ONLINE ELECTRICAL ENGINEERING STUDY SITE

Thermionic Emission

All materials are composed of atoms which inturn consists of nucleus, made of protons and neutrons, surrounded by electrons. These electrons are distributed in various levels around the nucleus and thus possess different levels of energy. Now, imagine that we start heating a particular material. The thermal energy so supplied increases the kinetic energy of the electrons within the material. This causes them to overcome the force of attraction which exists between them and the protons within the respective nuclei. As a result, they get knocked-out from the material and will be liberated into space surrounding the material (Figure 1). More is the heat supplied, more are the number of electrons ejected. This phenomenon is known as thermionic emission i.e. emission of ions called thermions due to the thermal energy supplied, first observed by Thomas Alva Edison in 1883.

From the discussion presented, it might appear that the number of thermions emitted can be increased upto a large value just by increasing the temperature of the substance in-hand. However this is not entirely true. The fact is that the number of thermions emitted is limited due to the effect of space charge – a phenomenon wherein the liberated thermions surround the electrode forming a shield, preventing the emission of further thermions.thermionic emission

Rate of Thermionic Emission

The number of thermions emitted per second from the substance considered is known as the rate of thermionic emission. This value depends on
  1. Nature of the Material
    In general, every element can be characterized by its electronic configuration i.e. by the distribution of electrons surrounding its nucleus. When we speak of thermionic emission, our particular interest is in the valence electrons (electrons in the outermost shell). This is because, these are the electrons which can be easily freed from the force of attraction so as to enable conduction. However the energy which must be supplied differs from element to element and is regarded to be its threshold energy or work function.
  2. Surface Temperature
    Higher is the temperature of the substance, greater is the rate of thermionic emission.
  3. Surface Area
    If the surface area of the material considered is larger, then there will be more number of thermions emitted. This means that the rate of thermionic emission is directly proportional to the surface area of the material.
By analyzing these factors, it can be concluded that the substance chosen to be a thermionic emitter should have low work function, larger surface area and high melting point. A few examples of this kind are metals like tungsten, thoriated tungsten, tantallum, etc and coated metals like barium oxide, strontium oxide, etc.

Thermionic Current

The flow of thermions gives rise to the flow of current known as thermionic current. Mathematically the thermionic equation which gives the current density of electrons is expressed as Where,
T is the absolute temperature
kB is the Boltzmann Constant
ΦW is the work function
e is the electron charge
A is a constant

Applications of Thermionic Emission

Thermionic emission forms the basic principle on which many of the devices used in the field of electronics and communication operate. Vacuum tubes, diode valves, cathode ray tube, electron tubes, electron microscopes, X-ray tubes, thermionic converters and electrodynamic tethers are a few among these.



Comments/Feedbacks






Closely Related Articles Electrical Engineering MaterialsWhat is an Atom?Atomic Energy LevelsClassification of Engineering MaterialsClassification of Electrical Engineering MaterialsElectron Configuration of AtomPhysical Properties of MaterialsMechanical Properties of Engineering MaterialsRutherford Atomic ModelBohrs Atomic Model Chemical Properties of MaterialsEnergy Bands in CrystalsElectrical Properties of Engineering MaterialsFermi Dirac Distribution FunctionMagnetic Properties of Engineering MaterialsSelection of Material for Engineering ApplicationBases of existence of properties in materialsQuantum NumbersLow Resistivity or High Conductivity of Conducting MaterialHigh Resistivity or Low Conductivity Conducting MaterialFactors Effecting the Resistivity of Electrical MaterialsMaterials used for Heating ElementsMaterials used for Precious WorksMaterials Used for Transmission Line ConductorElectrical Stranded ConductorsElectrical ConductorMaterials used for RheostatsMaterials for Lamp FilamentsClassification of Electrical Conducting MaterialsApplications of Carbon Materials in Electrical EngineeringSelection of Materials Used for Electrical ContactsBimetalsIonic PolarizationPiezoelectricityDielectric MaterialsMechanism of PolarizationDielectric Material as an Electric Field MediumOrientational PolarizationElectric Arc FurnaceThermal Conductivity of MetalsFree Electron Theory of MetalsMagnetostrictionAntiferroelectricityFerroelectric MaterialsElectronic PolarizationFerromagnetic MaterialsMore Related Articles Thomson Plum Pudding Model (1911)What is Superconductivity?Properties of SuperconductorsNew Articles Trees and Cotrees of Electric NetworkDifferentiatorIntegratorPhase Synchronizing Device or Controlled Switching DeviceDigital to Analog Converter or DACDifference Amplifier