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In practice, most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines.
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Introduction • Dielectrics are the materials having electric dipole moment permantly. • Dipole: A dipole is an entity in which equal positive andnegative • charges are separated by a smalldistance.. DIPOLE moment(µele):The product of either of the charges and the separation between them is called electric dipole moment. µe = q.x coul –m q -q • x • All dielectrics are electrical insulators and they are mainly usedto • store electricalenergy. • Ex: Mica, glass, plastic, water & polarmolecules
Dielectric constant • Dielectric constant is the ratio between the permittivity of the medium to the permittivity of the free space. • The characteristics of the dielectric material are determined by the dielectric constant and it has no units.
ElectricPolarization • The process of producing electric dipoles by an electric field is called polarization indielectrics. • Polarizability: • The induced dipole moment per unit electric field is called Polarizability. • The induced dipole moment is proportional to the intensityofthe electricfield. • Polarizability is aconstant
Polarizationvector: The dipole moment per unit volume of the dielectric material is called polarizationvector.
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
Electricsusceptibility: The polarization vector P is proportional to the total electric flux density and direction of electricfield. Thereforethe polarization vector can bewritten
TYPESOFDIELECTRICMATERIAL • Dielectric material can be solid, liquid orgas. • High vacuum can also be used as adielectric. • Solid dielectrics are most commonly use like glass, rubber, micaetc.. • As a liquid dielectric material Transformer oil, cable oil, Capacitor oil, Vegetable oil etc can beused. • Gaseous dielectric materials are used for bothas insulatorsand • also as a coolingagents. • For example: Air, Hydrogen, nitrogen, Helium, Sulphur- dioxide, Propen, methaneetc..
I)Mica • It is inorganic mineral material made up of silicate of aluminium with silicate of soda, potash andmagnesia. • It is rigid, tough and strong. It has high dielectric strength and is not affected by moisture. • It is widely used in irons, hot platesand toasters.
II)Glass • It is inorganic material made by the fusion of different oxides like SiO2, ZnO andMgO. • It is Brittle and hard material and hasgood dielectricstrength • It is mostly used in the capacitors. Also used as dielectric tubes in radios andtelevision.
III)Asbestos • It is naturally occurring material. In general it consist of magnesiumsilicate. • It has low dielectric strength. It is used as insulating material to prevent current flow in the outerbody. • It is widely used in the form of the paper, tap, clothetc.
IV)Rubber • It is organicpolymer.Itmaybenaturalor synthetic. • It has good electrical and thermal properties and also it has good tensilestrength. • It is used for the insulatingmaterialson electricalwires.
V)Ceramics • They aregenerallynon-metalic inorganic compounds such as silicates, aluminates, oxides, carbides, borides etc. • Ceramics can be classifiedas: • clay products, refractories, andglasses. • Ceramics are hard, strong and dense. They have exellent dielectric and mechanicalproperties. • They widely used as insulators in switches, plug holders etc. • They are also used as dielectric incapacitors
1. Mineral InsulatingOil • These oils are obtained from crude petroleum. • These have good thermal stability. • They are used in Transformers as coolingand insulating material and also inCapacitors. • Transformer oil, cable oil and capacitor oil belong to the category of mineral insulating oil.
2.Synthetic InsulatingOil • Askarels, aroclors, sovol and savtol are a few synthetic oils that are widelyused. • They are very much resistant to firehazards. • Due to longer life and safety in operating condition, these oils are used as coolants and insulators in high voltage transformers in place of Transformeroil.
3.Miscellaneous InsulatingOil • 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 inthe • H.Vtransformers.
1.Air • It is naturally available dielectricmaterial. • Dielectric loss is practically zero. • The dielectric constant of air linearly increase with increase in pressure. • It is used as dielectrics in aircondensers. • It can be used as an insulator only in low voltage applications.
2.Nitrogen • It is important gaseous dielectric material.It preventoxidation. • It is used in cables and capacitorsunder pressure.
3.Sulphure Hexafluoride • It is formed byburning of Sulphure in fluorineatmosphere. • It has superior cooling properties than air andnitrogen. • It is used in the transformers, electrical switches, voltage stabilizer and X-ray apparatus.
4.InertGases • They are used inelectronictubes, discharge tubes and insulators.
Properties of Good DielectricMaterial • It should have high resistivity to reduce the leakagecurrent. • It should have high dielectricstrength. • It should have high mechanicalstrength. • It should have high fireresistance. • It should have low thermalexpansion. • It should have high thermalconductivity. • It should have low dielectricloss. • It should have low water absorptionquality.
Applicationsof Dielectrics
