# Dielectric Insulator Material

Dielectric material basically is an insulating material, but it has no free electrons. When a source of e.m.f is connected across it, no current flows. The behavior of dielectric is primary electrostatic electrodynamic effects that also takes place due to their non-zero conductivity are quite insignificant in normal operating conditions. Since no dielectric is perfect it contains a small number of free electrons and a very small current called leakage current flows through it when electric field is applied. While the function of insulating material is to obstruct the flow of current, the function of dielectric material is to store electrical.

REQUIREMENTS FOR DIELECTRIC:

The dielectric material may be solid, liquid or gases. Whatever be the material, the basic function is the same i.e., to store electric charges. Since most of the dielectric materials are not perfect insulators in that they contain, few free electrons which result in leaking current. When a dielectric is placed between two parallel plates and a.c voltage is applied to the plates, if the dielectric is a perfect insulator with zero electrical conductivity an alternating electric field is set up in the dielectric. Thus in turn, sets up a flow of wattless capacitive current in the dielectric and may be found from the equation.

Ic = 2 Ï€ f CV amps

Where,
lc = capacitive current (Amps)
f = frequency (Hz)
C = Capacitance of the capacitor (Farads)
V = Applied voltage (Volts)

On putting the capacitor across AC voltage the capacitive current flows continuously till the voltage is applied, but if D.0 voltage is applied across the capacitor, the capacitive current will flow only until the electric field is set up in the dielectric. Besides the dielectric strength depends on the following factors.

(a) Thickness
(b) Temperature
(c) Applied frequency
(d) Wave form
(e) Electrode form
(f) Area
(g) Heat conductivity
(h) Surrounding medium
(i) Rate and time of application of voltage.

To increase the breakdown voltage, generally the thickness is increased. Therefore the essential requirement for a dielectric is

(a) Its dielectric strength for a given thickness should be high.
(b) The material should be able to withstand one and half times the operating temperature.
(c) It should not appreciably vary its performance for varied frequency.
(d) It should not get affected by surrounding atmosphere like moisture, acids, alkalies etc.
(e) The leakage frequency should be as least as possible.
(f) It should not breakdown due to mechanical vibrations.
(g) The material should have a uniform thickness such that spark or puncturing does not takes place.

Table gives the dielectric strength and permittivities of some of the medium.

 Material Dielectric strength Permittivity tan Î´ 1 Bakelite 22 kV / mm 6 0.02 2 Bitumen 14 kV / mm 4.5 -- 3 Cotton 4 kV / mm 5 0.2 4 Ebonite 25 kV / mm 2.8 0.01 5 Empire cloth 15 kV / mm 2 6 Fibre 5 kV / mm 6 7 Glass 8 kV / mm 6.5 0.004 8 Guttapercha 15 kV / mm 4 9 Rubber 17 kV / mm 4 0.016 10 Marble 6 kV / mm 8 11 Muscovite 95 kV / mm 6 0.0003 12 Mycalex 7 0.003 13 Dry paper 7 kV / mm 3 0.005 14 Paraffin wax 8 kV / mm 2.2 0.0003 15 Polystyrene 2.6 0.0002 16 Polythene 2.3 0.0001 17 Porcelain 15 kV / mm 6 0.002 18 Shellac 13 kV / mm 3 0.008 19 Slate 3 kV / mm 6 20 Steatite 5 0.002 21 Mineral oils 30 kV / mm 2 0.0002

Table shows the relative dielectric constant (Îµ) of dielectric material at different frequencies.

 Frequency in Hz 102 104 106 108 3 x 109 Pyranol 4.42 4.40 4.40 4.08 2.84 Cable oil 2.25 2.25 2.25 2.25 2.22 Teflon 2.1 2.1 2.1 2.1 2.1 Polystyrene 2.59 2.56 2.56 2.55 2.55 Polythylene 2.25 2.25 2.25 2.25 2.25 Bakelite 4.87 4.62 4.36 3.95 3.70 Porcelain 8.98 8.95 8.95 8.95 8.90 Nylon 3.88 3.60 3.33 3.16 3.03 Glass 6.68 6.57 6.43 6.33 6.10

Below Table shows the value of loss tangent of various dielectrics at different frequencies.

 Frequency in Hz 102 104 106 108 3 x 109 Pyranol x 10-4 36 4 25 1300 1200 Cable oil x 10-4 3 0.4 -- -- 18 Teflon x 10-4 5 3 2 2 1.5 Polystyrene x 10-4 0.5 0.5 0.7 1 3.3 Polythylene x 10-4 5 3 4 -- 3 Bakelite x 10-4 300 200 280 3 438 Porcelain x 10-4 22 6 2 4 11 Nylon x 10-4 144 233 257 210 128 Glass x 10-4 77 35 16 23 60