boundaries or any other interface such as

electrode-material interface (see (e and f))

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Dielectrics and Loss Tangent

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Ferroelectric –

A material that shows spontaneous

and reversible dielectric polarization.

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A material that develops voltage upon the application of a stress and develops strain when an electric field is applied.

Piezoelectric –

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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

The (a) direct and (b) converse piezoelectric effect. In the direct piezoelectric effect, applied stress causes a voltage to appear. In the converse effect (b), an applied voltage leads to development of strain.

Direct piezoelectric effect

Reverse (converse) piezoelectric effect

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E

E

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P = d

P = d

od

d: Piezoelectric coupling coefficient (piezoelectric charge coefficient)

Direct piezoelectric effect

Table: The piezoelectric constant d (longitudinal)

for selected materials

Material

Piezoelectric constant d

(C/N = m/V)

Quartz 2.3 x 10-12

BaTiO3 100 x 10-12

PbZrTiO6 250 x 10-12

PbNb2O6 80 x 10-12

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PZT: PbZrO3-PbTiO3 solid solution or lead zirconotitanate

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Table: Properties of commercial PZT ceramics

Property PZT-5H

(soft)

PZT4

(hard)

Permittivity ( at 1 kHz) 3400 1300

Dielectric loss (tan at 1 kHz) 0.02 0.004

Curie temperature (Tc, C) 193 328

Piezoelectric coefficients (10-12 m/V)

d33 593 289

d31 -274 -123

d15 741 496

Piezoelectric coupling factors

k33 0.752 0.70

k31 -0.388 -0.334

k15 0.675 0.71

Table: Measured longitudinal piezoelectric coupling coefficient d, measured relative

dielectric constant , calculated piezoelectric voltage coefficient g and calculated voltage

change resulting from a stress change of 1 kPa for a specimen thickness of 1 cm in the

direction of polarization.

Material

d (10-13 m/V)*

†

g (10-4 m2/C)†

Voltage change

(mV)†

Cement paste

(plain)

0.659 0.031 35 2.2 2.2

Cement paste with

steel fibers and

PVA

208 16 2700 8.7 8.7

Cement paste with

carbon fibers

3.62 0.40 49 8.5 8.5

PZT 136 1024 15 15

*Averaged over the first half of the first stress cycle †At 10 kHz

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Piezopolymer

Composites with piezoelectric/ferroelectric material sandwiched by metal faceplates fo enhancing the piezoelectric coupling coefficient

Moonie

Cymbal

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The ability of a material to spontaneously polarize and produce a voltage due to changes in temperature.

,dT

d

dT

dPo

p

p = pyroelectirc coefficient P = polarization

Material p(10-6 C/m2.K)

BaTiO3 20

PZT 380

PVDF 27

Cement paste 0.002

Pyroelectric –

o 1)-(

Px = V

d

d

d

d

d

d P

)1(

xx

1)-(

P =

V

oo

Voltage sensitivity

Compliance Piezoelectric coupling coefficient d

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Piezoelectric composite

• When any material undergoes polarization (due to an applied electric field), its ions and electronic clouds are displaced, causing the development of a mechanical strain in the material. polarization.

• This phenomenon is known as the electrostriction.

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Examples of ceramic capacitors.

(a)Single-layer ceramic capacitor (disk capacitors).

(b) Multilayer ceramic capacitor (stacked ceramic layers).

Types of Dielectric Materials Dielectric materials can be divided into following groups:

• Solid Dielectrics - are of following types: – • Mica – is inorganic material and is crystalline in nature.

– • Glass – is inorganic material made by fusion of different oxides.

– • Rubber – is a organic polymer, which can be natural or artificial.

– • Ceramic – is non-metallic organic compound such as silicates

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Liquid Dielectric – includes following: • Mineral Insulating Oils – obtained from crude petroleum & have high oxidation resistance. • Synthetic Insulating oil – are very much resistant to oxidation & fire hazards. • Miscellaneous Insulating oils – Vaseline, vegetable oils, silicon oils belongs to this.

Gaseous Dielectric – includes • Air • Nitrogen • Sulphur hexafluoride • Inert gases

Types of Dielectric Materials