The substance used for Electron Emission is known as an Emitter.An Emitter should have the following Properties.

01)High Melting Point:-

As Electron Emission takes place at very High Temperatures,therefore,the substance used as Emitter should have High Melting Point.

02)Low Work Function:-

The substance selected as an Emitter should have Low Work Function so that Electron Emission takes palce by applying small amount of Heat Energy.

03)High Mechanical Strength:-

The Emitter should have High Mechanical Strength to withstand the Bombardment of Positive Ions.

Important Thermionic Emitters:-

The High Temperatures needed for satisfactory Thermionic Emission in Vaccum Tubes limit the number of suitable Emitters to such substances as follows.

01)Tungsten:-

It was the earliest material used as a Cathode and has a slightly Higher Work Function.Below are the Important Factors in its Favour.

A)High Melting Point.
B)Greater Mechanical Strength.
C)Longer Life.

Besides this,Tungsten has following Disadvantages:-

A)High Operating Temperature.
B)High Work Function.
C)Low Emission Efficiency.

Tungsten can be used in application involving Voltage exceeding 5kV.

02)Thoriated Tungsten:-

A mixture of two metals may have a Lower Work Function than either of the pure metal alone.Thus,a Tungsten Emitter with a small quantity of Thorium has
a Work Function of 2.63 eV,compared with 3.4 eV for Thorium and 4.52 eV for Tungsten.

At the same time,Thoriated Tungsten provides Thermionic Emission at Lower Temperature with consequent reduction in the heating power required.
Thoriated Tungsten Emitters are used for Intermediate Power Tubes at Voltage between 500 to 5000 Volts.

03)Oxide-Coated Cathode:-

This type of Cathode consist of a Nickel ribbon coated with Barium and Strontium Oxides.The Oxide-Coated Cathode has Low Work function,operates at
comparatively Low Temperature and has Higher Emission Efficiency.

However,the principle limitation of Oxide-Coated Cathode is that it can not withstand High Voltages.Therefore,it is mostly used in Receiving Tubes or where
Voltage involved do not exceed 1000 Volts.

For more details,please visit
http://elctronicsengineering.blogspot.com/

Last 5 posts by electronicsguru

• What Are The Limitations In Operating Conditions Of Vacuum Tubes? - September 13th, 2008
• What Is Vacuum Tube Rectifires And Its Application In Electronic Devices? - September 13th, 2008
• Different types of Vaccum Tubes and its uses. - July 20th, 2008
• How you can construct Cathode? - July 16th, 2008
• Which are the Important Methods for obtaining Electron Emission? - July 15th, 2008