Dielectric Material, Properties & Applications

Dielectric Material

1. Unit II Dielectric Materials

2. What is a Capacitor? A capacitor is a two-terminal electrical device that possesses the ability to store energy in the form of an electric charge. It consists of two electrical conductors that are separated by a distance. The space between the conductors may be filled by vacuum or with an insulating material known as a dielectric. The ability of the capacitor to store charges is known as capacitance.

3. As V= 0 Volt. or if no voltage is applied across capacitor, net charge on conducting plates are zero. It means that capacitor plates has equal amount of positive as well as negative charge as shown in the figure V= 0 Volt

4. If finite voltage is applied across the capacitor, free electrons from one plate entered in positive terminal of the battery & plate remains with only positive charges. Also electrons from negative terminal of battery combines with positive charges on another plate remains only negative charges. At the end one plate consist only positive charges & another plate consist only negative charges which is called charged capacitor. Due to this charges voltage developed between two plates equal to supply voltage

5. Due to the voltage across the plate there is presence of electric field between the plates. E = V/d The ability of capacitor to stored the electric charge is called as capacitance C = Q / V Its unit is Farad. If you want to store more charge on the capacitor we need to increase the voltage across it but it has certain voltage limitation. https://youtu.be/R2QQ0yiqH_U

6. Factors affecting the value of capacitor C = 𝜺.A / d Where, A = Area of the plates d = distance between two plates 𝜺 = permittivity of the dielectric material

7. How permittivity affects the value of capacitance If no voltage is applied(V = 0) across the capacitor polar molecules are aligned randomly as shown figure.

8. As finite voltage applied across the capacitor then capacitor holds some charge across it & polar molecules aligned according to charges. So alignment of polar molecules are opposite to the electric field which decrease the electric field between the plates. Which results decrease in voltage. E = V/d Which result value of capacitor increases C = Q / V

9. Dielectric Losses

10. Dielectric Properties of Polymeric Material Dielectric Constant: The dielectric constant is a measure of the influence of a particular dielectric on the capacitance of a condenser. It measures how well a material separates the plates in a capacitor and is defined as the ratio of the capacitance of a set of electrodes with the dielectric material between them to the capacitance of the same electrodes with a vacuum between them. The dielectric constant for a vacuum is 1 and for all other materials it is greater than 1.

11. Power Factor The power factor is a measure of the energy absorbed by the material as the alternating current constantly changes direction and the dipoles try to align themselves with the field. As the dipoles try to align themselves with the external field they will always be slightly out of phase and will ‘lag’ behind the field. The amount of lagging is measured by the phase angle (q) and the power factor is defined as cos q. The power factor can be thought of as a measure of the internal friction created by the alternating current and will define how much a material heats up when placed in an alternating field.

12. Dielectric Strength The dielectric strength is the DC voltage between two electrodes at which dielectric breakdown occurs and is an indicator of how good an insulator the material is. The voltage is increased until the material breaks down, there is an arc across the electrodes and substantial current flows. Most plastics have good dielectric strengths (in the order of 100 to 300 kV/cm)

13. Surface Resistivity The surface resistivity is a measure of the resistance of the material to a surface flow of current. It is the ratio of the applied direct voltage and the resulting current along the surface of the material per unit width. Surface resistivity is measured in Ω.

14. Tracking and Arc Resistance These are measures of how long a material can resist forming a continuous conduction path under a high voltage/low current arc

15. Breakdown in Gases- Townsend discharge

16. The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field , collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field ; without both, the phenomenon does not occur.

17. Intrinsic or Ionic Breakdown Intrinsic breakdown depends upon the presence of free electrons which are capable of migration through the lattice of the dielectric. Usually, a small number of conduction electrons are present in solid dielectrics, along with some structural imperfections and small amounts of impurities. The impurity atoms, or molecules or both act as traps for the conduction electrons up to certain ranges of electric fields and temperatures. When these ranges are exceeded, additional electrons in addition to trapped electrons are released, and these electrons participate in the conduction process. Based on this principle, two types of intrinsic breakdown mechanisms have been proposed.

18. Electronic Breakdown Intrinsic breakdown occurs in time of the order of 10-8 s and therefore is assumed to be electronic in nature. The initial density of conduction (free) electrons is also assumed to be large, and electron-electron collisions occur. When an electric field is applied, electrons gain energy from the electric field and cross the forbidden energy gap from the valence band to the conduction band. When this process is repeated, more and more electrons become available in the conduction band, eventually leading to breakdown.

19. Requirements / Characteristics of a Good Insulating Material 1. Large insulating resistance. 2. High dielectric strength. 3. Uniform viscosity—it gives uniform electrical and thermal properties. 4. Least thermal expansion. 5. When exposed to arcing should be non-ignitable. 6. Should be resistance to oils or liquids, gas fumes, acids and alkalis . 7. Should have no deteriorating effect on the material, in contact with it.

20. 10. High mechanical strength. 11. High thermal conductivity. 12. Low permittivity. 13. High thermal strength. 14. Should be homogeneous to avoid local stress concentration. 15. Should be resistant to thermal and chemical deterioration.

21. Dielectric Materials - Mica chemically inert dielectric elastic flexible hydrophilic insulating lightweight Properties Applications electrical insulators thermal insulation gauge “glass” windows in stove and kerosene heaters dielectrics in capacitors insulation in motors and generator armatures field coil insulation

22. Ferroelectricity Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field.

23. If we take a ferroelectric material, and an electric field is given to it. We get a nonlinear polarization. It also exhibits nonzero spontaneous polarization without a peripheral field. We can also see that by inverting the direction of the applied electrical field, the direction of polarization can be inverted or changed.Thus, we can say that the polarization will depend on the present and the previous condition of the electric field. The hysteresis loop is obtained as in figure

24. Ferroelectric materials: BaTiO3 PbTiO3 Lead Zirconate Titanate (PZT) Triglycine Sulphate PVDF

25. Application of Ferroelectric Materials Thermistors Oscillators Non-volatile memory Filters Capacitors Light deflectors Transchargers Electro-optic materials Modulators Piezoelectrics Display etc

26. Piezoelectricity Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. The word Piezoelectric is derived from the Greek piezein, which means to squeeze or press, and piezo, which is Greek for “push”.

27. One of the unique characteristics of the piezoelectric effect is that it is reversible, meaning that materials exhibiting the direct piezoelectric effect (the generation of electricity when stress is applied) also exhibit the converse piezoelectric effect (the generation of stress when an electric field is applied).

28. Piezoelectric Material Barium Titanate. Lead zirconate titanate (PZT). Rochelle salt.

29. Piezoelectric Applications In microphones, the sound pressure is converted into an electric signal and this signal is ultimately amplified to produce a louder sound. Automobile seat belts lock in response to a rapid deceleration is also done using a piezoelectric material. It is also used in medical diagnostics. It is used in electric lighter used in kitchens. They are used for studying high-speed shock waves and blast waves. Used infertility treatment. Used in Inkjet printers