Ceramic insulators prevent leakage of
Paul Karl DRUDE (1863- electricity
1906), a German physicist, proposed in 1900 that tiny charged particles, now known as electrons, act as carriers of electricity across metals like copper. On applying an electric field across a material, these electrons gather speed. But as electrons move, they are obstructed by atoms present in the material.
The net obstructing effect, known as resistivity, varies from material to material. It depends upon the way the atoms are stacked up and the strengtn of links among them. Conduc- tance, or ability to conduct electricity, is the reverse of resis- tivity. That is, lesser the obstructing effect of the atoms in the material, faster is the movement of electrons.
Most traditional ceramics are poor conductors of electricity and, hence, were used originally as insulating materials. The reason for their poor conductivities lies in the nature of bonding among the atoms. The electrons, which bind the atoms, are not fixed to individual atoms in the metals. But, ceramics have ionic or covalent bonds in which electrons are tightly attached to atoms. Hence,the electrons are not as free to move about or act as carriers of charge as in metals.
No vacancy, no movement
Electrons can be thought of as individuals residing in
rooms at different floors in a multistoreyed building. The
rooms allotted to them are in accordance with the state of
their health, that is their energy. Let us say, energetic
electrons stay in rooms at the top floor of the building while
their weaker brethren are accommodated in rooms at the
ground floor. Of course, electrons can go up or down the
building on receiving or !osingenergy as and when an electric
(c) Valence
band Cond.band
(a) (b)
field is applied. A given floor in the building has rooms of similar energy and is called, in the lan- guage of scientists, a band of closely related energy states of electrons.
In a solid, a number of such bands exist, but only two of these have sig- nificant effect on electrical properties. These are the conduction band, which is highest in energy, and the
valence band, which is Energy gap between valence and
next lower in energy. The con~uction bands for a g<;>odconductor
cond u cti on band is (a), msulator (b) and semiconductor (c)
named so because electrons present in it are the source of electrical conductivity. Most of the rooms are empty on this highest energy floor providing ample opportunity to electrons for travel. The valence band, on the other hand, is fully occupied most of the time. The electrons present in this band can be transferred to or shared with other atoms. Thus, they determine the affinity of an atom towards other atoms.
This affinity is called the valency and hence the name valence band.
In good conductors, like metals, either the valence band is only partially full or the higher conduction band can be easily reached. The electrons can change rooms on the same floor or visit the next higher floor. In either case, since electrons can travel easily, the metals are good conductors.
In the case of insulators and semi-conductors, like most ceramics, the valence band is full. For conduction to occur, the electrons must reach the higher floor - the conduction band. Unfortunately, the lift between the two floors is not installed. And it requires very high energy to jump as high
as the next floor. Thus, there are practically no electrons in the conduction band to carry the electric charges. In semi- conductors le&serenergy is required to reach the conduction band and some electrons can do so. There is no lift to conduc- tion band in this case also, but there is a staircase which can be used by some of the more energetic electrons. The energy requirement for the travel of electrons from the valence band to the conduction band is very high in the case of insulators, medium in the case of semi-conductors and almost nil in the case of metals. The energy difference between the valence and conduction bands is called energy gap or band gap. It is characteristic of each material and determines its electrical conductivity - lower the band gap higher is the conduc- tivity.
Conductivities of Some Materials
Silver
Steel
Bakelite
6.3 x 107
4.25 x 107
-1
Diamond
Thus, metals have similar, very high, conductivities which are million million times greater than those of semi-conduc- tors like silicon. Typical insulators, like mica and diamond, have conductivities millions of times lower than those of semi -cond uctors.
The band gap in ceramics is quite high in accordance with their well deserved reputation as insulators. However, what is interesting is that the band gap in ceramics can be tinkered with and so also their electrical properties. This can be done by making slight changes in either their composition or crys- tal architecture, the way atoms are arranged in space. For example, normal nickel oxide is an insulator, but it can be made to conduct either by changing the nickel-to-oxygen ratio or by adding some metallic impurities. The net result is a smaller band gap. Such compounds are known as ceramic semi-conductors. On the other hand, zinc oxide is normally a semi-conductor, but doping it with certain impurities turns it into a good metal-like conductor. Further, a proper mix of insulating and conducting compositions can give ceramics that change their conducting ability in response to the ap- plied voltage.