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Saturday, 20 July 2019

Classification of Materials based on Energy Band Diagram

Classification of Materials based on Energy Level Concept :


Figure shows the energy band diagram of conductor, semi-conductor and insulator. In the case of insulator the forbidden zone between the valence band and conduction band is very large due to which it needs a large amount of additional energy for the electrons to become free so that they can move towards the conduction band. In the case of conductor, no forbidden gap exists between the valence band and conduction band and therefore electrons from the valence band can be moved easily into the conduction band even with a small applied energy.
Energy Band Diagram for Conductor, Insulator and Semi Conductor

Comparision of Conducting Materials :

Conducting materials having less than four electrons in the last orbit and its forbidden band is negligible or zero. Consider the three distinct elements whose orbital number vary. For example Gold. Silver and Copper. Their shell configuration is as given in Table.

Shell configuration of Gold, Silver and Copper


Shell

K
L
M
N
O
P
Gold
2
8
18
32
18
1
Silver
2
8
18
18
1

Copper
2
8
18
1




Even though in the last orbit the number of electrons is one, the energy level to detach the electrons from the last orbit is less for Gold than silver or copper since the bondage is less as the orbit level increases. As such Gold is a better element as a conductor than silver. In a similar way the energy required to detach the electron from the last orbit in silver is less than copper as the valence electron occupies the 'O' shell in silver and 'N' shell in copper. The higher is the energy level to detach the electron from the last orbit, higher is its resistivity and lower is its conductivity. This is the basis of grading any conducting material.



In conducting materials the free electrons move about with random velocity. The magnitude of velocity depends upon its temperature. When an electric field is applied to a conductor, the electrons acquire a systematic velocity. In the absence of electric field, the resultant drift current is zero. 

General Classification of Electrical Engineering Materials :

Based on the energy level concept, the materials are classified into three basic categories. viz. conductor, insulator and semi-conductor. Some of the conducting material do act as magnetic material. Similarly the insulators can also be used as Dielectric materials. With the applications in mind, the materials can be generally classified into :

(a) Conducting material : In which the outermost orbit has less than four valence electrons. The resistivity of such material is directly related to the total energy required to detach the electron. The energy required to detach is higher, the resistivity is also higher and vice-versa. Materials like silver, copper, aluminium, brass, carbon etc., are conducting materials.

(b) Magnetic material : Some of the conducting materials have the properly to get magnetised or are attracted by a magnet. The materials provide a path for magnetic flux for example Iron, Cobalt, Steel etc., are magnetic materials. Many alloys like Cobalt steel, Cadmium steel etc., have useful magnetic properties. Besides forming a medium for energy conversions in static and dynamic equipments, they are useful in making magnetic circuits in relays and transformers etc.

(c) Insulators : These materials have more than four valence electrons in the outermost orbit and offer high resistance to the flow of electric current. The resistivity ranges from 10^12 to 10^18 ohm - m and may be in the form of solids, liquid or gaseous state. They are used at places where a charged conductor has to be isolated as in the case of electric iron element, where nichrome wire is isolated with the use of mica. Some of these materials can also be used as Dielectric material.

(d) Dielectric material : When an insulating material is used to store electric energy, they are called dielectric materials. They can be defined as those insulating material in which an electric field can be sustained with a minimum dissipation of power. Mica, oil, paper etc:,  fall under dielectric material.

(e) Semi-conducting material : Semi-conducting materials have four valence electrons in the outer-most orbit and are neither a conducting material nor insulating material. But act in a peculiar way. It is a solid crystalline material having medium resistivity of l to 100 Ω – m. These materials have negative temperature coefficient over a part of the temperature range. Germanium, Silicon, etc., fail under this category and are used in electronic circuits as capacitors, rectifiers etc.

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