Topic-Dielectric Materials

Prepared By :-

RUSHIT PATEL

Introduction

• Dielectrics are the materials having electric dipole moment permantly.

• Dipole: A dipole is an entity in which equal positive and negative charges are separated by a small distance..

• DIPOLE moment (µele ):The product of magnitude of either of the charges and separation distance b/w them is called Dipole moment.µe = q . x coul – m

-q x

• All dielectrics are electrical insulators and they are mainly used to store electrical energy.

• Ex: Mica, glass, plastic, water & polar molecules

q

…

dipole

Dielectric Constant

Dielectric Constant is the ratio between the permittivity of the medium to the permittivity of free space.

The characteristics of a dielectric material are determined by the dielectric constant and it has no units.

Electric Polarization

The process of producing electric dipoles by an electric field is called polarization in dielectrics.

Polarizability:

• The induced dipole moment per unit electric field is called Polarizability.

• The induced dipole moment is proportional to the intensity of the electric field.

• Is a Polarizability constant

Polarization vector:The dipole moment per unit volume of the dielectric material is called polarization vector.

• Electric flux Density (D):• It is defined as the flux passing per unit area of a

section , held normal to the direction of flux.

• Electric flux density D at a point in a free space or air in terms of Electric field strength is

• At the same point in a medium is given by

Electric susceptibility:The polarization vector P is proportional to the total electric flux density and direction of electric field.Therefore the polarization vector can be written

TYPES OF DIELECTRIC MATERIAL

• Dielectric material can be solid, liquid or gas. • High vacuum can also be used as a dielectric. • Solid dielectrics are most commonly use like glass,

rubber, mica etc.. • As a liquid dielectric material Transformer oil, cable oil, Capacitor oil, Vegetable oil etc can be used.• Gaseous dielectric materials are used for both as

insulators and also as a cooling agents. For example: Air, Hydrogen, nitrogen, Helium,

Sulphur- dioxide, Propen, methane etc..

1) Solid Dielectric Materials

I) Mica• It is inorganic mineral material made up of

silicate of aluminium with silicate of soda, potash and magnesia.

• It is rigid, tough and strong. It has high dielectric strength and is not affected by moisture.

• It is widely used in irons, hot plates and toasters.

II) Glass• It is inorganic material made by the fusion of

different oxides like SiO2, ZnO and MgO.

• It is Brittle and hard material and has good dielectric strength

• It is mostly used in the capacitors. Also used as dielectric tubes in radios and television.

III) Asbestos• It is naturally occurring material. In general it consist of magnesium silicate.

• It has low dielectric strength. It is used as insulating material to prevent current flow in the outer body.

• It is widely used in the form of the paper, tap, cloth etc.

IV) Rubber• It is organic polymer. It may be natural or

synthetic.

• It has good electrical and thermal properties and also it has good tensile strength.

• It is used for the insulating materials on electrical wires.

V) Ceramics• They are generally non-metalic inorganic compounds

such as silicates, aluminates, oxides, carbides, borides etc.

• Ceramics can be classified as: clay products, refractories, and glasses.

• Ceramics are hard, strong and dense. They have exellent dielectric and mechanical properties.

• They widely used as insulators in switches, plug holders etc.

• They are also used as dielectric in capacitors

2) Liquid Dielectric Materials

1. Mineral Insulating Oil• These oils are obtained from crude petroleum.

• These have good thermal stability.

• They are used in Transformers as cooling and insulating material and also in Capacitors.

• Transformer oil, cable oil and capacitor oil belong to the category of mineral insulating oil.

2.Synthetic Insulating Oil• Askarels, aroclors, sovol and savtol are a few

synthetic oils that are widely used.

• They are very much resistant to fire hazards.

• Due to longer life and safety in operating condition, these oils are used as coolants and insulators in high voltage transformers in place of Transformer oil.

3.Miscellaneous Insulating Oil

• Vaseline, vegetable oils and silicon liquid belongs to these category. Silicon liquids has thermal stability upto 200 C and are very costly.

• The dielectric strength of these oils are same as mineral oils so they are also used in the H.V transformers.

3) Gaseous Dielectric Material

1.Air• It is naturally available dielectric material.

• Dielectric loss is practically zero.

• The dielectric constant of air linearly increase with increase in pressure.

• It is used as dielectrics in air condensers.

• It can be used as an insulator only in low voltage applications.

2.Nitrogen

• It is important gaseous dielectric material. It prevent oxidation.

• It is used in cables and capacitors under pressure.

3.Sulphure Hexafluoride• It is formed by burning of Sulphure in

fluorine atmosphere.

• It has superior cooling properties than air and nitrogen.

• It is used in the transformers, electrical switches, voltage stabilizer and X-ray apparatus.

4.Inert Gases

• They are used in electronic tubes and discharge tubes as insulators.

Properties of Good Dielectric Material

It should have high resistivity to reduce the leakage current.

It should have high dielectric strength.

It should have high mechanical strength.

It should have high fire resistance.

It should have low thermal expansion.

It should have high thermal conductivity.

It should have low dielectric loss.

It should have low water absorption quality.

Applications of Dielectrics

Capacitors

2.Transformers

3.Polymeric film

4.Electrolytic