Physics
Thermionic Electron Emission
Thermionic electron emission is the process of releasing electrons from a heated metal surface. This phenomenon occurs when the thermal energy of the metal exceeds the work function, which is the minimum energy required to remove an electron from the metal. The emitted electrons can be used in various applications, such as in vacuum tubes and electron microscopes.
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10 Key excerpts on "Thermionic Electron Emission"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 2 Thermionic Emission and Photoelectric Effect Thermionic emission Closeup of the filament on a low pressure mercury gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of barium, strontium and calcium oxides, the coating is sputtered away through normal use, often eventually resulting in lamp failure. ________________________ WORLD TECHNOLOGIES ________________________ Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the forces restraining it. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge will initially be left behind in the emitting region that is equal in magnitude and opposite in sign to the total charge emitted. But if the emitter is connected to a battery, then this charge left behind will be neutralized by charge supplied by the battery, as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission. The thermionic emission of electrons is also known as thermal electron emission. The classical example of thermionic emission is the emission of electrons from a hot metal cathode into a vacuum (archaically known as the Edison effect ) in a vacuum tube. However, the term thermionic emission is now used to refer to any thermally excited charge emission process, even when the charge is emitted from one solid-state region into another. This process is crucially important in the operation of a variety of electronic devices and can be used for power generation or cooling. The magnitude of the charge flow increases dramatically with increasing temperature.
- Christopher Lavers, Edmund G.R. Kraal(Authors)
- 2014(Publication Date)
- Thomas Reed(Publisher)
At a metal surface, ‘free’ electrons in the outer shells may leave the surface due to their increased velocity. However, they are attracted back by the unbalanced electric field created which sets up a potential barrier. If these free electrons can break through the potential barrier, a process termed electron emission results. Electron emission arises from various sources and is described by one of the following processes: (1) thermionic emission, (2) photoelectric emission, (3) secondary emission or (4) cold or field emission. Of these thermionic emission is considered first in some detail due to its relevance to various common electron-related devices operation and laying the principles for solid-state devices. Photoelectric and secondary emission will be discussed with respect to modern image intensifiers for ‘night vision’ while cold emission will not be discussed as significant here. Electronic Emission Processes and Devices • 273 Thermionic Emission When a metal is heated, energy is transferred to it and electrons acquire increased random velocities which may permit some to escape the metal surface’s potential barrier, in a similar manner to a rocket needing a certain minimum velocity to escape earth’s gravitational attraction. Such electrons can also be likened to vapour globules given off from a boiling water surface, where unless some means of collecting electrons as they are emitted is found, they will lose their velocities, forming a space charge . Such space charges or electron clouds give rise to a −ve charge which repels further electrons. We can picture electrons leaving a metal surface, repelled by the space charge which has built up, subsequently returning to the metal. To enable electrons to leave a heated metal surface, an additional electrode is placed adjacent to, but insulated from, the metal within a vacuum enclosure. A vacuum is needed as atmospheric pressure prevents electrons moving between –ve cathode and +ve anode.
- Yahachi Saito(Author)
- 2022(Publication Date)
- Jenny Stanford Publishing(Publisher)
Chapter 11 Theory of Thermionic Electron Emission for 2D Materials Y. S. Ang and L. K. Ang Science, Mathematics, and Technology, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, SingaporeThis chapter is intended to provide a brief overview on the recent development of the Thermionic Electron Emission models for two-dimensional (2D) materials, such as graphene. The overview is not meant to be comprehensive. Its main objective is for the readers to appreciate that the traditional models developed decades ago, like Richardson–Dushman (RD) law for thermionic emission and Fowler–Nordheim (FN) law for field emission, are no longer valid due to the unique physical properties of 2D materials. The focus is to highlight the different temperature scaling laws for thermionic emission from 2D materials at different conditions such as lateral or vertical emission and electron momentum conservation. A recently developed universal thermionic emission model for 2D electronic systems is introduced, which offers a unifying theoretical framework to understand the thermionic emission physics in 2D materials.11.1 Introduction
Electron emission from a material or charge injection from one medium to another medium is a fundamental process in cathode, diode, gas ionization, electrical contact, and many other areas. Depending on the energy used to release the electrons, it can be broadly characterized into three different processes known as thermionic emission (TE, by thermal energy), field emission (FE, by quantum tunneling), and photoemission (PE, by absorption of photons or optical tunneling of the light). The fundamental models for these emissions (TE, FE, PE) have been formulated many decades ago, known respectively, as the Richardson–Dushman (RD) law, Fowler–Nordheim (FN) law, and Fowler Dubrige (Fß) law or Keldysh model.
eBook - PDFThe Physical Basis of Electronics
An Introductory Course
- D. J. Harris, P. N. Robson, P. Hammond(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
Values of work function vary from 1-8 V for caesium to 6Ό V for platinum. There are several ways in which electrons in a conductor can be given energy to enable them to overcome the surface potential barrier and escape from the surface. The four main processes are thermionic emission, second-ary emission, photo-electric emission and field emission. In thermionic emission, thermal energy is given to the emitting material by heating it to a high temperature. Energy is transferred to the electrons, and if the energy transfer to an electron is sufficient, emission takes place. Although high temperatures are needed, this is the most commonly used emission method. Secondary emission of electrons results from bombarding the surface with high energy particles such as electrons or positive ions (gas molecules that have lost an electron and are therefore left with a net positive charge). Energy can be transferred from the incoming particle to one or more elec-trons in the surface of the material being bombarded. If any of these electrons acquire sufficient energy to enable them to overcome the retarding force at the surface, they will escape from the surface. More than one electron may be emitted for each incoming particle if the energy of this incident particle is sufficiently high. The ratio of the number of emitted electrons to incident particles is known as the secondary emission coefficient δ. Devices such as the electron-multiplier tube make use of this secondary emission phenome-non. Electrons emitted from a surface by photo-electric emission have had their energy increased by the absorption of radiation falling upon the surface. This radiation may be visible light or radiation in the invisible 10 The Physical Basis of Electronics where J is the current density in amperes per m 2 , A is a constant, Τ is the absolute temperature in degrees Kelvin (i.e. °C+273), and T 0 is equal to 11,600 φ.
eBook - PDF- H. S. W. Massey, E. W. McDaniel, B. Bederson, H. S. W. Massey, E. W. McDaniel, B. Bederson(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
1, electrons are emitted from a hot electrode and collected by a colder electrode at a high potential energy (lower potential). Part of the heat removed from the emitter by the evaporating electrons is rejected to the collector by the condensing electrons, and the remaining part is converted into electric power in the load as the electrons return to emitter potential. The various types of thermionic converters are identified primarily by the dominant EMITTER HEAT IN q £ O N -LOAD COLLECTOR HEAT OUT q r ELECTRON CURRENT J OUTPUT POWER = q E - q c Fig. 1 . Basic thermionic energy converter. electron transport mechanisms and the means for maintaining the inter-electrode plasma and electrode surface properties. Section II describes the properties of an ideal diode converter in which all interelectrode collisional and space-charge effects are absent. This establishes the basic thermodynamic characteristics and constraints under-lying the operation of all types of converters, and permits identification and characterization of all electrode surface and interelectrode effects in terms of the performance of an equivalent ideal diode converter. Section III introduces the effects of a plasma in the interelectrode space of a diode thermionic converter via an elementary analytical model, which I 1 T C 5. Thermionic Energy Conversion 171 includes the continuity of electron current, ion current, and energy flux at the plasma-electrode interfaces and across the plasma. Detailed collisional effects on plasma transport and maintenance are not included. This ele-mentary model identifies the overall effect of macroscopic plasma properties on converter performance and, through comparison with experimental converter data, permits characterization of the plasma by a few physically meaningful parameters.
- Richard G. Carter(Author)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
The energy required to enable electrons to escape may be supplied thermally, by photons, or by electron or ion bombardment. The height of the potential barrier can be reduced by a strong external electric field. These processes are discussed in the sections which follow. 18.2.1 Thermionic Emission We have seen that work functions are typically of the order of a few electron volts. It is therefore possible to approximate (18.2) for energies of the order of E F + φ by p E E E kT F ( ) = − − exp , (18.3) because E E kT F − . At 17 C for example kT = 0 025 . eV. The probability that an electron has just enough energy to escape from the metal is exp − ( ) φ kT . For example, the probability that an electron can escape from the surface of tungsten φ = ( ) 4 54 . eV is 2 10 76 × − at 300 °K but is 4 10 17 × − at 1300 °K. Since the density of conduction electrons is of the order of 10 23 cm −3 it can be seen that no emis- sion is possible at 300 °K but that appreciable emission may occur at the higher temperature. The current density of thermionic emission is governed by the Richardson- Dushman equation [1] J A T kT = − ( ) 0 2 exp . φ (18.4) Figure 18.2: Probability of occupation of states given by the Fermi-Dirac distribution function at 300 °K and 1300 °K when E F = 3 8 . eV. Emission of Electrons from Metal Surfaces 697 6 9 7 The theoretical value of the constant A 0 is A m ek h 0 0 2 3 4 120 = = π A cm K 2 2 − − (18.5) where h is Planck’s constant h = × ( ) − 6 626 10 34 . J s . The probability that an elec- tron will be reflected at the surface is negligible because of the smooth variation of potential outside the metal (see Figure 18.1). The work functions of metals depend upon temperature, and this can be modelled by rewriting (18.4) as J A T T kT = − + ( ) 0 2 exp , φ α (18.6) where α is the temperature coefficient of the work function.
eBook - PDFPhysical Electronics
Handbook of Vacuum Physics
- A. H. Beck(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
The presence of strong electric fields, caused by electrode geometry or the presence of positive ions or charged surface films, may also liberate electrons by sufficiently reducing the height and thickness of the surface barrier to enable electrons to escape by transmission through the barrier. 3.2. Photo-electric emission Photons of energy hv incident upon a metal may transfer their energy into electrons bound in the metal and if hv > φ an electron at the Fermi level could be ejected with kinetic energy, mV 2 = hv -φ. (27) ELECTRIC DISCHARGES IN GASES 35 The photo-electric emission current from a given surface is directly proportional to the intensity of illumination and the kinetic energy of the ejected electrons is a linear function of the frequency and is independent of the intensity of illumination. The photo-electric yield, i.e. number of photo-electrons liberated per incident photon depends upon the nature and state of the surface, the angle of incidence and plane of polarization of the illumination. Metal I Vacuum I 1 » Distance x = 0 FIG. 16. Potential energy of an electron outside a metal surface. In the presence of a field. In the absence of a field. 3.3. Thermionic emission At sufficiently elevated temperatures electrons may achieve energies which enable them to surmount the potential barrier. The current density J of escaping electrons when the collecting field is weak is related to the absolute temperature T of the metal by J = AT 2 e-* lKT (28) where A is a constant = 120 A cm -2 , Kis Boltzmann's constant and φ is the work function in electron volts of the surface at the temper-ature T. The energy distribution of emitted electrons is Maxwellian but may be modified by the velocity dependence of the electron reflection coefficient at the surface. This equation is due originally to Richardson and was modified by Laue and Dushman. Richardson also derived a similar equation for
eBook - PDFPhysics of Schottky Electron Sources
Theory and Optimum Operation
- Merijntje Bronsgeest(Author)
- 2016(Publication Date)
- Jenny Stanford Publishing(Publisher)
Nonclassically, there are small oscillations in the potential in the metal near the surface, even for a free-electron metal. The oscillations are associated with charge density oscillations (“Friedel” oscillations) in the direction perpendicular to the surface of a crystal that arise due to the breaking of the translational symmetry at the surface (Lang & Kohn, 1970). Because of these oscillations there is a weak reflecting region at the metal– vacuum interface, which will be addressed in more detail later (Section 1.4.1). 1.2 Emission by Heating One of the mechanisms to get electrons to overcome the potential barrier and escape from the surface is to provide them with enough energy by heating. We will discuss this mechanism using the free-electron theory of metals . (It is noted that tungsten , the main Schottky emitter material, actually is a transition metal with localized d electrons.) In the free-electron theory of metals electron states are described by plane waves with wave vectors k . The energy of an electron in a state with wave vector k is E m = planckover2pi 2 2 2 k . (1.2) The number of electron states per unit volume that have an energy between E and E + dE is r p p p p ( ) ( ) ( ) . E dE d d m EdE E dE E = = = Ê Ë Á ˆ ¯ ˜ + ÚÚÚ 2 2 2 2 4 1 2 2 3 3 3 2 2 2 3 2 k k k planckover2pi (1.3) The probability of an electron state with energy E being occupied is (Fermi–Dirac statistics) f E e E E T ( ) ( ) = + -1 1 F B k . (1.4) Emission by Heating 10 Electron Emission from a Surface At 0 K the Fermi energy is the highest of the energies of the electron states in the metal that can be occupied. At finite temperatures states with higher energies can become occupied, but at room temperature the occupation probability for an energy level as high as the potential barrier at a surface is still very small and the electron escape probability is generally negligible.
eBook - PDF- Aldo Vieira da Rosa(Author)
- 2005(Publication Date)
- Academic Press(Publisher)
For maximum efficiency, thermionic generators must operate at the maximum possible current den-sity and must, therefore, be emission limited. In a vacuum, such as may exist in the interelectrode space of some devices, there is no mechanism for scattering the emitted electrons. As a consequence, the electron motion is dictated in a simple manner by the local electric field, or, in other words, by the interelectrode potential. In the absence of charges in the interelectrode space of a parallel plate device—that is, if there are no emitted electrons between the plates—the electric field is constant and the potential varies linearly with the distance from the reference electrode (the emitter, in Figure 6.3). See curve “a” in Figure 6.4. A single electron injected into this space will suffer a constant acceleration. However, if the number of electrons is large, their collective charge will alter the potential profile causing it to sag—curve “b.” If the number of electrons in the interelectrode space is sufficiently large, the potential profile may sag so much that the electric field near the emitter becomes negative (curve “c”), thus applying a retarding force to the emitted particles. Only electrons that are expelled with sufficient initial velocity are able to overcome this barrier and find their way to the collector. This limits the current to a value smaller than that corresponding to the maximum emitter capacity. 6.7 206 Fundamentals of Renewable Energy Processes d x m V V CE V m a b c AVR Figure 6.4 Potential across a planar thermionic diode. 6.3.1 The Child-Langmuir Law Almost all thermionic generators operate under emission saturated conditions—that is, with no net space charges in the interelectrode space. Under such conditions the current does not depend on the voltage.
eBook - PDFPhysical Electronics
Handbook of Vacuum Physics
- A. H. Beck(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
The value of ψ depends on the configuration, on the areas and on the values of the work function of the different patches. 8. Noise from thermionic cathodes Noise is the statistical fluctuation in electric current, related to the special character of electricity—the existence of electrons—and to the thermal energy of these electrons. A change in the current density J = nev can be expressed by : AJ = evAn + neAv of which the first part is due to a change in the number of electrons, giving rise to the shot noise, and the second part is due to a change in the velocity of the electrons, resulting in thermal noise. The random fluctuations of the current are measured in terms of its mean square deviation from the mean current value: = (T^Tp = T 2 -CD 2 THERMIONIC EMISSION 215 Usually one is not only interested in the total noise, but even more in its frequency dependence given by the contribution AI% f in the frequency interval df. The thermionic emission from an ideal cathode is believed to be a Poisson process, which means that the probability of emission of one electron in an infinitesimal time interval at is a constant, and that the probability of emission of two or more electrons in this interval is proportional to at or to a corresponding higher power of at. In general, a large number of electrons are emitted per unit time, but it may be expected that this number will not be com-pletely constant in time. The small variations around the mean number are the origin of the shot noise and can be measured in a saturated diode. (AI^ f ) s , due to the shot noise, is independent of the frequency and has the value (31) : (KIf f ) s = 2Iedf The thermal noise plays an important part in current flow in circuits, composed of resistors, self-inductors and capacitors. Calcu-lation shows that ( 3 2 ) : ALT Wh)* = ~γ ά / in which R is the resistance of the circuit.
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