Electron emission from metal surface ( thermionic emission)

What is Electron Emission ?

The Liberation of electron from the surface of a substance is known as electron emission.
For electron emission metals are used because they have many free electrons. If a piece of metal is investigated at room temperature the random motion of electron. However this electrons are free only to the extent that they may transfer from one account to another within the matter but they cannot leave the metals her face to provide electron emission. It is because the free electron that start at the surface of metal find behind them positive nuclei pulling them back and none pulling forward. Thus at the surface of a metal, a free electron encounter forces that prevented to leave the metal. In other word the metallic surface offers a barrier to free electron and is known as surface barrier.
 However is sufficient external energy is given to the free electrons its kinetic energy increases and thus electron will cross over the surface area to leave the metal. This additional energy required by an electron to overcome the surface area of the metal is called work function of the metal.

Types of Electron Emission 

The electron emission from the surface of a metal is possible if sufficient additional energy is supplied from some external source.  This external energy may come from a varieties of sources of heat energy, energy stored in electric field, light energy, kinetic energy of the electric charges bombarding the metal surface. Accordingly,  there are following four principal methods of obtaining electron emission from the surface of a metal :-

(1) Thermionic emission = In this method the metal is heated to sufficient temperature( about 2500°C) to enable the free electrons to leave the metal surface. The number of electrons emitter depends upon the temperature. The higher the temperature, the greater is the emission of electrons. This type of emission is employed in vacuum tubes.

(2) Field emission = In this method is strong electric field is applied at the metal surface which pulls the free electrons out of metal because of attraction of positive field. The stronger the electric field, the greater in the electric emission.

(3) Photoelectric effect = In this method the energy of light falling on the metal surface is transferred to the free electron within the metal to enable them to leave the surface. The greater the intensity of light beam falling on the metal surface, the greater is the photo-electric emission.

(4) Secondary emission = In this method a high velocity beam of electrons strike the metal surface and causes the free electrons of the metal to be knocked out from the surface.

Thermionic Emission :-

Credit to - Electric4U.com

The process of electron emission from a metal surface by supplying thermal energy to it is known as thermionic emission
At ordinary temperature the energy possessed by free electron in the metal is inadequate to cause them to escape from the surface. When heat is applied to the metal, some of heat energy is converted into kinetic energy, causing accelerated motion of electrons. When the temperature rises sufficiently, this electrons acquire additional energy equal to the work function of the metals.
Consequently, they overcome the restraining surface barrier and leave the metalsurface.
 Metals with lower work function will require less additional energy and therefore, will emit electrons at lower temperatures. The commonly used material for electron emission are tungsten, thoriated tungsten and metallic oxides of barium and strontium. It may be added here that high temperature are necessary to cause therma emission. For example, pure tungsten must be heated to about 2300°C  to get electron emission. However oxide coated emitters need only 750°C to cause thermionic emission.

Thermionic Emitter :-

Thermionic emitter  (vaccum diode)
The substance used for electron  emission is known as an emitter or cathode. The cathode is heated in an evacuated space to emit electrons. If the cathode were heated to the required temperature in open air, it would burn up because of the presence of oxygen in the air. A cathode should have the following

• Low work function = The sabstance selected as cathode should have low work function so that electron emission takes place by applying small amount of heat energy  i.e. at low temperatures.

  •High melting point = As electron emission takes place at very high temperatures (I500°C), therefore, the substance used as a cothode should have high melting point. For a material such as copper which has the advantage of a low work function, it is seen that it cannot be used  as a cathode because it melts at 810°C. Consequently, it will vaporise before it begins to emit electrons.

  •High mechanical strength = The emitter should have high mechanical strength to withstand the bombardment of positive ions. In any vacuum tube, no matter how careful the evacuation, there are always present some gas molecules which may form ions by impact with electrons when current flows. Under the influence of electric field, the positive ions strike the cathode. If high voltages are used, the cathode is subjected to considerable bomburdment and may be damaged.

Post a Comment