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Introduction Thermionic effect Richardson Law

PH0101 UNIT-5 LECTURE 4. Introduction Thermionic effect Richardson Law Principle, construction and working of Thermionic power generator Advantage, disadvantage and applications. 1. Introduction.

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Introduction Thermionic effect Richardson Law

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  1. PH0101 UNIT-5 LECTURE 4 • Introduction • Thermionic effect • Richardson Law • Principle, construction and working of Thermionic power generator • Advantage, disadvantage and applications PH 0101 Unit-5 Lecture-4

  2. 1. Introduction Thermionic power generator (TPG) is a device that converts heat energy into electrical energy. Thermionic emission is the basis for the working of this system. The thermionic emission is the emission of electrons from metal surface due to heat. PH 0101 Unit-5 Lecture-4

  3. In 1873, the Britain professor Frederic Guthrie invented the Thermionic phenomenon. In 1883, Thomas A. Edison observed that the electrons are emitted from a metal surface when it was heated. This effect is called Edison effect. Later in 1904, a British physicist John Ambrose Fleming developed two-element vacuum tube known as diode. After Fleming, Owen Willans Richardson worked with thermionic emission and received a Nobel Prize in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him PH 0101 Unit-5 Lecture-4

  4. A thermionic energy converter (or) thermionic power gnerator is a device consisting of two electrodes placed near one another in a vacuum.   • One electrode is normally called the cathode, or emitter, and the other is called the anode, or plate.    • Ordinarily, electrons in the cathode are prevented from escaping from the surface by a potential-energy barrier.   • When an electron starts to move away from the surface, it induces a corresponding positive charge in the material, which tends to pull it back into the surface. • To escape, the electron must somehow acquire enough energy to overcome this energy barrier.  PH 0101 Unit-5 Lecture-4

  5. At ordinary temperatures, almost none  of  the  electrons can  acquire  enough  energy to escape.   • However, when the cathode is very hot, the electron energies are greatly increased by thermal motion. •   At sufficiently high temperatures, a considerable number of electrons are able to escape. • The liberation of electrons from a hot surface is called thermionic emission PH 0101 Unit-5 Lecture-4

  6. The idea of electrons leaving the surface is shown in figure PH 0101 Unit-5 Lecture-4

  7. 2. Thermionic effect Thermionic effect is the ejection of electron from the heated metal surface and forms as electron cloud at the cathode. The number of electron emitted from the metal surface depends on temperature and work function. • Work function(φ) • For Electrons to leave the surface of the metal, they have to be supplied with enoughEnergy. • We call the minimum energy required to allow an electron to be liberated from a material as its WORK FUNCTION(φ) • Work Function differs from material to material – usually a few eV PH 0101 Unit-5 Lecture-4

  8. 3. Richardson Law Richardson law states that the emission current density is exponentially depend on work function and inversely depends upon the absolute temperature. According to Richardson, the emission current density ‘J’ can be expressed as, J = At2 e (-/KT) Ampers/m3 Where A-Emission constant (A/m3/K2)  - Work function T – absolute temperature (Kelvin) K – Boltzman constant J - emission current density. PH 0101 Unit-5 Lecture-4

  9. 4. Principle, Construction and working of Thermionic power generator (TPG) • Principle • Thermionic power generator is based on the principles of Thermionic effect that the electrons are emitted from a hot metal surface and responsible for the production of electricity. • Construction • The TPG consist of tungsten metal, which is negatively charged cathode acts as an emitter. • There is positively charged electrode is called collector. It is collecting the ejected electrons. The emitter and collector are kept in a vacuum quartz tube. PH 0101 Unit-5 Lecture-4

  10. Emitter Quartz tube Collector Thermal energy VL RL Electrical energy Thermionic Power generator PH 0101 Unit-5 Lecture-4

  11. Working • The electrons within a metal can be treated as "electron gas" in which individual outer most electrons are capable of moving freely under the influence of a field. • This movement of electrons is responsible for the function of electric circuits. • At the surface of a metal, a potential barrier exists which prevents the electrons from escaping unless certain conditions are met. This concept can be explained as follows. PH 0101 Unit-5 Lecture-4

  12. Electrons escaped from cathode C Surface of cathode Electrons Promoted from EF  B EF A • It is known from the free electron theory, at 0K, all the energy levels up to EF (fermi energy) are completely filled and all the energy level above the EF are completely empty. PH 0101 Unit-5 Lecture-4

  13. The energy level from the surface of metallic cathode to the level of EF (BC in figure) is the potential energy barrier called work function (.). • If any electron wants to escape from the surface of the metallic cathode, they should cross this potential barrier. • At 0K, all the electrons are bound within fermi energy level and cannot escape from the surface of cathode (emitter). • When the thermal energy is supplied on the emitter side, some of the electrons are promoted to above the fermi level. PH 0101 Unit-5 Lecture-4

  14. These activated electrons can cross the potential energy barrier and escape from the surface of cathode and responsible for the current production. • As long as the temperature increased, the number of electrons escapes from the surface of emitter increases. Collector collects the emitted electrons and there is an external circuit through which the current flows • The thermionic emission current density is determined by the 'work function' of the material, which is basically the magnitude of the potential energy barrier. • Good emitters should have low work functions. PH 0101 Unit-5 Lecture-4

  15. It is generally accepted that the thoriated tungsten is the best cathode metal because of its lower value of work function • The metal can be heated in two different ways. • One is direct heating and second is the indirect heating. • In the direct heating where the filament itself is the cathode and the indirect heating where the cathode is heated by a separate filament. • Pure tungsten is used as main metal in the case of direct heating method • Nickel (or) Nickel alloys are used as main metal in the case of indirect heating. PH 0101 Unit-5 Lecture-4

  16. 5. Advantage, disadvantage and applications • Advantages • Higher efficiency and high power density • Compact to use • Disadvantages • There is a possibility of vaporization of emitter surface • Thermal breaking is possible during operation • The sealing is often gets failure PH 0101 Unit-5 Lecture-4

  17. Applications • They are used in space power application for spacecraft • They are used to power submarines and boats. • They used in water pump for irrigation, • They used in power plant for industry and domestic purpose PH 0101 Unit-5 Lecture-4

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