HIGH DIELECTRIC STRENGTH
An atom consist of a nucleus containing protons and neutrons surrounded by electrons moving in well defined orbits. The arrangement of electrons follows a certain pattern which can be explained on the basis of quantum mechanics. The electrons rotate around the nucleus in different energy level or orbits. An electron acquires energy from an applied field only if it can jump to a level of higher energy, not already occupied by an electron as in case of unfilled band or a band overlapping with an empty one. In such a case material behaves like a conductor. If all the bands of material are filled or completely empty , the electrons can acquire energy only by jumping the energy gap between the bands. When this energy gap is very large and the energy required to jump is very high the material behaves like insulator. Insulators are also known as dielectrics.
In the conductor, the filled and empty bands overlap and very little energy is required for electrons to move to the conductor band. In the case of semi-conductor there is a small gap between the filled and empty bands. Hence certain excitation is necessary for the conductor to jump to the conduction band. Thus some of the semi-conductors start behaving like conductors at high temperature. When the energy gap is 7 eV or more the material behaves like dielectric. In these materials the passage of electrons to the conduction band is rare and occurs because of defects in the structure or due to appearance of free moving ions following dissociation of molecules of the dielectric under the action of electric field.
When a material is subjected to electrical, thermal and other stresses and the subjected energy level is sufficiently high, the electrons jump to conductivity band and the conductivity of the material increases & insulation resistance decreases. This is an irreversible process beyond a certain limit. We call it dielectric breakdown.
Another aspect of structure is the number and behaviour of the electrons present in the outermost orbit of an atom. In cases other than inert gases the atom acquire stability by attracting or donating electrons with the another atom or share with adjacent atom as in the case of the homopolar bond. Due to high strength of homopolar bond large energy is required to dissociate these bonds.
Substitutions and attachments have varied effect on bond strength. When the material is subjected to stresses such as electrical, thermal, mechanical etc. and the subjected energy levels is higher than the bond energy the bond tends to rupture and degradation occurs along with loss of weight, failures flexibility decrease in dielectric strength and insulation resistance and ultimately the dielectric breaks.
Dielectric strength is defined as the voltage gradient at which dielectric material fails. As a test value it depends on the geometry of electrodes, thickness of the insulation. Dielectric strength is influenced by imperfections and chemical non homogeneity within the insulation itself.
